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The Unusual Redox Properties of Fluoroferrocenes Revealed through a Comprehensive Study of the Haloferrocenes
Abstract
We report the synthesis and full characterization of the entire haloferrocene (FcX) and 1,1′-dihaloferrocene (fcX2) series (X = I, Br, Cl, F; Fc = ferrocenyl, fc = ferrocene-1,1′-diyl). Finalization of this simple, yet intriguing set of compounds has been delayed by synthetic challenges associated with the incorporation of fluorine substituents. Successful preparation of fluoroferrocene (FcF) and 1,1′-difluoroferrocene (fcF2) were ultimately achieved using reactions between the appropriate lithiated ferrocene species and N-fluorobenzenesulfonimide (NFSI). The crude reaction products, in addition to those resulting from analogous preparations of chloroferrocene (FcCl) and 1,1′-dichloroferrocene (fcCl2), were utilized as model systems to probe the limits of a previously reported "oxidative purification" methodology. From this investigation and careful solution voltammetry studies, we find that the fluorinated derivatives exhibit the lowest redox potentials of each of the FcX and fcX2 series. This counterintuitive result is discussed with reference to the spectroscopic, structural, and first-principles calculations of these and related materials.
... The introduction of a chlorine instead of a methyl group increased the redox potential by 0.21 V (compound 8a), slightly more than the +0.16 V anticipated from the work of Albrecht and Long. 151 Probably due to the effect of the trimethylsilyl substituent, all three ferrocenes 5c, 8a and 10 underwent a oneelectron totally reversible oxidation. While we recorded no evolution of the E 1/2 value between ferrocenes 5c and 10, the expected effect of a methyl substituent (-0.05V) 145 was observed between the ferrocenes 3e and 11. ...
... The introduction of iodine with the compound 12 resulted in a smaller increase of the redox potential than expected (+0.11 V vs +0.15 V). 151 Upon the introduction of the dimethylaminomethyl substituent (compound 13), an irreversible oxidation process was observed at +0.64 V, which was attributed to the oxidation of the tertiary amine into an iminium ( Figure 6). 152 The latter also raised the redox potential of the ferrocene core by +0.21 V. 152 In this series of fluorinated ferrocenesulfonyl fluorides, it appeared that the evolution of the redox potential does not follow a purely additive effect of each substituent. ...
... Indeed, many other factors such as orbitals ordering, changes in hybridization and frontier orbital composition, through-space interaction between the metal and the substituents as well as electrostatic repulsion between the metal and the Cp ring orbitals need to be considered. 147,151,154 Therefore, a better understanding of the substituent effect on the redox potential of a hetero polysubstituted ferrocene would require a dedicated study, which falls out the scope of this work. ...
Ferrocenesulfonyl fluorides represent a new subfamily of metallocenes barely explored. Here, we describe how the use of deprotolithiation-electrophilic trapping sequences and ‘halogen dance’ reactions can afford up to 1,2,3,4,5-hetero pentasubstituted derivatives. While the orthogonal reactivity of ferrocenesulfonyl fluorides was demonstrated in a variety of transformations, SuFEx chemistry afforded various sulfur-based ferrocene derivatives. The electrochemical behaviour of many polysubstituted derivatives was studied and discussed as well as weak interactions identified in the solid state.
... A notre connaissance, le seul exemple de fluoroferrocène 1,1'-disubstitué a été décrit par Long. 192 Ainsi, après une double déprotométallation dans des conditions classiques (chélate n-BuLi-TMEDA), le seul agent de fluoration qui semble aujourd'hui compatible avec les lithioferrocènes, le NFSI, est ajouté. Le 1,1'-difluoroferrocène est ainsi isolé avec un rendement de seulement 2% que l'optimisation de la synthèse (nature du solvant, échange halogène/métal au lieu de déprotométallation) n'a pas permis d'améliorer. ...
... E vs. FcH/FcH + .Des travaux ont également été réalisés sur des ferrocènes polyhalogénés. On peut citer les études de Long sur des 1,1'-difluoro-et 1,1'-dichloroferrocènes,192 de Metzler-Nolte sur des 1fluoro-1'-halogénoferrocènes,191 de Lay sur le pentachloroferrocène, 217 et de Sünkel sur le pentafluoroferrocène(Figure 82).190 Ces études réalisées, dans des conditions proches établissent un lien direct entre la présence d'halogènes et le potentiel du ferrocène. ...
... Purification de l'iodoferrocène par lavage oxydant.Long a isolé un halogénoferrocène à partir d'un mélange composé de ferrocène et d'halogénoferrocène monosubstitué.192,219 Il est allé plus loin en montrant que la concentration en FeCl3 à employer dépend du produit à purifier et donc de son potentiel rédox. ...
Ce travail de thèse est majoritairement consacré au développement d’une voie d’accès originale vers des ferrocènes 1,3-disubstitués basée sur une réaction de migration d’halogène. Dans un premier temps, cette réaction a été optimisée à partir de ferrocène carboxamides. Malgré des rendements modestes, il s’agit des premiers exemples de migration d’halogène contrôlée en série ferrocène. Les réactions parasites identifiées ont permis une meilleure compréhension des facteurs à maîtriser pour le bon déroulement de la réaction. Dans un second temps, nous avons appliqué avec succès la réaction de migration d’halogène à une nouvelle famille de composés aux propriétés uniques, les fluoroferrocènes. Non seulement cette approche a permis d’obtenir des ferrocènes diversement trisubstitués, mais il a également été possible d’accéder à des ferrocènes tétrasubstitués et au premier exemple de ferrocène hétéro- pentasubstituté. Finalement, une étude électrochimique préliminaire a été réalisée sur des ferrocènes originaux. La détermination des potentiels rédox a permis une meilleure compréhension des effets de substituants sur les propriétés électroniques de ces composés et laisse entrevoir le développement d’une méthode de purification originale.
... The same year, Metzler-Nolte and Long independently reported the synthesis of 1,1′-difluoroferrocene, Fe(η 5 -C 5 H 4 F) 2 , with electrochemical studies highlighting the additive effect of the two fluorines on the metallocene redox potential. 42,43 However, this additive relationship observed for the simple 1,1′-difluoroferrocene is in stark contrast to anomalously low oxidation potential reported for Fe(η 5 -C 5 F 5 )Cp (+0.01 V vs. FcH/FcH + ), which was attributed to a large 'perfluoro effect'. 41 ...
... M aqueous solution) (Scheme 1). 43,47,48 Further deprotonation of 2 and trapping gave a mixture of di-, tri-and tetrafluoroferrocenes in a ∼1 : 8 : 1 ratio. 1,2-Difluoferrocene (2) was removed from the product mixture by oxidative purification (3 M aqueous solution of FeCl 3 ) with compounds 3 and 4 subsequently separated by column chromatography (63 and 5% yields, respectively). ...
... This observation indicates a solvent-dependent decomposition pathway, in which the modestly nucleophilic acetonitrile reacts with the highly electrophilic polyfluoroferrocene cation, explaining the loss of reversibility in this solvent. 52,53 The redox potentials of the 1,1′-and 1,2-isomers of difluoroferrocene, 1,1′-difluoroferrocene (E 1/2 = 0.24 V) 43 and 1,2difluoroferrocene (2; E 1/2 = 0.29 V) are similar. Therefore, at least for light halogens, whether the two substituents are on the same Cp ring or distributed over both rings have comparable effects on the redox potential. ...
The sequentially fluorinated ferrocenes (1-, 1,2-di, 1,2,3-tri, 1,2,3,4-tetra and 1,2,3,4,5-pentafluoroferrocene) have been synthesized from ferrocene. Rather than a ‘perfluoro’ effect, experimental and computational analysis of the complete series robustly demonstrates...
... The only known 1,2-heterodisubstituted fluoroferrocenes are 1-[(dimethylamino)methyl]-2-fluoroferrocene, the synthesis of which caused a violent explosion, and 1-fluoro-2-(2-pyridyl)ferrocene.[50][51][52]1,2-Heterodihaloferrocenes with one fluorine substituent have not been reported so far. Similar to the synthesis of rac-7 we treated 1,2-dibromoferrocene (3) with butyllithium in THF at –78 °C followed by addition of N-fluorobenzenesulfonimide (NFSI, 1 equiv.) to obtain rac-1-bromo-2-fluoroferrocene (rac-8) in about 60 % yield along with approximately 30 % of bromoferrocene (5) as a yellow oily liquid after workup (Scheme 5). ...
... Although the calculated HOMO/LUMO gaps significantly deviate from those experimentally observed for ferrocene,[54]they may serve for comparison within this group of compounds. While the electronic structures of 1,1′-dihaloferrocenes have been investigated experimentally[55]as well as more recently theoretically,[51]this is not the case for 1,2-dihaloferrocenes.Eur. J. Inorg. ...
... Therefore, they may more easily evade steric interactions caused by the size of the halogen atoms, and they may differ in overall polarizability and interaction with solvents as compared to 1,2-dihaloferrocenes with their fixed interhalogen distances. Interestingly, the HOMO stabilization caused by 1,2-difluoro substitution (0.33 eV) is smaller than that caused by 1,1′-difluoro substitution (0.46 eV[51]). The stabilizations are more similar for the respective chloro (0.50 vs. 0.56 eV[51]), bromo (0.53 vs. 0.60 eV[51]), and iodo substitutions (0.48 vs. 0.57 eV[51]). ...
Disubstituted ferrocenes, in particular 1,2-dihaloferrocenes, are important starting materials. Here we describe new and significantly improved syntheses of some 1,2-disubstituted ferrocenes providing these compounds in high yields. These include 1,2-dibromo- and 1,2-diiodoferrocene as well as 1-bromo-2-iodoferrocene, and 1-bromo-2-fluoroferrocene. DFT calculations show that 1,2-dihaloferrocenes do not differ much in their HOMO energies, however, their LUMO energies and thus the HOMO-LUMO gaps correlate with the sum of the halogen electronegativities. In addition, a synthesis of the rather sensitive 2-aminoferrocenecarboxylic acid, the ferrocene analogue of anthranilic acid, is presented, which starts from ferrocene-1,2-dicarboxylic acid. However, 2-aminoferrocenecarboxylic acid rapidly decomposed and thus could not be properly characterized. Ferrocene-1,2-dicarboxylic acid anhydride serves as the starting material for preliminary photochemical decarbonylation/decarboxylation experiments resulting in 1,2-dideuterioferrocene when carried out in d8-toluene.
... In classical acceptor-substituted compounds, e.g. haloferrocenes, the substituents mainly stabilize the HOMO while weakly destabilizing the LUMO, 41 resulting in high redox potentials for the reversible redox couple Fe +II/+III . 42 In silylated ferrocenes, e.g. ...
... Unsubstituted ferrocene exhibits an absorption band at λ = 441 nm in the visible regime corresponding to the unresolved 1 A1 → 1 E1 and 1 A1 → 1 E2 spin-allowed d-d transitions, explaining the orange color of this compound. 41,44 Incorporation of dimethylsilyl groups causes a bathochromic shift yielding a color transition from red to purple from heptakis-to decakis(dimethylsilyl)ferrocene. This shift has been observed in previously reported polysilylated ferrocenes 2,45,46 as well. ...
We report the preparation and structural characterization of the first persilylated metallocene via the metalation of decabromoferrocene. Although Grignard conditions turned out to be insufficient due to the steric and electronic effects of silyl groups causing a decreased nucleophilicity of the metalated intermediates, stepwise lithium-halogen exchange yields complex mixtures of polysilylated compounds FeC10DMSnH10-n (n = 10, 9, 8) including the targeted decasilylated ferrocene. These mixtures were successfully separated allowing a systematic study of silylation effects on ferrocene by XRD, CV, NMR and UV/vis spectroscopy supported by DFT calculations. The findings were used to develop a high-yielding and simple preparation method to generate a tenfold substituted overcrowded ferrocene, FeC10DMS8Me2.
... Similar results were previously reported by others who plotted the E 1/2 value against the sum of different Hammett parameters, depending on the substituent studied. 53,[121][122][123][124][125] From the compound 11, the introduction of a tertiary alcohol raised the redox potential by +0.10 V with the compound R p -2c while the introduction of two alcohol groups from the ferrocene 1 only raised the redox potential by +0.17 V (compound R p ,R p -2c). This additivity trend was confirmed with the difluorinated compound R p ,R p -2d (+0.10 V per fluorine atom) although an irreversible oxidation was also observed. ...
... However, when the iodine atoms were introduced remoted from another group (compound R P ,R P -6'hdesi), the redox potential increased by +0.13 V per iodine, in better agreement with the +0.15V expected value. 124 From the tetrasubstituted ferrocene R p ,R p -2f to the pentasubstituted compound R p ,R p -6b, an increase of +0.18 V was recorded, which is lower than the expected value (+0.22 V from the work of Jahn, 51 +0.26 V from the work of Heinze 53 ). Therefore, taking together, these results tend to highlight the importance of the substituent, and its position, already present on a ferrocene derivative on the redox potential. ...
The functionalization of (R,R)-S,S'-di-tert-butylferrocene-1,1'-disulfoxide by deprotolithiation-electrophilic trapping sequences was studied towards polysubstituted, enantiopure derivatives for which the properties were determined. While the 2,2'-disubstituted ferrocene derivatives were obtained as expected, subsequent functionalization of the 2,2'-di(phenylthio) and 2,2'-bis(trimethylsilyl) derivatives occurred primarily at the 4- or 4,4'-positions. This unusual regioselectivity was discussed in detail in light of pKa values and structural data. The less sterically hindered 2,2'-difluorinated derivative yielded the expected 1,1',2,2',3,3'-hexasubstituted ferrocenes by the deprotometallation-trapping sequence. Further functionalization proved possible, leading to early examples of 1,1',2,2',3,3',4,4'-octa, nona and even decasubstituted ferrocenes. Some of the newly prepared ferrocene-1,1'-disulfoxides were tested as ligands for enantioselective catalysis and their electrochemical properties were investigated.
... When present at C3, the carboxamide and, to a lesser extent, the ester are compatible (entries 26 and 23); this order is reversed when these groups are moved to the C1' position (entries 24 and 27). These results are difficult to rationalize due to the low stability of aldehyde-and ketone-containing (Table 1, entries 13-18) and amine-based ferrocenes (entries [28][29][30][31][32][33]. Furthermore, steric hindrance can be invoked in the case of CONiPr 2 at C2 (entry 25) while coordination can be advanced for CN (entries 19-21). ...
... HOMO and HOMO-1 of iodoferrocene (1 a) could be described as mainly d orbitals of iron. [30] For iodobenzene (PhI) and 2-iodopyridine (2-IPy), HOMOs are antibonding π orbitals. LUMO and LUMO + 1 of 2-IPy are antibonding π orbital and σ* (CÀ I), correspondingly; when coming to iodobenzene and 1 a, the LUMO/LUMO + 1 order is swapped (Scheme 2a). ...
Various 2‐, 3‐ and 1’‐substituted iodoferrocenes were reacted with acetamide in the presence of copper(I) iodide (1 equiv), N,N’‐dimethylethylenediamine (1 equiv), tripotassium phosphate (2 equiv) in dioxane at 90 °C for 14 h, and allowed a large range of original 1,2‐, 1,3‐ and 1,1’‐disubstituted ferrocenes to be obtained. The results were compared as a function of the substituent and its position on the ring. DFT calculations revealed higher activation barrier for the oxidative addition in the ferrocene series when compared with classical planar aromatics. Structure‐property relationships were applied to rationalize the reactivity of the different iodoferrocenes.
... In agreement with earlier reports, the Fe d-electrons dominate the frontier orbitals, and in all neutral ferrocenes the calculated HOMO is of 3d xy character. 74−76 As expected, a clear trend was observed when calculated HOMO orbital energies (E HOMO ) were correlated to the 33 33 − +193 (+150) 35 34 − +410 (+450) 60 35 − − 842 (+60) 33 36 − +471 (+635) 61 37 − − 346 (+550) 62 38 − +32 (+239) 35 39 − − 1220 (−340) 63 40 − +67 (+50) 33 ...
... In agreement with earlier reports, the Fe d-electrons dominate the frontier orbitals, and in all neutral ferrocenes the calculated HOMO is of 3d xy character. 74−76 As expected, a clear trend was observed when calculated HOMO orbital energies (E HOMO ) were correlated to the 33 33 − +193 (+150) 35 34 − +410 (+450) 60 35 − − 842 (+60) 33 36 − +471 (+635) 61 37 − − 346 (+550) 62 38 − +32 (+239) 35 39 − − 1220 (−340) 63 40 − +67 (+50) 33 ...
Hybrid density functionals have been regularly applied in state-of-the-art computational models for predicting reduction potentials. Benchmark calculations of the absolute reduction potential of ferricenium/ferrocene couple, the IUPAC-proposed reference in nonaqueous solution, include the B3LYP/6-31G(d)/LanL2TZf protocol. We used this procedure to calculate ionization energies and reduction potentials for a comprehensive set of ferrocene derivatives. The protocol works very well for a number of derivatives. However, a significant discrepancy (> 1 V) between experimental and calculated data was detected for selected cases. Three variables were assessed to detect an origin of the observed failure: density functional, basis set, and solvation model. It comes out that the Hartree-Fock exchange fraction in hybrid-DFT methods is the main source of the error. The accidental errors were observed for other hybrid models like PBE0, BHandHLYP, and M06-2X. Therefore, hybrid DFT methods should be used with caution, or pure functionals (BLYP or M06L) may be used instead.
... Still, nearly 1 V has been tuned on the same platform. 25,26 All of these complexes are positively charged, e.g., [Fe(C 5 Cl 5 ) 2 ] + or [Ru(bpy) 3 ] 2+ . Indeed, despite the fact that ligands are either negative or neutral, very few chemically stable and robust anionic complexes are available ready for E°tuning. ...
... Rights reserved. [16,26], [(C 5 H 4 F) 2 Fe] [23] and [(C 5 H 4 F)Ru(C 5 Me 5 )] [14]. The two reports on monofluoroferrocene have two different monoclinic unit cells. ...
The crystal and molecular structures of the fluorocymantrenes [(C5H4F)Mn(CO)3] and [(C5H5−nFn)Mn (CO)2(PPh3)] (n = 1–3) have been studied. The influence of the phosphine for carbonyl substitution on the bond parameters is larger than the influence of the increasing fluorine content. In most cases the Mn → P vector is in a transoid position relative to the fluorine substituents, and therefore the conformational parameters of the PPh3 propeller are in these cases very similar. The crystal structures show many intermolecular C–H⋯O hydrogen bonds and only very few C–H⋯F hydrogen bonds.
Graphic Abstract
The influence of the phosphine for carbonyl substitution on the bond parameters of the fluorocymantrenes [(C5H4F)Mn(CO)3] and [(C5H5−nFn)Mn (CO)2(PPh3)] (n = 1–3) is larger than the influence of the increasing fluorine content.
... Both molecules show an eclipsed molecular structure [85]. The charge distributions show negative areas around the halogen atoms (more pronounced for fluorine and with a small σ-hole in the case of chlorine) and over the Cp rings, with positive charges covering the hydrogen atoms (in Figure 1 and Table S1 it is possible to detect that the positive charges are slightly higher in H2 (7) and H5(10) for fluorine than in H3(8) and H4(9) for chlorine). ...
This paper focuses in the influence of halogen atoms in the design and structural control of the crystal packing of Group VIII halogenated metallocenes. The study is based on the present knowledge on new types of intermolecular contacts such as halogen (X⋯X, C-X⋯H, C-X⋯π), π⋯π, and C-H⋯π interactions. The presence of novel C-H⋯M interactions is also discussed. Crystal packings are analysed after database search on this family of compounds. Results are supported by ab initio calculations on electrostatic charge distributions; Hirshfeld analysis is also used to predict the types of contacts to be expected in the molecules. Special attention is given to the competition among hydrogen and halogen interactions, mainly its influence on the nature and geometric orientations of the different supramolecular motifs. Supramolecular arrangements of halogenated metallocenes and Group IV di-halogenated bent metallocenes are also compared and discussed. Analysis supports halogen bonds as the predominant interactions in defining the crystal packing of bromine and iodine 1,1′-halometallocenes.
... For ferrocene, three spin-allowed transitions in the UV-Vis spectra are expected ( 1 A 1gb 1 E 1g ; 1 A 1ga 1 E 1g and 1 A 1ga 1 E 2g ), where the latter two form an overlapping band in the range of 430 to 450 nm. [46][47][48][49] This area will be used as a reference in this study. ...
Ionic liquids and deep eutectic solvents have great potential in metallurgical applications as specialised solvents. In order to design ionometallurgical electrowinning and electrorefining processes, it is essential to characterise the electrochemical behaviour of metal complexes and compare potentials between relevant solvents. For such investigations, a universal reference redox couple would be desirable. In this study we investigate the speciation and electrochemical behaviour of ferrocenium/ferrocene and hexacyanoferrate(III/II) as possible reference couples for 15 different ionic media on platinum (Pt), glassy carbon (GC) and gold (Au) working electrodes. Amongst other parameters, formal electrode potentials, charge transfer coefficients, and rate constants were calculated. It was found that neither ferrocene nor hexacyanoferrate are universally suitable as redox standards in the liquids investigated. Nevertheless, hexacyanoferrate exhibits clear advantages in most of the strongly coordinating ionic liquids studied here.
... 17 For a scan at 200 mV s −1 from 0-1.4 V ( Fig. 3a), the half wave potentials, E 1/2 , of 1, 2, and 3 are 917, 877, and 907 mV, respectively. Based on electronegativity alone, the oxidation potentials should follow the order of Cl > Br > I, 18 yet in the CV data we find that the order becomes Cl > I > Br, which suggests that there is no simple correlation between E 1/2 and substituent electronegativity in these clusters. The separation (ΔE p ) of the anodic and cathodic peak potentials are 276, 153, and 208 mV, and the ratio of the cathodic and anodic peak currents i pc /i pa are 2.2, 2.2, and 1.9 for 1, 2, and 3, respectively, suggestive of a pseudoreversible electrochemical process for them. ...
The employment of halogen-substituted pyrazole ligands in nickel(II) clusters afforded three anionic Ni(II) cubes, (HNEt3)2[Ni8(Xpz)12(OH)6] (X = Cl, 1·2CH3CN; X = Br, 2·2CH3CN; X = I, 3·12CH3CN; pz = pyrazolate). Clusters 1–3 are isolated as dianionic compounds which have similar cube geometry with each vertex occupied by a Ni(II) atom and eight μ4-OH− capped on eight square faces. The Xpz ligands adopt a bidentate mode to bind paired Ni(II) atoms on the edge of the cube. Interestingly, the three Ni8 cubes have similar symmetry, size and shape but pack together to form different lattice symmetry, which should be dictated by the halogen-related supramolecular interactions (C–X⋯H, C–X⋯π and X⋯X halogen bonding) in different crystals. The electrochemistry of 1–3 showed a Ni(II)/Ni(I) redox couple with E1/2 at ca. 900 mV and the oxidization potential order is 1 > 3 > 2 depending on the halogen substituents. 1–3 exhibit excellent electrocatalytic performances toward the oxidation of nitrite. The different packing of Ni8 cubes in 1–3 also has an important influence on their magnetic behaviors. Complex 2 showed antiferromagnetic couplings between the Ni(II) ions within the cubes, whereas 1 and 3 exclusively exhibited mixed ferromagnetic and antiferromagnetic properties, leading to frustration and typical spin glass behavior.
The incorporation of fluorinated substituents in cyclopentadienyl anions leads to distinct changes in the properties of the corresponding metal complexes, e.g., with regards to electrochemical properties or metal–ligand bonding energies. This review summarizes the synthetic challenges of the preparation and coordination of fluorinated cyclopentadienyl anions.
1 Introduction
2 Fluorinated Cyclopentadienyls
3 Cyclopentadienyls with One CF3 Group
4 Cyclopentadienyls with Four CF3 Groups
5 Cyclopentadienyls with Five CF3 Groups
6 Cyclopentadienyls with ‘Fluorous Ponytails’
7 Cyclopentadienyls with C6F5 Groups
8 Trends in Interaction Energies
9 Conclusion
The reaction of [(C5H3Br2)2Fe] with lithium tetramethylpiperidinide (LiTMP) in a 1:10 molar ratio in tetrahydrofuran yields, after quenching with C2H2Br4, a mixture of the polybromoferrocenes [C10H10–nBrnFe] with n = 4–9, from which single crystals of bis(1,2,3-tribromocyclopentadienyl)iron(II), [Fe(C5H2Br3)2], and bis(1,2,3,4-tetrabromocyclopentadienyl)iron(II), [Fe(C5HBr4)2Fe], were obtained by a combination of chromatography and fractional crystallization. Treatment of ‘[C10(HgOAc)10Fe]’ with KBr3 yields a mixture of polybromoferrocenes [C10H10–nBrnFe] with n = 8–10 and bromomercurioferrocenes [C10H9–nBrn(HgBr)Fe] with n = 7–9, from which single crystals of (1-bromomercurio-2,3,4,5-tetrabromocyclopentadienyl)(1,2,3,4,5-pentabromocyclopentadienyl)iron(II), [FeHgBr(C5Br4)(C5Br5)], were obtained by fractional crystallization. The crystal structures of all the compounds show Br⋯Br, Br⋯H and sometimes Br⋯Cp⋯π (Cp is a ring centroid) interactions, as well as π–π interactions. The findings are supported by Hirshfeld analyses.
Over the last 15 years, research activity with ferrocene derivatives as main subject has afforded more than 12,000 references in major journals, an average of 800 publications per year. This survey chapter presents an overview on the organometallic facet of ferrocene including references of more exhaustive literature reviews that focus on particular research areas. A glimpse of recent advances on the application of ferrocene derivatives in other active fields of chemistry and neighboring disciplines such as catalysis, polymer science, bioorganometallic chemistry and material science is also addressed.
The synthesis, characterisation, and isolation of 1,1′,2-tribromoferrocene and 1,1′,2,2′-tetrabromoferrocene, which are key synthons in ferrocene chemistry, are described. These compounds are prepared using α-halide assisted lithiation. The crystal structures of 1,1′,2-tribromoferrocene, 1,1′,2,2′-tetrabromoferrocene, 1,1′-dibromoruthenocene, and 1,1′,2,2′-tetrabromoruthenocene have been determined and are reported together with a brief discussion of the intramolecular forces involved in the crystal structures.
Recent advances on substituent effects in transition metal bisarene complexes studied with high-resolution threshold ionization spectroscopy are reviewed to demonstrate new aspects of the ligand influence on electronic structures of sandwich molecules. Unprecedented accuracy in the determination of ionization energies provided by the laser techniques makes it possible to reveal and describe quantitatively such fine phenomena as isotope effects, the mutual substituent influence or variations of substituent effects on replacing the central metal atom with its Group analogues. In combination with DFT calculations, laser ionization spectroscopy unveils mechanisms of the ligand influence on unique redox properties of sandwich complexes which are of key importance for their practical use.
From 2‐iodo‐N,N‐diisopropylferrocene‐carboxamide, the halogen ‘dance’ reaction was applied to deliver gram quantities of the 3‐iodo isomer. The latter was successfully functionalized by Ullmann‐type and Suzuki‐Miyaura cross‐couplings to build C‐O and C‐C bonds, respectively. Lithium/iodine exchange‐electrophilic trapping sequences were also implemented to provide ferrocenes bearing valuable functional groups. Borane‐mediated carboxamide reduction was next studied on selected substrates to deliver the corresponding amino derivatives in moderate to excellent yields. One of them, 1‐( N , N ‐diisopropylaminomethyl)‐3‐iodoferrocene, was used as a substrate to access various iodoferrocene derivatives. Not only this work constitutes a general entry into the world of 1,3‐disubstituted ferrocenes, but it also extends the chemical space available around this original scaffold.
In spite in the growing interest for fluorine-containing compounds, and the improvements in materials, optical and biological properties that can arise from substitution of a phenyl ring by ferrocene within a molecular scaffold, synthetic strategies that allow the efficient preparation of fluoroferrocene derivatives are scarce. Following conversion of ferrocene to fluoroferrocene, we have developed routes to fluorine-containing di-, tri-, tetra- and penta-substituted ferrocene derivatives to extend the available chemical space. Our approach is based on the identification of suitable reagents and conditions to achieve fluorine-directed deprotometalation, and exploitation of the halogen ‘dance’ rearrangement in the ferrocene series.
The ferrocene halogen “dance” has been deemed hardly controllable. Here, we describe the first efficient examples of this reaction. Conversion of 1-fluoro-2-iodo (or 1-chloro-2-iodo) to 1-fluoro-3-iodo (or 1-chloro-3-iodo) ferrocenes was ensured by suitably protecting the free 5-position.
A series of four ferrocenyl ester compounds, 1-methoxycarbonyl- (1), 1,1'-bis(methoxycarbonyl)- (2), 1,1',3-tris(methoxycarbonyl)- (3) and 1,1',3,3'-tetrakis(methoxycarbonyl)ferrocene (4), has been studied with respect to their potential use as redox mediators. The impact of the number and position of ester groups present in 1-4 on the electrochemical potential E1/2 is correlated with the sum of Hammett constants. The 1/1
+
-4/4
+
redox couples are chemically stable under the conditions of electrolysis as demonstrated by IR and UV-vis spectroelectrochemical methods. The energies of the C=O stretching vibrations of the ester moieties and the energies of the UV-vis absorptions of 1-4 and 1
+
-4
+
correlate with the number of ester groups. Paramagnetic 1H NMR redox titration experiments give access to the chemical shifts of 1
+
-4
+
and underline the fast electron self-exchange of the ferrocene/ferrocenium redox couples, required for rapid redox mediation in organic electrosynthesis.
Two methods were compared to convert ferrocene into N,N-diisopropylferrocenecarboxamide, N,N-diethylferrocenecarboxamide, N,N-dimethylferrocenecarboxamide, and (4-morpholinocarbonyl)ferrocene, namely, deprotometalation followed by trapping using dialkylcarbamoyl chlorides and amide formation from the intermediate carboxylic acid. The four ferrocenecarboxamides were functionalized at C²; in the case of the less hindered and more sensitive amides, recourse to a mixed lithium–zinc 2,2,6,6-tetramethylpiperidino-based base allowed us to achieve the reactions. Halogen migration using lithium amides was next optimized. Whereas it appeared impossible to isolate the less hindered 3-iodoferrocenecarboxamides, 3-iodo-N,N-diisopropylferrocenecarboxamide proved stable and was converted to new 1,3-disubstituted ferrocenes by Suzuki coupling or amide reduction. DFT calculations were used to rationalize the results obtained.
Accurate computationally-derived reduction potentials are important for catalyst design. In this contribution, relatively inexpensive DFT methods are evaluated for computing reduction potentials of a wide variety of organic, inorganic, and organometallic complexes. Astonishingly, SCRF single points on B3LYP optimized geometries with a reasonably small basis set/ECP combination works quite well--B3LYP with the BS1 [modified-LANL2DZ basis set/ECP (effective core potential) for metals, LANL2DZ(d,p) basis set/LANL2DZ ECP for heavy nonmetals (Si, P, S, Cl, and Br), and 6-31G(d') for other elements (H, C, N, O, and F)] and implicit PCM solvation models, SMD (solvation model based on density) or IEFPCM (integral equation formalism polarizable continuum model with Bondi atomic radii and α=1.1 reaction field correction factor). The IEFPCM-Bondi-B3LYP/BS1 methodology was found to be one of the least expensive and most accurate protocols, among six different density functionals tested (BP86, PBEPBE, B3LYP, B3P86, PBE0, and M06) with thirteen different basis sets (Pople split-valence basis sets, Dunning correlation consistent basis sets, or Los Alamos National Laboratory ECP/basis sets) and four solvation models (SMD, IEFPCM, IPCM, and CPCM). The MAD (mean absolute deviation) values of SCRF-B3LYP/BS1 of 49 studied species were 0.263 V for SMD and 0.233 V for IEFPCM-Bondi; and the linear correlations had respectable R2 values (R2 = 0.94 for SMD and R2 = 0.93 for IEFPCM-Bondi). These methodologies demonstrate relatively reliable, convenient, and time-saving functional/basis set/solvation model combinations in computing the reduction potentials of transition metal complexes with moderate accuracy.
A general synthesis of highly functionalized ferrocenes, which includes (P, B)- and (N, B)-ambiphiles has been developed at multigram scale. Diastereoselective stepwise modification of di-tert-butylated ferrocene included the unprecedented separation of electroactive species. Bulky alkyl groups on ferrocene insure planar chirality of ambiphiles and enforce closer proximity of antagonist Lewis functions.
C10H9FeI, monoclinic, P1c1 (no. 7), a = 6.3676(2) Å, b = 9.8475(4) Å, c = 15.2298(6) Å, β = 93.015(2)°, V= 953.7 Å3, Z = 4, Rgt(F) = 0.018, wR ref(F2) = 0.038, T = 228 K.
A much improved route to 1,1’-bis(arylethynyl)ferrocenes comprising accessible thiolates on the aryl ring is reported. Unanticipated reactions between AcCl, TBAF/BBr3 and ferrocenyl-alkynes are also discussed, offering a rationale for previous synthetic difficulties.
The syntheses and electrochemical/optical properties of some branched and linear 1,1′-substituted ferrocene complexes for molecular electronics are described. Metal centers were extended (and where relevant, connected) by arylethynyl spacers functionalized with m-pyridyl, tert-butylthiol (StBu), and trimethylsilyl (TMS) moieties. Such systems provide two well-defined molecular pathways for electron transfer and hold interesting prospects for the study of new charge transport processes, such as quantum interference, local gating, and correlated hopping events.
Functionalized aryl- or heteroaryl-magnesium reagents, prepared from the corresponding bromides or iodides by using a halogen-magnesium exchange or by a direct magnesium insertion in the presence of LiCl, reacted smoothly with N-fluorobenzenesulfonimide, (PhSO2)2NF, in the mixed solvent (4:1 CH2Cl2-perfluorodecalin) to give the corresponding aromatic fluorides in moderate to good yields.
We report the synthesis of mono- and 1,1′-difluoro-substituted metallocenes (ferrocene, ruthenocene) and of asymmetrical 1,1′-disubstituted ferrocenes with one substituent being fluorine. Lithiation of metallocenes and subsequent addition of the fluorinating agent NFSI gave the fluorinated metallocenes after optimization of the experimental conditions. All new compounds were comprehensively characterized and the cyclic voltammograms of fluoro- and 1,1′-difluoroferrocene were recorded and compared to other mono- and dihalogenated ferrocenes. Half-wave potentials of +106 mV and +220 mV vs. FcH0/+ were obtained for monofluorinated species and difluorinated ferrocene, respectively. Both values are remarkably low compared to the other halogenated ferrocenes (Cl, Br, and I). Finally, 1-bromo-1'-fluoro-ferrocene turns out to be an ideal starting material for further fluoro-substituted ferrocene derivatives.
The structural properties of 1,1′-dibromoferrocene, [Fe(C 5H4Br)2, were investigated using single-crystal x-ray diffraction. It was found to crystallize in the non-centrosymmetric space group P21. Despite of the steric demand of the two bromine substituents, the two cyclopentaldienyl (Cp) rings were found to exhibit an eclipsed conformation in the solid state. The conformational arrangement found in compound seems not to be the result of general electronic preference within this class of compounds, but has to be attributed to crystal packing effects in the solid state.
Starting from ferrocene, pentafluoroferrocene [Fe(C5F5)(C5H5)] can be prepared in five steps via a one-pot lithiation–electrophilic fluorination strategy. Pentafluoroferrocene was characterized by multinuclear NMR and IR spectroscopy, by cyclovoltammetry as well as X-ray (solid) and electron diffraction (gas) and the experimental results compared with DFT calculations.
Two synthetic approaches to bis(pentachlorophenyl) boryl ferrocene have been explored. One mirrors that used in a novel approach to FcB(C6F5)(2) from FcBBr(2), but is less selective than its perfluorinated counterpart on account of the greater steric bulk of LiC6Cl5 over LiC6F5. This approach does, however, provide a viable route to unsymmetrical mono(pentachlorophenyl) derivatives of the type FcB(C6Cl5) Ar through the intermediacy of the mono-substituted species FcB(C6Cl5) Br. FcB(C6Cl5)(2) itself is best synthesized from ferrocenyllithum and ClB(C6Cl5)(2) and is a violet-blue species featuring an extremely electron deficient Fe(II) centre (E-1/2 = +550 mV with respect to ferrocene/ferrocenium). A combination of structural, spectroscopic and reactivity studies of these and related ferrocenylboranes allow some general comments to be made concerning the relative steric and electronic properties of the C6Cl5 group. Thus, in terms of their relative capabilities as electron-withdrawing groups the substituents examined can be ranked C6Cl5 > C6F5 > Mes, while steric properties are ordered Mes > C6Cl5 > C6F5. (c) 2014 Elsevier B.V. All rights reserved.
We report a simple method for purifying large amounts of iodoferrocene, synthesized in one step from ferrocene. Halogenated ferrocene derivatives have been known for some time, but are commonly not purified, as obtaining pure samples typically requires multiple steps and often involves use of highly toxic organomercury complexes. The purification described here takes advantage of the increased oxidation potential of iodoferrocene, relative to ferrocene.
Diastereomerically pure 1,1′-diiodoferrocenes with bulky alkyl groups in 3,3′ positions are obtained by a dilithiation strategy of 1,1′-di-tert-butylferrocene (1a) and 1,1′-bis(1-ethyl-1-methylpropyl)ferrocene (1b). The 3,3′-dilithiated species are converted to tri-n-butyltin derivatives (3a and 3b), which smoothly undergo a tin–iodine exchange reaction, yielding 1,1′-diiodo-3,3′-di-tert-butylferrocene (4a) and 1,1′-diiodo-3,3′-bis(1-ethyl-1-methylpropyl)ferrocene (4b). Negishi-type coupling reactions with trifluorovinylzinc chloride afforded the highly reactive 1,1′-bis(trifluorovinyl)ferrocenes with tert-butyl groups (5a) or 1-ethyl-1-methylpropyl groups (5b) in 3,3′-positions in excellent yields. 5a and 5b undergo under redox-conditions cyclizations to ferrocenophanes by a redox-autocatalytic mechanism. The main product four-carbon handle ferrocenophanes consist of CF2 moieties in β-positions and one carbonyl function and one C(F)(H) unit (6a and 6c) or two carbonyl functions (6b and 6d) in α-positions. A ferrocenophane with a highly fluorinated handle bearing seven fluorine atoms (6e) was isolated as a byproduct. Formation of cyclic ether derivatives by unusual intramolecular substitution of fluorine is demonstrated for the case of 6a. Several new fluorinated ferrocenes were isolated and analyzed, including 1H, 13C, and 19F nuclear magnetic resonance spectroscopies. Molecular structures of five fluorinated ferrocenophanes by tert-butyl groups were elucidated with X-ray single-crystal diffraction. Influences of the electron-withdrawing fluorous substituents and the electron-donating alkyl groups on the redox behavior of the iron center were investigated by cyclic voltammetry.
This paper describes the improved synthesis and purification of iodoferrocene (Fcl) and 1,1'-diiodoferrocene (Fcl(2)). Fcl and Fcl(2) were prepared by mono- and dilithiation of ferrocene followed by conversion into iodoferrocenes by reaction with iodine. Purification was accomplished by a simple sublimation/distillation procedure, affording Fcl and Fcl(2) in high yields (74 and 72%) and high purity (>99.9%). We determined the molecular structures of Fcl and Fcl(2) by X-ray single crystal diffraction.
1,1′-Dibromoferrocene and 1,1′-diiodoferrocene are readily converted into poly(1,1′-ferrocenylene) in a 45—77% yield by dehalogenation polymerization with magnesium, n(XC5H4FeC5H4X) + nMg (\\llap–C5H4FeC5H4) \\llap–n + nMgX2. The polymer obtained was fractionated into four parts by solubilities in organic solvents: Fraction I (soluble in hexane, 23—29 wt%), Fraction II (soluble in C6H6 at room temperature but nonsoluble in hexane, 6—11 wt%), Fraction III (soluble in C6H6 at 80°C but nonsoluble at room temperature, 6—12 wt%), and Fraction IV (insoluble in C6H6 at 80°C, 47—62 wt%). Infrared (IR) and nuclear magnetic resonance (NMR) spectra of Fractions I—IV indicate that they are constituted of regularly recurring 1,1′-ferrocenylene units. The number-average molecular weights of Fractions I and II are about 900 and 1500 respectively as determined by vapor pressure osmometry (VPO), whereas that of Fraction IV was calculated as about 4600 from analytical data, assuming that both polymer ends have halogen. The polymers have high thermal stabilities and the thermal stabilities are enhanced by preparing the polymer in the presence of a nickel compound, NiCl2(2,2′-bipyridine). The X-ray diffraction pattern of the powdery polymer shows fairly sharp peaks, indicating the presence of micro crystals. Doping of Fraction IV with acceptors such as iodine and 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) afforded semi-conducting materials having electric condutivities of 10−2—10−4S cm−1 at room temperature.
Regiospecific fluorination using N-fluorobenzenesulfonimide (NFSi) and N-fluoro-O-benzenedisulfonimide (NFOBS) reagents via directed ortho metalation (Scheme 1) is reported and exceptions of PhSO2 transfer from NFSi are described.
Lithiation of ferrocene with KOtBu and tBuLi in tetrahydrofuran at −78 °C, followed by treatment with hexachloroethane gives monochloroferrocene 1 in essentially quantitative yield. Lithiation of 1 with 1.2 equiv. lithium tetramethylpiperidide (LiTMP) in THF and chlorination with C2Cl6 gives 1,2-dichloroferrocene 2 contaminated with 1 and ferrocene. Treatment of this mixture with a tenfold excess of LiTMP and C2Cl6 yields after workup 1,2,3,4,5-pentachloroferrocene 3 in 29% yield based on ferrocene. Alternatively, from this mixture of 1 and 2 1,2,3-trichloroferrocene 4 and 1,2,3,4-tetrachloroferrocene 5 can be prepared in a stepwise fashion with yields of 40% and 46%, respectively. The molecular structures of 2 and 4 have been determined by X-ray diffraction and prove the postulated regiochemistry.
Solutions of n-butyllithium and N,N,N′,N′-tetramethylethylenediamine (TMEDA) readily dilithiate ferrocene in high yields. The 1,1′-dilithioferrocene can be used in situ, or when isolated as the pyrophoric adduct [(C5H4Li)2Fe]·TMEDA, to prepare the following complexes in high yields: ferrocene-1,1′-bis(dimethylarsine) (Fdma), ferrocene-1,1′-bis(diphenylarsine) (Fdpa), ferrocene-1,1′-bis(dimethylphosphine) (Fdmp), ferrocene-1,1′-bis(diphenylphosphine) (Fdpp). Elemental sulfur reacts with solutions of dilithioferrocene in refluxing glyme to give 1,2,3-trithia-[3]ferrocenophane. This can be converted quantitatively to ferrocene-1,1′-dithiol(Fdt) by reduction with lithium aluminum hydride.
A new method for the synthesis of the mixed-ligand compound FeCpCp′ (where Cp′ denotes the pentamethyl cyclopentadienyl group) is described from the reaction of a solution of NaCp′ and LiCp with FeCl2 in THF. The mixture of products includes FeCp2, FeCpCp′ and FeCp2′ in approximately a 1 : 3 : 1 mole ratio. Partial oxidation of the mixture leads to preferential precipitation of the ferricinium salts of the derivatives with the most negative reduction potentials. Subsequent treatment with a reducing agent regenerates the ferrocenes. Implementation of a couple of redox cycles allows the isolation of pure samples of each product, including FeCpCp′. Extension of the method to the preparation of other unsymmetrical ferrocenes is straightforward so long as there is a modest spread in the reduction potentials of the components. The development of new synthetic methods is worthwhile because substituted ferrocenes are useful reagents in studies of electron-transfer processes. The method described herein is attractive because it is quite simple and adaptable to large-scale preparations.
A new high-yield synthesis of 2-pyridylferrocene (1) without formation of the 1,1′-disubstituted product has been developed. Also the corresponding ruthenocene and cymantrene derivatives [C5H4(2-C5H4N)]MLn (MLn=Ru(C5H5) (2), Mn(CO)3 (3)) were prepared and fully characterized. Ortho-lithiation of 1 followed by electrophilic halogenation yielded [C5H3X(2-C5H4N)]Fe(C5H5) [X=F (4), Cl (5), Br (6), I (7)], with 4 only being the second reported and first fully characterized fluoroferrocene. The molecular structures of 1, 4 and 6 have been determined by X-ray crystallography.
The cyclic voltammetries of 1,2,3,4,5-pentachloro- and decachloro-ferrocene have been studied in acetonitrile. The complexes undergo an irreversible two-electron oxidation consistent with an electrochemical–chemical–electrochemical mechanism at scan rates up to 10 V s–1. However, at the faster scan rates (up to 160 V s–1) available to electrodes of small radius and microelectrodes, chemically reversible one-electron oxidations to the chlorinated ferrocenium ions, [C10H10–xClxFe]+ are obtained under ambient conditions. The reversible [C10H10–xClxFe]+/0 couples when x= 10,5,2 and 1 are observed at + 1.246, +0.774, +0.315 and +0.168 V vs. ferrocenium–ferrocene, respectively. A plot of Efvs.Σσp(σp= Hammett para coefficient for the chloro substituent) shows that the neutral molecules are stabilised with respect to the corresponding ferrocenium cations by 0.16–0.12 V per Cl. The rate constants of decomposition of the [Fe(η-C5Cl5)2] and [Fe(η-C5H5)(η-C5Cl5)]+ cations were calculated by both digital simulation and the method of Nicholson and Shain to be 40 ± 20 and 200 ± 50 s–1, respectively, at room temperature (ca. 20 °C). The complexes [Fe(η-C5H4Cl)2] and [Fe(η-C5H5)(η-C5H4Cl)] exhibit reversible oxidations at all scan rates down to 0.100 V s–1 under the same conditions. Both [Fe(η-C5Cl5)2] and [Fe(η-C5H5)(η-C5Cl5)] undergo a series of irreversible two-electron reductions at potentials negative of –1.8 V, which lead to reductive dechlorination consistent with an electrochemical–chemical–electrochemical–chemical reaction.
The effect of 29 commonly encountered substituents on the chemical shifts of α, β and C5H5 positions in monosubstituted ferrocenes are tabulated and employed for determining 1H NMR assignments in 1,2- and 1,1′-disubstituted ferrocene derivatives.
An SCF Xα SW calculation using overlapping spheres has been carried out for ferrocene. The agreement with the observed optical absorption spectrum, both for d<d transitions and charge<transfer excitations, is excellent. The calculated ionization spectrum agrees well with experimental measurements, leading, however, to a different interpretation for the two lowest ionization potentials. The level ordering for the highest occupied orbitals in ferrocene and the lowest unoccupied orbital is found to be: e1g(π-Cp) e1u(π-Cp) e2g(3d) a1g(3d) *1g(3d).
A series of partially fluorinated (pentamethylcyclopentadienyl)(η5-oxocyclohexadienyl)ruthenium complexes [RuCp*(2−6-η5-C5F5-nHnCO)] (n = 1−4; Cp* = C5Me5) has been prepared by treatment of the cation [RuCp*(MeCN)3]+ with thallium(I) salts of the corresponding phenols. Depending upon the number and location of the fluorine substituents, the complexes hydrogen bond to one-half of a molecule, one molecule, or no molecules of water. Subjection of these compounds to flash vacuum thermolysis (FVT) results in extrusion of CO and selective formation of the corresponding complexes [RuCp*(η5-C5F5-nHn)] containing the monofluorocyclopentadienyl, 1,2-difluorocyclopentadienyl, 1,3-difluorocyclopentadienyl, 1,2,3-trifluorocyclopentadienyl, 1,2,4-trifluorocyclopentadienyl, and tetrafluorocyclopentadienyl ligands. 1H, 19F, and 13C NMR spectra are reported for the new cyclopentadienyl ligands, and the 13C−19F coupling constants thus obtained are used to generate a full simulation of the 13C NMR spectrum of the pentafluorocyclopentadienyl ligand in [Ru(C5Me5)(C5F5)]. The solid-state structures of two complexes containing partially fluorinated cyclopentadienyl complexes have been determined by X-ray crystallography: [RuCp*(η5-C5-1,2-F2H3)] (15), and [RuCp*(η5-C5FH4)] (17).
A series of aromatic ethynyl-bridged ferrocenes with the general formula Fc–CC–R–CC–Fc (Fc=ferrocenyl, R=C6H2(-p-CH3)2 (1), C6H4-p-C6H4 (2), C5H3N (3), 9,10–C14H8 (4), C4H2S (5), (C4H2S)2 (6) and (C4H2S)3 (7)) has been synthesised by the reaction of ethynyl ferrocene with the appropriate dibromo-arenes. The new complexes have been characterised by spectroscopic techniques. The structures of 3 and 7 were determined via X-ray crystallography, and both show the trans–trans configuration of the two ethynyl ferrocene groups with respect to the central R group. The electronic properties of the compounds have been studied via optical spectroscopy and cyclic voltammetry.
We have measured the optical absorption of gaseous ferrocene, 1,1 prime -dimethylferrocene, 1,1 prime -dibromoferrocene, and 1,1 prime -dichloroferrocene using synchrotron radiation. From these data we have estimated the ligand field parameters and noted increasing e//2//g(d) to Cp( pi ) overlap with increasing charge transfer from the Cp ring to the substitution. The optical absorption spectra for ferrocene, dibromoferrocene, and dichloroferrocene are remarkably similar. The halogen substitutions result in greater Cp( pi ) to e//2//g-(d(x2-y2)) hybridization. The e//2//g orbitals become more bonding while the a//1//g and e//1//g orbitals become more non-bonding or antibonding. This change is reflected in a change of the ligand field parameters.
The first preparation of a 1,1′-dialkynylferrocene containing two thioacetyl units as alligator clips for the application as molecular wires by a Stille coupling is described. A building block principle for the synthesis of longer and more complicated molecular wires containing more than one ferrocene unit is presented. To cite this article: M. Vollmann, H. Butenschön, C. R. Chimie 8 (2005).
This review is a personal account of the research carried out in our laboratories on the design and synthesis of ferrocene compounds used in catalysis, primarily as ligands but also in material science applications. The emphasis of the research has always been to find the simplest possible synthetic routes towards these ferrocene compounds, which may be used as convenient starting materials. This is why lithiation and quench methodology has been used extensively and successfully in most applications. It has taken many years to achieve the convenient synthesis of some very simple compounds such as 1,2-dibromoferrocene and 1,2,3,4,5-pentabromoferrocene, and the reasons for this are highlighted. The review concentrates on the synthesis of ferrocenophanes, ferrocenylphosphanes, ferrocenylalkylamines and their precursor compounds, describing some of their early coordination chemistry. Subsequently, the successful use of ferrocene-based ligands in the Lucite Alpha process is discussed. The coordination chemistry of a selection of the ligands prepared is discussed, and the rationale behind the specific ligand design strategies is given. Included is some research material, previously unpublished, which serves to fill in gaps in the overview. Finally, some derivatives prepared for applications in material science are described from a synthetic standpoint.
Two complete series of novel polychlorinated derivatives of ferrocene, (C5H(5-n)Cl(n))2Fe and C5H(5-n)Cl(n)FeC5H5, were prepared and fully characterized. In one series, compounds in which both rings of the metallocene contain the same number (from two to five) of chlorine atoms were synthesized. Included in this group is decachloroferrocene, the first metallocene derivative completely substituted with electron-withdrawing atoms. The most highly chlorinated members of this series exhibited both a degree of oxidative stability heretofore unobserved in any metallocene and an unpredictedly high thermal stability. The second series of polychlorinated ferrocenes was composed of compounds in which one ring of the metallocene was substituted with from two to five chlorine atoms while the other ring was unsubstituted. This series of derivatives exhibited a decrease in thermal stability with increased chlorine content, indicating that symmetry effects were probably instrumental in providing thermal stability to the first series. As a first step toward the synthesis of fluorine analogs of the polychlorinated ferrocenes, the preparation of fluoroferrocene was accomplished. This compound represents the first known organometallic derivative in which a fluorine atom is directly bonded to a metallocene ring.
NUMEROUS investigators have studied the reaction of Grignard reagents
with anhydrous ferric chloride, but have failed to isolate any
organo-iron compounds or to produce any definite evidence for their
formation in such reactions.
We report the large scale syntheses and 'oxidative purification' of fcI(2), fcBr(2) and FcBr (fc = ferrocene-1,1'-diyl, Fc = ferrocenyl). These valuable starting materials are typically laborious to separate via conventional techniques, but can be readily isolated by taking advantage of their increased E(1/2) relative to FcH/FcX contaminants. Our work extends this methodology towards a generic tool for the separation of redox active mixtures.
A series of binuclear homometallic complexes [M-(µ-L)-M] containing ruthenium (M) [-Ru-(dppe) 2 Cl], dppe) 1,2-bis(diphenylphosphino)ethane) and iron (M) [-C 5 H 4 -Fe-C 5 H 5 ]) ferrocenyl) and trinuclear heterobimetallic complexes [M-(µ-L)-M′-(µ-L)-M] (M′) [Pd(P n Bu 3) 2 ], [Ru(dppe) 2 ]) in which the metals are bridged by arylethynyl ligands (µ-L) -CtC{(p-C 6 H 4)CtC} n -) of various lengths (n) 1-3) was prepared and investigated, focusing on their electrochemical behavior. Depending on the length and nature of the bridge the coupling of the fully reversible electrochemical oxidations varies from strong to zero. The comproportionation constant K C and hence the stability of the intermediate mixed-valent species is discussed in view of the nature of the bridge and compared to related systems with other types of unsaturated carbon ligands. The character of the oxidized states was examined using spectroelectrochemical techniques (UV/vis/near-IR, IR, or EPR) with special focus on the intervalence charge-transfer band (IVCT) of the mixed-valent monocations and the EPR behavior. Since the IVCT bands could not be assigned unequivocally and the EPR reveals marked alkynyl ligand contribution to the oxidized state for the ruthenium complexes, an alternative assignment (intraligand transitions) for the long-wavelength bands is discussed.
The reaction between Ru2(ap)4Cl (ap = 2-anilinopyridinate) and lithiated 1,1′-diethynylferrocene resulted in the expected product 1,1′-[Ru2(ap)4(C≡C)]2Fc (1) and 1-(Me3SiC≡C),1′-[Ru2(ap)4(C≡C)]Fc (2) as a byproduct. X-ray diffraction study of compound 1 revealed an anti arrangement of two Ru2(ap)4(σ-C≡C) fragments around the Fc unit. Cyclic and differential pulse volatmmetric (CV and DPV) measurements indicated that two Ru2 termini in 1 are weakly coupled in the reduced form. Also reported is the MO analysis of coupling mechanism based on the extended Hückel calculation of 1.
Regiospecific synthetic methods have been developed for the assembly of unsymmetric conjugated molecular frameworks containing 2,5-diethynylpyridyl- and 2,5-diethynylpyridinium-linked diferrocene structures and possessing either mono- or dithioacetate end-groups that are suitable for chemisorption onto Au(111) substrates after conversion to the corresponding thiol derivatives. Electronic spectra and solution electrochemistry of these and model compounds establish the electron-withdrawing character of a 2,5-dimethoxyphenylethynyl substituent on ferrocene that serves to shift the Fe(II)/Fe(III) redox couple to higher potentials. Further, while the unsymmetric nature of the 2,5-diethynylpyridyl bridge in 3 does not differentially perturb the redox couples of the two ferrocenes (ΔE1/2 < 10 mV), upon methylation, the corresponding pyridinium moiety of 4 now produces a large separation in the two redox potentials (ΔE1/2 = 190 mV). For the two regioisomeric monothioacetate compounds bearing a terminal 2,5-diethynylpyridyl-linked diferrocene unit, 5 and 6 (and their respective pyridinium counterparts, 7 and 8), redox potentials of the two ferrocenes are found to be either widely separated or similar in value depending upon the added influence of the 2,5-dimethoxyphenylethynyl group (e.g., ΔE1/2 = 310 mV in 7 vs 50 mV in 8).
The red solid obtained by treating ferrocene with n-BuLi in the presence of tetramethylethylenediamine (TMED) has the unexpected and novel stoichiometry [(n5-C5H4Li)2Fe] 3[TMED]2 as determined by an X-ray diffraction study. Crystal data: monoclinic, space group C2/c; a = 21.565 (2) Å, b = 10.8023 (7) Å, c = 17.9956 (13) Å, β = 99.840 (1)°, V = 4130.3 (5) Å, R = 0.041 for 3084 reflections. There are three distinct lithium environments, one essentially four-coordinate and two approximately three-coordinate. The four-coordinate lithium atoms Li(3) are bonded to the TMED nitrogen atoms and to two deprotonated carbon atoms C(1) and C(11) of different ferrocene groups. One three-coordinate lithium Li(1) bridges two carbon atoms C(1) and C(6) of one ferrocene group and is bonded to C(6)′ of a second. The second three-coordinate lithium Li(2) bridges C(11) and C(11)′ of one group and is bonded to C(6) of another. Li(2) and Li(2)′ are unique in that both bridge one ferrocene group. This unit sits like a "seesaw" on top of the rest of the molecule such that the four lithium atoms (1), (1)′, (2), and (2)′ form an approximate tetrahedron; a C5H4 group is situated asymmetrically over each face. Other interesting features such as short Li-C and Li-N bonds and short Li⋯Fe and Li⋯CH interactions are discussed.
The oxidation of the title compound (1) has been investigated in solution (electrochemistry in various solvents) and the gas phase (by using the electron- transfer equilibrium method). The C5F5 ligand is compared in its electronic effect to other known cyclopentadienyl ligands.
Treatment of ferrocene with mercuric trifluoroacetate (10 equiv) and mercuric oxide (5 equiv) in 1:1 diethyl ether/ethanol afforded decakis[(trifluoroacetoxy)mercurio]ferrocene (60%) as a yellow-orange powder. Reaction with cupric chloride dihydrate in acetone yielded mixtures of partially chlorinated ferrocenes, of which decachloroferrocene was a minor component. Treatment of ferrocene with mercuric acetate (10 equiv) in refluxing dichloroethane for 18 h afforded decakis(acetoxymercurio)ferrocene (95%). Halogenation of decakis(acetoxymercurio)ferrocene with cupric chloride dihydrate in acetone, potassium tribromide in water, or potassium triiodide in water afforded decachloroferrocene (27%), decabromoferrocene (60%), and decaiodoferrocene (67%), respectively. Examination of the H-1 NMR spectra of crude decachloroferrocene and decabromoferrocene revealed small amounts (less-than-or-equal-to 5%) of partially halogenated ferrocenes, which suggested that decakis(acetoxymercurio)ferrocene was not completely decamercurated. Treatment of ruthenocene with mercuric acetate (10 equiv) in refluxing dichloroethane afforded decakis(acetoxymercurio)ruthenocene (88%). Reaction of decakis(acetoxymercurio)ruthenocene with cupric chloride dihydrate in acetone, potassium tribromide in water, or potassium triiodide in water afforded decachlororuthenocene (73%), decabromoruthenocene (47%), and decaiodoruthenocene (39%), respectively. Inspection of the H-1 NMR spectra of crude decachlororuthenocene and decabromoruthenocene showed no resonances that could be attributed to partially halogenated ruthenocenes, which indicates that decakis(acetoxymercurio)ruthenocene was greater-than-or-equal-to 98% decamercurated. Treatment of pentamethylruthenocene with mercuric acetate in 1:1 diethyl ether/ethanol afforded pentakis(acetoxymercurio)pentamethylruthenocene (88%). Pentakis(acetoxymercurio)pentamethylruthenocene showed hindered rotation of the mercury-acetate groups in the H-1 NMR spectra. Halogenation afforded pentachloropentamethylruthenocene (67%), pentabromopentamethylruthenocene (35%), and pentaiodopentamethylruthenocene (60%). Treatment of (eta5-pentamethylcyclopentadienyl)(eta5-indenyl)ruthenium(II) with mercuric acetate (greater-than-or-equal-to 3 equiv) in 1:1 diethyl ether-ethanol afforded (eta5-1,2,3-tris(acetoxYmercurio)indenyl)(eta5-pentamethylcyclopentadienyl)ruthenium(II) (99%), which could be brominated and iodinated to afford (eta5-1,2,3-tribromoindenyl)(eta5-pentamethylcyclopentadienyl) ruthenium(II) (29%) and (eta5-1,2,3-triiodoindenyl)(eta5-pentamethylcyclopentadienyl)r uthenium(II) (66%). The structure of (eta5-1,2,3-triiodoindenyl)(eta5-pentamethylcyclopentadienyl)r uthenium(II) was determined, showing that it crystallized in the monoclinic space group P2(1)/c with cell dimensions a 15.934(3) angstrom, b = 10.308(4) angstrom, c = 12.530 (5) angstrom, beta = 93.53(2)-degrees, V = 2054(1) angstrom3, and Z = 4.
Reaction of [RuCp(eta4-butadiene)Cl] (Cp = eta5-C5H5) with [Tl(OC6F5)] in refluxing THF yields [RuCp((2-6-eta5)-C6F5O)] (2b), characterized by its H-1 and F-19 NMR, and infrared spectra. Flash vacuum pyrolysis of 2b (640-degrees-C, 10(-4) Torr) gives [RuCp(eta5-C5F5)] (1b) in 84% yield. Compound 1b contains only the second example of an eta5-pentafluorocyclopentadienyl ligand and was characterized by its H-1 NMR, F-19 NMR, (C{H})-C-13-H-1 NMR, and mass spectra. The structure was confirmed by a single-crystal X-ray diffraction study. The two rings are eclipsed, with the pentafluorocyclopentadienyl ligand being significantly closer to the metal than its hydrocarbon analogue.
A new strategy is presented for the construction of a ferrocene-based molecular diode that operates by an electron-hopping mechanism. Key to this design is implementation of the asymmetric 2,5-diethynylpyridine bridging unit that serves to reversibly switch the system between two states. Results of investigations of the molecular and electronic structures of the diferrocene complexes 1 and 2, which respectively serve as models for state 1 and state 2, establish that the criteria for the new strategy can likely be met with the 2,5-diethynylpyridine bridge.
Grignard reagents have been prepared by controlled reactions of magnesium with chloroferrocene, bromoferrocene, iodoferrocene, and 1,1′-dibromoferrocene, respectively, in tetrahydrofuran. Advantageous techniques involving methyl iodide and ethylene bromide have been developed. Ferrocenyl Grignard reagents decompose at elevated temperatures to give ferrocene and biferrocenyl; in the presence of cobaltous chloride, ferrocenylmagnesium bromide gives biferrocenyl in 80% conversion. These abnormal Grignard reactions apparently involve ferrocenyl radicals.
The synthesis of N-fluoro-o-benzenedisulfonimide (NFOBS, 2) and its use as an ''electrophilic'' fluorinating reagent with nucleophilic substrates is described and compared with that of N-fluorobenzenesulfonimide (NFSi, 3). NFOBS (2) is prepared in three steps in 81% overall yield from commercially available o-benzenedisulfonic acid (4) and involves treatment of o-benzenedisulfonimide (6) with dilute fluorine (10% F-2/N-2). Reaction of 2 with metal enolates, silyl enol ethers, and 1,3-dicarbonyl compounds affords the corresponding alpha-fluoro compounds in yields up to 95%, with good control of mono- and difluorination. Fluorination of ortho-metalated aromatic compounds was achieved in modest to good yields (10-80%). While the reactivities of 2 and 3 are similar, better yields were observed with the former reagent in the fluorination of metal enolates, Grignard and lithium reagents, while 3 gave better results with the ortho-lithiated aromatic substrates. The available evidence suggests an S(N)2-type mechanism for the fluorination of nucleophilic substrates by these reagents.
The first perhalo and oxidatively stable metallocene, decachloroferrocene, and a number of polychlorinated ferrocenes were prepared from 1,1′-dichloroferrocene by a series of repetitive metalation exchange-halogenation reactions. Structures of the polychlorinated intermediates were established unequivocally by nmr and high-resolution mass spectral analysis. Ruthenocene was converted analogously to decachlororuthenocene by a similar synthetic route. A number of substituted perchlorometallocenes were obtained from the parent perhalometallocenes by metalation exchange or nucleophilic substitution reactions.
The electronic absorption spectra of various low-spin d5 and d6 metallocene complexes have been studied in different environments (glasses, KBr pellets, and single crystals) and as a function of temperature. Ferrocene, phenylferrocene, ruthenocene, and cobalticenium ion were selected in the d6 study and several ferricenium, n-butylferricenium, and 1,1′-di-n-butylferricenium salts were examined in the d5 case. The electronic spectra of ferrocene and phenylferrocene single crystals at 4.2°K clearly indicate that the bands at 22,700 and 22,400 cm-1, respectively, consist of two electronic transitions each. The corresponding band systems in ruthenocene and cobalticenium ion also exhibit asymmetry at 77°K. Ligand-field theory has been successfully applied to the above d6 metallocenes, utilizing the three observed spin-allowed d-d absorption band positions. The visible absorption spectra of the various ferricenium complexes show a low-energy charge-transfer transition (16,200 cm-1 in the unsubstituted ion). Ring substitution and temperature effects have been used to assign this band to the ligand-to-metal 2E2g → 2E1u transition. The vibrational structure observed for this band at 77°K appears as a doubled progression of the lowest energy a1g vibration. The doubling has been attributed to splitting of the 2E1u excited state. Detailed assignments of this vibrational structure are given for [Fe(cp)2]PF6 and [Fe(cp)2](CCl3CO2H)3. The intense charge-transfer band of ferrocene (∼50,000 cm-1) as well as the corresponding bands in the other d6 metallocenes have been assigned as ligand-to-metal transitions. A similar assignment has been proposed for the three strong (f ≈ 0.1) bands observed in the ferricenium ultraviolet absorption spectrum.
It is argued that electronegativity is the third dimension of the Periodic Table, and that χspec = (m∈p + n∈s)/(m + n), for representative elements where ∈p, ∈s are the p, s ionization energies and m, n the number of p, s electrons. Values of spectroscopic χ are obtained to high accuracy from the National Bureau of Standards atomic energy level tables and closely match the widely accepted Pauling and Allred & Rochow scales. χspec rationalizes the diagonal separation between metals and non-metals in the Periodic Table, the formation of noble gas molecules, metallization of the elements as one descends groups I-V, and the force definition used by Allred & Rochow. Δχspec = χspecA - χspecB, the energy difference of an average electron in atom A and in atom B, is able to systematize properties of the vast array of known materials: ionic solids, covalent molecules, metals, minerals, inorganic and organic polymers, semiconductors, etc. Transition-metal electronegativity cannot be simply determined because of the nature of d-orbital radial distributions and this is reflected in its paucity of use among transition-metal chemists. Estimates for first transition series χspec are obtained and a computational method to address this problem is given. It also proves possible to translate free atom, ground-state χspec into the in situ molecular orbital definition of average one-electron energy for orbitals localized on an atomic center. This leads to an improved definition of group (or substituent) electronegativity, extension and refinements in the use of electronegativity perturbations in qualitative and semiquantitative molecular orbital theory, and understanding of hybrid orbital electronegativity ordering rules such as sp > sp2 > sp3.
The usefulness of the N-fluoropyridinium salt system as a source of fluorinating agents was examined by using substituted or unsubstituted N-fluoropyridinium triflates 1-11, N-fluoropyridinium salts possessing other counteranions 1a-d and 3a, and the counteranion-bound salts, N-fluoropyridinium-2-sulfonates 12 and 13. Electrophilic fluorinating power was found to vary remarkably according to the electronic nature of the ring substituents. This power increased as the electron density of positive nitrogen sites decreased, and this was correlated to the pKa values of the corresponding pyridines. By virtue of this variation, it was possible to fluorinate a wide range of nucleophilic substrates differing in reactivity. It is thus possible to fluorinate aromatics, carbanions, active methylene compounds, enol alkyl or silyl ethers, vinyl acetates, ketene silyl acetals, and olefins through the proper use of salts pentachloro 6 through 2,4,6-trimethyl 2, their power decreasing in this order. All the reactions could be explained on the basis of a one-electron-transfer mechanism. N-Fluoropyridinium salts showed high chemoselectivity in fluorination, the extent depending on the reactive moiety. In consideration of these Findings, selective 9α-fluorination of steroids was carried out by reacting 1 with tris(trimethylsilyl ether) 73 of a triketo steroid. Regio- or stereoselectivity in fluorination was determined by a N-fluoropyridinium salt structure. Steric bulkiness of the N-F surroundings hindered the ortho fluorination of phenols and aniline derivatives, while the capacity for hydrogen bonding on the part of the counteranions prompted this process, and the counteranion-bound salts 12 and 13 underwent this fluorination exclusively or almost so. Both bulky N-fluoropyridinium triflates 2 and 7 preferentially attacked the 6-position of the conjugated vinyl ester of a steroid from the unhindered β-direction to give a thermally unstable 6β-fluoro isomer. On the basis of these results, N-fluoropyridinium salts may be concluded to constitute a system that can serve as a source of the most ideal fluorinating agents for conducting desired selective fluorination through fluorinating capacity or structural alteration.
The d ..-->.. d electronic transitions of Fe(II) complexes in LiCl-DCl-D/sub 2/O solutions were studied over the concentration range 1-11 m Cl/sup -//sub tot/ at 25-82 /sup 0/C. Octahedral species including Fe(D/sub 2/O)/sub 6//sup 2 +/, FeCl(D/sub 2/O)/sub 5//sup +/, FeCl/sub 2/(D/sub 2/O)/sub 4/, FeCl/sub 3/(D/sub 2/O)/sub 3//sup -/, and FeCl/sub 4/(D/sub 2/O)/sub 2//sup 2 -/ were found to be present over this range as determined by factor analysis and from the rule of the average environment. A low-energy absorption band (A/sub max/ less than or equal to 4000 cm/sup -1/) is attributed to the formation of some tetrahedral complex, possibly FeCl/sub 4//sup 2 -/, at high temperature, high chloride concentration, or both. A ..delta..H value of 36-41 kJ/mol was found for the octahedral-tetrahedral equilibrium. 56 references, 10 figures, 1 table.