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Synthesis and structure of a monomeric aluminum(i) compound [{HC(CMeNAr)2}Al] (AR = 2,6-iPr2C6H3): A stable aluminum analogue of a carbene
Abstract
A monovalent aluminum monomer [{HC(CMeNAr)2}Al] was obtained by reduction of the corresponding aluminum(III) diiodide. The molecular structure shows a unique two-coordinate aluminum center and a planar heterocyclic Al-N-C-C-C-N six-membered ring system (see picture).A nonbonded lone pair of electrons on the Al atom indicates the Lewis base character of the aluminum center. Ar = 2,6-iPr2C6H3.
... Neutral monovalent, six-electron group 13 diyls LM (Al [1], Ga [2], In [3], Tl [4]; L = HC[C(Me)NDipp] 2 ; Dipp = 2,6-i Pr 2 C 6 H 3 ) are group 13 analogues of singlet NHCcarbenes. In particular, alanediyl LAl, and gallanediyl LGa have received steadily increasing interest in recent years due to their interesting ambiphilic electronic nature [5][6][7][8], resulting from the presence of both a filled donor (HOMO) and an empty (p-type) acceptor orbital (LUMO). ...
... Similarly, the digallane (dpp-Bian)Ga-Ga(dpp-Bian) (dpp-Bian = 1,2bis[(2,6-diisopropylphenyl)imino]acenaphthene) reacted with benzaldehyde to the respective 1,2-diphenyl-1,2-ethaneoate adduct (V) [18], while its reaction with 3,6-di-tert-butylortho-benzoquinone occurred with oxidation of the Ga (II) atoms and two dpp-Bian dianions to give the mononuclear catecholate VI [19], but this reaction most likely doesn't occur via cycloaddition. In contrast, to the best of our knowledge, the formation of [1,4]cycloaddition products with diketones is limited to trapping experiments of an in situ formed monomeric in compound (I) with benzil derivatives (VII) [20]. Our general interest in the reactivity of low valent group 13 diyls LM in σ-bond activation reactions [21][22][23][24][25][26] as well as of unsaturated main group element compounds in single electron transfer and cycloaddition reactions [27][28][29][30] let us now focus on reactions of group 13 diyls LM (M = Al, Ga, In, Tl) with 1,2-diketones. ...
... The reaction of LGa with one equivalent of butanedione proceeded with [1,4]-cycloaddition and formation of the expected 5-metalla-spiro [4.5]heterodecene 1 (Scheme 2), while no defined product was isolated from analogous reactions of LAl and LIn. LAl was found to be too reactive, resulting in the formation of a rather complex product mixture, ( Figure S50), while LIn is less reactive under these conditions, resulting in an incomplete conversion of LIn ( Figure S51). ...
Monovalent group 13 diyls are versatile reagents in oxidative addition reactions. We report here [1,4]-cycloaddition reactions of β-diketiminate-substituted diyls LM (M = Al, Ga, In, Tl; L = HC[C(Me)NDipp]2, Dipp = 2,6-iPr2C6H3) with various 1,2-diketones to give 5-metalla-spiro[4.5]heterodecenes 1, 4–6, and 8–10, respectively. In contrast, the reaction of LTl with acenaphthenequinone gave the [2,3]-cycloaddition product 7, with Tl remaining in the +1 oxidation state. Compound 1 also reacted with a second equivalent of butanedione as well as with benzaldehyde in aldol-type addition reactions to the corresponding α,β-hydroxyketones 2 and 3, while a reductive activation of a benzene ring was observed in the reaction of benzil with two equivalents of LAl to give the 1,4-aluminacyclohex-2,4-dien 12. In addition, the reaction of L’BCl2 (L = HC[C(Me)NC6F5]2) with one equivalent of benzil in the presence of KC8 gave the corresponding 5-bora-spiro[4.5]heterodecene 13, whereas the hydroboration reaction of butanedione with L’BH2 (14), which was obtained from the reaction of L’BCl2 with L-selectride, failed to give the saturated 5-bora-spiro[4.5]heterodecane.
... The isolation of a monomeric cyclopentadienylaluminylene was reported recently by Braunschweig and co-workers, but its monomeric nature was confirmed solely NMR spectroscopically from the crude product in solution 39 . Thus, structural authentication of a monomeric cyclopentadienylaluminylene has remained elusive, although a few monomeric aluminylenes with σ-bonded substituents have been structurally characterized [40][41][42][43][44][45][46][47] . ...
Homobimetallic dimetallocenes exhibiting two identical metal atoms sandwiched between two η⁵ bonded cyclopentadienyl rings is a narrow class of compounds, with representative examples being dizincocene and diberyllocene. Here we report the synthesis and structural characterization of a heterobimetallic dimetallocene, accessible through heterocoupling of lithium and aluminylene fragments with pentaisopropylcyclopentadienyl ligands. The Al–Li bond features a high ionic character and profits from attractive dispersion interactions between the isopropyl groups of the cyclopentadienyl ligands. A key synthetic step is the isolation of a cyclopentadienylaluminylene monomer, which also enables the structural characterization of this species. In addition to their structural authentication by single-crystal X-ray diffraction analysis, both compounds were characterized by multinuclear NMR spectroscopy in solution and in the solid state. Furthermore, reactivity studies of the lithium–aluminium heterobimetallic dimetallocene with an N-heterocyclic carbene and different heteroallenes were performed and show that the Al–Li bond is easily cleaved.
... 26,27 In most other cases, however, experimental evidence revealed the involvement of proton catalysis, the free carbene undergoing C−C coupling with its protonated salt to form a protonated dimer, which is then deprotonated to the corresponding alkene (Figure 1b). 28−33 Whereas the heavier group 14 R 2 E: (E = Si, Ge, Sn, Pb) carbene congeners and their dimerization to the corresponding trans-bent heavier alkene congeners (R 2 E�ER 2 ) were extensively studied in the 1990s, 34 40 and alumylene, 41 both stabilized by a bulky N,Nchelating β-diketiminate ligand and formally analogous to NHCs. 42, 43 Hill and Stasch later reported the dimerization of dicoordinate indylenes and gallylenes to their apparent "alkene" analogues (Scheme 1a). ...
... In a rare case, alumoles derived from Roesky's [HC(CMeNDipp) 2 ]Al (Dipp = 2,6-diisopropylphenyl) 58 have also found applications in aggregation-induced emission (AIE) in the solid state although such fluorescence emissions in benzene are almost negligible. 59 Despite such utility, synthetic routes towards alumoles are exceedingly limited and largely rely on the employment of organo-dilithium [60][61][62][63][64][65][66] or zirconocene-based 56,[67][68][69] reagents. ...
... In a rare case, alumoles derived from Roesky's [HC(CMeNDipp)2]Al (Dipp = 2,6-diisopropylphenyl) 58 have also found applications in aggregation-induced emission (AIE) in the solid state although such fluorescence emissions in benzene are almost negligible. 59 Despite such utility, synthetic routes towards alumoles are exceedingly limited and largely rely on the employment of organo-dilithium [60][61][62][63][64][65][66] ...
Synthetic chemistry targets building up molecular complexity using simple substrates through simple processes. We disclose an unprecedented Al-atom transfer strategy for the synthesis of aluminium heterocycles with high atom economy. This strategy involves an effective cycloaddition of a free carbazolyl-aluminylene with unsaturated hydrocarbons, followed by facile cleavage of the N-Al bond. The aluminylene formally behaves as an [Al+] cation transfer reagent in a two-step manner, and the only byproduct is a carbazolide salt that can be utilized to regenerate the aluminylene. The carbazolyl Al-heterocycles show unique luminescent properties, one of which exhibits dual emission. Our approach not only has a significant impact on the future design of single-atom addition reactions, but also paves the way for emissive materials based on Al-heterocycles.
In 2018, a new class of low‐valent aluminum compound was introduced to the chemical literature. Aluminyl anions [Al (I) (L 2 )] ⁻ consist of an Al(I) center supported by a range of (predominantly chelating) dianionic ligand scaffolds, [L 2 ] ²⁻ . The resulting negative charge is balanced by a group 1 metal cation with examples spanning all of the stable alkali metals Li, Na, K, Rb, and Cs. The nature and extent of cation···anion interactions can be controlled, affording three distinct structural types classified as a contacted dimeric pair (CDP), a monomeric ion pair (MIP), or a separated ion pair (SIP). The chemistry of these systems has proven to be very diverse, primarily driven by the oxidation of the aluminum to a more stable Al(III) center. Researchers have been able to harness this thermodynamically favored process to promote a number of different reactions. This article describes the oxidative addition reactions of X–Y sigma bonds to aluminyl anions, to form the corresponding aluminate products [Al (III) (L 2 )(X)(Y)] ⁻ , containing examples of new (AlH, AlB, AlC, AlN, AlO, AlF, AlSi, and AlP) bonds. The oxidation of aluminyl anions has been expanded to access compounds with new aluminum–element multiple bonds including examples of AlCR 2 ‐, AlO‐, AlS‐, AlSe‐, AlTe‐, and AlNR‐containing compounds. These terminal bonds react via [2+2] cycloaddition with unsaturated substrates to form new products, demonstrating a preference to react through formally double AlX bonds. Finally, a brief description of the application of aluminyl anions for the reductive coupling of small molecules is included. Examples involving the homocoupling of P 4 ; the homologation of CO; and the dimerization of ketones, isocyanides, and diazomethane are provided. Under favorable conditions, these couplings have been recently extended to include examples of heterocoupling of substrates, demonstrating a high degree of control of product formation.
Rare-earth metal complexes of the parent benzene tetraanion and neutral inverse-sandwich rare-earth metal arene complexes have remained elusive. Here, we report the first neutral inverse-sandwich rare-earth metal complexes of the parent benzene tetraanion supported by a monoanionic β-diketiminate (BDI) ligand. Reduction of the trivalent rare-earth metal diiodide precursors (BDI)MI2(THF) (BDI = HC(C(Me)N[C6H3-(3-pentyl)2-2,6])2; M = Y, 1-Y; M = Sm, 1-Sm) in benzene or para-xylene by potassium graphite yielded the neutral inverse-sandwich rare-earth metal arene complexes [(BDI)M(THF)n]2(μ-η⁶,η⁶-arene) (M = Y, Sm; arene = benzene, 2-M; arene = para-xylene, 3-M). Single crystal X-ray diffraction, spectroscopic and magnetic characterization studies, together with density functional theory (DFT) calculations confirm that these neutral rare-earth metal arene complexes possess an [M³⁺–(arene)⁴⁻–M³⁺] electronic structure with strong metal–arene δ interactions. The arene exchange reactivity shows that 2-Sm has higher stability than 3-Sm. Furthermore, 2-Sm can behave as a four-electron reductant to reduce unsaturated organic substrates. Particularly, while the reaction of 2-Sm with 1,3,5,7-cyclooctatetraene (COT) yielded (BDI)Sm(η⁸-COT) (4-Sm), 2-Sm reacted with 1,4-diphenylbutadiyne to afford (BDI)Sm(η⁴-C4Ph2) (5-Sm), the first rare-earth metallacyclopentatriene complex.
A low‐valent GaI complex with the superbulky b‐diketiminate ligand DIPePBDI (HC[C(Me)N‐DIPeP]2, DIPeP = 2,6‐C(H)Et2‐phenyl)) was obtained by reduction of (DIPePBDI)GaI2 (1) with KC8 in toluene. Considering that (BDI)GaI and analogue (BDI)AlI complexes are prone to decomposition and can generally only be obtained in low yields (20‐40%), the quantitative formation of (DIPePBDI)GaI (2) is remarkable and no doubt related to its excellent thermal stability even in refluxing toluene. Although the low‐valent metal center in 2 is sterically protected by the superbulky DIPePBDI ligand, it is readily oxidized by N2O to give intermediate (DIPePBDI)Ga=O which readily decomposed by abstracting a proton from the backbone Me‐substituent. Reaction with trimethylsilyl azide gave an intermediate imido complex (DIPePBDI)Ga=N(SiMe3)2 which reacted with a second equivalent of Me3SiN3 to a mixture of an azide/amido complex (DIPePBDI)GaN3[N(SiMe3)2] (4) and a tetrazagallole complex (DIPePBDI)Ga[N4(SiMe3)2] (5) in a 1:2 ratio. Whereas the azide/amido complex 4 could be structurally characterised, the tetraazagallole complex 5 was identified by NMR spectroscopy. DFT calculations on (DIPePBDI)GaI (2) and its reaction products complement this study.
The large steric profile of the N-heterocyclic boryloxy ligand, –OB(NDippCH)2, and its ability to stabilize the metal-centered HOMO, are exploited in the synthesis of the first example of a “naked” acyclic aluminyl complex, [K(2.2.2-crypt)][Al{OB(NDippCH)2}2]. This system, which is formed by substitution at AlI (rather than reduction of AlIII), represents the first O-ligated aluminyl compound and is shown to be capable of hitherto unprecedented reversible single-site [4 + 1] cycloaddition of benzene. This chemistry and the unusual regioselectivity of the related cycloaddition of anthracene are shown to be highly dependent on the availability (or otherwise) of the K⁺ countercation.
A series of EP(V)-imine phosphine-imine ligands (Dipp-NC(CH3)-(CH2)-PE(Ph2)) (Dipp = 2,6-diisopropylphenyl, E = O, S, Se) and corresponding dimethyl aluminum and indium complexes were prepared and characterized using multi-nuclear NMR spectroscopy and SC-XRD. Solution-state 1H and 31P NMR spectroscopy indicate a mixture of isomers in solution (C6D6, CDCl3, and CD3CN). Solid state structures of both the imine and (E)-enamine isomers of the oxygen-based ligand have been observed. These three ligands (L) were then deprotonated with M(CH3)3 (M = Al or In), with methane loss forming a 6-membered ring via bidentate chelation ((κ2-L)M(CH3)2). The indium/oxide complex also crystallized as a 12-membered ring.
The significance of small molecule activation spans diverse fields, including catalysis, medicine, and energy production. This article delves into the chemistry of low‐valent group 13 and 14 compounds as a more sustainable alternative for small molecule activation compared to the often toxic and expensive transition metal compounds. Emphasizing the pivotal role of computational chemistry, particularly density functional theory (DFT) calculations, the article explores how DFT methods contribute to comprehending and predicting various small molecule activation reactions by low‐valent group 13 and 14 compounds.
This review provides an overview of main group carbene analogues, covering recent advancements, synthesis strategies, and the diverse reactivity of elements in groups 13–15 based on their structural characteristics.
Amino imidazoline‐2‐imines (AmIm) are a new class of chelating, monoanionic N,N´‐donor ligands stronger than the analogous β‐diketiminate ligands and give rise to five‐membered metallacycles. The coordination chemistry of the monoanionic ligands 1,2‐NHC=N−C6H4−NDipp− (i), cis‐1,2‐NHC=N−CMe=CMe−NDipp− (ii) was towards magnesium was investigated with the focus on the stabilization of low‐valent magnesium(I) species, NHC = cyclo‐C(NiPr2CMe)2, Dipp = 2,6‐iPrC6H3. Both ligand systems (i) and (ii) form heteroleptic magnesium iodide species with were subjected to reduction. The complex [Mg(1,2‐NHC=N−C6H4−NDipp)]2 was found to be insoluble, which prevented further purification. In contrast, the yield of the analogous complex [Mg(cis‐1,2‐NHC=N−CMe=CMe−NDipp)]2 was very low. As a significant side product the homoleptic complex [Mg(cis‐1,2‐NHC=N−CMe=CMe−NDipp)2] was identified.
Carbocyclic aluminium halides [(ADC)AlX2]2 (2‐X) (X=F, Cl, and I) based on an anionic dicarbene (ADC=PhC{N(Dipp)C}2, Dipp = 2,6‐iPr2C6H3) framework are prepared as crystalline solids by dehydrohalogenations of the alane [(ADC)AlH2]2 (1). KC8 reduction of 2‐I affords the peri‐annulated Al(III) compound [(ADCH)AlH]2 (4) (ADCH=PhC{N(Dipp)C2(DippH)N}, DippH=2‐iPr,6‐(Me2C)C6H3)) as a colorless crystalline solid in 76 % yield. The formation of 4 suggests intramolecular insertion of the putative bis‐aluminylene species [(ADC)Al]2 (3) into the methine C−H bond of HCMe2 group. Calculations predict singlet ground state for 3, while the conversion of 3 into 4 is thermodynamically favored by 61 kcal/mol. Compounds 2‐F, 2‐Cl, 2‐I, and 4 have been characterized by NMR spectroscopy and their solid‐state molecular structures have been established by single crystal X‐ray diffraction.
Five new alkylaluminum complexes with different pyridinyl-substituted imines or cyclohexyl-substituted imines were synthesized and characterized successfully. The aluminum complex [FlCHNCH(CH3)Py]AlMe2(Py=2-Pyridyl) (1) was obtained by reacting with 9-[2-Pyridyl-CH(CH3)-N=CH]Fl(Fl=Fluorenyl) (L1) and equimolar...
Quantum‐chemical (DFT) calculations on hitherto unknown base(carbene)‐stabilized gallium monoiodides (LB→GaI) suggest that these systems feature one lone pair of electrons and a formally vacant p‐orbital – both centered at the central gallium atom – and exhibit metallomimetic behavior. The calculated reaction free energies as well as bond dissociation energies suggest that these LB→GaI systems are capable of forming stable donor‐acceptor complexes with group 13 trichlorides. Examination of the ligand exchange reactions with iron and nickel complexes indicates their potential use as ligands in transition metal chemistry. In addition, it is found that the title compounds are also able to activate various enthalpically robust bonds. Further, a detailed mechanistic investigation of these small molecule activation processes reveals the non‐innocent behavior of the carbene (base) moiety attached to the GaI fragment, thereby indicating the cooperative nature of these bond activation processes. The energy decomposition analysis (EDA) and activation strain model (ASM) of reactivity were also employed to quantitatively understand and rationalize the different activation processes.
A widely utilised class of ligands in synthesis and catalysis, β‐diketiminate (BDI) or NacNac compounds were initially considered innocent in the sense that they remained intact in all their applications. That changed when the γ‐C−H unit of their NCCCN backbone was found to engage in reactions with electrophiles. Here, we show that this special reactivity can be used advantageously to prepare tripodal modifications of the common NacNac ligand derived from 2,6‐diisopropylphenyl‐β‐methyldiketimine [NacNacH (Me, Dipp)]. Lithiation to give NacNacLi, followed by reactions with isocyanates, isothiocyanates and a carbodiimide, have afforded a series of tripodal NacNac variants having N,N,N,O; N,N,N,S; or N,N,N,N potential dentation sites, many of which have been crystallographically characterised. Distinct ligating modes of these new ligands have been elucidated through the crystal structures of their lithiated derivatives.
A non‐solvated alkyl‐substituted Al(I) anion dimer was synthesized by a reduction of haloalumane precursor using a mechanochemical method. The crystallographic and theoretical analysis revealed its structure and electronic properties. Experimental XPS analysis of the Al(I) anions with reference compounds revealed the lower Al 2p binding energy corresponds to the lower oxidation state of Al species. It should be emphasized that the experimentally obtained XPS binding energies were reproduced by delta SCF calculations and were linearly correlated with NPA charges and 2p orbital energies. image
In main group chemistry, the BDI ligand ([HC(C(Me)N(R))2]), more commonly known as NacNac, has proved indispensable over the last two decades. Within this work, we are introducing the hybrid NacNac‐akin ligand 3,3‐dimethyl‐2‐[2‐methyl‐2‐(2,6‐diisopropyl‐aniline)ethenyl]‐3H‐indolenine (DNI). On the basis of this novel ligand, alkali metal complexes [DNI(M¹)] [M¹=Li (3(OEt2)), Na (4(THF)3), and K (5(THF)3)] have been synthesized by facile deprotonation of the ligand's backbone. The reaction of DNI with AlH3 ⋅ NMe2Et led to the aluminium dihydride compound [DNI‐AlH2] (6) in excellent yield (90 %). Further synthesis of [DNI‐AlI2] (7), which is seen as a potential precursor for the aluminium(I) complex, was realized in the presence of 1.0 equiv. iodine with 6. All products 2–7 were completely characterized by 2D‐NMR, mass spectrometry, elemental analysis and SC‐XRD. Density functional theory calculations were performed to provide better understanding of their bonding characteristics and electronic structures.
With fluoroaromatic compounds increasingly employed as scaffolds in agrochemicals and active pharmaceutical ingredients, the development of methods which facilitate regioselective functionalisation of their C–H and C–F bonds is a frontier of modern synthesis. Along with classical lithiation and nucleophilic aromatic substitution protocols, the vast majority of research efforts have focused on transition metal-mediated transformations enabled by the redox versatilities of these systems. Breaking new ground in this area, recent advances in main group metal chemistry have delineated unique ways in which s-block, Al, Ga and Zn metal complexes can activate this important type of fluorinated molecule. Underpinned by chemical cooperativity, these advances include either the use of heterobimetallic complexes where the combined effect of two metals within a single ligand set enables regioselective low polarity C–H metalation; or the use of novel low valent main group metal complexes supported by special stabilising ligands to induce C–F bond activations. Merging these two different approaches, this Perspective provides an overview of the emerging concept of main-group metal mediated C–H/C–F functionalisation of fluoroarenes. Showcasing the untapped potential that these systems can offer in these processes; focus is placed on how special chemical cooperation is established and how the trapping of key reaction intermediates can inform mechanistic understanding.
Like the previously reported potassium-based system, rubidium and cesium reduction of [{SiNDipp}AlI] ({SiNDipp} = {CH2SiMe2NDipp}2) with the heavier alkali metals [M = Rb and Cs] provides dimeric group 1 alumanyl derivatives, [{SiNDipp}AlM]2. In contrast, similar treatment with sodium results in over-reduction and incorporation of a formal equivalent of [{SiNDipp}Na2] into the resultant sodium alumanyl species. The dimeric K, Rb, and Cs compounds display a variable efficacy toward the C–H oxidative addition of arene C–H bonds at elevated temperatures (Cs > Rb > K, 110 °C) to yield (hydrido)(organo)aluminate species. Consistent with the synthetic experimental observations, computational (DFT) assessment of the benzene C–H activation indicates that rate-determining attack of the Al(I) nucleophile within the dimeric species is facilitated by π-engagement of the arene with the electrophilic M⁺ cation, which becomes increasingly favorable as group 1 is descended.
A non-solvated alkyl-substituted Al(I) anion dimer was synthesized by a reduction of haloalumane precursor using a mechanochemical method. The crystallographic and theo-retical analysis revealed its structure and electronic proper-ties. Experimental and theoretical XPS analysis of the di-mer with reference compounds revealed the low oxidation state of Al(I) anions. It should be emphasized that the ex-perimentally obtained XPS binding energies were repro-duced by delta SCF calculations and were linearly correlat-ed with NPA charges and 2p orbital energies.
The potassium diamidoalumanyl, [K{Al(SiNDipp)}]2 (SiNDipp = {CH2SiMe2NDipp}2), reacts with the terminal B-O bonds of pinacolato boron esters, ROBpin (R = Me, i-Pr), and B(OMe)3 to provide potsassium (alkoxy)borylaluminate derivatives, [K{Al(SiNDipp)(OR)(Bpin)}]n (R = Me, n = 2; R = i-Pr, n = ∞) and [K{Al(SiNDipp)(OMe)(B(OMe)2)}]∞, comprising Al-B σ bonds. An initial assay of the reactivity of these species with the heteroallene molecules, N,N'-diisopropylcarbodiimide and CO2, highlights the kinetic inaccessibility of their Al-B bonds; only decomposition at high temperature is observed with the carbodiimide, whereas CO2 preferentially inserts into the Al-O bond of [K{Al(SiNDipp)(OMe)(Bpin)}]2 to provide a dimeric methyl carbonate species. Treatment of the acyclic dimethoxyboryl species, however, successfully liberates a terminal alumaboronic ester featuring trigonal N2Al-BO2 coordination environments at both boron and aluminum.
A general synthetic strategy for the systematic synthesis of group 4 MIV heterometallic complexes LMIII(H)(μ-O)Si(μ-O)(OtBu)2}nMIV(NR2)4-n (L = {[HC{C(Me)N(2,6-iPr2C6H3)}2; MIII = Al or Ga; n = 1 or 2; MIV = Ti, Zr, Hf; R = Me, Et), based on alumo- or gallosilicate hydride ligands bearing a Si-OH moiety, is presented. The challenging isolation of these metalloligands involved two strategies. On the one hand, the acid-base reaction of LAlH2 with (HO)2Si(OtBu)2 yielded LAlH(μ-O)Si(OH)(OtBu)2 (1), while on the other hand, the oxidative addition of (HO)2Si(OtBu)2 to LGa produced the gallium analog (2). These metalloligands successfully stabilized two hydrogen atoms with different acid-base properties (MIII-H and SiO-H) in the same molecule. Reactivity studies between 1 and 2 and group 4 amides MIV(NR2)4 (MIV = Ti, Zr, Hf; R = Me, Et) and tuning the reactions conditions and stoichiometry led to isolation and structural characterization of heterometallic complexes 3-11 with a 1:1 or 2:1 metalloligand/MIV ratio. Notably, some of these molecular heterometallic silicate complexes stabilize for the first time terminal (O3Si-O-)MIV(NR2)3 moieties known from single-site silica-grafted species. Furthermore, the aluminum-containing heterometallic complexes possess Al-H vibrational energies similar to those reported for modified alumina surfaces, which makes them potentially suitable models for the proposed MIV species grafted onto silica/alumina surfaces with hydride and dihydride architectures.
Since the discovery that main group elements can mimic transition metals, efforts have focussed on harnessing main group elements' abilities to break strong bonds to enable new sustainable methodologies. In this regard, use of small molecules, such as those found in greenhouse gases, is a prime target for incorporation into catalytic cycles to produce value‐added products. Initial challenges in main group chemistry focussed on the ability to isolate these highly reactive low‐oxidation state species, whilst also maintaining the reactive sites. An array of low valent compounds have been successfully isolated (i.e., multiple bonds, tetrylenes, anions, and cations), through careful ligand design, and have challenged traditional bonding models and preconceptions of the main group elements. Current advances have highlighted how reactive these low‐valent sites are with the ability to activate strong bonds, such as those found in small molecules (e.g., H 2 , CO, CO 2 , etc.). This represents the first step in a redox‐based catalytic cycle, as the low‐valent main group center undergoes oxidative addition reactions. Current challenges remain in reductive elimination, and thus functionalization of the small molecule, but examples of low‐valent main group elements in catalysis are starting to emerge and represent a bright future for main group chemistry. This article highlights the achievements of low valent main group elements, with a focus on group 2, 13, and 14 elements, in small molecule activation as well as the fundamental concepts that have aided the development of this rapidly advancing field.
Low‐valent MgI complexes like (BDI)Mg−Mg(BDI) have found wide‐spread application as specialty reducing agents (BDI=β‐diketiminate). Also their redox reactivity was extensively investigated. In contrast, attempts to isolate similar CaI complexes led to reduction of the aromatic solvents or N2. Complex (DIPePBDI)Ca(μ6,μ6‐C6H6)Ca(DIPePBDI) (VIII) should be regarded a CaII complex with a bridging C6H6²⁻ dianion (DIPePBDI=HC[C(Me)N‐DIPeP]2, DIPeP=2,6‐C(H)Et2‐phenyl). It can react as a CaI synthon by releasing benzene and two electrons. Herein we describe the reactivity of VIII with benzene, biphenyl, naphthalene, anthracene, COT, Ph3SiCl, PhSiH3, a (BDI)AlI2 complex, H2, PhX (X=F, Cl, Br, I), tBuOH and tBuCH2I. The C6H6²⁻ dianion in VIII can react as a 2e⁻ source, a nucleophile or a Brønsted base. In some cases radical reactivity cannot be excluded. Crystal structures of (DIPePBDI)Ca(μ8,μ8‐COT)Ca(DIPePBDI) (1) and [(DIPePBDI)CaX ⋅ (THF)]2 (X=F, Cl, Br, I) (2–5) are described.
Im Fischer–Tropsch (FT) Prozess werden C−C Bindungen zwischen C1 Bausteinen geknüpft. Die Knüpfung solcher C−C Bindungen ist von großem Interesse in vielen Katalysezyklen. In dieser Arbeit beschreiben wir die Reaktion zwischen dem neutralen Al I Komplex ( Me NacNac)Al ( 1 , Me NacNac=HC[(CMe)(NDipp)] 2 , Dipp=2,6‐diisopropylphenyl) und verschiedenen Isocyaniden, welche somit als Modelle für den FT‐Prozess dienen. Der Stufenmechanismus wurde mittels Tieftemperatur NMR Monitoring, Isotopenmarkierung sowie quantenchemischen Rechnungen untersucht. Drei verschiedene Produkte konnten nach der Reaktion von 1 mit dem sterisch anspruchsvollen 2,6‐bis(Benzhydryl)‐4‐Me‐Phenyl Isocyanid (BhpNC) isoliert werden und belegen transiente Carbenintermediate. Die Reaktion von 1 mit Adamantylisocyanid (AdNC) ergab ein Trimerisierungsprodukt, wobei ein Carbenintermediat in Form eines Mo ⁰ Komplexes abgefangen werden konnte. Tri‐, Tetra‐, sowie Pentamerisierungsprodukte wurden mit den sterisch weniger anspruchsvollen Phenyl‐ und p ‐Methoxyphenylisocyaniden (PhNC und PMPNC) unter Ausbildung von Quinolin‐ und Indolheterozyklen erhalten. Diese Arbeit belegt Carbenintermediate in der FT‐Chemie von Al I mit Isocyaniden.
The C−C bond formation between C1 molecules plays an important role in chemistry as manifested by the Fischer–Tropsch (FT) process. Serving as models for the FT process, we report here the reactions between a neutral AlI complex (MeNacNac)Al (1, MeNacNac=HC[(CMe)(NDipp)]2, Dipp=2,6‐diisopropylphenyl) and various isocyanides. The step‐by‐step coupling mechanism was studied in detail by low‐temperature NMR monitoring, isotopic labeling, as well as quantum chemical calculations. Three different products were isolated in reaction of 1 with the sterically encumbered 2,6‐bis(benzhydryl)‐4‐Me‐phenyl isocyanide (BhpNC). These products substantiate carbene intermediates. The reaction between 1 and adamantyl isocyanide (AdNC) generated a trimerization product, and a corresponding carbene intermediate could be trapped in the form of a molybdenum(0) complex. Tri‐, tetra‐, and even pentamerization products were isolated with the sterically less congested phenyl and p‐methoxyphenyl isocyanides (PhNC and PMPNC) with concurrent construction of quinoline or indole heterocycles. Overall, this study provides evidence for carbene intermediates in FT‐type chemistry of aluminium(I) and isocyanides.
The reaction of alumylene [(Dippnacnac)Al] (1) with C60 fashions the first example of a structurally characterised aluminium-fulleride complex, [{(Dippnacnac)Al}3C60] (2), in which the Al centres are covalently bound to significantly elongated 6 : 6 bonds. Hydrolysis of 2 yields C60H6 and the reaction of 2 with [{Mesnacnac)Mg}2] cleaved off the Al fragments by affording the fulleride [{Mesnacnac)Mg}6C60].
Comprehensive computational investigations were carried out to understand the electronic and ligand properties of skeletally substituted β-diketiminate stabilized Al(I) and Ga(I) carbenoids as well as to probe their potential in small molecule activation. All of the proposed group 13 carbenoids possess a stable singlet ground state, and the majority of them have a significantly enhanced electron donation ability compared to the experimentally reported systems. The evaluation of the energetics associated with the splitting of various strong bonds such as H-H, N-H, C-F, and B-H by these carbenoids indicates that many of the proposed Al and Ga carbenoids may be considered as suitable candidates for small molecule activation.
A transient dialumene based on an amidinate scaffold was synthesized. The complex was isolated as a cycloaddition product with toluene. The mechanism of the reaction investigated using the DFT method indicates a free energy barrier of 0.9 kcal/mol with respect to the monomeric AlI intermediate. image
The reactivity of the free aluminylene [N]‐Al (1) ([N]=1,8‐bis(3,5‐di‐tert‐butylphenyl)‐3,6‐di‐tert‐butylcarbazolyl) towards boron Lewis acids is investigated. A facile oxidative addition reaction of 1 with Ph2BOBPh2 furnishes an exceedingly scarce example of the free alumaborane [N]‐Al(BPh2)(OBPh2) (2) with an Al−B electron‐sharing bond. By contrast, complexation of 1 with B(C6F5)3 and HB(C6F5)2 gives rise to the corresponding Lewis adducts [N]‐Al→B(C6F5)3 (3) and [N]‐Al→BH(C6F5)2 (4), respectively, with an Al→B dative bond. Crystallization of 4 in Et2O produces the adduct [N]‐Al(Et2O)→BH(C6F5)2 (5). Quantum chemical calculations are carried out to understand the formation of 2 as well as the bonding situation of 3 and 5.
The poorly understood factors governing the small molecule activation reactions mediated by diazaborinines have been computationally explored in detail using quantum chemical tools. To this end, the activation of E−H σ‐bonds (E = H, C, Si, N, P, O, S) has been investigated. These reactions, which proceed in a concerted manner, are exergonic and, in general, associated with relatively low activation barriers. In addition, the barrier becomes lower for the E−H bonds involving the heavier element in the same group (ΔG≠: C>Si; N>P; O>S). This reactivity trend together with the mode of action of the diazaborinine system are quantitatively analyzed by means of the activation strain model of reactivity in combination with the energy decomposition analysis method.
The reduction of a carbene‐coordinated, sterically encumbered terphenyl‐substituted aluminium diiodide, (LRAlI2), yielded a “masked” dialumene (LRAl=AlRL), self‐stabilised through [2+2] cycloaddition with a peripheral aromatic group. During the course of the reaction, a carbene‐stabilised arylalumylene (LRAl:) was generated in situ, which was trapped using an alkyne, generating an aluminacyclopropene or a C−H activated product thereof, depending on the steric bulk of the alkyne. The masked dialumene also underwent intramolecular cycloreversion and dissociation into alumylene fragments, which reacted with various organic azides to yield monomeric or dimeric iminoalanes depending on the sterics of the azide substituent. The thermodynamics of monomeric and dimeric iminoalane formation were probed by theoretical calculations.
The synthesis of novel aluminyl anion complexes has been well exploited in recent years. Moreover, the elucidation of the structure and reactivity of these complexes opens the path toward a new understanding of low-valent aluminum complexes and their chemistry. This work computationally treats the substituent effect on aluminyl anions to discover suitable alternatives for H2 activation at a high level of theory utilizing coupled-cluster techniques extrapolated to the complete basis set. The results reveal that the simplest AlH2- system is the most reactive toward the activation of H2, but due to the low steric demand, severe difficulty in the stabilization of this system makes its use nonviable. However, the results indicate that, in principle, aluminyl systems with -C, -CN, -NC, and -N chelating centers would be the best choices of ligand toward the activation of molecular hydrogen by taking care of suitable steric demand to prevent dimerization of the catalysts. Furthermore, computations show that monosubstitution (besides -H) in aluminyl anions is preferred over disubstitution. So our predictions show that bidentate ligands may yield less reactive aluminyl anions to activate H2 than monodentate ones.
Electrophilic AlIII species have long dominated the aluminum reactivity towards arenes. Recently, nucleophilic low‐valent AlI aluminyl anions have showcased oxidative additions towards arenes C−C and/or C−H bonds. Herein, we communicate compelling evidence of an AlII radical addition reaction to the benzene ring. The electron reduction of a ligand stabilized precursor with KC8 in benzene furnishes a double addition to the benzene ring instead of a C−H bond activation, producing the corresponding cyclohexa‐1,3(orl,4)‐dienes as Birch‐type reduction product. X‐ray crystallographic analysis, EPR spectroscopy, and DFT results suggest this reactivity proceeds through a stable AlII radical intermediate, whose stability is a consequence of a rigid scaffold in combination with strong steric protection.
Electrophilic Al(III) species have long dominated the aluminum reactivity towards arenes. Recently, nucleophilic low‐valent Al(I) aluminyl anions have showcased oxidative additions towards arenes C‐C and/or C‐H bonds. Herein, we communicate compelling evidence of an Al(II) radical addition reaction to the benzene ring. The electron reduction of a ligand stabilized precursor with KC8 in benzene furnishes a double addition to the benzene ring instead of a C‐H bond activation, producing the corresponding cyclohexa‐1,3(orl,4)‐dienes as Birch‐type reduction product. X‐ray crystallographic analysis, EPR spectroscopy, and DFT results suggest this unprecedented reactivity proceeds through a stable Al(II) radical intermediate, whose stability is a consequence of a rigid scaffold in combination with strong steric protection.
The chemistry of low valent p-block metal complexes continues to elicit interest in the research community, demonstrating reactivity that replicates and in some cases exceeds that of their more widely studied d-block metal counterparts. The introduction of the first aluminyl anion, a complex containing a formally anionic Al(I) centre charge balanced by an alkali metal (AM) cation, has established a platform for a new area of chemical research. The chemistry displayed by aluminyl compounds is expanding rapidly, with examples of reactivity towards a diverse range of small molecules and functional groups now reported in the literature. Herein we present an account of the structure and reactivity of the growing family of aluminyl compounds. In this context we examine the structural relationships between the aluminyl anion and the AM cations, which now include examples of AM = Li, Na, K, Rb and Cs. We report on the ability of these compounds to engage in bond-breaking and bond-forming reactions, which is leading towards their application as useful reagents in chemical synthesis. Furthermore we discuss the chemistry of bimetallic complexes containing direct Al-M bonds (M = Li, Na, K, Mg, Ca, Cu, Ag, Au, Zn) and compounds with Al-E multiple bonds (E = NR, CR2, O, S, Se, Te), where both classes of compound are derived directly from aluminyl anions.
We report the oxidative addition of phenylsilane to the complete series of alkali metal (AM) aluminyls [AM{Al(NONDipp)}]2 (AM = Li, Na, K, Rb, and Cs). Crystalline products (1-AM) have been isolated as ether or THF adducts, [AM(L)n][Al(NONDipp)(H)(SiH2Ph)] (AM = Li, Na, K, Rb, L = Et2O, n = 1; AM = Cs, L = THF, n = 2). Further to this series, the novel rubidium rubidiate, [{Rb(THF)4}2(Rb{Al(NONDipp)(H)(SiH2Ph)}2)]+ [Rb{Al(NONDipp)(H)(SiH2Ph)}2]-, was isolated during an attempted recrystallization of Rb[Al(NONDipp)(H)(SiH2Ph)] from a hexane/THF mixture. Structural and spectroscopic characterizations of the series 1-AM confirm the presence of μ-hydrides that bridge the aluminum and alkali metals (AM), with multiple stabilizing AM···π(arene) interactions to either the Dipp- or Ph-substituents. These products form a complete series of soluble, alkali metal (hydrido) aluminates that present a platform for further reactivity studies.
The oxidative addition of C−C bonds in aromatic hydrocarbons by low valent main group species has attracted considerable attention from both theoretical and experimental chemists due to the big challenge in breaking their aromaticity. Herein, a general strategy to break the C−C bonds in benzene by cyclic (alkyl)(amino)aluminyl anion is demonstrated via density functional theory (DFT) calculations. The results suggest that the activation of the C−C bond of benzene by this anion is both kinetically and thermodynamically unfavorable whereas introducing electron‐withdrawing groups makes such C−C bond activation becomes favorable both kinetically and thermodynamically. Such a sharp change on the kinetics and thermodynamics could be rationalized by the frontier molecular orbital theory by decreasing the lowest unoccupied molecular orbitals of the mono‐ and disubstituted benzenes. Aromaticity is found to stabilize the transition state for the ring open step. All these findings can help develop the chemistry of small‐molecule activation.
The transmetalation reaction of picolyl-supported tridentate nacnac germylene monochloride [2,6-iPr2-C6H3NC(Me)CHC(Me)NH(CH2py)]GeCl (1) (py = pyridine) with SnCl2 results in an analogous stannylene chloride (2). The three-coordinated stannylenium cation [{2,6-iPr2-C6H3NC(Me)CHC(Me)NH(CH2py)}Sn]+ with SnCl3- as a counteranion (3) has been generated through the abstraction of chloride ligand from 2 using an additional equivalent of SnCl2. Instead of forming a donor-acceptor complex, 2 undergoes a facile redox transmetalation reaction with Ni(COD)2 (COD = cyclooctadiene) and CuCl to afford analogous nickel and copper complexes [2,6-iPr2-C6H3NC(Me)CHC(Me)NH(CH2py)]MCl [M = Ni (4) and Cu (5)]. The reactions of 4 with potassium tri-sec-butylborohydride (commonly known as K-selectride) and AgSbF6 provide access to monomeric Ni(II) hydride, [2,6-iPr2-C6H3NC(Me)CHC(Me)NH(CH2py)]NiH (6) and a Ni(II) cation, [{2,6-iPr2-C6H3NC(Me)CHC(Me)NH(CH2py)}Ni][SbF6] (7), respectively. 6 was found to be an effective catalyst for the hydroboration of amides.
A description of the ab initio quantum chemistry package GAMESS is presented. Chemical systems containing atoms through radon can be treated with wave functions ranging from the simplest closed-shell case up to a general MCSCF case, permitting calculations at the necessary level of sophistication. Emphasis is given to novel features of the program. The parallelization strategy used in the RHF, ROHF, UHF, and GVB sections of the program is described, and detailed speecup results are given. Parallel calculations can be run on ordinary workstations as well as dedicated parallel machines. © John Wiley & Sons, Inc.
Two intramolecular stabilized arylaluminum dihydrides, {2-(NEt2CH2)-6-MeC6H3}AlH2 (1) and {2,6-(NEt2CH2)2C6H3}AlH2 (2), were prepared by reducing the corresponding dichlorides with an excess of LiAlH4 in diethyl ether. Reactions of 1 and 2 with elemental selenium afforded the dimeric arylaluminum selenides [{2-(NEt2CH2)-6-MeC6H3}AlSe]2 (3) and [{2,6-(NEt2CH2)2C6H3}AlSe]2 (4). Reaction of 2 with metallic tellurium gave the dimeric arylaluminum telluride [{2,6-(NEt2CH2)2C6H3}AlTe]2 (5). The possible reaction pathway is discussed, and molecular structures determined by single-crystal X-ray analyses are presented for 3 and 5.
Recent developments in multiple bonding between heavier main group atoms are the focus of this review. Emphasis is placed on compounds with homonuclear bonds. It is clear that the Group 15 derivatives REER (E = P, As, Sb or Bi; R = alkyl or aryl ligand) display double bonding throughout the group. For the Group 14 species R2EER2 (E = Si, Ge, Sn or Pb, R = organo or related group), it is argued that, at present, only the silicon and certain germanium derivatives merit designation as ‘dimetallenes’. Data for multiply bonded heavier Group 13 compounds are currently very scarce. Nonetheless, the available structures of compounds such as (MR)n (M = Al, Ga, In or Tl; R = alkyl or aryl group; n = 1—6) indicate weakness of the M—M interaction especially for the gallium, indium and thallium compounds where monomeric species are obtained readily. The M—M bond order in the dimers RMMR is apparently less than 1 but can be increased by reduction to give [RMMR]2– but it is probable that the overall M—M bond order remains less than 2.
Al 4 Se 4 ‐ und Al 4 Te 4 ‐Würfel sind die zentralen Struktureinheiten der Heterocubane [(Cp*AlSe) 4 ] bzw. [(Cp*AlTe) 4 ] 1 (Strukturbild rechts), die bei der Umsetzung der tetrameren Al I ‐Verbindung [Cp*Al) 4 ] mit elementarem Selen bzw. Tellur entstehen. Triebkraft dieser glatt verlaufenden Umsetzungen ist die Umwandlung Al I → Al III . magnified image magnified image
Eine bei Raumtemperatur stabile Aluminium(I)‐Verbindung ist das tetramere, η ⁵ ‐koordinierte [{Al(C 5 Me 5 )} 4 ] 1 , das aus AlCl und [Mg(C 5 Me 5 ) 2 ] herstellbar ist. In diesem Cluster bilden vier Al‐Atome ein reguläres Tetraeder, und die Ebenen der C 5 Me 5 ‐Ringe liegen nahezu parallel zur jeweils gegenüberliegenden Basisfläche des Tetraeders. Siehe auch Highlight auf S. 559. magnified image
The inorganic and organometallic chemistry of indium(I) encompasses Lewis acid and base behaviour, metathesis, and oxidation reactions in which one-electron transfer has been identified; indium(II) compounds may be in the form of either ionic dimers or metal-metal bonded species. The relation between these structures illustrate the complexity of the chemistry of the lower oxidation states of the Main Group elements.
27Al-NMR spectroscopy to investigate and characterize new cyclopentadienylaluminum(I) derivatives formed in the reaction of AlX (X=Cl, Br, Cp* (=C5Me5)) with MRn (n=1: M=alkaline metal; n=2: M=Mg; R=cyclopentadienyl derivative (CpD), N(SiMe3)2): formation of new monomeric and tetrameric Al(I) species and X-ray structure analysis of a new aluminum(I) compound with an Al4-tetrahedron.
The tetrameric, eta-5-coordinated aluminum(I) compound \{Al(C5Me5)}4\ (1) is stable at room temperature. It was prepared from AlCl and [Mg(C5Me5)2]. In this cluster, four Al atoms form a regular tetrahedron and the planes of the C5Me5 rings lie nearly parallel to the respective faces of the tetrahedron.
The first neutral aluminium(I) compound Al4Cp*4 [Angew. Chem. 103 (1991) 594; Angew. Chem. Int. Ed. Engl. 30 (1991) 564] was published in 1991. Until now other neutral aluminum polyhedra, e.g. Al4[Si(t-Bu)3]4 [Organometallics 17 (1998) 1894], Al4[C(SiMe3)3]4 [Angew. Chem. 110 (1998) 2059; Angew. Chem. Int. Ed. Engl. 37 (1998) 1952] and Al4Cp*3[N(SiMe3)2] [J. Organomet. Chem. 561 (1998) 203], could be synthesized and structurally characterized. All of them form Al4 tetrahedra—obviously a preferred structural unit of aluminium(I) compounds. The reaction of [AlBr·NEt3]4 [Angew. Chem. 106 (1994) 1860; Angew. Chem. Int. Ed. Engl. 33 (1994) 1754] with [(Me3Si)3SiLi·3THF] leads, in a yield of 25%, to tetrakis[tris(trimethylsilyl)silylaluminium(I)] Al4[Si(SiMe3)3]41, another compound of this type containing an Al4 tetrahedron in an environment of four distorted tetrahedral Si4 units: Al4(Si4)4(CH3)36.
This review covers the organometallic derivatives of aluminium. There are two associated reviews which cover inorganic and heterometallic aluminium complexes. There are over 350 of organoaluminium X-ray crystallographic studies in which aluminium is found almost exclusively in the +3 oxidation state. The most common geometry around aluminium is tetrahedral, but other arrangements with from three to ten donor sites are also found. Dimeric derivatives are the most common, followed by monomeric derivatives. Higher degrees of oligomerization are also found, but there are only three examples which can be classified as polymers. Several examples exhibit metallic bonding between aluminium atoms. While the most common carbon-bonded ligand is the methyl group, a variety of other organic ligands are observed, including π-bonding moieties. The structures are classified according to structural type, and correlations between bonding parameters are noted.
1-Azaallylaluminum dihydride [RAlH(μ-H)]2 (1) (R = N(SiMe3)C(Ph)C(SiMe3)2) has been prepared in nearly quantitative yield from the reduction of RAlBr2 with an excess of LiAlH4 in diethyl ether. Reaction of 1 with elemental Se or Te in toluene afforded the novel organoaluminum chalcogenide [RAl(μ-E)]2 (E = Se (2); Te (3)) in good yield. The structures of compounds 1, 2, and 3 in the solid state have been characterized by X-ray diffraction analyses. Compounds 2 and 3 have a dimeric structure featuring three novel fused planar four-membered ring systems with a central Al2E2 core. Based on 1H and 29Si NMR data and crystal structural analysis of 2 and 3, an equilibrium of the trans and cis isomers in solution is proposed for the two compounds due to the relative orientation of the two chelating rings of the bidentate R ligands.
The synthesis and structures of new aluminum complexes using the 1-aza-allyl ligand R (R = [N(SiMe3)C(Ph)C(SiMe3)2]-) are described. The reaction of RLi·THF with AlMe2Cl, AlMeCl2, AlCl3, and AlBr3 in diethyl ether or n-hexane, after workup, afforded RAlMe2 (1), RAlMeCl (2), RAlCl2 (3), and RAlBr2 (4), respectively, while [RAlF(μ-F)]2 (5) or RAlI2 (6) was prepared in high yield by the reaction of RAlMe2(1) with 2 equiv of Me3SnF or I2, respectively, in toluene. The complex 2 or 3 reacts with an excess of THF to give the corresponding THF adduct RAlClMe·THF (7) or RAlCl2·THF (8). Compounds 7 and 8 are not stable; the coordinated THF can be easily removed in vacuo at ambient temperature. The molecular structures of 3, 5, and 8 have been established by X-ray crystallography. Compound 3 is a monomer with a chelating η3 1-aza-allyl ligand, while compound 8 forms an open structure with an η1 pendant ligand. Compound 5 is the first example of a dimeric aluminum difluoride, in which two bridging F atoms reside on the pseudo 2-fold axis of the approximately C2 symmetric molecule. The two pentacoordinated aluminum atoms can be described as having distorted trigonal-bipyramidal geometries, which have in common one bridging F atom in an apical position and the other bridging F atom in an equatorial position.
The reaction of (AlI·NEt3)4 with donor-free t-Bu3SiNa in toluene at −78 °C leads to the generation and crystallization of Al4[Si(t-Bu)3]4, whichafter Al4Cp*4 is the second structurally characterized Al4R4 compound. The obtained experimental data were compared with results of ab initio calculations.
The preparation and reaction chemistry of β-diketiminato aluminum complexes are described. (TTP)AlCl2 (1) (TTPH = 2-(p-tolylamino)-4-(p-tolylimino)-2-pentene) is formed by the treatment of AlCl3 with LiTTP. Sequential alkylation of 1 with CH3Li results in the formation of the mono- and dimethyl aluminum complexes (TTP)AlMeCl (2) and (TTP)AlMe2 (3), respectively. Only monoalkyl complexes are produced when more hindered alkyllithium reagents are used. Compounds 2 and 3 are more conveniently prepared by treating Al(CH3)3 with TTPH·HCl and TTPH, respectively. The more sterically hindered β-diketimine ligand 2-((2,6-diisopropylphenyl)amino)-4-((2,6-diisopropylphenyl)imino)-2-pentene (DDPH) also reacts smoothly with Al(CH3)3 to yield (DDP)Al(CH3)2 (4). Compound 3 undergoes methyl abstraction reactions upon addition of B(C6F5)3 or AgOTf. Cationic species formed from 3 and B(C6F5)3 are unstable and decompose to (TTP)Al(CH3)(C6F5) and MeB(C6F5)2. In contrast, (TTP)Al(CH3)(OTf) (6) is thermally stable, but the triflate group is surprisingly inert toward displacement by Lewis bases. Compounds 1, 3, 4, and 6 were crystallographically characterized. The structures all indicate that the β-diketiminato backbone is essentially planar. The pseudotetrahedral aluminum center is displaced from the plane formed by the ligand backbone in 4 by 0.72 Å.
The synthesis of new compounds containing Al−Al, Ga−Ga, and In−In bonds, the 1-aza-allyl ligand R (R = [(Me3Si)2C(Ph)C(Me3Si)N]), and halide substituents is described. The gallium(III) compound RGaCl2 (1) was prepared by reaction of GaCl3 with RLi·THF. The reduction of a mixture of RAlI2 and RAlClI with potassium afforded [RIAl−AlClR] (2). [RGaCl]2 (3) was synthesized by the reduction of RGaCl2 with sodium/potassium alloy, while [RInBr]2 (4) was prepared by reaction of RLi·THF with indium(I) bromide. The molecular structures of 1, 2, 3, and 4 have been established by X-ray crystallography.
Gas-phase electron diffraction data of Cp*Al = (C(5)Me(5))AL, Me = CH3, recorded with reservoir and nozzle temperatures of 139 +/- 4 degrees C, show that the gas consists of monomeric (eta(5)-Cp*)Al units. Least-squares refinements of a molecular model of C-5v symmetry yield the bond distances AI-C = 238.8(7), C-C(endocyclic) = 141.4(5), and C-C(exocyclic) = 152.9(5) pm and a perpendicular metal-to-ring distance of 206.3(9) pm.
We report ab initio calculations of Al-27 NMR chemical shifts for a variety of Al(I) compounds using the gauge-including atomic orbital method at the self-consistent-field and second-order Moller-Plesset perturbation theory levels. The calculated values, which include one of the most shielded (AlCp) and deshielded (AlSitBu3) NMR shifts known so far for aluminum compounds, are rationalized in terms of a molecular orbital picture of the bonding and compared to the available experimental data for Al(I) compounds. For AlCl, it is shown that solvation effects have to be included in order to reproduce the experimental NMR shift of +35 ppm of a AlCl/toluene/ether solution, while for AlCp and AlCp* the Al-27 chemical shifts (-111 and -80.7 ppm, respectively) at low temperatures (<30-degrees-C) can be assigned on the basis of the calculated values to the tetramers Al4Cp4 and Al4Cp*4. Increasing the temperature above 30-degrees-C led in the case of the AlCp* solution to a new NMR signal at -149 ppm, which is in good agreement with the theoretically predicted value of -143 ppm for the AlCp* monomer. An analysis of the temperature-dependent NMR spectra yields an estimate of about 150 +/- 20 kJ/mol for the dissociation energy of Al4Cp*4 into four AlCp*. For AlCp, a similar analysis was, however, not possible due to the thermal instability of the AlCp solution, which decomposes above -60-degrees-C. No assignment of the experimentally observed Al-27 NMR spectrum has been so far possible in the case of AlSitBu3, but the calculations show that both solvation and electron correlation effects have to be included in a reliable theoretical study of the Al-27 chemical shifts of AlSiR3 compounds.
A number of extensions to the multisolution approach to the crystallographic phase problem are discussed in which the negative quartet relations play an important role. A phase annealing method, related to the simulated annealing approach in other optimization problems, is proposed and it is shown that it can result in an improvement of up to an order of magnitude in the chances of solving large structures at atomic resolution. The ideas presented here are incorporated in the program system SHELX-90; the philosophical and mathematical background to the direct-methods part (SHELXS) of this system is described.
Bis(1-aza-allyl) aluminum compounds of formula R′2AlY (Y=Cl (1), Me (2); R′=N(SiMe3)C(But)CH(SiMe3)) were obtained in good yields by the reaction of (LiR′)2 with equivalent amounts of AlCl3 and AlMeCl2, respectively, in diethyl ether. The reaction of compound 1 with one equivalent of AlCl3 in toluene afforded the ionic species [R′2Al][AlCl4] (3). Treatment of RLi·THF (R=N(SiMe3)C(Ph)C(SiMe3)2) with GaCl3 and BiBr3 yielded mono(1-aza-allyl) complexes RGaCl2 (4) and RBiBr2 (5), respectively. Complexes 1, 2, 4 and 5 have been characterized by multinuclear NMR spectroscopy, mass spectroscopy and elemental analysis. The characterization of compound 3 is based on multinuclear NMR spectroscopy and physical properties, whereas single crystal X-ray diffraction data are provided for compound 1.