Figure 5 - uploaded by Yisong Guo
Content may be subject to copyright.
XANES spectra of 3(b)-Cl (-), 3(b)-Me 2 Im (---), and 5(b) (-· ·-). Inset: Magnified preedge features.
Source publication
The pentadentate ligand (n)Bu-P2DA (2(b), (n)Bu-P2DA = N-(1',1'-bis(2-pyridyl)pentyl)iminodiacetate) was designed to bind an iron center in a carboxylate-rich environment similar to that found in the active sites of TauD and other α-ketoglutarate-dependent mononuclear non-heme iron enzymes. The iron(II) complex (n)Bu(4)N[Fe(II)(Cl)((n)Bu-P2DA)] (3(...
Contexts in source publication
Context 1
... gain a greater understanding of the structural and electronic properties of the N-adducts of 3(b) we turned to X-ray absorption spectroscopy (XAS). Figure 5 shows the X-ray Absorption Near Edge Structure (XANES) features of 3(b)-Cl and 3(b)-Me 2 Im with their respective parameters listed in Table S2. The iron K-edge of 3(b)-Cl is downshifted by ca. 1 eV relative to that of 3(b)-Me 2 Im, suggesting that 3(b)-Cl has a lower ionization potential than 3(b)-Me 2 Im, as would be expected for the substitution of an anionic chloride by a neutral imidazole ligand. ...
Context 2
... extinction coefficient (ε) for this band was determined to be 220 M −1 cm −1 by parallel UV-vis analysis of the Mössbauer sample (see below). A maximum yield of 90% oxoiron(IV) was obtained with 3(b)-Me 2 Im as the precursor ( Figure S16 Figure S15) and their lower K-edges ( Figure 5). However, the use of Me 2 Im or DMAP as an ancillary ligand provided the appropriate balance between the rates of formation and decay to allow 5(b) to be formed in significant yield for detailed spectroscopic characterization. ...
Context 3
... gain a greater understanding of the structural and electronic properties of the N-adducts of 3(b) we turned to X-ray absorption spectroscopy (XAS). Figure 5 shows the X-ray Absorption Near Edge Structure (XANES) features of 3(b)-Cl and 3(b)-Me 2 Im with their respective parameters listed in Table S2. The iron K-edge of 3(b)-Cl is downshifted by ca. 1 eV relative to that of 3(b)-Me 2 Im, suggesting that 3(b)-Cl has a lower ionization potential than 3(b)-Me 2 Im, as would be expected for the substitution of an anionic chloride by a neutral imidazole ligand. ...
Context 4
... extinction coefficient (ε) for this band was determined to be 220 M −1 cm −1 by parallel UV-vis analysis of the Mössbauer sample (see below). A maximum yield of 90% oxoiron(IV) was obtained with 3(b)-Me 2 Im as the precursor ( Figure S16 Figure S15) and their lower K-edges ( Figure 5). However, the use of Me 2 Im or DMAP as an ancillary ligand provided the appropriate balance between the rates of formation and decay to allow 5(b) to be formed in significant yield for detailed spectroscopic characterization. ...
Citations
... Coordination complexes of tetrapodal pentadentate ligands have been studied in a variety of contexts including but not limited to hydrogen evolution, [1][2][3][4][5] water oxidation, [6][7][8] and enzymatic model complexes. [9][10][11][12] This topology provides a welldefined square-pyramidal coordination environment with good stability towards labile first-row transition metals by virtue of the chelate effect while restricting substrate access to a single coordination site. Both charge neutral and anionic frameworks have been prepared; an excellent overview of the variety of donor atoms that can be found in this topology is described by Axelson and coworkers. ...
We report the first tetrapodal pentadentate ligand composed of five N‐heterocyclic carbene donors. The proto‐ligand [CC4H5Me](OTf)3 (3) is formed in good yields from commercially available reagents. Upon removal of 5 proton equivalents from 3 with bulky non‐nucleophilic bases a dianionic penta‐carbene framework is provided in good yields as a dilithium complex CC4MeLi2 (4). Addition of FeCl3 to a solution of 4 formed in situ provides CC4MeFeCl (5) in moderate yield. Solution‐state magnetism measurements of 5 are consistent with a S=1/2 Fe(III) center. The related diamagnetic Fe(II) compound CC4MeFe (6) can be formed through reduction of 5 using KC8 though poor solubility characteristics have hampered its formation on a preparative scale and full characterization.
... Nowadays, thiosemi-and semicarbazide-based Schiff base transition metal complexes show structural diversity due to versatile coordination modes of thiosemi-and semicarbazides [15,75,76]. These types of metal complexes are very much important in different fields of catalysis such as catechol oxidase or phenoxazinone synthase activity [16,20,77,78]. In particular, semicarbazides are really interesting from the scientific point of view since they can coordinate the metal centre in both neutral and anionic forms [1,17]. ...
... In particular, semicarbazides are really interesting from the scientific point of view since they can coordinate the metal centre in both neutral and anionic forms [1,17]. As a result, model study of different metalloenzymes is useful in order to assess organic catalytic conversions [18,20,77,78]. Metalloenzymes normally activate kinetically inert triplet aerial oxygen to form extremely active singlet species in order to oxidize organic substances [10]. ...
Two novel cobalt(ii) complexes [Co(HL¹)2](NO3)2.2.5H2O (1) and [Co(HL²)2](NO3)2 (2) (where HL¹ = (E)-2-(1-(pyridin-2-yl)ethylidene)hydrazine-1-carboxamide and HL² = (E)-2-(pyridin-2-ylmethylene)hydrazine-1-carboxamide) have been synthesized and structurally characterized by spectroscopic techniques and single-crystal diffraction analysis. The complexes are close comparable with metals exhibiting the expected distorted octahedral geometry being chelated by two semicarbazone ligands via NNO donor set. The catecholase-like activity of complexes 1 and 2 was evaluated by using 3,5-di-tert-butylcatecholas substrate. The results showed that both the complexes are effective catalysts with Kcat values of 762 and 562, respectively. Theoretical DFT study and Hirschfeld surface analyses were also carried out to reveal the nature of intermolecular contacts and to integrate experimental observations.
Graphical abstract
Two novel cobalt(ii) complexes [Co(HL¹)2](NO3)2.2.5H2O (1) and [Co(HL²)2](NO3)2 (2) have been synthesized and structurally characterized by spectroscopic techniques and single-crystal diffraction analysis. The complexes are nearly akin with metals exhibiting the expected distorted octahedral geometry being chelated by two semicarbazone ligands via NNO donor set. The catecholase-like activity of complexes 1 and 2 was evaluated by using 3,5-DTBC as substrate. The result confirmed the formation of quinone or 3,5-DTBQ derivative and indicates that the complexes exhibit noticeable catalytic activity with Kcat values of 762 and 562, respectively. Theoretical DFT study and Hirschfeld surface analyses were also performed to reveal the nature of intermolecular contacts and to integrate experimental findings.
... Over recent years, a great number of carboxylate complexes of a variety of metallic ions have been synthesized and characterized, the interest for such compounds arising from the following reasons: (i) the versatility of the carboxylate group that presents various coordination modes [1][2][3][4][5] and generates interesting architectures; [6,7] (ii) carboxylate complexes present biological activities [8,9] and physiological effects; [10,11] and (iii) some carboxylate complexes mimic functional and structural features of enzymes. [12,13] In addition, special interest was given to designing and characterization of new metal complexes that contain mixed ligands with carboxylate groups and N donor ligands, compounds that would make a joint between the carboxylate groups and N donor ligands properties, making new compounds structurally more interesting and more effective. [14][15][16][17] Complexes of cadmium (II) and zinc (II) with imidazole/benzimidazole and various carboxylates [2-methyl-2-phenoxypropionate, 3-(4-methoxyphenyl)acrylate, 3,4-(methylenedioxy)benzoate, 1,4-cyclohexanedicarboxylate] were structurally characterized, and rich intra-and intermolecular non-covalent interactions were identified as responsible for different supramolecular structures (3D networks, 3D prismatic layer network, 3D layer network structure). ...
This paper presents the synthesis, physico‐chemical and biological properties of four new coordination compounds with mixed ligands: acrylate ion (acr) and benzimidazole/benzimidazole derivatives with the general formula [Co(L)2(acr)2]·nH2O [(1) L: benzimidazole (HBzIm), n: 0.5; (2) L: 2‐methylbenzimidazole (2‐MeBzIm), n: 0.5; (3) L: 5‐methylbenzimidazole (5‐MeBzIm), n: 0; (4) L: 5,6‐dimethylbenzimidazole (5,6‐Me2BzIm), n: 0]. Their chemical formulae were achieved correlating the chemical analysis with mass spectrometry data, the ligands coordination modes were assigned by Fourier transform‐infrared measurements, and the trigonal bipyramidal geometry of cobalt ion in complexes was assigned by data correlation of UV–Vis‐NIR spectra and magnetic moments measurements. Single‐crystal X‐ray diffraction reveals a mononuclear structure with a pentacoordinated cobalt (II) ion, connected to two acrylato coordinated in different modes and two unidentate 5,6‐dimethylbenzimidazole ligands for compound (4). The biological tests were performed against several microbial strains, the cytotoxicity was evaluated on HCT8 cellular lines and the cell cycle analysis was performed on HT29 cellular lines. Microbiological assays indicated that Co (II) complexes present a very good to good activity against Candida albicans 1760, Enterococcus faecium E5, Bacillus subtilis ATCC 6683 and Escherichia coli ATCC 25922. Predictive pharmacokinetic (ADME), toxicity and drug‐likeness profiles were evaluated for Co (II) complexes. Our results highlight that Co (II) complexes depicted in the present study are suitable to be used as efficient pharmacological agents.
... Overall, the non-heme iron(III) complexes 2, 2a,a nd 3 equilibrate rapidly with argon-purged methanol and show identical photochemical reduction to the Fe II oxidation state without ligand degradation. Both EPR spectroscopy and the wavelength dependence of F indicate that there are several species present in solution, not all of which are photochemically reactive.Innon-heme systems,the equilibrium between mononuclear and m-oxido-bridged dinuclear Fe III complexes with pentadentate ligands (N4Py,P 2DA, 6-OC 6 H 4 -TPA, etc.), [29,32,33] has been shown earlier to be rapid. Addition of base (NaOAc) and proton sources (H 2 Oo rTfOH) shifts the equilibrium towards complexes,s uch as mononuclear Fe III À OH and Fe III À OH 2 and dinuclear Fe III À O À Fe III complexes.In the present reaction, conditions which favor dimer formation (base addition) are accompanied by an increase in the rate of photoreduction, while an added proton source or added chloride favor the formation of mononuclear Fe III complexes and retard photoreduction. ...
The non-heme (L)FeIII and (L)FeIII-O-FeIII(L) complexes (L = 1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)ethan-1-amine) undergoes reduction under irradiation to the FeII state with concomitant oxidation of methanol to methanal, without the need for a secondary photosensitizer. Spectroscopic and Density Functional Theory (DFT) studies support a mechanism in which irradiation results in charge transfer excitation of a FeIII--O-FeIII complex to generate [(L)FeIV=O]2+ (observed transiently during irradiation in acetonitrile), and an equivalent of LFeII. Under aerobic conditions, irradiation accelerates reoxidation from the FeII to FeIII state with O2 closing the cycle of methanol oxidation to methanal.
... Calculations suggested that part of the reason for the low reactivity of triplet Fe IV (O) was because of steric clash between the incoming aromatic substrate and the equatorial ligands, which blocked access to the key π* acceptor orbitals on the Fe IV (O) unit. 6,7,25,32 Recently, we provided experimental evidence showing that Fe IV (O) complexes are capable of aromatic hydroxylation provided that the aromatic substrate can be oriented properly in the second coordination sphere. 15,33 In one case, we directly characterized the Fe IV (O) intermediate [Fe IV (O)(N4Py 2Ar 1 )] 2+ (Ar 1 = −2,6-difluorophenyl), and observed the intramolecular arene hydroxylation reaction for this system by UV-vis, Mössbauer, and cold-spray ionization mass spectrometry (CSIMS). ...
The synthesis and reactivity of a series of mononuclear nonheme iron complexes that carry out intramolecular aromatic C-F hydroxylation reactions is reported. The key intermediate prior to C-F hydroxylation, [FeIV(O)(N4Py2Ar1)](BF4)2 (1-O, Ar1 = -2,6-difluorophenyl), was characterized by single-crystal X-ray diffraction. The crystal structure revealed a nonbonding C-H···O=Fe interaction with a CH3CN molecule. Variable-field Mössbauer spectroscopy of 1-O indicates an intermediate-spin (S = 1) ground state. The Mössbauer parameters for 1-O include an unusually small quadrupole splitting for a triplet FeIV(O) and are reproduced well by density functional theory calculations. With the aim of investigating the initial step for C-F hydroxylation, two new ligands were synthesized, N4Py2Ar2(L2, Ar2 = -2,6-difluoro-4-methoxyphenyl) and N4Py2Ar3(L3, Ar3 = -2,6-difluoro-3-methoxyphenyl), with -OMe substituents in the meta or ortho/para positions with respect to the C-F bonds. FeII complexes [Fe(N4Py2Ar2)(CH3CN)](ClO4)2 (2) and [Fe(N4Py2Ar3)(CH3CN)](ClO4)2 (3) reacted with isopropyl 2-iodoxybenzoate to give the C-F hydroxylated FeIII-OAr products. The FeIV(O) intermediates 2-O and 3-O were trapped at low temperature and characterized. Complex 2-O displayed a C-F hydroxylation rate similar to that of 1-O. In contrast, the kinetics (via stopped-flow UV-vis) for complex 3-O displayed a significant rate enhancement for C-F hydroxylation. Eyring analysis revealed the activation barriers for the C-F hydroxylation reaction for the three complexes, consistent with the observed difference in reactivity. A terminal FeII(OH) complex (4) was prepared independently to investigate the possibility of a nucleophilic aromatic substitution pathway, but the stability of 4 rules out this mechanism. Taken together the data fully support an electrophilic C-F hydroxylation mechanism.
... Theoretical studies of such biomimetic models may not only identify the key elements that determine their chemical reactivities, but may also provide insight into intermediates and reactivities of parent enzymes (Shaik et al., 2007a;de Visser et al., 2013). To date, DFT calculations have been applied extensively to various types of non-heme iron species (Scheme 1) (Bassan et al., 2002(Bassan et al., , 2005aRoelfes et al., 2003;Decker and Solomon, 2005;Kumar et al., 2005;Quinonero et al., 2005;Berry et al., 2006;Bernasconi et al., 2007Bernasconi et al., , 2011de Visser, 2006de Visser, , 2010Hirao et al., 2006aHirao et al., , 2008aHirao et al., ,b, 2011Rohde et al., 2006;Decker et al., 2007;de Visser et al., 2007de Visser et al., , 2011Johansson et al., 2007;Noack and Siegbahn, 2007;Sastri et al., 2007;Sicking et al., 2007;Baerends, 2008, 2013;Comba et al., 2008;Dhuri et al., 2008;Fiedler and Que, 2009;Klinker et al., 2009;Wang et al., 2009aWang et al., , 2013bCho et al., 2010Cho et al., , 2012aCho et al., , 2013Geng et al., 2010;Chen et al., 2011;Chung et al., 2011b;Seo et al., 2011;Shaik et al., 2011;Vardhaman et al., 2011;Wong et al., 2011;Ye and Neese, 2011;Gonzalez-Ovalle et al., 2012;Gopakumar et al., 2012;Latifi et al., 2012;Mas-Ballesté et al., 2012;McDonald et al., 2012;Van Heuvelen et al., 2012;Ansari et al., 2013;Kim et al., 2013;Lee et al., 2013;Sahu et al., 2013;Tang et al., 2013;Ye et al., 2013;Hong et al., 2014;Sun et al., 2014). The intriguing reactivity patterns of SCHEME 1 | Some of the (oxygen-bound) non-heme iron complexes studied by DFT calculations. ...
The past decades have seen an explosive growth in the application of density functional theory (DFT) methods to molecular systems that are of interest in a variety of scientific fields. Owing to its balanced accuracy and efficiency, DFT plays particularly useful roles in the theoretical investigation of large molecules. Even for biological molecules such as proteins, DFT finds application in the form of, e.g., hybrid quantum mechanics and molecular mechanics (QM/MM), in which DFT may be used as a QM method to describe a higher prioritized region in the system, while a MM force field may be used to describe remaining atoms. Iron-containing molecules are particularly important targets of DFT calculations. From the viewpoint of chemistry, this is mainly because iron is abundant on earth, iron plays powerful (and often enigmatic) roles in enzyme catalysis, and iron thus has the great potential for biomimetic catalysis of chemically difficult transformations. In this paper, we present a brief overview of several recent applications of DFT to iron-containing non-heme synthetic complexes, heme-type cytochrome P450 enzymes, and non-heme iron enzymes, all of which are of particular interest in the field of bioinorganic chemistry. Emphasis will be placed on our own work.
... 27 Most of these complexes are supported by polydentate ligands with N-donors. For a handful of complexes, one or two N donors are replaced by carboxylates, [28][29][30] hydroxide, 31 or thiolate. 32 A very recent example even used a macrocycle with four N-heterocyclic carbene donors L NHC ) to support the Fe IV =O unit. ...
... In principle, this goal could be achieved by weakening the ligand field in existing complexes, which would decrease the energy difference between the d xy and the d x2-y2 orbitals on the iron center and discourage spin pairing at the d xy orbital. To this end, we replaced two pyridine donors in [Fe IV (O) (N4Py)] 2+ with weaker-field carboxylate donors but were unsuccessful in changing its spin state to S = 2. 30 An alternative approach was to employ ligands that would enforce trigonal symmetry, which would make the d xy and the d x2-y2 orbitals degenerate. This strategy proved successful for the three tripodal ligands shown in Scheme 3, all of which were based on a tris(2-aminoethyl)amine (tren) framework. ...
... The Fe-N py and Fe-O RCOO distances are comparable to related N 3 O 2 pyridine-carboxylate Fe 2+ complexes. 52 Moreover, FT-IR analysis of the complex in solid state showed absorptions at 1600 cm −1 and 1360 cm −1 , which are assigned to the symmetric and asymmetric vibrations of the carboxylate ligand, while the 1 H NMR spectrum of 6-FeCl (CD 3 CN, 298 K, 400 MHz) showed paramagnetically shifted resonances comparable to similar S=2 Fe 2+ complexes. 51 The relatively simple spectrum suggests the mononuclear structure persists in solution, which is also supported by the detection of the ...
Iron is an essential metal for living organisms, but misregulation of its homeostasis at the cellular level can trigger detrimental oxidative and/or nitrosative stress and damage events. Motivated to help study the physiological and pathological consequences of biological iron regulation, we now report a reaction-based strategy for monitoring labile Fe(2+) pools in aqueous solution and living cells. Iron Probe 1 (IP1) exploits a bioinspired, iron-mediated oxidative C-O bond cleavage reaction to achieve a selective turn-on response to Fe(2+) over a range of cellular metal ions in their bioavailable forms. We show that this first-generation chemical tool for fluorescence Fe(2+) detection can visualize changes in exchangeable iron stores in living cells upon iron supplementation or depletion, including labile iron pools at endogenous, basal levels. Moreover, IP1 can be used to identify reversible expansion of labile iron pools by stimulation with vitamin C or the iron regulatory hormone hepcidin, providing a starting point for further investigations of iron signaling and stress events in living systems as well as future probe development.
Numerous synthetic models of the FeMo‐co cluster of nitrogenases have been proposed to find the simplest structure with relevant reactivity. Indeed, such structures are able to perform multi‐electrons reduction processes, such as the conversion of N2 to ammonia, and of CO2 into methane and alkenes. The most challenging parameter to imitate is indeed the central carbide ligand, which is believed to maintain the integrity of iron sulfide assembly during the course of catalytic cycles. The study proposes the use of bis(diphenylthiophosphinoyl)methanediide (SCS)²⁻ as an ideal platform for the synthesis of bi‐ and tetra‐metallic iron complexes, in which the iron‐carbon interaction is maintained upon structural diversification and redox state changes.
There has been a recent explosion of activity in the chemistry of synthetic nonheme high-valent iron-oxo complexes, which serve to mimic high-valent intermediates involved in the action of corresponding nonheme iron oxygenases in biology. This effort began with Karl Wieghardt in 2000 with the characterization of the first nonheme FeIV = O complex, and interest was boosted by the first crystal structure of an FeIV = O complex reported by Que and Nam in 2003, so that there are now over a hundred such Fe = O complexes identified in a variety of coordination environments. Moreover, they occur in two different spin states and exhibit CH bond cleavage rates that span a range of over six orders of magnitude as mimics of nonheme iron oxygenases. More recently, spectroscopic evidence for FeV = O oxidants has also emerged, mainly due to efforts of Talsi and Costas with the goal of identifying bio-inspired nonheme iron catalysts for hydrocarbon oxidation. Examples of corresponding diiron-oxo complexes have also emerged to model high-valent oxidants in the action of diiron enzymes like methane monooxygenase. This latter subset augments the structural complexity among complexes with the inclusion of the attractive diamond core motif and its open-core isomer in the reactivity consideration.
