Hydride-containing models for the active site of the nickel-iron hydrogenases

The [NiFe]-hydrogenase model complex NiFe(pdt)(dppe)(CO)(3) (1) (pdt = 1,3-propanedithiolate) has been efficiently synthesized and found to be robust. This neutral complex sustains protonation to give the first nickel-iron hydride [1H]BF(4). One CO ligand in [1H]BF(4) is readily substituted by organophosphorus ligands to afford the substituted derivatives [HNiFe(pdt)(dppe)(PR(3))(CO)(2)]BF(4), where PR(3) = P(OPh)(3) ([2H]BF(4)); PPh(3) ([3H]BF(4)); PPh(2)Py ([4H]BF(4), where Py = 2-pyridyl). Variable temperature NMR measurements show that the neutral and protonated derivatives are dynamic on the NMR time scale, which partially symmetrizes the phosphine complex. The proposed stereodynamics involve twisting of the Ni(dppe) center, not rotation at the Fe(CO)(2)(PR(3)) center. In MeCN solution, 3, which can be prepared by deprotonation of [3H]BF(4) with NaOMe, is about 10(4) stronger base than is 1. X-ray crystallographic analysis of [3H]BF(4) revealed a highly unsymmetrical bridging hydride, the Fe-H bond being 0.40 ? shorter than the Ni-H distance. Complexes [2H]BF(4), [3H]BF(4), and [4H]BF(4) undergo reductions near -1.46 V vs Fc(0/+). For [2H]BF(4), this reduction process is reversible, and we assign it as a one-electron process. In the presence of trifluoroacetic acid, proton reduction catalysis coincides with this reductive event. The dependence of i(c)/i(p) on the concentration of the acid indicates that H(2) evolution entails protonation of a reduced hydride. For [2H](+), [3H](+), and [4H](+), the acid-independent rate constants are 50-75 s(-1). For [2H](+) and [3H](+), the overpotentials for H(2) evolution are estimated to be 430 mV, whereas the overpotential for the N-protonated pyridinium complex [4H(2)](2+) is estimated to be 260 mV. The mechanism of H(2) evolution is proposed to follow an ECEC sequence, where E and C correspond to one-electron reductions and protonations, respectively. On the basis of their values for its pK(a) and redox potentials, the room temperature values of ΔG(H?) and ΔG(H-) are estimated as respectively as 57 and 79 kcal/mol for [1H](+).     

Active-site models for iron hydrogenases: reduction chemistry of dinuclear iron complexes

Reduction of Fe2(mu-S2C3H6)(CO)6 (1) in tetrahydrofuran with 1 equiv of decamethylcobaltocene (Cp*2Co) affords a tetranuclear dianion 2. The IR spectra of samples of 2 in solution and in the solid state exhibit a band at 1736 cm(-1), suggestive of the presence of a bridging carbonyl (CO) ligand. X-ray crystallography confirms that the structure of 2 consists of two Fe2 units bridged by a propanedithiolate moiety formulated as [Fe2(mu-S2C3H6)(CO)5(SCH2CH2CH2-mu-S)Fe2(mu-CO)(CO)6](2-). One of the Fe2 units has a bridging CO ligand and six terminal CO ligands. The second subunit exhibits a bridging propanedithiolate moiety. One CO ligand has been replaced by a terminal thiolate ligand, replicating the basic architecture of Fe-only hydrogenases. The reduction reaction can be reversed by treatment of 2 with 2 equiv of [Cp2Fe][PF6], reforming complex 1 in near-quantitative yield. Complex 2 can also be oxidized by acids such as p-toluenesulfonic acid, regenerating complex 1 and forming H2.     

Active-site structure and electron-transfer reactivity of plastocyanins

The active-site structures of Cu(II) plastocyanins (PCu's) from a higher plant (parsley), a seedless vascular plant (fern, Dryopteris crassirhizoma), a green alga (Ulva pertusa), and cyanobacteria (Anabaena variabilis and Synechococcus) have been investigated by paramagnetic (1)H NMR spectroscopy. In all cases the spectra are similar, indicating that the structures of the cupric sites, and the spin density distributions onto the ligands, do not differ greatly between the proteins. The active-site structure of PCu has remained unaltered during the evolutionary process. The electron transfer (et) reactivity of these PCu's is compared utilizing the electron self-exchange (ESE) reaction. At moderate ionic strength (0.10 M) the ESE rate constant is dictated by the distribution of charged amino acid residues on the surface of the PCu's. Most higher plant and the seedless vascular plant PCu's, which have a large number of acidic residues close to the hydrophobic patch surrounding the exposed His87 ligand (the proposed recognition patch for the self-exchange process), have ESE rate constants of approximately 10(3) M(-)(1) s(-)(1). Removal of some of these acidic residues, as in the parsley and green algal PCu's, results in more favorable protein-protein association and an ESE rate constant of approximately 10(4) M(-)(1) s(-)(1). Complete removal of the acidic patch, as in the cyanobacterial PCu's, leads to ESE rate constants of approximately 10(5)-10(6) M(-)(1) s(-)(1). The ESE rate constants of the PCu's with an acidic patch also tend toward approximately 10(5)-10(6) M(-)(1) s(-)(1) at higher ionic strength, thus indicating that once the influence of charged residues has been minimized the et capabilities of the PCu's are comparable. The cytochromes and Fe-S proteins, two other classes of redox metalloproteins, also possess ESE rate constants of approximately 10(5)-10(6) M(-)(1) s(-)(1) at high ionic strength. The effect of the protonation of the His87 ligand in PCu(I) on the ESE reactivity has been investigated. When the influence of the acidic patch is minimized, the ESE rate constant decreases at high [H(+)].     

Diferrous cyanides as models for the Fe-only hydrogenases

The first systematic study of diferrous dicyano dithiolates is described. Oxidation of [Fe2(S2C2H4)(CN)2(CO)4](2-) in the presence of cyanide and tertiary phosphines and of Fe2(S2C2H4)(CO)4(PMe3)2 in the presence of cyanide affords a series of diferrous cyanide derivatives that bear a stoichiometric, structural, and electronic relationship to the H(ox)(air) state of the Fe-only hydrogenases. With PPh3 as the trapping ligand, we obtained an unsymmetrical isomer of Fe2(S2C2H4)(mu-CO)(CN)2(PPh3)2(CO)2, as confirmed crystallographically. This diferrous cyanide features the semibridging CO-ligand, with Fe-muC bond lengths of 2.15 and 1.85 A. Four isomers of Fe2(S2C2H4)(mu-CO)(CN)2(PMe3)2(CO)2 were observed, the initial product again being unsymmetrical but more stable isomers being symmetrical. DFT calculations confirm that the most stable isomers of Fe2(S2C2H4)(mu-CO)(CN)2(PMe3)2(CO)2 have cyanide trans to mu-CO. Oxidative decarbonylation also afforded the new tetracyanide [Fe2(S2C2H4)(mu-CO)(CN)4(CO)2]2-. Insights into the oxidative decarbonylation mechanism of these syntheses come from the spectroscopic characterization of the tetracarbonyl [Fe2(S2C2H4)(mu-CO)(CN)3(CO)3](-). This species reacts with PEt3 to produce the stable adduct [Fe2(S2C2H4)(mu-CO)(CN)3(CO)2(PEt3)](-).     

Synthesis and reactivity of iridium complexes with pyridine and piperidine ligands: models for hydrodenitrogenation

The complexes [Ir(H)2(eta1-N-L)2(PPh3)2]PF6, L = py (1), iQ (2) and pip (3) (py = pyridine, iQ = isoquinoline, pip = piperidine) have been synthesized in high yields by hydrogenation of [Ir(cod)(PPh3)2]PF6 in the presence of the appropriate nitrogen compound. When hydrogen is bubbled through 1,2-dichloroethane solutions of 1 or 2, two new species were formed in each case by C-Cl bond activation of the solvent, Ir(H)2Cl(eta1-N-L)(PPh3)2 (L = py, 4; iQ, 5) and IrH(Cl)2(eta1-N-L)(PPh3)2 (L = py, 6; iQ, 7). Reaction of 3 with py or iQ yielded complexes 1 and 2, respectively, while under a slow stream of carbon monoxide the complex [Ir(H)2(eta1-N-pip)(CO)(PPh3)2]PF6 (8) was produced. Complex 3 also reacts with halide and 4-bromothiophenolate anions leading to the corresponding neutral species Ir(H)2(X)(eta1-N-pip)(PPh3)2, X = Cl (9), I (10) and 4-BrC6H4S (11), or with [MoS4]2- to yield the hetero-bimetallic complex [Ir(H)(PPh3)2(mu-S)2MoS2]- (13). All the new complexes were characterized by analytical and spectroscopic methods. The X-ray structures of , 2 and 8 consist of distorted octahedra with a mutually cis disposition of the two hydrides and mutually trans phosphines. Complexes 1, 2 and 3 and their derivatives are of interest as models for the chemisorption step in hydrodenitrogenation reactions on solid catalysts.     

Gas pressure effects on the rates of catalytic H(2) oxidation by hydrogenases

During catalysis by hydrogenases, entities no larger than H(2) or H(+) reach and leave a deeply buried active site, by as yet unidentified pathways. Novel experiments, conducted mainly with the membrane-bound [NiFe]-hydrogenase from Ralstonia eutropha, explore why small excess gas pressures (H(2) or He) attenuate the rate of H(2) oxidation.     

Heterobimetallics of nickel-iron dinitrosyl: electronic control by chelate and diatomic ligands

Reaction of [PPN][Fe(NO)2(SePh)2] (1) with dimeric [Ni(mu-SCH2CH2SCH2CH2S)]2 in the presence of additional NO2- produced the neutral heterobimetallic [(ON)Ni[(mu-SCH2CH2)2S]Fe(NO)2] complex (2). The X-ray crystal structures of 1 and 2 show distorted tetrahedral iron dinitrosyl groups, assigned according to the Feltham-Enemark notation as [Fe(NO)2]9 The Fe-NO bonds are off linearity by an average of approximately equals 10 degrees for compounds 1 and 2, while a more linear Ni-NO coordination with a Ni-NO distance of 1.644(2) A was found in 2. The v(NO) value of complex 2 is consistent with an assignment for [Ni(NO)]9 of Ni0(NO)+ as is known for analogous phosphine derivatives, P3Ni0(NO)+. EPR signals of g values = 2.02-2.03 confirmed the existence of the odd electron in the chalcogenated [Fe(NO)2]9 compounds. Two [Fe(NO)2]10 complexes coordinated by the nickel(II) dithiolate, (bismercaptoethanediazacyclooctane)nickel(II), (Ni-1), (Ni-1)Fe(CO)(NO)2 and (Ni-1)Fe(NO)2, were prepared for comparison to the Ni0(NO)+ derivative and other monomeric and homodimetallic derivatives of the Fe(NO)2 fragment. While the oxidation level of Fe(NO)2 is the primary determinant of v(NO) values, they are also highly sensitive to ancillary ligands and, thereby, the distal metal influence through the bridging thiolate donor.     

Proton electroreduction catalyzed by cobaloximes: functional models for hydrogenases

Cobaloximes have been examined as electrocatalysts for proton reduction in nonaqueous solvent in the presence of triethylammonium chloride. [Co(III)(dmgH)2pyCl], working at moderate potentials (-0.90 V/(Ag/AgCl/3 mol x L(-1) NaCl) and in neutral conditions, is a promising catalyst as compared to other first-row transition metal complexes which generally function at more negative potentials and/or at lower pH. More than 100 turnovers can be achieved during controlled-potential electrolysis without detectable degradation of the catalyst. Cyclic voltammograms simulation is consistent with a heterolytic catalytic mechanism and allowed us to extract related kinetic parameters. Introduction of an electron-donating (electron-withdrawing) substituent in the axial pyridine ligand significantly increases (decreases) the rate constant of the catalytic cycle determining step. This effect linearly correlates with the Hammet coefficients of the introduced substituents. The influence of the equatorial glyoxime ligand was also investigated and the capability of the stabilized BF2-bridged species [Co(dmgBF2)2(OH2)2] for electrocatalyzed hydrogen evolution confirmed.     

Novel chiral dendritic diphosphine ligands for Rh(I)-catalyzed asymmetric hydrogenation: remarkable structural effects on catalytic properties

[reaction: see text] A series of dendritic ligands with a chiral diphosphine located at the focal point have been synthesized through coupling of pyrphos 2 with Fréchet-type polyether dendron 3. The relationship between the primary structure of the dendrimer and its catalytic properties was established in the Rh-catalyzed asymmetric hydrogenation of alpha-acetamido cinnamic acid 4. A remarkable structural effect on catalytic activity was observed.     

Alkyne-coordinating tridentate ligands: structural properties and reactivity of their rhodium complexes

Complexes of novel alkyne-chelating tridentate ligands bound to a rhodium atom were isolated and characterised. The present alkyne-rhodium complex underwent dimerisation simply by heating to afford the unprecedented cyclobutadiene dirhodium complex. It is also found that the ligands at the trans positions influence the π-coordination of alkynes.     

Phosphorus-nitrogen-phosphorus ligands: cooperative effects between nitrogen and phosphorus substituents on catalytic activity

A new generation of PNP compounds bearing different diarylphosphine groups were prepared and used as ligands in palladium-catalysed Suzuki cross-coupling reactions. Rates of oxidative addition of iodobenzene to (PNP)Pd[0] complexes were measured using UV spectroscopy. Synergistic effects between the N- and P- substituents were identified and correlated in redox and catalytic chemistry.     

Research on soluble metal sulfides: from polysulfido complexes to functional models for the hydrogenases

Results from this laboratory are surveyed, emphasizing the synthesis of metal sulfides. Four themes are described. Continuing studies exploit the exothermic desulfurization of polysulfido complexes as a means to generate new clusters and rings. Illustrative inorganic rings prepared in this way include 1,5-[L(2)M](2)(S(3))(2) and 1,4-[L(2)M](2)(S(2))(2), where L(2)M = CpRu(PPh(3)) and Cp(2)Ti. Fundamentally new clusters prepared in this project included the cubanes [(C(5)R(5))MS](4) for M = Ti, V, Ru, Ir. Associated redox studies led to the discovery of the phenomenon of mobile metal-metal bonds, as manifested in [Cp(4)Ir(4)S(4)](2+) wherein the localized Ir-Ir bond migrates over the six Ir- - -Ir edges of the cluster. Other desulfurization experiments led to the preparation of the reactive species Ir(II)(2)S(2)(PPh(3))(4) from [IrS(16)](3)(-) and the synthesis of the first high polymers of ferrocene, [(RC(5)H(3)S)(2)Fe](n) (n approximately 500). A second theme uncovered the useful role of donor solvents on the reaction of metals with sulfur. It was found that pyridine accelerates the low temperature conversion of Cu to crystalline CuS via the intermediacy of the cluster Cu(4)(S(5))(2)L(4). Related synthetic methodology led to a family of amine-stabilized zinc polysulfides, e.g. ZnS(6)(tmeda), an efficient sulfur-transfer agent. A third theme explored the organic and organometallic chemistry of the tetrathiometalates. The sulfido analogue of OsO(4), ReS(4)(-) was shown to be broadly reactive toward unsaturated organic substrates such as alkenes, alkynes, nitriles, and isocyanides. The final and still emerging theme focuses on the preparation of functional and structural models for bio-organometallic reaction centers. Studies on models for the Fe-only hydrogenases began with the synthesis of the highly reducing species [Fe(2)(SR)(2)(CN)(2)(CO)(4)](2)(-) where (SR)(2) also includes the proposed azadithiolate cofactor HN(CH(2)S(-))(2). Systematic studies on the cyanide substitution process led to the preparation of [HFe(2)(SR)(2)(CN)(CO)(4)(PMe(3))], which efficiently catalyzes the reduction of protons to H(2). Work on the hydrogenases was expanded to include modeling of acetyl Co-A synthase, leading to the preparation of mixed valence Ni(2) models containing bound CO substrate.     

Density functional calculations for modeling the active site of nickel-iron hydrogenases. 2. Predictions for the unready and ready States and the corresponding activation processes

ZORA relativistic DFT calculations are presented which aim to model the geometric and electronic structure of the active site of NiFe hydrogenases in its EPR-active oxidized states Ni-A (unready state) and Ni-B (ready state). Starting coordinates are taken from the X-ray structure of a mutant of Desulfovibrio fructosovorans hydrogenase refined at 1.81 A resolution. Nine possible candidates for Ni-A and Ni-B are analyzed in terms of their geometric and electronic structure. Comparison of calculated geometric and magnetic resonance parameters with available experimental data indicates that both oxidized states have a micro-hydroxo bridge between the two metal centers. The different electronic structures of both forms can be explained by a modification of a terminal cysteine in Ni-B, best modeled by protonation of the sulfur atom. A possible mechanism for the activation of both oxidized forms is presented.     

Steric and electronic effects on the reactivity of rh and ir complexes containing P-S,P-P,and P-O ligands. Implications for the effects of chelate ligands in catalysis

Kinetic studies of the reactions of [M(CO)(L-L)I] [M = Rh, Ir; L-L = Ph(2)PCH(2)P(S)Ph(2) (dppms), Ph(2)PCH(2)CH(2)PPh(2) (dppe), and Ph(2)PCH(2)P(O)Ph(2) (dppmo)] with methyl iodide have been undertaken. All the chelate ligands promote oxidative addition of methyl iodide to the square planar M(I) centers, by factors of between 30 and 50 compared to the respective [M(CO)(2)I(2)](-) complexes, due to their good donor properties. Migratory CO insertion in [Rh(CO)(L-L)I(2)Me] leads to acetyl complexes [Rh(L-L)I(2)(COMe)] for which X-ray crystal structures were obtained for L-L = dppms (3a) and dppe (3b). Against the expectations of simple bonding arguments, methyl migration is faster by a factor of ca. 1500 for [Rh(CO)(dppms)I(2)Me] (2a) than for [Rh(CO)(dppe)I(2)Me] (2b). For M = Ir, alkyl iodide oxidative addition gives stable alkyl complexes [Ir(CO)(L-L)I(2)R]. Migratory insertion (induced at high temperature by CO pressure) was faster for [Ir(CO)(dppms)I(2)Me] (5a) than for its dppe analogue (5b). Reaction of methyl triflate with [Ir(CO)(dppms)I] (4a) yielded the dimer [[Ir(CO)(dppms)(mu-I)Me](2)](2+) (7), which was characterized crystallographically along with 5a and [Ir(CO)(dppms)I(2)Et] (6). Analysis of the X-ray crystal structures showed that the dppms ligand adopts a conformation which creates a sterically crowded pocket around the alkyl ligands of 5a, 6, and 7. It is proposed that this steric strain can be relieved by migratory insertion, to give a five-coordinate acetyl product in which the sterically crowded quadrants flank a vacant coordination site, exemplified by the crystal structure of 3a. Conformational analysis indicates similarity between M(dppms) and M(2)(mu-dppm) chelate structures, which have less flexibility than M(dppe) systems and therefore generate greater steric strain with the "axial" ligands in octahedral complexes. Ab initio calculations suggest an additional electronic contribution to the migratory insertion barrier, whereby a sulfur atom trans to CO stabilizes the transition state compared to systems with phosphorus trans to CO. The results represent a rare example of the quantification of ligand effects on individual steps from catalytic cycles, and are discussed in the context of catalytic methanol carbonylation. Implications for other catalytic reactions utilizing chelating diphosphines (e.g., CO/alkene copolymerization and alkene hydroformylation) are considered.     

Redox properties of Tanaka's water oxidation catalyst: redox noninnocent ligands dominate the electronic structure and reactivity

[Ru(2)(OH)(2)(3,6-(t)Bu(2)Q)(2)(btpyan)](2+) ((t)Bu(2)Q, 3,6-di-tert-butyl-1,2-benzoquinone; btpyan, 1,8-bis(2,2':6',2'-terpyridyl)anthracene) is one of a handful of structurally well-defined homogeneous catalysts that can electrocatalytically oxidize water at room temperature. Unfortunately, the exact composition and the chemical properties of the redox intermediates leading to the catalytically competent species remains poorly resolved. On the basis of the UV-vis spectra the catalyst was previously speculated to lose two protons spontaneously to form an intermediate containing the key O-O bond in water. We evaluated this mechanistic scenario computationally and found that the associated pK(a) values are in the range of 21, much too high to justify spontaneous deprotonation under experimental conditions of pH = 4. In later work, the O-O bond formation was speculated to occur after removal of two protons and two electrons. Extensive exploration of the various oxidation and protonation states that the diruthenium complex may access during catalyst activation reveals surprisingly complex electronic structure patterns in several redox intermediates: the quinone and tpy ligands become redox noninnocent, i.e., they participate actively in the electron transfer processes by temporarily storing redox equivalents. On the basis of this new insight into the electronic structure we propose a novel alternative explanation of the spectroscopic observations reported previously and characterize the electronic structure of the key intermediates in detail. Finally, the redox potential for the first two-electron oxidation is evaluated based on our proposed intermediates and predicted to be 0.411 V, which compares well with the experimentally observed broad two-electron wave at ～0.32 V.     

A QM/MM investigation of the activation and catalytic mechanism of Fe-only hydrogenases

Fe-only hydrogenases are enzymes that catalyze dihydrogen production or oxidation, due to the presence of an unusual Fe(6)S(6) cluster (the so-called H-cluster) in their active site, which is composed of a Fe(2)S(2) subsite, directly involved in catalysis, and a classical Fe(4)S(4) cubane cluster. Here, we present a hybrid quantum mechanical and molecular mechanical (QM/MM) investigation of the Fe-only hydrogenase from Desulfovibrio desulfuricans, in order to unravel key issues regarding the activation of the enzyme from its completely oxidized inactive state (Hoxinact) and the influence of the protein environment on the structural and catalytic properties of the H-cluster. Our results show that the Fe(2)S(2) subcluster in the Fe(II)Fe(II) redox state - which is experimentally observed for the completely oxidized form of the enzyme - binds a water molecule to one of its metal centers. The computed QM/MM energy values for water binding to the diferrous subsite are in fact over 70 kJ mol(-1); however, the affinity toward water decreases by 1 order of magnitude after a one-electron reduction of H(ox)(inact), thus leading to the release of coordinated water from the H-cluster. The investigation of a catalytic cycle of the Fe-only hydrogenase that implies formation of a terminal hydride ion and a di(thiomethyl)amine (DTMA) molecule acting as an acid/base catalyst indicates that all steps have reasonable reaction energies and that the influence of the protein on the thermodynamic profile of H(2) production catalysis is not negligible. QM/MM results show that the interactions between the Fe(2)S(2) subsite and the protein environment could give place to structural rearrangements of the H-cluster functional for catalysis, provided that the bidentate ligand that bridges the iron atoms in the binuclear subsite is actually a DTMA residue.     

Mixed-valence nickel-iron dithiolate models of the [NiFe]-hydrogenase active site

A series of mixed-valence nickel-iron dithiolates is described. Oxidation of (diphosphine)Ni(dithiolate)Fe(CO)(3) complexes 1, 2, and 3 with ferrocenium salts affords the corresponding tricarbonyl cations [(dppe)Ni(pdt)Fe(CO)(3)](+) ([1](+)), [(dppe)Ni(edt)Fe(CO)(3)](+) ([2](+)) and [(dcpe)Ni(pdt)Fe(CO)(3)](+) ([3](+)), respectively, where dppe = Ph(2)PCH(2)CH(2)PPh(2), dcpe = Cy(2)PCH(2)CH(2)PCy(2), (Cy = cyclohexyl), pdtH(2) = HSCH(2)CH(2)CH(2)SH, and edtH(2) = HSCH(2)CH(2)SH. The cation [2](+) proved unstable, but the propanedithiolates are robust. IR and EPR spectroscopic measurements indicate that these species exist as C(s)-symmetric species. Crystallographic characterization of [3]BF(4) shows that Ni is square planar. Interaction of [1]BF(4) with P-donor ligands (L) afforded a series of substituted derivatives of type [(dppe)Ni(pdt)Fe(CO)(2)L]BF(4) for L = P(OPh)(3) ([4a]BF(4)), P(p-C(6)H(4)Cl)(3) ([4b]BF(4)), PPh(2)(2-py) ([4c]BF(4)), PPh(2)(OEt) ([4d]BF(4)), PPh(3) ([4e]BF(4)), PPh(2)(o-C(6)H(4)OMe) ([4f]BF(4)), PPh(2)(o-C(6)H(4)OCH(2)OMe) ([4g]BF(4)), P(p-tol)(3) ([4h]BF(4)), P(p-C(6)H(4)OMe)(3) ([4i]BF(4)), and PMePh(2) ([4j]BF(4)). EPR analysis indicates that ethanedithiolate [2](+) exists as a single species at 110 K, whereas the propanedithiolate cations exist as a mixture of two conformers, which are proposed to be related through a flip of the chelate ring. M?ssbauer spectra of 1 and oxidized S = 1/2 [4e]BF(4) are both consistent with a low-spin Fe(I) state. The hyperfine coupling tensor of [4e]BF(4) has a small isotropic component and significant anisotropy. DFT calculations using the BP86, B3LYP, and PBE0 exchange-correlation functionals agree with the structural and spectroscopic data, suggesting that the SOMOs in complexes of the present type are localized in an Fe(I)-centered d(z(2)) orbital. The DFT calculations allow an assignment of oxidation states of the metals and rationalization of the conformers detected by EPR spectroscopy. Treatment of [1](+) with CN(-) and compact basic phosphines results in complex reactions. With dppe, [1](+) undergoes quasi-disproportionation to give 1 and the diamagnetic complex [(dppe)Ni(pdt)Fe(CO)(2)(dppe)](2+) ([5](2+)), which features square-planar Ni linked to an octahedral Fe center.     

Carbene complexes of rhodium and iridium from tripodal N-heterocyclic carbene ligands: synthesis and catalytic properties

Two tripodal trisimidazolium ligand precursors have been tested in the synthesis of new N-heterocyclic carbene rhodium and iridium complexes. [Tris(3-methylbenzimidazolium-1-yl)]methane sulfate gave products with coordination of the decomposed precursor. [1,1,1-Tris(3-butylimidazolium-1-yl)methyl]ethane trichloride (TIMEH(3)(Bu)) coordinated to the metal in a chelate and bridged-chelate form, depending on the reaction conditions. The crystal structures of two of the products are described. The compounds resulting from the coordination with TIME(Bu) were tested in the catalytic hydrosilylation of terminal alkynes.     

Density functional calculations for modeling the oxidized states of the active site of nickel-iron hydrogenases. 1. Verification of the method with paramagnetic Ni and Co complexes

ZORA relativistic DFT calculations are presented which aim to reproduce geometric structures and EPR properties of [Ni(mnt)(2)](-) (H(2)mnt = maleonitrildithiol), two other paramagnetic low-spin Ni(III) complexes, and an asymmetric paramagnetic Co(II) complex. The study tests the accuracy of the computational method as a prior step to the modeling of the geometric and electronic structure of the active site of NiFe hydrogenases in its EPR-active oxidized states Ni-A and Ni-B. Systematic deviations from experiment are found for the calculated g-values; relative differences among them are, however, well reproduced. Because no significant improvements have been achieved by using larger basis sets or more sophisticated functionals, g-values may be calculated rather rapidly at the VWN level. This is most important for the modeling of the active site of NiFe hydrogenases because its complexity does not permit calculations at high levels of theory. For [Ni(mnt)(2)](-), excellent agreement between calculated and experimental results is obtained for the (14)N quadrupole coupling, whereas the calculated hyperfine couplings are not always in good agreement with experimental data.