Comparative kinetics and mechanism of oxygen and sulfur atom transfer reactions mediated by bis(dithiolene) complexes of molybdenum and tungsten

Although the kinetics and mechanism of metal-mediated oxygen atom (oxo) transfer reactions have been examined in some detail, sulfur atom (sulfido) transfer reactions have not been similarly scrutinized. The reactions [M(IV)(O-p-C(6)H(4)X')(S(2)C(2)Me(2))(2)](1-) + Ph(3)AsQ --> [M(VI)Q(O-p-C(6)H(4)X')(S(2)C(2)Me(2))(2)](1-) + Ph(3)As (M = Mo, W; Q = O, S) with variable substituent X' have been investigated in acetonitrile in order to determine the relative rates of oxo versus sulfido transfer at constant structure (square pyramidal) of the atom acceptor and of atom transfer at constant structure of the atom donor and metal variability of the atom acceptor. All reactions exhibit second-order kinetics and entropies of activation (-25 to -45 eu) consistent with an associative transition state. At parity of atom acceptor, k(2)(S) (0.25-0.75 M(-1)s(-1)) > k(2)(O) (0.023-0.060 M(-1)s(-1)) with M = Mo and k(2)(S) (4.1-66.7 M(-1)s(-1)) > k(2)(O) (1.8-9.8 M(-1)s(-1)) with M = W. At constant atom donor and X', k(2)(W) > k(2)(Mo) with reactivity ratios k(2)(W)/k(2)(Mo) = 78-184 (Q = O) and 16-89 (Q = S). Rate constants refer to 298 K. At constant M and Q, rates increase in the order X' = Me less, similar OMe < H < Br < COMe < CN; increasing electron-withdrawing propensity accelerates reaction rates. The probable transition state involves significant Ph(3)AsQ...M bond-making (X' rate trend) and concomitant As-Q bond weakening (bond energy order As-O > As-S). Orders of oxo and sulfido donor ability of substrates and complexes are deduced on the basis of qualitative reactivity properties determined here and elsewhere. This work complements previous studies of the reaction systems [M(IV)(O-p-C(6)H(4)X')(S(2)C(2)Me(2))(2)](1-)/XO where the substrates are N-oxides and S-oxides and k(2)(W) > k(2)(Mo) at constant substrate also applies. The reaction order of substrates is Me(3)NO > (CH(2))(4)SO > Ph(3)AsS > Ph(3)AsO. This research provides the first quantitative information of metal-mediated sulfido transfer.     

Generation of bis(dithiolene)dioxomolybdenum(VI) complexes from bis(dithiolene)monooxomolybdenum(IV) complexes by proton-coupled electron transfer in aqueous media

Electron transfer oxidation reaction of bis(dithiolene)monooxomolybdenum(iv) (Mo(IV)OL(x)) complexes is studied as a model of oxidative-half reaction of arsenite oxidase molybdenum enzymes. The reactions are revealed to involve proton-coupled electron transfer. Electrochemical oxidation of Mo(IV)OL(x) yields the corresponding bis(dithiolene)dioxomolybdenum(vi) complexes in basic solution, where the conversion of Mo(IV)OL(dmed) supported by a smaller electron donating dithiolene ligand (1,2-dicarbomethoxyethylene-1,2-dithiolate, L(dmed)) to Mo(VI)O(2)L(dmed) is faster than that of Mo(IV)OL(bdt) with a larger electron donating dithiolene ligand (1,2-benzenedithiolate, L(bdt)) under the same conditions. Titration experiments for the electrochemical oxidation reveal that the reaction involves two-electron oxidation and two equivalents of OH(-) consumption per Mo(IV)OL(x). In the conversion process of Mo(IV)OL(x) to Mo(VI)O(2)L(x), the five-coordinate bis(dithiolene)monooxomolybdenum(v) complex (Mo(V)OL(x)) being a one-electron oxidized species of Mo(IV)OL(x) is suggested to react with OH(-). Mo(V)OL(x) reacts with OH(-) in CH(3)CN or C(2)H(5)CN in a 2?:?2 ratio to give one equivalent Mo(IV)OL(x) and one equivalent Mo(VI)O(2)L(x), which is confirmed by the UV-vis and IR spectroscopies. The low temperature stopped-flow analysis allows investigations of the mechanism for the reaction of Mo(V)OL(x) with OH(-). The kinetic study for the reaction of Mo(V)OL(dmed) with OH(-) suggests that Mo(V)OL(dmed) reacts with OH(-) to give a six-coordinate oxo-hydroxo-molybdenum(v) species, Mo(V)O(OH), and, then, the resulting species undergoes successive deprotonation by another OH(-) and oxidation by a remaining Mo(V)OL(dmed) to yield the final products Mo(IV)OL(dmed) and Mo(VI)O(2)L(dmed) complexes in a 1?:?1 ratio. In this case, the Mo(V)O(2) species are involved as an intermediate in the reaction. On the other hand, in the reaction of Mo(V)OL(bdt) with OH(-), coordination of OH(-) to the Mo(V) centre to give a six-coordinate Mo(V)O(OH)L(bdt) species becomes the rate limiting step and other intermediates are not suggested. On the basis of these results, the ligand effects of the dithiolene ligands on the reactivity of the bis(dithiolene)molybdenum complexes are discussed.     

Desoxo molybdenum(IV) and tungsten(IV) bis(dithiolene) complexes: monomer-dimer interconversion involving reversible thiol bridge formation

Two series of thiol-bridged dimeric desoxo molybdenum(IV) and tungsten(IV) bis(dithiolene) complexes, [Et(4)N](2)[M(IV)(2)(SR)(2)(mnt)(4)] [M = Mo, R = (1) -Ph, (2) -CH(2)Ph, (3) -CH(2)CH(3), (4) -CH(2)CH(2)OH; M = W, R = (1a) -Ph, (2a) -CH(2)Ph, (3a) -CH(2)CH(3), (4a) -CH(2)CH(2)OH] and one monomeric desoxo complex, [Et(4)N](2)[WIV(SPh)(2)(mnt)(2)] (5a) are reported. These complexes are diamagnetic, and crystal structures of each of the complex (except 5a) exhibits a dimeric {M(IV)(2)(SR)(2)} core without any metal-metal bond where each metal atom possesses hexa coordination. The M-SR distance ranges from 2.437 to 2.484 Angstrom in molybdenum complexes and from 2.418 to 2.469 Angstrom in tungsten complexes. These complexes display Mo-S(R)-Mo angles ranging from 92.84 degrees to 96.20 degrees in the case of 1-4 and W-S(R)-W angles ranging from 91.20 degrees to 96.25 degrees in the case of 1a-4a. Interestingly, both the series of Mo(IV) and W(IV) dimeric complexes respond to an unprecedented interconversion between the dimer and the corresponding hexacoordinated monomer upon change of pH. This pH-dependent interconversion establishes the fact that even the pentacoordinated Mo(IV) and W(IV) bis(dithiolene) moieties are forced to dimerize; these can easily be reverted back to the corresponding monomeric complex, reflecting the utility of dithiolene ligand in stabilizing the Mo(IV)/W(IV) moiety in synthesized complexes similar to the active sites present in native proteins.     

Nonclassical oxygen atom transfer reactions of oxomolybdenum(VI) bis(catecholate)

Mechanistic studies indicate that the oxomolybdenum(vi) bis(3,5-di-tert-butylcatecholate) fragment deoxygenates pyridine-N-oxides in a reaction where the oxygen is delivered to molybdenum but the electrons for substrate reduction are drawn from the bound catecholate ligands, forming 3,5-di-tert-butyl-1,2-benzoquinone.     

Bis(dithiolene)molybdenum analogues relevant to the DMSO reductase enzyme family: synthesis, structures, and oxygen atom transfer reactions and kinetics.

A series of dithiolene complexes of the general type [Mo(IV)(QR')(S(2)C(2)Me(2))(2)](1)(-) has been prepared and structurally characterized as possible structural and reactivity analogues of reduced sites of the enzymes DMSOR and TMAOR (QR' = PhO(-), 2-AdO(-), Pr(i)()O(-)), dissimilatory nitrate reductase (QR' = 2-AdS(-)), and formate dehydrogenase (QR' = 2-AdSe(-)). The complexes are square pyramidal with the molybdenum atom positioned 0.74-0.80 A above the S(4) mean plane toward axial ligand QR'. In part on the basis of a recent clarification of the active site of oxidized Rhodobacter sphaeroides DMSOR (Li, H.-K.; Temple, C.; Rajagopalan, K. V.; Schindelin, H. J. Am. Chem. Soc. 2000, 122, 7673), we have adopted the minimal reaction paradigm Mo(IV) + XO right arrow over left arrow Mo(VI)O + X involving desoxo Mo(IV), monooxo Mo(VI), and substrate/product XO/X for direct oxygen atom transfer of DMSOR and TMAOR enzymes. The [Mo(OR')(S(2)C(2)Me(2))(2)](1)(-) species carry dithiolene and anionic oxygen ligands intended to simulate cofactor ligand and serinate binding in DMSOR and TMAOR catalytic sites. In systems with N-oxide and S-oxide substrates, the observed overall reaction sequence is [Mo(IV)(OR')(S(2)C(2)Me(2))(2)](1)(-) + XO --> [Mo(VI)O(OR')(S(2)C(2)Me(2))(2)](1)(-) --> [Mo(V)O(S(2)C(2)Me(2))(2)](1)(-). Direct oxo transfer in the first step has been proven by isotope labeling. The reactivity of [Mo(OPh)(S(2)C(2)Me(2))(2)](1)(-) (1) has been the most extensively studied. In second-order reactions, 1 reduces DMSO and (CH(2))(4)SO (k(2) approximately 10(-)(6), 10(-)(4) M(-)(1) s(-)(1); DeltaS(double dagger) = -36, -39 eu) and Me(3)NO (k(2) = 200 M(-)(1) s(-)(1); DeltaS(double dagger) = -21 eu) in acetonitrile at 298 K. Activation entropies indicate an associative transition state, which from relative rates and substrate properties is inferred to be concerted with X-O bond weakening and Mo-O bond making. The Mo(VI)O product in the first step, such as [Mo(VI)O(OR')(S(2)C(2)Me(2))(2)](1)(-), is an intermediate in the overall reaction sequence, inasmuch as it is too unstable to isolate and decays by an internal redox process to a Mo(V)O product, liberating an equimolar quantity of phenol. This research affords the first analogue reaction systems of biological N-oxide and S-oxide substrates that are based on desoxo Mo(IV) complexes with biologically relevant coordination. Oxo-transfer reactions in analogue systems are substantially slower than enzyme systems based on a k(cat)/K(M) criterion. An interpretation of this behavior requires more information on the rate-limiting step(s) in enzyme catalytic cycles. (2-Ad = 2-adamantyl, DMSOR = dimethyl sulfoxide reductase, TMAOR = trimethylamine N-oxide reductase)     

Homoleptic tris(dithiolene) and tetrakis(dithiolene) complexes of uranium(IV)

Reaction of UCl4 with 3 or 4 mol equiv of Na2dddt (dddt = 5,6-dihydro-1,4-dithiine-2,3-dithiolate) in THF afforded the first example of a tetrakis(dithiolene) metal compound, [Na4(THF)8U(dddt)4](infinity) (1). The red crystals of 1 are composed of infinite zigzag chains in which Na2(micro-THF)3 fragments ensure the linking of Na2(THF)5U(dddt)4 moieties; the uranium atom is in a dodecahedral environment of eight sulfur atoms. Treatment of UCl4 with 3 mol equiv of Na2dddt in pyridine gave a mixture of tris- and tetrakis(dithiolene) compounds. After addition of 18c6 (18-crown-6), only the tris(dithiolene) complex was obtained and crystallized as orange crystals of [Na(18c6)(py)2]2[U(dddt)3].2py (2.2py) in which the isolated [U(dddt)3]2- anion adopts a slightly distorted trigonal prismatic configuration. A few red crystals of the unsolvated complex 2 and the trinuclear anionic compound [Na(18c6)(py)2]3[Na{U(dddt)3}2] (3) were also obtained along with orange crystals of 2.2py. All the tris(dithiolene) compounds exhibit large folding of the dddt ligand and significant interaction between the C=C double bond and the metal center.     

Synthesis, structure and redox properties of bis(cyclopentadienyl)dithiolene complexes of molybdenum and tungsten

The compounds [Cp(2)M(S(2)C(2)(H)R)] (M = Mo or W; R = phenyl, pyridin-2-yl, pyridin-3-yl, pyridin-4-yl or quinoxalin-2-yl) and [Cp(2)Mo(S(2)C(2)(Me)(pyridin-2-yl)] have been prepared by a facile and general route for the synthesis of dithiolene complexes, viz. the reaction of [Cp(2)MCl(2)] (M = Mo or W) with the dithiolene pro-ligand generated by reacting the corresponding 4-(R)-1,3-dithiol-2-one with CsOH. These Mo compounds were reported previously (Hsu et al., Inorg. Chem. 1996, 35, 4743); however, the preparative method employed herein is more versatile and generates the compounds in good yield and all of the W compounds are new. Electrochemical investigations have shown that each compound undergoes a diffusion controlled one-electron oxidation (OX(I)) and a one-electron reduction (RED(I)) process; each redox change occurs at a more positive potential for a Mo compound than for its W counterpart. The mono-cations generated by chemical or electrochemical oxidation are stable and the structures of both components of the [Cp(2)Mo(S(2)C(2)(H)R)](+)/[Cp(2)Mo(S(2)C(2)(H)R)] (R = Ph or pyridin-3-yl) redox couples have been determined by X-ray crystallography. For each redox related pair, the changes in the Mo-S, S-C and C-C bond lengths of the {MoSCCS} moiety are generally consistent with OX(I) involving the loss of an electron from a π-orbital that is Mo-S and C-S antibonding and C-C bonding in character. These results have been interpreted successfully within the framework provided by DFT calculations accomplished for [Cp(2)M(S(2)C(2)(H)Ph)](n) (M = Mo or W; n = +1, 0 or -1). The HOMO of the neutral compounds is derived mainly from the dithiolene π(3) orbital (65%); therefore, OX(I) is essentially a dithiolene-based process. The similarity of the potentials for OX(I) (ca. 30 mV) for analogous Mo and W compounds is consistent with this interpretation and the EPR spectra of each of the Mo cations show that the unpaired electron is coupled to the dithiolene proton but relatively weakly to (95,97)Mo. The DFT calculations indicate that the unpaired electron is more localised on the metal in the mono-anions than in the mono-cations. In agreement with this, the EPR spectrum of each of the Mo-containing mono-anions manifests a larger (95,97)Mo coupling (A(iso)) than observed for the corresponding mono-cation and RED(I) for a W compound is significantly (ca. 300 mV) more negative than that of its Mo counterpart. [Cp(2)W(S(2)C(2)(H)(quinoxalin-2-yl))] is anomalous; RED(I) occurs at a potential ca. 230 mV more positive than expected from that of its Mo counterpart and the EPR spectrum of the mono-anion is typical of an organic radical. DFT calculations indicate that these properties arise because the electron is added to a quinoxalin-2-yl π-orbital.     

Mono-oxo bis(dithiolene) Mo(IV)/W(IV) complexes as building blocks for sulfide bridged Bi- and Tri-nuclear complexes

A new electron precise, six-electron, sulfide-bicapped trinuclear cluster complex [Et4N]4[Mo(IV)3(mu3-S)2(mnt)6] (1) has been synthesized, where each Mo(IV) atom is seven coordinated. Identical reaction conditions yielded a dimeric complex, [PNP]2[W(V)2(mu2-S)2(mnt)4] (2) from the starting W(IV) analogue due to oxidation by sulfur formed by the auto-oxidation of H2S. Two stepwise reversible reductions and no oxidation of 2 as observed by cyclic voltammetry are correlated with the nonbonding nature of the lowest unoccupied molecular orbital and deeply buried highest occupied molecular orbital by theoretical calculations at the density-functional theory level.     

X-ray spectroscopy of enzyme active site analogues and related molecules: bis(dithiolene)molybdenum(IV) and -tungsten(IV,VI) complexes with variant terminal ligands

The X-ray absorption spectra at the molybdenum and selenium K-edges and the tungsten L2,3-edges are acquired for a set of 14 Mo(IV) and W(IV,VI) bis(dithiolene) complexes related to the active sites of molybdo- and tungstoenzymes. The set includes square pyramidal [MoIVL(S2C2Me2)2]- (L = O2-, R3SiO-, RO-, RS-, RSe-) and [WIV(OR)(S2C2Me2)2]-, distorted trigonal prismatic [MoIV(CO)(SeR)(S2C2Me2)2]- and [WIV(CO)L(S2C2Me2)2]- (L = RS-, RSe-), and distorted octahedral [WVIO(OR)(S2C2Me2)2]-. The dithiolene simulates the pterin-dithiolene cofactor ligand, and L represents a protein ligand. Bond lengths are determined by EXAFS analysis using the GNXAS protocol. Normalized edge spectra, non-phase-shift-corrected Fourier transforms, and EXAFS data and fits are presented. Bond lengths determined by EXAFS and X-ray crystallography agree to < or = 0.02 A as do the M-Se distances determined by both metal and selenium EXAFS. The complexes [MoIV(QR)(S2C2Me2)2]- simulate protein ligation by the DMSO reductase family of enzymes, including DMSO reductase itself (Q = O), dissimilatory nitrate reductase (Q = S), and formate dehydrogenase (Q = Se). Edge shifts of these complexes correlate with the ligand electronegativities. Terminal ligand binding is clearly distinguished in the presence of four Mo-S(dithiolene) interactions. Similarly, five-coordinate [ML(S2C2Me2)2]- and six-coordinate [M(CO)L(S2C2Me2)2]- are distinguishable by edge and EXAFS spectra. This study expands a previous XAS investigation of bis(dithiolene)metal(IV,V,VI) complexes (Musgrave, K. B.; Donahue, J. P.; Lorber, C.; Holm, R. H.; Hedman, B.; Hodgson, K. O. J. Am. Chem. Soc. 1999, 121, 10297) by including a larger inventory of molecules with variant physiologically relevant terminal ligation. The previous and present XAS results should prove useful in characterizing and refining metric features and structures of enzyme sites.     

Monoanionic molybdenum and tungsten tris(dithiolene) complexes: a multifrequency EPR study

Numerous Mo and W tris(dithiolene) complexes in varying redox states have been prepared and representative examples characterized crystallographically: [M(S(2)C(2)R(2))(3)](z) [M = Mo, R = Ph, z = 0 (1) or 1- (2); M = W, R = Ph, z = 0 (4) or 1- (5); R = CN, z = 2-, M = Mo (3) or W (6)]. Changes in dithiolene C-S and C-C bond lengths for 1 versus 2 and 4 versus 5 are indicative of ligand reduction. Trigonal twist angles (Θ) and dithiolene fold angles (α) increase and decrease, respectively, for 2 versus 1, 5 versus 4. Cyclic voltammetry reveals generally two reversible couples corresponding to 0/1- and 1-/2- reductions. The electronic structures of monoanionic molybdenum tris(dithiolene) complexes have been analyzed by multifrequency (S-, X-, Q-band) EPR spectroscopy. Spin-Hamiltonian parameters afforded by spectral simulation for each complex demonstrate the existence of two distinctive electronic structure types. The first is [Mo(IV)((A)L(3)(5-?))](1-) ((A)L = olefinic dithiolene, type A), which has the unpaired electron restricted to the tris(dithiolene) unit and is characterized by isotropic g-values and small molybdenum superhyperfine coupling. The second is formulated as [Mo(V)((B)L(3)(6-))](1-) ((B)L = aromatic dithiolene, type B) with spectra distinguished by a prominent g-anisotropy and hyperfine coupling consistent with the (d(z(2)))(1) paramagnet. The electronic structure disparity is also manifested in their electronic absorption spectra. The compound [W(bdt)(3)](1-) exhibits spin-Hamiltonian parameters similar to those of [Mo(bdt)(3)](1-) and thus is formulated as [W(V)((B)L(3)(6-))](1-). The EPR spectra of [W((A)L(3))](1-) display spin-Hamiltonian parameters that suggest their electronic structure is best represented by two resonance forms {[W(IV)((A)L(3)(5-?))](1-) ? [W(V)((A)L(3)(6-))](1-)}. The contrast with the corresponding [Mo(IV)((A)L(3)(5-?))](1-) complexes highlights tungsten's preference for higher oxidation states.     

Phosphine-substituted dithiolene complexes as ligands: communication between ruthenium(II) centers through a dimolybdenum bis(dithiolene) core

The reaction of Mo2(SCH2CH2S)2Cp2 (1; Cp=eta-C5H5) with an excess of an alkyne in refluxing dichloromethane affords the bis(dithiolene) complexes Mo2(micro-SCR1=CR2S)2Cp2 (2a, R1=R2=CO2Me; 2b, R1=R2=Ph; 2c, R1=H, R2=CO2Me) whereas with 1 equiv of alkyne at room temperature the mixed dithiolene-dithiolate species Mo2(micro-SCR1=CR2S)(micro-SCH2CH2S)Cp2 (3a, R1=R2=CO2Me; 3b, R1=R2=Ph) are formed. The remaining dithiolate ligand in 3 can then be converted into a different dithiolene by reaction with a second alkyne. Applying this methodology, we have used bis(diphenylphosphino)acetylene to prepare the first examples of complexes containing phosphine-substituted dithiolene ligands: Mo2{micro-SC(CO2Me)=C(CO2Me)S}{micro-SC(PPh2)=C(PPh2)S}Cp2 (2g) and Mo2{micro-SC(PPh2)=C(PPh2)S}2Cp2 (2h). Tri- and tetrametallic complexes can then be assembled by coordination of these diphosphines to CpRuCl units by reaction with CpRu(PPh3)2Cl. Electrochemical studies of the Ru(II)/Ru(III) couple in Mo2{micro-SC(PPh2)=C(PPh2)S}2Cp2(RuClCp)2 (4b) reveals that the two separate ruthenium centers are oxidized electrochemically at different potentials, demonstrating communication between them through the dimolybdenum bis(dithiolene) core. Density functional theory calculations were carried out to explore the electronic structures of these species and to predict and assign their electronic spectra.     

Stereochemical consequences of oxygen atom transfer and electron transfer in imido/oxido molybdenum(IV, V, VI) complexes with two unsymmetric bidentate ligands

Two equivalents of the unsymmetrical Schiff base ligand (L(tBu))(-) (4-tert-butyl phenyl(pyrrolato-2-ylmethylene)amine) and MoCl(2)(NtBu)O(dme) (dme = 1,2-dimethoxyethane) gave a single stereoisomer of a mixed imido/oxido Mo(VI) complex 2(tBu). The stereochemistry of 2(tBu) was elucidated using X-ray diffraction, NMR spectroscopy, and DFT calculations. The complex is active in an oxygen atom transfer (OAT) reaction to trimethyl phosphane. The putative intermediate five-coordinate Mo(IV) imido complex coordinates a PMe(3) ligand, giving the six-coordinate imido phosphane Mo(IV) complex 5(tBu). The stereochemistry of 5(tBu) is different from that of 2(tBu) as shown by NMR spectroscopy, DFT calculations, and X-ray diffraction. Single-electron oxidation of 5(tBu) with ferrocenium hexafluorophosphate gave the stable cationic imido phosphane Mo(V) complex [5(tBu)](+) as the PF(6)(-) salt. EPR spectra of [5(tBu)](PF(6)) confirmed the presence of PMe(3) in the coordination sphere. Single-crystal X-ray diffraction analysis of [5(tBu)](PF(6)) revealed that electron transfer occurred under retention of the stereochemical configuration. The rate of OAT, the outcome of the electron transfer reaction, and the stabilities of the imido complexes presented here differ dramatically from those of analogous oxido complexes.     

Substitution and oxidation reactions of bis(dithiolene)tungsten complexes of potential relevance to enzyme sites

Structurally characterized tungstoenzymes contain mononuclear active sites in which tungsten is coordinated by two pterin-dithiolene ligands and one or two additional ligands that have not been identified. In this and prior investigations (Sung, K.-M.; Holm, R. H. Inorg. Chem. 2000, 39, 1275; J. Am. Chem. Soc. 2001, 123, 1931), stable coordination units of bis(dithiolene)tungsten(IV,V,VI) complexes potentially related to enzyme sites have been sought by exploratory synthesis. In this work, additional members of the sets [WL(S2C2Me2)2](2-,-) and [WLL'(S2C2Me2)2](2-,-) have been prepared and structurally characterized. Tungsten(IV) complexes obtained by substitution are carbonyl displacement products of [W(CO)2(S2C2Me2)2] and include those with the groups W(IV)S (4), W(IV)(O2CPh) (5), and W(IV)(2-AdQ)(CO) (Q = S (6), Se (7); Ad = adamantyl). Those obtained by oxidation reactions contain the groups W(V)O (9), W(V)(QPh)2 (Q = S (10), Se (11)), W(VI)S(OPh) (12), and W(VI)O2 (14). The latter two complexes were obtained from W(IV) precursors using sulfur and oxygen atom transfer reactions, respectively. Complexes 4 and 9 are square pyramidal; 6, 7, 10, and 11 are distorted trigonal prismatic with cis ligands LL'; and 12 and 14 are distorted octahedral. Complexes 4, 10, and 11 support three-membered electron transfer series. Attempts to oxidize 4 to the W(V)S complex results in the formation of binuclear [W2(mu2-S)2(S2C2Me2)4](2-) having distorted octahedral coordination. The 21 known functional groups WL and WLL' in mononuclear bis(dithiolene) complexes prepared in this and prior investigations are tabulated. Of those with physiological-type ligands, it remains to be seen which (if any) of these ligation modes are displayed by enzyme sites.     

Synthesis and characterization of oxorhenium(v) diamido pyridine complexes that catalyze oxygen atom transfer reactions

The detailed syntheses of complexes 1-4, Re(O)(X)(DAP) (X = Me, 1; Cl, 2; I, 3; OTf (triflate), 4) incorporating the diamido pyridine (DAP) ancillary ligand (2,6-bis((mesitylamino)methyl)pyridine) are described and shown to be effective catalysts for oxygen atom transfer (OAT) reactions of PyO to PPh(3). The catalytic activities are as follows: 4≈3 > 2 > 1. The observed electronic trend is consistent with the turnover limiting reduction of the proposed Re(VII) dioxo intermediate, Re(O)(2)(X)(DAP), during the catalytic cycle. The catalytic activity of complexes 1-3 was compared to previously published diamido amine (DAAm) oxorhenium complexes of the type Re(O)(X)(DAAm) (X = Me, 5; Cl, 6; I, 7 and DAAm = N,N-bis(2-arylaminoethyl)methylamine) which exhibit hydrolytic degradation during the catalytic reaction. Complexes 1-3 displayed higher turnover frequencies compared to 5-7. This higher catalytic activity was attributed to the more rigid DAP ligand backbone, which makes the complexes less susceptible to decomposition. However, another decomposition pathway was proposed for this catalytic system due to the observation of Re(O)(3)((MesNCH(2))(MesNCH)NC(5)H(3)) 8 in which one arm of the DAP ligand is oxidized.     

A new class of sulfido/oxo(dithiolene)-molybdenum(IV) complexes derived from sulfido/oxo-bis(tetrasulfido)molybdenum(IV) anions

Mono(dithiolene)sulfidomolybdenum(IV) complexes, [MoS(S4)(bdt)](2-) (2) and [MoS(S4)(bdtCl2)](2-) (3) (1,2-benzenedithiolate = bdt, 3,6-dichloro-1,2-benzenedithiolate = bdtCl2), were prepared by the substitution reaction of a tetrasulfido ligand in known [MoS(S4)2](2-) (1) with the corresponding dithiol. Complexes 2 and 3 were irreversibly oxidized to give bis(mu-sulfido) dimolybdenum(V) species, {[MoS(bdt)]2(mu-S)2}(2-) (4) and {[MoS(bdtCl2)]2(mu-S)2}(2-) (5), in aerobic acetonitrile. Mono(dithiolene)oxomolybdenum(IV) complexes, [MoO(S4)(bdt)](2-) (7) and [MoO(S4)(bdtCl2)](2-) (8), that are oxo derivatives of 2 and 3 were also synthesized from a known [MoO(S4)2](2-) (6) of an oxo derivative of 1 and the corresponding dithiol. Further, the electrophilic addition of dimethyl acetylenedicarboxylate to 7 gave [MoO(bdt)(S2C2(COOMe)2)](2-) (9), and ligand substitution of the tetrasulfido group of 7 with bdt and bdtCl2 yielded [MoO(bdt)2](2-) ( 10) and [MoO(bdt)(bdtCl2)](2-) (11), respectively. New sulfido/oxo molybdenum complexes were characterized by (1)H NMR, IR, ESI-MS, Raman, and UV-vis spectroscopies; cyclic voltammetry; and elemental analysis, and crystal structures of 2, 3, 5, 7, and 8 were determined by X-ray analysis.     

Synthesis and structures of bis(dithiolene)-tungsten(IV) complexes related to the active sites of tungstoenzymes

Recent protein crystallographic results on tungsten enzymes and primary sequence relationships between certain molybdenum and tungsten enzymes provoke interest in the generalized bis(dithiolene) complexes [WIV(QR)(S2C2R'2)2]1- and [WVIO(QR)(S2C2R'2)2]1- (Q = O, S, Se) as minimal representations of enzyme sites. The existence and stability of W(IV) complexes have been explored by synthesis. Reaction of [W(CO)2(S2C2Me2)2] (1) with PhO- results in complete CO substitution to give [W(OPh)(S2C2Me2)2]1- (2). Reaction of 1 with PhQ- affords the monocarbonyls [W(CO)(QPh)(S2C2Me2)2]1- (Q = S (3), Se (5)). The use of sterically demanding 2,4,6-Pri3C6H2Q- also yields monocarbonyls, [W(CO)(QC6H2-2,4,6-Pri3)(S2C2Me2)2]1- (Q = S (4), Se (6)). The X-ray structures of square pyramidal 2 and trigonal prismatic 3-6 (with unidentate ligands cis) are described. The tendency to substitute one or both carbonyl ligands in 1 in the formation of [MIV(QAr)(S2C2Me2)2]1- and [MIV(CO)(QAr)(SeC2Me2)2]1- with M = Mo and W is related to the M-Q bond length and ligand steric demands. The results demonstrate a stronger binding of CO by W(IV) than Mo(IV), a behavior previously demonstrated by thermodynamic and kinetic features of zerovalent carbonyl complexes. Complexes 3-6 can be reversibly reduced to W(III) at approximately -1.5 V versus SCE. On the basis of the potential for 2(-2.07 V), monocarbonyl ligation stabilizes W(III) by approximately 500 mV. This work is part of a parallel investigation of the chemistry of bis(dithiolene)-molybdenum (Lim, B. S.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 2000, 39, 263) and -tungsten complexes related to enzyme active sites.     

Molecular oxygen activation by a molybdenum(IV) monooxo bis(beta-ketiminato) complex

Molybdenum(IV) monooxo compound that contains bis(beta-ketiminato) ligands activates molecular oxygen forming a molybdenum(VI) monooxo peroxo compound, representing a new entry into molybdenum peroxo derivatives.     

Synthesis and structures of bis(dithiolene)tungsten(IV,VI) thiolate and selenolate complexes: approaches to the active sites of molybdenum and tungsten formate dehydrogenases

Formate dehydrogenases are molybdenum- or tungsten-containing enzymes that catalyze the oxidation of formate to carbon dioxide. Among the significant characteristics of the mononuclear active sites are coordination of two pyranopterindithiolene ligands and selenocysteinate to the metal in oxidation states IV-VI. The first detailed investigation of the synthesis and structures of bis(dithiolene)tungsten selenolate and analogous thiolate complexes of relevance to formate dehydrogenases has been undertaken. Some 17 complexes of the types [WIV(QR)(S2C2Me2)2]-, [WVIO(QR)(S2C2Me2)2]-, and [WVIS(QR)(S2C2Me2)2]- (Q = S, Se; R = tert-butyl, 1-adamantyl) and the desoxo species [WVI(SR)(OSiR'3)(S2C2Me2)2] (R' = Me, Ph) were prepared. Ten structures of representative members of these types were determined; WIV complexes are square-pyramidal and WVI complexes are six-coordinate, with geometries intermediate between octahedral and trigonal-prismatic. Selenolate complexes are less stable than similar thiolate species; decomposition products were identified as [WV2(mu2-Q)2(S2C2Me2)2]2- and [WIV,V2(mu2-Se)(S2C2Me2)4]-. The several [MoIV(QR)(S2C2Me2)2]- complexes prepared earlier and the tungsten compounds synthesized in this work form a family of molecules whose overall stereochemistry and metric features are those expected in the absence of protein structural constraints.     

Computational study of iron bis(dithiolene) complexes: redox non-innocent ligands and antiferromagnetic coupling

The molecular and electronic structure of monomeric ([Fe(S2C2H2)2](z), [Fe(S2C2(C6H4-p-OCH3)2)2](z)) and dimeric ([{Fe(S2C2H2)2}2](z)) iron bis(dithiolene) complexes, and of their phosphine adducts ([(PH3)Fe(S2C2H2)2](z), [(P(C6H5)3)Fe(S2C2H2)2](z), [(PH3)Fe(S2C2(C6H4-p-OCH3)2)2](z)), carrying various charges (z = 0, 1-, 2-), have been investigated by density functional theory (DFT). Net total spin polarization values S of zero, two, and four have been considered for all neutral model compounds and their dianions, whereas all monoanions have been examined with net total spin polarization values S of one, three, and five. The DFT calculations utilized the pure functional BP86, as well as the hybrid functionals B3LYP and B3LYP*. For the monomers, the calculations reveal the presence of redox non-innocent dithiolene ligands and antiferromagnetic coupling between the ligands and the metal center. For the dimers, complexes with antiferromagnetically coupled iron centers have been found to represent structures of low energy, if not lowest energy structures. The spin-coupling constant of [{Fe(S2C2H2)2}2](2-) is calculated as J = -230 cm(-1). On the basis of the computational results, a model for reversible, electrochemically controlled binding and release of phosphine ligands to iron bis(dithiolene) complexes is proposed. Only BP86 and B3LYP* results, but not those of B3LYP calculations, are in qualitative agreement with experimental findings. BP86 calculations provide the best quantitative match in comparison with the experiment.