D. Coucouvanis

Syntheses, Structures, and Reactions of Binary and Tertiary Thiomolybdate Complexes Containing the (O)Mo(Sx) AND (S)Mo(Sx) Functional Groups (x = 1, 2, 4)

Publisher Summary This chapter discusses syntheses, structures, and reactions of binary and tertiary thiomolybdate complexes containing the (O)Mo(Sx,) and (S)Mo(Sx,) functional Groups. Molybdenum is one of the most abundant elements on the planet (found as molybdenite—MoS 2 and wulfenite—PbMoO), and in the oceans molybdenum is the most abundant of the redox-active transition elements. The importance of molybdenum sulfur compounds in nonbiological catalysis is exemplified by the hydrodesulfurization (HDS) reaction. The chemistry of molybdenum ions with sulfur ligands is unique when compared to other transition metal ions. At times, the ground states of electron-redistribution isomers of isoelectronic complexes are close enough in energy so that minor perturbations (like crystal packing forces, solvent dielectrics, and ion pairing) are sufficient to stabilize preferentially one isomer over another. A multitude of heterometallic complexes are obtained whereby the thiomolybdate anions serve as ligands. The Mo atoms found at the edges of the MoS 2 crystallites may be bound by any of several oxo, thio, or sulfide ligands and these units may be directly or indirectly involved in the catalytic process.

Unique reactivity characteristics of Mo-coordinated S2-2 and S2-4 ligands

Abstract The synthesis of the new dithiolene complexes, [{(MeOOC)2C2S2}2Mo(μ2-S)]−2 and [OMo(S2C2(COOMe)2)2]−, is reported. These complexes are obtained by the reaction of dicarbomethoxyacetylene (DMA) with either [(S4)Mo(S)(μ2-S)2Mo(S)(S4)]2− or [(CS4)Mo(S)(μ2-S)2Mo(S)(CS4)]2− and [OMo(S4)2]2−, respectively. The reaction of [(S4)Mo(O)(μ2-S)2Mo(O)(S2)]2− with DMA results in the new dithiolene complex [{(MeOOC)2C2S2}Mo(O)(μ2-S)]−2, which is the isomeric form of the vinyl disulfide complex obtained in the reaction of the [(S2)Mo(O)(μ2-S)2Mo(O)(S2)]2− complex with DMA. The difference in reactivity between the two complexes that contain the same [Mo2O2S2]2+ core is attributed to the intrinsically different reactivity characteristics of the S2−4 and S2−2 ligands. As a result of Mo-S dπ-pπ bonding an alternation in the SS bond lengths is observed in virtually all of the structurally characterized Mo-S4 units. The consequent weakening of the SS bonds adjacent to the MoS bonds allows for the ready dissociation of S02 from the Mo-coordinated S2−4 ligands. This weakening also accounts for the facile formation of dithiolenes in cycloaddition reactions of alkynes with the Mo-S4 units. By comparison, the SS bond in side-on Mo-coordinated S2−2 ligands is strengthened as a result of depopulation of the ligand π*-orbitals. Reactions of the latter with alkynes do not proceed by cycloaddition. Instead, insertion into the MoO bond has been reported for at least one such reaction. The importance of activated polysulfide ligands in the hydrodesulfurization reaction is discussed.

Functional analogs for the reduction of certain nitrogenase substrates. Are multiple sites within the Fe/Mo/S active center involved in the 6e – reduction of N 2 ?

Abstract Reactivity studies of clusters that contain the MFe3S4 cores (M = Mo, V) with catecholate, multicarboxylate (or DMF) ligands coordinated to the Mo (or V) atoms, and Cl ligands coordinated to the Fe atoms have been carried out. These studies show the M/Fe/S single cubane clusters to be effective catalysts in the reduction of nitrogenase substrates such as hydrazine, acetylene and protons to give ammonia, ethylene and dihydrogen respectively. The same molecules do not activate or catalyze the reduction of dinitrogen. The results indicate that the observed catalyses are occurring at the Mo (V) sites by a process that, in the case of hydrazine, involves substrate protonation prior to reduction. The facile catalytic reduction of hydrazine by clusters that contain coordinatively saturated polycarboxylate-bound Mo atoms is rationalized in terms of a possible protonation/proton delivery function of the coordinated polycarboxylate ligands. The reactivity characteristics of the M/Fe/S clusters (structurally quite similar to the nitrogenase cofactor) have led to the suggestion that the Mo (V) atoms may be involved in the reduction of hydrazine in the later stages of dinitrogen reduction.

Catalytic and stoichiometric multielectron reduction of hydrazine to ammonia and acetylene to ethylene with clusters that contain the MFe3S4 cores (MMo, V). Relevance to the function of nitrogenase

Abstract A functional model for nitrogenase is currently sought in our laboratory in reactivity studies using various single cubane clusters that possess the [MFe 3 S 4 ] n + core, (MMo, n = 3; MV, n = 2). These clusters are used as catalysts for the reduction of enzymatically relevant substrates. Substrates such as hydrazine and acetylene are catalytically reduced by (NEt 4 ) 2 [(Cl 4 -cat) (CH 3 CN) MoFe 3 S 4 Cl 3 ], I , to ammonia and ethylene respectively, in the presence of added protons and reducing equivalents. Hydrazine also is catalytically reduced by the (NEt 4 ) [(DMF) 3 VFe 3 S 4 Cl 3 ] cubane under similar conditions. Gas chromatography was employed to monitor the reduction of acetylene to ethylene and a trace of ethane. Catalysis in excess of 100 turnovers (for hydrazine reduction) and in excess of 15 turnovers (in acetylene reduction) has been demonstrated over a period of 24 h. A study of the acetylene reduction reveals saturation kinetics to be operating at high substrate concentrations. A variable temperature kinetic study of acetylene reduction shows a moderate activation energy ( E act = 9(1) kcal mol −1 ) but a large entropy of activation ( ΔS ‡ = −32(2) cal K −1 mol −1 ) which extrapolates to a significant Gibbs free energy ( ΔG ‡ = 19(1) kcal mol −1 ). These results are consistent with an ordered transition state. Considerable evidence has been amassed which directly implicates the Mo and V atoms as the primary catalytic sites. Replacement of the Mo or V bound, kinetically labile, solvent molecules with non-labile ligands acts to suppress the observed rates of reaction. The Fe sites on I are totally inactive in the reduction of hydrazine, however they have been found to effect acetylene reduction albeit at a markedly reduced rate compared to the Mo site. Catalyst integrity has also been demonstrated by a variety of techniques, primarily EPR spectroscopy which identifies the characteristic S = 3 2 signals of the Mo and V cubanes after at least 18 h reaction time.

ESR of spin 5/2 systems with axial symmetry and moderately large zero‐field splittings. Application of line‐shape calculations to the interpretation of randomly oriented microcrystallite spectra

Magnetic resonance line positions and intensities are calculated for the simplified spin‐Hamiltonian H=D[Sz2−S(S+1)/3]+g μBH·Swith S=5/2. The resonance line positions and intensities are found by direct diagonalization of the matrix of ℋ and are plotted versus the axial angle θ and the magnetic field H. The region 1/2≲|D|/h  ν ≲ 2 is explored, where v=the microwave frequency, and line shapes integrated over all orientations are calculated using a Gaussian line shape with isotropic linewidth. The results are compared with the experimental EPR spectra of two crystallographically isomorphic high‐spin trisdithio‐oxalato‐Fe(III) salts, one having |D|=0.26 GHz (treated by perturbation theory, including quartic spin operators), and the other |D|=6.8 GHz. Single crystal measurements were made for the former case, but only powder measurements of the latter system were possible. Calculation of the powder line shape for the |D|=6.8 GHz spectrum for both X‐band and Ka‐band EPR spectra gives good agreement with experi...

Mössbauer effect in synthetic analogs of rubredoxin

Abstract Mossbauer measurements have been performed on three recently synthesized Fe(II) complexes with tetrahedral sulphur coordination, simulating the active center in reduced rubredoxin. One of the complexes has virtually identical Mossbauer parameters with reduced rubredoxin.

Ground-state electronic and magnetic properties of MS2−4 derived trinuclear M-Fe-S complexes (M = Mo, W)

Abstract Three trinuclear complex anions, [(FeCl 2 ) 2 MS 4 ] 2− (M = Mo, W) and [(MoS 4 ) 2 Fe] 3− , were studied by Mossbauer and magnetic measurements. The results imply a high electron affinity of the MS 2− 4 group and intramolecular spin coupling. These properties and structural features suggest these molecular groups as possible building blocks of the Fe-Mo co-factor of the nitrogen fixing enzyme nitrogenase.

Mössbauer, magnetic susceptibility, and EPR studies of intermediate spin iron (III) dithiooxalato halides

Mossbauer, magnetic susceptibility, and EPR measurements have been performed in the series of intermediate spin [Fe(S2C2O2)2X]2−, (X=Cl, Br, I) complex anions. All members of the series are paramagnetic down to 1.4 °K. Mossbauer and EPR results establish the axial character of the electronic spin Hamiltonian for the iodide derivative and increasing rhombicity for Br and Cl. Axial zero field splitting parameters D are positive and near 5 °K. The quadrupole splittings are large in the range 3.4–3.6 mm/sec and typical for the S=3/2 state of Fe(III). Isomer shifts are near 0.30 mm/sec indicating substantial d electron delocalization in the π system of the ligand. The magnetic hyperfine constant decreases from the Cl to the I derivative indicating increasing covalency of the metal–halide bond. Pronounced relaxation effects are present in the Mossbauer spectra varying with the halogen ligand. Spectra in applied fields up to 60 kG have seen fitted with a relaxation theory based on a stochastic model.

Reactivity and kinetic studies of (NH4)2(MoS4) in acidic aqueous solution: Possible relevance to the angiostatic function of the MoS42- ligand

Although addition of mineral acids to WS(4)(2-) in water is known to lead to aggregation and formation of various polynuclear thiotungstate anions, acid hydrolysis of the MoS(4)(2-) anion is reported to give mainly MoS(3) or MoS(2) as hydrolysis products. Knowledge of the resulting product(s) from such reactions has implications on the use of tetrathiomolybdate (MoS(4)(2-)) as both a potential anti-tumor drug and for the treatment of Wilsons disease. In this investigation, reaction of HCl with MoS(4)(2-) in water was monitored as a function of time. Reaction mixtures of both 1:1 and 2:1 mole ratios of the acid to MoS(4)(2-) were examined, as well as MoS(4)(2-) reactions in simulated human stomach fluids at pH of approximately 2 and 3. Monitoring by electrospray mass spectrometry (ESMS), Fourier transform infrared (FTIR), and UV-visible spectroscopy clearly has revealed the formation of complex mixtures of polynuclear thiomolybdates (Mo(2)-Mo(18)). Generally, a two-stage consecutive reaction sequence occurs. A faster stage (k=7.0-7.9 x 10(-2)min(-1)), which seems to extend to trinuclear thiomolybdate species, followed by a slower second stage (k=5.4-15.2 x 10(-4)min(-1)) to higher polynuclear thiomolybdates. Under acidic conditions (e.g. pH approximately 3) that could also mimic some human stomach fluids, and under anaerobic atmosphere where the generated hydrogen sulfide is prevented from escaping from the reaction vessel, Mo(3)S(9)(2-) predominates over an extended reaction period. In similar reactions under aerobic conditions and where the hydrogen sulfide is irretrievably lost from the reaction mixture the binuclear (Mo(2)O(a)S(10-a)(2-);a=0-3) and trinuclear (Mo(3)O(b)S(9-b)(2-);b=1-3) anions predominate.

The synthesis and structural characterization of the [Nb(Se)3StBu]2−, [Nb(O)(Se2)SeH]2−, [(Nb(O)(Se2)2)2Se4]4− and {[Nb(Se)3]2O}4− anions. New elementary selenoniobates

Abstract The reaction of NbCl5, NaStBu, Se and Et4NCl in CH3CN in a 1:6:1:1.6 molar ratio affords crystalline (Et4N)2[Nb(Se)3(StBu)], 1. The same salt is obtained in about 25% yield by the oxidation of [Nb(S)(StBu)4]− with 1 equiv. of Se. Oxidation of 1, or its Ph4P+ analog with 1.5 equiv. of Se in the presence of water, results in the formation of the [Nb(O)(Se2)2SeH)]2− anion, 2, that can be isolated as Ph4P+ or Et4N+ salts. The SeH vibration in the Et4N+ salt of 2 is found at 2526 cm−1 and the proton in the anion is easily exchanged with D3COD. The (Et4N)4[(Nb(O)(Se2)2)2Se4] dimer, 3 was obtained by the oxidation of (Et4N)2[Nb(O)(Se2)2SeH)] with 2 equiv. of elemental selenium. The reaction of (Et4N)2[Nb(Se)3(StBu)] with water in CH3CN, led to isolation of the oxo-bridged dimer, (Et4N)4[(Se)3NbONb(Se)3], 4. The crystal structures of 1–4 have been determined. Single crystal X-ray diffraction data for 1–4 were collected on a Nicolet P3/F diffractometer using MoKα radiation and the structures solved using a combination of heavy atom Patterson techniques, direct methods and Fourier techniques. The number of parameters refined and the final R factors obtained for each structure at the conclusion of refinement are: (1) 280, 0.0463; (2), 528, 0.0941; (3), 318, 0.0886 and (4), 205, 0.0625 respectively. The anion in 1 exhibits tetrahedral geometry while the structure of the anion in 2 shows the Nb5+ ion in a pentagonal pyramidal geometry with two η2-Se22− and the η1-SeH− ligands in the equatorial plane and the oxo ligand in the apical position. The Nb atom is located 0.70 A above the plane defined by the five equatorial Se atoms. In 2, the NbO, NbSeH and NbSe2 bonds are found at 1.671(11), 2.608(4) and 2.60(1) A, respectively. The structure of the anion in 3 shows 2 [Nb(O)(Se2)2] units, bridged by a tetraselenido ligand to give a dimer with pentagonal pyramidal [Nb(O)(Se2)2Se] subunits and the oxo ligands in an anti conformation. In 4 the μ-oxo ligand of the anion is located on a crystallographic centre of symmetry and the NbO bond, in the linear NbONb unit is 1.916(1) A. The two Nb(Se)3 units are oriented in a staggered fashion to give an idealised D3d symmetry.