Synthetic Analogues and Reaction Systems Relevant to the Molybdenum and Tungsten Oxotransferases

Uranium(VI) alkoxide complexes are prepared via metathesis reactions of [UO2Cl2(THF)2]2 with potassium alkoxides in nonaqueous media. The dark red compound U[OCH2C(CH3)3]6, 1, results from redistributive exchange of oxo and neopentoxide ligands between more than one uranium species. Single-crystal X-ray diffraction analysis of 1 reveals a monomer in which the uranium is coordinated in a pseudooctahedral fashion by six neopentoxide ligands. Imposition of steric congestion at the metal center prevents oxo-alkoxide ligand exchange in the reactions using more sterically demanding alkoxides. Simple metathesis between uranyl chloride and alkoxide ligands occurs in the synthesis of golden yellow-orange UO2(OCHPh2)2(THF)2, 2, and yellow UO2[OCH(tBu)Ph]2(THF)2, 3. Single-crystal X-ray diffraction analysis of 2 reveals a monomer in which the uranium is coordinated in a pseudooctahedral fashion by two apical oxo ligands, two diphenylmethoxide ligands occupying trans positions, and two tetrahydrofuran ligands. Coordination of diisopropylmethoxide allows for synthesis of a more complex binary alkoxide system. Single-crystal X-ray diffraction analysis of watermelon red [UO2(OCH(iPr)2)2]4, 4, reveals a tetramer in which each uranium is coordinated in a pseudooctahedral fashion by two apical oxo ligands, one terminal alkoxide, two bridging alkoxide ligands, and one bridging oxo ligand from a neighboring uranyl group. These compounds are characterized by elemental analysis, 1H NMR, infrared spectroscopy, and, for 1, 2, and 4, single-crystal X-ray diffraction analysis. Luminescence spectroscopy is employed to evaluate the extent of aggregation of compounds 2-4 in various solvents. Vibrational spectroscopic measurements of 2-4 imply that, in contrast to the case of uranyl complexes prepared in aqueous environments, coordination of relatively strongly donating alkoxide ligands allows for enhancement of electron density on the uranyl groups such that the uranyl U=O bonds are weakened. Crystal data are as follows. 1: monoclinic space group C2/m, a = 10.6192(8) A, b = 18.36(1) A, c = 10.6151(8) A, beta = 109.637(1) degrees, V = 1949.1(3) A3, Z = 2, dcalc = 1.297 g cm-3. Refinement of 2065 reflections gave R1 = 0.045. 2: monoclinic space group P2(1)/c, a = 6.1796(4) A, b = 15.669(1) A, c = 16.169(1) A, beta = 95.380(1) degrees, V = 1558.7(2) A3, Z = 2, dcalc = 1.664 g cm-3. Refinement of 3048 reflections gave R1 = 0.036. 4: tetragonal space group I4, a = 17.8570(6) A, b = 17.8570(6) A, c = 11.4489(6) A, V = 3650.7(3) A3, Z = 2, dcalc = 1.821 g cm-3. Refinement of 1981 reflections gave R1 = 0.020.

Basicity of uranyl oxo ligands upon coordination of alkoxides.

Synthesis, structure, and reactivity of some new dipyridyl and diamine-bridged dinuclear oxomolybdenum(VI) complexes

Thermodynamic Scales for Sulfur Atom Transfer and Oxo-for-Sulfido Exchange Reactions

The oxo-sulfido functional group cis-MoVIOS is essential to the activity of the xanthine oxidase family of enzymes but has proven elusive to synthesis in molecules containing no other four-electron ligands. A direct route to molecules containing this group has been achieved. The reaction system [MoO2(OSiPh3)2]/L in dichloromethane yields the complexes [MoVIO2(OSiPh3)2L] (L = phen (1), Me4phen (2), 4,4'-Me2bpy (3), 5,5'-Me2bpy (4), 2 py (5)) (74-96%), which are shown to have a distorted octahedral structure of crystallographically imposed C2 symmetry (1, 5) with cis oxo and trans silyloxy ligands. The related reaction system [MoO3S]2-/2Ph3SiCl/L in acetonitrile affords the complexes [MoVIOS(OSiPh3)2L] (L = phen (6), Me4phen (7), 4,4'-Me2bpy (8), 5,5'-Me2bpy (9)) (36-69%). From the collective results of elemental analysis, mass spectrometry, 1H NMR, and X-ray structure determinations (6, 7), complexes 6-9 are shown to contain the cis-MoVIOS group in molecules with the same overall stereochemistry as dioxo complexes 1-5. The crystal structures of 6 and 7 exhibit O/S disorder, which was modeled in refinements with 50% site occupancies. The Mo=O (1.607(5) (6), 1.645(5) (7) A) and Mo=S (2.257(3) (6), 2.203(2) (7) A) bond distances obtained in this way are somewhat shorter and longer, respectively, than expected. Distances obtained by molybdenum EXAFS analysis using the GNXAS protocol for 6-9 (Mo=O 1.71-1.72 A; Mo=S 2.18-2.19 A) are considered more satisfactory and are in good agreement with EXAFS values for xanthine oxidase. Molybdenum K-edge data for 1 and 6-9 are reported. Reaction of 7 with Ph3P in dichloromethane results in sulfur abstraction and formation of [MoVOCl(OSiPh3)2(Me4phen)] (10), which has a distorted octahedral structure with cis O/Cl and cis silyloxy ligands. Sulfur rather than oxygen abstraction is favored by relative Mo=O/Mo=S bond strengths. Complexes 6-9 should allow exploration of the biologically significant cis-MoVIOS group. (Less)

The Unperturbed Oxo-Sulfido Functional Group cis-Mo(VI)OS, Related to that in the Xanthine Oxidase Family of Molybdoenzymes: Synthesis, Structural Characterization and Reactivity Aspects

Reaction of MoIVO(S2PR2)2 with K{HB(Pripz)3} [HB(Pripz)3- = hydrotris(isopropylpyrazol-1-yl)borate] in refluxing toluene affords green L*MoIVO(S2PR2-S,S‘) complexes [L* = HB(3-Pripz)2(5-Pripz)- = hydrobis(3-isopropylpyrazol-1-yl)(5-isopropylpyrazol-1-yl)borate; R = Pri, Ph], which are converted upon reaction with boron sulfide in dichloromethane to the yellow thio analogues L*MoIVS(S2PR2-S,S‘). Crystals of L*MoIVO(S2PPri2) are monoclinic, space group P21/n, with a = 10.024(2) A, b = 20.999(9) A, c = 15.368(5) A, β = 100.57(2)°, V = 3180(2) A3, and Z = 4. Crystals of L*MoIVS(S2PPh2) are monoclinic, space group P21, with a = 10.801(8) A, b = 13.100(5) A, c = 12.023(9) A, β = 99.56(10)°, V = 1678(2) A3, and Z = 2. The mononuclear, distorted-octahedral complexes are isostructural and are composed of a terminal chalcogenide ligand [MoO = 1.671(3) A, MoS = 2.126(3) A], a bidentate dithiophosphinate-S,S‘ ligand, and a facial, tridentate L* ligand. In both cases the 5-isopropylpyrazole group is bound trans to the...

Oxygen Atom Transfer, Sulfur Atom Transfer, and Correlated Electron−Nucleophile Transfer Reactions of Oxo- and Thiomolybdenum(IV) Complexes: Synthesis of Oxothiomolybdenum(VI) and (Hydroxo)oxomolybdenum(V) Species

Silvano Colmanet

Papers

Oxygen Atom Transfer, Sulfur Atom Transfer, and Correlated Electron−Nucleophile Transfer Reactions of Oxo- and Thiomolybdenum(IV) Complexes: Synthesis of Oxothiomolybdenum(VI) and (Hydroxo)oxomolybdenum(V) Species