Dioxomolybdenum(VI) Complexes of Tripodal Nitrogen-Donor Ligands: Syntheses and Spectroscopic, Structural, and Electrochemical Studies, Including the Generation of EPR-Active Molybdenum(V) Species in Solution

Article abstract of DOI:10.1021/ic00128a005

The dioxo-Mo(VI) complexes LMoO2X [L = hydrotris(3,5-dimethylpyrazol-1-yl)borate (L(a)), hydrotris(3-isopropylpyrazol-1-yl)borate (L(b)), hydrotris(3,5-dimethyl-1,2,4-triazol-1-yl)borate (L(c)); X = Cl, Br, NCS, OMe, OEt, OPh, SPr(i), SPh, SCH2Ph] have been synthesized and characterizedby spectroscopic and structural methods. The infrared spectra of the complexes exhibit ν(MoO2) bands at 940-920 and 910-890 cm**-1, and the (1)H NMR spectra are indicative of molecular Cs symmetry in solution. The X-ray crystal structures of three complexes are reported. L(a)MoO2(SPh): monoclinic space group P21/c, a = 18.265(6) ?, b = 8.110(3) ?, c = 18.299(3) ?, β = 117.06(2)°, V = 2414(1) ?**3 with Z = 4. L(b)MoO2(OMe): monoclinic space group C2/c, a = 30.365(4) ?, b = 8.373(1) ?, c = 19.646(2) ?, β = 113.28(1)°, V = 4588(1) ?**3 with Z = 8. L(c)MoO2(SPh): orthorhombic space group P212121, a = 7.9302(13) ?, b = 16.627(2) ?, c = 17.543(2) ?, V = 2313.1(9) ?**3 with Z = 4. The structures were refined by full-matrix least-squares procedures to R values of 0.043, 0.027, and 0.039, respectively. The mononuclear complexes feature facially tridentate N-donor ligands, mutually cis oxo and X ligands, and distorted octahedralgeometries. The alkoxy and thiolate complexes undergo a reversible, one-electron reduction to form the corresponding dioxo-Mo(V) anions [LMo(V)O2X](1-). The requirements for reversible, one-electron electrochemical reduction of dioxo-Mo(VI) complexes appear to be (i) minimal conformational change, restricting substitution trans to the oxo groups, upon reduction and (ii) a steric or electrostatic barrier to the close approach and dinucleation of the reduced species. A number of oxo-hydroxo-Mo(V) complexes of the type LMo(V)O(OH)X were generated by protonation of the anions [LMo(V)O2X](1-). Chemical reduction by Bu(n)4NSH results in the sequential generation of [LMo(V)O2X](1-) and [LMo(V)OSX](1-) anions (except for X = OPh, SPh, and SPr(i), when only [LMo(V)O2X](1-) is formed). The Mo(V) complexes have been characterized by EPR spectroscopy.