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

The dioxo-Mo(VI) complexes LMoO(2)X [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, N CS, OMe, Oft, OPh, SPri, SPh, SCH(2)Ph] have been synthesized and char acterized by spectroscopic and structural methods. The infrared spectr a of the complexes exhibit nu(MoO2) bands at 940-920 and 910-890 cm(-1 ), and the H-1 NMR spectra are indicative of molecular C-s symmetry in solution. The X-ray crystal structures of three complexes are reporte d. L(a)MoO(2)(SPh): monoclinic space group P2(1)/c, a = 18.265(6) Angs trom, b = 8.110(3) Angstrom, c = 18.299(3) Angstrom, beta = 117.06(2)d egrees, V = 2414(1) Angstrom(3) with Z = 4. L(b)MoO2(OMe): monoclinic space group C2/c, a = 30.365(4) Angstrom, beta = 8.373(1) Angstrom, c = 19.646(2) Angstrom, beta = 113.28(1)degrees, V = 4588(1) Angstrom(3) with Z = 8. L(c)MoO(2)(SPh): orthorhombic space group P2(1)2(1)2(1), a = 7.9302(13) Angstrom, b = 16.627(2) Angstrom, c = 17.543(2) Angstro m, V = 2313.1(9) Angstrom(3) with Z = 4. The structures were refined b y full-matrix least-squares procedures to R values of 0.043, 0.027, an d 0.039, respectively. The mononuclear complexes feature facially trid entate N-donor ligands, mutually cis oxo and X ligands, and distorted octahedral geometries, The alkoxy and thiolate complexes undergo a rev ersible, one-electron reduction to form the corresponding dioxo-Mo(V) anions [LMo(V)O(2)X](-). 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 o xo groups, upon reduction and (ii) a steric or electrostatic barrier t o 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 generat ed by protonation of the anions [LMo(V)O2X](-). Chemical reduction by Bu(4)(n)NSH results in the sequential generation of [LMo(V)O(2)X](-) a nd [LMo(V)O(2)X](-) anions (except for X = OPh, SPh, and SPri, when on ly [LMo(V)O(2)X](-) is formed). The Mo(V) complexes have been characte rized by EPR spectroscopy.