Synthesis and characterization of redox-active tris(pyrazolyl)borate cobalt complexes.

The reaction of CoX2 (X = Cl, Br, NO3) with KTp(Ph2) in tetrahydrofuran (THF) yields the half-sandwich compounds [Tp(Ph2)CoX] (X = Cl 1, Br 2, NO3 3). The reaction of [Tp(Ph2)CoBr] with NaX (X = N3, NO2) or potassium thiocyanate (KNCS) permits isolation of [Tp(Ph2)CoX] (X = N3 4, NCS 5, NO2 6). In contrast, the reaction of cobalt(II) acetate with KTp(Ph2) yields [Tp(Ph2)CO(OAc)(Hpz(Ph2))] 7 as a result of B-N bond cleavage. Subsequent reaction of 7 with a range of beta-diketones in the presence of NaOMe produces the beta-diketonate complexes, [Tp(Ph2)Co(beta-diketonate)] (beta-diketonate = acac 8, hfac 9, dbm 10, tmhd 11). IR spectroscopy suggests that the Tp(Ph2) ligands are kappa3-coordinated and that the beta3-diketonate ligands adopt a bidentate coordination mode. Electronic spectra are consistent with four- or five-coordinate species in solution. X-Ray crystallographic studies of 7 reveal an intermediate five-coordinate cobalt centre with a hydrogen bonding interaction between the pyrazole hydrogen and the acetate carbonyl oxygen. The molecular structures of 9 and 10 show cobalt centres with square pyramidal coordination geometries and kappa2-coordinated beta-diketonate ligands. Cyclic voltammetric studies of 6 reveal irreversible one-electron reduction to Co(I). However, the beta-diketonate complexes, 8, 10 and 11 undergo irreversible one-electron oxidation. The redox potential and reversibility increases as the steric bulk of the substituent on the beta-diketonate ligand increases.