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

Abstract

The reaction of CoX<inf>2</inf> (X = Cl, Br, NO<inf>3</inf>) with KTp ^Ph2 in tetrahydrofuran (THF) yields the half-sandwich compounds [Tp^Ph2CoX] (X = Cl 1, Br 2, NO<inf>3</inf>3). The reaction of [Tp^Ph2CoBr] with NaX (X = N<inf>3</inf>, NO<inf>2</inf>) or potassium thiocyanate (KNCS) permits isolation of [Tp^Ph2CoX] (X = N <inf>3</inf>4, NCS 5, NO<inf>2</inf>6). In contrast, the reaction of cobalt(ii) acetate with KTp^Ph2 yields [Tp^Ph2Co(OAc)(Hpz ^Ph2)] 7 as a result of B-N bond cleavage. Subsequent reaction of 7 with a range of β-diketones in the presence of NaOMe produces the β-diketonate complexes, [Tp^Ph2Co(β-diketonate)] (β-diketonate = acac 8, hfac 9, dbm 10, tmhd 11). IR spectroscopy suggests that the Tp^Ph2 ligands are κ³-coordinated and that the β-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 κ²- coordinated β-diketonate ligands. Cyclic voltammetric studies of 6 reveal irreversible one-electron reduction to Co(i). However, the β-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 β-diketonate ligand increases. © 2009 The Royal Society of Chemistry.