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

Abstract

The reaction of CoX₂ (X = Cl, Br, NO₃) with KTp^Ph₂ in tetrahydrofuran (THF) yields the half-sandwich compounds [Tp^Ph₂CoX] (X = Cl 1, Br 2, NO₃3). The reaction of [Tp^Ph₂CoBr] with NaX (X = N₃, NO₂) or potassium thiocyanate (KNCS) permits isolation of [Tp^Ph₂CoX] (X = N₃4, NCS 5, NO₂6). 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.