Steric control of the reactivity of moderately hindered tris(pyrazolyl)borates with copper(II) salts

The co-ordination chemistry of the anions tris(3-phenylpyrazolyl)borate ([TpPh]−), tris(3-cyclohexylpyrazolyl)borate ([TpCy]−) and tris(3,5-diphenylpyrazolyl)borate ([TpPh2]−) with Cu(O2CMe)2·H2O, CuCl2 and Cu(BF4)2·6H2O has been investigated. The complex [Cu(O2CMe)(TpPh)] (1) transforms in solution to the B–N cleavage product [Cu(O2CMe)(HpzPh)(TpPh)] (2), whose crystal structure shows two square pyramidal Cu(II) centres in the asymmetric unit, each with a monodentate acetate ligand. The analogous complex [Cu(O2CMe)(TpCy)] (3) does not undergo this reaction. Reaction of K[TpCy] or K[TpPh2] with one equivalent of CuCl2 in CH2Cl2 yields mixtures of [CuCl(TpR)] (R = Cy, Ph2) and [CuCl(HpzR)(TpR)] (R = Cy, 5; R = Ph2, 6). Reaction of Cu(BF4)2·6H2O with one equivalent of K[TpPh] in CH2Cl2 gives [Cu(HpzPh)4](BF4)2 (7) in low yield as the only isolable product. An identical reaction with K[TpCy] affords [Cu(HpzCy)2(TpCy)]BF4 (8) in moderate yield. The single crystal structure of 8·CHCl3 contains a square pyramidal complex with two N–H⋯FBF3 and one Cl3C–H⋯FBF3 hydrogen bonds within each formula unit. Complexation of CuCl2 by two equivalents of K[TpPh] in MeOH affords [Cu(TpPh)2] (9) in high yield. In contrast, identical reactions employing K[TpCy] or K[TpPh2] yield [Cu(pzCy)(HpzCy)(TpCy)] (10) or 6 as the major products. The single crystal structure of 10 shows a square pyramidal Cu(II) centre with metric parameters very similar to 8; although not crystallographically located, the presence of a N–H⋯N hydrogen bond between the HpzCy and [pzCy]− ligands can be inferred from the close approach of the pzCy pyrrolic N atoms. All complexes were characterised by FAB mass spectrometry, microanalysis, IR, UV/vis and EPR spectroscopies.