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 ([Tp(Ph)](-)), tris(3-cyclohexylpyrazolyl)borate ([Tp(Cy)](-)) and tris(3,5-diphenylpyrazolyl)borate ([Tp(2)(Ph)](-)) with Cu(O2CMe)(2). H2O, CuCl2 and Cu(BF4)(2). 6H(2)O has been investigated. The complex [Cu(O2CMe)(Tp(Ph))] (1) transforms in solution to the B-N cleavage product [Cu(O2CMe)(Hpz(Ph))(Tp(Ph))] (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)(Tp(Cy))] (3) does not undergo this reaction. Reaction of K[Tp(Cy)] or K[Tp(2)(Ph)] with one equivalent of CuCl2 in CH2Cl2 yields mixtures of [CuCl(Tp(R))] (R=Cy, Ph-2) and [CuCl(Hpz(R))(Tp(R))] (R=Cy, 5; R=Ph-2, 6). Reaction of Cu(BF4)(2). 6H(2)O with one equivalent of K[Tp(Ph)] in CH2Cl2 gives [Cu(Hpz(Ph))(4)](BF4)(2) (7) in low yield as the only isolable product. An identical reaction with K[Tp(Cy)] affords [Cu(Hpz(Cy))(2)(Tp(Cy))]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[Tp(Ph)] in MeOH affords [Cu(Tp(Ph))(2)] (9) in high yield. In contrast, identical reactions employing K[Tp(Cy)] or K[Tp(2)(Ph)] yield [Cu(pz(Cy))(Hpz(Cy))(Tp(Cy))] (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 Hpz(Cy) and [pz(Cy)](-) ligands can be inferred from the close approach of the pz(Cy) pyrrolic N atoms. All complexes were characterised by FAB mass spectrometry, microanalysis, IR, UV/vis and EPR spectroscopies.