Copper complexes of 2,6-bis(iminomethyl)pyridine derivatives and of 1,3-bis(pyridin-2-yl)pyrazole. Effects of ligand bulk and conformational strain on the ground state of a six-co-ordinate copper(II) ion

Complexation of hydrated Cu(BF4)(2) by 2 molar equivalents of (LR)-R-2 ((LM e)-Me-2 = 2,6-bis{methyliminomethyl}pyridine, (LCy)-Cy-2 = 2,6-bis{cyclohex yliminomethyl}pyridine, (LBut)-Bu-2 = 2,6-bis{tert-butyliminomethyl}pyridin e) afforded [Cu((LR)-R-2)(2)][BF4](2) (R = Me, 1; Cy, 2; or Bu-t, 3) in mod erate yields. EPR spectroscopy in solution and the solid state demonstrates that 1 and 2 adopt the expected {d(y2 - z2)}(1) electronic ground state, a nd that the pseudo-Jahn-Teller elongation axis is fluxional in solid 1 and static in solid 2. In contrast, 3 exhibits a {d(z2)}(1) ground state by EPR . The crystal structures of 2.MeNO2 and 3.1/2Me(2)CO contain rhombic six-co -ordinate copper(II) ions, which differ principally in the length of the Cu -N bonds close to the molecular y axis. Treatment of [Cu(NCMe)(4)]BF4 with varying ratios of (LR)-R-2 yields [Cu((LR)-R-2)(2)]BF4 (R = Me, 4; Mes, 5; or Cy, 6: L(2)Mes = 2,6-bis{2,4,6-trimethylphenyliminomethyl}pyridine) or [ {Cu((LBut)-Bu-2)}(2)][BF4](2) 7. The single crystal structure of 5.MeCN sho ws a flattened tetrahedral copper(I) centre with two bidentate L(2)Mes liga nds. Reaction of hydrated Cu(ClO4)(2) with 2 molar equivalents of L-3 (1,3- bis{pyridin-2-yl}pyrazole) yields [Cu(L-3)(2)][ClO4](2) 8, whose crystal st ructure demonstrates a six-co-ordinate copper(II) ion with long Cu-N{pyridi ne} bonds. Powder EPR spectroscopy demonstrates a {d(z2)}(1) ground state f or 8 in the solid.