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

Abstract

Complexation of hydrated Cu(BF₄)₂ by 2 molar equivalents of L²R (L²Me = 2,6-bis{methyliminomethyl}pyridine, L²Cy = 2,6-bis{cyclohexyliminomethyl}pyridine, L²Bu^t = 2,6-bis{tert-butyliminomethyl}pyridine) afforded [Cu(L²R)₂][BF₄]₂ (R = Me, 1; Cy, 2; or Bu^t, 3) in moderate yields. EPR spectroscopy in solution and the solid state demonstrates that 1 and 2 adopt the expected {d_{y² − z²}}¹ electronic ground state, and 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 3·½Me₂CO 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 L²R yields [Cu(L²R)₂]BF₄ (R = Me, 4; Mes, 5; or Cy, 6: L²Mes = 2,6-bis{2,4,6-trimethylphenyliminomethyl}pyridine) or [{Cu(L²Bu^t)}₂][BF₄]₂7. The single crystal structure of 5·MeCN shows a flattened tetrahedral copper(I) centre with two bidentate L²Mes ligands. Reaction of hydrated Cu(ClO₄)₂ with 2 molar equivalents of L³ (1,3-bis{pyridin-2-yl}pyrazole) yields [Cu(L³)₂][ClO₄]₂8, whose crystal structure demonstrates a six-co-ordinate copper(II) ion with long Cu–N{pyridine} bonds. Powder EPR spectroscopy demonstrates a {d_z²}¹ ground state for 8 in the solid.