Temperature dependence of the electronic ground states of two mononuclear, six-coordinate copper(ii) centres

Abstract

Powdered [Cu(L²NH₂)₂][ClO₄]₂ (1; L²NH₂ = 2,6-bis{hydrazonomethyl}pyridine) and [Cu(L²OH)₂][ClO₄]₂ (2; L²OH = 2,6-bis{oximomethyl}pyridine) exhibit EPR spectra that are consistent with {d_z²}¹ Cu(II) centres at 295 K. These slowly transform upon cooling to 5 K, to show g-values that more closely resemble {d_y²−z²}¹, pseudo-Jahn–Teller elongated structures. Single crystal X-ray structures of 1·2(CH₃)₂CO and 22(CH₃)₂CO show the expected six-coordinate Cu(II) centres, with Cu–N bond lengths that are consistent with a {d_z²}¹ configuration according to DF calculations. Importantly, the structure of 2 is temperature-dependent between 100–300 K, its Cu–N bond lengths varying in a manner consistent with the EPR data. The EPR spectra, and TLS analyses of the crystal structures, strongly imply that these changes are not a consequence of dynamic Jahn–Teller disorder. These results contrast with [Cu(L²Me)₂]²⁺ (L²Me = 2,6-bis{N-methylcarbaldimino}pyridine) and other [Cu(L²R)₂]²⁺ complexes with small alkyl or aryl ‘R’ substituents, which adopt the more usual {d_y²−z²}¹ ground states at all temperatures.