Electron paramagnetic resonance and crystallographic resolution of an axially compressed octahedral complex [Cu(terpyR8)2](ClO4)2 containing the C8 alkyl-chain ligand terpyR8; 4′-octoxy-2,2′:6′,2″-terpyridine

A copper(II) complex containing the ligand 4′-octoxy-2,2′:6′,2″-terpyridine (terpyR8), [Cu(terpyR8)2](ClO4)2 (1) has been synthesized and characterized by X-ray crystallography and X-band EPR spectroscopy. Structural analysis at 123 K revealed a compressed octahedral geometry, whereby the axial Cu–N bond lengths are significantly shorter than the equatorial ones. The mononuclear units in 1 are close packed such that there is no included solvent. This close packed nature is highlighted through a series of π–π interactions between mononuclear units and an array of hydrogen-bonding interactions between mononuclear units, the long C8 alkyl chains and ClO 4 - anions. Detailed EPR studies were carried out on both polycrystalline powder and frozen solution samples of 1, over a temperature range from 2.5 K up to 200–260 K. The polycrystalline powder exhibits a spectrum characteristic of a d x 2 - y 2 ground configuration at temperatures below ∼10 K. Above ∼100 K, the spectrum had the appearance of originating from a d z 2 ground configuration, but it is best interpreted in terms of fluxional disorder with a d x 2 - y 2 ground state. The spectra of the frozen solutions are consistent with the d x 2 - y 2 configuration (elongated tetragonal) at all temperatures, although the spectra show complex behaviour below ∼30 K, due to microwave power saturation effects, and above ∼120 K due to motional averaging effects. Apparent contradictions between the structural and EPR work of the kind seen here on the solid sample have recently been discussed elsewhere for related bis-tridentate Cu(II) complexes and are resolved here such that complex 1 has a Jahn–Teller elongated configuration masked by a structural disorder.