A crystallographic and EPR study of the fluxional Cu(II) ion in [CuL2][BF4]2 (L = 2,6-dipyrazol-1-ylpyridine)

This paper reports a crystallographic and EPR study of pseudo-Jahn–Teller fluxionality in [Cu(L1)2][BF4]2 (1; L1 = 2,6-dipyrazol-1-ylpyridine). For 50 ≤ T ≤ 350 K, the Cu(II) ion in crystalline 1 is fluxional, with its axis of pseudo-Jahn–Teller elongation being disordered about the two N{pyrazole}–Cu–N{pyrazole} axes. The crystallographic data for 1 at these temperatures are well reproduced by a two-state model that neglects intermolecular interactions, but which yields an unusually small pseudo-Jahn–Teller radius (SpJT) for the compound. This was confirmed by measuring SpJT independently from the mean-square displacement amplitudes (MSDAs) in 1 and [Zn(L1)2][BF4]2 (2). At 41 K, 1 undergoes a phase transformation to a new polymorph containing three molecules per asymmetric unit, which have static structures and exhibit more normal SpJT values. Q-band EPR data show that the proportion of static spins in a powdered sample of 1 grows in relatively slowly as the temperature is lowered below 40 K.

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