A bis(diimidazole)Copper Complex Possessing a Reversible CuII/CuI Couple with a High Redox Potential

The new diimidazole ligand, bis (1‐methyl‐4,5‐diphenylimidazol‐2‐yl)ketone (BIMDPK), has been synthesised, characterised and shown to form four‐coordinate bis(diimidazole) CuII and CuI complexes in the salts [Cu(bimdpk)2]‐[BF4]2 and [Cu(bimdpk)2][PF6], the structures of which have been determined by X‐ray crystallography. The cations of these salts have a very similar geometry with CuII–Nav = 1.949 and CuI–Nav = 1.999 Å; the N‐Cu‐N interbond angles are constrained by 1) the bite angle of the BIMDPK ligand to 94 ± 2° and 2) the interligand steric interactions, which lead to the dihedral angle of the intraligand CuN2 planes of 68.2° for CuII and 74.9° for CuI–that is, a CuN4 geometry intermediate between tetrahedral and square planar. The X‐band EPR spectrum for the powdered CuII compound is typical of an approximately D2 CuN4 centre possessing a dxy ground state (gx = 2.080, gy = 2.075, gz = 2.291; Az = 112.3 × 10−4 cm−1). The UV/vis spectra are dominated by charge‐transfer bands, and both the CuII and CuI systems are intensely coloured. The EPR and electronic spectra indicate that these cations have a very similar structure in the solid state and in solution; the potential of the [Cu(bimdpk)2]2+/[Cu(bimdpk)2]+ couple is 0.59 V vs. SCE in MeCN and 0.80 V vs. SCE in CH2Cl2, and the electron self‐exchange constant in MeCN is 1.9 × 104M−1S−1. Comparisons are made between the properties of the [Cu(bimdpk)2]2+/+ centres and related Cu centres in chemical and biological systems; the results of this study reinforce the view that a [Cu(His)4] centre should not be precluded from consideration in biological electron transport.