A BIS(DIIMIDAZOLE)COPPER COMPLEX POSSESSING A REVERSIBLE CU-II CU-I COUPLE WITH A HIGH REDOX POTENTIAL/

The new diimidazole ligand, bis (1-methyl-4,5-diphenylimidazol-2-yl)ke tone (BIMDPK), has been synthesised, characterised and shown to form f our-coordinate bis(diimidazole) Cu-II and Cu-I complexes in the salts [Cu(bimdpk)(2)][BF4](2) and [Cu(bimdpk)(2)][PF6], the structures of wh ich have been determined by X-ray crystallography. The cations of thes e salts have a very similar geometry with Cu-II-N-av = 1.949 and Cu-I- N-av 1.999 Angstrom; the N-Cu-N interbond angles are constrained by 1) the bite angle of the BIMDPK ligand to 94+/-2 degrees and 2) the inte rligand steric interactions, which lead to the dihedral angle of the i ntraligand CuN2 planes of 68.2 degrees for Cu-II and 74.9 degrees for Cu-I-that is, a CuN4 geometry intermediate between tetrahedral and squ are planar. The X-band EPR spectrum for the powdered Cu-II compound is typical of an approximately D-2 CuN4 centre possessing a d(xy) ground state (g(x) = 2.080, g(y) = 2.075, g(z) = 2.291; A(z) = 112.3 x 10(-4 ) cm(-1)). The UV/vis spectra are dominated by charge-transfer bands, and both the Cu-II and Cu-I systems are intensely coloured. The EPR an d electronic spectra indicate that these cations have a very similar s tructure in the solid slate and in solution, the potential of the [Cu( bimdpk)(2)](2+)/[Cu(bimdpk)(2)](+) couple is 0.59 V vs. SCE in MeCN an d 0.80 V vs. SCE in CH2Cl2, and the electron self-exchange constant in MeCN is 1.9 x 10(4) M(-1) s(-1). Comparisons are made between the pro perties 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 consi deration in biological electron transport.