Copper Complexes with NH-Imidazolyl and NH-Pyrazolyl Units and Determination of Their Bond Dissociation Gibbs Energies

We synthesized two dinuclear copper
complexes, which have ionizable N imidazole and N pyrazole protons in the ligand, respectively, and determined
the BDFE of the hypothetical H atom transfer reactions CuII(LH–1) + H• ↔ CuI(L) in MeOH/H2O (BDFE: bond dissociation Gibbs (free)
energy). The ligands have two adjacent N,N′,O-binding pockets, which differ
in one N-heterocycle: La has an imidazole
unit and Lc, a pyrazole unit. The copper­(II) complexes
of La and Lc have been characterized, and the
substitution pattern has only little influence on the structural properties.
The BDFEs of the hypothetical PCET reactions have been determined
by means of the species distribution and the redox potentials of the
involved species in MeOH/H2O (80/20 by weight). The pyrazole
copper complex 3 exhibits a lower BDFE than the isoelectronic
imidazole copper complex 1 (1, 292(3) kJ
mol–1; 3, 279(1) kJ mol–1). The difference is mainly caused by the higher acidity of the N pyrazole proton of 3 compared to the N imidazole proton of 1. The redox potentials
of 1 and 3 are very similar.