Tuning of copper(I)-dioxygen reactivity by bis(guanidine) ligands

A series of bis(guanidine) ligands designed for use in biomimetic coordination chemistry, namely bis(tetramethylguanidino)-, bis(dipiperidinoguanidino)-, and bis(dimethylpropyleno)propane (btmgp, DPipG(2)p and DMPG(2)P, respectively), has been extended to include bis(dimethylethyleneguanidino)propane (DMEG(2)P), which has both N-amine atoms of each guanidine functionality connected by a short ethylene bridge, as a member. From this series, a family of novel bis(guanidine)copper(i) compounds - [CU2(btmgp)(2)[PF6](2) (1), [CU2(DPipG(2)P)(2)[PF6](2) (2), [CU2(DMPG(2)P)(2)[PF6](2) (3), and [CU2(DMEG(2)P)(2)[PF6](2).2MeCN (4) - has been synthesised. Single-crystal X-ray analysis of 1-4 demonstrated that these compounds contain dinuclear complex cations that contain twelve-membered heterocyclic CU2N4C6 rings with the Cu atoms being more than 4 angstrom apart. Each copper atom is surrounded by a set of two N-donor functions from different ligands, resulting in linear N-Cu-N coordination sites. De pending on their individual substitution patterns, the guanidine moieties deviate from planarity by characteristic propeller-like twists of the amino groups around their N-C-imine bonds. The influence of these groups on the reactivity of the corresponding complexes 1-4 with dioxygen was investigated at low temperatures by means of UV/Vis spectroscopy. The reaction products can be classified into mu-eta(2):eta(2)-peroxodicopper(ii) or bis(mu-oxo)dicopper(iii) complex cations that contain the {Cu2O2}(2+) core portion as different isomers. The electronic properties of the specific bis(guanidine) ligands are discussed from the viewpoint of their a-donor and pi-acceptor capabilities, and it is shown that mu-eta(2):eta(2)-peroxodicopper(ii) complexes are stabilised relative to the bis(mu-oxo)dicopper(iii) ones if pi conjugation within the guanidine moieties is optimised. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)