Tuning of Copper(I)–Dioxygen Reactivity by Bis(guanidine) Ligands

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Abstract
A series of bis(guanidine) ligands designed for use in biomimetic coordination chemistry, namely bis(tetramethylguanidino)‐, bis(dipiperidinoguanidino)‐, and bis(dimethylpropyleno)propane (btmgp, DPipG2p and DMPG2p, respectively), has been extended to include bis(dimethylethyleneguanidino)propane (DMEG2p), which has both Namine 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(DPipG2p)2][PF6]2 (2), [Cu2(DMPG2p)2][PF6]2 (3), and [Cu2(DMEG2p)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 Å 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. Depending on their individual substitution patterns, the guanidine moieties deviate from planarity by characteristic propeller‐like twists of the amino groups around their N–Cimine 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 μ‐η2:η2‐peroxodicopper(II) or bis(μ‐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 σ‐donor and π‐acceptor capabilities, and it is shown that μ‐η2:η2‐peroxodicopper(II) complexes are stabilised relative to the bis(μ‐oxo)dicopper(III) ones if π conjugation within the guanidine moieties is optimised. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

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