Copper Complexes of Nitrogen-Anchored Tripodal N-Heterocyclic Carbene Ligands

Incorporation of a nitrogen functionality into a tripodal N-heterocyclic carbene ligand system affords the first N-anchored tetradentate tris-carbene ligands TIMENR I (R = Me (5a), t-Bu (5b), Bz (5c)). Treatment of the Me derivatized H3TIMENMe3 imidazolium salt (H35a) with silver oxide yields the silver complex (TIMENMe)2Ag33 (9), which, in a ligand transfer reaction, reacts with copper(I) bromide to give the trinuclear copper(I) complex (TIMENMe)2Cu33 (10). Deprotonation of the tert-Bu and benzyl derivs. H3TIMENt-Bu3 and H3TIMENBz3 yields the free tris-carbenes TIMENt-Bu (5b) and TIMENBz (5c), which react readily with copper(I) salts to give mononuclear complexes (TIMENt-Bu)Cu (11b) and [(TIMENBz)Cu]Br (11c). The solid-state structures of 10, 11b, and 11c were detd. by single-crystal x-ray diffraction. While the TIMENMe ligand yields trinuclear complex 10, with both T-shaped three-coordinate and linear two-coordinate copper(I) centers, the TIMENt-Bu and TIMENBz ligands induce mononuclear complexes 11b and 11c, rendering the cuprous ion in a trigonal planar ligand environment of three carbenoid carbon centers and an addnl., weak axial nitrogen interaction. Complexes 11b and 11c exhibit reversible one-electron redox events at half-wave potentials of 110 and -100 mV vs Fc/Fc+, resp., indicating sufficient electronic and structural flexibility of both TIMENR ligands (R = t-Bu, Bz) to stabilize copper(I) and copper(II) oxidn. states. Accordingly, a copper(II) NHC complex, (TIMENBz)Cu2 (12), was synthesized. Paramagnetic complex 12 was characterized by elemental anal., EPR spectroscopy, and SQUID magnetization measurements. [on SciFinder (R)]