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 (R = Me (5a), t-Bu (5b), Bz (5c)). Treatment
of the methyl derivatized [H3TIMENMe](PF6)3 imidazolium salt (H35a) with silver oxide yields the silver complex
[(TIMENMe)2Ag3](PF6)3 (9), which, in a ligand transfer reaction, reacts with copper(I) bromide to give the
trinuclear copper(I) complex [(TIMENMe)2Cu3](PF6)3 (10). Deprotonation of the tert-butyl and benzyl
derivatives [H3TIMENt-Bu](PF6)3 and [H3TIMENBz](PF6)3 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](PF6)
(11b) and [(TIMENBz)Cu]Br (11c). The solid-state structures of 10, 11b, and 11c were determined 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 additional, 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+,
respectively, indicating sufficient electronic and structural flexibility of both TIMENR ligands (R = t-Bu, Bz)
to stabilize copper(I) and copper(II) oxidation states. Accordingly, a copper(II) NHC complex, [(TIMENBz)Cu](OTf)2 (12), was synthesized. Paramagnetic complex 12 was characterized by elemental analysis, EPR
spectroscopy, and SQUID magnetization measurements.