Copper (I) Complexes of Zwitterionic Imidazolium- 2-Amidinates, A Promising Class of Electroneutral, Amidinate-Type ligands

Abstract

The first complexes containing imidazolium-2-amidinates as ligands (betaine-type adducts of imidazolium-based carbenes and carbodiimides, NHC-CDI) are reported. Interaction of the sterically hindered betaines ICyCDIDiPP and IMeCDIDiPP [both bearing 2,6-diisopropylphenyl (DiPP) substituents on the terminal N atoms] with Cu(I) acetate affords mononuclear, electroneutral complexes 1a and 1b, which contain NHC-CDI and acetate ligands terminally bound to linear Cu(I) centers. In contrast, the less encumbered ligand ICyCDIp-Tol, with p-tolyl substituents on the nitrogen donor atoms, affords a dicationic trigonal paddlewheel complex, [Cu2(μ-ICyCDIp-Tol)3]2+[OAc–]2 (2-OAc). The nuclear magnetic resonance (NMR) resonances of this compound are broad and indicate that in solution the acetate anion and the betaine ligands compete for binding the Cu atom. Replacing the external acetate with the less coordinating tetraphenylborate anion provides the corresponding derivative 2-BPh4 that, in contrast with 2-OAc, gives rise to sharp and well-defined NMR spectra. The short Cu–Cu distance in the binuclear dication [Cu2(μ-ICyCDIp-Tol)3]2+ observed in the X-ray structures of 2-BPh4 and 2-OAc, ca. 2.42 Å, points to a relatively strong “cuprophilic” interaction. Attempts to force the bridging coordination mode of IMeCDIDiPP displacing the acetate anion with BPh4– led to the isolation of the cationic mononuclear derivative [Cu(IMeCDIDiPP)2]+[BPh4]− (3b) that contains two terminally bound betaine ligands. Compound 3b readily decomposes upon being heated, cleanly affording the bis-carbene complex [Cu(IMe)2]+[BPh4–] (4) and releasing the corresponding carbodiimide (C(═N-DiPP)2).