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

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).