Shorter Argentophilic Interaction than Aurophilic Interaction in a Pair of Dimeric {(NHC)MCl}2 (M = Ag, Au) Complexes Supported over a N/O-Functionalized N-Heterocyclic Carbene (NHC) Ligand

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Abstract
Synthesis, structure, bonding, and photoluminescence studies of a pair of neutral dimeric silver and gold complexes of a N/O-functionalized N-heterocyclic carbene ligand exhibiting closed-shell d10···d10 argentophilic and aurophilic interactions, are reported. In particular, dimeric complexes of the type {[1-(benzyl)-3-(N-tert-butylacetamido)imidazol-2-ylidene]MCl}2 [M = Ag (2); Au (3)] displayed attractive metallophilic interaction in the form of a close ligand-unsupported metal···metal contact [3.1970(12) Å in 2; 3.2042(2) Å in 3] as observed from X-ray diffraction study and also was further verified by low temperature photoluminescence study at 77 K that showed the characteristic emission [527 nm for 2; 529 nm for 3] owing to the metal···metal interaction. The nature of the metallophilic interaction in these complexes was further probed using computational studies that estimated the metal···metal interaction energy to be 12.8 (2) and 8.6 kcal/mol (3). Notably, the argentophilic interaction was found to be stronger than the aurophilic interaction in this series of neutral dimeric complexes. The complexes 2 and 3 were synthesized sequentially, with the silver 2 complex prepared by the reaction of the 1-(benzyl)-3-(N-tert-butylacetamido)imidazolium chloride with Ag2O in 66% yield, while the gold 3 complex was obtained by the transmetallation reaction of the silver 2 complex with (SMe2)AuCl in 86% yield.

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