C–H amination chemistry mediated by trinuclear Cu(i) sites supported by a ligand scaffold featuring an arene platform and tetramethylguanidinyl residues

Abstract

Tripodal ligands that can encapsulate single or multiple metal sites in C₃-symmetric geometric configurations constitute valuable targets for novel catalysts. Of particular interest in ligand development are efforts toward incorporating apical elements that exhibit little if any electron donicity, to enhance the electrophilic nature of a trans positioned active oxidant (e.g., metal-oxo, -nitrene). The tripodal ligand TMG₃trphen-Arene has been synthesized, featuring an arene platform 1,3,5-substituted with phenylene arms possessing tetramethylguanidinyl (TMG) residues. Compound [(TMG₃trphen-Arene)Cu₃(μ-Cl)₃] has been subsequently synthesized by extracting a Cu₃(μ-Cl)₃ cluster from anhydrous CuCl and shown to encapsulate a crown-shaped Cu₃(μ-Cl)₃ fragment, supported by Cu–N_TMG bonds and modest Cu₃⋯arene long-range contacts. Energy decomposition analysis (EDA) indicates that electrostatic contributions to the total interaction energy far exceed those due to orbital interactions. The latter involve orbital pairings largely associated with the N_TMG stabilization of the Cu₃(μ-Cl)₃ cluster. The independent gradient model based on the Hirshfeld partition (IGMH) corroborates that contacts between the arene platform and the Cu₃ triangle are noncovalent in nature. Catalyst [(TMG₃trphen-Arene)Cu₃(μ-Cl)₃] enables amination of sec-benzylic and tert-C–H bonds of a panel of substrates by pre-synthesized PhINTces in solvent matrices that incorporate small amounts of HFIP. The involvement of an electrophilic aminating agent is evidenced by the better yields obtained for electron-rich benzylic sites and is further supported by Hammett analysis that reveals the development of a small positive charge during C–H bond activation. A rather modest KIE effect (2.1) is obtained from intramolecular H(D) competition in the amination of ethylbenzene, at the borderline of reported values for concerted and stepwise C–H amination systems. DFT analysis of the putative copper–nitrene oxidant indicates that the nitrene N atom is bridging between two copper sites in closely spaced triplet (ground state) and broken-symmetry singlet electronic configurations.