Hydrogen bonds and dispersion forces serving as molecular locks for tailored Group 11 bis(amidine) complexes

Abstract

A flexible polydentate bis(amidine) ligand LH₂, LH₂ = {CH₂NH(^tBu)CN-2-(6-MePy)}₂, operates as a molecular lock for various coinage metal fragments and forms the dinuclear complexes [LH₂(MCl)₂], M = Cu (1), Au (2), the coordination polymer [{(LH₂)₂(py)₂(AgCl)₃}(py)₃]_n (3), and the dimesityl-digold complex [LH₂(AuMes)₂] (4) by formal insertion of MR fragments (M = Cu, Ag, Au; R = Cl, Mes) into the N–H⋯N hydrogen bonds of LH₂ in yields of 43–95%. Complexes 1, 2, and 4 adopt C₂-symmetrical structures in the solid state featuring two interconnected 11-membered rings that are locked by two intramolecular N–H⋯R–M hydrogen bonds. QTAIM analyses of the computational geometry-optimized structures 1a, 2a, and 4a reveal 13, 11, and 22 additional bond critical points, respectively, all of which are related to weak intramolecular attractive interactions, predominantly representing dispersion forces, contributing to the conformational stabilization of the C₂-symmetrical stereoisomers in the solid state. Variable-temperature ¹H NMR spectroscopy in combination with DFT calculations indicate a dynamic conformational interconversion between two C₂-symmetrical ground state structures in solution (ΔG^‡_c = 11.1–13.8 kcal mol⁻¹), which is accompanied by the formation of an intermediate possessing C_i symmetry that retains the hydrogen bonds.