Ligation-Enhanced π-Hole···π Interactions Involving Isocyanides: Effect of π-Hole···π Noncovalent Bonding on Conformational Stabilization of Acyclic Diaminocarbene Ligands.

The reaction of cis-[PdCl2(CNXyl)2] (Xyl = 2,6-Me2C6H3) with the aminoazoles [1 H-imidazol-2-amine (1), 4 H-1,2,4-triazol-3-amine (2), 1 H-tetrazol-5-amine (3), 1 H-benzimidazol-2-amine (4), 1-alkyl-1 H-benzimidazol-2-amines, where alkyl = Me (5), Et (6)] in a 2:1 ratio in the presence of a base in CHCl3 at RT proceeds regioselectively and leads to the binuclear diaminocarbene complexes [(ClPdCNXyl)2{μ-C(N-azolyl)N(Xyl)C═NXyl}] (7-12; 73-91%). Compounds 7-12 were characterized by C, H, N elemental analyses, high-resolution ESI+-MS, Fourier transform infrared spectroscopy, 1D (1H, 13C) and 2D (1H,1H-COSY, 1H,1H-NOESY, 1H,13C-HSQC, 1H,13C-HMBC) NMR spectroscopies, and X-ray diffraction (XRDn). Inspection of the XRDn data and results of the Hirshfeld surface analysis suggest the presence in all six structures of intramolecular π-holeisocyanide···πarene interactions between the electrophilic C atom of the isocyanide moiety and the neighboring arene ring. These interactions also result in distortion of the Pd-C≡N-Xyl fragment from the linearity. Results of density functional theory calculations [M06/MWB28 (Pd) and 6-31G* (other atoms) level of theory] for model structures of 7-9 followed by the topological analysis of the electron density distribution within the framework of Bader's theory (QTAIM method) reveal the presence of these weak interactions also in a CHCl3 solution, and their calculated strength is 1.9-2.2 kcal/mol. The natural bond orbital analysis of 7-9 revealed that π(C-C)Xyl → π*(C-N)isocyanide charge transfer (CT) takes place along with the intramolecular π-holeisocyanide···πarene interactions. The observed π(C-C)Xyl → π*(C-N)isocyanide CT is due to ligation of the isocyanide to the metal center, whereas in the cases of the uncomplexed p-CNC6H4NC and CNXyl species, the effects of CT are negligible. Available CCDC data were processed from the perspective of isocyanide-involving π-hole···π interactions, disclosed the role of metal coordination in the π-hole donor ability of isocyanides, and verified the π-holeisocyanide···πarene interaction effect on the stabilization of the in-conformation in metal-bound acyclic diaminocarbenes.