| Abstract |
The reaction of cis-[PdCl2(CNXyl)(2)] (Xyl = 2,6-Me2C6H3) with the aminoazoles [1H-imidazol-2-amine (1), 4H-1,2,4-triazol-3-amine (2), 1H-tetrazol-5-amine (3), 1H-benzimidazol-2-amine (4), 1-alkyl-1H-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)\mu-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 (H-1, C-13) and 2D (H-1, H-1-COSY, H-1,H-1-NOESY, H-1,C-13-HSQC, H-1,C-13-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 hole(isocyanide) center dot center dot center dot pi(arene) 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 6-3IG* (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 pi(C-C)Xyl -> pi*(C-N)isocyanide charge transfer (CT) takes place along with the intramolecular, pi-hole(isocyanide)center dot center dot center dot pi(arene) interactions. The observed pi(C-C)xyl -> pi*(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 pi-hole center dot center dot center dot pi interactions, disclosed the role of metal coordination in the pi-hole donor ability of isocyanides, and verified the pi-hole(isocyanide)center dot center dot center dot pi(arene) interaction effect on the stabilization of the in-conformation in metal-bound acyclic diaminocarbenes. |
