Intramolecular hydrogen bonding stabilizes trans-configuration in a mixed carbene/isocyanide PdII complexes

Aromatic amidines 1a-e undergo facile reaction with one isocyanide in PdCl2(CNBut)2 giving carbene complexes 2b-d (Scheme 2) in high isolated yields (79–95%). The structures of 2b–d were confirmed by the 1H and 13C NMR spectroscopies, high resolution electrospray ionization mass spectrometry (HRESI-MS), IR, the elemental analyses (C, H, N), and X-ray diffraction analysis for 2c, which revealed that the carbene and unreacted isocyanide ligands were located in a mutually trans position. Such arrangement was unexpected since it did not fit trans effect rule. Stabilization of the unfavorable isomer was rationalized by intramolecular hydrogen bonding. The nature of the intramolecular non-covalent interactions, which were responsible for the stabilization of the trans-isomer, was studied theoretically using DFT calculations and topological analysis of the electron density distribution within the framework of Bader's theory (QTAIM method). © 2020