Novel Reactivity Mode of Metal Diaminocarbenes: Palladium(II)-Mediated Coupling between Acyclic Diaminocarbenes and Isonitriles Leading to Dinuclear Species

Abstract

Metal-mediated coupling between equimolar amounts of cis-[PdCl2(CNR1)2] [R1 = 2,6-Me2C6H3 (Xyl) 1, 2-Cl,6-Me-C6H32, cyclohexyl (Cy) 3] and H2NC5H3R2N [R2 = H, 2-aminopyridine 4; R2 = NH2, 2,6-diaminopyridine 5] proceeds smoothly for 12 h at 20–25 °C and leads to the diaminocarbene species [PdCl{C(NHC5H3R2N)═N(H)R1}(CNR1)]Cl (6–9). In the reaction of 2 with 5 (1:1 molar ratio), corresponding carbene 10 was detected only by high-resolution ESI+-MS in a mixture with other yet unidentified products. Addition of each of 6–8 to starting 1 or 2 (1:1 molar ratio) in the presence of excess solid K2CO3 in CHCl3 and heating of the reaction mixture for 12 h at 40 °C led to a novel type of dinuclear complexes, 11 (75% isolated yield) and 13 (65%). Similar dinuclear complexes 12 and 14, formed by addition of 7 or 10 to 2, were identified by high-resolution ESI+-MS in the mixture with other species (e.g., 7 and 10). Generation of 11–14 proceeds via a cascade reaction including addition of the amino group of a 2-aminopyridine to the metal-activated isonitrile, ring-closure, and coupling of the derived acyclic diaminocarbene complex with the yet unreacted starting material. Complexes 6–9, 11, and 13 were fully characterized by elemental analyses (C, H, N), high-resolution ESI+-MS, IR, and 1H and 13C{1H} NMR spectroscopies, while 10, 12, and 14 were identified by ESI+-MS. In addition, the structures of five complexes [6, 6a (the latter is a neutral species derived from the deprotonation of 6), 9, 11, and 13] were elucidated by single-crystal X-ray diffraction.