Syntheses and characterizations of Pd(II) complexes incorporating a N-heterocyclic carbene and aromatic N-heterocycles

Reaction of the bromo-bridged dimeric monocarbene complex [PdBr2(Pr-i(2)-bimy)](2) (1) with various bidentate N-heterocycles afforded novel linear dinuclear Pd(II) complexes {[PdBr2(Pr-i(2)-bimy)](2)(mu-L)}-{mu-L = 4,4'-bipyridine (2); mu-L = 4,4'-bipyridylethane (3); mu-L = 4,4'-bipyridylethylene (4)}. The mononuclear counterpart [PdBr2(Pr-i(2)-bimy)(pyridine)] (5) has also been synthesized by reaction of 1 with pyridine. All compounds have been fully characterized by multi-nuclei NMR spectroscopies, FAB mass spectrometry, and X-ray diffraction analyses. Molecular structures of 2-5 show a fixed orientation of the C-H protons in the N-isopropyl substituents toward the metal center, suggesting interesting C-H center dot center dot center dot Pd preagostic interactions. These interactions are retained in solution as indicated by the large downfield shift of these C-H protons in the H-1 NMR spectrum. UV-vis and CV studies revealed that the dinuclear complexes 2-4 have electrochemical behavior similar to that of the mononuclear species 5.