Palladium(II) and Platinum(II) Complexes Featuring a Nitrile-Functionalized N-Heterocyclic Carbene Ligand

The transmetalation reaction of 2 equiv of trans-PdCl2(CH3CN)2 or Pd(cod)Cl2 with a nitrile-functionalized N-heterocyclic carbene complex of silver(I), bis[1-(2-cyanophenyl)-3-methylimidazol-2-ylidene]silver(I) tetrafluoroborate ([Ag(m-CN)2]BF4, 1), and 1 equiv of AgBF4 afforded a bimetallic complex formulated as [(m-CN)2Pd(μ-Cl)2Pd(CH3CN)2](BF4)2 (2a). The interesting trimetallic complex 2b was obtained during an attempt to crystallize 2a from wet diethyl ether and acetonitrile. The solid-state structure of 2b reveals the presence of a novel C−N−N−C donor ligand providing two bridging imido nitrogens to adjacent palladium(II) centers in the trimetallic complex [{Pd(CH3CN)2}3(C−N−N−C)](BF4)4 (2b). The C−N−N−C ligand, which has a central −NC−O−CN− linkage, was formed from the hydrolysis and condensation of the nitrile groups of two carbene ligands. Palladium(II) and platinum(II) complexes bearing chelating olefin and N-heterocyclic carbene ligands were synthesized by transmetalation reactions of N-heterocyclic carbene complexes of silver(I), complex 1 and bis[1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene]silver(I) tetrafluoroborate ([Ag(IMes)2]BF4, 7), with [Pd(η1:η2-coe-OMe)(μ-Cl)]2 and Pt(cod)Cl2 (coe-OMe = 2-methoxycyclooct-5-enyl, cod = 1,5-cyclooctadiene) or by addition of a methoxide anion to the coordinated diolefin ligand of the novel platinum(II) complex, chloro[1-(2-cyanophenyl)-3-methylimidazol-2-ylidene](η4-1,5-cyclooctadiene)platinum(II) tetrafluoroborate ([Pt(m-CN)(cod)Cl]BF4, 4). The complexes bearing 1-(2-cyanophenyl)-3-methylimidazol-2-ylidene and 2-methoxycyclooct-5-enyl ligands ([M(m-CN)(η1:η2-coe-OMe)]+: 3, M = Pd; 6, M = Pt) exists in both monomeric and dimeric forms depending on the choice of recrystallization solvents. All of these complexes were isolated and studied by NMR and infrared spectroscopy. Different ratios of rotamers were observed for the complexes bearing a 2-methoxycyclooct-5-enyl ligand owing to its orientation relative to the carbene ligand. Rotamers are believed to form because of steric restrictions to rotation about the M−Ccarbene bond (M = Pd, Pt).