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|Title:||Palladium(II) complexes of a sterically bulky, benzannulated N-heterocyclic carbene with unusual intramolecular C-H⋯Pd and C carbene⋯Br interactions and their catalytic activities|
|Authors:||Huynh, H.V. |
|Source:||Huynh, H.V., Han, Y., Ho, J.H.H., Tan, G.K. (2006-06-19). Palladium(II) complexes of a sterically bulky, benzannulated N-heterocyclic carbene with unusual intramolecular C-H⋯Pd and C carbene⋯Br interactions and their catalytic activities. Organometallics 25 (13) : 3267-3274. ScholarBank@NUS Repository. https://doi.org/10.1021/om060151w|
|Abstract:||The sterically bulky carbene precursor 1,3-diisopropylbenzimidazolium bromide (iPr2-bimyH+Br-) (A) has been prepared by an improved method in 84% yield. Reaction of A with Pd(OAc)2 and NaBr gave the dimeric Pd(II) benzimidazolin-2-ylidene complex [PdBr2(iPr2-bimy)]2 (1), which can be easily cleaved by CH3CN, another equivalent of salt A, and triphenylphosphine to afford the novel benzannulated monocarbene complexes ?rans-[PdBr2(CH3CN)(iPr2-bimy)] (2), (iPr2-bimyH)[PdBr3OPr2-bimy)] (3), trans[PdBr2(iPr2-bimy)(Ph3P)] (trans-4), and cw-[PdBr2(iPr2-bimy)(Ph 3P)] (cis-4), respectively. All compounds have been fully characterized by multinuclei NMR spectroscopies and mass spectrometries (FAB, ESI). X-ray diffraction studies on single crystals of 1-3 and cis-4 revealed a square planar geometry and a fixed orientation of the N-isopropyl substituents with the C-H group pointing to the metal center to maximize C-H⋯Pd interactions. The large downfield shift of the C-H protons in the 1H NMR spectrum compared to the precursor A indicates that these C-H⋯Pd interactions are retained in solution and better described as weak hydrogen bonds, rather than as agostic interactions. Furthermore, the molecular structures of especially complexes 2 and 3 clearly show a bending of the bromo ligands toward the carbene carbon atom in order to maximize intramolecular Ccarbene⋯Br interactions. The nature of these interactions can be attributed to a form of back-bonding to the formally vacant p-orbital of the Ccarbene atom with the electron density originating from the bromo ligands' lone pairs. A detailed study on the trans-cis isomerization of the mixed NHC-phosphine complexes 4 revealed that a cis arrangement in such complexes is thermodynamically favored. Furthermore, a preliminary catalytic study shows that complex 1 is highly active in the Suzuki-Miyaura coupling of aryl bromides and chlorides in pure water as environmentally benign solvent. © 2006 American Chemical Society.|
|Appears in Collections:||Staff Publications|
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