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Changes in Coordination of Sterically Demanding Hybrid Imidazolylphosphine Ligands on Pd(0) and Pd(II)

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posted on 18.01.2006, 00:00 by Douglas B. Grotjahn, Yi Gong, Lev Zakharov, James A. Golen, Arnold L. Rheingold
Low-coordinate organometallic complexes are important in structure and catalysis, and hemilability or secondary interactions such as hydrogen bonding enabled by hybrid ligands are receiving increasing attention. To study the factors controlling these phenomena, three new imidazol-2-ylphosphine ligands, L, were made. In these ligands, the bulk around P and the hindrance at the basic and potentially coordinating imidazole N-3 were varied. Remarkably, L2Pd(0) complexes 3ac were shown to be two-coordinate, 12-electron species, despite the availability of imidazole N-3 to enter into η2-P,N chelation. In oxidative additions of C−X bonds to the Pd(0) complexes, reaction rates and products could be controlled by the nature of the C and X groups and the R groups on the phosphine. Most significantly, whereas 4c-PhI and 4c-MeOTf from 3c are normal trans-bis(phosphine)Pd(R)(X) species, 5a-PhI, 5a-PhBr, and 5b-PhI from 3a and 3b were shown by X-ray diffraction to be a monomeric species with a single η2-P,N-chelating phosphine. From 3a and methyl triflate, an ionic complex [6a-Me]+[OTf]- with one chelating and one nonchelating phosphine was formed, with temperature-dependent windshield-wiper exchange of the two, showing hemilability. Thus, large phosphine substituents (R = tert-butyl rather than isopropyl) favor chelation. The chelate Pd−imidazole N-3 bond is longer when the heterocyclic nitrogen is hindered by an adjacent tert-butyl group at C-4 (comparing 5a-PhI and 5b-PhI). Finally, whereas in [8b-Ph]+[OTf]- from 5b-PhI and isopropylamine, the amine coordinates without chelate opening or hydrogen bonding, in [10c-Me]+[OTf]- made from 4c-MeOTf and isopropylamine, the amine is not only coordinated at N but also donates a hydrogen bond to each phosphine imidazol-2-yl substituent.