Synthesis, Reactivity, and Catalytic Applications of Ruthenium and Palladium Complexes Supported by New Pincer Ligands
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Cyclometalated phosphine-based PNP and PCP ‘pincer’ complexes of the platinum group metals have been the subject of intense research in recent years, owing to the remarkable stoichiometric and catalytic reactivity exhibited by such complexes. With the goal of discovering new metal-mediated reactivity patterns and extending the versatility of metal pincer chemistry, significant effort has been devoted to the synthesis of structurally and/or electronically related systems where strategic alterations have been introduced to the pincer ligand architecture, including variation of the central and peripheral donor fragments, as well as the ancillary ligand backbone. In this context, the synthesis and study of Ru and Pd complexes supported by pincer-like tridentate ancillary ligands that feature a central anionic phosphorus ([NPN]) or silicon ([PSiP]) donor in the pincer ligand backbone are described herein. The decreased propensity for forming ?-bonds to P was anticipated to lead to a higher degree of electronic unsaturation in complexes supported by tridentate phosphido ligation relative to structurally related metal amido (M-NR2) species. In the case of [PSiP] ligation, the reduced electronegativity of Si relative to C should promote the formation of electron-rich late metal species that can readily undergo oxidative addition reactions. The trans-labilizing silyl donor was also expected to stabilize coordinatively and electronically unsaturated late metal complexes. The synthesis and reactivity of Ru complexes featuring bis(phosphino)silyl ligation of the type [?3-(2-R2PC6H4)2SiMe] ([R-PSiP]; R = Ph, Cy) are described. The 5-coordinate complex [Ph-PSiP]RuCl(PPh3) was shown to be catalytically active for the transfer hydrogenation of ketones in basic isopropanol. These transfer hydrogenation studies are among the first catalytic studies of silyl-pincer complexes and establish [R-PSiP]M species as viable candidates for catalysis. The synthesis and reactivity of 4- and 5-coordinate RuII complexes featuring the [Cy-PSiP] ligand were explored. Reaction of [Cy-PSiP]H with [(p-cymene)RuCl2]2 in the presence of NEt3 and PCy3 resulted in the formation of ([Cy-PSiP]RuCl)2, which serves as a precursor to a series of unprecedented 4-coordinate, formally 14-electron [Cy-PSiP]RuX (X = NHAr, N(SiMe3)2, OtBu) complexes that feature an unusual trigonal pyramidal geometry at Ru. The reactivity of these novel diamagnetic complexes is described, including the reaction of [Cy-PSiP]RuOtBu with amine-boranes resulting in the formation of rare bis(?-BH) complexes. Computational studies confirmed the key role of the strongly ?-donating silyl group of the Cy-PSiP ligand in facilitating the synthesis of such low-coordinate Ru species and enforcing the unusual trigonal pyramidal geometry. The mechanism of ammonia-borane activation was also examined computationally. Lastly, the synthesis and structural characterization of PdII complexes supported by the pincer-like bis(amino)phosphido ligand [?3-(2-Me2NC6H4)2P]- ([NPN]) is described. Examples of ?1-, ?2-, and ?3-NPN coordination to Pd are described, as is the catalytic activity of ([NPN]PdX)2 (X = Cl, OAc, OTf) complexes in the Heck olefin arylation reaction. In an effort to discourage the formation of phosphido-bridged dinuclear complexes, pre-coordination of the Lewis acid BPh3 to [NPN] was pursued. Upon reaction of [N(P?BPh3)N]K with [PdCl(C3H5)]2, the ?1-allyl complex [?3-N(P?BPh3)N]Pd(?1-C3H5) was isolated, which establishes the coordination of a Lewis acid to the phosphido donor of the [NPN] ligand as a viable strategy for encouraging the formation of mononuclear ?3-NPN complexes.