Ruthenium Complexes with Cooperative PNP-Pincer Amine, Amido, Imine, and Enamido Ligands: Facile Ligand Backbone Functionalization Processes

The quantitative formation of enamido complex [Ru(H)PMe3(PNP')] (3; PNP1= N(CHCHPiPr(2))(CH(2)CH(2)PiPr(2))) from the reaction of [RuCl2PMe3(HPNP)] (5; HPNP = HN((CH2CH2PPr2)-Pr-i)(2)) with an excess of base (KOtBu) can be explained by beta-hydride migration from an intermediate amido complex [RuCIPMe3(PNP)] (6; PNP=N(CH2CH2PiPr2)2). Resulting imine complex [RuCl(H)PMe3(PNP*)] (7; PNP*. N(CHCH2P/Pr-2)((CH2CH2PPr2)-Pr-i)) could be independently synthesized and gives 3 with KOSu. A computational examination of the reversible double H-2 addition and elimination equilibria of enamide 3, amido complex [Ru(H)PMe3(PNP)] (1), and amine complex [Ru(H)(2)PMe3(HPNP)] (2) explains why [Ru(H)2PMe3(PNP*)] (8) is not observed experimentally. The distinctly different molecular and electronic structures of related complexes 1 and 3, which feature a Y-shaped distorted trigonal-bipyramid (Y-TBP) for amide 1 but T-shaped TBP for enamide 3, respectively, can be attributed to considerably reduced N M-->pi-donation for the PNP' ligand due to delocalization of the N-lone pair into the unsaturated pincer backbone. The resulting lowlying LUMO of 3 explains its Lewis-acidic behavior, as documented by the formation of octahedral complex [RuH(PMe3)(2)(PNP')] (14) upon the addition of PMe3. In comparison, the reaction of 1 with PMe3 gives a mixture of 2 and 14 via a base-assisted hydrogen elimination pathway. On the other hand, with electrophiles, such as MeOTf, predominant N-methylation is observed for both 1 and 3, producing [RuH(OTf)PMe3(MePNP)] (11) and [RuH(OTf)PMe3(MePNP')] (12), respectively. This reactivity of 3 contrasts with pyridine-based cooperative pincer analogues and can be attributed to the high flexibility of the aliphatic PNP' pincer ligand. The structural and reactivity patterns place this novel ligand between the parent PNP and aromatic pincer ligands.