Late Transition Metal Complexes Supported By Pincer Ligands PDF Download

Late transition-metal pincer complexes of primarily palladium(II) and platinum(II) have been investigated for their application as catalysts in partial oxidation reactions. The epoxidation of higher olefins using molecular oxygen as the oxidant has been targeted, and the individual reaction steps needed to accomplish this overall transformation are described herein, including: (1) hydrogenolysis of a metal hydroxide (M-OH) species to yield a metal hydride (M-H), (2) insertion of O2 into the M-H bond to form a metal hydroperoxide (M-OOH), and (3) O-atom transfer from the M-OOH to epoxides, yielding a M-OH and completing the catalytic cycle. Previous results from our group on these individual transformations using (tBuPCP)Pd and (tBuPCO)Pd fragments are also reviewed. The requirements for O2 insertion into PdII and PtII hydrides are discussed. An array of cationic, neutral, and anionic Pd-H and Pt-H complexes supported by a tBuPNP backbone were synthesized and evaluated for O2 insertion (tBuPNP = 2,6-bis-(di-tbutylphosphinomethyl)pyridine). Metal-ligand cooperation was observed in the activation of H2 to form neutral hydride complexes. The effect of ligand protonation/deprotonation on the trans influence experienced by the hydride ligand was investigated. No reaction with O2 was observed with the cationic hydrides, while the neutral and anionic forms reacted with O2 at the tBuPNP backbone. The synthesis and characterization of mono- and dinuclear Pd-OH complexes supported by a PCNR pincer ligand (PCNR = (1-(3-((di-tert-butylphosphino)methyl)phenyl)-1H-5-R-pyrazole), R = H, Me) is presented. When R = H, ligand pyrazole "rollover" C-H activation was observed, forming a mixed ligand (PCNH)Pd(μ-OH)Pd(PCC) dinuclear structure. This "rollover" was investigated using DFT computations. The mono- and dinuclear hydroxide species were evaluated for hydrogenolysis. The dinuclear compounds {[(PCNR)Pd]2(μ-OH)}[OTf] reacted under an H2 atmosphere to yield the corresponding dinuclear hydrides {[(PCNR)Pd]2(μ-H)}[OTf]. A mechanistic study on the hydrogenolysis of the μ-bridged hydroxide {[(PCNMe)Pd]2(μ-OH)}[OTf] revealed first order kinetics in both [Pd] and [H2]. Terminal hydrides were not detected, and reduction of the mononuclear hydroxide complexes (PCNR)Pd-OH to Pd0 was observed under H2. The reduction was proposed to proceed through displacement of the pyrazole arm, and was examined by DFT computations. Lastly, a new strategy to promote O-atom transfer from M-OOH to epoxides, the final step in the targeted catalytic cycle, is proposed. Preliminary studies on NNNPyz, NNNEt, and NNMe ligated PdII and PtII are discussed (NNNPyz = 2,6-bis(5-tbutyl-1H-pyrazol-3-yl)pyridine; NNNEt = 2-(5-tbutyl-1H-pyrazol-3-yl)-6-(diethylaminomethyl)pyridine; NNMe = 2-(5-tBu-1H-pyrazol-3-yl)-6-methylpyridine). The NNNPyz ligand, containing two acidic sites in proximity to the fourth site in the square plane, was found to protonate M-O2 complexes, chelate to the metal center and oxidize phosphine substrates. Similar reactivity was observed with NNNEt and NNMe, however hemilability of these ligands resulted in undesired coordination modes.