Novel phosphatriptycene and NHC supported late transition metal complexes for small molecule transformations

Abstract

This thesis focuses on the synthesis and characterization of late transition metal complexes supported by novel caged phosphine and NHC ligands, and the study of their reactivity in small molecule transformations. The first part of the thesis focuses on the design of synthetic routes to, and the coordination chemistry of, novel sterically bulk π-acceptor phosphine molecules based on the class of 9-phosphatriptycene (Chapter 2). While the synthesis of phosphatriptycene bearing functional groups ortho to the phosphorus position proved elusive, we have succeeded in the synthesis of several phosphatriptycene derivatives and in the elaboration of an azaphosphatriptycene with meta substituents. The new 2,7,14-tri-tert-butyl-10-aza-9-phosphatriptycene has been synthesized in high yield, on gram-scale, by a concise route from inexpensive precursors. The solid-state structure of the copper complexes shows that meta substitution on the phosphatriptycene framework introduces significant steric bulk around the supported metal centers, with consequences to the aggregation state of the copper chloride complexes. The phosphatriptycene readily dissociates from copper center during attempted transformation reactions, but forms an isolable rhodium complex that can be applied as a highly efficient catalyst precursor in the homogeneous hydroformylation of alkenes (Chapter 3). Infrared spectroscopy of the rhodium(I) carbonyl complex bearing the ligand reflects the strong π-acceptor character of the azaphosphatriptycene, and the solid-state structure reveals its moderate steric demand. Consistent with the expected weak σ-basicity and strong π-acidity of the ligand, the rhodium complex shows remarkable activity in the hydroformylation of less-reactive internal cyclic alkenes, including heterocyclic alkenes. The second part of the thesis (Chapter 4) focuses on the design of novel NHC-supported dinuclear nickel complexes. Salts of a mono-cationic, μ-hydrido dinickel(I) complex have been synthesized and the solid-state structure of the cation features a symmetric linear structure with a Ni•••Ni distance of 3.0076(5) Å. The μ-hydrido dinickel(I) monocation complexes activate the C−CN bonds in nitriles to form the corresponding alkane, accompanied by the formation of a nickel(II) cyanide complex in acetonitrile reactions, as well as dinuclear Ni(I)/Ni(0) mixed-valence radical complexes and homoleptic nickel(I) complexes in longer-chain nitriles reactions. The dinuclear Ni(I)/Ni(0) complexes can be synthesized through the comproportionation of Ni(II) and Ni(0), or by the one-electron oxidation of two Ni(0) centers with a mild oxidant. It can also be generated and detected through hydrogen abstraction from the μ-hydrido dinickel(I) monocation complexes using a stable 2,4,6-tri-tert-butylphenoxyl radical. The solid-state structure of the dinuclear Ni(I)/Ni(0) mixed-valence cation features a short Ni•••Ni distance of 2.6356(6) Å.