Activation of Carbon Dioxide by Pincer Supported Transition Metal Complexes

The synthesis, characterization, and catalytic activity of a highly active carbon dioxide hydrogenation PNPir catalyst (PNP = HN((alkyl) 2PCH2CH2)2) is detailed. Computational and X-ray diffraction experiments indicate a secondary coordination sphere H-bond interaction is crucial for carbon dioxide insertion reactivity. The synthesis and characterization of a family of PNPFe complexes is presented. Substitution of the pincer isopropyl groups for larger cyclohexyl and tert-butyl groups results in a dramatic attenuation in reactivity. Dihydrogen bonding is present in a number of these PNPFe complexes and has lead to the isolation of the first example of a bimetallic transition metal borohydride structure in which each of the two Fe atoms are bridged by one B-H bond of the borohydride ligand (mu,eta1-H2BH2). A pathway of formation for this unique dinuclear complex is proposed based on experimental and x-ray diffraction data. A family of PCPNi complexes (PCP = bis-2,6-di-tert-butylphosphinomethylbenzene) was also synthesized and their reactivity with carbon dioxide was investigated. A number of these PCPNi complexes were found to insert carbon dioxide to provide the corresponding unidentate carboxylate complexes while other PCPNi complexes proved inert toward carbon dioxide. A number of disparate insertion mechanisms for the family of PCPNi complexes based on experimental and computational data is proposed and important structural requirements for carbon dioxide insertion reactivity are detailed.