A nonclassical dihydrogen species of cobalt(I) is an important key intermediate in homogeneous hydrogenations and dehydrogenation reactions. As an elementary mechanistic step, the initial interaction of H2 with a low-valent metal center provide a pathway to produce a Co(H2) species. Typical conversion of such species involves 2-electron oxidative addition to generate cobalt(III)-dihydride. This process requires the necessary increase in the coordination number by one or two. As an alternative pathway, few studies showed homolytic cleavage of dihydrogen occurs between two monovalent cobalt centers, in which the coordination number is not necessarily changed. By minimizing the geometric alteration of a cobalt complex, the homolysis of H2 can be effectively controlled. To test the effect of metal’s structural change on the small molecule activation including H2, we are currently exploring two types of cobalt pincer systems. Here, we will present syntheses and characterization of two cobalt complexes supported by a PNP ligand (PNP− = bis(2-(diisopropylphosphino)-4-methylphenyl)amide) and an acridane adapted acriPNP ligand (acriPNP− = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide). The PNP pincer ligands can accommodate a square planar cobalt(II) center in the presence of various anionic and neutral donors as a fourth ligand. Dinuclear cobalt(I) dinitrogen species {(PNP)Co}2(µ-N2) and {(acriPNP)Co}2(µ-N2) were prepared from the reduction of corresponding cobalt(II) bromide complexes. Upon addition of H2(g), a square planar cobalt(II) monohydride species was generated involving a homolytic cleavage of H2 via a single electron process at cobalt. During the conversion, nonclassical cobalt dihydrogen species was spectroscopically detected. Further investigation on the transformation of a cobalt-H2 species will be discussed in detail.

Dihydrogen activation on a monovalent cobalt center supported by PNP ligands

256th National Meeting (2018)

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