The development of vast global natural gas reserves has increased the supply of ethylene and related downstream alpha-olefins. This has rejuvenated interest in the functionalization and conversion of these building blocks into more valuable carbon architectures for applications in the petrochemical and fine chemicals industries. Earth-abundant, reduced pyridine(diimine) (PDI) molybdenum complexes have been synthesized and studied for several applications, such as the dehydrogenative coupling of ethylene to butadiene and the selective hydrogenation of arenes and olefins. The synthesis, characterization, electronic structure determination, and reactivity of (PDI) molybdenum olefin, diene, and bis(alkyl) complexes will be presented. Reduction of (iPrPDI)MoCl3 in the presence of either 1,5-cyclooctadiene or ethylene resulted in the isolation of (iPrPDI)Mo(1,5-COD) and (iPrPDI)Mo(ethylene)2, respectively. Both complexes are best described as low spin Mo(II) compounds arising from significant π-backdonation to the ligand from a reduced molybdenum center. Reducing the size of the N-aryl substituents to mesityl resulted in isolation of (MesPDI)Mo(butadiene)(ethylene) following sodium amalgam reduction of the corresponding MoCl3 complex in the presence of ethylene. Mechanistic studies indicated that butadiene formation is consistent with a pathway involving ethylene coupling to form 1-butene (via a metallacyclic intermediate) followed by allylic dehydrogenation to produce butadiene. Excess ethylene serves as the hydrogen acceptor. (PDI) molybdenum complexes were also explored for their catalytic activity in the hydrogenation of benzene and a selection of substituted arenes. Bis(alkyl) molybdenum complex (4-tBu-iPrPDI)Mo(CH2SiMe3)2, exhibited the highest activity for hydrogenating benzene to cyclohexane under 4 atm of H2 at 23 C. Substituted arenes such as toluene, xylenes (ortho - 80:20 d.r.), biphenyl, and naphthalene were also successfully hydrogenated at 4 or 32 atm of H2. (4-tBu-iPrPDI)Mo(CH2SiMe3)2 was also shown to be an active alkene hydrogenation catalyst, supporting stepwise reduction of benzene to cyclohexadiene and cyclohexene during molybdenum-catalyzed arene hydrogenation. To our knowledge, this is the first instance of a mild (23 °C, 4 atm), molybdenum-catalyzed hydrogenation of olefins. Deuterium labelling studies reveal many isotopologues and stereoisomers, indicating H/D scrambling through reversible hydride insertion and allylic C-H activation.

Pyridine(diimine) molybdenum complexes and their applications to ethylene upconversion and catalytic arene and olefin hydrogenation

256th National Meeting (2018)

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