Synthesis and Reactivity of Pyridine(diimine) Molybdenum Olefin Complexes: Ethylene Dimerization and Alkene Dehydrogenation

Abstract

Reduced pyridine(diimine) molybdenum olefin complexes have been synthesized and structurally characterized. Examples with 1,5-cyclooctadiene, (PDI)Mo(η2:η2-1,5-COD) (COD = 1,5-cyclooctadiene) adopt a distorted-trigonal-bipyramidal geometry and are best described as low-spin Mo(II) compounds arising from significant π back-donation to the ligand from a reduced molybdenum center. With the 2,6-diisopropyl N-aryl-substituted variant of the pyridine(diimine) ligand, a molybdenum bis(ethylene) complex was obtained. Reducing the size of the N-aryl substituents to 2,4,6-trimethyl resulted in isolation of (MesPDI)Mo(η4-butadiene)(η2-ethylene) following sodium amalgam reduction of the corresponding molybdenum(III) trichloride complex in the presence of excess ethylene. Analysis of the byproducts of the reaction and olefin addition experiments demonstrate that butadiene formation is consistent with a pathway involving ethylene coupling to form 1-butene followed by allylic dehydrogenation to produce butadiene. Excess ethylene serves as the hydrogen acceptor. The dehydrogenation reaction was also compatible with α-olefins, as reduction of either (iPrPDI)MoCl3 or (MesPDI)MoCl3 in the presence of 1-hexene resulted in isolation of (PDI)Mo(η4-1,3-hexadiene)(η2-1-hexene) complexes. An α,ω-diene complex, (iPrPDI)Mo(η2:η2-1,6-heptadiene), was also synthesized and importantly displayed no cycloaddition chemistry, suggesting that first-row metal pyridine(diimine) complexes are thus far unique in promoting cyclobutane synthesis.