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

• Published in: Organometallics Vol. 36; no. 21; pp. 4215 - 4223
• Main Authors: Joannou, Matthew V, Bezdek, Máté J, Al-Bahily, Khalid, Korobkov, Ilia, Chirik, Paul J
• Format: Journal Article
• Language: English
• Published: American Chemical Society 13.11.2017
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/acs.organomet.7b00653

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.