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Synthesis, Structures, and Reactions of Manganese Complexes Containing Diphosphine Ligands with Pendant Amines

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posted on 2010-10-25, 00:00 authored by Kevin D. Welch, William G. Dougherty, W. Scott Kassel, Daniel L. DuBois, R. Morris Bullock
Addition of the pendant amine ligand PNRP (PNRP = Ph2PCH2NRCH2PPh2; R = Me, Ph, n-Bu) to Mn(CO)5Br gives fac-Mn(PNRP)(CO)3Br. Photolysis of fac-Mn(PNRP)(CO)3Br with dppm [dppm = 1,2-bis(diphenylphosphino)methane] provides mixed bis(diphosphine) complexes, trans-Mn(PNRP)(dppm)(CO)(Br). Reaction of trans-Mn(PNRP)(dppm)(CO)(Br) with LiAlH4 leads to trans-Mn(PNRP)(dppm)(CO)(H), which has been characterized by crystallography. Mn(P2PhN2Bn)(dppm)(CO)(H) [P2PhN2Bn = 1,5-diphenyl-3,7-dibenzyl-1,5-diaza-3,7-diphosphacyclooctane] can be prepared in a similar manner; its structure has one chelate ring in a chair conformation and the second in a boat conformation. The boat-conformer ring directs the nitrogen of the ring toward the carbonyl ligand, and the N···C distance between one N of the P2PhN2Bn ligand and CO is 3.171(4) Å, indicating a weak interaction between the N of the pendant amine and the CO ligand. Reaction of NaBArF4 (ArF = 3,5-bis(trifluoromethyl)phenyl) with Mn(P−P)(dppm)(CO)(Br) produces the cations [Mn(P−P)(dppm)(CO)]+. The crystal structure of [Mn(PNMeP)(dppm)(CO)][BArF4] shows two very weak agostic interactions between C−H bonds on the phenyl ring and the Mn. The cationic complexes [Mn(P−P)(dppm)(CO)]+ react with H2 to form dihydrogen complexes [Mn(H2)(P−P)(dppm)(CO)]+ (Keq = 1−90 atm−1 in fluorobenzene, for a series of different P−P ligands). Similar equilibria with N2 produce [Mn(N2)(P−P)(dppm)(CO)]+ (Keq generally 1−3.5 atm−1 in fluorobenzene).

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