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Coordinatively Diverse ortho-Phosphinoaniline Complexes of Ruthenium and Isolation of a Putative Intermediate in Ketone Transfer Hydrogenation Catalysis

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journal contribution
posted on 03.05.2010 by Lindsay J. Hounjet, Matthias Bierenstiel, Michael J. Ferguson, Robert McDonald, Martin Cowie
Amine-functionalized mono- and diphosphines have been used to prepare a series of ruthenium complexes which exhibit a variety of coordination modes depending on the number of donors possessed by the ligands, the degree of amine methylation, the solvent system used, and the oxidation state of the metal. Reactions of the monophosphinoanilines, Ph2PAr or Ph2PAr′ (Ar = o-C6H4NHMe, Ar′ = o-C6H4NMe2), with 0.5 equiv of [RuCl(μ-Cl)(η6-p-cymene)]2 in dichloromethane result in the formation of [RuCl26-p-cymene)(P-Ph2PAr)] or [RuCl(η6-p-cymene)(P,N-Ph2PAr′)]Cl, respectively. In refluxing methanol, [RuCl26-p-cymene)(P-Ph2PAr)] gradually undergoes chloride ion dissociation to afford the P,N-chelate, [RuCl(η6-p-cymene)(P,N-Ph2PAr)]Cl. This chelate can then be deprotonated to afford the amido complex, [RuCl(η6-p-cymene)(P,N-Ph2PAr)] (Ar = o-C6H4NMe), which is an active ketone transfer hydrogenation catalyst. Reactions of the diphosphines, Ar2PCH2PAr2 (mapm) or Ar′2PCH2PAr′2 (dmapm) with 0.5 equiv of [RuCl(μ-Cl)(η6-p-cymene)]2 result in the formation of [RuCl2(P,P′,N,N′-mapm)] or [RuCl(η6-p-cymene)(P,P′-dmapm)]Cl, respectively, in which increased methyl substitution in the latter actually inhibits amine coordination with retention of the p-cymene fragment. Reaction of mapm with 1 equiv of [Ru(CO)42-C2H4)] in dichloromethane initially produces [Ru(CO)4(P-mapm)] which, over a 24 h period with exposure to ambient light, is completely converted to the P,P′-chelate, [Ru(CO)3(P,P′-mapm)], by photodissociation of carbon monoxide. The same reaction with 2 equiv of [Ru(CO)42-C2H4)] generates a mixture of [Ru3(CO)10(μ-P,P′-mapm)] and the mononuclear P,P′-chelate. The trinuclear complex can also be synthesized by direct reaction of mapm with 1 equiv of [Ru3(CO)12].