Dihydridoamine and Hydridoamido Complexes of Ruthenium(II) with a Tetradentate P−N−N−P Donor Ligand

The new diphosphinediimine ligand PPh2C6H4CHNCMe2CMe2NCHC6H4PPh2 {tmeP2N2}
is prepared by reacting NH2CMe2CMe2NH2 with 2 equiv of PPh2(C6H4-2-CHO). The diamine
derivative PPh2C6H4CH2NHCMe2CMe2NHCH2C6H4PPh2 {tmeP2(NH)2} is prepared by reducing tmeP2N2 with LiAlH4. The reaction of RuHCl(PPh3)3 with tmeP2N2 in THF produces the
complex trans-RuHCl{tmeP2N2}, while a similar reaction of RuHCl(PPh3)3 with tmeP2(NH)2
gives the complex trans-RuHCl{tmeP2(NH)2}, as a mixture of two isomers. The isomer with
two N−H bonds syn to the Ru−H bond is converted on heating to an isomer thought to have
one N−H syn to the Ru−H. The reaction of the two isomers with KOtBu under Ar in toluene
produces the novel hydridoamido complex RuH{tmeP2NNH}, where the ligand is deprotonated at one nitrogen. Reaction of this amido complex with H2 gives exclusively the
dihydridoamine complex trans-Ru(H)2{tmeP2(NH)2}. The complexes have been characterized
by X-ray crystallography, NMR, IR, elemental analysis, and a deuterium exchange study.
Complex RuH{tmeP2NNH}, or a combination of RuHCl{tmeP2(NH)2} and KOtBu, slowly
catalyzes the hydrogenation of acetophenone (6 atm H2, 20 °C, benzene or 2-propanol). The
catalytic cycle is thought to be similar to that for the ketone hydrogenation precatalyst trans-RuHCl{(S,S)-cyP2(NH)2)}. However in the present work both of the proposed catalysts trans-RuH2{tmeP2(NH)2} and RuH{tmeP2NNH} have been completely characterized. The tmeP2(NH)2 system, with hindering methyl groups, is less active than the cyP2(NH)2 system with
a trans-1,2-substituted cyclohexyl backbone. The variable configuration of the amine
nitrogens that is observed for the tmeP2(NH)2 complexes might also occur in the cyP2(NH)2
systems and could explain the inconsistent selectivity of such catalysts. The diimine complex
RuHCl{tmeP2N2} reacts with KOtBu under H2 to produce the dihydridodiamine complex
trans-Ru(H)2{tmeP2(NH)2}, the catalyst for the hydrogenation of acetophenone.