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Synthesis of an Iron Parent Amido Complex and a Comparison of Its Reactivity with the Ruthenium Analog

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posted on 2004-04-12, 00:00 authored by Daniel J. Fox, Robert G. Bergman
The iron amido complex (dmpe)2Fe(H)(NH2) (2; dmpe = 1,2-bis(dimethylphosphino)ethane) has been prepared from the hydrido chloride complex using sodium amide in a liquid ammonia/THF solvent mixture. The complex is a mixture of trans and cis isomers in solution, although it crystallizes as the trans isomer exclusively. Complex 2 is a strong base, although somewhat weaker than the ruthenium amido complex trans-(dmpe)2Ru(H)(NH2) (1). Amido complex 2 deprotonates several weak acids, and some of these deprotonations result in displacement of ammonia, while others do not. The basicity of complex 2 is further demonstrated with systems in which acidic substrates are reversibly deprotonated. Complex 2 undergoes H/D exchange with toluene-d8, and it catalyzes H/D exchange between toluene-d8 and several other extremely weak acids. An investigation into the mechanism of the H/D exchange and isomerization reactions of trans-9,10-diisopropyl-9,10-dihydroanthracene (trans-4) using 1, 2, and cis-(PMe3)4Ru(H)(NH2) (5) as catalysts demonstrates that the H/D exchange reactions proceed through formation of ion pairs, although it is surprising that displacement of ammonia is never observed. Amido complexes 1 and 2, as well as the hydroxo complex trans-(dmpe)2Ru(H)(OH), are also good nucleophiles. For example, they ring-open 1,3-di-tert-butylaziridinone in a nucleophilic manner to form trans-(dmpe)2M(H)(XC(O)CH(t-Bu)NH(t-Bu)) (M = Fe, X = NH; M = Ru, X = NH, O). A second example is an unusual insertion of carbon monoxide into the N−H bond of 2 to form the carboxamide complex trans-(dmpe)2Fe(H)(NHCHO) (6). Although CO is a poor electrophile, mechanistic experiments suggest that this reaction proceeds through direct attack of the amide nitrogen on the CO carbon.