Toward direct synthesis of organonitrogen compounds from dinitrogen: The chemistry of diazoalkane complexes derived from dinitrogen complexes

The hydrazido complexes trans-[MX(NNH2)(dppe)(2)](+) (3) and cis,mer-[MX(2)(NNH2)(PMe(2)Ph)(3)] (4) (M = Mo, W; X = halogen; dppe = 1,2-bis(diphenylphosphino)ethane), which are readily derived from dinitrogen complexes trans-[M(N-2)(2)(dppe)(2)] (1) and cis-[M(N-2)(2)(PMe(2)Ph)(4)] (2) by protonation with acid, undergo condensation with aldehydes or ketones (RR'C=O) to give a series of diazoalkane complexes, trans-[MX(NNCRR')(dppe)(2)](+) and cis,mer-[MX(2)(NNCRR')(PMe(2)Ph)(3)]. The diazoalkane ligand in those complexes has a singly-bent structure with the N-N-C bond angle of ca. 120 degrees and the essentially linear N-N-M bond, and may be regarded as a four-electron-donating ligand. Owing to the conjugate structure including the metal center M=N-N=CRR', the diazoalkane complexes exhibit various reactivities including nucleophilic addition, deprotonation, conjugate addition, and reductive dimerization. Condensation of hydrazido complexes 3 and 4 with dialdehydes or their equivalents leads to complexes with nitrogen heterocyclic ligands incorporating the terminal nitrogen atom by way of the cyclization of intermediary diazoalkane complexes. By this method are prepared complexes containing phthalimidine, 1,3-dihydro-2H-pyrrol-2-one, 1,5-dihydro-2N-pyrrol-2-one, and pyrrole moieties. Interestingly, the (l-pyrrolyl)imido complexes with dppe ligands trans-[MX(NNCH = CHCH = CH)(dppe)(2)](+) (26; M=Mo, W) undergo unusual electrophilic substitution reactions at the beta-position of the pyrrole ring. Various types of organonitrogen compounds such as acetone azine, pyrazoles, phthalimidines, pyrroles, and N-aminopyrrole are liberateded from the above complexes in good yields. As a typical reaction, pyrrole and N-aminopyrrole are formed by the LiAlH4 reduction of complexes 26. In this reduction, tetrahydrido complexes [MH(4)(dppe)(2)] (M = Mo, W) are obtained as the metal products, which can be converted back to the starting dinitrogen complexes 1. Thus, a synthetic cycle for the formation of pyrrole and N-aminopyrrole from dinitrogen has been accomplished by using dinitrogen complexes 1.