Structural and dynamic studies on amido-bridged rhodium and iridium complexes

Treatment of [{M(μ-Cl)(diolefin)}2] with the lithium salts of primary and secondary amines (LiNRR′) in diethyl ether affords the complexes [{M(μ-NRR′)(diolefin)}2] (M=Rh, Ir; diolefin=1,5-cyclooctadiene (cod), tetrafluorobenzobarrelene (tfb); R′=H, R=tBu, Ph, 4-MeC6H4; R=R′=Ph, 4-MeC6H4). Mixed-bridged chloro/amido complexes are intermediates in these syntheses, two of which, [{Rh(cod)}2(μ-NHR)(μ-Cl)] (R=tBu, 4-MeC6H4), have been isolated. Replacement of the diolefin ligands by carbon monoxide or tert-butyl isocyanide in selected compounds takes place with retention of the binuclear structure to give the corresponding complexes [{M(μ-4-HNC6H4Me)(CO)2}2], [{Rh(μ-4-HNC6H4Me)(CNtBu)2}2] (12), and [{Rh(μ-NPh2)(CNtBu)2}2] (13). Single-crystal X-ray diffraction analyses of the complexes [{Rh(μ-NRR′)(cod)}2] (R′=H, R=4-MeC6H4 (3); R=R′=4-MeC6H4 (5)), 12, and 13 have shown that the conformation of the “RhN2Rh” four-membered metallacycle is planar in 5 and folded in 3, 12, and 13. The complexes with primary amides, 3 and 12, were found to exist as the syn,endo stereoisomers. The fluxionality of the complexes with secondary amides is due to rotation of the aromatic substituents about the NCipso bond and, in the case of 13, to the inversion of the “RhN2Rh” metallacycle as well. The complexes [{M(μ-NHR)(cod)}2] (R=Ph, 4-MeC6H4) were found to exist as isomeric mixtures in solution, the syn/anti ratio being 2:3 for the rhodium derivatives and 1:1 for their iridium counterparts. Again, the motion detected was due to rotation of the aromatic substituents, and could be frozen only in the case of the syn isomers. The complex [{Rh(μ-NHtBu)(cod)}2] with aliphatic amido ligands was found to be the anti folded isomer and proved to be nonfluxional. The most common conformation of the “RhN2Rh” metallacycle in these compounds is folded, and the preferred configuration varies from syn for the less encumbered compounds to anti on increasing the bulkiness of the bridging and ancillary ligands.Generous financial support from the Dirección General de Enseñanza Superior e Investigación (DGES) and the Ministerio de Ciencia y Tecnología (MCyT-PNI) (Projects PB 98–641 and BQU 2000–1170) is gratefully acknowledged.Peer reviewe