An Unusual Coordination Mode for Amides: Lone-Pair Binding via Nitrogen

Article abstract of DOI:10.1021/ic00129a014

A variety of 8-amino- and amido-substituted quinolines react with [Ir(PPh3)2H2(acetone)2]SbF6 (1) to give a series of complexes in which the amine or amide nitrogen binds to the metal. The X-ray crystal structures of [Ir(H)2(PPh3)2(C9H6N{HNCO(t)Bu})]SbF6 (4b) and [Ir(H)2(PPh3)2(C9H6N{HNSi(t)BuMe2})]SbF6 (4c) were obtained: 4b, a = 9.904(4) ?, b = 13.480(5) ?, c = 19.346(9) ?, α = 83.79(4)°, β = 81.64(4)°, γ = 76.52(3)°, Z = 2, triclinic P1-, R = 5.37%: 4c, a = 45.24(1) ?, b = 10.536(3) ?, c = 25.450(7) ?, β = 111.11(2)°, Z = 8, monoclinic C2/c, R = 5.30%. The X-ray data do not allow a definite distinction between the two possible bindingmodes, N binding via the lone pair and N-H agostic (2e, 3-center) binding. Definitive evidence for the lone-pair-bound structure was obtained from natural-abundance (15)N heteronuclear multiple-quantum coherence (HMQC) spectroscopy and proton nuclear Overhauser effect (NOE) data. The amine or amide N therefore rehybridizes to sp(3) on binding, a previously unreported complexation mode for undeprotonated amides that normally coordinate via O. Fluxional exchange processes are interpreted on the basisof N lone-pair decoordination and inversion with the possible intermediacy of an agostic species or iminol tautomer. The Ir-N bond distances (4b, 2.313(9) ? and 4c, 2.320(10) ?) are very long. The NH of the coordinated amide is much more acidic than that of the free amide and can be deprotonated with NEt3 to give an N-bound complex of the deprotonated amide. In a conformationally restricted pyrrole where the N lone paircannot bind to Ir, we find that deprotonation occurs spontaneously to give a pyrrolyl complex.