A Comparative Experimental and Computational Study of Heterometallic Fe-M (M = Cu, Ag, Au) Carbonyl Clusters Containing N-Heterocyclic Carbene Ligands

The [Fe(CO)(4){M(NHC)}](-) (M = Cu, NHC = IMes, 1; M = Cu, NHC = IPr, 2; M = Ag, NHC = IMes, 3; M = Ag, NHC = IPr, 4; IMes = C3N2H2(C6H2Me3)(2); IPr = C3N2H2(C(6)H(3)iPr(2))(2)) mono-anions were obtained from the reaction of Na-2[Fe(CO)(4)]center dot 2thf with one equivalent of M(NHC)Cl (M = Cu, Ag; NHC = IMes, IPr) in dmso. Furthermore, the reaction of Na-2[Fe(CO)(4)]center dot 2thf with two equivalents of M(NHC)Cl in thf afforded the neutral compounds Fe(CO)(4){M(NHC)}(2) (M = Cu, NHC = IMes, 11; M = Cu, NHC = IPr, 12; M = Ag, NHC = IMes, 13; M = Ag, NHC = IPr, 14). 2 and 4 further reacted with one equivalent of M(IPr)Cl (M = Cu, Ag, Au) resulting in the trimetallic clusters Fe(CO)(4){Cu(IPr)}{Ag(IPr)} (18), Fe(CO)(4){Cu(IPr)}{Au(IPr)} (19), and Fe(CO)(4){Ag(IPr)}{Au(IPr)} (20). 1-4, 11-14 and 18-20 have been spectroscopically characterized by IR, H-1 and(13)C{H-1} NMR techniques. The molecular structures of 2, 12, 18, 19 and 20 have been determined through single-crystal X-ray diffraction. The structure, bonding and stability of the copper and silver IMes derivatives were compared to the related Fe-Au clusters previously reported on the basis of theoretical calculations. Stability of the Fe-M bonds decreases in the order Au > Cu > Ag, and the same trend was found for what concerns the M-IMes interactions. The decomposition products of 1-4, 11-14 and 18-20 have been studied allowing, among the others, the structural characterization of the new species [Fe-2(CO)(8){Ag(IPr)}](-) (10) and Fe(CO)(4)(CH(2)IMes) (21).