Thermal Growth of Au-Fe Heterometallic Carbonyl Clusters Containing N-Heterocyclic Carbene and Phosphine Ligands

The thermal reactions of [NEt4][Fe(CO)4(AuNHC)] [NHC = IMes ([NEt4][1]) or IPr ([NEt4][2]); IMes = C3N2H2(C6H2Me3)2; IPr = C3N2H2(C6H3iPr2)2], Fe(CO)4(AuNHC)2 [NHC = IMes (3) or IPr (4)], Fe(CO)4(AuIMes)(AuIPr) (5), and Fe(CO)4(AuNHC)(AuPPh3) [NHC = IMes (6) or IPr (7)] were investigated in different solvents [CH2Cl2, CH3CN, dimethylformamide, and dimethyl sulfoxide (dmso)] and at different temperatures (50-160 °C) in an attempt to obtain higher-nuclearity clusters. 1 and 2 completely decomposed in refluxing CH2Cl2, resulting in [Fe2(CO)8(AuNHC)]- [NHC = IMes (10) or IPr (11)]. Traces of [Fe3(CO)10(CCH3)]- (12) were obtained as a side product. Conversely, 6 decomposed in refluxing CH3CN, affording the new cluster [Au3{Fe(CO)4}2(PPh3)2]- (15). The relative stability of the two isomers found in the solid state structure of 15 was computationally investigated. 4 was very stable, and only after prolonged heating above 150 °C in dmso was limited decomposition observed, affording small amounts of [Fe3S(CO)9]2- (9), [HFe(CO)4]- (16), and [Au16S{Fe(CO)4}4(IPr)4]n+ (17). A dicationic nature for 17 was proposed on the basis of density functional theory calculations. All of the other reactions examined led to species that were previously reported. The molecular structures of the new clusters 11, 12, 15, and 17 were determined by single-crystal X-ray diffraction as their [NEt4][11]·1.5toluene, [Au(IMes)2][15]·0.67CH2Cl2, [NEt4][12], and [17][BF4]n·solvent salts, respectively.