Synthesis of the Highly Reduced [Fe6C(CO)15]4– Carbonyl Carbide Cluster and Its Reactions with H+ and [Au(PPh3)]+

The reduction of [Fe6C(CO)(16)](2-) (1) with NaOH in DMSO or with Na/naphthalene in THF affords the highly reduced [Fe6C(CO)(15)](4-) (2) monocarbide tetraanion. The molecular structure of 2 has been crystallographically determined as its [Et4N](4)[Fe6C(CO)(15)]center dot CH3CN salt, revealing an octahedral structure as expected for a hexanuclear cluster possessing 86 valence electrons. The stepwise protonation of 2 with strong acids such as HBF4 center dot Et2O results first in the monohydride trianion [HFe6C(CO)(15)](3-) (3) and then the purported dihydride dianion [H2Fe6C(CO)(15)](2-) (4), as indicated by H-1 NMR spectroscopy. Both 3 and 4 are not stable enough to be isolated and rapidly decompose, yielding the parent dianion 1. The reaction of 2 with a slight excess of Au(PPh3)Cl affords the aurated dianion [Fe6C(CO)(15)(AuPPh3)(2)](2-) (5), which has been isolated and structurally characterized as its [Et4N](2)[Fe6C(CO)(15)(AuPPh3)(2)]center dot 2CH(3)CN salt. DFT calculations allowed us to study, from a computational point of view, the electronic features of the new compounds and possible reaction intermediates.