Synthesis, structure & diphenylacetylene reduction reactivity of a carbide-supported Fe4Mo2 carbonyl cluster: a higher Fe-valence hydride intermediate for enhanced selectivity.

The novel four-iron, two-molybdenum cluster [(µ6-C)Fe4Mo2(CO)18]2- (2) containing an interstitial carbide has been structurally characterized and prepared from the corresponding Fe4 dianion [(µ4-C)Fe4(CO)12]2- (1) supported by two crowned alkali units in [K(benzo-18-crown-6)]+. In the X-ray structure of 2, the two molybdenum atoms share an internal geometry of a cis orientation about the central carbide, which indicates the stability of the well-known 'butterfly' Fe4C core found in the precursor 1, and this finding is consistent with other bis-heterometal variants of general formula [Fe4M2] (M = Ni, Cu, Rh, Au). Reactivity studies monitoring the reduction of diphenylacetylene (DPA) catalyzed by 2 showed selective reduction to cis-diphenylethylene. The effect of proton source pKa and steric bulk on DPA reduction demonstrated that product selectivity is enhanced with increased steric bulk near the protonation site, and that conversion increases with more acidic proton sources. Overall, increased selectivity is observed with 2 compared with catalyst-free reactions, Fe-only clusters and the Fe5Mo variant. We attribute the beneficial reactivity profile of cluster 2 to the electronic effect of the two Mo(0) centers, which lead to higher valent iron sites in the hydride intermediate cluster(s), thus decreasing non-specific reduction and increasing selectivity.