The class of iron carbonyl clusters predominantly investigated in the late twentieth century in organometallic fields are of new interest to bioinorganic chemists since the identification of the interstitial carbide in the active site of nitrogenase, FeMoco. These clusters contain a true, inorganic carbide, but the carbonyls limit their usefulness as FeMoco models. As neutral, pi-accepting, strong-field ligands, the carbonyls enforce highly reduced, low-spin iron sites and are highly recalcitrant towards substitution. Previously, our group has developed novel synthetic pathways towards sulfide and thiolate insertion onto the anionic cluster [NEt4]2[Fe6(µ6-C)(CO)16] with electropositive sources (S2Cl2 and sulfenyl chloride), reporting the first known iron carbide thiolato cluster, [NEt4][Fe5(µ5-C)(CO)13(µ2-STol]. We have now developed a new route for direct insertion of thiolates onto neutral clusters, showing that reaction of the known penta-iron neutral cluster Fe5C(CO)15 with p-toluyl-thiolate yields [NEt4][Fe5(µ5-C)(CO)13(µ2-STol], verified by XRD and FT-IR. This same result is also reached via the reaction of the in situ oxidized hexa-iron, dianionic, and more synthetically accessible cluster [NEt4]2[Fe6(µ6-C)(CO)16] with p-toluyl-thiolate. We propose that the in situ oxidation of [NEt4]2[Fe6(µ6-C)(CO)16] with an outer-sphere oxidant, ferrocenium, generates a putatively 84-electron neutral cluster and we report our spectroscopic and computational investigations of this species. The use of the in situ oxidation of stable anionic clusters to labilize the carbonyl ligands is a facile synthetic route for otherwise difficult or intractable ligand insertions onto iron carbonyl carbide clusters. This method has yielded the first known multi-iron-chelated phosphine carbido iron cluster, Fe4(µ4-C)(CO)10Triphos. We will show how we have explored the potential of the in situ oxidation method with both the dianionic hexa-iron cluster and the tetraanionic hexa-iron cluster, [NEt4]4[Fe6(µ6-C)(CO)15], to target the insertion of multipodal ligands, particularly thiolates, onto these clusters.

Multipodal phosphine and thiolate insertion onto iron carbonyl carbide clusters via cluster oxidation

ACS Spring 2022

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