SYNTHETIC NICKEL-CONTAINING HETEROMETAL CUBANE-TYPE CLUSTERS WITH NIFE3Q4 CORES (Q = S, SE)

Reaction of the linear trinuclear Fe(III) clusters [Fe3Q4(SEt)4]3- with Ni(PPh3)4 in acetonitrile solution affords the products [NiFe3Q4(PPh3)(SEt)3]2- (Q = S (9), Se (10)) and [NiFe3Q4(SEt)4]3- (Q = S (11), Se (12)) in ca. 30% yield. The reactions involve reductive rearrangement of the initial cluster to a cuboidal fragment and capture of the nickel atom. The compounds (Et4N)2[9/10] are isomorphous and contain cluster anions with the cubane-type [NiFe3Q4]1+ core units and Ni-PPh3 and Fe-SEt terminal ligation. The compound (Et4N)3[12] is isomorphous with (Et4N)3[Fe4Se4(SEt)4] and contains the same cubane unit but with all-thiolate terminal ligation and disordered Ni and Fe subsites. The dimensions of the [NiFe3Q4]1+ cores are closely similar to those of the more familiar [Fe4Q4]2+,1+ Cluster cores, rendering separation of the salts of the same cation and cluster charge difficult. Collective structural, magnetic, and spectroscopic results are consistent with the simplified charge distribution [Fe3Q4]1- (S = 5/2) + Ni2+ (S = 1) and an S = 3/2 ground state that arises from antiparallel coupling of the two fragment spins. This mode of spin coupling causes oppositely signed isotropic shifts of the identical ligands at Ni and Fe subsites. Subsite-differentiated cluster 9 undergoes regiospecific substitution reactions to afford products with phosphines, cyanide, and isonitrile at the Ni subsite. These reactions are often accompanied by formation of [Fe4S4(SEt)4]2-,3- minority products. Comparison of properties of the synthetic clusters and of a reconstituted NiFe3S4 Species formed with Pyrococcus furiosus ferredoxin (Conover, R. C.; Park, J.-B.; Adams, M. W.; Johnson, M. K. J. Am. Chem. Soc. 1990, 112, 4562) reveals that they are isoelectronic with the same ground state. Consequently, the protein-bound species almost certainly has the cubane-type structure of the synthetic cluster. Some seven types of heterometal cubane clusters MFe3S4 (M = V, Nb, Mo, W, Re, Co, Ni) have now been prepared. Reductive rearrangement reactions are likely to provide routes to additional members of the set, several of which are good structural models for the immediate heterometal coordination environment in enzymes (M = V, Mo), but none of which has as yet been shown to occur naturally.