Metal carbonyl analogues of iron–sulfur clusters found in metalloenzyme chemistry

Abstract

Many iron–sulfur metalloenzymes are based on Fe 4S 4 cubes in which the iron sites have high-spin tetrahedral coordination. Such Fe 4S 4 cubes can be obtained by dimerization of Fe 2S 2 rhombuses. Similar cluster aggregation processes can be observed for model FeS systems in which the high-spin tetrahedral iron sites are replaced by low-spin octahedral Fe(CO) 3S 3 sites. The Fe 2S 2 analogue for the metal carbonyl system is Fe 2(CO) 6S 2, whose ground-state structure contains a Fe 2S 2 tetrahedron with FeFe and SS bonds as well as four FeS bonds. However, much of the observed photochemistry of Fe 2(CO) 6S 2 can be rationalized by a coordinatively unsaturated rhombus photointermediate in which the FeFe and SS bonds of the ground state have broken in a concerted process in accordance with molecular orbital calculations. Additional insight into the chemistry of these iron carbonyl sulfides is obtained by replacement of the sulfur atoms by isoelectronic RP groups. Thus the stable Fe 2(CO) 6(P t Bu) 2 tetrahedron and the stable rhombus Fe 2(CO) 6(POC 6H 2-4-Me-2,6- t Bu 2) 2 can both be isolated and characterized. In addition, the reaction of Na 2Fe(CO) 4 with i Pr 2NPCl 2 gives the phosphorus-bridging carbonyl derivative ( i Pr 2NP) 2COFe 2(CO) 6, which undergoes thermal reactions with unsaturated organic molecules to give adducts of their CC, CO, or CN bonds to the phosphorus atoms of a Fe 2(CO) 6(PN i Pr 2) 2 unit generated by extrusion of the phosphorus-bridging carbonyl group. A low-temperature IR spectroscopic study of ( i Pr 2NP) 2COFe 2(CO) 6 photolyzed in a Nujol matrix provides evidence for an unstable nearly square Fe 2(CO) 6(PN i Pr 2) 2 rhombus.