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

Many iron-sulfur metalloenzymes are based on Fe4S4 cubes in which the iron sites have high-spin tetrahedral coordination. Such Fe4S4 cubes can be obta ined by dimerization of Fe2S2 rhombuses. Similar cluster aggregation proces ses can be observed for model Fe-S systems in which the high-spin tetrahedr al iron sites are replaced by low-spin octahedral Fe(CO)(3)S-3 sites. The F e2S2 analogue for the metal carbonyl system is Fe-2(CO)(6)S-2, whose ground -state structure contains a Fe2S2 tetrahedron with Fe-Fe and S-S bonds as w ell as four Fe-S bonds. However, much of the observed photochemistry of Fe- 2(CO)(6)S-2 can be rationalized by a coordinatively unsaturated rhombus pho tointermediate in which the Fe-Fe and S-S bonds of the ground state have br oken in a concerted process in accordance with molecular orbital calculatio ns. Additional insight into the chemistry of these iron carbonyl sulfides i s obtained by replacement of the sulfur atoms by isoelectronic RP groups. T hus the stable Fe-2(CO)(6)((PBu)-Bu-t)(2) tetrahedron and the stable rhombu s Fe-2(CO)(6)(POC6H2-4-Me-2,6-Bu-t(2))(2) can both be isolated and characte rized. In addition, the reaction of Na2Fe(CO)(4) with (Pr2NPCl2)-Pr-t gives the phosphorus-bridging carbonyl derivative ((Pr2NP)-Pr-t)(2)COFe2(CO)(6), which undergoes thermal reactions with unsaturated organic molecules to gi ve adducts of their C=C, C=O, or C=N bonds to the phosphorus atoms of a Fe- 2(CO)(6)((PNPr2)-Pr-t)(2), unit generated by extrusion of the phosphorus-br idging carbonyl group. A low-temperature IR spectroscopic study of ((Pr2NP) -Pr-t)(2)COFe2(CO)(6) photolyzed in a Nujol matrix provides evidence for an unstable nearly square Fe-2(CO)(6)((PNPr2)-Pr-t)(2) rhombus. (C) 2000 Else vier Science S.A. All rights reserved.