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Iron-sulfur proteins are found in all life forms. Most frequently, they contain Fe2S2, Fe3S4, and Fe4S4 clusters. These modular clusters undergo oxidation-reduction reactions, may be inserted or removed from proteins, can influence protein structure by preferential side chain ligation, and can be interconverted. In addition to their electron transfer function, iron-sulfur clusters act as catalytic centers and sensors of iron and oxygen. Their most common oxidation states are paramagnetic and present significant challenges for understanding the magnetic properties of mixed valence systems. Iron-sulfur clusters now rank with such biological prosthetic groups as hemes and flavins in pervasive occurrence and multiplicity of function.

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Contents 1. Introduction 527 2. Rubredoxin Site Analogues 530 2.1. Preparation 530 2.2. Structures 530 2.3. Properties 531 3. Analogues of Binuclear (Fe 2 S 2 ) Sites 532 3.1. Preparation 532 3.2. Structures 533 3.3. Properties 533 3.4. Heteroligated Clusters 534 4. Analogues of Trinuclear (Fe 3 S 4 ) Sites 535 4.1. Linear Clusters 535 4.2. Cuboidal Clusters 536 4.3. Reactivity 537 5. Analogues of Tetranuclear (Fe 4 S 4 ) Sites 538 5.1. Electron-Transfer Series 538 5.2. [Fe 4 S 4 ] 3+ Clusters 540 5.3. [Fe 4 S 4 ] 2+ Clusters 542 5.4. [Fe 4 S 4 ] + Clusters 543 5.5. [Fe 4 S 4 ] 0 Clusters 543 5.6. Principal Structural Features 544 5.7. Specialized Clusters 545 5.7.1. Peptide Clusters 545 5.7.2. Site-Differentiated Clusters 546 5.7.3. Bridged Assemblies 549 6. Mo ¨ssbauer Parameters and Oxidation States 551 7. Structural Conversions 552 8. Perspective 554 9. Acknowledgement 555 10. Abbreviations 555 11. References 555Richard H. Holm was born in Boston, MA, spent his younger years on Nantucket Island and Cape Cod, and graduated from the University of Massachusetts (B.S.) and Massachusetts Institute of Technology (Ph.D.) He has served on the faculties of the University of Wisconsin, Massachusetts Institute of Technology, and Stanford University. Since 1980, he has been at Harvard University, where he has been Chair of the Department of Chemistry and, from 1983, Higgins Professor of Chemistry. His research interests are centered in inorganic and bioinorganic chemistry, with particular reference to the synthesis and properties of molecules whose structures and reactions are pertinent to biological processes. Venkateswara Rao Pallem was born in Hyderabad, India. He received his M.Sc. degree in chemistry in 1995 from Osmania University, Hyderabad, and his Ph.D. in chemistry in 2001 at the Indian Institute of Technology, Bombay, India, under the guidance of Professor Chebrolu P. Rao. He is currently working as a postdoctoral fellow with Professor Richard H. Holm. His research interests include the design and study of synthetic analogues of biologically related molecules.

A general method for accessing the solution chemistry of cluster constituents of solid phases exhibiting extended cluster frameworks is demonstrated. The approach is described in terms of simple metal−anion (M−X) frameworks and involves the formal incorporation of AX into a parent structure, resulting in termination of the X bridges between M centers while balancing the charge of the resulting framework with external cations A. The new structures obtained display frameworks of reduced connectedness and dimensionality. By replacing single metal centers with multinuclear cluster cores, this dimensional reduction approach is extended to cluster-containing frameworks. Its utility is demonstrated via application to the phases Re6Q8Cl2 (Q = S, Se), exhibiting three- and two-dimensional arrays of face-capped octahedral [Re6(μ3-Q)8]2+ cluster cores covalently linked through extremely tight Re2Q2 rhombic interactions of the type common to many intractable cluster frameworks (including the Chevrel phases). Stoichiometric solid-state reactions incorporating TlCl supplement the cores with additional terminal ligands, producing less connected frameworks:  two-dimensional [Re6Se8Cl3]1- sheets, one-dimensional [Re6Q8Cl4]2- chains, and ultimately, isolated [Re6Q8Cl6]4- clusters. The connectivities for such [M6Q8] frameworks are enumerated; of the 28 possibilities, three previously unknown frameworks are achieved in the structures of TlRe6Se8Cl3, CsRe6Se8I3, and Cs2Re6Se8Br4, described herein. Alternatively, employing cesium halide as a dimensional reduction agent directly provides the unprecedented molecular clusters in water-soluble form as the phases Cs5Re6S8X7 (X = Cl, Br), Cs6Re6S8I8, and Cs4Re6Se8I6. The species [Re6S8X6]4- (X = Cl, Br, I) are precipitated from aqueous base upon addition of (Bu4N)X to give the soluble molecular products (Bu4N)4[Re6S8Cl6], (Bu4N)4[Re6S8Br6]·H2O, and (Bu4N)4[Re6S8I6]·H2O. Treatment of yellow acetonitrile solutions of these compounds with anhydrous acid induces an immediate color change to red owing to the formation of the protonated clusters [Re6S7(SH)X6]3-. Reversible uptake of a single proton is confirmed by the single-crystal X-ray structure determinations of (Bu4N)3[Re6S7(SH)Cl6], (Bu4N)3[Re6S7(SH)Br6]·2Me2CO, and (Bu4N)3[Re6S7(SH)I6]·2Me2CO, as well as spectrophotometric titrations and elemental analyses. The pK a of [Re6S7(SH)Br6]3- in acetonitrile is estimated at 20. An analogous workup of red Cs4Re6Se8I6 affords (Bu4N)3[Re6Se7(SeH)I6]·2Me2CO.

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