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.

Sulfur bridging interactions between three cis-planar NiII-S2N2 complexes and NiII, CuI,II, ZnII, and HgII reactants were investigated by synthesis and X-ray crystal structures of some 24 complexes. This work was stimulated by recent crystallographic structures of the A-cluster of carbon monoxide dehydrogenase/acetylcoenzyme A synthase. This bridged biological assembly has the minimal formulation [Fe4S4]-(micro2-SCys)-[M((micro2-SCys)2Gly)Ni] with M = NiII, CuI, and ZnII at sites distal and proximal, respectively, to the iron-sulfur cluster. Bridges supported by representations of the distal nickel site were sought by reactions of the complexes [NiII(LH-S2N2)]2- and [NiII(LR-S2N2)], with 5-5-5 chelate ring patterns. Reaction products implicate the bridges Ni-(micro2-S)1,2-M in a variety of molecular structures, some with previously unknown connectivities of bridge atoms. The most frequently encountered bridge units are the nonplanar rhombs Ni(2-S)2M involving both sulfur atoms of a given complex. Those with M = NiII are biologically relevant inasmuch as the catalytic metal at the proximal site is nickel. The complex [Ni(L-655)]2-, containing the 6-5-5 ring pattern and coordination sphere of the distal nickel site, was prepared and structurally characterized. It was shown to sustain Ni2(micro2-S)2 rhombic interactions in the form of trinuclear [[Ni(L-655)]2Ni]2- and [[Ni(L-655)]Ni(R2PCH2CH2PR2)] (R = Et, Ph) in which the second NiII simulates the proximal site. Bridging interactions of NiII-S2N2 complexes are summarized, and geometrical features of Ni2(2-S)2 rhombs in these complexes, as dependent on ring patterns, are considered (LH-S2N2 = N,N'-ethylenebis(2-mercaptoisobutyramide)(4-); LR-S2N2 = trans-rac-N,N'-bis(2-mercapto-2-methylprop-1-yl)-1,2-cyclohexanediamine(2-); L-655 = N-(2-mercaptopropyl)-N'-(2'-mercaptoethyl)glycinamide(4-)).

The construction of a synthetic analogue of the A-cluster of carbon monoxide dehydrogenase/acetylcoenzyme synthase, the site of acetylcoenzyme A formation, requires as a final step the formation of an unsupported [Fe(4)S(4)]-(mu(2)-SR)-Ni(II) bridge to a preformed cluster. Our previous results (Rao, P. V.; Bhaduri, S.; Jiang, J.; Holm, R. H. Inorg. Chem. 2004, 43, 5833) and the work of others have addressed synthesis of dinuclear complexes relevant to the A-cluster. This investigation concentrates on reactions pertinent to bridge formation by examining systems containing dinuclear and mononuclear Ni(II) complexes and the 3:1 site-differentiated clusters [Fe(4)S(4)(LS(3))L'](2-) (L' = TfO(-) (14), SEt (15)). The system 14/[{Ni(L(O)-S(2)N(2))}M(SCH(2)CH(2)PPh(2))](+) results in cleavage of the dinuclear complex and formation of [{Ni(L(O)-S(2)N(2))}Fe(4)S(4)(LS(3))]- (18), in which the Ni(II) complex binds at the unique cluster site with formation of a Ni(mu(2)-SR)(2)Fe bridge rhomb. Cluster 18 and the related species [{Ni(phma)}Fe(4)S(4)(LS(3))](3)- (19) are obtainable by direct reaction of the corresponding cis-planar Ni(II)-S(2)N(2) complexes with 14. The mononuclear complexes [M(pdmt)(SEt)]- (M = Ni(II), Pd(II)) with 14 in acetonitrile or Me(2)SO solution react by thiolate transfer to give 15 and [M(2)(pdmt)(2)]. However, in dichloromethane the Ni(II) reaction product is interpreted as [{Ni(pdmt)(mu(2)-SEt)}Fe(4)S(4)(LS(3))](2-) (20). Reaction of Et(3)NH(+) and 15 affords the double cubane [{Fe(4)S(4)(LS(3))}(2)(mu(2)-SEt)](3-) (21). Cluster 18 contains two mutually supportive Fe-(mu(2)-SR)-Ni(II) bridges, 19 exhibits one strong and one weaker bridge, 20 has one unsupported bridge (inferred from the (1)H NMR spectrum), and 21 has one unsupported Fe-(mu(2)-SR)-Fe bridge. Bridges in 18, 19, and 21 were established by X-ray structures. This work demonstrates that a bridge of the type found in the enzyme A-clusters is achievable by synthesis and implies that more stable, unsupported single thiolate bridges may require reinforcement by an additional covalent linkage between the Fe(4)S(4) and nickel-containing components. (LS(3) = 1,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(p-tolylthio)benzene(3-); L(O)-S(2)N(2) = N,N'-diethyl-3,7-diazanonane-1,9-dithiolate(2-); pdmt = pyridine-2,6-methanedithiolate(2-); phma = N,N'-1,2-phenylenebis(2-acetylthio)acetamidate(4-); TfO = triflate.).

Gd Sc O 3 films were deposited on hydrogenated silicon substrates by atomic layer deposition. The films were pure and amorphous, both as-deposited and after a 5min anneal at 950°C. Cross-sectional transmission electron microscopy revealed a sharp, smooth interface between GdScO3 and Si. Capacitance and leakage current measurements on metal oxide semiconductor capacitors made from atomic layer deposited WN∕GdScO3 stacks showed that the amorphous GdScO3 films have a high dielectric constant (∼22), low fixed charge density, and low interface trap density. A film with 1nm equivalent oxide thickness also demonstrated that the leakage current density is less than 2mA∕cm2 at 1V gate bias.

A method has been devised that creates a planar Ni(II) site from a tetrahedral site in a NiFe(3)S(4) cubane-type cluster. Reaction of [(Ph(3)P)NiFe(3)S(4)(LS(3))](2)(-) (2) with 1,2-bis(dimethylphosphino)ethane affords [(dmpe)NiFe(3)S(4)(LS(3))](2)(-) (3), isolated in ca. 45% yield as (Et(4)N)(2)[3a].2.5MeCN and (Et(4)N)(2)[3b].0.25MeCN, both of which occur in triclinic space group P. Each crystalline form contains two crystallographically inequivalent clusters with the same overall structure but slightly different dimensions. The cluster is bound by three thiolate terminal ligands to semirigid cavitand ligand LS(3). The NiFe(3)S(4) core contains three tetrahedral sites, one Fe(micro(3)-S)(3)(SR) and two Fe(micro(3)-S)(2)(micro(2)-S)(SR) with normal metric features, and one distorted square planar Ni(micro(3)-S)(2)P(2) site in a Ni(micro(3)-S)(2)Fe face with mean bond lengths Ni-P = 2.147(9) A and Ni-S = 2.29(2) A. The opposite Fe(2)(micro(3)-S)(micro(2)-S) face places the micro(2)-S atom at nonbonding and variable distances (2.60-2.90 A) above the nickel atom. Binding of the strong-field ligand dmpe results in a planar Ni(II) site and deconstruction of the full cubane geometry. The structure approximates that established crystallographically in the C-cluster of C. hydrogenoformans carbon monoxide dehydrogenase whose NiFe(4)S(4) core contains a planar NiS(4) site and three tetrahedral FeS(4) sites in a fragment that is bridged by sulfide atoms to an exo iron atom. Mössbauer studies of polycrystalline samples containing both clusters 3a and 3b reveal the presence of at least two cluster types. The spectroscopically best defined cluster accounts for ca. 54% of total iron and exhibits hyperfine interactions quite similar to those reported for the S = (5)/(2) state of the protein-bound cubane-type cluster [ZnFe(3)S(4)](1+), whose Mössbauer spectrum revealed the presence of a high-spin Fe(2+) site and a delocalized Fe(2.5+)Fe(2.5+) pair. Development of reactions leading to a planar nickel and a sulfide-bridged iron atom is requisite to attainment of a synthetic analogue of this complex protein-bound cluster. This work demonstrates a tetrahedral (2) --> planar (3) Ni(II) stereochemical conversion can be effected by binding of ligands that generate a sufficiently strong in-plane ligand field (dmpe = 1,2-bis(dimethylphosphino)ethane, LS(3) = 1,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(p-tolylthio)benzene(3-)).