The tripeptide, Ac-CysGlyCys-CONH2, is utilized as a ligand to bind Ni in a fashion identical to that found at the active site of acetyl coenzyme A synthase. The Ni-peptide construct is a suitable metalloligand for the preparation of larger structures formed via bridging Cys side chains. The complexes Ni(CysGlyCys)Ni(dppe) and Ni(CysGlyCys)Ni(depe) serve as close structural representations for the binuclear subcluster, exhibiting electrochemical properties that demonstrate facile access to the reduced mixed valent Ni(II)Ni(I) state, which binds CO.

Supramolecular complexes coupling Ru(II) or Os(II) polyazine light absorbers through bridging ligands to Rh(III) or Ir(III) allow the study of factors impacting photoinitiated electron collection and multielectron water reduction to produce hydrogen. The [{(bpy)(2)Ru(dpb)}(2)IrCl(2)](PF(6))(5) system represents the first photoinitiated electron collector in a molecular system (bpy = 2,2'-bipyridine, dpb = 2,3-bis(2-pyridyl)benzoquinoxaline). The [{(bpy)(2)Ru(dpp)}(2)RhCl(2)](PF(6))(5) system represents the first photoinitiated electron collector that affords photochemical hydrogen production from water in the presence of an electron donor, N,N-dimethylaniline (dpp = 2,3-bis(2-pyridyl)pyrazine). The complexes [{(bpy)(2)Ru(dpp)}(2)RhCl(2)](PF(6))(5), [{(bpy)(2)Ru(dpp)}(2)RhBr(2)](PF(6))(5), [{(phen)(2)Ru(dpp)}(2)RhCl(2)](PF(6))(5), [{(bpy)(2)Os(dpp)}(2)RhCl(2)](PF(6))(5), [{(tpy)RuCl(dpp)}(2)RhCl(2)](PF(6))(3), [{(tpy)OsCl(dpp)}(2)RhCl(2)](PF(6))(3), and [{(bpy)(2)Ru(dpb)}(2)IrCl(2)](PF(6))(5) are herein evaluated with respect to their functioning as hydrogen photocatalysts (tpy = 2,2':6',2''-terpyridine, phen = 1,10-phenanthroline). With the exceptions of [{(bpy)(2)Ru(dpb)}(2)IrCl(2)](PF(6))(5) and [{(tpy)OsCl(dpp)}(2)RhCl(2)](PF(6))(3), all other complexes demonstrate photocatalytic activity. The functioning systems possess a rhodium localized lowest unoccupied molecular orbital that serves as the site of electron collection and a metal-to-ligand charge-transfer ((3)MLCT) and/or metal-to-metal charge-transfer ((3)MMCT) excited-state with sufficient driving force for excited-state reduction by the electron donor. The lack of photocatalytic activity by [{(bpy)(2)Ru(dpb)}(2)IrCl(2)](PF(6))(5), although photoinitiated electron collection occurs, establishes the significance of the rhodium center in the photocatalytic system. The lack of photocatalytic activity of [{(tpy)OsCl(dpp)}(2)RhCl(2)](PF(6))(3) is attributed to the lower-energy (3)MLCT state that does not possess sufficient driving force for excited-state reduction by the electron donor. The variation of electron donor showed the photocatalysis efficiency to decrease in the order N,N-dimethylaniline > triethylamine > triethanolamine. The general design considerations for development of supramolecular assemblies that function as water reduction photocatalysts are discussed.

A series of binuclear NiNi complexes supported by a single thiolate bridge and containing a methylnickel moiety have been prepared and fully characterized. The complexes represent structural analogs for the proposed organonickel intermediate in the acetyl coenzyme A synthase catalytic cycle. Variable temperature 31 P NMR spectroscopy was used to examine dynamic behavior of the thiolate bridging interaction in two of the derivatives. Kinetic analyses, independent exchange and crossover experiments support an intermolecular exchange mechanism. Carbonylation results in thioester formation via a reductive elimination pathway.Organonickel species have been implicated as catalytic intermediates in several enzyme reactions, 1 specifically transformations that shuttle one carbon species within the anaerobic world. Archaea and anaerobic bacteria grow autotrophically using carbon dioxide or sulfate as their terminal electron acceptors. 2 Examples of proposed bioorganometallic enzyme intermediates include the methylnickel species in methyl coenzyme reductase (MCR) and acetyl coenzyme A synthase (ACS), and Ni-CO adducts in carbon monoxide dehydrogenase (CodH) and ACS. Until very recently, the evidence supporting the involvement of such novel biological species has been indirect, i.e. direct spectroscopic or structural authentication of such species is lacking. However, several recent reports provide the first direct evidence for a methylnickel(III) state in MCR 3 and a Ni(CO 2 ) adduct in COdH. 4The ACS enzyme, found in archaea and sulfate-reducing bacteria, possesses an unprecedented active site cluster of [Fe 4 S 4 ]Ni 2 composition. 5,6 The two Ni sites are structurally distinct with consensus building towards Ni p (proximal to the Fe 4 S 4 cluster) as the locus for methyl binding. 7,8 Since the first protein structure report in 2002, a number of laboratories including ours 9, 10 have modeled aspects of the binuclear NiNi site. 11 Herein, we report initial efforts to prepare binuclear complexes containing a methylnickel moiety and explore their dynamic properties and reactivity to provide further understanding of organonickel catalysis in biology. 12 A series of binickel complexes containing a methylnickel moiety were prepared via condensation of monomer precursors. This condensation strategy has been applied extensively using Ni(N 2 S 2 ) complexes leading to their apt moniker "metalloligands." In the present context riordan@udel.edu. The structure of the anion, 1a is depicted in Fig. 1 (left). A single thiolato bridge supports the binuclear core with slightly different Ni-S distances, 2.192(1) Å and 2.230(1) Å. The Ni-C distance is typical for a square planar site, 1.966(2) Å. The orientation of the two square planes defined by the respective metal coordination spheres places the methyl group in proximity of the other Ni. The Ni---Ni distance, 2.952(1) Å, is most similar to the distance found in the structure of the reduced A cluster from C. hydrogenoformans, 3.0 Å an unmethylated enzyme state containing two sq...

Photocatalytic generation of hydrogen from water is an integral part of the next generation clean fuel technologies. The conversion of solar energy into useful chemical energy is of great interest in contemporary investigations. The splitting of water is a multi-electron process involving the breaking and making of chemical bonds which requires multi-component photocatalytic systems. Supramolecular complexes [{(TL) 2 Ru(BL)} 2 RhX 2 ](Y) 5 (where TL ¼ terminal ligand, BL ¼ bridging ligand, X ¼ Cl À or Br À , and Y ¼ PF 6À or Br À ) have been synthesized and studied for their light absorbing, electrochemical and photocatalytic properties. The supramolecular complexes in this investigation are multi-component systems comprised of two ruthenium based light absorbers connected through bridging ligands to a central rhodium, which acts as an electron collecting center upon excitation. These complexes absorb light throughout the ultraviolet and visible regions of the solar spectrum. The supramolecular complexes possess ruthenium based highest occupied molecular orbitals (HOMO) and a rhodium based lowest unoccupied molecular orbital (LUMO). These molecular devices have been investigated and shown to function as photoinitiated electron collectors at the reactive rhodium metal center, and explored as photocatalysts to generate hydrogen from water in an aqueous solution in the presence of an electron donor.

Reaction of the nickel metalloligands [EtN2S2]Ni (EtN2S2, N,N'-diethyl-3,7-diazanonane-1,9-dithiolate) or K2[Ni(phmi)] (phmi, N,N'-1,2-phenylenebis(2-sulfanyl-2-methylpropionamide)) with [Cu(CH3CN)4]BF4 yields polynuclear complexes in which two copper(I) ions are bridged by the nickel metalloligands. Alternatively, reaction with the Cu(I) source, [(PhTttBu)Cu] (PhTttBu, phenyltris((tert-butylthio)methyl)borate), generates discrete binuclear NiCu complexes that may serve as models of the acetyl coenzyme A synthase active site. The binuclear species react reversibly with CO via rupture of the thiolate bridges.