Binuclear metal cores

The ability to modify the backbones of LMP structures with phosphonates allows for wide flexibility in the design of new materials containing photoactive binuclear metal cores. The goal of our initial studies has been to demonstrate that a ligating functionality within the gallery is accessible for reaction with a bimetallic core. To demonstrate these initial objectives, we have chosen to study zirconium phosphate modified with alkyl carboxylate, which is a good ligand of bimetallic cores. 

Monodentate or multidentate thiols as the ligand coordinate are anionic donors in form of thiolates. The metal core of their complexes is normally [M =0] 3 +. The charge requirements of this core are easily met by the ligands. In the case of pure thiolato coordination, anionic complexes result. In a few cases, neutral binuclear oxo complexes and oxo-free species also occur. 

Figure 9.3 Recognition of cAMP by PDE4. A bidentate interaction between Glu443 and two of the adenine nitrogens is key to nucleotide discrimination for this cAMP-specific PDE. The core binding pocket contributes hydrophobic interactions with the heterocyde. The cyclic phosphate is thought to interact with the binuclear metal center, which may deliver an activated water molecule.

Complexation of 1,3-butadiynes In the reactions of complex 1 with various butadiynes, binuclear complexes with intact C4 units between the two metal centers are found. The former diynes are transformed to zig-zag butadiene ligands or g-r (l-3),r (2-4)-trans,-trans-tetradehydrobutadiene moieties between two metallocene cores. The bond type in 19 is unknown for the corresponding zirconocene complexes. 

Among the earliest compounds investigated are those containing the binuclear di-/i-oxo core represented by (1). The complexes are isolated as mixed-valent Mn(IV,III) species, with four N-donor atoms from two bipyridyl or two phenanthroline ligands bound to each metal. The cyclic voltammetric response [80-82] of the more stable phen system, [Mn202(phen)2] is reproduced in Fig. 9. It consists of a one-electron oxidation to the Mn(IV,IV) species, [Mn202(phen)2]", and a one-electron reduction to the Mn(III,III) species, [Mn202(phen)2] " " ... 

In the earlier examples, the binuclear complexes with a M02S3 core clearly cannot be seen as modeling the metal sites in FeMo—co since it is known to contain a single molybdenum center [74]. However, what is suggested by these studies is that several steps (if not all) of the reduction of dinitrogen to ammonia by Mo-nitrogenase could take place at a... 

The key synthons M(OR)5 are all dimeric in solution when R is primary but usually monomeric when R is secondary or tertiary.292 A monomer iimer equilibrium is present for R = Pr1 at lower temperatures. Recent synthetic aspects include improved procedures for anodic dissolution of the metals into alcohols containing electrolytes. For example, binuclear Ta2(OR)i0 (R = Me, Et, Bun) and Ta20(0Pr1)g-Pri0H were isolated (electrolyte, LiCl).276 The latter molecule features a Ta2(q2-0)-(q2-OPr ) core and may be hydrolyzed to heptanuclear Ta709(0Pr )i7. 

Various iron salts and mononuclear Fe or binuclear Fe complexes with a N,0 environment, biomimetic to methane monooxygenase complexes, have been applied to the oxidation of cyclohexane with various oxidants [6u,v,7a-g], but their catalytic activity is usually modest, with the exception of a hexanuclear Fe(III) compound derived from p-nitrobenzoic acid, which gives the highest total yield to Ol/One of about 30% [7a]. Moreover, most of these complexes are often unstable and very expensive. A hexanuclear heterotrimetallic Fe/Cu/Co complex bearing two Cu(p-0)2Co(p-0)2Fe cores, prepared by self-assembly, oxidizes cyclohexane with aqueous HP, with a maximum yield to Ol/One of 45%, virtually total selectivity to the two compounds, and preferred formation of cyclohexanol [7hj. The remarkable activity of the Fe/Cu/Co cluster was associated with the synergic effect of the three metals. 

Electron transfers may lead to modifications of the metal-sulfur core of binuclear sulfur-bridged complexes. The CV of [M2S2(cp )2(/i-S)2] 11 (M = Mo or W, cp = /j-CsHs nMe,) [30] showed that the first reduction was a one-electron process. 

X-ray crystal structures of CODH/ACS proteins show that they are large (300 kDa) homodimeric proteins with the two CODH domains j3 subunits) at the core, and the two ACS domains (a subunits) tethered to the side of each of these (3 subunits. The (3 subunits each contain one Ni and 10 Fe ions that are arranged into three FeS clusters (the so-called B-, C-, and D-clusters), whereas the a subunits each contains the active site A-cluster, a [Fe4S4] " cubane that is bridged by the sulfur of a cysteinyl residue to the proximal metal (Mp) of a binuclear center. This binuclear center contains a square planar nickel ion, referred to as the distal Ni (Ni(j), which is coordinated by the two thiolates and two backbone amides of a Cys-Gly-Cys motif There has been some debate as to the identity of Mp, as Ni, Cu, and Zn ions have been shown to occupy this site however, it is now generally accepted that the A-cluster is a binuclear Ni—Ni center bridged by a cysteine thiol to a [4Fe— 4S] cluster. " ... 

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