Binuclear catalysis reactions

Most catalysts that have been mentioned so far are mononuclear. The few binuclear compounds utilized Co2(CO)8 or phosphinesubstituted derivatives) did not give evidence of any unusual type of binuclear catalysis. However, new products could result with catalysts producing two active centers in close vicinity which would not dissociate in the course of the reaction. The expected difference between mononuclear and binuclear catalysis is shown in the accompanying diagram (52). A series of metal salts of cobalt carbonyl hydride of composition M[Co(C0)4]n (M=Zn, Cd, Hg, n = 2 M = In, = 3) were tested as potential binuclear catalysts. The complex salts are relatively easily accessible Zn[Co(CO)4]2, for instance, may be prepared from cobalt carbonyl, metallic zinc, and CO (at 3000 psi initial pressure) using toluene as the solvent and a temperature of 200°. The compound may also be synthesized directly from metallic cobalt, zinc, and CO... 

The diversity of results of studies of hydrogenation and hydroformylation catalysis have prompted a report on the factors governing the different mechanisms of binuclear elimination reactions of complexes leading to carbon-hydrogen bond formation. 

Ribonucleotide reductase is notable in that its reaction mechanism provides the best-characterized example of the involvement of free radicals in biochemical transformations, once thought to be rare in biological systems. The enzyme in E. coli and most eukaryotes is a dimer, with subunits designated R1 and R2 (Fig. 22-40). The R1 subunit contains two lands of regulatory sites, as described below. The two active sites of the enzyme are formed at the interface between the R1 and R2 subunits. At each active site, R1 contributes two sulfhydryl groups required for activity and R2 contributes a stable tyrosyl radical. The R2 subunit also has a binuclear iron (Fe3+) cofactor that helps generate and stabilize the tyrosyl radicals (Fig. 22-40). The tyrosyl radical is too far from the active site to interact directly with the site, but it generates another radical at the active site that functions in catalysis. 

Copper has an essential role in a number of enzymes, notably those involved in the catalysis of electron transfer and in the transport of dioxygen and the catalysis of its reactions. The latter topic is discussed in Section 62.1.12. Hemocyanin, the copper-containing dioxygen carrier, is considered in Section 62.1.12.3.8, while the important role of copper in oxidases is exemplified in cytochrome oxidase, the terminal member of the mitochondrial electron-transfer chain (62.1.12.4), the multicopper blue oxidases such as laccase, ascorbate oxidase and ceruloplasmin (62.1.12.6) and the non-blue oxidases (62.12.7). Copper is also involved in the Cu/Zn-superoxide dismutases (62.1.12.8.1) and a number of hydroxylases, such as tyrosinase (62.1.12.11.2) and dopamine-jS-hydroxylase (62.1.12.11.3). Tyrosinase and hemocyanin have similar binuclear copper centres. 

Several diverse metal centres are involved in the catalysis of monooxygenation or hydroxylation reactions. The most important of these is cytochrome P-450, a hemoprotein with a cysteine residue as an axial ligand. Tyrosinase involves a coupled binuclear copper site, while dopamine jS-hydroxylase is also a copper protein but probably involves four binuclear copper sites, which are different from the tyrosinase sites. Putidamonooxin involves an iron-sulfur protein and a non-heme iron. In all cases a peroxo complex appears to be the active species. 

Scheme 3 forms a catalytic cycle for the water-gas shift reaction (63) employing [Rh2(/i-CO)(CO)2(dpm)2] in the presence of acid as a catalyst (62). It should be reiterated that alternative cycles might be written which do not involve formate intermediates. For example, a possible mechanism for catalysis of the water-gas shift reaction involving the binuclear metal species, [Pt2H2( -HXdpm)2]+, is outlined below (Scheme 4) (64). We have critically discussed the role of formate versus carboxylic acid intermediates in homogeneous catalysis of the water-gas shift reaction by mononuclear metal catalysts elsewhere (34).

Attempts have been made to mimic proposed steps in catalysis at a platinum metal surface using well-characterized binuclear platinum complexes. A series of such complexes, stabilized by bridging bis(diphenyl-phosphino)methane ligands, has been prepared and structurally characterized. Included are diplati-num(I) complexes with Pt-Pt bonds, complexes with bridging hydride, carbonyl or methylene groups, and binuclear methylplatinum complexes. Reactions of these complexes have been studied and new binuclear oxidative addition and reductive elimination reactions, and a new catalyst for the water gas shift reaction have been discovered. 

In contrast to haem proteins there is no direct spectroscopic evidence for FeIV=0 involvement in catalysis by non-haem iron enzymes. Both the absence of a highly coloured prosthetic group and the short lifetimes of the proposed intermediates make the task of detection difficult. However, analysis of possible reaction pathways and the nature of the products formed has provided some indirect evidence for FeIV=0 formation, both in binuclear and mononuclear non-haem iron enzymes. 

There has been considerable interest in binuclear and polynuclear metal complexes as models for intermediates proposed to be formed during reactions which are heterogeneously catalysed by transition metals (1). Since platinum is one of the most versatile catalysts, we have begun an investigation into the synthesis, and chemical and catalytic properties of some binuclear organo-platinum complexes. In this article some hydrido and methyl complexes will be described, and a preliminary account of catalysis with binuclear complexes given. In addition, structural studies indicate that Pt-Pt bonding interactions may take several different forms in these complexes and so the nature of the Pt-Pt bond will also be discussed. 

These complexes are the first examples of multifunctional catalysts and demonstrate impressively the opportunities that can reside with the as yet hardly investigated bimetallic catalysis. The concept described here is not limited to lanthanides but has been further extended to main group metals such as gallium [31] or aluminum [32]. In addition, this work should be an incentive for the investigation of other metal-binaphthyl complexes to find out whether polynuclear species play a role in catalytic processes there as well. For example, the preparation of ti-tanium-BINOL complexes takes place in the presence of alkali metals [molecular sieve ( )]. A leading contribution in this direction has been made by Kaufmann et al, as early as 1990 [33], It was proven that the reaction of (5)-la with monobromoborane dimethyl sulfide leads exclusively to a binuclear, propeller-like borate compound. This compound was found to catalyze the Diels-Alder reaction of cyclopentadiene and methacrolein with excellent exo-stereoselectivity and enantioselectivity in accordance with the empirical rule for carbonyl compounds which has been presented earlier. 

Scheme 41 outlines the essence of chiral catalysis. The chiral catalysts in general work homogeneously which means that they are small molecules, mostly monomeric and contain one (mononuclear) or sometimes two (binuclear) metal atoms in a chelate complex with chiral organic ligands. Typical metals are Pd(0), Pd(II), Rh(I), Rh(II), Cu(II) which are used for essentially non-polar reactions... 

Enzymes that contain binuclear metal centers are also well suited to catalyze hydrolysis reactions, including a number of the reactions described above for the mononuclear metallohydrolases. Additionally, several of the examples that are discussed here belong to the enzyme superfamilies described above, specifically the amidohydrolase, zinc a,/3-hydrolase, and metallo-/31 superfamilies. The substrates for the binuclear metallohydrolases are also biologically diverse, including proteins, peptides, nucleotides, polyamines, and xenobiotics. The binuclear nature of these metal centers produces an active site with altered properties compared to the mononuclear counterparts, and therefore catalysis by these enzymes occurs with alternative reaction mechanisms. Readers are referred to the preceding sections for background information pertaining to enzymes that have already been discussed. 

Figures Proposed CODH mechanism. This mechanism proposes that the active state is Credl, consistent with recent electrochemical studies/ which contains a bridging hydroxide at the binuclear NiFe center that serves as the nucleophile to attack a Ni-bound carbonyl forming a Ni-carboxylate. This OH, which is formed by acid-base catalysis by indicated acid-base residues, is in the position of the bridging sulfide in the C. hydrogenoformans structure. The enzyme is proposed to remain in the Credl redox state until formation of CO2 when it becomes two-electrons reduced to the Cred2 state. Conversion of Credl to Cred2 occurs faster than elechon hansfer from Cred2 to the FeS clusters, which in turn reduce external electron acceptors. The electron transfer reactions are proposed to occur through a diamagnetic Cint state.

In this chemistry, it is natural to focus on models for the catalytic reactions that are most important economically or which are most poorly understood because of difficulties of direct study. One which best fits these criteria is the catalytic hydrotreatment of petroleum feedstocks, which is used to remove sulfur and other heteroatoms, which interfere with subsequent catalytic reactions such as petroleum reforming, from the hydrocarbons. Molybdemun sulfide is the most common metal sulfide used in this catalysis, and hydrogen activation and C-S bond hydrogenolysis are known to be key reactions occurring at the catalyst surface but details are difficult to obtain. Study of model binuclear and cluster complexes has elucidated mechanisms of several of the key reactions and Section 2.6 describes important recent advances in this field, with the focus being on models for hydrodesulfurization catalysts. 

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