Binuclear catalytic intermediates

Binuclear Organoplatinum Complexes as Models for Catalytic Intermediates... 

Mononuclear platinum complexes often have been used as models for catalytic intermediates since systematic studies of synthesis, reactivity, and mechanism are often convenient and because metallic platinum is a very important catalyst. However, using binuclear or polynuclear platinum complexes as models for proposed intermediates in heterogeneous catalysis has not been studied, probably because planned routes to such complexes have not been available. This chapter describes our first studies in this area. 

The observation that the rate of reaction is linear with concentration of (I) strongly suggests that the catalytic intermediates are binuclear but more work is needed before a detailed catalytic cycle can be deduced with confidence. It is likely that the mechanism defined by equations (3) and (9) is an oversimplification. Attempts to prove directly that reaction (11) can occur have been unsuccessful since, in the absence of CO, (VIII) decomposes faster than it reacts with water. However, the dicationic [Pt2(C0)2(y-dppm)2]2+ reacts readily with water according to equation (10). 

The most fundamental reaction is the alkylation of benzene with ethene.38,38a-38c Arylation of inactivated alkenes with inactivated arenes proceeds with the aid of a binuclear Ir(m) catalyst, [Ir(/x-acac-0,0,C3)(acac-0,0)(acac-C3)]2, to afford anti-Markovnikov hydroarylation products (Equation (33)). The iridium-catalyzed reaction of benzene with ethene at 180 °G for 3 h gives ethylbenzene (TN = 455, TOF = 0.0421 s 1). The reaction of benzene with propene leads to the formation of /z-propylbenzene and isopropylbenzene in 61% and 39% selectivities (TN = 13, TOF = 0.0110s-1). The catalytic reaction of the dinuclear Ir complex is shown to proceed via the formation of a mononuclear bis-acac-0,0 phenyl-Ir(m) species.388 The interesting aspect is the lack of /3-hydride elimination from the aryliridium intermediates giving the olefinic products. The reaction of substituted arenes with olefins provides a mixture of regioisomers. For example, the reaction of toluene with ethene affords m- and />-isomers in 63% and 37% selectivity, respectively. 

A mechanistic study of acid and metal ion (Ni2+, Cu2+, Zn2+) promoted hydrolysis of [N-(2-carboxyphenyl)iminodiacetate](picolinato)chromate (III) indicated parallel H+- or M2+-dependent and -independent pathways. Solvent isotope effects indicate that the H+-dependent path involves rapid pre-equilibrium protonation followed by rate-limiting ring opening. Similarly, the M2+-dependent path involves rate-determining Cr-0 bond breaking in a rapidly formed binuclear intermediate. The relative catalytic efficiencies of the three metal ions reflect the Irving-Williams stability order (88). 

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).

The ability of the binuclear complex [Cp RuCl(p2-SR)2RuCl(Cp )] to generate cationic allenylidene complexes by activation of terminal prop-2-ynols in the presence of NH4BF4 as a chloride abstractor opens the way to a variety of catalytic transformations of propargylic alcohols involving nucleophilic addition at the Cy atom of the ruthenium allenylidene intermediate (Scheme 19). This leads to the formation of a functional ruthenium vinylidene species which tau-tomerizes into an -coordinated alkyne that is removed from the ruthenium centre in the presence of the substrate. 

Complexes of type [LyM (CH2)IIX ], where n > 1, have been shown to be useful precursors for hetero- and homobimetallic n(a.,a>) alkanediyl complexes, [LjcM(CH2)bM L, ] (where ML, is not necessarily the same as M Lj,). Such hydrocarbon-bridged binuclear compounds have been proposed as models for intermediates in the Fischer-Tropsch reaction (18,19) and other significant catalytic processes (20-23). Some [LyM (CH2)BX ] complexes are precursors to cyclic carbene complexes (Section III), whereas others have been shown to have synthetic utility in organic chemistry (24). 

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. 

FIGURE 8. Postulated mechanism for MMO. The inner cycle are postulated intermediates in the catalytic cycle (only the binuclear iron cluster of the MMOH component is shown). The outer cycle represents the intermediates detected during a single turnover beginning with diferrous MMOH and ending with diferric MMOH. The rate constants shown are for 4 C and pH 7.7. The rate shown for the substrate reaction RH with Q is that for methane. The alignment of the two cycles shows the postulated structures for the intermediates. 

The preparation and characterization of novel man-ganese(III) complexes of various porphyrin and porphyrin-likes macrocycles have continued to attract strong attention especially because of their importance in catalytical oxidation processes through the formation of a Mn(V)0 intermediate (see Section 6) and as model for metalloenzymes. In this line, an artificial enzyme formed through a directed assembly of a molecular square that encapsulated a Mn porphyrin has been prepared and investigated as a catalyst. In contrast to symmetrical binuclear bis(phenoxo) bridged macrocyclic Mn(III)Mn(III) complexes, unsymmetrical ones are rare. A new series of these kinds of carboxylate-free complexes has been described and their redox properties investigated. ... 

Mn(II) centre (c, d) Following a proton transfer to the leaving amino group mediated by Asp 128, the tetrahedral intermediate collapses to yield the products L-ornithine and urea, (e) A water molecule enters to bridge the binuclear Mn cluster, causing the urea product to move to a terminal coordination site on Mn. Product dissociation facilitates ionisation of the metal-bridging water molecule to yield the catalytically active hydroxide ion. Proton transfer from the metal-bridging water to the bulk solvent is mediated by His-141, followed by the release of the two products. 

In electron transfer between metal ions, a metal-ion catalyst normally reacts by nonassociative activation, in which the species do not form long-lived binuclear intermediates. The catalytic process often can be rationalized by reactivity patterns e.g., the Cu catalyses of the oxidation "of V(III) by Fe(III). This catalysis by Cu occurs by outer-sphere mechanisms as in ... 

One binuclear complex may be involved in the catalytic cycle for butadiene oligomerization. Allylpalladium acetate reacts with butadiene to form an acetate-bridged allyl complex. Heptadiene is displaced from this intermediate when it is treated with additional butadiene, and a binuclear, acetate-bridged complex of the 2,6,10-dode-catriene-l,12-diyl ligand is claimed to be formed ... 

Stereochemistry is another powerful tool for determining the net reaction pathway of phosphatases and sulfatases. These enzymes catalyze the net transfer of a phosphoryl or sulfuryl group to water from a monoester, producing inorganic phosphate or sulfate. Inversion results when the reaction occurs in a single step (Scheme 2, pathway a). Phosphatases that transfer the phosphoryl group directly to water with inversion typically possess a binuclear metal center and the nucleophile is a metal-coordinated hydroxide. Examples of phosphatases that follow this mechanism are the purple acid phosphatases (PAPs) and the serine/threonine phosphatases (described in Sections 8.09.4.3 and 8.09.4.4.1). Net retention of stereochemistry occurs when a phosphorylated or sulfiirylated enzyme intermediate is on the catalytic pathway, which is hydrolyzed by the nucleophilic addition of water in a subsequent step (Scheme 2, pathway b). 

Autoxidation of iron(II) chloride in nonaqueous solvents is much faster than in water. The rate is first order in oxygen, and under controlled conditions, second order in iron(II). Various additives have powerful catalytic or inhibitory effects. The inhibition by iron(III) disappears in the presence of excess lithium chloride, so inhibition is attributed to competition between iron(II) and iron(III) for chloride ions. Induced autoxidation of benzoin to benzil has the same rate-limiting step as the autoxidation of iron(II) without cosubstrate. The data can be accommodated by a mechanism in which the rate-limiting step is production of iron(IV) by dissociation of a binuclear complex having the composition Cl FeOOFeCl. In the presence of excess lithium chloride, intermediates containing more chloride bound to iron become involved. 

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