Complex multimetal

In the industrial production of acetic acid the main production routes are based on the carbonylation of methanol, a process which was first developed in the 1940s. Although the process remains cheap, the availability and pricing of raw materials have forced the development of new processes based on the direct oxidation of ethylene to form acetic acid (Table 3, entry 24). Complex multimetal oxide... 

Numerous investigators have attempted to control the precursor structure and related solution chemistry effects with varying degrees of success, to influence subsequent processing behavior, such as crystallization tempera-ture.40-42,78,109 110 Particular attention has been given to precursor characteristics such as structural similarity to the desired product and the chemical homogeneity of the precursor species. For multicomponent films, this latter factor is believed to influence the interdiffusional distances associated with the formation of complex crystal structures, such as perovskite compounds. Synthetic approaches have been geared toward the preparation of multimetal species with cation stoichiometry identical to that of the desired crystalline phase.40 42 83 84... 

Several diynyl complexes react with other metal substrates, the proximity of the a-bonded metal to a multimetal system often resulting in further cluster condensation. 

Rasmussen, S.C., Richter, M.M., Yi, E., Place, H. and Brewer, KJ. (1990) Synthesis and characterization of a series of novel rhodium and iridium complexes containing polypyridyl bridging ligands Potential uses in the development of multimetal catalysts for carbon dioxide reduction. Inorg. Chem., 29, 3926—3932. 

The salts with organic cations are important, since they can be used for the synthesis of heteronu-clear multimetal complexes in organic solvents. Most of them can be obtained simply from aqueous solutions of their alkali metal salts. 

Thioanions have remarkable ligand properties, e.g. versatile coordination behavior and very low-lying unoccupied orbitals. Thus, in general, complexes show strong metal-ligand interactions. Multimetal complexes of this type exist with a variety of electron populations and some with... 

Table 1 Multimetal Complexes with Thiometalate Ligands (Averaged M—S and M [ M Distances in Ay ...

Figure 4 Examples of multimetal complexes in the system M +/M04 aSj /PPh3 (n = 3, 4 M = Cu, Ag M = Mo, W) (PPh3)3Cu2(MoS4), (PPh3)4Ag2(MoS4), (Cu3MoS3a)(PPh3)3S and (Ag4Mo2S6XPPh3)4S2...

Rapid development of this area followed the discovery of routes to these complexes, either by ready conversion of terminal alkynes to vinylidene complexes in reactions with manganese, rhenium, and the iron-group metal complexes (11-14) or by protonation or alkylation of some metal Recent work has demonstrated the importance of vinylidene complexes in the metabolism of some chlorinated hydrocarbons (DDT) using iron porphyrin-based enzymes (15). Interconversions of alkyne and vinylidene ligands occur readily on multimetal centers. Several reactions involving organometallic reagents may proceed via intermediate vinylidene complexes. 

Salt-elimination reactions also produce multimetal complexes, e.g. 73 (equation 67)221. 

A limited number of thermal redistribution reactions are reported, involving redistribution at both the tin and metal centers of the Sn-M complex (equations 136 and 137)329-331. The product in equation 137 contains a cyclic Os3Sn3 skeleton. A similar multimetal product 109 was obtained by the pyrolysis of the stannylene complex [/71-Cp2SnFe(CO)4]2332. 

The potential for CO insertion in an sp-hybridized carbon melal bond may exist in multimetal complexes in which stabilization and reactivity enhancement could be assumed (9. 57, 58, 59). The different type of bonding in IIOs>(CO)9 (CNBu )COR and HOsafCOliofCOR) has already been mentioned... 

Another multimetal complex is one of the first examples of C-H insertions with a transition metal. The iridium starting material is mononuclear, although the final product is a dimer. 

The synthesis of multimetallic transition metal complexes where the metals are held at specific distances from each other is an important objective because of their potential role in multimetal-centered catalysis in both biological and industrial reactions (1). Moreover, such systems, through cooperative electronic and/or steric effects between metal centers, might give rise to distinct reactivity patterns for both their stoichiometric and catalytic reactions, which are not available to their monometallic analogues (2). Of the ligands that are able to maintain the metal centers in close proximity, the pyrazolate ion (pz ) appears to be a particularly suitable candidate. Pyrazoles (Hpz ) are weak bases (3, 4) and behave as 2-monohapto ligands. 

Binary pyrazolate complexes containing divalent metals bridged by two pz groups are known for many transition metals. The increasing interest for this class of complexes stems from the expectation that they may provide useful insights in the field of magnetostructural correlations as well as in multimetal centered catalysis. 

A large number of synthetic multimetal complexes have been prepared as potential photoinduced electron-transfer systems [53]. In these designs the metal complexes can act both as structural templates and as photoactive units in their own right. 

---