Heterobimetallic system

Heterobimetallic systems have not been investigated extensively. An astonishing result was, however, observed when treating osnnium(VIII) oxide with a chromium(0)-NHC complex OSO4 added stoichiometrically to the C=C double bond of the NHC instead of oxidizing chromium(O) [Eq. (38)]. ... 

Treatment of the mononuclear complex Au(Spy)(PPh2py) with [Cu(NCMe)4](PF6) resulted in the isolation of the heterobimetallic system [AuCu( i-Spy)(p.-PPh2py)]2(PF6)2 417. Its tetranuclear cation can be considered as a dimer resulting... 

In the case of the heterobimetallic systems, the few examples known so far are [ClPt(p-dppm)2(p-C=N-Me)Ni(CNMe)]Cl (64), reported by Kubiak and collaborators, which is obtained via a transmetallation of [(CNMe)]Ni (p,-dppm)2(p,-C = N-Me)Ni(CNMe)] with [Pt(dppm)Cl2].80 The other examples are the mixed M-Pt systems 65—74 (M = W, Fe, Mo, Cr) containing a dis-phosphine backbone (dppm or dppa), as described by Knorr and collaborators81 -84 (Fig. 31). These complexes were obtained via substitution of the bridging carbonyl by an isocyanide ligand. 

Until the mechanism of formation of heterobimetallics, such as 15, in these one-step reactions is understood better, it is difficult to devize a reliable general synthesis of this type. Consequently, our attempts to synthesize such systems by this method have b n somewhat hit and miss. Thus, our as yet limited attempts to produce heterobimetallics by co-reduction of Co and metal ions other than Rh have so far been unproductive. Such a system has, however, been produced (37) from a related Ni°/Cu° reaction. In this reaction, an Ni°/dppm mixture was treated in the usual manner with NaBH3CN (reactant ratios 1 3.6 4.8)) under CO with an addition time of 10 minutes. The mixture was stirred for two hours after which CuCl2 (1 molar equivalent) was added. From this mixture was obtained 16 (66% yield, P NMR AA XX pattern, 5Ni-P 23.0, 8Cu-P -17.1 (v. broad signals). IR Wco 2000, 1958, Vcn 21W cm ). As will be seen shordy, complex 10 is a probable intermediate in this reaction. An X-ray crystal structure has shown that in the solid state, the molecule has the cradle-like geometry shown in 16. While this is a heterobimetallic system, it is of less interest than homo- and heterobimetallic systems such as 2, 3, 11 and 15 since the metal-metal bond which is so useful in reactions which mimic those which take place on metal surfaces is absent. 

In 1987 we published the first review on the magnetism of heteropoly-metallic systems (3). This article was largely devoted to the mechanism of the interaction in heterobinuclear complexes. The field of heteropoly-metallic compounds was covered up to the first attempts to obtain molecular-based magnets. The very last sentence of this article was One may anticipate that in a few years another review will be necessary, and that its main emphasis will be on systems of higher nuclearity and extended systems with subtle spin orders. In the present case, our prediction was quite correct, which is not so frequent in science. The field of heterobimetallic systems has developed tremendously since 1987, particularly in relation to the synthesis of molecular-based magnets. It is now timely to write a second review concerning this topic. 

Scheme 11 Heterobimetallic system for intermolecular allylic oxidative amination...

Since these initial publications, the considerable efforts of Shibasaki and coworkers have lead to lanthanide element-binaphtholato complexes as being the most developed and potent of asymmetric phospho-aldol catalysts. From their early work on catalysis in the nitro-aldol reaction [28], Shibasaki and co-workers have published widely and in considerable detail on the use of hetero-bimetallic catalysis, developing the field to such a degree that enzyme-like comparisons have been made. Much of the success of the Shibasaki team in hetero-bimetallic catalysis has been reviewed recently [29], the key to which has been the delineation of the solid state structures of the catalytic precursors via single crystal X-ray diffraction studies [30] (Fig. 1) and the development of improved synthetic routes to this class of heterobimetallic system (Scheme 12) which emphasise the important role of added water [31]. 

This section will focus on the reactivity and catalytic applications of homobimetallic iridiiun complexes containing the two metal centers in close proximity, purposely excluding examples of heterobimetallic systems that contain an iridium center, since their rich chemistry makes it impossible to deliver a comprehensive description within the scope of this chapter. However, for the interested reader, catalytic cooperativity in heterobimetallic complexes has been recently reviewed [69]. 

From an in situ spectroscopic and chemometric view point, the lack of measurable quantities of Rh2(CO)g and Rh6(CO)ig in heterobimetallic CBERs is one of the most obvious differences with homometallic rhodium hydroformylations. This is clearly related to the very aggressive nature of the hydrides HMn(CO)5, HRe(CO)5, HWCp(CO)3 and HMoCp(CO)3 towards higher nuclearity rhodium carbonyls. It also suggests that in the heterobimetallic systems, there is slightly better utilization of rhodium due to the deduced concentrations of these di- and multinuclear reservoirs. 

Linkers of the tetraphenylmethane type, such as depicted in Figure 2a, shield the phosphine groups and their coordinated metal complexes from the surface by the rigidity of the backbone. Such ligand systems allow the investigation of cooperative effects, even with heterobimetallic systems. These materials are also well suited to study the mobility of the surface species. 

A heterobimetallic system has been observed by Geofiroy s group to oxidatively add and reductively eliminate H2 in two ways, reactions (96) and (97)/ ... 

This chapter seeks to update the recent literature concerning organometallic compounds of Al(III) [1-19]. Heterobimetallic systems are essentially excluded from the present contribution, even where there is no inter-metal interaction and the aluminum is in the +111 oxidation state. Some of these aspects are discussed in detail in [286]. Whilst this eliminates the extensive subsection of heterometallic aluminum compounds known as ate complexes, these have been reviewed elsewhere [20]. 

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