Heterobimetallic complexes early-late

Casey, C.P., Jordan, R.F. and Rheingold, A.L. (1983) Metal-metal-bonded zirconium-ruthenium and zirconium-iron complexes. /. Am. Chem. Soc., 105, 665-667, and references therein. For more recent reviews see (a) Stephan, D.W. (1989) Early-late heterobimetallics. Coord. Chem. Rev., 95, 41-107. (b) Wheatley, N. and Kalck, P. (1999) Structure and reactivity of early-late heterobimetallic complexes. Chem. Rev, 99, 3379-3419. 

Structure and reactivity of early-late heterobimetallic complexes ... 

Finally, the main application of zirconium phosphanido complexes is to use them as metalloligands for coordination with the late transition metals to form phosphoms-bridged early-late heterobimetallic complexes. 

Fig. 4. Early-late heterobimetallic complexes with unsupported metal-metal bonds stabilized by tripodal amido ligands.

Ligands bearing phosphonato functionalities are useful in coordination chemistry. Early-late heterobimetallic complexes can be prepared with the presence of functionalized bridging ligands bearing P=0 unit. The compound shown in Scheme 706 has been prepared.1775... 

Abstract By combining an ever-increasing number of catalysts or catalytic functions, cooperative catalysis is a research area that grows fast. In the field, early-late heterobimetallic complexes are rather old objects but they still continue to fasdnate chemists because of their latent reactivity. After a brief and ccaicise overview of cooperative catalysis, this review focuses on early-late heterobimetallic complexes that were used in catalysis over the last decades. Examples of dual catalysis using early and late metal partners are also described. This chapter ends with an opening towards therapeutic applications of early-late heterobimetallic complexes. 

Early-Late Heterobimetallic Complexes as New Anticancer Agents. 179... 

The combination of hard oxophilic early transition metals and soft nucleophilic late transition metals with opposite functionalities, provided they do not inhibit one another, is a priori ideal for promoting cooperative effect. A proof of concept can be found in the stoichiometric reactivity of early—late heterobimetallic complexes featuring a metal-metal bond [76]. It has been shown that such complexes are good candidates to realize the heterolytic cleavage of a bond in polar and apolar substrates. An illustrative example by Bergman et al. is the reaction of the Zr-lr complex 20 with carbon dioxide which leads to the rupture of the metal—metal bond (Scheme 18) [77]. The CO2 insertion occurs in the expected fashion with the CO2 bridging the two metals, the carbon atom coordinated to iridium, and the oxygen atom on the zirconium center. 

In a pioneering paper [97], Osakada has reported ethylene polymerization trials with a series of early-late heterobimetallic complexes, including a Co/Zr combination, and showed that some of these complexes enable the enchainment of the a-olefin or of the oligomer formed at the late metal center to the polymer grown at the Zr center. The synthesis of the Co/Zr heterobimetallic complex 49 is elegant and involves as key step the Ru-catalyzed cross metathesis of an ansa-zirconocene complex 47 with an allyl substituent and a Co complex 48 having a pendant acrylate (Scheme 31). In the solid state, both Zr and Co atoms were found far away from... 

The first example of application of a chiral early-late heterobimetallic complex in asymmetric catalysis was reported by Bomer in 1999 and deals with hydroformylation of activated olefins (Scheme 39) [122]. The chiral bimetallic complex 66 was generated in situ by reacting the (R,R)-Diph-salenophos-Ti(0 Pr)2 ligand (67) with [Rh(acac)(CO)2]. This complex gives rise to a diminished conversion in aldehyde (21 % vs 99%) as well as a lower selectivity (i n = 77/23 vs 99/1) with respect to the monometallic salenophos-Rh complex generated in situ from the free-metal ligand 68 and [Rh(acac)(CO)2] but affords the branched aldehyde with 30% ee. 

In related research, Kalck and Gautheron have been interested in carbonylation of ethylene in the presence of the early-late heterobimetallic complex 69 (Scheme 40) [123]. The latter was synthesized by addition of [Cp 2Zr(SH)2] to [RhCl2(CO)2]. The structure of 69 in the solid state has been determined by X-ray diffraction study. It shows a usual tetrahedral and planar geometry arotmd the Rh and Zr atoms, respectively, a planar ZrS2Rh core, and a rather long Rh-Zr distance of 3.161(3) A. Complex 69 promoted the direct carbonylation of ethylene with low efficiency (TON = 2) in the presence of PPha and triethyl orthoformate. The analogous reaction in the absence of triethyl orthoformate gave acrolein in... 

As we already mentioned earlier in this chapter, there have been a huge variety of multi-catalytic systems and cooperative effects that have been described. Among them, examples of the simultaneous use of separated early and late transition metal catalysts however remain somewhat scarce. Nonetheless (me could use these examples as an inspiration to design early-late heterobimetallic complexes with the hope to encompass performances of separated systems. 

In a fifth contribution, Pierre Le Gendre et al. review the catalytic performances of early-late bimetallic complexes. After an introduction dealing with bifunctional and cooperative dual catalysis in which the cooperative effects arise from two catalytic functions present in the same or two different molecules, this paper focuses on an inventory of early-late heterobimetallic complexes in catalysis. It includes those where the two metal centers belong to the same complex, with bridging ligands as in the following example in which the chiral information is provided by the titanium metal center ... 

The Chalk-Harrod type mechanism is proposed to be operative in the reaction of ethene with HSiMes, HSiEts and HSiPhs catalyzed by an early-late heterobimetallic complex, Cp2Ta(CH2)2lr(CO)2 . The proposed catalytic cycle is very similar to the one shown in Scheme 5, wherein [M] is Cp2Ta(CH2)2lr(CO), except for the dissociation of CO at the oxidative addition of HSiR3 and the coordination of CO at the reductive elimination of EtSiRs. This catalyst, however, only promotes the hydrosilylation of ethene, and higher alkenes are isomerized ". ... 

Metal-Metal Bond Polarity in Early-Late Heterobimetallic Complexes Involving Group 4 Metals... 

Early-late heterobimetallic complexes containing unsupported metal-metal bonds (M = Fe... 

This and subsequent extended Huckel MO calculations on various early-late heterobimetallic complexes [15] offered a qualitative analysis of the principal bonding interactions between the early and late met2il centers. However, although providing some valuable insights into the nature of this class of compounds, they were unsuitable for addressing issues such as metal-metal bond polarity even... 

S Early-Late Heterobimetallic Complexes of Croup 4 Metals as Potential Catalysts I 8S... 

There are a considerable number of reports of catalytic activity of early-late heterobimetallic complexes, which is believed to differ substantially from that of the mononuclear and/or homometal-lic reference systems. The hydrosilylation of ketones and the Kumada couplings catalyzed by the Zr-Co complex 20 have already been mentioned earlier. Previously, Fandos etaL reported the Ti/Rh heterometallic complex [(CjMejjTifp-OjjIRhfCODjjj], which they regarded as a molecular model for SMSI in TiOj-supported Fischer-Tropsch catalysts. In the presence of this material, CO was indeed reduced at 250 °C and 20 bar (H2 CO 3 1) to give, inter alia, methane and short-chain alcohols. However, neither the catalytically active species nor the importance of a possible bonding interaction between the metal centers has been established [39]. 

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