Early-late heterodinuclear complexes

Figure 4.5 Unsupported group 4 metal early-late heterodinuclear complexes stabilized by tripodal amido ligands.

The same strategy and ligand sets as described above for the synthesis of early-late heterodinuclear complexes involving group 4 metrJs have been recently successfully used for the preparation of stable U-Ru heterobimetallic compounds (see Chapter 3) [24]. 

Basic Patterns of Reactivity Observed for Metal-Metal Bonded Early-Late Heterodinuclear Complexes... 

The insertion of isocyanides provides another example of this reactive behavior. Insertion reactions of isocyanides and heteroallenes carried out with the tripod-stabilized early-late heterodinuclear complexes (represented in Figure 4.5) that led to stable products are summarized in Scheme 4.8. 

Scheme 4.13 Mechanistic scheme of a nonsymmetrical activation of H, across a polar metal-metal bond in an early-late heterodinuclear complex (containing a bridging ligand) and the subsequent hydrigenation of coordinated CO.

Scheme 29 Preparation of early-late heterodinuclear complexes as precatalysts for double hydrophosphinylation of alkynes...

Mizuta and coworkers have synthesized early-late heterodinuclear transition metal complexes Cp2M(/r-OPPh2)2PdMe2 (M=Ti (14), Zr (15), and Hf (16)) (Scheme 29) and found that 15 and 16 catalyze, highly efficiently when a phosphine ligand (PPh2Me, PPhMes) is added, double addition of HP(0)Ph2 to 1-octyne to mainly give l,2-bis(diphenylphosphinyl)octane 17 in >95% yield under mild conditions (typically 5 mol% catalyst, 40°C, 1 h) (Scheme 30) [36]. The activity decreases in the order of Hf > Zr > Ti, which is partially associated with the... 

The hydrosilylation of ethylene by the early-late transition-metal heterodinuclear complexes [CpTa( t-CH2)2lr(CO)2] has been studied mainly in a bid to recognize the mechanism of reaction, which occurs via a predominant alkene/Ir—H insertion pathway over a minor insertion of ethylene into the Ir—Si bonds [21]. 

Rapid desulfurization of thiiranes hy heterodinuclear complexes is achieved by using an early-late dinuclear transition-metal complex. Cp2Zr(/r-N Bu)IrCp reacts with thiiranes in almost diffusion controlled rates to give /r-sulfido complex and corresponding olefin with retention of configuration at the alkenyl carbon center (Scheme 3.68) [132]. Two mechaiusms such as completely concerted sulfur transfer reaction and insertion of the C-S bond into the Zr-Ir bond followed by concerted four-center ehmination are proposed. 

In the simplest case, heterodinuclear complexes will react as pairs of metal electrophiles and nucleophiles. Cooperative reactivity is particularly likely with polar substrates. In this case, the electropositive early transition metal center may react with the more Lewis-basic part of the substrate, while the nucleophilic late transition metal complex fragment will attack the more Lewis-acidic part of the substrate molecule. For metal- metal single bonds between the two metal centers, this interaction wiU go along with the cleavage of the metal-metal bond. In many cases, it is difficult to establish whether the scission of the metal-metal bond precedes the transformation of the substrate or whether it occurs at a later stage. 

---