Bimetallic complexes elimination from

Stable zirconium, platinum, molybdenum, and tungsten complexes of cyclooctyne, a zirconium complex of cydoocta-5-enyne, and a bimetallic molybdenum complex of cyclocta-3,7-dienyne have been discussed in earlier reviews.28 More recently, two stable zirconocene complexes of cycloocta-trienyne (275 and 276) have been prepared101 by /3-hydride elimination from 274 in the presence of PMe2R [Eq. (45)]. 

The dissociation pathway particular to the bimetallic complexes 99 and 100 is methane elimination from the decarbonylated ions... 

Addition of a toluene solution of the cuprate, Cu2Li2(p-tolyl)4-2Et20 (1), to a solution of palladium(II) acetate in toluene leads to the in situ formation of a thermally unstable organocopper complex in which the lithium atom of 1 has been replaced by the more electronegative precious metal. Subsequent reductive elimination of the organic tolyl group from the unstable bimetallic complex in the presence of silica affords supported bimetallic particles, which without further treatment are an active catalytic system. (Equation 3)... 

Carbon-heteroatom reductive elimination from dinuclear transition metal complexes, as was proposed by us [96,109] as the product-forming step in Pd-catalyzed C-H acetoxylation and chlorination reactions, is rare. The two formulations of the high-valent, dinuclear Pd intermediate in arylation proposed by Sanford (60 and 61) highlight that reductive elimination from dinuclear Pd structures could, in principle, proceed with either redox chemistry at both metals (bimetallic reductive elimination reductive elimination from 60) or with redox chemistry at a single metal (monometallic redox chemistry reductive elimination from 61). While structures 60 and 61 do not differ in composition, they do differ in their respective potentials for metal-metal redox cooperation to be involved in C-C bond-forming reductive elimination. 

In 2009, this possibility was realized by Ritter and coworkers. The two-electron oxidation of dipalladium(II) compound 148 at low temperature (-30 °C) afforded the dipalladium(lll) compound 149 with significant Pd-Pd distance contraction from 2.84A in 148 to 2.57A in 149 (Entry 1, Table 10.9) (Scheme 10.68) [108]. The existence of a Pd(III)-Pd(III) bond was further proven by the diamagnetism of 149, which was the result of spin pairing of two d Pd(III) centers. Warming 149 to ambient temperature led to bimetallic reductive elimination to form a C-Cl bond, along with unidentified Pd(II) species. This was the first clearly defined example of carbon-heteroatom reductive elimination from a binuclear transition metal complex, and created a new horizon of palladium organometallic chemistry based on synergetic Pd(III)-Pd(III) bond [113]. 

Thermal reductive eliminations are faster for di-gold complexes and show evidence for bimetallic cooperation. Moreover, reductive elimination from a dibromide complex is accelerated by buildup or addition of the corresponding product. The authors do not speculate on the microscopic origin of any metal-metal cooperation, but aurophilic assistance is a plausible hypothesis. 

Electron-richer dM compounds can also be considered as H2-activating alternatives to compounds with the unfavorable dM configuration. In the case of the bis-dppm bridged Rh(I)Ir(-I) complex 14, the d d configuration has been found to result in a metal-metal bonded species in which the coordination around the rhodium center is similar to that in planar homovalent d compounds. [47] The kinetic product of dihydrogen addition to 14 is consistent with the occurrence of a single-metal oxidative addition to the Rh(I) (Scheme 12). This kinetic product is thermally unstable and reductively eliminates methane from the iridium center. The overall reaction constitutes a clear example of bimetallic cooperation, since the oxidative addition to one center provokes a reductive elimination in the other metal. 

Fig. 18. Such relatively high barrier can be ascribed to a closed-shell bimetallic core PtAu". The subsequent hydrogen transfer results in 9 with a barrier of 33.8 kcal moP Association of two hydrogen atoms in 9 gives a dihydrogen complex 10. This process is endothermic by 18 kcal moP The elimination of D2 from 10 leads to the ion product 11, requiring an energy of 2.4 kcal mol As Fig. 20 and Table 8 show, the overall reaction has an endothermicity of 8.6 kcal moP and free energies of reaction AG of 7.1 kcal moP (298.15 K).

If high chloride concentrations are present in these reactions, the selectivity of the second step is strongly increased for the formation of C-Cl bonds, and P-hydride elimination is inhibited. This effect seems to originate from the saturation of the Pd coordination sites by the Cl and consequent inhibition of hydrogen coordination and thus prevention of the ehmination reactions [108]. The observation that the use of Pd pyridine complexes favored chlorination even at lower chloride concentrations gave rise to an asymmetric chlorohydrin synthesis from allyl ether 154 with the use of chiral bimetallic Pd complex 156 (Scheme 16.42). 

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