Bimetallic cooperativity

Even more efficient bimetallic cooperativity was achieved by the dinuclear complex 36 [53]. It was demonstrated to cleave 2, 3 -cAMP (298 K) and ApA (323 K) with high efficiency at pH 6, which results in 300-500-fold rate increase compared to the mononuclear complex Cu(II)-[9]aneN at pH 7.3. The pH-metric study showed two overlapped deprotonations of the metal-bound water molecules near pH 6. The observed bell-shaped pH-rate profiles indicate that the monohydroxy form is the active species. The proposed mechanism for both 2, 3 -cAMP and ApA hydrolysis consists of a double Lewis-acid activation of the substrates, while the metal-bound hydroxide acts as general base for activating the nucleophilic 2 -OH group in the case of ApA (36a). Based on the 1000-fold higher activity of the dinuclear complex toward 2, 3 -cAMP, the authors suggest nucleophilic catalysis of the Cu(II)-OH unit in 36b. The latter mechanism is comparable to those of protein phosphatase 1 and fructose 1,6-diphosphatase. 

The above complexes have been shown to mimic the second step of RNA hydrolysis as well, i.e. the-efficient cleavage of ribonucleoside 2, 3 -cyclic monophosphates [55] with bell-shaped pH-rate profile. With these substrates 37 showed much higher bimetallic cooperativity the dime/2 m0nomer ratios range between 64 and 457 for the different 2, 3 -NMPs used, while for 38 this ratio varies between 1 and 26. Since the mononuclear complexes have nearly the same activity toward the different 2, 3 -NMPs, these kinetic data indicate a notable base-selectivity of the dimer complexes. Since no correlation was observed with the size,... 

Even more interesting is the observed regioselectivity of 37 its reaction with 2, 3 -cCMP and 2, 3 -cUMP resulted in formation of more than 90% of 2 -phosphate (3 -OH) isomer. The postulated mechanisms for 37 consists of a double Lewis-acid activation, while the metal-bound hydroxide and water act as nucleophilic catalyst and general acid, respectively (see 39). The substrate-ligand interaction probably favors only one of the depicted substrate orientations, which may be responsible for the observed regioselectivity. Complex 38 may operate in a similar way but with single Lewis-acid activation, which would explain the lower bimetallic cooperativity and the lack of regioselectivity. Both proposed mechanisms show similarities to that of the native phospho-monoesterases (37 protein phosphatase 1 and fructose 1,6-diphosphatase, 38 purple acid phosphatase). 

This observation led to the proposal that tethering two rhodium centers together via the bisphosphine ligands was producing some sort of bimetallic cooperativity between the two metal centers. An intramolecular hydride transfer, analogous to the intermolecular hydride transfer proposed by Heck (Scheme 1), enhanced by the proximity of the two metal centers, seemed a very likely possibility. 

This led Stanley to propose the bimetallic cooperativity mechanism shown in Scheme 16. The key bimetallic cooperativity step involves rotation about the central bridging methylene group to give a H /CO double-bridged intermediate (or transition state) species, which directly leads to an... 

These studies indicate once more the importance of multimetallic cooperative effects for desulfurization to be accomplished. In another clear case of bimetallic cooperative C-S bond activation, Bianchini and coworkers have provided very pertinent information involving a complex containing a Group 8/Group 6 metal couple (RhAV). The reaction of BT with [(triphos)RhH] leads to ring-opening followed by hydride transfer as shown in Eq. 4.18 ... 

Two different binuclear copperdi) complexes have been prepared recently, one with a bridging phenoxy ligand having two bis-benzi-midazole arms (12, Fig. 14), and the second having a bis-cyclen-naphthalene ligand (13, Fig. 15) (352, 353). Both of them show bimetallic cooperativity for the hydrolysis of phosphate diesters, contrary to studies with the dinuclear cobalt complex (354). The pseudo-first-order rate constants for hydrolysis of the para-nitrophenylphosphate ester of propylene glycol by bis-benzimidazole-based copper complexes... 

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. 

Another case of bimetallic cooperative C-S bond activation involves the reaction the triphos-Rh BT-derived metallacycle with W(CO)s to yield the heterobimetallic sulfur-bridged species 47, which upon thermolysis under H2 (30 atm) induced HDS of BT to ethylbenzene, plus [(triphos)RhH(CO)] together with an insoluble W-S material (Equation (9)). This shows that the S-bridged Rh-W couple switches the reactivity from hydrogenolysis... 

Our work into bimetallic cooperativity in homogeneous catalysis has concentrated on the binucleating tetraphosphine ligands meso- and racemic-et,ph-P4, shown in Scheme 3 [25, 26]. These ligands are designed to chelate two metal centers via a single, conformationally flexible, methylene bridge. 

The following observations support the proposed bimetallic cooperativity. Model monometallic [Rh(nbd)(P2)](BF4) (P2 = Et2PCH2CH2PEt2,... 

Further persuasive evidence for bimetallic cooperativity came from bimetallic model systems where the central methylene group in the et,ph-P4 ligand has been replaced by p-xylylene (6) or propyl groups (7), thus limiting the ability of the two rhodium centers to interact with one another. 

These spaced bimetallic precursors, 6 and 7, are also extremely poor hydroformylation catalysts (1/2-6 tumovers/h, 3 1 linear to branched aldehyde regioselectivity, 50-70% alkene isomerization and hydrogenation side reactions). Complex 7, however, is about three times faster than the monometallic analogs, consistent with Sanger s proposal (vide supra) of some bimetallic cooperativity... 

The proposed bimetallic hydroformylation mechanism for 15r is shown in Fig. 13 and is based on our DFT calculations. In many ways the mechanistic steps parallel those for the dicationic catalyst, and bimetallic cooperativity once... 

But all the monometallic catalysts deactivate before they can complete the 1,000 turnovers (100% conversion of alkene). This is tied into Rh-induced phosphine orthometalations and P-Ph or P-benzyl bond cleavage reactions that lead to monometallic catalyst deactivation [4], Bisbi is especially susceptible to deactivation under these conditions, barely making it past 50% conversion of the alkene. The mainly alkylated et,ph-P4 ligand does not seem to suffer from Rh-induced phosphine fragmentations under these conditions, but does tend to lose a rhodium center and fragment losing the bimetallic cooperativity. 

This review is not intended to be fully comprehensive but instead should serve to highlight current understanding of bimetallic cooperative catalysis as it applies to the activation of the alkyne triple bond. We have divided the review into four sections, separated by reaction type, which emphasise different aspects of the bimetallic alkyne activation mechanism. These four sections are as follows ... 

The bridge between the two metal centers is too large in ligands 6 and 7, which prevents the cooperation between the metal centers. Indeed activities and selectivities were found that were similar to the monometallic species. Another important indication supporting this bimetallic cooperativity was an... 

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. 

Finally, the efficient Negishi coupling of unactivated aryl, heterocyclic, vinyl chlorides as well as aryl dichlorides catalyzed by binuclear and mononuclear NHC-Ni complexes was recently described by Chen and co-workers (Equation (10.18)). Elaborated heteroarene-functionalized NHC were found to be highly effective for the preparation biaryls and terphenyls in good to excellent yields under mild conditions. Notably, the binuclear nickel catalysts showed higher activities than mononuclear analogues, probably because of a bimetallic cooperative effect. 

In the hydrolysis of dinucleotides, bimetallic cooperation of Zn(II) with Sn(rV), In(III), Fe(III) or Al(III) leads to an increased rate of reaction, and a mechanism was proposed for the cooperactivity. The same group have also shown that a dinuclear Zn(II) complex efficiently cleaves ApA at S0°C and... 

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