Cleavage mechanism, dinuclear complexes

In dinuclear complexes containing a metal-metal single bond, the major consequence of irradiation is homolytic cleavage to form reactive metal-carbonyl radicals. Many simple substitutions can be explained by a mechanism involving thermal substitution of these radicals. This mechanism is especially helpful in explaining the formation of a disubstituted product ... 

The simplest mechanism, and the one adopted in the papers mentioned, was that of bridge cleavage, i.e. successor-complex dissociation. Consequently, the observed first-order rate constants for the disappearance of dinuclear complex were denoted A , and at 25 °C 10 Are/s" = 22.1, 5,4, and 3.17 for Y = cydta, edta, and pdta respectively. 

Carbon-carbon bond formation by reductive elimination from Ni, Pd, or Pt complexes is widespread. In many cases it is presumed to occur as the final step in a catalytic cycle, whereby the organic product is expelled from the metal center, but in others it is a well-defined, mechanistically studied reaction. Elimination takes place from Ni, Pd, and Pt complexes in their - - 2 or + A oxidation states, and it may be promoted by thermolysis, by photolysis, or by nucleophilic attack at the metal center. The reaction may proceed by heterolytic or homolytic metal-carbon bond cleavage, reductive elimination, or dinuclear elimination, and more than one mechanism may operate. 

Essentially no incorporation of deuterium label was observed when the same reaction was carried out in toluene-dg. Since the formation of more than one equivalent of CO(n) hydroxide requires the transfer of hydrogen atoms, we hypothesized that the mechanism of this transformation involved the formation of a dinuclear p-peroxo complex, followed by rapid cleavage of the 0-0 bond and hydrogen atom abstraction from the Tp-ligand. The inferred activation of the primary C-H bonds of the rm-butyl groups of the ligand requires a highly reactive species thus the "activation" of O2 seems to have been accomplished. 

Another dinuclear Zn complex with A(,A/ ,A(, N -tetrakis[(2-pyridyl) methyl]-2-hydroxy-l,3-diaminopropane was shown to be able to achieve phosphodiester cleavage of diribonucleotide ApA at 50°C and neutral pH 355) (16, Fig. 17). The hydrol5dic cleavage of ApA was significantly accelerated by the cooperation of two metals in the binu-clear complex because the mononuclear complex was inactive in comparable experimental conditions. The pseudo-first-order rate constant is 1.9 X 10 sec (half-life 10 hr) (ApA, 100 /aM ZnCL, 5 mM ligand, 2.5 mM). Deprotonation of 2 -OH by a metal hydroxo would be involved in the mechanism of reaction because the products of cleavage were adenosine, adenosine 2 - or 3 -phosphate, and 2, 3 -cAMP. 

Kinetic studies of the oxidation of a variety of mono- and dinuclear ruthenium(II) amine complexes by S20 have been interpreted on the basis of a mechanism including ion-pair formation followed by a one-electron transfer reaction involving the reductive cleavage of the peroxide bond in S2OI". The reaction of the dinuclear species [Ru2(NH3)iopz]" yields, as kinetically controlled products. 

A DFT study of the cleavage of a DNA model, p-nitrophenyl methyl phosphate (47), and two RNA models, p-nitrophenyl 2-hydroxypropyl phosphate (48) and phenyl 2-hydroxypropyl phosphate (48 H for NO2), promoted by the dinuclear Zn(II) complex of l,3-bis(l,5,9-triazacyclododec-l-yl)propane formulated with a bridging methoxide (49), was undertaken to determine possible mechanisms for the tfansesterification processes that are consistent with experimental data. The initial substrate-bound state of... 

Fig. 24.27 (a, b) Mechanism for catalytic phosphate diester cleavage by the synergetic action of two Cu(II) centers in a dinuclear cis-diaqua Cu(II) calix[4]arene enzyme model, (c, d) Schematic representation of possible mechanisms for Zn(II)-mediated HPNP and GpG cleavage, (e, f) Proposed mechanism of HPNP cleavage by 1,2-vicinal bimetallic copper(II) complex and model for cooperative binding and selective eleavage of UpU and UpG by a 1,2-vicinal coppeT (II) eomplex [84-89]... 

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