Binuclear breaking

A mechanistic study of acid and metal ion (Ni2+, Cu2+, Zn2+) promoted hydrolysis of [N-(2-carboxyphenyl)iminodiacetate](picolinato)chromate (III) indicated parallel H+- or M2+-dependent and -independent pathways. Solvent isotope effects indicate that the H+-dependent path involves rapid pre-equilibrium protonation followed by rate-limiting ring opening. Similarly, the M2+-dependent path involves rate-determining Cr-0 bond breaking in a rapidly formed binuclear intermediate. The relative catalytic efficiencies of the three metal ions reflect the Irving-Williams stability order (88). 

The formation of p-hydroxo diphosphine complexes by protonolysis of the (5-keto chelates with water (Figure 7.9) is another factor that contributes to a decrease in the copolymerisation activity (Scheme 7.28) [5f]. In general, the p-hydroxo complexes can re-enter the copolymerisation cycle by reaction with CO that breaks the binuclear structure to give Pd-H via Pd-C(0)0H [5a, 13, 36]. The contribution... 

Nitric oxide binds to the Cu(II) ion in the binuclear center of fully oxidized cytochrome oxidase (Brudvig et al., 1980). The binding of NO creates an even spin copper center and effectively breaks the spin coupling between the heme and copper metal ions. As a result, the high-spin heme EPR signal is visible at g = 6. 

Metal-Metal Bond Making and Breaking in Binuclear Complexes with Phosphine Bridging Ligands... 

The flexibility of the bridging dpm ligand allows for considerable variation in the metal-metal separation. The range of metal-metal separations runs from 2.138(1) A in the quadruple metal-metal bonded Mo2(dpm)2CU (13) to 3.492(1) A in the A-frame Pd2(dpm)2(y-C2 CF3 2)2C12. (14,15 ) This flexibility allows metal-metal bond making and breaking to occur without other disruptions within a variety of binuclear complexes. 

No dimers were formed in the controlled thermal decomposition a binuclear ir complex Co2(CO)4(C7Hg)2(57). However, when the decomposition of this complex was performed in the presence of AlBrs, Binor-S was obtained in almost quantitative yield (52). Interaction of the complex with AlBrs may cause breaking of the bridging carbonyl groups and thus cause free rotation in the Co-Co axis, which may be necessary to obtain Binor-S selectivity. Similarly, CosfCOls becomes an effective Binor-S catalyst in the presence of as little as 0.5-1 mole of AlBi s or BFs O(C2H5)2 per mole of the carbonyl. These catalysts are very active and produce Binor-S essentially with quantitative conversion and selectivity (52). Other metal carbonyls and their mixtures with Lewis acids were evaluated as well. Low conversion to dimers other than Binor-S took place in some instances in the majority of cases, however, polymerization of norbornadiene occurred. The catalyst systems studied included Ni(CO)4, Fe(CO)5, Mn2(CO)io, Cr(CO)g, Mo(C0)fl, and W(CO)g in combination with BPg O(C2H5)2 cocatalyst (52). 

A second example of a unique transformation in a bimetallic system is the observation of facile carbon-carbon bond making and breaking reactions in the binuclear complexes shown in reaction 1 ... 

Reaction kinetics and bonding energies are additional tools useful for examination of ligand bonding to oxide surfaces. Non-specific bonds should result in rapid desorption reaction kinetics, while monodentate and bidentate and binuclear bonds should be correspondingly more difficult to break. 

On treatment with sodium amalgam, followed by methyl iodide, (azulene)-Mo2(CO)6 gives a complex of the unexpected formula [CH3Mo(CO)3azu-lene]2, which apparently contains one azulene ring per metal atom (93). Presumably, sodium breaks the bond between metal and seven-membered ring, giving a binuclear, substituted (CH3)(7r-cyclopentadienyl)Mo(CO)3... 

Table V represents the relationship betwenn half wave potentials and acidity for the binuclear complexes with a completely different sequence of protonated compounds. All of these structures are different from type I described above and they cannot be oxidized to the tin (IV) chelate without bond breaking and bond forming. This explains the higher stability of the binuclear chelates in acidic solution since their rates of oxidation are determined by the rates of rearrangement to the octahedral or pseudo-octahedral configuration (type I) of the mononuclear compound. Additional proof for this unsymmetric protonation sequence has been the isolation of several compounds which are presently being studied and one of which has been identified as H Y.SnCl, 2 H,0.

The simplest mechanism for CO migration in a tetranuclear carbonyl is represented on Figure 25. This process involves a interconversion of the M4(CO)i2 clusters. There is a simple bridge-break, bridge-make mechanism of the type employed in binuclear complexes. 

Human prolidase is a 54-kDa binuclear Mn+ -dependent enz)une that breaks the amide bond in dipeptides containing proline or hydroxyproline as the C-terminal amino acid. It plays a crucial role in the recycling of proline. Prolidase is found in most tissues and in several animal species. Deficiency of this enzyme in humans results in a syndrome with a highly variable clinical phenotype, such as chronic recurrent infections, mental retardation, splenomegaly, skin lesion, and the excretion of massive amounts of iminopeptides in urine (Wang et al., 2006 Chandrasekaran et al., 2013). 

For/> 1 the chain acquirs net charge, it expands and water molecules break down the binuclear complexes. This results in complexes in which each Cu ion is surrounded b two carboxylate groups and additional H2O molecules. In this, more symmetrical ligand field the transitions leading to the observed UV/visible absorbtion are less intense the selection rule forbidding these transitions is less strongly perturbed. 

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