Bimetallic activation

The bimetallic mechanism is illustrated in Fig. 7.13b the bimetallic active center is the distinguishing feature of this mechanism. The precise distribution of halides and alkyls is not spelled out because of the exchanges described by reaction (7.Q). An alkyl bridge is assumed based on observations of other organometallic compounds. The pi coordination of the olefin with the titanium is followed by insertion of the monomer into the bridge to propagate the reaction. 

Towards Catalytic Relevance Bimetallic Activation of Acyl Ligands and Transition Organometallic Hydrides as Reducing Agents... 

Bimetallic activation of acetyl and alkoxyacetyl ligands — through formation of cationic P2 acyl complexes — to reaction with nucleophilic hydride donors was established. Cationic transition metal compounds possessing an accessible coordination site bind a neutral T -acyl ligand on another complex as a cationic P2 acyl system. These i2 3icyl systems activate the acyl ligand to reduction analogous to carbocation activation. Several examples of i2-acyl complexation have been reported previously. 

It should be added that MS-02 is not necessarily a mono-nuclear complex. It could be shown in a few cases that the catalytic activity of the metal ion is due to the formation of dinuclear metal-substrate complexes. Presumably in these species each oxygen atom of dioxygen coordinates to a different metal center. Such systems were extensively used to model the reactivity patterns of various enzymes containing a bimetallic active center. 

It has recently been established that carbozirconation with organylzirconocene derivatives evidently requires dipolar (and mostly bimetallic) activation and/or small-ring zirconacycles, especially three-membered ones, that can undergo cyclic carbozirconation (Generalization 6 ) [149]. One of the recent examples has been shown to involve both bimetallic and cyclic organo-zirconium species [150], These reactions can be either stoichiometric or catalytic in Zr. 

Electrocatalyst see also specific catalysts adsorbate-support interactions, 30 273-279 adsorption, 30 240-264 isotherms, 30 241-243 bimetallic activity, 30 275... 

In D2O, HD was found instead of 0-H2. It is presently assumed that binding of hydrogen to a metal ion in the bimetallic active site weakens the H-H bond sufficiently to enable this reaction. Oxidation of the hydride is expected to be a two-electron process, and hydrogenases should, therefore, contain a redox unit capable of accepting these two electrons simultaneously. I assume here that the bimetallic center plus the conserved proximal Fe-S cluster perform this task. 

Nickel-iron hydrogenases [NiFe] (Figure 8.2) are present in several bacteria. Their structure is known [22, 23] to be a heterodimeric protein formed by four subunits, three of which are small [Fe] and one contains the bimetallic active center consisting of a dimeric cluster formed by a six coordinated Fe linked to a pentacoordinated Ni (III) through two cysteine-S and a third ligand whose nature changes with the oxidation state of the metals in the reduced state it is a hydride, H, whereas in the oxidized state it may be either an oxo, 0, or a sulfide,... 

Although bimetallic active centers with activator molecules complexed to the transition metal were suggested and observed, experimental evidence indi-... 

Likewise, at elevated temperatures complex 21 promotes the stoichiometric cleavage of unactivated amides (e.g., dmf) or the hydrolytic ring opening of cyclic esters (e.g., butyrolactone. Scheme 4). These reactions are not catalytic, however, since the formed carboxylate in 27 and 28 acts as a tight bridge that blocks the bimetallic active site (70). 

Recently, Doi152) speculated on the presence of two types of bimetallic active centers, based on 13C NMR analysis of the structure and stereochemistry of polypropylene fractions obtained with different Ziegler-Natta catalyst systems (see Fig. 44). Site A produces highly isotactic polypropylene, site B atactic polypropylene consisting of isotactic and syndiotactic stereoblocks. The formation of the latter fraction would be due to the reversible migration of the aluminum alkyl, made... 

A CNDO analysis of the insertion step of coordinated ethylene for the bimetallic active center showed that this step proceeds with an activation energy which is lower than the heat of formation of the rt-complex. A low activation energy 1 kcal/mol) for the insertion step was calculated also for the monometallic active center a mutual influence of Ti-ethylene and Ti-alkyl being taken into account. 

We have shown that metal activation of o-,77-coordinated ligands in bimetallic complexes may facilitate carbon-carbon bond formation. However, only a few examples of C-C coupling reactions of organic substrates have been recorded (Sections I1,C-E), and this aspect of bimetallic activation has not yet been fully exploited. Furthermore, activation of a bridging ligand in both the cr-frame and 77-frame by different transition... 

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