Multi-metal center

The synthesis of di- and polynuclear complexes having ligands which maintain the metal centers in close proximity is an important objective in transition metal chemistry, owing to their potential role in multi-metal centered catalysis. Metal assisted reactions of dioxygen are very important processes and there is increasing interest in their products and mechanisms. 

Since the presence of multi-metal centers makes the reaction channels become much complicated, it is important to contrive subsequent reaction processes. In the reaction of PtCH2" with O2 [73], it was noted that the branch channels to products of H2O/CO and HCOOH request the occurrence of the 0-0 bond activation of dioxygen and the 0-H bond coupling, while the generation of H2/CO2 does not. On the basis of the analysis of product varieties, two types of reaction mechanisms for the loss of H2 and the oxidation of CH2 to H2O/CO or HCOOH have been considered here for the reaction of Pt4CH2" with O2. 

Toward Novel Organic Synthesis on Multi-metallic Centers Synthesis and Reactivities of Polynuclear Transition-Metal-Sulfur Complexes. 

Although some multi-metal complexes can be utilized as hosts, efficient catalysts and functional materials, hetero multi-metal complexes are expected to show more sophisticated functions based on their more complex structures. In biological systems, multi-metal centers provide very high catalytic activities and selectivi-ties that are not achieved by singular metal systems. Combinations of homo- and hetero-metal systems produce more elegant functional systems as seen in photosynthetic cascades. In artificial molecules and molecular assemblies, such combinatorial strategies should create a novel and useful variety of cooperative and synergistic supramolecular functional systems. 

Alkali metal derivatives of 2-(trimethylsilyl)aminopyridines can be further derivatized by insertion of 1,3-dicyclohexylcarbodiimide. Functionalized guani-dinates are formed in this reaction via a 1,3-silyl shift. Scheme 170 illustrates the reaction sequence as well as the preparation of an aluminum complex of the modified ligand, which exhibits pseudo jS-diketiminate binding of the metal center, thus exemplifying the coordinative versatility of this new multi-N-donor system. ... 

Apparently very little has been done with simple barium, strontium or calcium alkyl catalysts. One can predict that they should be similar to sodium and potassium catalysts which require special conditions and selected monomers to obtain sufficient cationic attack on the monomer for stereoregular addition. Furukawa (251, 252) has obtained low crystallinity isotactic polymers from styrene and acrylic esters by using complexes of these alkyl metals with ZnR2. However, stereospecificity was attributed to multi-centered coordination involving the two metallic centers (250). 

Since butadiene can also undergo coordinated anionic polymerizations, some of the differences in polymer microstructure are attributable to changes in mechanism. Based on the catalysts reported to date, the isotactic and syndiotactic 1,2-polybutadienes appear to arise from coordinated anionic mechanisms. Qs and trans 1,4-polybutadienes can probably be made by all mechanisms, with cis arising from soluble catalysts which are capable of multi-coordination at one metal site. Trans structure is favored by cationic mechanism and by anionic mechanism involving coordination at two metal centers. 

Fig. 6. Schematic representation of the biosynthetic pathway associated with the assembly of the NiFe-hydrogenase catalytic metal center. The precursor to the large subunit is represented by pre-HycE-Fe, and the CO (and/or CN) ligands are provided by the multi-protein complex formed between HypE, HypC and HypD. The source of the ligands to HypE is carbamoyl phosphate, and the insertion reaction is catalyzed by HypF. The Ni atom is inserted into the pre-HycE-Fe-CO-CN-HypC complex in a reaction catalyzed by HypA and HypB. Finally, the Hycl endopeptidase processes the C-terminus of the pre-HycE-Fe-CO-CN-HypC complex, yielding the mature NiFe-hydrogenase large subunit.

The most studied of all copper-confaining oxidases is cytochrome c oxidase of mitochondria. This multi-subxmif membrane-embedded enzyme accepts four elecfrons from cytochrome c and uses them to reduce O2 to 2 HjO. It is also a proton pump. Its structure and functions are considered in Chapter 18. However, it is appropriate to mention here that the essential catalytic centers consist of two molecules of heme a a and 3) and three Cu+ ions. In the fully oxidized enzyme two metal centers, one Cu + (of the two-copper center Cu ) and one Fe + (heme a), can be detected by EPR spectroscopy. The other Cu + (Cug) and heme exist as an EPR-silent exchange-coupled pair just as do the two copper ions of hemocyanin and of other t57pe 3 binuclear copper centers. 

Bridging ligands capable of binding more than two metal centers are less studied. Some examples were highlighted earlier. Polymetallic ruthenium complexes have been synthesized using the tetra-bidentate ligands (62) and (63) as multi-electron transfer agents.181,182... 

Coordination chemistry is a multi-disciplinary research area, which has led to the design of various new organic-inorganic hybrid species, with intriguing new chemical and physical behavior. These results have been boosted by the tremendous capabilities of organic synthesis, applied in the context of non-traditional electronic behavior of the metal centers. 

An increase of the reactant pressure merely displaees the relative peak heights toward higher values of n. No additional peak is observed. It can be easily deduced from such a spectrum that the reaction proceeds through multi-step attachment of the reactant to each metal center ... 

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