Metal bridge complexes

Carboxamides containing alkylamine arms like (34), synthesized from dialkylmalonate and ethylene-diamine, act as chelating dianionic N4-donors,44,45 whereas the geometric isomer (35) was prepared from 1,3-diaminopropane and chloroacetyl chloride.46 The hexadentate bis(pyrazine)-carboxamides (36) and (37) can be obtained by the addition of 1,3-propanediamine to the appropriate carboxylic acid.47 Compound (36), in the dianionic form, can be used to obtain homo- and hetero-metal bridged complexes, whereas (37) has been shown to yield tetranuclear copper(II) complexes involving opened-up dianionic ligand molecules.47... 

The coupling constant is inconsistent with carboxyl coordination but consistent with carbonyl coordination 15). Similar data for -ketobu-tyrate 15) and oxalacetate (19) have been fit by exchange contributions (1/tm) and inner sphere contributions Tm and T2m)- The rates of formation of these metal bridge complexes from an outer sphere complex ( 3,4) are limited predominantly by the rate of dissociation of a water molecule from the coordination sphere of the enzyme-bound manganese (Figure 3, Table V) (15,19), as required by the Sj l-outer sphere mechanism of Eigen and Tamm (20),... 

A third example of a activation occurs with D-xylose isomerase. The enzymes from lactobacillus (28) and streptomyces (29) require a divalent cation for activity. Both enzymes form metal bridge complexes to the C-1 hydroxyl group of -D-xylose by the NMR method since these enzymes enhance the eflFect of Mn on the relaxation rates of the C-1 proton of a-D-xylose (Table II) (7, 30, 3J). The calculated Mn -to-proton distances (Figure 1) and coupling constants (Table IV) are consistent with the following structure. 

Pyruvate kinase, phosphopyruvate kbtase (EC 2.7.1.40) a widely distributed, metal-ion dependent phosphotransferase, present in yeast, muscle, liver, erythrocytes and other organs and cells. It catalyses the last reaction of glycolysis Phosphoeno/pyruvate (PEP) + ADP -> Pyruvate + ATP (substrate level phosphorylation). Each subunit of P. k. forms an intermediate, cyclic, ternary metal bridge complex ... 

A new type of xenophilic metal cluster that was termed by Whittlesey was synthesized and studied. These clusters are compounds that contain two very different types of metal centers that are bonded together through substantial covalent interaction. The neutral cluster 57 was obtained readily from the reaction of the metal-bridged complex Mn[Mn7(THF)6(GO)i2]2 with Fe(CO)s in THF, and is an important example of this class of novel clusters. ... 

Titanium imido complexes supported by amidinate ligands form an interesting and well-investigated class of early transition metal amidinato complexes. Metathetical reactions between the readily accessible titanium imide precursors Ti( = NR)Cl2(py)3 with lithium amidinates according to Scheme 84 afforded either terminal or bridging imido complexes depending on the steiic bulk of the amidinate anion. In solution, the mononuclear bis(pyridine) adducts exist in temperature-dependent, dynamic equilibrium with their mono(pyiidine) homologs and free pyridine. 

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