Bound-Ion-Coordination Mechanism

Problem 9.13 The coordinate and bound-ion-radical mechanisms, although apparently quite dissimilar, have many features in common. If, in the bound-radical hypothesis, the surface involved decreases to the limiting case of three points of contact, the two mechanisms would appear to be quite similar (Friedlander, 1959). Explain this similarity by applying the idea of growth on the surface (used in the bound-radical hypothesis) to the coordinate mechanism, considering a surface with only three points of contact. 

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Two general mechanisms have been proposed to explain the formation of polymers with precipitated catalysts (a) the bound-ion-radical mechanism and (b) the bound-ion-coordinate mechanism. The bound-ion-radical mechanism involves chain growth in a chemisorbed layer of monomer molecules initiated by radicals or ion-radicals bound to the surface of the catalyst, while the coordinate mechanism involves chain growth from a complex ionic center in the catalyst. 

Figure 9.9 The process of chain growth in bound-ion-coordination mechanism for polymerization on a catalyst surface with the growth taking place from a single active site and the monomer being supplied from the liquid phase. Con-secutive propagation steps are represented in (a), (b), and (c). (After Natta, 1955.)...

Unlike ATXl, where the chemistry of copper transfer is well understood, the mechanism of copper transfer from CCS to SODl remains unclear. For example, a dramatic change in metal ion coordination chemistry must accompany the copper transfer process in this case. Copper is bound to CCS in the reduced Cu(I) form via sulfhydral ligands, while in SODl, copper is oxidized to the cupric Cu(II) state and is coordinated in an all-nitrogen environment to four histidines. The driving force for this large change in coordination chemistry is still poorly understood, but may involve an external oxidant, such as the superoxide substrate itself... 

From this point, MEP is coupled to cytidine triphosphate to produce 4-diphosphocytidyl-2C-methyl-D-erythritol (CDP-ME) in a reaction, which is catalyzed by 4-diphospho-cytidyl-2C-methyl-D-erythritol transferase (IspD) [2.7.7.60] (Scheme 5.2). IspD structure-mechanism analysis showed that a sequential mechanism applies with a Mg ion coordinating first, followed by CTP binding and then MEP [22]. The cytosine base portion of the substrate or the product is hydrogen bonded to Ala 14, Alai 5, Gly82, Asp 83, and Sep88 based on smdies conducted on product-bound cocrystal of IspD from E. coli [23]. 

The specific feature of polymerization as a catalytic reaction is that the composition and structure of the polymer molecule formed show traces of the mechanism of the processes proceeding in the coordination sphere of the transition metal ion to which a growing polymer chain is bound. It offers additional possibilities for studying the intimate mechanism of this heterogeneous catalytic reaction. 

Thiocarbamate (tc, RHNCSO-) is a monodentate ambidentate ligand, and both oxygen- and sulfur-bonded forms are known for the simple pentaamminecobalt(III) complexes. These undergo redox reactions with chromium(II) ion in water via attack at the remote O or S atom of the S- and O-bound isomers respectively, with a structural trans effect suggested to direct the facile electron transfer in the former.1045 A cobalt-promoted synthesis utilizing the residual nucleophilicity of the coordinated hydroxide in [Co(NH3)5(OH)]2+ in reaction with MeNCS in (MeO)3PO solvent leads to the O-bonded monothiocarbamate, which isomerizes by an intramolecular mechanism to the S-bound isomer in water.1046... 

The crystal structure of a CODH/ACS enzyme was reported only in 2002.43,44 It reveals a trio of Fe, Ni, and Cu at the active site (6). The Cu is linked to the Ni atom through two cysteine-S, the Ni being square planar with two terminal amide ligands. Planarity and amide coordination bear some resemblance to the Ni porphinoid in MCR. A two-metal ion mechanism is likely for acetyl CoA synthesis, in which a Ni-bound methyl group attacks an adjacent Cu—CO fragment with formation of a Cu-acyl intermediate. A methylnickel species in CODH/ACS has been identified by resonance Raman spectroscopy.45... 

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