Electron transfer remote attack

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... 

More subtle factors that might affect k will be the sites structures, their relative orientation and the nature of the intervening medium. That these are important is obvious if one examines the data for the two copper proteins plastocyanin and azurin. Despite very similar separation of the redox sites and the driving force (Table 5.12), the electron transfer rate constant within plastocyanin is very much the lesser (it may be zero). See Prob. 16. In striking contrast, small oxidants are able to attach to surface patches on plastocyanin which are more favorably disposed with respect to electron transfer to and from the Cu, which is about 14 A distant. It can be assessed that internal electron transfer rate constants are =30s for Co(phen)3+, >5 x 10 s for Ru(NH3)jimid and 3.0 x 10 s for Ru(bpy)3 , Refs. 119 and 129. In the last case the excited state Ru(bpy)3 is believed to bind about 10-12 A from the Cu center. Electron transfer occurs both from this remote site as well as by attack of Ru(bpy)j+ adjacent to the Cu site. At high protein concentration, electron transfer occurs solely through the remote pathway. 

The neutral carboxyl group is not very effective in increasing the reduction rate of the complex. However, when the proton is removed from the carboxyl, the effect can increase and is greatest when the carboxyl ion is in a configuration favorable to chelation. Thus, the inverse (H+) path is not even observable for acid succinate in the same acidity range as that for which this path is important in the acid malonato reaction. The acid dissociation constants are known well enough so that the behavior difference between acid malonato and acid succinato can not be entirely ascribed to different acidities of the complexes. The results obtained with the acid malonate complexes, as reported in Table II, incidentally provide no support for the hypothesis (22) that electron transfer takes place by remote attack across hydrogen bonds. 

Invoking the reducibility of the ligand in certain instances helps explain some other comparisons. Thus, it has been a matter of concern that electron transfer by remote attack through an esterified carboxyl group is accompained by such drastic effects as hydrolysis of the ester, while electron transfer through a carbonyl, when this is part of a ketonic or aldehydic function, takes place easily. This occurs even though there are no opportunities here for the kind of chemistry which accompanies... 

A related example of inner-sphere reaction is shown in reaction (5), where an additional mechanistic subtlety appears.10 As shown by the products, inner-sphere electron transfer also occurs but now by remote attack in which the sites of bridging ligand binding to the reductant and oxidant are at different atoms on the bridging ligand. 

Two separate but somewhat interwoven themes have emerged from the study of inner-sphere reactions. The first is the use of product and rate studies to establish the existence of inner-sphere pathways and then the exploitation of appropriate systems to demonstrate such special features as remote attack . In the second theme the goal has been to assemble the reactants through a chemical bridge and then to study intramolecular electron transfer directly following oxidation or reduction of the resulting dimer (note equation 7). It is convenient to turn first to chemically prepared, intramolecular systems since many of the theoretical ideas and experimental results for outer-sphere reactions can be carried over directly as an initial basis for understanding the experimental observations. 

The reduction of [(NH3)5Co02CH2/o N]2+ apparently has not been studied. The general experience with reduction of Co (III) by Cr2+ however is that when CH2 is inserted into a conjugated system, the reaction by remote attack is much less rapid than it is by the outer-sphere path, and this latter path is usually much slower than that involving remote attack, when this can occur.) Our limited capacity to predict the results of the experiments on intramolecular electron transfers is a strong motivation for continuing the work. 

Both monomeric (213), chelated (214) and bridged dimeric (215) and (216) complexes are known. The latter were first prepared by Werner689 and in the last decade have received some attention in relation to remote attack and outer-sphere electron transfer (see below). The monomeric complexes... 

The Fixation of Carbon Dioxide and Nitrogen.—As reduced carbon compounds are convenient fuels, the possibility of achieving the reductive fixation of COa in vitro is appealing, if remote. There are very few data on photochemical reactions involving carbon dioxide, for it has no low-lying excited states and has not historically been of much interest to the photochemist. However, what appears to be the first example of photofixation of COa in a non-biological system has been briefly reported.18 Photoirradiation (with a high-pressure mercury lamp) of phenanthrene in the presence of an amine and C02 in a polar solvent (MeaSO or HCONMea) yielded 9,10-dihydrophenanthrene-9-carboxylic acid, in unspecified quantum yield. The mechanism appears to involve formation of COaT by electron transfer from the photoexcited amine, followed by attack of COaT on position 9 of phenanthrene. Similar reductive carboxylation of anthracene, pyrene, naphthalene, and biphenyl was observed. 

These results confirm (c/. ref. 145) that terephthalate ion is not an effective electron-transfer bridge. Remote attack does occur in the reactions of (23 R=C=CCOjjH) with Cr + (ref. 146) and V + (ref. 147), and of (23 K = trans-CH=CHCOaH) with The rate laws are equation (70) for the acetylene-... 

The preparation of europium metasilicate hydrate from solutions of europium bromide and sodium metasilicate has been described.The kinetics and mechanism of the reduction of thiocyanato- and isothiocyanato-penta-amminecobalt(iii) ions by europium(ii) in acid solution have been discussed in terms of europium(ii) attack on the ambidentate bridging ligand at the end remote from the cobalt centre. The differences in the activation enthalpies for the reduction of the complexes were attributed to (a) differences in enthalpy of formation of the precursor complex Eu - X - Co (NH3)5, and (b) ease of stretching of the Co—S or Co—N bond in the precursor complex. The low-temperature Mossbauer spectrum of EUH2 suggested the covalent transfer of electron density. to the metal 6s orbital to be more marked in EuH2 than EuO. ... 

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