Bipyridine formation rates

In the formation of the highly colored Fe(bpy) ion from Fe ion and excess bipyridine (bpy) in acid solution, the following rate law has been demonstrated... 

The formation of N,iV -dialkyl-4,4 -bipyridine cation-radical proceeds through the reduction of the corresponding dication (alkylviologen) by tributylphosphine. Yasui et al. (2001) measured the reaction kinetics in the acetonitrile-methanol mixture. The rate of cation-radical formation decreases in all of the cases when the alkyl groups are deuterated. 

The gentle reflux is maintained while, using cannula filtration techniques, the dimethylcadmium solution in the 2-L Schlenk is now run into the rapidly stirred 2,2 -bipyridine solution through the central Suba-Seal, forming a clear bright yellow solution of the adduct. It should be noted that formation of this adduct solution is slightly exothermic, so that the rate of addition of the dimethylcadmium solution needs to be controlled to prevent too violent... 

Rate equations of this type are normally associated with the formation of associative intermediates or the involvement of deprotonated ligand forms in reactions with two (or more) competitive pathways. However, in the case of octahedral metal complexes of ligands such as 2,2 -bipyridine or 1,10-phenanthroline, such mechanisms do not appear to be likely. Associative mechanisms would involve seven-co-ordinate intermediates, which are likely to be sterically strained and electronically disfavoured on ligand field grounds. Furthermore, this type of ligand does not appear to contain any strongly acidic protons which are likely to be involved in reactions with aqueous hydroxide ion (but see later). 

Over recent years there have been a number of publications concerned with formation constants of [Fe(a,a -diimine)3]2+ complexes at various temperatures. Consequently additional AH° and AS° values are now available (Table 13),425428 and some data concerning mixed solvents have also been reported.424 459 4,1 Most of the substitution reactions of the tris ligand, low-spin, intensely coloured complexes proceed at rates conveniently monitored by conventional spectrophotometric techniques, and a considerable body of literature dealing with these kinetic and mechanistic aspects has been published. The most important a,a -diimine ligands are 2,2 -bipyridine and 1,10-phenanthroline, and their iron(II) complexes are dealt with first before considering complexes of other a,a -diimines. 

Here, MV " is methylviologen and the reaction rate of each process is denoted above. The MFEs on the formation of free radicals (Te) was measured by photostationary illumination in a continuous flow system. The observed relative MFEs (AR B)=[Ye(B)- Ye(PTY]/ Ye OTY) on the yield of photoinduced MV" radical with the four complexes (la-ld) is depicted in Fig. 12-13. This figure shows that the Te value of each complex decreases with increasing B from 0 T to 15 T and that it is almost saturated above 15 T. It is noteworthy that the AR(E) value decreases regularly for every replacement of a bipyridine ligand by a phenanthroline one. 

Some electron transfer reactions have been studied in supercritical xenon. Two of them have been shown to be diffusion controlled and two are energy controlled. These reactions have been followed by changes in the optical absorption after the pulse. To carry out these studies requires that the rate of electron attachment to the solute be suffidendy fast to compete with ion recombination, which occurs on the picosecond time scale in pulse radiolysis. The solute hexafluo-robenzene satisfies this criterion the rate constant is sufficiently large (see Fig. 6) that millimolar concentrations will allow formation of anions. The rate constant for attachment to 4,4 -bipyridine (bipy) is also sufficiently large to satisfy this need. ° Another requirement for making these studies is to quench the excimers whose optical absorptions are strong and can interfere with detection of ions. As mentioned under Sec. 2, a small concentration of ethane (0.4%) is sufficient for this purpose. 

The successful synthetic application of this electroenzymatic system has been demonstrated for the in situ electroenzymatic reduction of pyruvate to D-lactate using the NADH-dependent D-lactate dehydrogenase. Electrolysis at —0.6 V vs. Ag/AgCl reference electrode of 50 ml of a 0.1 M tris-YLC buffer of pH 7.5 containing pentamethylcyclopen-tadienyl-2,2 -bipyridine-chloro-rhodium(III) (1 x 10 M), NAD (2 x 10 M), pyruvate (2 X 10 M), and 1300 units D-lactate dehydrogenase (divided cell, carbon foil electrode) after 3 h resulted in the formation of D-lactate (1.4 x 10 M) with an enantiomeric excess of 93.5%. This means that the reaction occurred at a rate of 5 turnovers per hour with respect to the mediator with a 70% turnover of the starting material. The current efficiency was 67% [62] (Fig. 12). 

Most researchers have assumed that electron transfer between Fe3+ and SO " is an outersphere process. However, formation of an inner-sphere complex appears to be a necessary step in this mechanism when open coordination sites are available in the octahedral coordination sphere of Fe(III) (Conklin and Hoffmann, 1988a-c). On the other hand, Fe(2,2 -bipyridine)3 +, which has a fully occupied coordination sphere, oxidizes SO3- via an outer-sphere electron transfer to give SOj with the following rate law ... 

Kinetic data on complexing of other +2 transition metal ions is sparse and scattered. However, sufficient information is available to allow a comparison to be made of the rates of formation of complexes of Mn Fe Co " " and Cu with the common ligands bipyridine, terpyridine and phenanthroline (Table 13). There follows an account of some of the more recent studies. 

The formation and dissociation rates of complexes of Mn(II) with phenanthroline, bipyridine, and terpyridine in anhydrous methanol have been measured by Benton and Moore [71], using a low-temperature... 

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