Reactions in Nonaqueous Solvents

Rate constants have been measured for the binding of carbon monoxide to a series of five-coordinate capped iron(II) porphyrin com- 

Proton NMR studies show that DMF exchange on five-coordinate [Cu(Me6tren)(DMF)] has a rate constant (298 K) = 555 s, AH = 43.3 kJ mor, AS = -47.0 J K mol , and A(365 K) = 6.1 cm mol , suggesting a dissociative mode of activation. Anation of this complex by NCS , N3, and Br is characterized by kinetic data consistent with an Id mechanism. The observance of comparatively slow solvent exchange on the zinc analog also emphasizes the importance of steric hindrance in these compounds. 

A NMR study of DMSO exchange in two [U02L2(DMS0)] complexes (L = 2,4-pentanedione and dibenzoylmethanate) suggests the operation of a D mechanism in contrast to the A mechanism proposed for [U02(DMS0)5] .  

A rather comprehensive review of the ligand substitution reactions of low-valent organometallic complexes has appeared and is a most welcome addition to the literature. Another useful review deals with ligand substitution processes in metal carbonyls. Brief accounts of ligand effects on organometallic substitution reactions and of the mechanistic behavior of metal-metal bonded carbonyls have also been published. 

The observed rate constant for the reaction between phthalocyaninato-iron(II) and an excess of CO in DMSO has the form in equation (7) 

Convincing kinetic evidence has been presented for the existence of a solvent path [similar to that well known in platinum(II) chemistry] in ligand substitution at four-coordinate trans-N2O2 chelate complexes of nickel(II). Thus, for the substitution of a bidentate ligand HB (acetyl-acetone, benzoylacetone, dibenzoylmethane, trifluoroacetylacetone, 8-hydroxyquinoline, or A/ -ethylsalicylaldimine) into bis(iV-i -salicyl-aldiminato)nickel(II) complexes (with R = Et, /-Pr, or r-Bu) in methanol, 2-propanol, or toluene, a two-term rate law [equation (8)] is observed. In 

A similar two-term rate law has been found, and the kinetic parameters determined, for ligand substitution in four bis(AT-alkylsalicy-lideneiminato)copper(II) complexes (a) with HB = iV-ethyl- or N-phenylsalicylideneimine and alkyl = ethyl, isopropyl, or /-butyl in alcoholic medium and (b) with HB = A-ethylsalicylideneimine and alkyl = /-butyl in various aprotic organic solvents. Mass-action retardation has also been observed. 

Rate constants and activation parameters for the axial ligation of zinc(II) meso -tetraphenylporphyrin by pyridine, 2-methylpyridine, and imidazole in aprotic solvents are close to, but significantly different from, the diffusion-controlled values, and it appears that desolvation (either of the base or the metalloporphyrin) is an important step in these reactions. 

The mechanism of axial ligand substitution with a series of ruthenium(II) phthalocyanine adducts is dissociative D), the five-coordinate intermediate possessing little or no ability to discriminate between nucleophiles. The cis effect of tetraphenylporphyrin is considerably less in Ru(III) complexes than in their Fe(II) analogs. An estimate of the relative labilizing effects of the ligands L—L and Cl in the complexes [Pd(L—LiCb] (where L—L = l,2-bis(diphenylphosphino)ethane, o-phenylenebis(dimethylarsine), en, phen, bipy, or l,2-bis(phenylthio)-ethane) have been made through a study of the reaction of these complexes with en. 

There are certain experiments done in hydrogen-labile solvents which could economically have been discussed in relation to the corresponding experiments in water. However, work in solvents as little different from H2O as CH3OH promises such new and striking results, that it seemed preferable to emphasize the importance of the work by collecting the relevant data in one section. 

The experiments of Ward and Weissman 135) have already been referred to. They represent the only work on redox reactions of highly polar substances in solvents of low dielectric constant and are to be regarded as the beginning of a field of work which can be expected to develop considerably. An interesting feature of the results is not only the magnitude of the specific rates they were able to measure but also the dependence of rate on the nature of the cation. Table VIII contains a summary of their data. 

Interesting differences in the kinetics of the Sn(II) — Sn(IV) and Sb(III) — Sb(IV) reactions are produced by changing from water to media in which the species exist as discrete molecules. For the conditions 

The exchange reaction of SnCb with SnCU in C2H5OH (86) takes place by the rate law 

AS is calculated as —0.4 e.u. The activation energy for the reaction is much higher than that in concentrated HCl presumably, some use is made in the latter case of the large fund of additional Cl available. When CH3OH is the solvent (S7) the rate is given by 

The results of a stopped-flow study of the substitution of axial acetonitrile by imidazole and A -methylimidazole in two complexes of the type [FeL(AN)2] (where L is a 14-membered tetra-aza macrocyclic ligand) in acetonitrile (AN) and acetone are consistent with a dissociative (D) mechanism. The ratedetermining step in the reaction between ammineaquonickel(II) complexes and the tridentate ligand l-(2-hydroxyphenyl)-3,5-diphenylformazan in a 50% (by weight) ethanol-water mixture appears to be ring closure. 

Following a temperature-jump study of the tetrahedral ion [CoCU] solubilized in a reversed micelle system, it is suggested that the rate-determining step in the tetrahedral-octahedral conversion (8) is the final step (9)  

The coordinated dithiocarbamate ion (dtc ) in the mono complex of nickel(II) (in DMSO) exerts a significant labilizing effect on the remaining DMSO molecules in the inner coordination sphere the rate constant for dtc + [Ni(dtc)] is 4 X 10 dm mol s compared with a value of 3 x 10 dm mol s for dtc + NF.  

The high-pressure nmr technique has been used to confirm the associa-tive(/ )-dissociative(D) crossover for the ligand exchange reaction (10)  

Acetonitrile solvent exchange at the five-coordinate complex ions [ML(AN)] (with M = Co or Ni and L = l,4,8,ll-tetramethyl-l,4,8,ll-tetraazacyclote-tradecane) appears to follow an la mechanism, with large, negative entropies of activation (-69.5 and —47.8 J K mol respectively) and low enthalpies of activation (19.5 and 20.3 kJ mol ). Interestingly, the nickel complexes [NiL(AN)] and [NiL(OH2)] are significantly more labile than the cobalt(II) analogs. 

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Table 8.11 pK, Values for Proton-Transfer Reactions in Nonaqueous Solvents 8.81... 

I think Dr. Tobe s paper clearly suggests that concepts developed in connection with reactions in nonaqueous solvents, preassociation, and its importance for reaction, may have important applications in studying reactions in aqueous solution. In that context I would like to offer a few thoughts on the more or less classic reaction of water with the chloropentamminecobaltic ion. 

Mann, C. K., Barnes, K. K. Electrochemical Reactions in Nonaqueous Solvents, Marcel Delcker, New York, 1970. 

Burger, K. Solvation, Ionic and Complex Formation Reactions in Nonaqueous Solvents, Elsevier, Amsterdam, 1983. 

Less commonly, other experimental conditions may need to be controlled. For example, it may be that the presence of O2 in solution affects the reaction, in which case stock solutions and the reaction mixture should be flushed and then kept saturated with an inert gas (nitrogen or, preferably, argon). For reactions in nonaqueous solvents, of course, water may need to be rigorously excluded. And sometimes, a chemical process is affected by light if any of the species involved is light sensitive. In this event, stock solutions and the... 

Several studies of chemical oxidation-reduction reactions in nonaqueous solvents have been made.8 12 Not all dipolar aprotic solvents exhibit good stability toward oxidation or reduction. While some solvents extend the range... 

The chapters in this volume address challenging problems associated with the observation and interpretation of anodic dissolution of semiconductors, electrode reactions in nonaqueous solvents, and charge-transfer across the interface between two immiscible electrolytes. In-situ FTIR spectroscopy of surface reactions, and a review of electrochemical methods of pollution abatement complete the range of timely topics included. 

Enzymatic reactions in nonaqueous solvents have generated a great deal of interest, fueled in part by the commercial application of enzymes as catalysts in specialty synthesis. The increasing demand for enantiopure pharmaceuticals has accelerated the study of enzymatic reactions in organic solvents containing... 

The analytical chemistry of redox reactions in nonaqueous solvents has received less attention than acid-base reactions in these solvents. It should be a fruitful subject for future study. Thus far the Karl Fischer titration for water has been the most... 

Copper(II) and cerium(IV) have been studied as oxidants in acetonitrile. The copper(II)-copper(I) couple has an estimated electrode potential of 0.68 V relative to the silver reference electrode. It has been studied as an oxidant for substances such as iodide, hydroquinone, thiourea, potassium ethyl xanthate, diphenylbenzidine, and ferrocene. Cerium(IV) reactions are catalyzed by acetate ion. Copper(I) is a suitable reductant for chromium(VI), vanadium(V), cerium(IV), and manganese(VII) in the presence of iron(III). For details on many studies of redox reactions in nonaqueous solvents, the reader is referred to the summary by Kratochvil. ... 

Marty, A., Dossat, V., and Condoret, J. S., Continuous operation of hpase-catalyzed reactions in nonaqueous solvents influence of the production of hydrophilic compounds, Biotechnol. Bioeng., 56, 232-237, 1997. 

The first metallocene radical isolated was cobaltocene—the importance of which is highlighted by its inclusion in an early preparative organometallic text.12 This 19 e complex remains a reagent of choice for electron transfer reactions in nonaqueous solvents.13 A key question in delocalized organometallic radical complexes is where is the unpaired electron. An early example of this is RhCp2 (Cp = T 5-C5H5), which... 

A number of workers (8, 13, 14, 15, 20, 21, 22, 23, 33, 34, 35) have reported studies of the rates of air oxidation of iron (II) to iron (III) in aqueous solution and have discussed the mechanistic implications of the kinetic studies. Only Pound (28) has reported the results of a preliminary study of the reaction in nonaqueous solvents. We have investigated the reaction in organic solvents because the faster rates are more convenient to follow and because a wider range of organic cosubstrates can be included. The ultimate aim of this study is to increase our understanding of the various catalytic and inhibitory roles played by metallic compounds in autoxidations. 

The relationship between the reaction in nonaqueous solvents and that in water is obscure. Since the kinetics are different, it is likely that the mechanism is also different. In methanol the chloride-catalyzed reaction is second order with respect to iron (II) whereas ligand-catalyzed reactions show only first-order dependence on ferrous ion in aqueous solution. In water, rate-limiting solvolysis of species such as XFe 02, to produce H02, may occur simply because formation of the binuclear compound, required for the rate-limiting step in methanol, is inhibited by strong solvation of iron (II) in aqueous solution. 

The most general view of acids and bases was advanced by G. N. Lewis. In this model, acids are substances which have an affinity for lone electron pairs, and bases are substances which possess lone electron pairs. Water and ammonia are the most common substances which possess lone electron pairs, and therefore behave as bases in the Lewis scheme. The reaction of silver ion, Ag with cyanide ion, CN , and boron trifluoride, BF3 (an electron-deficient compound), with ammonia, NH3, are two examples of Lewis acid-base reactions. The Lewis acid-base concept is most useful in chemical reactions in nonaqueous solvents. We will not find it useful in our study of ionic equilibria in water. 

One important aspect of the Lowry-Bronsted theory is that, because it involves proton transfers, it does not necessarily have to involve water. It is possible to describe reactions in nonaqueous solvents, such as liquid ammonia, in terms of acid-base reactions. 

If counter ions have an effect on the electron-transfer rates, then Marcus theory would have a problem because these ions are not included in the theory. For most cationic reactants, such as those in Table 6.2, anions affect the rate through normal ionic strength effects and ion pairing. The latter has been observed generally to inhibit reaction in nonaqueous solvents. The Co(phen)3 " system is somewhat unusual in that N03 seems to have some catalytic effect. For anionic reactants the situation is quite different and cations often provide significant catalysis. One of the most widely studied of these is the Fe(CN)g system for which Wahl and... 

Although the Lewis theory can be used to explain the acidic or basic property of some species in protic solvents, the most important use of the Lewis theory is for acid-base reactions in nonaqueous solvents and with transition metal complexes. 

Investigation of free radicals generated in the electrode reactions in nonaqueous solvents and their interactions with cations of different background electrolytes. Stability of the ion pairs formed in these solvents, their change in the presence of water, and their other interactions. 

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