Reactions in Non-aqueous Solvents

Reactions in Non-aqueous Solvents.—Lincoln and West have detennined the kinetic parameters for the exchange of acetonitrile in a series of substituted nickel(n) species which were chosen so that the relative effects of oxygen- and of primary- and tertiary-nitrogen-binding could be assessed. The complexes are shown in structures (21)—(25) (AN = acetonitrile) and the results in Table 11. The 555-fold variation 

Structural interconversions of dichloro-l,l,7,7-tetraethyldiethylenetriamine-nickel(n) (NiLClg) in acetonitrile have been studied by relaxation methods in which the equilibria were suddenly perturbed by an electric-field jump or by means of a pulse of radiation from a Q-switched neodymium laser. The results are interpreted in terms of the mechanism 

The exchange processes involving the five-co-ordinate nickel(n) complex of 2,6-lutidine 1-oxide and the tributylphosphine adduct of bis(diethyldithiophosphato)-nickel(n) in benzene both appear to be dissociative in character. 

Kildahl and Drago have compared the enthalpy of formation of the adduct of 4-methylpyridine (4-Mepy) with the Ni complex of (26) and the activation enthalpy 

Clear evidence for the operation of the kinetic trans effect has been foundin [Co(MeOH)5py] + in that the rm/w-site exchange rate constant per co-ordinated methanol molecule (1200 s at — 30 °C) is 2.5— 3.0 times that for the cis site (410 s ). The values of AlT and A are, respectively, 11.9 kcal mol- and 2.9 cal K mol for the cis site and 11.2 kcalmol and 2.9 cal K mol- for the trans site. 

Reactions in Non-aqueous Solvents.—Coetzee and co-workers have determined the ratio N of the rate constants of the two reactions (a) p iXS ]+ -f Y- -temary complex and (b) [NiS6] + + Y- simple complex (where X is a monoanion, S the solvent, and Y a neutral aromatic ligand) for the solvents acetonitrile and methanol. The accelerating effects produced by the inner-sphere substituents vary quite considerably from case to case (Table 11) and, in particular, between the two solvents. The fact that the rate of reaction (a) in acetonitrile is much less sensitive to the nature of Y than is the case for reaction (b) supports the authors previous view on the importance of an extra stabilization of the outer-sphere complex with this solvent (the latter being brought about by the interaction of the ligand with the polarized acetonitrile molecules of [NiSe] +) since the S molecules vrill be less polarized in [NiXS ]+ than in [NiSe] +. On the basis of these and other results, Coetzee and co-workers conclude that the accelerating influence of anionic inner-sphere substituents in non-aqueous solvents can be attributed to the net effect of solvent labiliza-tion and outer-sphere destabilization. 

The lability of pyridine in bis-(/S-alkanedionato)dipyridinenickel(n) complexes (Table 12) decreases as the electron-withdrawing power of the jS-alkanedionato- 

The mechanism of ligand substitution on the five-co-ordinate 1,2-dithiolene complexes [M SaC2(CF3)a 2X] (M is Fe or Co, X is a phosphine or phosphite) in benzene is associative when 2 = 0 and dissociative when z — — 1second-order rate constants are given in Table 13. 

From n.m.r. studies on eight tetrahedral Co complexes of thiourea (tu) and substituted thioureas in acetone, it is concluded that, in each case, two mechanisms are involved. The first is a direct associative exchange, 

As the authors point out, the lability of the complexes arises from the low enthalpi of activation (Table 14) indeed, the values are so low in many cases that the Reporter is forced to wonder whether we have heard the full mechanistic story about these interesting reactions. 

Reactions in Non-aqueous Solvents.—By studying the kinetics of replacement of water in [Fe(CN)5(OH2)] with a series of unidentate ligands in ethylene glycol containing 0.055 mol 1 water, ASperger and co-workers have been able to distinguish between h and D mechanisms for the general substitution reaction 

Both mechanisms are consistent with the observation of a limiting rate at sufficiently high [Y], but they may be distinguished by working in a poorly co-ordinating solvent containing a certain amount of water. If the D mechanism, 

Rate constants and activation parameters have been reported for the reactions of Ni + with 4-phenylpyridine, bipy, phen, and terpy in propan-l-ol, propan-2-ol, and isobutyl cyanide as solvents. The donor strength of the solvent appears to be of particular importance in the formation as well as the dissociation reaction (see also ref. 8 and Vol. 4, p. 211). It has been shown by n.m.r. measurements that, in DMF-methanol mixtures, nickel(n) ions are preferentially solvated by two DMFmolecules the remaining solvent molecules in the inner sphere are then distributed on a statistical basis according to the composition of the bulk solvent. The methanol exchange rate at Ni + is enhanced by co-ordinated DMF, logA ex ° being linearly 

Three relaxation processes observed during the Q-switched laser photolysis of dibromo-[l,3-bis(diphenylphosphino)propane]nickel(ii) in acetonitrile have been assigned to the relaxation of coupled equilibria involving planar and tetrahedral isomers of the complex together with ion pairs and free ions. The kinetics of the rapid square-planar-octahedral interconversion accompanying the addition of two unidentate ligands L to (14) in chlorobenzene are consistent with a two-step mechanism, 

Rate constants for the substitution reactions of square-planar dithiophosphate and dithiocarbamate complexes of Ni , Pd, and Pt, with en and CN as nucleophiles, have been reported with methanol as the solvent the general mechanism 

Reactions in Non-aqueous Solvents.— N.m.r. studies on five co-ordinate complexes with identical ligands indicate that the barriers to intramolecular exchange are very low (ca. 5 kcal mol ), although it has been reported that the ion [Co(CNCMe3)s]+ is stereochemically rigid at - 30 °C. Muetterties has recently shown that reaction (34) has a relatively low activation energy 

The interconversion between octahedral [Co(py)4Br2] and tetrahedral [Co(py)aBr2] in nitromethane has also been studied by n.m.r. The kinetic data are found to be consistent with the following mechanism, in which reaction (35) equilibrates rapidly compared with reaction (36). 

Proton n.m.r. has been used to measure the exchange rate of methanol from cis and trans co-ordination sites of [Co(NCS)(MeOH)5]+. It is concluded that the exchange occurs from both types of co-ordination site without internal rearrangement of the complex, and that the cis and trans exchange rates are equal in addition, the exchange occurs exclusively between the bulk and bound environments. These results are markedly different from those reported previously for similar systems e.g. [Co(OH2)(MeOH)6] + and [CoCl(MeOH)5]+ in which the mean lifetime of a methanol molecule in the trans site is 0.59 times that for the cis). The reason for the difference is unclear. 

The kinetics of exchange of acetonitrile on the complexes of cobalt(n) and nickel(n) with tren and Megtren have been measured. The two acetonitrile exchange rates observed for the non-equivalent sites on [Ni(tienXMeCN)2] + (Table 17) are ascribed to the cis (more labile) and trans (less labile) positions 

The exchange kinetics of tren with [Nd(tren 2] + in acetonitrile indicate that the reaction is first-order with respect to both the complex and the ligand. The proposed mechanism involves the simultaneous partial attachment of the free ligand and the partial unwrapping of co-ordinated tren from the metal ion. 

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K. Burger, Solvation, Ionic and Complex Formation Reactions in Non-Aqueous Solvents Experimental Methods for Their Investigation, Akademiai Kiado, Budapest, 1983. 

K. Burger, Solvation, Ionic and Complex Formation Reactions in Non-Aqueous Solvents (Experimental Methods for their Investigation), Studies in Analytical Chemistry, Vol. 6, Elsevier, Amsterdam, 1983, Ch. 2 and 3 and Ch. 9, pp. 256-257. 

Microwave heating is achieved with a pulse of microwave energy generated within a magnetron. The only requirement for the use of this heating method is that the solvent has a permanent dipole moment.24 This technique can be employed to study reactions in non-aqueous solvents since the presence of electrolytes is not necessary.30 However, the temperature changes attained were much lower than in the case of Joule or laser heating.24... 

Acid-base reactions in non-aqueous solvents have been extensively studied. Many books and reviews are available concerning acid-base equilibria and acid-base titrations in non-aqueous solvents. References [1-3] are particularly useful. 

Like metal chelates of some other dithio ligands, e.g. maleonitrile dithiolate, dithio-/J-diketonato complexes undergo reversible redox reactions in non-aqueous solvents. The square-planar complexes [ML2] (M = Co, Ni, Pd and Pt) exhibit two consecutive one-electron reductions as shown in equation (5).252,253... 

The reaction of organic phosphorus thioloesters with halogenating agents has been known for twenty years. This reaction has been shown to proceed via different pathways depending on the reaction medium ( 1 - 3). The reaction in non-aqueous solvents has been applied successfully to the synthesis of optically active phosphino, phosphono and phosphorochloridates I I I (4 7) ... 

Intermediate formation mechanisms indicated in the monooxygenation diagram relate to the class of reactions in non-aqueous solvents. This is the reason why the hemin form of iron porphyrin is absent in it. Hence, hemin is present, of which the intermediate formation shaped as Hm+0 (where Hm is heme) is typical. 

Enzymatic reactions in non-aqueous solvents are subjected to a wide interest. A particular class of these solvents is the supercritical fluid (1) such as carbon dioxide that has many advantages over classical organic solvents or water no toxicity, no flammability, critical pressure 7.38 Mpa and temperature 31°C, and allowing high mass transfer and diffusion rates. 

The polymerization of N-carboxy anhydrides (NCA s) is a complicated process that Is difficult to study. The sensitivity of NCA s to moisture and other Impurities, the limited solubility of the products of NCA polymerizations In most solvents that are suitable for anionic polymerizations, the tendency of NCA s to associate with polypeptides, leading to enhanced monomer concentrations In the vicinity of growing polypeptide chains, the general cosplexlty of Ionic reactions in non-aqueous solvents and the diversity of possible mechanisms for amide bond formation or destruction. Including catalysis by COj, acids, bases, etc., collectively make It difficult to establish mechanisms for NCA polymerizations. 

LLC phases as reaction media have also been explored for biocatalysis. The LLC phases of commercial surfactants have been used to successfully stabilize enzymes and cofactors for reaction in non-aqueous solvent environments. In addition to significant stabilization of the enzyme biocatalyst in the bipha-sic LLC system, product recovery has also been found to be facilitated [107]. 

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