Solvents and counterions

Counterions and solvent molecules compete for a place in the vicinity of every ion. The relative strength of interaction and the space available in the neighborhood of the ion determine which species are formed. 

The competing reactions are isomerization of the cationic chain end, transfer reactions to monomer, counterion and solvent, and also termination reactions. The actual process of propagation depends on the concrete interactions between the reactants present in the polymerizing system. A synopsis of interactions expected is given in Table 7. For the most important of them quantum chemical model calculations were carried out. 

These Mo catalysts with a C2-tether connecting the phosphine and cyclopenta-dienyl ligand provide an example of the use of mechanistic principles in the rational design of improved catalysts, in this case based on information about a decomposition pathway for the prior generation of catalysts. The new catalysts offer improved lifetimes, higher thermal stability, and low catalyst loading. The successful use of a triflate counterion and solvent-free conditions for the hydrogenation are additional features that move these catalysts closer to practical utility. 

In the case of crystals, both intramolecular (conformational) and packing energies should be taken into account simultaneously. Such a total energy minimization method, with suitable crystallographic constraints, has been applied in different steps of the analysis of crystalline structures of three different synthetic polymers. Structures of these molecules, namely, isotactic trans-1,4-poly-penta-1,3-diene (ITPP), poly-pivalolactone (PPVL), and isotactic cis-1,4-poly(2-methyl-penta-1,3-diene)(PMPD), do not have troublesome features such as charged groups, counterions, and solvent molecules. 

Cyclic phosphonamides (tetrahydro-2//-l, 3,2-oxazaphosphorine 2-oxides) were obtained by reaction of / -amino alcohols with benzylphosphonic dichloride in moderate yield. In the case of the 6-monomethyl derivative the diastereomers were separated by chromatography. Deprotonation with /erz-butyllithium followed by alkylation occurred in a highly stereoselective manner (d.r. 96 4-99 1), independent of the nature of the alkylating reagent, counterion and solvent, as demonstrated for the 6,6-dimethyl derivative6. 

Hogen-Esch, T. E and J. Smid Studies of contact and solvent-separated ion pairs of carbanions. I. Effect of temperature, counterion, and solvent. J. Am. Chem. Soc. 88, 307 (1966). 

Attachment of bulky chiral barriers to achieve enantiomeric selection is exemplified by ligands (61),204 (63)69 and (64).19 Ligands of type (61) are especially efficient chiral compounds with binap-thyl hinges . The orientation of the binaphthyl rings for (61) is shown more explicitly in (62).234 With optically pure (61), complete enantiomeric separation of ammonium salt racemates such as (67) is obtained.235-237 The structure has been determined crystallographically for the less stable d form of the PF6- salt of (67) with the (S,S) host (61). The results indicate that the receptor, substrate, counterion and solvent have undergone several steric concessions in order to relieve the strain imposed on the system by the visit of an unwanted guest.238... 

The reduction of carbonyl compounds to form pinacol dimers can be accomplished photochemically, electrochemically or with chemical reducing agents. When conducted under acidic conditions or in protic solvents, pinacols are likely produced by coupling of two neutral ketyl radicals (vs radical anions). The electrochemical reduction is especially complicated in terms of the role of the electrode surface, counterion and solvent, and an excellent review has appeared on the subject32. 

H. Gordon and S. Goldman,/. Pbys. Cbem., 96,1921 (1992). Simulations on the Counterion and Solvent Distribution Functions Around Two Simple Models of a Poly electrolyte. 

Ab initio SCF-MO calculations327 and Hartree-Fock-Slater studies328 predict that the polarization of coordinated N2 will be Ru—N—N. Bands in the vibrational spectrum have been assigned329,330 to the N=N and Ru—N(N2) stretching modes. The frequency and intensity of rN=N are found to be dependent upon counterion and solvent, due to electrostatic interactions in solution.188,331 Mossbauer and 15N NMR studies on [Ru(NH3) N2]2+ have been reported.270 120,332,333 Linkage isomerism of coordinated N2 has been detected in the solid state (t1/2 = 2 1 days at 22°C in the dark) and solution (t1/2 = 2h at 25°C),318 a weak 7i-bonded intermediate being postulated318 (equation 34). 

Octaethyl-l,l -biarsole/stibole 19 (E = As, Sb) can be reduced by alkali metals such as sodium, potassium, rubidium, and cesium in DME or TMEDA yielding metal arsolides 69 (E = As) or stibolides 69 (E = Sb) with corresponding counterion and solvents being bidentate co-ligands (Equation 11) <20040M3417>. 

Figure 2.54 ORTEP diagrams of complexes [(dfppy)2lr(iJL-L )]3EuCl Cl2, thermal ellipsoids shown at the 20% probability level. The hydrogen atoms, counterions, and solvent molecules are omitted for clarity [32]. (Reprinted with permission from F.F. Chen, Z.Q. Bian, Z.W. Liu et al, Flighly efficient sensitized red emission from europium(III) in Ir-Eu bimetaUic complexes by MLCT energy transfer, Inorganic Chemistry, 47, 2507-2513, 2008. 2008 American Chemical Society.)...

Reference probes, usually employed as external standards, are VOCI3 (neat) in the case of NMR, and [Et4N][NbCl6] ([Et4N]Cl + freshly sublimed NbCls, saturated in ultrapure acetonitrile and sealed under N2 or Ar) in the case of Nb NMR. All data presented in this text are referenced against these standards. Since there are distinct temperature, counterion and solvent isotope effects upon shielding (cf. Section 2.1.3), these should be taken into account in order to provide reproducible data. 

Once the combinations of counterion and solvent(s) are identified, studies at a slightly larger scale (usually 10-50 mg, occasionally up to 500mg) can be initiated to confirm the suitability and viability of the crystalline salts produced. 

Remove counterions and solvent molecules in order to obtain single-compound records. 

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