Detailed experimental and computational studies of the high selectivity for functionalization of primary over secondary sp(3) C-H bonds in alkanes by borane reagents catalyzed by Cp*Rh complexes are reported. Prior studies have shown that Cp*Rh(X)(Bpin) (X = H or Bpin), generated from Cp*Rh(H)(2)(Bpin)(2) and Cp*Rh(H)(2)(Bpin)(3), are likely intermediates in these catalytic reactions. To allow analysis of the system by H/D exchange, the current studies focused on reactions of Cp*Rh(D)(2)(Bpin)(2) through the 16-electron species Cp*Rh(D)(Bpin). Density functional theory (DFT) calculations of the reaction between Cp*Rh(H)(BO(2)C(2)H(4)) and the primary and secondary C-H bonds of propane indicate that the lowest energy pathway for C-H bond cleavage occurs to form an isomer in which the alkyl and boryl groups are trans to each other, while the lowest energy pathway for functionalization of the primary C-H bond occurs by formation of the isomer in which these two groups are cis to each other. The barrier for formation of the rhodium complex by cleavage of secondary C-H bonds is higher than that by cleavage of primary C-H bond. The alkyl intermediates are formed reversibly, and steric effects cause the barrier for B-C bond formation from the secondary alkyl intermediate to be higher than that from the primary alkyl intermediate. Experimental studies are consistent with this computational analysis. H/D exchange occurs between (Cp*d(15))Rh(D)(2)(Bpin)(2) and n-octane, indicating that C-H bond cleavage occurs reversibly and occurs faster at primary over secondary C-H bonds. The observation of small amounts of H/D exchange into the secondary C-H bonds of linear alkanes and the clear observation of H/D exchange into the secondary positions of cyclic alkanes without formation of products from functionalization are consistent with the high barrier calculated for B-C bond formation from the secondary alkyl intermediate. A series of kinetic experiments are consistent with a mechanism for H/D exchange between (Cp*d(15))Rh(D)(2)(Bpin)(2) and n-octane occurring by dissociation of borane-d(1) to form (Cp*d(15))Rh(D)(Bpin). Thus, the origin of the selectivity for borylation of primary over secondary C-H bonds is due to the cumulative effects of selective C-H bond cleavage and selective C-B bond formation.
With an interest in exploring the limits of relative cation/anion mobilities in nonaqueous electrolyte solutions, we have measured the diffusivities of Li-and F-containing species in 0.5 M solutions of the new lithium salt, lithium bis͑perfluoropinacolato͒ borate, LiBPFPB, which contains a giant anion with 24 fluorine atoms. Using the pulsed field gradient spin echo method on the NMR resonances of 7 Li and 19 F in the temperature range 30-95°C we find, for the first time in nonaqueous salt-in-molecular solvent solutions, lithium diffusivities that are higher than those of the anion-containing species. Furthermore, solutions in propylene carbonate ͑PC͒ appear to be fully dissociated, since the conductivities calculated from the Nernst-Einstein equation exceed the measured conductivities by only 23% at ambient temperature and 41% at 95°C. These values are comparable with those observed for molten salts such as LiNO 3 , NaNO 3 , and aqueous LiCl solutions. Since such deviations are known to be due to interionic friction alone, transport numbers for Li ϩ may be calculated from the diffusivities without correction for neutral species. We obtain a value of 0.55 for PC solutions at 50°C. In the lower dielectric constant 1,2-dimethoxyethane solutions the ratio of calculated to measured conductivity is much higher. Here it would appear that ion association is still a problem and must be corrected for in calculating the transport number. For this case we obtain the value 0.53. We discuss means of increasing this value toward unity and show that this must involve abandoning simple salt solutions as electrolytes.The optimal functioning of a high-rate discharge battery demands the achievement of very specific characteristics for the electrolyte. Its performance depends not only on the ionic conductivity of the electrolyte but also on a high cationic transport number, t ϩ , since this condition minimizes the overpotentials due to the increase of concentration gradients in the vicinity of the electrodes and the depletion of electrolyte inside porous electrodes. 1 However, values of t ϩ for lithium ions commonly found in nonaqueous solutions fall below 0.5. 2,3 Furthermore, much of the transport of lithium is unfruitfully performed by neutral species according to the large deviations from the Nernst-Einstein equation that are usually found. Extensive ion-pairing has so far been the source of diffusivity-based transport numbers that fall near 0.5. 3 In an effort to obtain better electrolytes we have been attempting to design systems which combine high lithium mobilities with high lithium transport numbers and low ion-pairing.In this paper we give evidence of some progress in this direction, using a combination of self-diffusivity and conductivity studies on solutions containing a new lithium salt reported recently, 4 lithium bis͓1,2-tetrakis͑trifluoromethyl͒ ethylenediolato (2-)-O,OЈ͔ borate, LiB͓OC͑CF 3 ͒ 2 ͔ 4 , or more simply, lithium bis͑perfluoropinacolato͒ borate ͑LiBPFPB͒, which has a particularly large perfluorinated anio...
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