Transmission of Electronic Effects through the {closo-1-CB9} and {closo-1-CB11} Cages: Apparent Dissociation Constants for Series of [closo-1-CB9H8-1-COOH-10-X] and [closo-1-CB11H10-1-COOH-12-X] Acids

The apparent ionization constants pK(a)' for series of carboxylic acids [closo-1-CB9H8-1-COOH-10-X](-) (1) and [closo-1-CB11H10-1-COOH-12-X](-) (2), where X = H, I, n-C6H13, +NMe3, N-+(2), +SMe2, OC5H11, were measured in EtOH/H2O (1/1, v/v) at 24 degrees C. Correlation analysis of the pK(a)' values using Hammett substituent constants sigma(P)(X) gave the reaction constant rho = 0.87 +/- 0.04 for series 1 and rho = 1.00 +/- 0.09 for series 2. These values are higher than for derivatives of PhCH=CHCOOH (rho = 0.70 +/- 0.09 in 55% EtOH) and correspond to 56% and 65% efficiencies in transmission of electronic effects by [closo-1-CB9H10](-) (E) and [closo-1-CB11H12](-) (F), respectively, as compared to benzene (A). Experimental results were supported with DFT calculations of relative acidity for series of acids derived from A, E, and F in aqueous medium.