Transmission of Electronic Substituent Effects across the 1,12-Dicarba-doso-dodecaborane Cage: A Computational Study Based on Structural Variation, Atomic Charges, and 13C NMR Chemical Shifts

The ability of the 1,12-dicarba-closo-dodecaborane cage to transmit long-range substituent effects has been investigated by analyzing the structural variation of a phenyl probe bonded to C1, as caused by a remote substituent X at C12. The geometries of 41 Ph-CB10H10C-X molecules, including 11 charged species, have been determined by MO calculations at the B3LYP/6-311++G** level of theory. The structural variation of the phenyl probe is best represented by a linear combination of the internal ring angles, termed S-F(CARB). Multiple regression analysis of S-F(CARB), using appropriate explanatory variables, reveals the presence of resonance effects, superimposed onto the field effect of the remote substituent. The ability of the para-carborane cage to transmit resonance effects is, on average, about one-half of that of the para-phenylene frame in coplanar para-substituted biphenyls. Analysis of the pi-charge variation of the phenyl probe confirms that the para-carborane frame is less capable than the coplanar para-phenylene frame of transmitting p-electrons from the remote substituent to the phenyl probe, or vice versa. The para-carborane cage is a better pi-acceptor than pi-donor; this makes pi-donor substituents less effective than pi-acceptors in exchanging p-electrons with the phenyl probe across the cage. When the remote substituent is an uncharged group, the para-carborane cage acts as a very weak pi-acceptor toward the phenyl probe. The structural variation of the para-carborane cage has also been investigated. It consists primarily of a change of the C1 center dot center dot center dot C12 nonbonded separation, coupled with a change of the five B-C-B angles at C12. This concerted geometrical change is controlled by the electronegativity of the substituent and the resonance interactions occurring between substituent and cage. These, however, appear to be important only when pi-donor substituents are involved. The C-13 NMR chemical shifts of the para-carbon of the phenyl probe correlate nicely with S-F(CARB), pointing to the reliability of these quantities as measures of long-range substituent effects. On the contrary, the B-11 and C-13 chemical shifts of the cage atoms do not convey information on electronic substituent effects.