Transmission of Electronic Substituent Effects across the 1,12-Dicarba-closo-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 π-charge variation of the phenyl probe confirms that the para-carborane frame is less capable than the coplanar para-phenylene frame of transmitting π-electrons from the remote substituent to the phenyl probe, or vice versa. The para-carborane cage is a better π-acceptor than π-donor; this makes π-donor substituents less effective than π-acceptors in exchanging π-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 π-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···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 π-donor substituents are involved. The ¹³C 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 ¹¹B and ¹³C chemical shifts of the cage atoms do not convey information on electronic substituent effects.