C-C plasticity in boron chemistry:: Modulation of the Cc•••Cc distance in mixed pyrrolyl/dicarbollide complexes

The single, double, and triple bond distances are well defined in conventional organic compounds. Cluster boron chemistry, in particular the o-carboranes, provides the possibility to modulate the C-C distance in an almost continuous way within the same family of compounds. As an example, mixed pyrrolyl/dicarbollide sandwich cobalt complexes derived from closo-[3-Co(eta (5)-NC4H4)-1,2-C2B9H11] (1) display a continuous range of C-c. . .C-c distances between 1.640(7) for the parent compound and 1.919(6) for closo- [3-Co(eta (5)-NC4H4)-1,2-(SCH3)(2)-1,2-C2B9H9] (5). Here, C, represents the cluster carbon atom. The C-c. . .C-c modulation was achieved merely by modifying the substituents on the carbon of the cluster compound. The complexes studied were synthesized by reaction of the appropriate closo cluster, e.g., 1,2-(SCH3)2-1,2-C2B10H10 with a suspension of K[NC4H4] and anhydrous COCl2. The new compounds are closo- [3-Co(eta (5)-NC4H4)-1,2-(SCH3)(2)-1,2-C2B9H9] (5), closo-[3-Co(eta (5)-NC4H4)-1,2-(SCH3)(2)-1,2-C2B9H9] (6), and closo-[3-Co(eta (5)-NC4H4)-1-C6H5-1,2-C2B9H10] (7). Their crystal structures were elucidated by single-crystal X-ray diffraction and are compared with those of the parent compound closo-[3-Co(eta (5)-NC4H4)-1,2-C2B9H11] (1) and earlier prepared compounds closo-[3-Co(eta (5)-NC4H4)-1-CH3-2-C4H9-1,2-C2B9H9] (2) and closo-[3-Co(eta (5)-NC4H4)-1-C6H5-2-C3H5-1,2-C2B9H9] (3). Purely alkyl substituents alter the C-c. . .C-c distance of the parent compound very little, which implies that steric effects, although relevant, are not the major cause of the lengthening. In contrast, substituents with lone pairs alter the C-c. . .C-c distance substantially. Computational methods suggest that the cause of the elongation is the transfer of electron density from the available lone pairs on the substituents to the Psi* orbitals on C-c, producing a decrease in the C-c. . .C-c bond order and, thereby, an increase in the C-c. . .C-c distance.