C−C Plasticity in Boron Chemistry:  Modulation of the Cc···CcDistance 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(η 5 -NC 4 H 4 )-1,2-C 2 B 9 H 11 ] ( 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(η 5 -NC 4 H 4 )-1,2-(SCH 3 ) 2 -1,2-C 2 B 9 H 9 ] ( 5 ). Here, C 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-(SCH 3 ) 2 -1,2-C 2 B 10 H 10 with a suspension of K[NC 4 H 4 ] and anhydrous CoCl 2 . The new compounds are closo- [3-Co(η 5 -NC 4 H 4 )-1,2-(SCH 3 ) 2 -1,2-C 2 B 9 H 9 ] ( 5 ), closo -[3-Co(η 5 -NC 4 H 4 )-1,2-(SC 6 H 5 ) 2 -1,2-C 2 B 9 H 9 ] ( 6 ), and closo -[3-Co(η 5 -NC 4 H 4 )-1-C 6 H 5 -1,2-C 2 B 9 H 10 ] ( 7 ). Their crystal structures were elucidated by single-crystal X-ray diffraction and are compared with those of the parent compound closo -[3-Co(η 5 -NC 4 H 4 )-1,2-C 2 B 9 H 11 ] ( 1 ) and earlier prepared compounds closo -[3-Co(η 5 -NC 4 H 4 )-1-CH 3 -2-C 4 H 9 -1,2-C 2 B 9 H 9 ] ( 2 ) and closo -[3-Co(η 5 -NC 4 H 4 )-1-C 6 H 5 -2-C 3 H 5 -1,2-C 2 B 9 H 9 ] ( 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 Ψ* 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.