(Tetramethylcyclobutadiene)cobalt Complexes with Phosphacarborane Ligands^†

Reactions of the 11-vertex phosphadicarbollide anions [10-R-7,8,9-PC2B8H9]- (R = H (1a) and Cl (1b)) and [9-Cl-7,8,11-PC2B8H9]- (7) with [Cb*Co(MeCN)3]PF6 or [Cb*Co(C6H6)]PF6 (Cb* = C4Me4) give the expected cobaltaphosphadicarbollides 1-Cb*-5-R-1,2,3,4-CoPC2B8H9 (R = H (2a) and Cl (2b)) and 1-Cb*-4-Cl-1,2,3,6-CoPC2B8H9 (5b), respectively. 2a rearranges to a mixture of 1-Cb*-1,2,4,5-CoPC2B8H10 (4a) and 1-Cb*-1,2,3,6-CoPC2B8H10 (5a) at 110 °C and further to 1-Cb*-1,2,3,5-CoPC2B8H10 (6a) at 160 °C. Heating of the Cl-substituted derivative 2b at 65 °C results in 1-Cb*-5-Cl-1,2,4,8-CoPC2B8H9 (3b). Both 2b and 5b rearrange at 160 °C, giving 1-Cb*-5-Cl-1,2,3,10-CoPC2B8H9 (8b) as a main product. The observed rearrangement sequence of Cb*CoPC2B8H10 isomers correlates well with the relative stabilities of their nonmethylated analogues estimated by DFT calculations. Reaction of the diphosphacarbollide anion [7,8,10-P2CB8H9]- (11) with Cb*Co(CO)2I at 160 °C affords the nonrearranged complex 1-Cb*-1,2,3,5-CoP2CB8H9 (12). The structures of 2a, 3b, and 4a were determined by X-ray diffraction. The electrochemical study revealed that phosphadicarbollide ligands are stronger acceptors compared with tricarbollide [C3B8H11]- and charge-compensated dicarbollide [9-L-7,8-C2B9H10]-.