Isomeric Dipyrrinato and Dipyrromethanato Boranes

Reaction of the dipyrrin ligand 1,3,7,9-tetramethyl-2,8-diethyldipyrrin with BH3 center dot SMe2 initially gives a dipyrrin-BH3 adduct, which eliminates dihydrogen to form the dipyrrin-supported BH2 borane (LBH2)-B-H. This species is unstable toward isomerization to its dipyrromethane isomer 1, via a (possibly BH3 catalyzed) hydride transfer from boron to the meso position of the dipyrrin core. Computations suggest that 1 is similar to 4 kcal mol(-1) more stable than the dipyrrin isomer. These conclusions were supported by low-temperature NMR and UV-vis/fluorescence spectroscopic experiments and labeling studies using BD3 center dot SMe2. Borane 1 is susceptible to hydride abstraction to form the previously characterized borenium ion [(LBH)-B-H][B(C6F5)(4)] and takes up hydride to form a dipyrromethane borohydride, which was characterized spectroscopically and crystallographically.