Rhodium-catalyzed dehydrocoupling of the sterically encumbered phosphine-borane adduct tBu2PH•BH3:: Synthesis of the linear dimers tBu2PH-BH2-tBu2P-BH3 and tBu2PH-BH2-tBu2P-BH2Cl

The dehydrocoupling of the sterically hindered phosphine-borane adduct tBu(2)PH . BH3 above 140 degreesC is catalyzed by the rhodium complexes [Rh(1,5-cod)(2)][OTf] or Rh-6(CO)(16) to give the four-membered chain tBu(2)PH-BH2-tBu(2)P-BH3 (1), which was isolated in 60%. yield and. characterized by multinuclear NMR spectroscopy, mass spectrometry, and elemental analysis. Thermolysis of 1 in the temperature range 175-180 degreesC led to partial decomposition and the formation of,tBu(2)PH . BH3. When the dehydrocoupling of tBu(2)PH . BH3 was performed in the presence of [{Rh(mu -Cl)(1,5-cod)}2] or RhCl3 hydrate, the chlorinated compound tBu(2)PH-BH2-tBu(2)P-BH2-Cl (2) was formed which could not be obtained free of 1. The molecular structures of tBu(2)PH . BH3, tBu(2)PH-BH2-tBu(2)P-BH3 (1), and tBu(2)PH-BH2-tBu(2)P-BH2Cl (2) together with 1 were determined by single-crystal X-ray diffraction studies.