Transition metal-catalyzed formation of phosphorus-boron bonds: A new route to phosphinoborane rings, chains, and macromolecules

A novel catalytic dehydrocoupling route for the synthesis of linear, cyclic, and polymeric phosphinoboranes has been developed. The dehydrocoupling of neat Ph2PH . BH3, which is otherwise very slow below 170 degrees C, is catalyzed by [{Rh(mu-Cl)(1,5-cod)}(2)] or [Rh(1,5-cod)(2)][OTf] (0.5-1 mol % Rh) to give the linear compound Ph2PH-BH2-PPh2-BH3 (1) at 90 degrees C, and a mixture of the cyclic trimer [Ph2P-BH2](3) (2a) and tetramer [Ph2P-BH2](4) (2b) at 120 degrees C. In addition, the catalytic potential of other (e.g., Ti, Ru, Rh, Ir, Pd, Pt) complexes toward the dehydrocoupling of Ph2PH . BH3 was investigated and was in many cases demonstrated. The molecular structures of 1 and 2b, and of the primary phosphine-borane adduct PhPH2. BH3, were determined by single-crystal X-ray analysis. The dehydrogenative coupling of PhPH2. BH3 gave low-molecular-weight poly(phenylphosphinoborane) [PhPH-BH2](n) (3) when performed in toluene (110 degrees C) with ca. 0.5 mol % [Rh(1,5-cod)(2)][OTf] as catalyst. The absolute weight-average molecular weight was determined by static light scattering (SLS) in THF which showed that M-w = 5600. Samples of high-molecular-weight polymer 3 (M-w = 31 000 or 33 300 by SLS) were synthesized using neat conditions at 90-130 degrees C in the presence of [{Rh(mu-Cl)(1,5-cod)}(2)], anhydrous RhCl3, or RhCl3 hydrate (ca. 1 mol % Rh). Poly(phosphinoborane) 3 was thereby obtained in ca. 75% yield as an air-stable, off-white solid and was structurally characterized by H-1, B-11, C-13, and P-31 NMR and IR spectroscopy and elemental analysis. The hydrodynamic diameters for polymers 3 in THF were also determined by dynamic light scattering (DLS). Catalytic dehydrocoupling of the alkyl-substituted phosphine-borane adduct iBuPH(2). BH3 was also investigated and was found to be much slower than that of PhPH2. BH3. This produced poly(isobutylphosphinoborane) [iBuPH-BH2](n) (4) under neat conditions at 120 degrees C in the presence of [{Rh(mu-Cl)(1,5-cod)}(2)] in 80% yield. Multinuclear NMR spectroscopy and DLS were also used to characterize polymer 4, and the latter indicated that M-w = ca. 10 000-20 000, Prolonged heating of polymers 3 and 4 at elevated temperatures in the presence of catalyst led to insoluble but solvent-swellable gels possibly due to light interchain cross-linking through P-B bonds. In the absence of rhodium catalyst thermally induced dehydrocoupling of PhPH2. BH3 and iBuPH(2). BH3 proceeds very slowly and forms only low-molecular-weight materials with complex NMR spectra and which probably possess a branched structure.