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 Ph 2 PH·BH 3, which is otherwise very slow below 170 °C, is catalyzed by [{Rh(μ-Cl)(1,5-cod)} 2 ] or [Rh(1,5-cod) 2 ][OTf] (0.5−1 mol % Rh) to give the linear compound Ph 2 PH−BH 2 −PPh 2 −BH 3 ( 1 ) at 90 °C, and a mixture of the cyclic trimer [Ph 2 P−BH 2 ] 3 ( 2a ) and tetramer [Ph 2 P−BH 2 ] 4 ( 2b ) at 120 °C. In addition, the catalytic potential of other (e.g., Ti, Ru, Rh, Ir, Pd, Pt) complexes toward the dehydrocoupling of Ph 2 PH·BH 3 was investigated and was in many cases demonstrated. The molecular structures of 1 and 2b, and of the primary phosphine−borane adduct PhPH 2 ·BH 3, were determined by single-crystal X-ray analysis. The dehydrogenative coupling of PhPH 2 ·BH 3 gave low-molecular-weight poly(phenylphosphinoborane) [PhPH−BH 2 ] n ( 3 ) when performed in toluene (110 °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 °C in the presence of [{Rh(μ-Cl)(1,5-cod)} 2 ], anhydrous RhCl 3, or RhCl 3 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 1 H, 11 B, 13 C, and 31 P 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 i BuPH 2 ·BH 3 was also investigated and was found to be much slower than that of PhPH 2 ·BH 3 . This produced poly(isobutylphosphinoborane) [ i BuPH−BH 2 ] n ( 4 ) under neat conditions at 120 °C in the presence of [{Rh(μ-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 PhPH 2 ·BH 3 and i BuPH 2 ·BH 3 proceeds very slowly and forms only low-molecular-weight materials with complex NMR spectra and which probably possess a branched structure.