A mild, catalytic dehydrocoupling route to aminoboranes and borazine derivatives from either primary or secondary amine-borane adducts has been developed using late transition metal complexes as precatalysts. The adduct Me(2)NH.BH(3) thermally eliminates hydrogen at 130 degrees C in the condensed phase to afford [Me(2)N-BH(2)](2) (1). Evidence for an intermolecular process, rather than an intramolecular reaction to form Me(2)N=BH(2) as an intermediate, was forthcoming from "hot tube" experiments where no appreciable dehydrocoupling of gaseous Me(2)NH.BH(3) was detected in the range 150-450 degrees C. The dehydrocoupling of Me(2)NH.BH(3) was found to be catalyzed by 0.5 mol % [Rh(1,5-cod)(mu-Cl)](2) in solution at 25 degrees C to give 1 quantitatively after ca. 8 h. The rate of dehydrocoupling was significantly enhanced if the temperature was raised or if the catalyst loading was increased. The catalytic activity of various other transition metal complexes (Ir, Ru, Pd) for the dehydrocoupling of Me(2)NH.BH(3) was also demonstrated. This new catalytic method was extended to other secondary adducts RR'NH.BH(3) which afforded the dimeric species [(1,4-C(4)H(8))N-BH(2)](2) (2) and [PhCH(2)(Me)N-BH(2)](2) (3) or the monomeric aminoborane (i)Pr(2)N=BH(2) (4) under mild conditions. A new synthetic approach to the linear compounds R(2)NH-BH(2)-NR(2)-BH(3) (5: R = Me; 6: R = 1,4-C(4)H(8)) was developed and subsequent catalytic dehydrocoupling of these species yielded the cyclics 1 and 2. The species 5 and 6 are postulated to be intermediates in the formation of 1 and 2 directly from the catalytic dehydrocoupling of the adducts R(2)NH.BH(3). The catalytic dehydrocoupling of NH(3).BH(3), MeNH(2).BH(3), and PhNH(2).BH(3) at 45 degrees C to give the borazine derivatives [RN-BH](3) (10: R = H; 11: R = Me; 12: R = Ph) was demonstrated. TEM analysis of the contents of the reaction solution for the [Rh(1,5-cod)(mu-Cl)](2) catalyzed dehydrocoupling of Me(2)NH.BH(3) together with Hg poisoning experiments suggested a heterogeneous catalytic process involving Rh(0) colloids.

In depth, comparative studies on the catalytic dehydrocoupling of the amine-borane adduct Me(2)NH.BH(3) (to form [Me(2)N-BH(2)](2)) and the phosphine-borane adduct Ph(2)PH.BH(3) (to form Ph(2)PH-BH(2)-PPh(2)-BH(3)) with a variety of Rh (pre)catalysts such as [[Rh(1,5-cod)(micro-Cl)](2)], Rh/Al(2)O(3), Rh(colloid)/[Oct(4)N]Cl, and [Rh(1,5-cod)(2)]OTf have been performed in order to determine whether the dehydrocoupling proceeds by a homogeneous or heterogeneous mechanism. The results obtained suggest that the catalytic dehydrocoupling of Me(2)NH.BH(3) is heterogeneous in nature involving Rh(0) colloids, while that of Ph(2)PH.BH(3) proceeds by a homogeneous mechanism even when starting with Rh(0) precursors such as Rh/Al(2)O(3). The catalytic dehydrocoupling reactions are thought to proceed by different mechanisms due to a combination of factors such as (i) the greater reducing strength of amine-borane adducts, (ii) the increased ease of dissociation of phosphine-borane adducts, and (iii) phosphine ligation and/or poisoning of active catalytic sites on metal colloids.

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-molecularweight 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 iBuPH 2 ‚BH 3 was also investigated and was found to be much slower than that of PhPH 2 ‚BH 3 . This produced poly(isobutylphosphinoborane) [iBuPH-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 iBuPH 2 ‚BH 3 proceeds very slowly and forms only low-molecular-weight materials with complex NMR spectra and which probably possess a branched structure.

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[structure: see text]. Phenanthrene analogues with internalized B-N moieties were found to afford blue light emission with good quantum efficiencies, whereas the isomeric species with peripheral B-N moieties displayed only UV emission behavior, like the all-carbon framework.