Spontaneous dehydrocoupling in peri-substituted phosphine–borane adducts

Abstract

Bis(borane) adducts Acenap(PiPr2·BH3)(PRH·BH3) (Acenap = acenaphthene-5,6-diyl; 4a, R = Ph; 4b, R = ferrocenyl, Fc; 4c, R = H) were synthesised by the reaction of excess H3B·SMe2 with either phosphino-phosphonium salts [Acenap(PiPr2)(PR)](+)Cl(-) (1a, R = Ph; 1b, R = Fc), or bis(phosphine) Acenap(PiPr2)(PH2) (3). Bis(borane) adducts 4a-c were found to undergo dihydrogen elimination at room temperature, this spontaneous catalyst-free phosphine-borane dehydrocoupling yields BH2 bridged species Acenap(PiPr2)(μ-BH2)(PR·BH3) (5a, R = Ph; 5b, R = Fc; 5c, R = H). Thermolysis of 5c results in loss of the terminal borane moiety to afford Acenap(PiPr2)(μ-BH2)(PH) (14). Single crystal X-ray structures of 3, 4b and 5a-c are reported.

Hydride Abstraction and Deprotonation – an Efficient Route to Low Coordinate Phosphorus and Arsenic Species

“…This 1 J P,H coupling constant is comparable to that observed in the primary phosphine 1b ( 1 J P,H = 204 Hz). [30] Similar to the arsenic analogue 2a, the 2 J H,P through-bond coupling of the phosphorus-bound hydrogen, as observed in the 1 H NMR spectrum ( 2 J H,P = 9.3 Hz), is actually considerably smaller than the related through-space coupling in compound 1b ( 5 J H,P = 47.8 Hz). [30] The crystal structure of 2b is shown in Figure 1 and Table 1.…”

“…[30] Similar to the arsenic analogue 2a, the 2 J H,P through-bond coupling of the phosphorus-bound hydrogen, as observed in the 1 H NMR spectrum ( 2 J H,P = 9.3 Hz), is actually considerably smaller than the related through-space coupling in compound 1b ( 5 J H,P = 47.8 Hz). [30] The crystal structure of 2b is shown in Figure 1 and Table 1. Once again, the compound shows a relatively relaxed acenaphthene backbone, with a P9-C9···C1-P1 torsion angle of only 4.0(1)°.…”

“…Primary phosphine 1b, which was recently synthesised by our group, [30] was subjected to heating in both acetonitrile and toluene to determine if 3b could be formed directly. However, it was found that hydrogen evolution does not occur and no 3b was detected after prolonged heating.…”

“…Subjecting 1b to sequential hydride abstraction and deprotonation, however, seemed a viable route to 3b. This is due in part to the success of these reactions on the arsenic congener, but also on account of an [M − H] + peak observed in the ES+ MS spectrum of compound 1b [30] , suggesting that loss of hydride from 1b is facile. Treatment of 1b with Ph 3 C·BF 4 under the same conditions as 1a led to the formation of phosphino-phosphonium salt 2b as a white precipitate in high yield (75%) and purity.…”

“…Compounds 1a and 1b were synthesised according to literature procedures. [23,30] Where possible, new compounds were fully characterized by 31 …”