Synthesis and Reactivity of the Phosphinoboranes R₂PB(C₆F₅)₂

Abstract

The phosphinoboranes [R(2)PB(C(6)F(5))(2)](2) (R = Et 1, Ph 2) and R(2)PB(C(6)F(5))(2) (R = tBu 3, Cy 4, Mes 5) were synthesized from the reaction of (C(6)F(5))(2)BCl and the corresponding lithium phosphide. The relationships between B-P distance, P pyramidality, and the extent of BP multiple bonding were further explored computationally. Natural Bond Order (NBO) analyses of 3 and 4 showed that the π-bonding highest occupied molecular orbitals (HOMOs) were highly polarized. In addition the Lewis acid-base adducts, R(2)(H)P·B(H)(C(6)F(5))(2) (R = Et 6; Ph 7; tBu 8; Cy 9; Mes 10) were prepared via the reaction of the phosphines R(2)PH with the borane HB(C(6)F(5))(2). Compounds 1 and 2 showed no signs of reaction with H(2); however, reaction of compounds 3 and 4 with H(2) was observed to give 8 and 9. In a related set of reactions compounds 3 and 4 were reacted with H(3)NBH(3) or Me(2)(H)NBH(3) also led to the generation of 8 and 9, respectively. The reaction profile of the reaction of (CF(3))(2)BPR(2) with H(2) was examined computationally and shown to be exothermic. Efforts to effect the reverse reaction, that is, dehydrogenation of adducts 6-10 were unsuccessful. Compound 4 was also shown to react with 4-tert-butylpyridine to give Cy(2)PB(C(6)F(5))(2)(4-tBuC(5)H(4)N) 11 while reactions of 3 and 4 with the Lewis acid BCl(3) gave the dimers (R(2)PBCl(2))(2) (R = tBu 12, Cy 13) and the byproduct ClB(C(6)F(5))(2).

Hydroboration of Phosphaalkynes by HB(C6F5)2

“…This reactivity contrasts with that of phosphinoborane dimers [(C6F5)2BPR2]2 which proved unreactive toward donor molecules. [24,29] Collectively these data are consistent with the formulation of 3a as tBuCH=PB(C6F5)2(tBuNC). The adduct 3a was consistently isolated as a pale oil, so analogous chemistry with 2b was undertaken with the goal of obtaining crystallographic data.…”

“…The P-B bond lengths average 2.040(7) Å in 3b and was found to be 2.029(2) Å in 4a. These P-B distances are significantly longer than those of (C6F5)2BPR2 (R = tBu, Cy), [24] which were found to be 1.786(4) Å and 1.762(4) Å. This difference presumably reflects the poorer sigma-donor ability of sp 2 vs sp 3 phosphorus.…”

“…P-B-P bond angles of 81.14(8)° and 81.01(8)°, and B-P-B bond angles of 96.17(9)° and 96.24(9)° were found in the central ring. The P-B bond lengths in 2a ranged from 2.009(2) -2.013(2) Å; these distances are at the short end of those of phosphinoborane dimers, [R2B-PR′2]2, which range from 2.004(4) to 2.096(5) Å, [24] but are longer than those of P2B2 diradicaloids, which range from 1.8904(15) to 1.900(2) Å.…”

Experimental and Theoretical Studies of the Potential Interconversion of the Amine–Borane iPr2NH·BH(C6F5)2 and the Aminoborane iPr2N=B(C6F5)2 Involving Hydrogen Loss and Uptake

“…[51] In 2008, Stephan and co-workers reported the irreversible activation of dihydrogen by the phosphanylborane systems, tBu 2 P-B(C 6 F 5 ) 2 and Cy 2 P-B(C 6 F 5 ) 2 (Scheme 3). [52,53] Scheme 3. Hydrogenation of R 2 P-B(C 6 F 5 ) 2 .…”

“…Hydrogenation of R 2 P-B(C 6 F 5 ) 2 . [52,53] Aminoboranes and phosphanylboranes are formally isoelectronic, although the bonding within these systems is markedly different, with the 3p phosphorus centre leading to poorer orbital overlap with the vacant 2p-orbital at boron, and more significant lone-pair character at phosphorus. [54] The demonstration of hydrogen uptake by phosphanylboranes, coupled with our own computational studies of aminoborane hydrogenation provided the impetus for www.eurjic.orgour current investigation.…”