Frustrated and classical Lewis pairs arising from combinations of Lewis acids and phosphines react with terminal alkynes either via C-H activation forming an alkynylborate salt or by addition to alkyne giving a zwitterionic phosphonium borate.

Catechol borane reacts with the frustrated Lewis pairs tBu2RP (R = tBu, 2-C6H4(C6H5)) and B(C6F5)3 to give the species [(C6H4O2)BPtBu2R][HB(C6F5)3] that can formally be described as either borenium cation or boryl-phosphonium salts; the nature of these species was probed with DFT calculations.

Reaction of Piers' borane, HB(C(6)F(5))(2), with either tert-butyl or isopropyl carbodiimide cleanly affords the boron amidinates HC(RN)(2)B(C(6)F(5))(2) [R = iPr (1), tBu (2)]. These species undergo a variety of insertion reactions. For example, treatment of 1 with CO(2) or excess carbodiimide gives HC(iPrN)(2)(CO(2))B(C(6)F(5))(2) (3) or HC(iPrN)(2)C(iPrN)(2)B(C(6)F(5))(2) (4), respectively. Similarly, exposure of 1 or 2 to 1 atm CO gives HC(RN)(2)(CO)B(C(6)F(5))(2) [R = iPr (5), tBu (6)], while reaction of 1 with CNtBu gives HC(RN)(2)(CNtBu)B(C(6)F(5))(2) [R = iPr (7), tBu (8)]. Compounds 1 and 2 also react with benzaldehyde, resulting in the formation of HC(RN)(2)(PhHCO)B(C(6)F(5))(2) [R = iPr (9), tBu (10)]. Compound 1 also reacts with MeCN to give HC(iPrN)(2)(MeCN)B(C(6)F(5))(2) (11) and effects heterolytic C-H cleavage to afford HC(iPrN)(iPrNH)(PhCC)B(C(6)F(5))(2) (12). In contrast, the species PhC(iPrN)(2)B(C(6)F(5))(2) (14) derived from PhC(iPrN)(2)BCl(2) (13) failed to react with any of the above substrates. These data, in addition to the isolation of HC(iPrN)(2)(C(6)F(4))BF(C(6)F(5)) (15), the product of thermolysis of 1, provide further support for the notion that the transient "open-chain" form of these amidinates is present in solution.

Deprotonation of terminal alkynes is effected by the treatment with tBu3P and B(C6F5)3 to give [tBu3PH][PhCCB(C6F5)3] (1a) and [tBu3PH][RCCB(C6F5)3] (R = nBu 2, tBu 3, Me3Si 4, and CpFe(C5H4) 5). In a similar fashion, FLP deprotonation of 1,4-diethynylbenzene was employed to prepare the salt [tBu3PH]2[(C6F5)3BCC(C6H4)CCB(C6F5)3] (6). As well, use of differing Lewis acids afforded the species [tBu3PH][PhCCEAr3] (EAr3 = Al(C6F5)3 7, PhB(C6F5)2 8, BPh3 9). The corresponding reaction of Me3SiCCSiMe3 afforded [tBu3PSiMe3][Me3SiCCB(C6F5)3] (10). For less basic phosphines, phosphine/borane addition reactions were observed with alkynes. Thus the species E-R3P(Ph)CC(H)B(C6F5)3 (R = o-tol 11, Ph 12) were prepared. In the latter case, Ph3P·B(C6F5)3 was employed. Similarly, E-Ph3P(CpFe(C5H4))CC(H)B(C6F5)3 (13), E-Ph3P(Ph)CC(Me)B(C6F5)3 (14), E-R2PH(Ph)CC(H)B(C6F5)3 (R = Ph 15, C6H2Me3 16), and E-(C6H2 tBu3)PH2(Ph)CC(H)B(C6F5)3 (17) were prepared. Again, variation in the Lewis acid afforded E-Ph3P(Ph)CC(H)EAr3 (EAr3 = PhB(C6F5)2 18, Al(C6F5)3 19), E-Ph3P(nBu)CC(H)Al(C6F5)3 (20), and E-(o-tol)3P(Ph)CC(H)Al(C6F5)3 (21). The macrocyclic [(H)CC(Ph)Mes2PC6F4B(C6F5)2]2 (22) was prepared from the analogous alkyne addition to Mes2PC6F4B(C6F5)2, while E-Ph2PCH2CH2PPh2(Ph)CC(H)B(C6F5)3 (23) and E-(CH2PPh2(Ph)CC(H)B(C6F5)3)2 (24) were derived from the reactions of Ph2PCH2CH2PPh2. The related addition reaction involving 1,4-diethynylbenzene gave E-HCCC6H4C(PPh3)C(H)B(C6F5)3 (25), while subsequent reaction with tBu3P and B(C6F5)3 yielded the unusual salt/zwitterion [tBu3PH][(C6F5)3BCCC6H4C(PPh3)C(H)-B(C6F5)3] (26). Reaction of PhCH2NMe2 with PhCCH and B(C6F5)3 gave give a 84:16 mixture of [PhCH2NMe2H][PhCCB(C6F5)3] (27a) and PhCH2NMe2(Ph)CC(H)B(C6F5)3 (27b), while imines were used to prepare [(tBu)HNCHPh][PhCCB(C6F5)3] (28) and [(tBu)HNCPh2][PhCCB(C6F5)3] (29). The corresponding reaction of tBuNCNtBu, B(C6F5)3, and two equivalents of PhCCH led to the unusual product [tBuNCN(H)C(Ph)C(H)tBu][PhCCB(C6F5)3] (30). Finally, non-pnictogen Lewis bases were explored. The reaction of the N-heterocyclic carbene ItBu with B(C6F5)3 and PhCCH was shown to yield the deprotonation product [ItBuH][PhCCB(C6F5)3] (31), while the sulfides R2S gave E-R2S(Ph)CC(H)B(C6F5)3 (R = Me 32, PhCH2 33). The formation of these latter sulfide zwitterions was demonstrated to be reversible.

The addition of diphenyl disulfide (PhSSPh) to tBu(2)P(C(6)F(4))B(C(6)F(5))(2) (1) affords the zwitterionic phosphonium borate [tBu(2)P(SPh)(C(6)F(4))B(SPh)(C(6)F(5))(2)] (2), while the addition of a base or donor solvent to 2 effected the liberation of disulfide and the formation of [tBu(2)P(C(6)F(4))B(donor)(C(6)F(5))(2)]. The reaction of 1 with S(8) gave tBu(2)P(S)(C(6)F(4))B(C(6)F(5))(2) (3). In a similar fashion, the frustrated Lewis pair of tBu(3)P/B(C(6)F(5))(3) reacts with RSSR to give [tBu(3)P(SR)][(RS)B(C(6)F(5))(3)] (R = Ph (4), p-tolyl (5), iPr (6)). In contrast, the corresponding reaction of BnSSBn yields a 1:1:1 mixture of tBu(3)P horizontal lineS, Bn(2)S, and B(C(6)F(5))(3). Species 4 reacts with p-tolylSSp-tolyl to give a mixture of 4, 5, PhSSPh, and p-tolylSS p-tolyl, while treatment of 5 with PhSSPh afforded a similar mixture. To probe this, a crossover experiment between [tBu(3)P(SPh)][B(C(6)F(5))(4)] (7) and [NBu(4)][(p-tolylS)B(C(6)F(5))(3)] (9) was performed. The former species was prepared by a reaction of 4 with [Ph(3)C][B(C(6)F(5)) (4)], while cation exchange of [(Et(2)O)(2)Li( p-tolylS)B(C(6)F(5))(3)] (8) with [NBu(4)]Br gave 9. The reaction of compounds 7 and 9 gave a statistical mixture of the cations [tBu(3)P(SR)](+) and anions [(RS)B(C(6)F(5))(3)](-), R = Ph, Sp-tolyl. The mechanism of this exchange process was probed and is proposed to be an equilibrium involving disulfide and the frustrated Lewis pair. Crystallographic data are reported for compounds 4-8, and the natures of the P-S cations are examined via DFT calculations.