Contrasting E−H Bond Activation Pathways of a Phosphanyl‐Phosphagallene

Abstract

The reactivity of the phosphanyl‐phosphagallene, [H2C{N(Dipp)}]2PP=Ga(Nacnac) (Nacnac=HC[C(Me)N(Dipp)]2; Dipp=2,6‐iPr2C6H3) towards a series of reagents possessing E−H bonds (primary amines, ammonia, water, phenylacetylene, phenylphosphine, and phenylsilane) is reported. Two contrasting reaction pathways are observed, determined by the polarity of the E−H bonds of the substrates. In the case of protic reagents (δ−E−Hδ+), a frustrated Lewis pair type of mechanism is operational at room temperature, in which the gallium metal centre acts as a Lewis acid and the pendant phosphanyl moiety deprotonates the substrates. Interestingly, at elevated temperatures both NH2iPr and ammonia can react via a second, higher energy, pathway resulting in the hydroamination of the Ga=P bond. By contrast, with hydridic reagents (δ+E−Hδ−), such as phenylsilane, hydroelementation of the Ga=P bond is exclusively observed, in line with the polarisation of the Si−H and Ga=P bonds.

Selective 1,2 addition of polar X–H bonds to the Ga–P double bond of gallaphosphene L(Cl)GaPGaL

“…Addition of NH 3 gas (1 atm) to a red solution of 1 in toluene at ambient temperature instantaneously gave a colorless solution, from which LGa(Cl)PH(NH 2 )GaL 2 was isolated as colorless solid in 92% isolated yield (Scheme 2). The selective 1,2 addition of the N-H bond to the Ga-P double bond of 1 stands in remarkable contrast to the reaction of phosphanylphosphagallene, 19 which reacts at ambient temperature in a FLP-type mechanism at the 1,3 positions of the Ga-P-P unit, and only thermal treatment of this 1,3 addition product to 80 °C resulted in 1,2 proton migration and formation of the 1,2 addition product. 19 Addition of Lewis acidic B(C 6 F 5 ) 3 to the 1,3 addition product proceeded with regeneration of the phosphanyl-phosphagallene.…”

“…The selective 1,2 addition of the N-H bond to the Ga-P double bond of 1 stands in remarkable contrast to the reaction of phosphanylphosphagallene, 19 which reacts at ambient temperature in a FLP-type mechanism at the 1,3 positions of the Ga-P-P unit, and only thermal treatment of this 1,3 addition product to 80 °C resulted in 1,2 proton migration and formation of the 1,2 addition product. 19 Addition of Lewis acidic B(C 6 F 5 ) 3 to the 1,3 addition product proceeded with regeneration of the phosphanyl-phosphagallene. 19 The 1 H NMR spectrum of 2 shows a broad resonance at 0.48 ppm for the NH 2 moiety, which agrees well with that of P′ (H)PGa(NH 2 )L (0.47 ppm, P′ = (H 2 C) 2 (NAr) 2 P), 19 whereas it is shifted to lower field compared to that of LGa(NH 2 ) 2 (−0.58 ppm).…”

“…19 Addition of Lewis acidic B(C 6 F 5 ) 3 to the 1,3 addition product proceeded with regeneration of the phosphanyl-phosphagallene. 19 The 1 H NMR spectrum of 2 shows a broad resonance at 0.48 ppm for the NH 2 moiety, which agrees well with that of P′ (H)PGa(NH 2 )L (0.47 ppm, P′ = (H 2 C) 2 (NAr) 2 P), 19 whereas it is shifted to lower field compared to that of LGa(NH 2 ) 2 (−0.58 ppm). 21 In addition, 2 displayed the expected doublets in the 1 H NMR (−0.84 ppm, 1 J P-H = 174.5 Hz; Fig.…”

“…borapnictenes with B-Pn (Pn = N, P, As) 11 and metallaimines with M-N double bonds (M = Al, Ga, In), 12 the heavier congeners with M-Pn (M = Al, Ga; Pn = P, As, Sb) double bonds have been reported only recently, [13][14][15][16][17] and their reactivity studies are in its infancy. [16][17][18][19][20] Such compounds possess polarized double bonds due to the different electronegativities of the group 13 and 15 elements, hence, revealing a chemical reactivity that differs largely from homonuclear heavier group 14 analogues of alkenes. We recently reported on a series of reactions of gallaarsenes and -stibenes L(Cl)GaPnGaL (Pn = As, Sb; L = HC[C(Me)N(Dipp)] 2 , Dipp = 2,6-i-Pr 2 C 6 H 3 ) with both electrophilic and nucleophilic reagents, 18 while Goicoechea et al reported on frustrated Lewis pair-type (FLP-type) reactions of phosphanyl-substituted phosphagallene (P′)PGaL (P′ = (H 2 C) 2 (NAr) 2 P) with a variety of small molecules (H 2 , CO 2 , NH 3 , RNH 2 , H 2 O etc.).…”

“…The reactions typically occurred at ambient temperature at the 1,3 positions of the Ga-P-P moiety rather than at the Ga-P double bond, 16a,19 however, thermal treatment of some 1,3 addition products resulted in a 1,2 proton migration and subsequent formation of the 1,2 addition products. 19 Very recently, we reported on reversible [2 + 2] and [2 + 2 + 2] cycloaddition reactions of gallaphosphene LGa(Cl)PGaL 1 with heteroallenes (CO 2 , isocyanates, carbodiimides) at the Ga-P double bond, 17,20 while the reactions with ketones proceeded via a C(sp 3 )-H bond activation at the Ga-P double bond. 17 This unprecedented C(sp 3 )-H bond activation reaction as well as the scarcity of π-bonded metallapnictenes to activate X-H bonds encouraged us to investigate X-H bond activation reactions of gallaphosphene 1 with compounds with different X-H bond polarity, and we herein report on reactions with ammonia, primary amines, water, phenol, thiophenol, and selenophenol, respectively (Scheme 1e).…”