Metal-Free Hydrogen Activation by the Frustrated Lewis Pairs of ClB(C₆F₅)₂ and HB(C₆F₅)₂ and Bulky Lewis Bases

Abstract

The frustrated Lewis pair (FLP) derived from ClB(C6F5)2 and the bulky Lewis bases 2,2,6,6-tetramethylpiperidine (TMP), tri-tert-butylphosphine, and tris(2,4,6-trimethylphenyl)phosphine cleaved H2 heterolytically to form the intermediate anion [HClB(C6F5)2]−, which quickly underwent hydride/chloride exchange with the remaining ClB(C6F5)2 to give the known compound [HB(C6F5)2] n (n = 1 or 2) and the anion [Cl2B(C6F5)2]− present in the products [TMPH][Cl2B(C6F5)2] (1a), [t-Bu3PH][Cl2B(C6F5)2] (2a), and [Mes3PH][Cl2B(C6F5)2] (3a). [HB(C6F5)2] n forms Lewis adducts with TMP and t-Bu3P: TMP-BH(C6F5)2 (1b) and t-Bu3P-BH(C6F5)2 (2b). The Lewis adduct t-Bu3P-BH(C6F5)2 was found capable of generating a FLP at elevated temperature and was reacted with H2, producing the splitting product [t-Bu3PH][H2B(C6F5)2] (2c). Mes3P forms no Lewis adduct with [HB(C6F5)2] n , but a FLP, which was also capable of splitting H2 to yield initially [Mes3PH][H2B(C6F5)2]. The [H2B(C6F5)2]− anion underwent disproportionation to form [Mes3PH][HB(C6F5)3] (3b), Mes3P, [H2B(C6F5)]2, and H2. Similarly, 2,4,6-tri-tert-butylpyridine (TTBP) and [HB(C6F5)2] n gave in the presence of H2 the final products [TTBPH][HB(C6F5)3] salt and [H2B(C6F5)]2. The contrasting reactivities of the t-Bu3P/[BH(C6F5)2] n , Mes3P/[HB(C6F5)2] n , and TTBP/[HB(C6F5)2] n pairs were explained on the basis of the different pK a’s of the [LBH]+ cations. After disproportionation of the [H2B(C6F5)2]− anion to give [Mes3PH][HB(C6F5)3] (3b) or [TTBPH][HB(C6F5)3] (4a), the also formed [H3B(C6F5)]− anion reacted with the more acidic cations ([Mes3PH]+, [TTBPH]+) to give H2 and syn- and anti-[H2B(C6F5)]2 (3c). 1a, 2a, 3a, and 4a were studied by single-crystal X-ray diffraction analysis.

Intramolecular Frustrated Lewis Pair with the Smallest Boryl Site: Reversible H2 Addition and Kinetic Analysis

“…[19] 1,2-Linked (vicinal) FLPs that do not form intramolecular dative bonds are rare, but known. [15,23] Our results indicate that dimerization by B(m-H) 2 B bridges leads to a stable species for aminoborane 1 as well, for which trans-(1) 2 was found to be the prevailing form both in the solid state and in solution. On the other hand, fairly stable fourmembered heterocycles are already observed when the steric hindrance is slightly decreased.…”

“…According to the Gutmann-Beckett and Childs acidity scales, the Lewis acidity of inorganic boranes BX 3 (X = H or halogen) is comparable to that of the commonly used borane B(C 6 F 5 ) 3 . [14,15] Another critical issue in our approach is the reduced size of the BX 2 unit, which may restrict the reactivity by different quenching modes. [14,15] Another critical issue in our approach is the reduced size of the BX 2 unit, which may restrict the reactivity by different quenching modes.…”

Steric and electronic effects on the heterolytic H2‐splitting by phosphine‐boranes R3B/PR′3(R = C6F5, Ph; R′ = C6H2Me3,tBu, Ph, C6F5, Me, H): A computational study

“…Although the Repo group [28] described the H 2 activation using amines and boranes, Erker and co-workers [29][30][31] developed the FLP catalysts using linked phosphonium borates. A number of exciting applications related to FLPs have already emerged [32][33][34][35][36][37][38][39][40][41].…”

Phosphonium salts and P-ylides

“…The reversibility of H 2 activation for a few P/B pairs was examined with various experiments in order to apply the reactivity of FLPs to develop new approaches to H 2 storage. 18 The FLP derived from ClB(C 6 F 5 ) 2 and the bulky Lewis bases 2,2,6,6-tetramethylpiperidine (TMP), tri-tert-butylphosphine, and tris(2,4,6-trimethylphenyl)phosphine was found to cleave H 2 heterolytically to form the intermediate anion [HClB(C 6 F 5 ) 2 ] À , which quickly underwent hydride/ chloride exchange with the remaining ClB(C 6 F 5 ) 2 to give the known compound [HB(C 6 F 5 ) 2 ] n (n = 1 or 2) with the anion [Cl 2 B(C 6 19 In continuation, para-substitution found previously for classical phosphine adducts of B(C 6 F 5 ) 3 under warming to yield the air-and moisture-stable zwitterions R 3 P-C 6 H 4 B(F)(C 6 F 5 ) 2 , was observed for smaller phosphines after the combined toluene solutions of the reagents were heated under reflux, confirming the general character of this reaction. 20 In computational studies of H 2 activation by FLPs, Pa´pai and co-workers suggested generation of a phosphine-borane ''encounter complex'', stabilized by HÁÁÁF interactions.…”

“…24 The reactions of FLPs with alkenes, aldehydes, and a variety of other small molecules, including carbon dioxide, in cooperative three-component processes, offer new strategies and perspectives for synthetic chemistry. Thus, the FLP formed by the combination of tBu 3 P and B(C 6 F 5 ) 3 reacts with 1,3-dienes to give zwitterionic 1,4-addition phosphonium borates, e.g., (19), the geometry of which appears to be dominated by the steric demands of the substrate. 25 Estimation of the ability of phosphonium borates of the form [R 3 PH][B(C 6 F 5 ) 4 ], R 2 PHC 6 F 4 BF(C 6 F 5 ) 2 and R 2 PHC 4 H 8 OB(C 6 F 5 ) 3 (R 2= Cg 2 , Mes 2 , tBu(Mes), tBu 2 ) as well as the phosphine-boranes R 2 PC 6 F 4 B(C 6 F 5 ) 2 to activate CpTiMe 2 (NPtBu 3 ) for olefin polymerization both in stoichiometric and catalytic reactions, suggest that metal-based FLPs are uniquely reactive.…”