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 pKa’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.