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