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

The frustrated Lewis pair (FLP) derived from CIB(C6F5)(2) and the bulky Lewis bases 2,2,6,6-tetramethylpiperidine (TMP), tri-tert-butylphosphine, and tris(2,4,6-trimethylphenyl)phosphine cleaved H-2 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 H-2, 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 H-2 to yield initially [MeS3PH][H2B(C6F5)(2)]. The [H2B(C6F5)(2)](-) anion undcrwentdisproportionation toform [(MeSPH)-P-3][HB(C6F5)(3)] (3b), MeS3P, [H2B(C6F5)](2), and H-2. Similarly, 2,4,6-tri-tert-butylpyridine (TTBP) and [HB(C6F5)(2)](n), gave in the presence of H-2, 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 H-2 and syn- and anti-[H2B(C6F5)](2) (3c). 1a, 2a, 3a, and 4a were studied by single-crystal X-ray diffraction analysis.