Catalytic CO2 Activation Assisted by Rhenium Hydride/B(C6F5)3 Frustrated Lewis PairsMetal Hydrides Functioning as FLP Bases

Reaction of 1 with B­(C6F5)3 under 1 bar of CO2 led to the instantaneous formation of the frustrated Lewis pair (FLP)-type species [ReHBr­(NO)­(PR3)2(η2-OCO–B­(C6F5)3)] (2, R = iPr a, Cy b) possessing two cis-phosphines and OCO2-coordinated B­(C6F5)3 groups as verified by NMR spectroscopy and supported by DFT calculations. The attachment of B­(C6F5)3 in 2a,b establishes cooperative CO2 activation via the Re–H/B­(C6F5)3 Lewis pair, with the Re–H bond playing the role of a Lewis base. The Re­(I) η1-formato dimer [{Re­(μ-Br)­(NO)­(η1-OCHO–B­(C6F5)3)­(PiPr3)2}2] (3a) was generated from 2a and represents the first example of a stable rhenium complex bearing two cis-aligned, sterically bulky PiPr3 ligands. Reaction of 3a with H2 cleaved the μ-Br bridges, producing the stable and fully characterized formato dihydrogen complex [ReBrH2(NO)­(η1-OCHO–B­(C6F5)3)­(PiPr3)2] (4a) bearing trans-phosphines. Stoichiometric CO2 reduction of 4a with Et3SiH led to heterolytic splitting of H2 along with formation of bis­(triethylsilyl)­acetal ((Et3SiO)2CH2, 7). Catalytic reduction of CO2 with Et3SiH was also accomplished with the catalysts 1a,b/B­(C6F5)3, 3a, and 4a, showing turnover frequencies (TOFs) between 4 and 9 h–1. The stoichiometric reaction of 4a with the sterically hindered base 2,2,6,6-tetramethylpiperidine (TMP) furnished H2 ligand deprotonation. Hydrogenations of CO2 using 1a,b/B­(C6F5)3, 3a, and 4a as catalysts gave in the presence of TMP TOFs of up to 7.5 h–1, producing [TMPH]­[formate] (11). The influence of various bases (R2NH, R = iPr, Cy, SiMe3, 2,4,6-tri-tert-butylpyridine, NEt3, PtBu3) was studied in greater detail, pointing to two crucial factors of the CO2 hydrogenations: the steric bulk and the basicity of the base.