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)(eta(2)-O=C=O-B(C6F5)(3))] (2, R = iPr a, Cy b) possessing two cis-phosphines and O-CO2-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) eta(1)-formato dimer [{Re(mu-Br)(NO)(eta(1)-OCH=O-B-(C6F5)(3))(PiPr(3))(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 H-2 cleaved the mu-Br bridges, producing the stable and fully characterized formato dihydrogen complex [ReBrH2(NO)(eta(1)-OCH=O-B(C6F5)(3))(PiPr(3))(2)] (4a) bearing trans-phosphines. Stoichiometric CO2 reduction of 4a with Et3SiH led to heterolytic splitting of H-2 along with formation of bis(triethylsilyl)acetal ((Et3SiO)(2)CH2, 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 H-2 ligand deprotonation. Hydrogenations of CO2 using 1a,b/B(C6F5)(3), 3a, and 4a as catalysts gave in the presence of TAP 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.