Catalytic CO2 activation assisted by rhenium hydride/B(C 6F5)3 frustrated Lewis pairs - Metal hydrides functioning as FLP bases

Article abstract of DOI:10.1021/ja402381d

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(C₆F ₅)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(C ₆F₅)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)(PiPr ₃)₂] (4a) bearing trans-phosphines. Stoichiometric CO ₂ reduction of 4a with Et₃SiH led to heterolytic splitting of H₂ along with formation of bis(triethylsilyl)acetal ((Et ₃SiO)₂CH₂, 7). Catalytic reduction of CO ₂ 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, PtBu ₃) was studied in greater detail, pointing to two crucial factors of the CO₂ hydrogenations: the steric bulk and the basicity of the base.