132001-45-7Relevant academic research and scientific papers
Catalytic CO2 activation assisted by rhenium hydride/B(C 6F5)3 frustrated Lewis pairs - Metal hydrides functioning as FLP bases
Jiang, Yanfeng,Blacque, Olivier,Fox, Thomas,Berke, Heinz
, p. 7751 - 7760 (2013)
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.
