The Hydroboration of CO 2 with 9-BBN to Methoxyborane Catalyzed by Pincer Carbene Palladium(II) and Nickel(II) Thiolates with High Turnover Numbers

The reactions between 1,3-bis((diphenylphosphaneyl)methyl)imidazolidine L1 or 1,3-bis((di- tert -butylphosphaneyl)methyl)imidazolidine L2 and [PdCl 2 (COD)], in the presence or absence of KPF 6 in CH 3 CN, afforded complexes [( Ph PCP)PdCl]PF 6 1, [( t Bu PCP)PdCl]Cl 2, and [( t Bu PCP)PdCl]PF 6 3 in excellent isolated yields. Interestingly, the reaction between L2 and [PdCl 2 (COD)] in CH 2 Cl 2 yielded the unexpected noncarbene amine-nitrogen-coordinated complex [( t Bu PNP)PdCl] PF 6 4 . Complexes 1 and 3 react with PhSNa to give their thiolate complexes, [( Ph PCP)Pd(SPh)]PF 6 5 and [( t Bu PCP)Pd(SPh)]PF 6 6, in good yields. Complex 3 reacts with an excess amount of NaBH 4 in THF to give the hydride complex [( t Bu PCP)Pd(H)]PF 6, 7 . Of these, the palladium thiolate and the previously reported analogous nickel thiolate complexes were found to be efficient catalysts for the hydroboration of CO 2 with 9-BBN to selectively give methoxyborane in excellent yields under mild conditions, compared to the corresponding halide and hydride complexes. Complex 6 and complex 8, [( tBu PCP)Ni(SPh)]PF 6, exhibit TONs of over 25,000 and 7,000, respectively. A combined proposed mechanism, based on control experiments, shows that the thiolate complexes form reactive adducts, exhibit two pathways for MeOBBN formation, and hence outperform the hydride complexes.