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 [PdCl2(COD)], in the presence or absence of KPF6 in CH3CN, afforded complexes [(PhPCP)PdCl]PF6 1, [(tBuPCP)PdCl]Cl 2, and [(tBuPCP)PdCl]PF6 3 in excellent isolated yields. Interestingly, the reaction between L2 and [PdCl2(COD)] in CH2Cl2 yielded the unexpected noncarbene amine-nitrogen-coordinated complex [(tBuPNP)PdCl] PF6 4. Complexes 1 and 3 react with PhSNa to give their thiolate complexes, [(PhPCP)Pd(SPh)]PF6 5 and [(tBuPCP)Pd(SPh)]PF6 6, in good yields. Complex 3 reacts with an excess amount of NaBH4 in THF to give the hydride complex [(tBuPCP)Pd(H)]PF6, 7. Of these, the palladium thiolate and the previously reported analogous nickel thiolate complexes were found to be efficient catalysts for the hydroboration of CO2 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, [(tBuPCP)Ni(SPh)]PF6, 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.