Ph₂PCH₂CH₂B(C₈H₁₄) and Its Formaldehyde Adduct as Catalysts for the Reduction of CO₂ with Hydroboranes

We study two metal-free catalysts for the reduction of CO₂ with four different hydroboranes and try to identify mechanistically relevant intermediate species. The catalysts are the phosphinoborane Ph₂P(CH₂)₂BBN (1), easily accessible in a one-step synthesis from diphenyl(vinyl)phosphine and 9-borabicyclo[3.3.1]nonane (H-BBN), and its formaldehyde adduct Ph₂P(CH₂)₂BBN(CH₂O) (2), detected in the catalytic reduction of CO₂ with 1 as the catalyst but properly prepared from compound 1 and p-formaldehyde. Reduction of CO₂ with H-BBN gave mixtures of CH₂(OBBN)₂ (A) and CH₃OBBN (B) using both catalysts. Stoichiometric and kinetic studies allowed us to unveil the key role played in this reaction by the formaldehyde adduct 2 and other formaldehyde-formate species, such as the polymeric BBN(CH₂)₂(Ph₂P)(CH₂O)BBN(HCO₂) (3) and the bisformate macrocycle BBN(CH₂)₂(Ph₂P)(CH₂O)BBN(HCO₂)BBN(HCO₂) (4), whose structures were confirmed by diffractometric analysis. Reduction of CO₂ with catecholborane (HBcat) led to MeOBcat (C) exclusively. Another key intermediate was identified in the reaction of 2 with the borane and CO₂, this being the bisformaldehyde-formate macrocycle (HCO₂){BBN(CH₂)₂(Ph₂P)(CH₂O)}₂Bcat (5), which was also structurally characterized by X-ray analysis. In contrast, using pinacolborane (HBpin) as the reductant with catalysts 1 and 2 usually led to mixtures of mono-, di-, and trihydroboration products HCO₂Bpin (D), CH₂(OBpin)₂ (E), and CH₃OBpin (F). Stoichiometric studies allowed us to detect another formaldehyde-formate species, (HCO₂)BBN(CH₂)₂(Ph₂P)(CH₂O)Bpin (6), which may play an important role in the catalytic reaction. Finally, only the formaldehyde adduct 2 turned out to be active in the catalytic hydroboration of CO₂ using BH₃·SMe₂ as the reductant, yielding a mixture of two methanol-level products, [(OMe)BO]₃ (G, major product) and B(OMe)₃ (H, minor product). In this transformation, the Lewis adduct (BH₃)Ph₂P(CH₂)₂BBN was identified as the resting state of the catalyst, whereas an intermediate tentatively formulated as the Lewis adduct of compound 2 and BH₃ was detected in solution in a stoichiometric experiment and is likely to be mechanistically relevant for the catalytic reaction.