Density Functional Theory Mechanistic Study of the Reduction of CO2 to CH4 Catalyzed by an Ammonium Hydridoborate Ion Pair: CO2 Activation via Formation of a Formic Acid Entity

Density functional theory computations have been applied to gain insight into the CO2 reduction to CH4 with Et3SiH, catalyzed by ammonium hydridoborate 1 ([TMPH](+)[HB(C6F5)(3)](-), where TMP = 2,2,6,6-tetramethylpiperidine) and B(C6F5)(3). The study shows that CO2 is activated through the concerted transfer of H delta+ and H delta- of 1 to CO2, giving a complex (IM2) with a well-formed HCOOH entity, followed by breaking of the O-H bond of the HCOOH entity to return H delta+ to TMP, resulting in an intermediate 2 ([TMPH](+)[HC(=O)OB(C6F5)(3))](-)), with CO2 being inserted into the B-H bond of 1. However, unlike CO2 insertion into transition-metal hydrides, the direct insertion of CO2 into the B-H bond of 1 is inoperative. The computed CO2 activation mechanism agrees with the experimental synthesis of 2 via reacting HCOOH with TMP/B(C6F5)(3). Subsequent to the CO2 activation and B(C6F5)(3)-mediated hydrosilylation of 2 to regenerate the catalyst (1), giving HC(=O)OSiEt3 (5), three hydride-transfer steps take place, sequentially transferring H delta- of Et3SiH to 5 to (Et3SiO)(2)CH2 (6, the product of the first hydride-transfer step) to Et3SiOCH3 (7, the product of the second hydride-transfer step) and finally resulting in CH4. These hydride transfers are mediated by B(C6F5)(3) via two S(N)2 processes without involving 1. B(C6F5)(3) acts as a hydride carrier that, with the assistance of a nucleophilic attack of 5-7, first grabs H delta- from Et3SiH (the first S(N)2 process), giving HB(C6F5)(3)(-), and then leave H delta- of HB(C6F5)(3)(-) to the electrophilic C center of 5-7 (the second S(N)2 process). The SN2 processes utilize the electrophilic and nucleophilic characteristics possessed by the hydride acceptors (5-7). The hydride-transfer mechanism is different from that in the CO2 reduction to methanol catalyzed by N-heterocyclic carbene (NHC) and PCP-pincer nickel hydride ([Ni]H), where the characteristic of possessing a C=O double bond of the hydride acceptors is utilized for hydride transfer. The mechanistic differences elucidate why the present system can completely reduce CO2 to CH4, whereas NHC and [Ni]H catalysts can only mediate the reduction of CO2 to [Si]OCH3 and catBOCH(3), respectively. Understanding this could help in the development of catalysts for selective CO2 reduction to CH4 or methanol.