Mechanistic Studies on NaHCO3 Hydrogenation and HCOOH Dehydrogenation Reactions Catalysed by a FeII Linear Tetraphosphine Complex

We present a theoretical extension of the previously published bicarbonate hydrogenation to formate and formic acid dehydrogenation catalysed by FeII complexes bearing the linear tetraphosphine ligand tetraphos-1. The hydrogenation reaction was found to proceed at the singlet surface with two competing pathways: A) H2 association to the Fe-H species followed by deprotonation to give a Fe(H)2 intermediate, which then reacts with CO2 to give formate. B) CO2 insertion into the Fe-H bond, followed by H2 association and subsequent deprotonation. B was found to be slightly preferred with an activation energy of 22.8 kcal mol-1, compared to 25.3 for A. Further we have reassigned the Fe-H complex, as a Fe(H)(H2), which undergoes extremely rapid hydrogen exchange.