Synthesis and reactivity of iron complexes with a new pyrazine-based pincer ligand, and application in catalytic low-pressure hydrogenation of carbon dioxide

A novel pincer ligand based on the pyrazine backbone (PNzP) has been synthesized, (2,6-bis(di(tert-butyl)phosphinomethyl)pyrazine), tBu-PNzP. It reacts with FeBr₂ to yield [Fe(Br)₂(tBu-PNzP)], 1. Treatment of 1 with NaBH₄ in MeCN/MeOH gives the hydride complex [Fe(H)(MeCN)₂(tBu-PNzP)][X] (X = Br, BH₄), 2·X. Counterion exchange and exposure to CO atmosphere yields the complex cis-[Fe(H)(CO)(MeCN)(tBu-PNzP)][BPh₄] 4·BPh₄, which upon addition of Bu₄NCl forms [Fe(H)(Cl)(CO)(tBu-PNzP)] 5. Complex 5, under basic conditions, catalyzes the hydrogenation of CO₂ to formate salts at low H₂ pressure. Treatment of complex 5 with a base leads to aggregates, presumably of dearomatized species B, stabilized by bridging to another metal center by coordination of the nitrogen at the backbone of the pyrazine pincer ligand. Upon dissolution of compound B in EtOH the crystallographically characterized complex 7 is formed, comprised of six iron units forming a 6-membered ring. The dearomatized species can activate CO₂ and H₂ by metal-ligand cooperation (MLC), leading to complex 8, trans-[Fe(PNzPtBu-COO)(H)(CO)], and complex 9, trans-[Fe(H)₂(CO)(tBu-PNzP)], respectively. Our results point at a very likely mechanism for CO₂ hydrogenation involving MLC.