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-buty1)phosphinomethyl)pyrazine), tBu-PNzP. It reacts with FeBr2 to yield [Fe(Br)(2)(tBu-PNzP)], 1. Treatment of 1 with NaBH4 in MeCN/Me0H gives the hydride complex [Fe(H)(MeCN)2(tBu-13NzP)][X] (X = Br, BH4), 2X. Counterion exchange and exposure to CO atmosphere yields the complex cis[Fe(H)(C0)(MeCN)(tBu-PNzP)][BPh4] 4.BPh4, which upon addition of Bu(4)NC1 forms [Fe(H)(C1)(C0)(tBu-PNzP)] 5. Complex 5, under basic conditions, catalyzes the hydrogenation of CO2 to formate salts at low Hy 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 Et0H the crystallographically characterized complex 7 is formed, comprised of six iron units forming a 6-membered ring. The dearomatized species can activate CO2 and Hy by metal ligand cooperation (MLC), leading to complex 8, trans-[Fe(PNzPtBuCOO) (H) (CO)], and complex 9, trans-[Fe(H)(2)(C0)(tBu-PNzP)], respectively. Our results point at a very likely mechanism for CO2 hydrogenation involving MLC.