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.