title: Synthesis and reactivity of iron complexes with a new pyrazine-based pincer ligand, and application in catalytic low-pressure hydrogenation of carbon dioxide.
abstract: 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.
authors: †Department of Organic Chemistry and ‡Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel.
authors: Orestes
authors: O
authors: Rivada-Wheelaghan
authors: †Department of Organic Chemistry and ‡Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel.
authors: Alexander
authors: A
authors: Dauth
authors: †Department of Organic Chemistry and ‡Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel.
authors: Gregory
authors: G
authors: Leitus
authors: †Department of Organic Chemistry and ‡Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel.
authors: Yael
authors: Y
authors: Diskin-Posner
authors: †Department of Organic Chemistry and ‡Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel.
authors: David
authors: D
authors: Milstein