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.

No abstract

A ferraquinone–ferrahydroquinone organometallic redox couple was prepared and characterized. Intricate cooperativity of the metal was observed with different positions on the ligand. This allowed cooperative activation of small molecules like molecular hydrogen, oxygen, and bromine. Likewise, dehydrogenation of alcohols was achieved through 1,6 metal–ligand cooperation.

2-Azametallacyclobutanes have been reported for a number of metals. This Perspective provides a brief overview of 2-azametallacyclobutanes, focusing on complexes in which the ring nitrogen is anionic (amido).

Herein, we report experimental, spectroscopic, and computational data that indicate that a rhodium ethylene complex, formally described as rhodium(I) and which forms a 2-rhoda(III) oxetane following reaction with H2O2, is more accurately described as a rhodium(III) metallacyclopropane. X-ray absorption spectroscopy clearly demonstrates a change in the oxidation state at rhodium following ligand coordination with tris(2-pyridylmethyl)amine. Both NMR and density functional theory studies suggest a high energy barrier to rotation of the coordinated ethylene, which is attributed to large geometric and electronic reorganization resulting from the loss of π-back-bonding. These results imply that the role of H2O2 in the formation of 2-rhoda(III) oxetanes is to oxidize the C2H4 fragment rather than the metal center, as has been previously suggested.