[Cp*Rh] complexes (Cp* = pentamethylcyclopentadienyl) supported by bidentate chelating ligands are a useful class of compounds for studies of redox chemistry and catalysis. Here, we show that the bis(2-pyridyl)methane ligand, also known as dipyridylmethane or dpma, can support [Cp*Rh] complexes in the formally + iii and + ii rhodium oxidation states. Specifically, two new rhodium complexes ([Cp*Rh(dpma)(L)] n+ , L = Cl − , CH 3 CN) have been isolated and structurally characterized, and the properties of the complexes have been compared with those of [Cp*Rh] complexes bearing the related dimethyldipyridylmethane (Me 2 dpma) ligand. Complex [Cp*Rh(dpma)(NCCH 3 )] 2+ displays a quasireversible rhodium( iii / ii ) reduction by cyclic voltammetry; related electron paramagnetic resonance (EPR) spectroscopic studies confirm access to the unusual rhodium( ii ) oxidation state. Further reduction to the formally rhodium( i ) oxidation state, however, is followed by deprotonation of dpma, as observed in electrochemical studies and chemical reduction experiments. This reactivity can be understood to occur as a consequence of the presence of doubly benzylic protons in the dpma ligand, since use of the analogous Me 2 dpma enables reduction to rhodium( i ) without involvement of ligand deprotonation. These findings highlight the important role of the ligand backbone substitution pattern in influencing the stability of highly-reduced complexes, a key class of metal species for study of electron and proton management in catalysis.
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This content will become publicly available on July 19, 2024
Catalytic reduction of dinitrogen to ammonia using molybdenum porphyrin complexes
Porphyrin complexes are well-known in O 2 and CO 2 reduction, but their application to N 2 reduction is less developed. Here, we show that oxo and nitrido complexes of molybdenum supported by tetramesitylporphyrin (TMP) are effective precatalysts for catalytic N 2 reduction to ammonia, verified by 15 N 2 labeling studies and other control experiments. Spectroscopic and electrochemical studies illuminate some relevant thermodynamic parameters, including the N–H bond dissociation free energy of (TMP)MoNH (43 ± 2 kcal mol −1 ). We place these results in the context of other work on homogeneous N 2 reduction catalysis.
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- NSF-PAR ID:
- 10435561
- Date Published:
- Journal Name:
- Faraday Discussions
- Volume:
- 243
- ISSN:
- 1359-6640
- Page Range / eLocation ID:
- 429 to 449
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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