Now, Takebayashi, Gellrich, Fayzullin and co-workers have reported a 21-electron cobaltocene derivative Co(CpNCp) (pictured, left and centre) that can be stabilized in solid state and in solution (Nat. Commun. 14, 4979; 2023). Key to its preparation is the use of a pyridine-based chelating ligand that coordinates the metal through two Cp rings as well as a nitrogen lone pair donor. The researchers thoroughly characterized the complex in the solid state and in solution by a variety of methods including single-crystal X-ray diffraction (pictured, middle), nuclear magnetic resonance spectroscopy, electron paramagnetic resonance spectroscopy, vibrating sample magnetometry, and theoretical calculations. They also analysed both the experimental and theoretical distributions of the Laplacian of electron density (pictured, right) and, all taken together, their data reveals Co(CpNCp) to be a 21-electron cobaltocene where the metal is coordinated to the pyridine via a Co–N bond and the η5-coordination mode of the two Cp ligands is maintained.
The researchers also tried to prepare a 20-electron analogue of Co(CpNCp) by adding one equivalent of oxidant to Co(CpNCp), however, this yielded a cobaltocenium complex that does not contain Co‒N bonds and is best described as a 18-electron complex. They also applied their methodology to the preparation of other d-block metallocenes, attempting to synthesize an analogous 22-electron nickel complex with CpNCp. In this case, however, the reaction yields a dimeric complex lacking Ni‒N bonds. Nevertheless, the synthetic strategy developed may inspire the preparation of other d-block metallocenes with high valence electron counts.
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