NbCl2(pyridine)4 provides a practical gateway into niobium(II) chemistry, a very rare oxidation state for the element. It can be synthesized via reduction according to the procedure of Cotton and Murillo.[1]
Procedure: Rigorous exclusion of air and moisture are required. In the glovebox, a 50 mL round bottom flask was charged with a THF/pyridine mixture (15:1, 16mL). To the mixture were added, with stirring, KC8 (0.214g, 1.58mmol) and NbCl4(thf)2 (0.30g, 0.79mmol), which led to an immediate color change from yellow to royal blue. After stirring for 2h at room temperature, the mixture was filtered through celite to remove insoluble materials (see note), and transferred into a separate round bottom flask into a swivel frit apparatus. After attaching the setup to a vacuum line, solvent was removed in vacuo and pentane (30mL) subsequently added. The product was filtered and dried in vacuo to yield a blue powder. Authors report a 0.23g, 61% yield. Product could be recrystallized from diethyl ether with a few drops of pyridine at -40degC.
Notes:
- THF and pyridine should be dried (molecular sieves) prior to use. KC8 can be prepared via the following procedure.
- Prior addition of pyridine to the reaction is essential for its success. Without it, the formed niobium(II) species is too reactive and will form “unidentified niobium-containing solids”. It is possible that pyridine coordination prevents the niobium(II) from getting further reduced into its metallic form
- Product was found to form three different polymorphs depending on the crystallization conditions used.
- Low-valent niobium chemistry is very underexplored and only a handful of examples ranging from Nb(II-0) are known.[2-7] These compounds were mostly reported in the 1990s and seem to have lost interest at the turn of the century.
[1] Araya, M. A.; Cotton, F. A.; Matonic, J. H.; Murillo, C. A. Inorg. Chem. 1995, 34, 5424-5428.
[2] Green, M. L. H.; O’Hare, D.; Watkin, J. G. J. Chem. Soc., Chem. Commun. 1989, 698-701.
[3] a) Calderazzo, F.; Pampaloni, G.; Rocchi, L.; Strähle, J.; Wurst, K. Angew. Chem. Int. Ed. 1991, 30, 1, 102-103. b) Calderazzo, F.; Pampaloni, G.; J. Organomet. Chem. 1995, 500, 47-60. c) Calderazzo, F.; Gingl, F.; Pampaloni, G.; Rocchi, L.; Strähle, J. Chem. Ber. 1992, 125, 5, 1005-1010.
[4] Clark, D. L.; Gordon, J. C.; McFarlan, J. T.; Vincent-Hollis, R. L.; Watkin, J. G.; Zwick, B. D. Inorg. Chim. Acta, 1996, 244, 2, 269-272.
[5] Cotton, F. A.; Matonic, J. H.; Murillo, C. A. J. Am. Chem. Soc. 1997, 119, 33, 7889-7890.
[6] Kucera, B. E.; Roberts, C. J.; Young, V. G.; Brennessel, W. W.; Ellis, J. E. Acta Cryst. C 2019, 75, 9, 1259-1265.
[7] Unkrig, W.; Zhe, F.; Tamim, R.; Oesten, F.; Kratzert, D.; Krossing, I. Chem. Eur. J. 2020, 27, 2, 758-765.