NbCl4(thf)2 provides a convenient gateway into niobium(IV) complexes due to its easily displaced THF ligands. While the chemistry of these types of complexes is underexplored, Nb(IV) has found use in reductive homocouplings of imines and nitriles.[1] Its reactivity is more fundamentally studied - Noteworthy are various niobocene derivatives synthesized from tetrahaloniobium(IV) complexes. NbCl4(thf)2 can be conveniently synthesized by one of two ways: 1) Reduction of the NbCl5 with trimethyltin hydride in the presence of THF, according to an air/moisture-free 1992 Inorganic Syntheses procedure;[2] or by reduction of NbCl5 with aluminum powder in acetonitrile, allegedly forming the red-colored NbCl4(MeCN)3, followed by acetonitrile displacement with THF.[3]
Notes:
- NbCl4(thf)2 is an air/moisture sensitive yellow solid poorly soluble in THF and hydrocarbon solvents (yet most people react it in THF). It is stable indefinitely at room temperature when stored in the glovebox. However, there are accounts of it decolorating (to gray) on long-term standing, likely due to THF loss. In these cases, the complex is regenerated by stirring in THF for 30 minutes and filtering.
- NbCl5 starting material is moisture sensitive. It can be prepared from niobium oxide hydrate and thionyl chloride.[4] It can also be purchased from chemical vendors (Fisher Scientific).
- Anhydrous acetonitrile can be dried with molecular sieves. Anhydrous tetrahydrofuran can be purchased from chemical vendors (Millipore-Sigma, VWR, Fisher Scientific). It can also be dried with molecular sieves and distilled (to remove inhibitor) prior to use. Tetrahydrofuran forms peroxides over time when exposed to air. For a nice list of peroxide-forming routine chemicals, see the document below from University of Louisville:
Peroxide-Forming Chemicals Guide
or a more comprehensive list from UC Santa Cruz:
Classification List of Peroxide Forming Chemicals - Careful when handling tributyltin hydride. It is toxic and readily absorbed upon contact with skin, as expected of organotin compounds.
[1] Herndon, J. W. Niobium(IV) Chloride. EROS, 2001. DOI: 10.1002/047084289X.rn019.
[2] Pedersen, S. F.; Hartung, J. B.; Roskamp, E. J.; Dragovich, P. S. Niobium(III) and (IV) Halide Complexes. Inorg. Synth . 1992, 28, 119–123. DOI: 10.1002/9780470132609.ch28.
[3] Scott, P. Reduction of niobium pentachloride; niobium tetrachloride NbCl4(THF)2. SyntheticPage, 2001, 17. DOI: 10.1039/SP17.
[4] Brown, D.; Cavell, R. G. Niobium(V) Chloride and Hexachloroniobates(V). Inorg. Synth. 1967, 9, 88-92.