Nb(mesitylene)2 is a rare instance of niobium(0). This deep red, paramagnetic, 17 valence-electron complex has been used in the materials community as a source of niobium atoms for the synthesis of Nb-chalcogenide solids, for example.[1] There are also instances of its use in catalysis, namely in hydroboration reactions.[2] Nb(mesitylene)2 is formed via the Fischer-Hafner synthesis, where NbCl5 is reduced by Al/AlCl3 in the presence of mesitylene; a more practical pathway than the vapor-metal method.
Procedure:[3] In the glovebox, a 150mL round-bottom flask was charged with AlCl3 (4.69g, 35.2mmol), Al powder (5.7g, 211mmol), NbCl5 (9.45g, 35.0mmol), and 40mL of mesitylene. The flask was capped, taken out of the glovebox, and attached to a swivel frit setup on a double-manifold high-vacuum line. The mixture was stirred at room temperature overnight, and subsequently at 125degC for 4h. The mixture was cooled to -15degC and 50mL of DME were slowly added under vigorous stirring. 50mL of hexanes were added, and the deep-red suspension stirred overnight. The mixture was filtered to remove byproduct salts and unreacted materials, and the filtrate evaporated under high vacuum. The setup was filled with argon and the flask with the residue was transferred to a second swivel frit setup. Residue was extracted with 50mL of hexanes and filtered. Filtrate was concentrated in vacuo to yield the product, which could be recrystallized by dissolving in minimal hexane or pentane and cooling in the glovebox freezer. Yield was reported as 5.03g, 43%.
Notes:
- Solvents were dried and degassed by standard procedures. AlCl3 should be sublimed prior to use. NbCl5 can be purchased from chemical vendors or synthesized via reaction of niobium oxide hydrate with thionyl chloride.
- Nb(mesitylene)2 is a highly air/moisture sensitive, dark-red/brown solid. It is soluble in ethereal and hydrocarbon solvents. Although stable at room-temperature, it should be kept in the glovebox freezer for long-term storage.
- The role of AlCl3 is that of a halogen scavenger, facilitating the role of Al as reductant. Reaction is believed to proceed via a dicationic Nb(II)-(AlCl4)2 species which further reduces upon addition of THF or DME to form the product.
- The Fischer-Hafner synthesis is the classical method of forming homoleptic metal-arene (metal = transition-metal, lanthanide, actinide, main-group element) complexes. It gained popularity due to its ability to circumvent vapor-metal synthesis.
[1] Miller, G. J. Chemistry and properties of novel niobium cluster compounds. *J. Alloys Compd. 1995, 229, 93-106.
[2] Burgess, K.; Jaspars, M. Hydroboration reactions mediated by bis(mesityl)niobium: Beware of the trojan horse. Tet. Lett. 1993, 34, 43, 6813-6816.
[3] Calderazzo, F.; Pampaloni, G.; Rocchi, L. J. Organomet. Chem. 1991, 413, 91-109.