Trimesitylvanadium
Trimesitylvanadium is one of the organovanadium complexes with vanadium in an oxidation state of 3, first synthesized by W. Seidel and G. Kreisel in 1974.
Synthesis and structure
To prepare this compound, VCl33 was reacted with Grignard reagent MesMgBr to form a blue solution at room temperature. It is precipitated by the addition of dioxane, which results in a blue solid. It is thermally stable, but it is also an air-sensitive compound.A crystal structure revealed VMes3 with trigonal pyramidal or pseudo-tetrahedral geometry.
Solvates
Synthesis of trimesitylvanadium usually gives an adduct of the complex with one molecule of solvent. The fact that trimesitylvanadium is recrystallized with THF adduct is due to the strong interaction between vanadium and oxygen. The bond length of the V-O bond is 2.069 Å. According to Pyykkö's atomic radii periodic trend, 1.97 Å would be expected for a single bond between vanadium and oxygen. This suggests that this V-O bond is not fully of single bond, but it is still close enough that it is considered a strong interaction, resulting in the formation of an adduct in recrystallization.However, THF can be easily dissociated. Experiments found that THF in trimesitylvanadium was exchanged with either pyridine or 2,2'-bipyridine when the product was exposed to either chemical. Lithium mesitylide adjoins to trimesitylvanadium to give a tetramesitylate complex that air oxidizes to the neutral V(IV) salt.
Applications in organic synthesis
Insertion reaction by using reactive V-C bond
The V-C σ bond in trimesitylvanadium is so reactive that it undergoes insertion reaction of several molecules. Rozzoli et al. investigated the reactivity of V3THF with CO, CO2, and tBuCN. When V3THF is reacted with CO, it undergoes reductive elimination and forms MesCMes as a product. Excess of CO will also result in the formation of V(CO)6 as a side product. For CO2 and pivalonitrile as a reagent, they are inserted between the V-C bond. Since V3THF is air- and water-sensitive, when the product from the insertion of tBuCN is exposed to water and/or O2, it undergoes reductive elimination to form imine and amine. These reactions reveal examples of small molecule activation reactions.Deoxygenation with vanadium
Vanadium is known to be oxophilic transition metal. In vanadium species, V3 undergoes deoxygenation of styrene oxide. The styrene oxide turns into styrene while vanadium species becomes vanadyl species. This product with V3 can form μ-oxo complex in toluene as a solution. This unique compound has a magnetic moment of 1.65 μB per vanadium at 288 K and a V-O-V stretch vibration of 680 cm−1. However, this μ-oxo complex is decomposed under polar coordinating solvent such as pyridine, in which it forms tetramesitylvanadium and pyridine-coordinated complex with C2 symmetry. For, the bond length of the V-C bond is much longer than trimesitylvanadium and trimesitylvanadyl complexes. The μ-oxo complex, tetramesitylvanadium, and pyridine-coordinated complex are examples of vanadium complexes.Deoxygenation by trimesitylvanadium can also be done for coordinated nitric oxide. In Cr3, introducing V3THF in toluene leads to cleavage of N=O bond to form CrΞO complex and μ-oxo vanadium complex. This reaction reduces and cleaves the NO bond by using five electrons.