Cationic Vanadium(IV) Methyl Complexes [Cp 2 VMe(CH 3 CN)][B(C 6 H 5 ) 4 ] and [Cp 2 VMe(THF)][MeB(C 6 F 5 ) 3 ] Robert Choukroun,* Christian Lorber, and Bruno Donnadieu Equipe Pre ´ curseurs Mole ´ culaires et Mate ´ riaux, Laboratoire de Chimie de Coordination, CNRS UPR 8241, 205 Route de Narbonne, 31077 Toulouse Cedex 4, France Received October 12, 2001 The cationic vanadocene alkyl complex [Cp 2 VMe(CH 3 CN)] + with [BPh 4 ] - counteranion was structurally established. The reaction of Cp 2 VMe 2 in THF in the presence of B(C 6 F 5 ) 3 is also described: EPR evidence of the intermediate formation of the [Cp 2 VMe(THF)] + species, which gives, via a disproportionation redox reaction, the V III species [Cp 2 V(THF)][Me B(C 6 F 5 ) 3 ], is demonstrated. The chemistry of cationic complexes of group 4 is very well documented, and literature is abundant concerning the structural characterization of nearly all these complexes. 1 On the other hand, the chemistry and the X-ray structure of cationic complexes of vanadium is less developed 2 despite their potential application in Zie- gler-Natta catalysis. Efforts have been made by our group to obtain types of cationic vanadium complexes from the protonolysis of vanadium alkylamido or vana- dium alkyl complexes with [NHMe 2 Ph][BR 4 ] (R ) C 6 H 5 or C 6 F 5 ). 3,4 Recently we carried out the reaction between Cp 2 VMe 2 (1), and [NHMe 2 Ph][B(C 6 H 5 ) 4 ] in acetonitrile, and [Cp 2 VMe(CH 3 CN)][B(C 6 H 5 ) 4 ] was isolated and spec- troscopically and analytically characterized. 4 Its analogy with the well-known Ti and Zr analogue complexes 5-8 as well as its inertness toward ethylene polymerization in contrast to group 4 led us to ascertain the X-ray structure of this adduct vanadium complex to ensure that the formulated complex is sor is notssimilar to the other structures already resolved and established in the Ti and Zr cationic chemistry. On the other hand the reactivity of 1 with the tris(perfluorophenyl)borane B(C 6 F 5 ) 3 is hereby presented. In the course of our studies of the reactivity of [Cp 2 - VMe] + , we have recently crystallized [Cp 2 VMe(CH 3 CN)]- [B(C 6 H 5 ) 4 ](2), which was confirmed by an X-ray crystal structure determination (Figure 1). The structure of 2 does not present new features in its tetrahedral struc- tural geometry. Bond angles around the vanadium atom give a pseudo-tetrahedral geometry where both planes [Cp1, V, Cp2] and [N-V-C(3)] are nearly perpendicular (89.82°). Cyclopentadienyl rings are in a staggered position. V-C (2.221 Å) and V-N (2.096 Å) distances are consistent with other complexes containing V-C and V(NCCH 3 ) units. 9-13 Some chemistry of 1 with the tetraphenyl borate ammonium salt [NHMe 2 Ph][B(C 6 H 5 ) 4 ] in THF was previously published. 4 Redox and disproportion reac- tions were demonstrated, according to Scheme 2. Simi- lar pathways could be observed in the reaction of 1 with tris(pentafuorophenyl)borane, B(C 6 F 5 ), in toluene or THF, and the reactions can be monitored by EPR and NMR spectroscopies. In toluene, the reaction of 1 with B(C 6 F 5 ) 3 gives an immediate insoluble blue-violet oil with a nearly color- less supernatant solution. This is a rapid reaction that precluded observation of intermediates. Nevertheless, formation of C 2 H 6 is observed by 1 H NMR when the reaction is performed in deuterated toluene or benzene. An NMR tube containing a weighted amount of 1 (0.05 mmol) and B(C 6 F 5 ) 3 (0.05 mmol) is filled with deuterated solvent at -80 °C and sealed. After the temperature is allowed to reach room temperature, the tube is moni- tored by 1 H NMR. The presence of a peak at δ(C 6 D 5 - (1) (a) Halterman, R. L. In Metallocenes; Togni, A., Halterman, R. L., Eds.; Wiley-VCH: New York, 1998; Vol. 1, Chapter 8, p 455. Some recent contributions: (b) Grimmer, N. E.; Coville, N. 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