Solid State Structure of Bis[(bromo) (dicyclopentadienyl)vanadium(μ2-fluoro)][(bromo) (cyclopentadienyl)vanadium][tetrafluoroborate]

The new complex [V(C 5 H 5 ) 2 Br] 2 (μ 2 -F) 2 [V(C 5 H 5 )Br][BF 4 ] has been isolated from the reaction of vanadocene monobromide with the ferrocenium cation.The complex is a mixed valence compound composed of two V(IV) and one V(III) centers.The V(III) center has one cyclopentadienyl ligand in its coordination sphere, as well as a bromide and two fluoride ligands. Each fluoride ligand is also attached to one of the V(IV) centers, which additionally is coordinated by a bromide and two cyclopentadienyl ligands. The complex crystallizes in the monoclinic space group P2 1 /m, with a = 7.66490(10) Å, b = 15.2457(2) Å, c = 13.3185(2) Å, and β = 101.2721(8) at 150(1) K.


Introduction
While numerous M(C 5 H 5 ) 2 X 2 (X = halogen) complexes are known, species with two different halide ligands are significantly less common.For vanadium, the dichloride [1], dibromide [2], and diiodide [3] are all known and the dibromide [4], the dichloride, and some alkyl-substituted analogues [5][6][7][8] have been crystallographically characterized.No structures have been reported for any mixed species, nor for fluoride complexes, with the exception of species having two AsF 5 or SbF 5 fragments linked to the fluorides [9,10].Herein a reaction product isolated from an attempt to generate a mixed V(C 5 H 5 ) 2 BrF complex is reported.

Materials and Methods
2.1.Synthesis.All operations were conducted under a nitrogen atmosphere using dried and deoxygenated solvents.0.202 g of V(C 5 H 5 ) 2 Br [11] was dissolved in 20 mL of methylene chloride in a 100mL Schlenk flask.0.211g of [Fe(C 5 H 5 ) 2 + ]BF 4 − [12] was then added, leading to a brownish-yellow solution.After the solution was allowed to stir for several hours, the solvent was removed in vacuo, and the residue was washed with hexane to remove ferrocene, leaving a greenish colored solid.This was not very soluble in toluene and was redissolved in a small amount of methylene chloride.The solution exhibited a dark green color by reflection but pale brownish by transmission.After about a week at −60 ∘ in a freezer, a small amount of air-and moisture-sensitive crystals had formed.

X-Ray
Crystallography.Single crystals were selected under Paratone oil and transferred to an Enraf-Nonius Kappa CCD diffractometer for low temperature unit cell determination and data collection.A cold nitrogen stream was used to maintain a nearly constant temperature and to protect the compound from air.The structure was solved by SIR 97 [13] and improved from difference Fourier maps and leastsquares refinements using SHELXL97 [14], using published scattering factors [15].Only one cyclopentadienyl ligand was found not to be disordered (C1-5 on V1), with the other two adopting two orientations each.For V1, its second C 5 H 5 ligand images appear nearly equally well defined, as judged by their thermal parameters.In contrast, one of the two images for the single C 5 H 5 ligand, with imposed  symmetry, on V2, is significantly better defined than the other and will be used exclusively for subsequent discussions of bonding parameters.All nonhydrogen atoms were refined anisotropically, while the hydrogen atoms were modeled isotropically and were all allowed to ride on their attached carbon atoms.Pertinent crystallographic information is contained in Table 1, while selected bonding parameters are given in Table 2.The largest peak and hole observed in the final difference Fourier map are each within 0.75 Å of Br2.Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC, number 1430708.Copies of this information may be obtained free of charge from The Director, CCDC, 12, Union Road, Cambridge CB2 1EZ FAX [+44(1223)336-033] or e-mail deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk/.

Results and Discussion
The structure of the cationic portion of the isolated product is shown in Figure 1, while selected bonding parameters are given in Table 2.This portion may be formulated as are equivalent due to a crystallographic mirror plane.As a result, the three vanadium centers have a total charge of +11, and the only reasonable formulation would have the two end vanadium centers (V1, V1  ) being tetravalent and the central one (V2) trivalent.This is consistent with the fact that the tetravalent V1 centers are bound by one more anionic ligand as compared to V2.Given the fact that V2 has lost a C 5 H 5 ligand, the formation of the product appears to have required a one electron oxidation for each of the three vanadium centers, presumably forming V(C 5 H 5 ) 2 Br + ions.One of these could then abstract a fluoride ion from BF 4 − to give V(C 5 H 5 ) 2 FBr, which subsequently lost one of its C 5 H 5 ligands, yielding V(C 5 H 5 )FBr.Its subsequent linking up with two other oxidized vanadium units, one in the form of V(C 5 H 5 ) 2 Br + and the other as V(C 5 H 5 ) 2 FBr, would lead to the observed product, in which the two V(IV) centers might presumably be stabilized through their sharing their fluoride ions with the central V(III) center.
Support for the proposed metal oxidation states may be obtained from a comparison with V(C 5 H 5 ) 2 Br 2 [4], whose V-Br bonds are ca.0.28 Å longer than its V-C bonds (2.58 versus 2.30 Å).This is quite consistent with our observed V1-Br1 and average V1-C distances (2.5600(5) and 2.30 Å).The V2-C distances average 2.337(15) Å, only slightly longer than the V1-C distances.While a greater difference might have been expected based on data for V(C 5 H 5 ) 2 Cl 2 [5-8] and V(C 5 H 5 ) 2 Cl [16], the reduced steric crowding in the present case for V2 should lead to some shortening.The V2-Br2 bond is somewhat shorter than the V1-Br1 bond, 2.4293(8) versus 2.5600(5) Å.In contrast, the V2-F1 bonds are much shorter than the V1-F1 bond, 1.680(2) versus 2.008(2) Å, opposite to expectations based upon relative metal ion acidities.Given that the V2 center can be formulated as having a 14-electron configuration, arising from the central [ being further coordinated by a fluorine atom lone pair from each of the two V(C 5 H 5 ) 2 FBr units, it would be quite feasible for these two fluorine atoms to serve as  donors also, thereby generating an 18-electron configuration for V2 and accounting for the V2-F bonds being shorter than the V1-F bonds.In this approach, the fluorides would each be serving as four electron donors to V2.One could alternatively invoke a resonance form in which fluoride lone pairs from the central [V(C 5 H 5 )F 2 Br] − unit coordinate to the two terminal [V(C 5 H 5 ) 2 Br] + units.This would not change the electron counts at the vanadium centers but would lead to the fluoride coordination at V2 to be considered to be three electron interactions.

Conclusions
The accessibility of both V(C 5 H 5 ) 2 X and V(C 5 H 5 ) 2 X  2 (X, X  = halides) complexes makes it possible to isolate V(C 5 H 5 ) 2 XX  complexes.This work demonstrates how a combination of fluoride and bromide ligands may be added to the vanadocene fragment, yielding a complex that is of further interest due to its mixed valence character.Additional routes, such as those developed for pentadienyl complexes [22], may also be suitable for such preparations, as has also been demonstrated by the use of interhalogen molecules [3].

Table 1 :
Crystal and experimental data.
having five C 5 H 5 ligands, three terminal bromide ligands and two doubly bridging fluoride ligands.The first two vanadium centers