Journal of Crystallography
Volume 2015 (2015), Article ID 312963, 4 pages
http://dx.doi.org/10.1155/2015/312963

Research Article

Solid State Structure of Bis[(bromo)(dicyclopentadienyl)vanadium(μ ₂-fluoro)][(bromo)(cyclopentadienyl)vanadium][tetrafluoroborate]

Lisa M. Chan, Atta M. Arif, and Richard D. Ernst

Department of Chemistry, University of Utah, 315 S. 1400 E., Room 2020, Salt Lake City, UT 84112-0850, USA

Received 14 October 2015; Accepted 12 November 2015

Academic Editor: Mehmet Akkurt

Copyright © 2015 Lisa M. Chan et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The new complex [V(C₅H₅)₂Br]₂(μ ₂-F)₂[V(C₅H₅)Br]⁺[BF₄]⁻ 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 , with (10) Å, (2) Å, (2) Å, and (8)° at 150(1) K.

1. Introduction

While numerous M(C₅H₅)₂X₂ (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 [5–8] have been crystallographically characterized. No structures have been reported for any mixed species, nor for fluoride complexes, with the exception of species having two AsF₅ or SbF₅ fragments linked to the fluorides [9, 10]. Herein a reaction product isolated from an attempt to generate a mixed V(C₅H₅)₂BrF complex is reported.

2. Materials and Methods

2.1. Synthesis

All operations were conducted under a nitrogen atmosphere using dried and deoxygenated solvents. 0.202 g of V(C₅H₅)₂Br [11] was dissolved in 20 mL of methylene chloride in a 100 mL Schlenk flask. 0.211 g of [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.

2.2. 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 least-squares 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 C₅H₅ ligand images appear nearly equally well defined, as judged by their thermal parameters. In contrast, one of the two images for the single C₅H₅ 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.

Table 1: Crystal and experimental data.

Empirical formula
C₂₆H₂₇BBr₃Cl₂F₆V₃

Formula weight
927.74

Temperature (K)
150(1)

Wavelength ()
0.71073

Crystal system
Monoclinic

Space group

Unit cell dimensions

7.66490(10)

15.2457(2)

13.3185(2)

    β (°)
101.2721(8)

Volume ()
1526.33(4)

2

(g cm⁻³)
2.019

(MoKα) (mm⁻¹)
5.056

900

θ range (°)
2.67–27.46

Limiting indices

Reflections collected/unique
6956/3614

Completeness (%)
99.9

Goodness of fit on
1.072

indices []
,

indices (all data)
,

(int)
0.0177

2.374, −2.321

Table 2: Selected bond lengths () and angles (°) for [V(C₅H₅)₂Br]₂-F)₂[V(C₅H₅)Br]⁺[BF₄]⁻.

Bond distances

V1–Br1
2.5600(5)
V2–Br2
2.4293(8)

V1–F1
2.008(2)
V2–F1
1.680(2)

V1–C1
2.326(3)
V1–C6
2.31(2)

V1–C2
2.322(3)
V1–C7
2.29(2)

V1–C3
2.278(3)
V1–C8
2.28(2)

V1–C4
2.278(3)
V1–C9
2.30(2)

V1–C5
2.293(3)
V1–C10
2.29(2)

V2–C11
2.277(5)
V1–C6′
2.31(3)

V2–C12
2.350(5)
V1–C7′
2.34(3)

V2–C13
2.354(6)
V1–C8′
2.28(3)

V2–C11′
2.41(2)
V1–C9′
2.27(3)

V2–C12′
2.16(2)
V1–C10′
2.31(3)

V2–C13′
2.33(2)

Bond angles

Br1–V1–F1
87.74(6)
Br2–V2–F1
99.48(7)

V1–F1–V2
172.78(12)
F1–V2–F1′
107.28(13)

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/.

3. 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 [V(C₅H₅)₂Br]₂(μ ₂-F)₂V(C₅H₅)Br]⁺ (1 ⁺), having five C₅H₅ ligands, three terminal bromide ligands and two doubly bridging fluoride ligands. The first two vanadium centers 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 C₅H₅ ligand, the formation of the product appears to have required a one electron oxidation for each of the three vanadium centers, presumably forming V(C₅H₅)₂Br⁺ ions. One of these could then abstract a fluoride ion from to give V(C₅H₅)₂FBr, which subsequently lost one of its C₅H₅ ligands, yielding V(C₅H₅)FBr. Its subsequent linking up with two other oxidized vanadium units, one in the form of V(C₅H₅)₂Br⁺ and the other as V(C₅H₅)₂FBr, 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.

Figure 1: Structure of the cationic portion of [V(C₅H₅)₂Br]₂(μ ₂-F)₂[V(C₅H₅)Br]⁺[BF₄]⁻.

Support for the proposed metal oxidation states may be obtained from a comparison with V(C₅H₅)₂Br₂ [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(C₅H₅)₂Cl₂ [5–8] and V(C₅H₅)₂Cl [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 [V(C₅H₅)Br]⁺ unit being further coordinated by a fluorine atom lone pair from each of the two V(C₅H₅)₂FBr 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(C₅H₅)F₂Br]⁻ unit coordinate to the two terminal [V(C₅H₅)₂Br]⁺ 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.

The local environments about V1 and V2 may be favorably compared with related species. For V1, some comparisons have already been made for its V–Br1 and V–C bonds. In addition, the F1–V1–Br1 angle of 87.74(6)° can be compared to those for V(C₅H₅)₂Br₂ and V(C₅H₅)₂Cl₂, 86.60(4)° and 87.20(3)°, respectively. The V1–F1 distance of 2.008(2) Å is slightly shorter than the V–F distances in V(C₅H₅)₂[(μ ₂-F)AsF₅]₂ and V(C₅H₅)₂[(μ ₂-F)SbF₅]₂ (2.03(1) and 2.04(1) Å, resp.) [9, 10], which likely reflects the high Lewis acidities of AsF₅ and SbF₅.

For the piano stool coordination environment of V2, one observes F1–V2–F1′ and F1–V2–Br2 angles of 107.28(13)° and 99.48(7)° and angles between the cyclopentadienyl ligand centroid and F1 and Br2 of 122.5° and 98.8°. For [V(C₅H₅)Cl₃]⁻ [17] and [V(C₅H₄Me)Cl₃]⁻ [18], the Cl–V–Cl angles ranged from 95.25(4)° to 103.19(4)° and from 95.0(1)° to 100.0(1)°, while the centroid–V–Cl angles ranged from 117.4(2)° to 120.1(2)° and from 118.8° to 120.7(4)°, respectively. For the V(IV) complexes V(C₅H₄Me)Cl₃ [19] and V(C₅Me₄Et)Cl₃ [20], the Cl–V–Cl angles ranged from 98.7(1)° to 103.3(1)° and from 99.69(4)° to 102.02(3)°, while the centroid–V–Cl angles ranged from 116.4(5)° to 117.8(5)° and from 115.13(9)° to 117.65(5)°, respectively. For V(C₅H₅)I₃ [21], the I–V–I angles ranged from 99.5(1)° to 101.6(1)°, while the centroid–V–I angles ranged from 115.6(1)° to 119.5(2)°. These values show little difference as compared to the V(IV) chloride complexes. The V2 environment in 1 ⁺ differs from the above primarily in its large F1–V2–F1′ angle of 107.28(13)° and small centroid–V–Br2 angle of 98.8°. These would likely seem to be a result of the fact that F1 is also coordinated to V1 and perhaps furthermore to its apparent donor interaction with V2.

4. Conclusions

The accessibility of both V(C₅H₅)₂X and (X, = halides) complexes makes it possible to isolate V(C₅H₅)₂XX′ 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].

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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