A highly disordered cobaltocenium salt

Cobaltocenium tetra­fluoro­borate, [Co(C₅H₅)₂]BF₄, crystallizes in space group Cccm and packs in a way that causes the Cp₂Co moiety to sit just off of the mirror plane, causing extreme disorder.

Key indicators

• Single-crystal X-ray study
• T = 213 K
• Mean [sigma](F-B) = 0.002 Å
• Disorder in main residue
• R factor = 0.028
• wR factor = 0.084
• Data-to-parameter ratio = 12.1

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry


 Alert Level B:



PLAT_301  Alert B Main Residue Disorder ........................      50.00 Perc.

Author response: The nature of the disorder is fully discussed in the comment section.

 0 Alert Level A = Potentially serious problem

 1 Alert Level B = Potential problem

 0 Alert Level C = Please check

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S1. Comment

Cobaltocene has often been used as a convenient, mild reducing agent (Connelly & Geiger, 1996), thus leading to the appearance of the cobaltocenium cation in many crystal structures [over 80 times in the Cambridge Database (Allen & Kennard, 1993)]. We have also found the cobaltocenium cation to be useful as a mild oxidizing agent, and often use the BF₄^- salt whenever a weakly coordinating anion is desired in the target synthetic product. While structures of the Cp₂Co⁺ ion with many different anions are known, the BF₄^- salt, (I), is not, and its structure is reported here.

Space group Cccm was strongly indicated by intensity statistics, and while the solution and refinement of the structure was expected to be straightforward, the initial solution from direct methods suggested a position for the Co atom that was on the twofold axis at 1/4, 1/4, z but only approximately 0.2 Å from the 2/m site at 1/4, 1/4, 1/2. (The BF₄^- anion resides on a site of 222 symmetry at 0, 1/2, 1/4 in a way that generates an almost perfectly tetrahedral moiety.) Fourier peaks corresponding to the C atoms of the Cp rings were therefore badly mixed by the symmetry operations. At this point, it seemed apparent that the mirror plane should be removed to reduce the symmetry and change the space group to Ccc2. However, two slightly separated positions for the Co atom persisted and the Cp rings continued to appear as a badly disordered cluster of atoms. The solution and refinement were therefore continued in space group Cccm, using a disordered model in which the Cp₂Co⁺ unit resides on a twofold axis perpendicular to the main axis of the molecule, but slightly displaced from the mirror plane which almost contains the main molecular axis. A view of the model showing one orientation of the Cp₂Co group is shown in Fig. 1.

In order to refine the model effectively, the Cp ring was constrained to be exactly pentagonal, but was allowed to shrink or expand freely. This resulted in a C—C distance of 1.397 (4) Å, which is shorter than the expected distance for π-bonded Cp, but is due to librational error. Librational analysis (Schomaker & Trueblood, 1968) of the Cp ring gives corrected C—C bond lengths of 1.42 Å, which is the expected value of the distances.

Atom C1 lies almost on the mirror plane and appears to be shared by both orientations in Fig. 2, although it is actually not. The Cp plane is 1.634 (1) Å from the Co atom, and the two Cp rings form an angle of 1.5 (4)°. The Cp rings are slightly staggered, with an approximate offset of 21°.

S2. Experimental

All synthetic procedures were carried out under a nitrogen atmosphere. Cobaltocene (51 mg, 0.27 mmol) and AgBF₄ (52 mg, 0.27 mmol) were weighed into separate Schlenk flasks. Methylene chloride (10 ml) was added to each flask, and the cobaltocene solution was transferred to the flask containing the AgBF₄ via cannula. The mixture was allowed to react, with stirring, for 1 h. The solution was then transferred with a filter-equipped cannula into a 50 ml Schlenk tube and layered with 20 ml hexanes. Crystals formed after one week.

Computing details

Data collection: SMART (Bruker, 2000); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Siemens, 1996); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The molecular structure of [(C5H5)2Co]BF4, drawn with 40% displacement ellipsoids.
	Fig. 2. A representation of the entire model used in the refinement of the disordered structure. The twofold axis is horizontal in this view, passing through the Co atoms. The mirror is vertical, perpendicular to the plane of the drawing, passing between the Co atoms.

cobaltocenium tetrafluoroborate

Crystal data

[Co(C5H5)2]·BF4	Dx = 1.680 Mg m^−3
Mr = 275.92	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Cccm	Cell parameters from 1619 reflections
a = 7.3586 (7) Å	θ = 2.4–27.4°
b = 11.9176 (11) Å	µ = 1.59 mm^−1
c = 12.4369 (13) Å	T = 213 K
V = 1090.68 (18) Å^3	Block, yellow
Z = 4	0.30 × 0.28 × 0.20 mm
F(000) = 552

Data collection

Bruker SMART diffractometer	665 independent reflections
Radiation source: normal-focus sealed tube	485 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.018
ω scans	θmax = 27.5°, θmin = 3.3°
Absorption correction: multi-scan (SADABS; Blessing, 1995)	h = −9→9
Tmin = 0.609, Tmax = 0.727	k = −15→10
3263 measured reflections	l = −16→13

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.028	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.084	H-atom parameters not refined
S = 1.09	w = 1/[σ^2(Fo^2) + (0.036P)^2 + 0.6P] where P = (Fo^2 + 2Fc^2)/3
665 reflections	(Δ/σ)max = 0.003
55 parameters	Δρmax = 0.19 e Å^−3
0 restraints	Δρmin = −0.18 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
Co	0.2500	0.2500	0.4821 (4)	0.0622 (10)	0.50
C1	0.3131 (4)	0.4145 (3)	0.4953 (10)	0.0860 (16)	0.50
H1	0.2311	0.4730	0.5050	0.103*	0.50
C2	0.3970 (13)	0.3527 (7)	0.5769 (8)	0.113 (4)	0.50
H2	0.3806	0.3629	0.6504	0.136*	0.50
C3	0.5098 (10)	0.2730 (6)	0.5287 (5)	0.101 (7)	0.50
H3	0.5817	0.2208	0.5645	0.122*	0.50
C4	0.4956 (9)	0.2854 (4)	0.4173 (5)	0.092 (4)	0.50
H4	0.5563	0.2429	0.3659	0.110*	0.50
C5	0.3740 (14)	0.3729 (7)	0.3967 (6)	0.0721 (18)	0.50
H5	0.3397	0.3989	0.3292	0.087*	0.50
F	0.1069 (3)	0.4337 (2)	0.18803 (19)	0.1734 (10)
B	0.0000	0.5000	0.2500	0.124 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Co	0.0360 (3)	0.0488 (3)	0.102 (3)	−0.00996 (18)	0.000	0.000
C1	0.065 (2)	0.0539 (17)	0.139 (4)	−0.0181 (18)	−0.030 (9)	0.008 (11)
C2	0.095 (9)	0.140 (11)	0.104 (6)	−0.077 (7)	0.000 (4)	−0.025 (5)
C3	0.040 (2)	0.096 (4)	0.17 (2)	−0.023 (2)	−0.018 (5)	0.043 (8)
C4	0.050 (3)	0.061 (3)	0.164 (11)	−0.010 (2)	0.025 (5)	0.006 (4)
C5	0.056 (3)	0.051 (2)	0.110 (5)	−0.012 (2)	0.013 (3)	0.005 (3)
F	0.217 (3)	0.1521 (18)	0.1513 (18)	−0.0482 (15)	0.0620 (17)	−0.0701 (14)
B	0.156 (8)	0.119 (6)	0.095 (5)	0.000	0.000	0.000

Geometric parameters (Å, º)

Co—C1	2.021 (4)	C1—C5	1.397 (4)
Co—C2	2.015 (11)	C2—C3	1.397 (4)
Co—C3	2.016 (7)	C3—C4	1.397 (4)
Co—C4	2.023 (7)	C4—C5	1.397 (4)
Co—C5	2.027 (10)	B—F	1.355 (2)
C1—C2	1.397 (4)
C1—C2—C3	108.0	C2—C3—Co	69.7 (3)
C2—C3—C4	108.0	C4—C3—Co	70.0 (5)
C3—C4—C5	108.0	C5—C4—Co	69.9 (3)
C4—C5—C1	108.0	C3—C4—Co	69.5 (5)
C2—C1—C5	108.0	C1—C5—Co	69.6 (3)
C2—C1—Co	69.5 (4)	C4—C5—Co	69.70 (15)
C5—C1—Co	70.0 (4)	F^i—B—F	108.7 (2)
C1—C2—Co	70.0 (3)	F^i—B—F^ii	110.7 (2)
C3—C2—Co	69.77 (15)	F—B—F^ii	109.05 (19)

Symmetry codes: (i) −x, y, −z+1/2; (ii) x, −y+1, −z+1/2.