A new combination of donor and acceptor: bis(η6-benzene)chromium and hexafluorobenzene form a charge-transfer stacked crystal

Article abstract of DOI:10.1039/a900919a

Bis(η⁶-benzene)chromium reacts with hexafluorobenzene to yield a red charge-transfer complex [Cr(η⁶-C⁶H₆)2·C₆F₆] which contains stacks of alternating donor and acceptor molecules with close inter- and intrastack contacts; in addition to the charge-transfer complex, formation of [Cr(η⁶-C₆H₆)2]⁺ is demonstrated by EPR and IR spectroscopy.

Full text of DOI:10.1039/a900919a

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A new combination of donor and acceptor: bis(h -benzene)chromium and
hexafluorobenzene form a charge-transfer stacked crystal
Catherine J. Aspley,a Clive Boxwell,a Maria L. Buil,a Catherine L. Higgitt,a Conor Longb and Robin N.
Perutz*a
a Department of Chemistry, University of York, York, UK YO10 5DD. E-mail: rnp1@york.ac.uk
b Department of Chemistry, Dublin City University, Dublin, Ireland
Received (in Oxford, UK) 3rd February 1999, Accepted 23rd April 1999
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Bis(h -benzene)chromium reacts with hexafluorobenzene to
C6H6 rings in a neighbouring stack. The minimum C···C
distance between C6H6 and C6F6 lies within a stack, while the
minimum Cr···F distance lies between stacks (Fig. 2, Table 1).
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yield a red charge-transfer complex [Cr(h -C6H6)2•C6F6]
which contains stacks of alternating donor and acceptor
molecules with close inter- and intrastack contacts; in
addition to the charge-transfer complex, formation of
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The regularly spaced DA stacks contrast with those of [Fe(h -
C5Me5)2•C14F10]13 and [1][TCNE].8
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[Cr(h -C6H6)2]+ is demonstrated by EPR and IR spectros-
We have also attempted to solve the structure of the thin
yellow plates at 2100 °C. Although the crystal did not diffract
well, we found that the structure was little changed from that of
the red–pink blocks. A compression along b and c led to a
contraction of 1.9% in the minimum intrastack C···C distance
and 3.7% in the minimum interstack Cr···F distance.
copy.
Non-covalent interactions have a major role in determining
structures of molecular assemblies. The formation of stacks of
donors alternating with acceptors can lead to special electronic
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and magnetic properties as in [Fe(h -C5Me5)2][TCNE] or
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[Fe(h -C6Me3H3)2][C6(CN)6].1,2 Stacking of aromatic rings (p-
stacks) represents another motif, this time driven by quad-
rupolar interactions, e.g. in co-crystals of benzene and hexa-
fluorobenzene.3 Complex analogues of C6H6•C6F6 are
attracting interest for their optoelectronic properties.4,5
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Bis(h -benzene)chromium 1 has potential for both these types
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of interaction: it has featured as a donor in [Cr(h -
C6H6)2][S2O6•2SO2] and has been employed for crystal engi-
neering.6,7 Donor-acceptor (DA) complexes are also formed by
6
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1 in [Cr(h -C6H6)2][TCNE] and [Cr(h -C6Me3H3)2][TCNQ]
but they are arranged in stacks of the type ···A2DD··· or in
chains.8
Several authors have proposed that an electron-transfer
process precedes C–F bond activation of perfluoroalkanes and
perfluoroarenes by transition metal complexes,9–11 although
experimental support for redox initiation is sparse. If this
proposal is to be reconciled with the redox potentials, there must
be either preassociation of the fluorocarbon and transition metal
complex, or the products must be stabilised by rapid irreversible
2
dissociation (e.g. C6F6 to C6F5 + F2), or more likely both.12
5
Crabtree et al. recently reported that [Fe(h -C5R5)2] (R = H or
Me) associates with perfluoronaphthalene or perfluorophenan-
threne in the solid state to form an irregular DA stack.13
However, there was no optical evidence for a charge-transfer
(CT) interaction. We report that 1 reacts with hexafluoro-
benzene to form a red CT complex in solution which crystallises
as a donor–acceptor stack, and additionally that 1 undergoes
one-electron oxidation by C6F6 to a limited extent.
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Fig. 1 Packing diagram for [Cr(h -C6H6)2•C6F6], viewed down the body
diagonal of the triclinic cell (hydrogen atoms omitted).
When a large excess of dry hexafluorobenzene is condensed
onto a freshly sublimed sample of 1 and thawed under argon, a
claret-coloured solution is obtained from which crystals grow
over several weeks at room temperature. The crystals have two
habits, red–pink blocks and thin yellow plates, both of which are
stable only in the mother liquor. An X-ray crystal structure of a
red–pink crystal mounted in its mother liquor in a capillary at
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room temperature revealed it to be [Cr(h -C6H6)2•C6F6].† The
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structure contains face-to-face stacks of [Cr(h -C6H6)2] moie-
ties alternating with C6F6 units at regular intervals of ca. 3.5 Å,
a value similar to that in [Fe(h -C5Me5)2][TCNE].2 The stacks
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Fig. 2 ORTEP diagram (ref. 15) showing the key intra- and inter-stack
spacings within the crystal structure of [Cr(h -C6H6)2•C6F6], viewed down
the c axis. The hydrogen atoms are omitted for clarity. The average C–C and
Cr–C bond lengths within a Cr(C6H6)2 unit are 1.397 and 2.134 Å. For the
C6F6 molecule, the C–C and C–F bond lengths average 1.365 and 1.341
Å.
lie approximately parallel to the body diagonal of the triclinic
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unit cell (Fig. 1) such that the benzene rings of the [Cr(h -
C6H6)2] units eclipse the C6F6 rings in the same stack. The angle
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between the ring normals of the C6F6 and [Cr(h -C6H6)2] units
is 5.9°. Each C6F6 lies approximately in line with one of the
Chem. Commun., 1999, 1027–1028
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Table
C6H6)2•C6F6]
1 Minimum contact distances in the structure of [Cr(h -
Notes and references
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† Crystal data for [Cr(h -C6H6)2•C6F6]: C18H12CrF6, M = 394.28, red–
¯
Labela
Type of contact
Contact atoms
Distance/Å
pink blocks, triclinic, space group P1, a = 7.3254(8), b = 9.2627(9), c =
6.6102(8) Å, a = 100.026(9), b = 112.327(8), g = 97.532(9)°, V =
398.87(8) Å3, Z = 1, T = 293(2) K, m(Mo2Ka) = 0.778 mm21, F(000)
A
B
C
D
Intrastack C6H6···C6F6
Intrastack Cr···C6F6
Intrastack Cr···C6F6
Interstack Cr···C6F6
r(C···C)
r(Cr···C)
r(Cr···F)
r(Cr···F)
3.480(4)
5.074(3)
5.312(3)
4.490(2)
=
198, 1477 reflections measured, 1401 unique (Rint = 0.040), 116
parameters. The crystal was mounted in a capillary in its mother liquor. The
structure was solved by direct methods (SHELX) (ref. 14) and refined by
full-matrix least-squares on F2. Goodness of fit on F2 1.083, final R1
a labels A, B, C, D refer to Fig. 2.
[I > 2s(I)] R1
= 0.0392, wR2 = 0.1027. CCDC 182/1238. See
http://www.rsc.org/suppdata/cc/1999/1027/ for crystallographic files in .cif
format.
‡ [Cr(C6H6)2] is yellow–green with a weak band at 640 nm and an intense
band at 320 nm. The corresponding cation exhibits weak bands in the near-
IR region and a more intense band at 340 nm (ref. 16). The CT band of
When the reaction of 1 with C6F6 was carried out with higher
concentrations of 1, a fine yellow precipitate separated from the
claret solution. A UV–VIS absorption spectrum of the solution,
measured after filtering off the precipitate, showed an absorp-
tion at 503 nm and a shoulder at 390 nm.16‡ On freezing in
liquid nitrogen, the claret solution turned to a yellow glass, but
the claret colour returned on melting. The corresponding
absorption maxima lie to longer wavelength (542, 414 nm) on
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Cr(h -C6H6)2•SO2 is observed at 540 nm (ref. 6).
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§ The short-wavelength shoulder will overlap the bands of [Cr(h -C6H6)2]
in C6F5H.
¶ The half-wave potentials of C6F6/C6F6 and [Cr(h -C6H6)2]+/ [Cr(h -
C6H6)2] are 22.55 and 20.68 V respectively vs. SCE. (ref. 18).
2
6
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∑ Characterisation of the anion in the yellow precipitate was complicated by
paramagnetic 1+. The precipitate was dissolved in CD2Cl2, Me3SiOTf
added and the volatiles condensed into an NMR tube. Me3SiF was identified
by the eight central lines of the decet at d 2157.96 (JHF 7.6 Hz) in the 19
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reaction of [Cr(h -1,4-C6H4Me2)2] with C6F6 and to shorter
F
wavelength (424 nm)§ on reaction of 1 with C6F5H. These
NMR spectrum (ref. 19). A control experiment with C6F6, CD2Cl2 and
Me3SiOTf generated only traces of Me3SiF.
absorption bands are assigned to charge-transfer transitions of
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the complexes [Cr(h -arene)2•C6F6–nHn] (n = 0, 1) which must
** Complex 1 was freshly sublimed before use. Solutions in toluene (0.047
mol dm23) were made up in an argon-filled glove-box; C6F6 (99.9%),
previously dried over molecular sieves and degassed by freeze-pump-thaw
methods, was added in the box with a microsyringe. Typical impurities are
C6F5Cl and C6F5Cl2. Since the addition of C6F6 causes slight precipitation,
the values of the percentage oxidation are lower limits.
be present in solution. Redox potentials suggest that the ground
state of the complex will be close to the A•D description and the
excited state close to A+•D2.¶
The solutions from reaction of 1 with C6F6 (whether dilute or
more concentrated) gave a broad EPR signal at g = 1.987
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consistent with formation of [Cr(h -C6H6)2]+ 1+.6 Solid-state
1 J. S. Miller and A. J. Epstein, Angew. Chem., Int. Ed. Engl., 1994, 33,
385.
2 J. S. Miller, J. C. Calabrese, H. Rommelmann, S. R. Chittipeddi, J. H.
Zhang, W. M. Reiff and A. J. Epstein, J. Am. Chem. Soc., 1987, 109,
769; M. D. Ward, Organometallics, 1987, 6, 754; M. D. Ward and J. C.
Calabrese, Organometallics, 1989, 8, 593.
3 J. H. Williams, Acc. Chem. Res., 1993, 26, 593; A. P. Weat, S. Mecozzi
and D. A. Dougherty, J. Phys. Org. Chem., 1997, 10, 347.
4 G. W. Coates, A. R. Dunn, L. M. Henling, J. W. Ziller, E. B. Lobkovsky
and R. H. Grubbs, J. Am. Chem. Soc., 1998, 120, 3641.
5 P. Kirsch and K. Tarumi, Angew. Chem., Int. Ed., 1998, 37, 484.
6 C. Elschenbroich, R. Gondrum and W. Massa, Angew. Chem., Int. Ed.
Engl., 1985, 24, 967.
7 D. Braga, A. L. Costa, F. Grepioni, L. Scaccianoce and E. Tagliavini,
Organometallics, 1997, 16, 2070.
8 J. S. Miller, D. M. O’Hare, A. Chakraborty and A. J. Epstein, J. Am.
Chem. Soc., 1989, 111, 7853; D. O’ Hare, M. D. Ward and J. S. Miller,
Chem. Mater., 1990, 2, 758.
9 J. Burdeniuc and R. H. Crabtree, J. Am. Chem. Soc., 1996, 118, 2525;
Organometallics, 1998, 17, 1582.
10 B. K. Bennett, R. G. Harrison and T. G. Richmond, J. Am. Chem. Soc.,
1994, 116, 11165.
EPR spectra of the yellow precipitate from reaction of 1 with
C6F6 also revealed the presence of 1+ (g|| = 2.002, g^ = 1.983),
while IR spectra showed characteristic bands of both 1 and 1+.17
The presence of fluoride as a corresponding anion was revealed
by its reaction with Me3SiOTf yielding Me3SiF.∑ In order to
ascertain the proportion of 1 which is oxidised, we investigated
the effect of addition of C6F6 to toluene solutions of 1. A control
sample of 1 in toluene showed only traces of 1+ . On addition of
2 and 5 equiv. of C6F6, well-resolved resonances (AH = 3.4 G)
for 1+ were observed with intensities ca. 20 fold and 48 fold
greater than the control, respectively. Comparison with the
resonance of a standard solution of TEMPO (10-4 mol dm-3 in
toluene) provided lower-limiting estimates that the solutions of
1 were 0.5 and 3.3% oxidised, respectively.** The extent of
oxidation is appreciably higher than expected to arise from
impurities in the hexafluorobenzene. The formation of
[Cr(C6H6)2]+F2 is reminiscent of the reaction of cobaltocene
with perfluoroalkanes.10
These experiments lead to the following conclusions. (i) A
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donor–acceptor complex is formed between [Cr(h -C6H6)2] 1
11 M. K. Whittlesey, R. N. Perutz and M. H. Moore, Chem. Commun.,
1996, 787.
and C6F6 with a long-wavelength absorption in solution,
12 B. L. Edelbach and W. D. Jones, J. Am. Chem. Soc., 1997, 119, 7734.
13 C. M. Beck, J. Burdeniuc, R. H. Crabtree, A. L. Rheingold and G. P. A.
Yap, Inorg. Chim. Acta, 1998, 270, 559.
14 G. M. Sheldrick, SHELXL 93, Program for the Refinement of Crystal
Structures, University of Go¨ttingen, 1995.
15 C. K. Johnson, ORTEP, Report ORNL-5138, Oak Ridge National
Laboratory, Oak Ridge, TN, 1976.
16 K. D. Warren, Struct. Bonding (Berlin), 1976, 27, 45.
17 H. P. Fritz, W. Lu¨ttke, H. Stammreich and R. Forneris, Spectrochim.
Acta, 1961, 17, 1068.
18 J. A. Marsella, A. G. Galicinski, A. M. Coughlin and G. P. Pez, J. Org.
Chem., 1992, 57, 2856; D. Astruc, Electron Transfer and Radical
Processes in Transition Metal Chemistry, VCH, Weinheim, 1995.
19 D. C. England, F. J. Weigart and J. C. Calabrese, J. Org. Chem., 1984,
49, 4816.
assigned to a CT transition between 1 and C6F6. (ii) The
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complex crystallises as [Cr(h -C6H6)2•C6F6] with a ···DADA···
stacked structure with close intra- and inter-stack contacts. The
claret colour is observed in the larger crystals. The structure is
probably stabilised by charge-transfer and p–p interactions. (iii)
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In addition to [Cr(h -C6H6)2•C6F6], salts including [Cr(h -
C6H6)2]+F— are formed in low conversion although half-wave
potentials suggest that 1 is incapable of reducing C6F6.¶ (iv) The
formation of the donor–acceptor complex and the oxidation of
1 provide support for related mechanisms for reactions of
metal–hydride complexes with fluoroarenes.11, 12
We thank Dr T. Braun and Professors N. Connelly and W. E.
Geiger for helpful discussions, the referees for helpful criticism,
S. Foxon and Dr A. C. Whitwood for experimental help and the
EPSRC and the European Commission for support.
Communication 9/00919A
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Chem. Commun., 1999, 1027–1028