Zinc(II) η1- and η2-toluene complexes: Structure and bonding in Zn(C6F5)2· (toluene) and Zn(C6F4-2-C6F5) 2·(toluene)

Article abstract of DOI:10.1021/om060429k

The first structural characterization of toluene complexes of zinc is reported. The (perfluoroaryl)zinc compounds Zn(C₆F₅) ₂·(toluene) and Zn(C₆F₄-2-C ₆F₅)₂·-(toluene)

Full text of DOI:10.1021/om060429k

Organometallics 2006, 25, 3311-3313
3311
Zinc(II) η1- and η2-Toluene Complexes: Structure and Bonding in
Zn(C6F5)2‚(toluene) and Zn(C6F4-2-C6F5)2‚(toluene)
Antonio Guerrero,† Eddy Martin,† David L. Hughes,† Nikolas Kaltsoyannis,‡ and
Manfred Bochmann*,†
Wolfson Materials and Catalysis Centre, School of Chemical Sciences and Pharmacy,
UniVersity of East Anglia, Norwich NR4 7TJ, U.K., and Department of Chemistry,
UniVersity College London, 20 Gordon Street, London WC1H 0AJ, U.K.
ReceiVed May 17, 2006
Summary: The first structural characterization of toluene com-
plexes of zinc is reported. The (perfluoroaryl)zinc compounds
Zn(C6F5)2‚(toluene) and Zn(C6F4-2-C6F5)2‚(toluene) show tolu-
ene in η2 and η1 coordination modes, respectiVely. On the other
hand, charge distribution calculations, Mulliken oVerlap popu-
lation analysis, and bonding energy data suggest that the
interactions between the Zn and the toluene ring are rather
similar in both cases.
The propensity of zinc to form π complexes with anionic
organometallic moieties is, of course, well-known.1-7 Probably
the best example of this type of complex is bis(cyclopentadi-
enyl)zinc, with its polymeric [Zn(η2-C5H5)2]∞ structure.5 Similar
asymmetric η2 interactions have also been documented for zinc
allyls7 and, most recently, for zinc divinyl compounds, where
η2 coordination leads to the formation of coordination polymers
in the solid state.8 In contrast, although the Zn2+ ion and many
zinc(II) compounds are known for their Lewis acid behavior
and readily bind polar molecules, well-characterized complexes
of zinc coordinated to neutral CdC π systems are remarkable
for their scarcity. In early pioneering studies on the system
Zn(SbF6)2/arene in liquid SO2, Damude and Dean9 isolated solid
1:1 products of the composition Zn(SbF6)2‚(arene) for electron-
rich arenes (C6HMe5 and C6Me6); solution spectroscopic studies
showed labile zinc-arene interactions which, in analogy to the
known copper(I)10 and silver(I)11 systems, were interpreted as
localized bonding of the arene to the metal cation. Weak
Figure 1. View of a molecule of Zn(C6F5)2‚(toluene) (1), indicating
the atom-numbering scheme. Hydrogen atoms have been omitted
for clarity. Thermal ellipsoids are drawn at the 50% probability
level. Selected bond distances (Å) and angles (deg): Zn-C(1) )
1.9436(14), Zn-C(11) ) 1.9419(15), Zn-C(25) ) 2.784(2), Zn-
C(26) ) 2.6847(15); C(11)-Zn-C(1) ) 162.35(6).
intramolecular interactions between Zn(II) centers and aryl
substituents have been structurally identified, as in Power’s
thiolato complexes Zn(SC6H3-2,6-Ar2)2 (Ar ) 2,4,6-R3C6H2,
R ) Me, Pri)12 or in core-modified porphyrin complexes.13
Adducts between zinc porphyrinato complexes and toluene are
also known, although in this case this is a π-stacking interaction
with the planar porphyrinato system, rather than specific toluene
coordination to the metal center.14
A few years ago we described the synthesis of Zn(C6F5)2‚
(toluene), where toluene could be readily displaced by more
electron rich arenes such as hexamethylbenzene.15 At the time,
no crystals suitable for X-ray diffraction could be obtained, and
the nature of the zinc-arene bonding, if any, could therefore
not be ascertained. We now report the first structurally
confirmed examples of isolable zinc complexes with η1- and
η2-coordinated toluene.
As reported earlier, the reaction of B(C6F5)3 with ZnMe2 in
toluene provides a convenient route to Zn(C6F5)2‚(toluene) (1)
in high yield. The compound is usually isolated as colorless
needles.15 Leaving a toluene solution of 1 standing in an NMR
tube has now provided colorless block-shaped crystals of the
compound that proved suitable for crystallographic examina-
tion.16 The structure (Figure 1) shows a slightly angled Zn(C6F5)2
molecule (angle C(11)-Zn-C(1) 162.35(6)°). The two C6F5
ligands are oriented almost perpendicular to the C(1)-Zn-C(11)
plane. The third coordination site on zinc is occupied by a
toluene molecule that is asymmetrically bonded to the metal
center, with the bond distance to the C atom in an ortho position
being shorter than that to meta C. Although the distances Zn-
C(25) ) 2.784(2) Å and Zn-C(26) ) 2.6847(15) Å are 0.74
† University of East Anglia.
‡ University College London.
(1) Aoyagi, T.; Shearer, H. M. M.; Wade, K.; Whitehead, G. J.
Organomet. Chem. 1978, 146, C29.
(2) Haaland, A.; Samdal, S.; Seip, R. J. Organomet. Chem. 1978, 153,
187.
(3) Blom, R.; Boersma, J.; Budzelaar, P. H. M.; Fisher, B.; Haaland,
A.; Volden, H. V.; Weidlein, J. Acta Chem. Scand., Ser. A 1986, 40, 113.
(4) Budzelaar, P. H. M.; Boersma, J.; van der Kerk, G. J. M.; Spek, A.
L. Organometallics 1984, 3, 1187.
(5) Budzelaar, P. H. M.; Boersma, J.; van der Kerk, G. J. M.; Spek, A.
L.; Duisenberg, A. J. M. J. Organomet. Chem. 1985, 281, 123.
(6) Walker, D. A.; Woodman, T. J.; Schormann, M.; Hughes, D. L.;
Bochmann, M. Organometallics 2003, 22, 797.
(7) Benn, R.; Grondey, H.; Lehmkuhl, H.; Nehl, H.; Angermund, K.;
Kru¨ger, C. Angew. Chem., Int. Ed. Engl. 1987, 26, 1279.
(8) Wooten, A.; Carroll, P. J.; Maestri, A. G.; Walsh, P. J. J. Am. Chem.
Soc. 2006, 128, 4624.
(9) (a) Damude, L. C.; Dean, P. A. W. J. Organomet. Chem. 1977, 125,
C1. (b) Damude, L. C.; Dean, P. A. W. J. Organomet. Chem. 1979, 168,
123.
(10) (a) Turner, R. W.; Amma, E. L. J. Am. Chem. Soc. 1966, 88, 1877.
(b) Dines, M. B.; Bird, P. J. Chem. Soc., Chem. Commun. 1973, 12. (c)
Dattelbaum, A. M.; Martin, J. D. Inorg. Chem. 1999, 38, 6200. (d) Xu, F.
B.; Li, Q. S.; Wu, L. Z.; Leng, X. B.; Li, Z. C.; Zeng, X. S.; Chow, Y. L.;
Zhang, Z. Z. Organometallics 2003, 22, 633. (e) Sundararaman, A.;
Lalancette, R. A.; Zakharov, L. N.; Rheingold, A. L.; Ja¨kle, F. Organo-
metallics 2003, 22, 3526.
10.1021/om060429k CCC: $33.50 © 2006 American Chemical Society
Publication on Web 06/07/2006

3312 Organometallics, Vol. 25, No. 14, 2006
Communications
Scheme 1
Figure 2. View of a molecule of Zn(C6F4-2-C6F5)2‚(toluene) (2),
indicating the atom-numbering scheme. Thermal ellipsoids are
drawn at the 50% probability level. Selected bond distances (Å)
and angles (deg): Zn-C(1a) ) 1.951(3), Zn-C(1b) ) 1.949(3),
Zn-C(24) ) 2.524(3); C(1b)-Zn-C(1a) ) 153.26(11), C(1a)-
Zn-C(24) ) 112.96(11), C(1b)-Zn-C(24) ) 93.55(11).
and 0.84 Å longer, respectively, than the average Zn-C6F5
bonds, they are shorter than the sum of the zinc and carbon
covalent radii and are clearly within bonding range.
Attempts were made to prepare a bulkier analogue of
Zn(C6F5)2, bis(2-perfluorobiphenylyl)zinc. Initial reactions be-
tween LiC6F4-2-C6F517 and ZnCl2 gave only intractable products.
However, the reaction of EtZnCl with LiC6F4-2-C6F5, generated
The bonding of toluene to zinc in these systems was explored
in more detail using molecular orbital calculations.20 In par-
ticular, we wished to establish whether in the η1-toluene complex
there was a significant contribution from a zwitterionic reso-
nance structure (Scheme 2), akin to the formation of the well-
known silylium ion [Et3Si‚(toluene)]+, which contains σ-bonded
n
in situ from 2-bromononafluorobiphenyl and BuLi in a light
petroleum/diethyl ether mixture, gave a clean white solid. We
assume that the initial product, EtZn(C6F4-2-C6F5), dispropor-
tionates into Et2Zn and an ether adduct of Zn(C6F4-2-C6F5)2,
which was not isolated. Residual diethyl ether was removed by
heating this solid with toluene at 90 °C while passing a nitrogen
stream over the solution (Scheme 1).18
Cooling to -26 °C afforded colorless crystals of Zn(C6F4-
2-C6F5)2‚(toluene) (2) (Figure 2).19 The Zn-aryl bonds are
comparable to those in 1. However, the toluene ligand in 2 is
bonded to zinc via the para C atom, not via the ortho and meta
C atoms, and the Zn-C(toluene) bond length in 2 is significantly
shorter than in 1, 2.524(3) Å. In agreement with a stronger
metal-toluene interaction, the C(1a)-Zn-C(1b) angle is more
acute, 153.26(11)°.
(18) To a stirred solution containing 2-bromononafluorobiphenyl (2.26
g, 18.3 mmol) in light petroleum/diethyl ether (1/1, 250 mL) at -78 °C
was added dropwise nBuLi in hexanes (11.4 mL, 18.3 mmol). The initially
colorless solution turned green and was stirred for a further 3 h. Solid EtZnCl
(2.38 g, 18.3 mmol) was added at -78 °C, and the solution was slowly
allowed to reach room temperature. A white solid formed, and the mixture
was stirred for a further 30 min before the solvent was evaporated at reduced
pressure. Toluene (250 mL) was added, and the mixture was heated to 90
°C while a stream of nitrogen was passed over the solution. After 2 h the
hot solution was filtered, the solvent volume was reduced to 40 mL, and
the solution was allowed to crystallize at -26 °C to afford the title
compound as colorless plates (4.46 g, 62%). Anal. Calcd for C31H8F18Zn:
C, 47.26; H, 1.02. Found: C, 46.85; H, 0.76. 1H NMR (C6D6): δ 7.02
(5H, C6H5), 2.10 (3H, CH3). 19F NMR (C6D6): δ -119.03 (dd, 2F, JFF
26.5 Hz, JFF ) 11.8 Hz, o-F), -134.98 (m, 2F), -141.59 (dt, 4F, JFF
)
)
(11) (a) Andrews, L. J.; Keefer, R. M. J. Am. Chem. Soc. 1949, 71, 3644.
(b) Smith, H. G.; Rundle, R. E. J. Am. Chem. Soc. 1958, 80, 5075. (c)
Turner, R. W.; Amma, E. L. J. Am. Chem. Soc. 1966, 88, 3243. (d) Hall,
E. A.; Amma, E. L. J. Chem. Soc., Chem. Commun. 1968, 622. (e) Hall
Griffith, E. A.; Amma, E. L. J. Am. Chem. Soc. 1971, 93, 3167. (f) Taylor,
I. F.; Hall, E. A.; Amma, E. L. J. Am. Chem. Soc. 1969, 91, 5745. (g) Hall
Griffiths, E. A.; Amma, E. L. J. Am. Chem. Soc. 1974, 96, 743. (h) Shelly,
K.; Finster, D. C.; Lee, Y. J.; Scheidt, R.; Reed, C. A. J. Am. Chem. Soc.
1985, 107, 5955. (i) Ogawa, K.; Kitagawa, T.; Ishida, S.; Komatsu, K.
Organometallics 2005, 24, 4842.
(12) (a) Ellison, J. J.; Power, P. P. Inorg. Chem. 1994, 33, 4231. (b)
Nguyen, T.; Panda, A.; Olmstead, M. M.; Richards, A. F.; Stender, M.;
Brynda, M.; Power, P. P. J. Am. Chem. Soc. 2005, 127, 8545.
(13) Stepien, M.; Latos-Grazynski, L. Inorg. Chem. 2003, 42, 6183.
(14) Scheidt, W. R.; Kastner, M. E.; Hatano, K. Inorg. Chem. 1978, 17,
706.
22.3 Hz, JFF ) 9.0 Hz, o′-F), -151.63 (t, 2F, JFF ) 21.7 Hz), -151.99
(ddd, 2F, JFF ) 23.4 Hz, JFF ) 20.0 Hz, JFF ) 5.6 Hz), -152.78 (td, 2F,
JFF ) 20.0 Hz, JFF ) 3.1 Hz), -160.07 (tdd, 4F, JFF ) 22.3 Hz, JFF ) 7.0
Hz, JFF ) 2.0 Hz, m-F′). 13C NMR (C6D6): δ 148.37, 148.05, 145.89,
140.07, 139.44, 137.86, 129.28 (C6H5), 128.51 (C6H5), 125.64 (C6H5),
114.93, 21.37 (CH3).
(19) Crystal data for 2: C31H8F18Zn, fw ) 787.7, triclinic, space group
P1h (No. 2), a ) 10.5464(12) Å, b ) 11.5033(12) Å, c ) 12.2157(16) Å,
R ) 78.844(10)°, â ) 77.811(10)°, γ ) 74.448(10)°, V ) 1380.7(3) Å3. Z
) 2, Dc ) 1.895 g cm-3, F(000) ) 772, T ) 180(1) K, µ(Mo KR) ) 10.4
cm-1. The total number of reflections recorded, to θmax ) 25°, was 15 221,
of which 4846 were unique (Rint ) 0.064); 3623 were “observed” with I >
2σI. wR2 ) 0.091 and R1 ) 0.055 for all 4846 reflections weighted with
w ) [σ2(Fo ) + (0.0536P)2]-1 and P ) (Fo2 + 2Fc )/3; for the “observed”
data only, R1 ) 0.037.
2
2
(20) Single-point calculations on 1 and 2 were performed at their
crystallographic geometries using the Gaussian 03 (revision D.01; Frisch,
M. J. et al. Gaussian Inc., Wallingford, CT, 2004) and Amsterdam Density
Functional codes (version 2005.01b: te Velde, G.; Bickelhaupt, F. M.; van
Gisbergen, S. J. A.; Fonseca Guerra, C.; Baerends, E. J.; Snijders, J. G.;
Ziegler, T. J. Comput. Chem. 2001, 22, 931). The PBE GGA exchange
and correlation functionals were employed. For the Gaussian 03 calculations,
the 6-311++G** basis set was employed, and the SCF convergence
criterion was set to “tight”. For the single-point ADF calculations, the all-
electron TZP basis sets were used. Geometry optimizations were also
performed using the ADF code. The frozen core approximation was
employed (Zn(2p), C(1s), F(1s)), and the energy gradient convergence
criterion was set to 0.001 au/Å. All other parameters were employed at
their default values.
(15) Walker, D. A.; Woodman, T. J.; Hughes, D. L.; Bochmann, M.
Organometallics 2001, 20, 3772.
(16) Crystal data for 1: C19H8F10Zn, fw ) 491.6, monoclinic, space group
P21/n (No. 14), a ) 14.7241(12) Å, b ) 6.7216(4) Å, c ) 18.7794(15) Å,
â ) 108.099(7)°, V ) 1766.6(2) Å3. Z ) 4, Dc ) 1.848 g cm-3, F(000) )
968, T ) 140(2) K, µ(Mo KR) ) 14.9 cm-1. The total number of reflections
recorded, to θmax ) 27.5°, was 22 516, of which 4043 were unique (Rint
)
0.034); 3289 were “observed” with I > 2σI. The structure was determined
by direct methods routines. wR2 ) 0.064 and R1 ) 0.033 for all 4043
2
reflections weighted with w ) [σ2(Fo ) + (0.0381P)2 + 0.073P]-1 and P
2
2
) (Fo + 2Fc )/3; for the “observed” data only, R1 ) 0.023.
(17) Fenton, D. E.; Park, A. J.; Shaw, D.; Massey, A. G. J. Organomet.
Chem. 1964, 2, 437.

Communications
Organometallics, Vol. 25, No. 14, 2006 3313
Scheme 2
The zinc-toluene bonds are weak: the interaction energy
between the toluene ring and the Zn-containing fragment in 1
is 25.4 kJ mol-1. The value for the η1 complex 2 is rather
similar, 27.9 kJ mol-1. To probe this further, geometry
optimizations of 1 and 2 in the gas phase were performed, using
the ADF code and starting from the crystallographic coordinates.
Both calculations readily converged to geometries that were very
similar to the starting structures, except for the Zn-C(toluene)
distances, which were ca. 0.2 Å longer than in the crystal
structures. A further geometry optimization of 1, in which the
toluene was initially placed in an η1 arrangement, also readily
converged to a structure in which the η1 coordination was
retained. This structure is only 1 kJ mol-1 less stable than the
η2 coordinated optimized 1, indicating that the potential surface
for toluene coordination is very flat.
π stacking between toluene and C6F5, as suggested by a
reviewer, is unlikely to be significant in these structures. In 1
the best planes between C6F5 and toluene form an angle of 16.2°,
with a centroid-centroid distance of 4.05 Å. There are no near-
parallel oriented arene moieties in 2. To further probe the
possible contribution of toluene-C6F5 attractive forces, the
energies of interaction between the toluene ring and the closest
perfluorophenyl ring in both systems were calculated, yielding
2.7 kJ mol-1 for 1 and 3.1 kJ mol-1 for 2. We believe these
values are too small to be structure directing.
toluene.21,22A single-point calculation on 2 gives a natural charge
for the zinc atom of 1.37. The natural charges on the C atoms
of the toluene ring, starting from C(Me), are 0.00 (ipso), -0.19
(ortho), -0.22 (meta), and -0.33 (para), with the last being
the C atom bonded to the Zn. There is therefore no evidence
for a Wheland-type structure, with would imply positive charge
delocalization over the toluene ring. For the η2 complex, the
Zn has a charge of 1.35. As one goes around the toluene ring,
starting from C(Me), the charges are -0.02, -0.20, -0.19,
-0.21, -0.25, and -0.28, where the last two are the C atoms
bonded to the Zn. Hirshfeld charge analysis, while producing
absolute values different from those of the natural approach,
also finds little difference between the charges on corresponding
atoms in 1 and 2. Hence, the calculations suggest that, despite
the differences in Zn-C distances and bonding modes apparent
from the solid-state structure, the charge distributions in
compounds 1 and 2 are quite similar.
Mulliken overlap populations may be considered to be the
number of electrons covalently bonding between two atoms.
The Zn-C(C6F5) overlap populations in 2 are 0.74 and 0.77 e,
not unsurprising for single bonds. In contrast, the overlap
populations between the Zn atom and the C of the toluene rings
in 1 and 2 are much smaller; 0.12 e in 2 and 0.06 and 0.08 e in
1. This suggests not only that there is comparatively little Zn-
toluene covalent bonding in both 1 and 2 but also that, as noted
in the charge analyses, the interactions between the Zn and the
toluene rings are rather similar in both cases.
The overall conclusion, therefore, is that, while the crystal-
lographic data point to a distinct difference between the Zn-
toluene bonding in 1 and 2, the computed electronic structures
indicate relatively weak zinc-arene π bonding and emphasize
the similarity of arene bonding in both cases.
Acknowledgment. This work was supported by the Engi-
neering and Physical Sciences Research Council (EPSRC). A.G.
thanks Lanxess Inc. of Canada for a studentship. N.K. thanks
the EPSRC for computing resources under Grant No. GR/
S06233.
Supporting Information Available: Tables giving crystal-
lographic data and structure refinement details for 1 and 2; crystal
data are also given as CIF files. This material is available free of
charge via the Internet at http://pubs.acs.org.
(21) Lambert, J. B.; Zhang, S.; Stern, C. L.; Huffman, J. C. Science 1993,
260, 1917.
(22) The related aluminum compound Al(C6F5)3(η1-toluene) is better
represented by a π coordination model: Hair, G. S.; Cowley, A. H.; Jones,
R. A.; McBurnett, B. G.; Voigt, A. J. Am. Chem. Soc. 1999, 121, 4922.
OM060429K