Synthesis, structure, and supramolecular architecture of benzonitrile and pyridine adducts of bis(pentafluorophenyl)zinc: Pentafluorophenyl-aryl interactions versus homoaromatic pairing

Article abstract of DOI:10.1021/om701127p

Treatment of (C₆F₅)₂Zn(toluene) with 2 equiv of a series of benzonitrile or pyridine derivatives yielded the complexes (C₆F₅)₂Zn(L)₂ (where L = benzonitrile, 4-(phenyl)benzonitrile, 4-(N-pyrroryl)benzonitrile, pyridine, 4-(phenyl)pyridine, and 4-(N-pyrrolyl)pyridine). The four-coordinate solution-phase nature of these complexes was confirmed by a series of variable-temperature ¹⁹F NMR experiments and comparison to (C ₆F₅)₂Zn(2,2′-bipy). The solvent-free solid-state structures of each of the four-coordinate adducts and the toluene solvate of (C₆F₅)₂Zn(NCC₆H ₄C₆H₅)₂ were determined by single-crystal X-ray diffraction and have distorted tetrahedral geometries. Analysis of the crystal packing revealed a preponderance of offset face-to-face homo-aryl and embrace-like interactions over the hetero-aryl, pentafluorophenyl-phenyl, interaction. These aryl-aryl synthons serve to assemble paired, one- and three-dimensional supramolecular architectures.

Full text of DOI:10.1021/om701127p

1436
Organometallics 2008, 27, 1436–1446
Synthesis, Structure, and Supramolecular Architecture of
Benzonitrile and Pyridine Adducts of Bis(pentafluorophenyl)zinc:
Pentafluorophenyl–Aryl Interactions versus Homoaromatic Pairing
Eddy Martin,† Claire Spendley,† Andrew J. Mountford,† Simon J. Coles,‡ Peter N. Horton,‡
David L. Hughes,† Michael B. Hursthouse,‡ and Simon J. Lancaster*,†
Wolfson Materials and Catalysis Centre, School of Chemical Sciences and Pharmacy, UniVersity of East
Anglia, Norwich, NR4 7TJ U.K., and School of Chemistry, UniVersity of Southampton, Highfield,
Southampton, SO17 1BJ U.K.
ReceiVed NoVember 9, 2007
Treatment of (C6F5)2Zn(toluene) with 2 equiv of a series of benzonitrile or pyridine derivatives yielded
the complexes (C6F5)2Zn(L)2 (where L ) benzonitrile, 4-(phenyl)benzonitrile, 4-(N-pyrrolyl)benzonitrile,
pyridine, 4-(phenyl)pyridine, and 4-(N-pyrrolyl)pyridine). The four-coordinate solution-phase nature of
these complexes was confirmed by a series of variable-temperature 19F NMR experiments and comparison
to (C6F5)2Zn(2,2′-bipy). The solvent-free solid-state structures of each of the four-coordinate adducts
and the toluene solvate of (C6F5)2Zn(NCC6H4C6H5)2 were determined by single-crystal X-ray diffraction
and have distorted tetrahedral geometries. Analysis of the crystal packing revealed a preponderance of
offset face-to-face homo-aryl and embrace-like interactions over the hetero-aryl, pentafluorophenyl-phenyl,
interaction. These aryl-aryl synthons serve to assemble paired, one- and three-dimensional supramolecular
architectures.
recourse to dative bonds.7–14 Despite the challenges associated
Introduction
with depending upon such weak interactions, this approach
promises materials more readily processed than metal organic
The attraction of coordination complexes to supramolecular
chemists derives from the combination of highly directional
dative bonding and metal ion nodes with geometries not found
in organic chemistry.1 In particular, supramolecular coordination
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optics2 and as gas storage materials.3,4 While dative bonding
continues to dominate supramolecular coordination chemistry,
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interactions, such as hydrogen bonding and aryl-aryl pairing,
play in determining the three-dimensional assembly of metal
complexes.5 These are the supramolecular synthons familiar to
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plexes using the tools of organic crystal engineering and without
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* To whom correspondence should be addressed. Fax: 44 1603 592003.
Tel: 44 1603 592009. E-mail: S.Lancaster@uea.ac.uk.
† University of East Anglia.
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10.1021/om701127p CCC: $40.75
2008 American Chemical Society
Publication on Web 03/05/2008

Adducts of Bis(pentafluorophenyl)zinc
Organometallics, Vol. 27, No. 7, 2008 1437
Scheme 1
frameworks and with properties more closely related to discrete
molecules than coordination polymers.
There is also a limited number of reports of organometallic
compounds exhibiting aryl-perfluoroaryl stacking and pairing
interactions.23,24
Despite their opposite quadrupole moments, the offset face-
to-face (off) interaction between perfluoroaromatic groups is
comparable to the often observed supramolecular motif in which
aromatics form off pairs or stacks.25,26 Such off interactions
between pairs or stacks of C6F5 groups have been reported for
organometallic and coordination compounds with pentafluo-
rophenyl substituents.13,27
During a study of N-H · · · F-C hydrogen bonding in protic
amine adducts of tris(pentafluorophenyl)-aluminum and -boron
we found examples of intramolecular phenyl-pentafluorophenyl
pairing interactions.16d In order to establish a one-to-one
correspondence between the number of C6F5 groups and Lewis
acidic sites, favoring the formation of infinite supramolecular
assemblies, our subsequent investigations have focused upon
the supramolecular architectures of adducts of bis(penta-
fluorophenyl)zinc.28,29 Determination of the structures of a series
of protic amine adducts revealed their assembly through
examples of N-H · · · F-C contacts, off phenyl-phenyl (PhH · · ·
PhH) and off pentafluorophenyl-pentafluorophenyl interactions
(PhF · · · PhF), while there were relatively few instances of
phenyl-pentafluorophenyl interactions (PhH · · · PhF).28 This was
surprising in light of the prominence this interaction has attained
in the literature. We concluded that the fine balance between
competing intermolecular interactions yielded a series in which
no one supramolecular interaction predominated.
Interest in pentafluorophenyl complexes originated with the
highly electron-withdrawing nature of the group, rendering the
metal center highly Lewis acidic.15 We were initially alerted to
the potential of organofluorine groups in supramolecular chem-
istry by a series of structural and spectroscopic observations
revealing significant intramolecular interactions.16,17 The three
most pertinent intermolecular interactions in which the pen-
tafluorophenyl group participates are pairing or stacking with
aryl groups, offset face-to-face arrangements with other per-
fluoroaryl rings, and acting as a hydrogen bond acceptor. The
first is perhaps the best known of these prospective supramo-
lecular synthons and results from the opposing quadrupoles of
the perfluorophenyl and phenyl groups.18 Since the original
demonstration of the elevation in melting point on mixing
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A. J.; Hughes, D. L.; Lancaster, S. J. Chem. Commun. 2003, 2148. (d)
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D. L.; Hursthouse, M. B.; Light, M. E. Inorg. Chem. 2005, 44, 5921. (e)
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Lancaster, S. J. Chem. Commun. 2005, 2044. (f) Mountford, A. J.; Clegg,
W.; Coles, S. J.; Harrington, R. W.; Horton, P. N.; Humphrey, S. M.;
Hursthouse, M. B.; Wright, J. A.; Lancaster, S. J. Chem.-Eur. J. 2007, 13,
4535. (g) Mountford, A. J.; Lancaster, S. J.; Coles, S. J. Acta Crystallogr.,
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(17) For a review of the supramolecular chemistry of organofluorine
see: Reichenbächer, K.; Süss, H. I.; Hulliger, J. Chem. Soc. ReV. 2005, 34,
22.
(18) (a) Williams, J. H. Acc. Chem. Res 1993, 26, 593. (b) West, Jr.,
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Grubbs, R. H. Angew. Chem., Int. Ed. Engl. 1997, 36, 248. (b) Coates,
G. W.; Dunn, A. R.; Henling, L. M.; Ziller, J. W.; Lobkovsky, E. B.; Grubbs,
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Chem. Soc. 2003, 42, 3903.
Figure 1. Molecular structure of 1. Displacement ellipsoids are
shown at the 50% probability level. Hydrogen atoms have been
omitted for clarity.

1438 Organometallics, Vol. 27, No. 7, 2008
Martin et al.
Figure 2. Molecular structures and relative orientations of the two, very similar but crystallographically independent, molecules in the
toluene-free lattice of 2. Displacement ellipsoids are shown at the 50% probability level. Hydrogen atoms have been omitted for clarity.
Figure 3. Molecular structure of 4. Displacement ellipsoids are
shown at the 50% probability level. Hydrogen atoms have been
omitted for clarity.
We now report the synthesis, structure, and supramolecular
architecture of 4-substituted benzonitrile and pyridine adducts
Figure 4. Molecular structure of 6. Displacement ellipsoids are
of bis(pentafluorophenyl)zinc. The absence of protic hydrogens
was intended to simplify the interplay of intermolecular interac-
tions and allow us to determine the relative preference for
phenyl-phenyl (I), pentafluorophenyl-phenyl (II), and penta-
fluorophenyl-pentafluorophenyl (III) interactions in these
systems. In addition, while intramolecular aryl-pairing interac-
tions were geometrically possible and occurred in the benzyl
amine adducts we reported earlier,16d this complication should
be absent in the pyridine and benzonitrile adducts.
shown at the 50% probability level. Hydrogen atoms have been
omitted for clarity. Compound 6 has a two-fold symmetry axis
passing through the zinc atom and is isostructural with com-
pound 5.
Results and Discussion
Treatment of a toluene solution of (C6F5)2Zn with the
appropriate benzonitrile or pyridine donor resulted in conversion
to adducts 1-6 in excellent yield (Scheme 1), with elemental
analyses in good agreement with the expected compositions.
As was the case for the previously reported amine adducts of
bis(pentafluorophenyl)zinc,28 the 1H and 19F NMR data proved
indicative of adduct formation. The ortho-fluorine resonances
are all within (2 ppm of that for (C6F5)2Zn (–118.3 ppm in
C6D6, where the species present in solution is presumably an
arene adduct).30 However, the value of ∆δ (m-F – p-F) is a
more sensitive indicator of the metal coordination environment
and reduces from ca. 8 ppm for (C6F5)2Zn(arene) to less than 5

Adducts of Bis(pentafluorophenyl)zinc
Organometallics, Vol. 27, No. 7, 2008 1439
Table 1. Selected Bond Lengths and Angles (in Å and deg) for Compounds 1–6, with Esd’s in Parentheses
compound
Zn-C
2.011(2)
Zn-N
2.067(2)
C-Zn-C
N-Zn-N
Zn-N-C
Zn-N3-C3
1
Zn-C11
Zn-C21
Zn-C11
Zn-C21
Zn-C51
Zn-C61
Zn-C11
Zn-C21
Zn-C11
Zn-C21
Zn-C11
Zn-C21
Zn-C11
Zn-C11
Zn-N3
Zn-N4
Zn1-N3
Zn1-N4
Zn2-N7
Zn2-N8
Zn-N3
Zn-N4
Zn-N3
Zn-N4
Zn-N31
Zn-N41
Zn-N31
Zn-N31
129.67(8)
97.47(7)
171.8(2)
156.7(2)
162.1(3)
157.2(3)
158.0(3)
159.9(3)
157.3(2)
157.7(2)
175.60(14)
174.76(14)
178.79(8)
178.17(8)
161.07(11)
162.68(1)
2.012(2)
2.002(4)
2.002(3)
2.009(3)
2.018(3)
2.005(2)
2.007(2)
2.0292(16)
2.0209(15)
2.032(2)
2.024(2)
2.020(3)
2.018(3)
2.112(2)
2.105(3)
2.110(3)
2.098(3)
2.097(3)
2.093(2)
2.090(2)
2.0660(14)
2.1206(15)
2.0998(13)
2.0983(14)
2.099(2)
2.099(2)
Zn-N4-C4
Zn1-N3-C3
Zn1-N4-C4
Zn2-N7-C7
Zn2-N8-C8
Zn-N3-C3
Zn-N4-C4
Zn-N3-C3
Zn-N4-C4
Zn-N31-C34
Zn-N41-C44
Zn-N31-C34
Zn-N31-C34
2 Mol. A
135.30(13)
132.68(13)
128.17(9)
123.55(6)
120.14(6)
97.41(11)
96.25(12)
99.11(8)
95.75(6)
92.97(5)
Mol. B
2 · tol
3
4
5
6
118.07(14)
117.45(15)
87.91(12)
87.59(13)
ppm for each of the adducts.31 Addition of further donor ligand
to NMR samples did not result in duplication of 1H NMR
resonances but did produce a chemical shift change, whereas
cooling the samples to –80 °C led only to a slight broadening
of the resonances. These observations support the assumption
that, in solution, these adducts participate in dynamic dissocia-
tion-association equilibria with free ligand.
The fact that we normally obtain four-coordinate crystalline
adducts from solution suggests, but does not prove, that the bis-
donor complex is the major constituent of these equilibria. In
order to address this point, we prepared the 2,2′-bipyridine (bipy)
adduct, (C6F5)2Zn(bipy) (7). The 19F NMR spectrum of 7 is
very similar to those of adducts 1-6 and almost superimposable
upon that of compound 4, (C6F5)2Zn(py)2. Since the chelating
bipy ligand must give rise to a four-coordinate complex, we
regard the spectral similarity as further compelling evidence that
four-coordinate adducts are the predominant species in solution.
Furthermore, treatment of (C6F5)2Zn(toluene) with less than 2
equiv of monodentate donor and observation at –60 °C resulted
in 19F NMR spectra with multiple C6F5 environments suggesting
that under these conditions three- and four-coordinate complexes
can be spectroscopically distinguished.
However, the toluene-free crystal structure of compound 2 is
the only example in the series in which there is more than one
crystallographically independent molecule in the lattice (for 2,
Z′ ) 2). Selected bond lengths and angles for each member of
the series are collated in Table 1.
The Zn-C bond length is a rather insensitive parameter, and
there is little variation between complexes in either the
benzonitrile (1-3) or pyridine (4-6) series of adducts. Indeed,
the average Zn-C bond lengths for the benzonitrile, pyridine,
and previously reported amine adducts at 2.012, 2.023, and 2.032
Å, respectively, are rather similar. The average Zn-N bond
lengths are also remarkably consistent in the benzonitrile (2.096
Å) and pyridine (2.097 Å) adducts; here the greatest variation
is observed between asymmetrically coordinated ligands in
complexes 1 and 3. In general, the Zn-N bond lengths are
slightly shorter than the bond lengths we observed for
amine adducts (2.094-2.187 Å), where the differences reflected
more pronounced steric and electronic contrast than is present
in the current study.
Whereas there is little variation in the bond length distribution
in complexes 1–6, the C-Zn-C angles cover a range of some
12° for the benzonitriles (123–135°), while for the pyridines
(117–120°) they are somewhat more acute and cover a smaller
range. In contrast, the N-Zn-N angles vary over only 3° for
the benzonitriles (96–99°) and cover a slightly wider spread of
ca. 6° for the pyridines (87–93°). A number of comparisons
suggest that these differences are not simply due to the steric
demands of the donor ligands, which should to a first ap-
proximation be rather similar. Table 1 lists three significantly
different C-Zn-C angles separated by some 7° for compound
2, two from crystallographically independent molecules in the
solvent-free lattice and one from the toluene solvate; the
supramolecular origin of these differences is discussed below.
The most striking anomaly in Table 1 is the somewhat
counterintuitive relationship between the C-Zn-C and N-Zn-N
angles. As expected, in each adduct the C-Zn-C angle is the
Molecular Structures. For compounds 1, 2, 4, and 5, X-ray
quality crystals were afforded by cooling concentrated toluene
solutions to –25 °C. Of these, 2 was the only one to crystallize
with a solvent molecule in the lattice, giving 2 · toluene. Solvent-
free single crystals of 2 and 6 suitable for X-ray diffraction were
obtained by layering light petroleum over dichloromethane
solutions of the crude products and cooling to –25 °C overnight.
The preparation of compound 3, its crystallization from 1,2-
difluorobenzene, and its molecular structure have been com-
municated and are included here for comparison.29 The solid-
state structures of 1–6 and 2 · toluene were determined by X-ray
crystallography and are described below. No evidence for the
presence of more than one type of crystalline product was
observed, under magnification, during the crystal selection
process.
Each of the compounds 1–6 has the expected essentially
tetrahedral ligand coordination about the zinc center (selected
ORTEPs are presented in Figures 1, 2, 3, 4). Only compounds
5 and 6 retain the molecular C2 symmetry axis in the solid state.
Figure 5. Packing in compound 4.

1440 Organometallics, Vol. 27, No. 7, 2008
Martin et al.
Figure 9. Illustrating the off PhH · · · PhH and PhF · · · PhF pairing
interactions, connecting adjacent sheets, in 1, viewed along the a
axis. The carbon atoms are colored blue and red to indicate different
sheets.
Figure 6. View down the a axis, showing the packing in one layer
of compound 5.
Figure 7. View down the a axis of the packing in one layer of
compound 6.
Figure 10. Representation showing pentafluorophenyl rings in a
column of molecules of 2A stacking along a 21 symmetry axis.
lowest N-Zn-N angles in the study. More generally, there is
not the inverse correlation between C-Zn-C and N-Zn-N
angles that might have been anticipated. The Zn-N-C bond
angles determined for the benzonitrile adducts also show much
greater variation, between 156.7° and 175.6°, than one would
initially anticipate. Similarly, the Zn-N-Cpara angles for the
pyridine adducts, which one would also expect to be near linear,
cover a range from 161.1° to 178.8°. Further insight into the
angular distribution is provided by consideration of these
structures as supramolecular assemblies.
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Robins, E. G.; Batsanov, A. S.; Stimson, L. M.; Howard, J. A. K.; Clark,
S. J.; Marder, T. B. New J. Chem. 2002, 26, 1740. (d) Smith, C. E.; Smith,
P. S.; Thomas, R. L.; Robins, E. G.; Collings, J. C.; Dai, C. Y.; Scott,
A. J.; Borwick, S.; Batsanov, A. S.; Watt, S. W.; Clark, S. J.; Viney, C.;
Howard, J. A. K.; Clegg, W.; Marder, T. B. J. Mater. Chem. 2004, 14,
413. (e) Collings, J. C.; Smith, P. S.; Yufit, D. S.; Batsanov, A. S.; Howard,
J. A. K.; Marder, T. B. Cryst. Eng. Commun. 2004, 6, 25. (f) Watt, S. W.;
Dai, C.; Scott, A. J.; Burke, J. M.; Thomas, R. L.; Collings, J. C.; Viney,
C.; Clegg, W.; Marder, T. B. Angew. Chem., Int. Ed. 2004, 43, 3061. (g)
Collings, J. C.; Burke, J. M.; Smith, P. S.; Batsanov, A. S.; Howard, J. A. K.;
Marder, T. B. Org. Biomol. Chem. 2004, 2, 3172.
Figure 8. Illustrating the off PhH · · · PhH and PhF · · · PhF stacks in 1
running parallel to the a axis.
more obtuse because of the strong repulsion between the
pentafluorophenyl groups, while the N-Zn-N is the most acute.
From a purely molecular standpoint, it is therefore somewhat
surprising that complexes 5 and 6, which have the smallest
C-Zn-C angle distortions from tetrahedral, also exhibit the

Adducts of Bis(pentafluorophenyl)zinc
Organometallics, Vol. 27, No. 7, 2008 1441
Figure 11. Projection of overlapping rings onto the plane of the lower ring for the pentafluorophenyl-phenyl interactions in 2 (a, b).
Displacement ellipsoids are drawn at 30% probability.
Table 2. Hydrogen · · · Fluorine Contacts
symmetry operation
(CHF)/ for intermolecular
compound C-H · · · F d(H · · · F)a /Å
deg
interaction
2
F52 · · · H72
F26 · · · H46
2.28
2.45
2.46
2.34
2.46
2.42
2.46
2.38
150
140
152
126
159
150
129
138
x, y–1, z
x, y–1, z
2–x, y, 0.5–z
x–1, y, z
1–x, y–0.5, 1.5–z
–x, –y, 1–z
2 · (toluene) F25 · · · H42
3
4
F16 · · · H43
F12 · · · H45
F16 · · · H34
F13 · · · H32
F13 · · · H36
5
6
1–x, y+0.5, 0.25–z
–0.5–x, y, 0.75–z
a Only those contacts < 2.50 Å are listed here.
Supramolecular Architecture. Adduct 4 has the simplest
supramolecular architecture in this study. There are no infinite
structural elements, and this is the only example in which there
are no offset face-to-face homoaryl interactions between C6F5
rings (off PhF · · · PhF). Instead noteworthy supramolecular
features are restricted to an off interaction between two
centrosymmetrically related pyridine ligands (Figure 5).
The packing of 5 is depicted in Figure 6. Molecules of 5 are
paired through a 4-fold pentafluorophenyl embrace (FPFE) of
Figure 12. Packing diagram for 2, viewed along the b axis. The
aryl ring in dark blue interacts with the red ring (pentafluorophenyl)
in a PhF · · · ArH interaction, while a second PhF · · · PhH interaction
occurs between the aryl rings in light blue and the pentafluorophenyl
ring in orange.
(22) (a) Renak, M. L.; Bartholemew, G. P.; Wang, S.; Ricatto, P. J.;
Lachicotte, R. J.; Bazan, G. C. J. Am. Chem. Soc 1999, 121, 7787. (b)
Gdaniec, M.; Jankowski, W.; Milewska, M. J.; Poloñski, T. Angew. Chem.
Int. Ed. 2003, 42, 3903. (c) El-azizi, Y.; Schmitzer, A.; Collins, S. K. Angew.
Chem. Int. Ed. 2006, 45, 968.
(23) (a) Beck, C. M.; Burdeniuc, J.; Crabtree, R. H.; Rheingold, A. L.;
Yap, G. P. A. Inorg. Chim. Acta 1998, 270, 559. (b) Aspley, C. J.; Boxwell,
C.; Buil, M. L.; Higgitt, C. L.; Long, C.; Pertutz, R. N. Chem. Commun.
1999, 1027. For examples of stacking involving C6F5 substituents on
cyclopentadienyl rings see: (c) Blanchard, M. D.; Hughes, R. P.; Concolino,
T. E.; Rheingold, A. L. Chem. Mater 2000, 12, 1604. (d) Thornberry, M. P.;
Slebodnick, C.; Deck, P. Organometallics 2000, 19, 5352. (e) Thornberry,
M. P.; Slebodnick, C.; Deck, P. Organometallics 2001, 20, 920.
(24) (a) For examples with intramolecular interactions involving metal-
bonded C6F5 groups see: Parks, D. J.; Piers, W. E.; Parvez, M.; Atencio,
R.; Zaworotko, M. J. Organometallics 1998, 17, 1369. (b) Blackwell, J. M.;
Piers, W. E.; Parvez, M.; McDonald, R. E. Organometallics 2002, 21, 1400.
(25) orenzo, S.; Lewis, G. R.; Dance, I. New J. Chem. 2000, 24, 295.
(26) unter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525.
(27) (a) For acknowledged examples see: Adams, N.; Cowley, A. R.;
Dubberley, S. R.; Sealey, A. J.; Skinner, M. E. G.; Mountford, P. Chem.
Commun. 2001, 2738. (b) Hair, G. S.; Cowley, A. H.; Gorden, J. D.; Jones,
J. N.; Jones, R. A.; Macdonald, C. L. B. Chem. Commun. 2003, 424.
Figure 13. One-Dimensional Network of 2 · (toluene) viewed along
the c* axis.
the sort documented by Dance and co-workers.25 Each molecule
¯
is related to its partner by a 4 center where four C6F5 ligands
are closely bound. The (4-phenyl)pyridine ligands also appear
to form an embrace, but this is essentially a shape-based
intermeshing, allowing close packing of the molecules; with
no short interligand contacts between the four ligands involved,

1442 Organometallics, Vol. 27, No. 7, 2008
Martin et al.
is a second PhF · · · PhH interaction with an interplanar angle of
19.7° and a centroid–centroid distance of 3.85 Å (Figure 11a).
Thus three of the four aryl substituents on each zinc center and
one of the outer phenyl rings participate in aryl-aryl intermo-
lecular interactions. In addition, we note that there are two
intermolecular C-H · · · F-C contacts within the van der Waals
radii (Table 2).
The supramolecular architecture is built up from two
independent columns of molecules linked through the
PhF · · · PhF stacks. Each column has an equivalent half a unit
cell away along the c axis. Individual columns of 2A or 2B
molecules are assembled into a three-dimensional structure
by the hetero-aryl interactions (Figure 12).
When 2 crystallizes from toluene as a solvate, its supramo-
lecular structure is quite different from that of the solvent-
free analogue. Although, as in 2, the supramolecular arrange-
ment of 2 · (toluene) is directed by PhF · · · PhF and PhH · · · PhH
off interactions, here a one-dimensional chain, linked by
alternating offset pentafluorophenyl and offset biphenyl
interactions (Figure 13), is formed. Unlike in 2, there are no
pentafluorophenyl-aryl interactions. The two offset C6F5
rings are related by an inversion center. One of the 4-(phe-
nyl)benzonitriles on each molecule is also related through
an inversion center to an overlapping ligand in an off
interaction (Figure 15a).
Figure 14. View of the diamondoid packing of 3, showing the off
interactions of one molecule with four neighbors.
it is difficult to single out individual attractive contributions.
Since every Zn atom lies on a 2-fold symmetry axis which
¯
passes through the 4 centers and is parallel to the c axis, the
supramolecular structure of 5 consists of columns of molecules
along this axis connected through the two alternating embraces.
Each column is symmetry-related to four others by 2-fold axes
parallel to the a and b axes. Adjacent columns interact through
an off PhF · · · PhF pairing. In addition, the (4-phenyl)pyridine
ligands pair through an off-like interaction whereby each
pyridine ring overlays a phenyl ring (and vice versa).
The lattice of adduct 6 (Figure 7) exhibits essentially the
same supramolecular architecture as 5. Whereas in compound
5 the Zn to Zn distance in the FPFE is 6.851(2) Å, in this
structure the separation between corresponding zinc centers
is 7.0058(2) Å.
In adduct 3, there are two pairs of centrosymmetrically related
off PhF · · · PhF interactions (Figure 16c,d). Similarly, each 4-(N-
pyrrollyl)benzonitrile ligand interacts with another about a center
of symmetry such that the phenyl ring of one ligand overlaps
the pyrrole ring of the second ligand, and vice versa (Figure
15b). Each of the four ligands thus interacts with a different
neighboring molecule, resulting in a three-dimensional diamond-
like network (Figure 14). This arrangement includes a short
C-H · · · F-C contact (the F16 · · · H45′ distance is 2.34 Å)
between molecules.
Discussion. When we began our investigations into the
supramolecular architecture of amine adducts of (C6F5)2Zn, we
anticipated that the introduction of a phenyl group into the donor
would be highly likely to lead to intermolecular quadrupolar
phenyl-pentafluorophenyl interactions. In initial results, ex-
amining amine adducts, this intermolecular interaction did not
predominate, but was found only occasionally in competition
with the intramolecular variety, homo-aryl and X-H · · · F-C
interactions.28
In adduct 1, every C6F5 (PhF) and C6H5 (PhH) ring is
involved in a homoaryl off interaction. The major supramo-
lecular motif comprises sheets of PhH · · · PhH and PhF · · · PhF
stacks running parallel to the crystallographic a axis, linked
through Zn atoms and alternating along c (Figure 8). Each
sheet is related to the next by a translation in the b direction
and connected through PhH · · · PhH and PhF · · · PhF pairing
interactions (Figure 9), thus generating a three-dimensional
architecture.
In this study we have employed ligands without potent X-H
hydrogen bond donors. As a result, such interactions do not
play a major role in determining the supramolecular architectures
described herein. However, some noteworthy C-H · · · F-C
interactions were found, and these are summarized in Table 2.
Weak hydrogen-bond-like interactions, such as those present
in compound 2, will make a small, but non-negligible, contribu-
tion to directing the assembly of molecules like those studied
here, where their significance is enhanced in the absence of
The 4-(phenyl)benzonitrile adduct, 2, crystallizes from dichlo-
romethane solution with two molecules, 2A and 2B, in the
asymmetric unit. Despite the assembly of the lattice generating
two crystallographically distinct molecules, there are a number
of close similarities between the intermolecular interactions in
which they participate. In both cases one of the pentafluorophe-
nyl rings is part of an infinite off PhF · · · PhF stack parallel to
the b axis, each ring along the stack related to the next by a
crystallographic 21 screw axis (Figure 10). The second C6F5
ring of 2A engages in a pentafluorophenyl-aryl (PhF · · · PhH)
interaction with a para-phenyl ring of molecule 2B: the rings
have an interplanar angle of 3.5° and centroid–centroid distance
of 3.65 Å (Figure 11b). Between molecules 2B and 2A, there
(29) Martin, E.; Hughes, D. L.; Lancaster, S. J. Eur. J. Inorg. Chem.
2006, 4037.
(30) Guerrero, A.; Martin, E.; Hughes, D. L.; Kaltsoyannis, N.;
Bochmann, M. Organometallics 2006, 25, 3311.
(31) Horton, A. D.; de With, J.; van der Linden, A. J.; van de Weg, H.
Organometallics 1996, 15, 2672. (b) Duchateau, R.; Cremer, U.; Harmsen,
R. J.; Mohamud, S. I.; Abbenhuis, H. C. L.; van Santen, R. A.; Meetsma,
A.; Thiele, S. K.-H.; van Tol, M. F. H.; Kranenburg, M. Organometallics
1999, 18, 5447.
(28) Mountford, A. J.; Lancaster, S. J.; Coles, S. J.; Horton, P. N.;
Hughes, D. L.; Hursthouse, M. B.; Light, M. E. Organometallics 2006, 25,
3837.

Adducts of Bis(pentafluorophenyl)zinc
Organometallics, Vol. 27, No. 7, 2008 1443
Figure 15. Selected projections of overlapping aromatic rings, (PhH · · · PhH) (the upper ring is in the plane of the page), for the off interactions
in 2 · toluene (a), 3 (b), 5 (c), and 6 (d). Displacement ellipsoids are drawn at 30% probability.
stronger interactions.32 It is also worth noting that the use of
rigid benzonitrile or pyridine donors, projecting the phenyl
substituent out from the zinc center, has ensured that there are
no intramolecular aryl-aryl interactions.
Including the toluene solvate of 2, six of the seven supramo-
lecular architectures reported here have offset face-to-face
interactions between hydrogen-substituted aromatics (off
PhH · · · PhH). The variation in the degree of overlap (extent of
offset) between selected instances of these interactions is
illustrated in Figure 15. Hunter and Sanders have shown that
the offset or slipped geometry optimizing σ-π attractions is
the most favorable for face-to-face aryl-aryl interactions.25 The
offset face-to-face interaction is not restricted to single phenyl
groups. In 2 · (toluene) (Figure 15a) there is an effective off
interaction encompassing the whole biphenyl group, while in
3, 5, and 6 (Figure 15b,c, and d) the overlap is somewhat less
efficient. The centroid–plane distances observed for our off
homo-aryl series lie in the range 3.41–3.63 Å, which is typical
for these interactions.
The offset or slipped geometry has been calculated to again
be the most favorable face-to-face arrangement for perfluoroaryl
groups.26 In this series the analogous off interaction between
pentafluorophenyl rings is equally prevalent in the aryl-aryl
and is observed for at least one of the two C6F5 groups in six
of the seven crystal lattices (selected illustrations are presented
in Figure 16). The centroid–plane distances vary between 3.35
and 3.62 Å, and although not as low as that predicted for the
gas-phase dimerization of C6F6,26 they are in the same range
that we and others have previously reported.27,28
(32) (a) Borwick, S. J.; Howard, J. A. K.; Lehmann, C. W.; O’Hagan,
D. Acta. Crystallogr., Sect. E 1997, 53, 124. (b) Weiss, H.-C.; Boese, R.;
Smith, H. L.; Haley, M. M. Chem. Commun. 1997, 2403. (c) Thalladi, V. K.;
Weiss, H.-C.; Bläser, D.; Boese, R.; Nangia, A.; Desiraju, G. R. J. Am.
Chem. Soc. 1998, 120, 8702. (d) Pham, M.; Gdaniec, M.; Polon´ski, T. J.
Org. Chem. 1998, 63, 3731. (e) Barbarich, T. J.; Rithner, C. D.; Miller,
S. M.; Anderson, O. P.; Strauss, S. H. J. Am. Chem. Soc. 1999, 121, 4280.
For estimates of the strength of O-H · · · F-C hydrogen bonds see: (f)
Takemura, H.; Kotoku, M.; Yasutake, M.; Sinmyozu, T. Eur. J. Org. Chem.
2004, 2019. (g) Caminanti, W.; Melandri, S.; Maris, A.; Ottaviani, P. Angew.
Chem. Int. Ed. 2006, 45, 2438, and references therein. For a discussion of
weakly attractive C-H · · · F-C interactions in polymerization catalysis see:
(i) Chan, M. C. W.; Kui, S. C. F.; Cole, J. M.; McIntyre, G. J.; Matsui, S.;
Zhu, N.; Tam, K.-H. Chem.-Eur. J. 2006, 12, 2607.
In contrast, the only examples of hetero-aryl pairing found
in this study were those in the solvent-free supramolecular
architecture of 2 (Figure 11). Of these interactions, only that
depicted in Figure 11a, between a C6F5 ring and the C6H4 ring
of 4-(phenyl)benzonitrile, shows particularly good overlap. As
is typical for hetero-aryl interactions of this type, the rings are
somewhat further apart than is the case for homo-aryl contacts:
the centroid of the C6F5 is 3.84 Å from the plane of the
benzonitrile ring, while the C6H4 centroid is 3.52 Å from the
plane of the C6F5 ring.

1444 Organometallics, Vol. 27, No. 7, 2008
Martin et al.
Figure 16. Selected projections of overlapping PhF · · · PhF rings (the upper ring is in the plane of the page) in 1 (a), 2 (b), and 3 (c, d).
Displacement ellipsoids are drawn at 30% probability.
Despite the fact that the hetero-aryl interaction has been
estimated to be potentially twice as strong an attraction, the
preference of these complexes for assembly through the offset
face-to-face homo-aryl rather than PhF · · · ArH hetero-aryl
supramolecular synthons is easily rationalized. An effective
quadrupolar aryl-perfluoroaryl interaction requires one ring to
sit almost directly above the other.26 Clearly the zinc substituent
presents a steric impediment to this arrangement. The 4-pyrrolyl
substituent, as well as being more electron-rich, was intended
to reduce this steric congestion through the smaller ring size
and the absence of a projecting para group. Evidently, this
strategy was not successful. The off interactions avoid the steric
constraints imposed by the metal complex and therefore
predominate.
The solid-state structures of compounds 1-6 provide a
number of instances where it is readily apparent that the
variation in bond angles observed is as much a consequence of
the supramolecular as the coordination chemistry. The anoma-
lously acute C-Zn-C angles in 5 and 6 result from the
attractive interactions of the FPFE embrace and the adoption of
an optimal geometry to accommodate the four off interactions
in which each molecule participates. The observed variation in
individual Zn-N-C bond angles for coordinated benzonitrile
ligands (Table 1) can also be attributed to supramolecular
influences. There is a considerable difference between the
Zn-N3-C3 (171.8(2)°) and Zn-N4-C4 (156.66(2)°) angles
in compound 1. Both the phenyl rings are engaged in off
interactions. However, it is apparent that there is a trade-off
between optimal Zn-N-C angle and off overlap, such that, if
Zn-N4-C4 were more linear, the offset would be much less
favorable.
The three distinct molecular structures we have determined
for the 4-(phenyl)benzonitrile adduct (2) provide a further
example of the influence of supramolecular structure on
observed molecular geometry. The differences between the
C-Zn-C angles in molecules 2A and 2B, which are the largest
in this series, result from contrasting PhF · · · ArH interactions,
while 2 · (toluene), in which there is no PhF · · · ArH interaction,
has a significantly smaller value.
Conclusions
Bis(pentafluorophenyl)zinc forms essentially tetrahedral ad-
ducts with 2 equiv of a broad range of nitrogen donors. The
benzonitrile and pyridine complexes studied here retain their
four-coordinate characters in solution. In the solid state their
supramolecular architectures are largely assembled through face-
to-face intermolecular interactions between the aromatic groups.
The results reported here suggest that homo-aryl offset face-
to-face interactions of the type Zn-PhF · · · PhF are favored over
hetero-aryl, Zn-PhF · · · ArH. This reflects the steric role of the
zinc substituent, which impedes efficient face-to-face hetero-aryl
overlap, apparently even with the electron-rich five-membered
pyrrolyl ring. In contrast, the offset homo-aryl face-to-face
interaction can readily accommodate the zinc substituent and
therefore predominates.
Although Zn-C and Zn-N bond lengths show little depen-
dence on the nature of the donor, the distorted tetrahedral
geometry varies considerably between ostensibly similar mo-
lecular species. These geometrical variations can be traced to
the supramolecular architectures, or, put alternatively, these
systems display substantial packing effects.

Adducts of Bis(pentafluorophenyl)zinc
Organometallics, Vol. 27, No. 7, 2008 1445
We are currently exploring approaches that either utilize the
propensity toward offset face-to-face interactions of the M-C6F5
group or provide a more favorable steric environment for the
ArF · · · ArH supramolecular synthon, in order to direct the
assembly of related organometallics.
Experimental Section
General Procedures. Syntheses were performed under nitrogen
using standard Schlenk techniques. Solvents were distilled over
sodium-benzophenone (diethyl ether, tetrahydrofuran), sodium
(toluene), sodium-potassium alloy (light petroleum, bp 40–60 °C),
or CaH2 (dichloromethane, 1,2-difluorobenzene). NMR solvents
(CDCl3, C6D6, C7D8) were dried over activated 4 Å molecular sieves
and degassed by several freeze-thaw cycles. NMR spectra were
recorded using a Bruker DPX300 spectrometer. Chemical shifts
are reported in ppm and referenced to residual solvent resonances
(1H, 13C); 19F is relative to CFCl3. Elemental analyses were
performed by the in-house service at the University of East Anglia
or at the Department of Health and Human Sciences, London
Metropolitan University. (C6F5)2Zn(toluene), 4-(N-pyrrolyl)pyridine,
and 4-(N-pyrrolyl)benzonitrile were prepared according to the
literature procedures.33,34 The remaining benzonitriles and pyridines
were purchased from Aldrich or Lancaster, dried over 4 Å molecular
sieve, and used without further purification.
Crystal Structure Analyses. Suitable crystals were selected and
data for 1, 2, 2 · C7H8, 4, and 5 were measured at 120 K at the
National Crystallography Service on a Bruker Nonius KappaCCD
area detector equipped with a Bruker Nonius FR591 rotating anode
(λMo-KR ) 0.71073 Å) driven by COLLECT35 and processed by
DENZO36 software. For 3 and 6, data were collected at 140 K at
UEA on an Oxford Diffraction Xcalibur-3 CCD diffractometer
equipped with Mo KR radiation and graphite monochromator, and
the data were processed using the CrysAlis-CCD and -RED37
programs. The structures were determined with SHELXS-97 and
refined using SHELXL-97.38 Crystal data and refinement results
for all samples are collated in Table 3.
General Procedure for the Synthesis of Adducts 1-7. A
solution of (C6F5)2Zn · (toluene) (0.50 g, 1.0 mmol) in toluene (10
mL) was treated with the corresponding ligand. The reaction mixture
was stirred for 10 min at room temperature before removing the
volatiles under reduced pressure, yielding the crude solid.
(C6F5)2Zn(NCC6H5)2 (1). 1 was prepared following the general
procedure outline above, using benzonitrile (0.21 mL, 2 mmol).
The crude solid was crystallized from toluene at –25 °C, affording
colorless blade-like crystals of 1 (0.54 g, 0.89 mmol, 89%) suitable
for X-ray analysis. 1H NMR (300 MHz, 293 K, C6D6): δ 6.90 (dd,
2H, J ) 7.8 and 1.3 Hz, o-H), 6.73 (tt, 2H, J ) 7.6 and 1.3 Hz,
p-H), 6.51 (dd, 4H, J ) 7.8 and 7.6 Hz, m-H). 19F NMR (282
MHz, 293 K, C6D6): δ –118.1 to –118.2 (m, 4F, o-F), –157.94 (t,
2F, J ) 20 Hz, p-F), –162.40 to –162.66 (m, 4F, m-F). Anal. Found:
C, 51.03; H, 1.41; N, 4.57. Calc for C26H10F10N2Zn: C, 51.55; H,
1.66; N, 4.62.
(C6F5)2Zn(NCC6H4C6H5)2 (2). 2 was prepared in a similar
fashion to 1, using 4-(phenyl)benzonitrile (0.36 g, 2 mmol). The
(33) (a) Sun, Y.; Piers, W. E.; Parvez, M. Can. J. Chem 1998, 76, 513.
(b) Weidenbruch, M.; Herrndorf, M.; Schäfer, A.; Pohl, S.; Saak, W. J.
Organomet. Chem. 1989, 361, 139. (c) Day, V. W.; Campbell, D. H.;
Michejda, C. J. J. Chem. Soc., Chem. Commun. 1975, 118. (d) Garratt, S.;
Guerrero, A.; Hughes, D. L.; Bochmann, M. Angew. Chem. Int. Ed. 2004,
43, 2166.
(34) Sanchez, I.; Pujol, J. K. Tetrahedron 1999, 55, 5593.
(35) Hooft, R., Nonius, B. V. COLLECT: Data collection software; 1998.
(36) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307.
(37) Programs CrysAlis-CCD and -RED; Oxford Diffraction Ltd.:
Abingdon, UK, 2005.
(38) Sheldrick, G. M. SHELX-97, Programs for crystal structure
determination (SHELXS) and refinement (SHELXL); University of Göt-
tingen: Germany, 1997.

1446 Organometallics, Vol. 27, No. 7, 2008
Martin et al.
crude solid was crystallized by diffusion of petroleum ether in
7.12 to 7.09 (m, 10H), 7.03 (dd, 4H, J ) 6.7 and 1.4 Hz), 6.89
(dd, 4H, J ) 6.7 and 1.4 Hz). 19F NMR (282 MHz, 293 K, C6D6):
δ –118.2 to –118.4 (m, 4F, o-F), 157.4 (t, 2F, J ) 20 Hz, p-F),
–162.1 to –162.4 (m, 4F, m-F). Anal. Found: C, 57.29; H, 2.40; N,
3.90. Calc for C34H18F10N2Zn: C, 57.53; H, 2.57; N, 3.95.
dichloromethane at 2 °C, affording crystals of 2 as colorless needles
1
(0.54 g, 0.71 mmol, 71%) suitable for X-ray analysis. H NMR
(300 MHz, 293 K, C6D6): δ 7.12 to 7.07 (m, 10H), 7.03 (dd, 4H,
J ) 6.6 and 1.9 Hz), 6.90 (dd, 4H, J) 6.6 and 1.9 Hz). 19F NMR
(282 MHz, 293 K, C6D6): δ –118.2 to –118.4 (m, 4F, o-F), -157.4
(t, 2F, J) 20 Hz, p-F), -162.1 to -162.4 (m, 4F, m-F). Anal.
Found: C, 60.16; H, 2.27; N, 3.79. Calc for C38H18F10N2Zn: C,
60.21; H, 2.39; N, 3.70.
Crystals of 2 · (C7H8)0.5 suitable for X-ray diffraction were
obtained as colorless slabs by cooling a toluene solution of 2 to
–25 °C overnight.
(C6F5)2Zn(NCC6H4NC4H4)2 (3). A solution of (C6F5)2Zn-
(toluene) (0.44 g, 0.9 mmol) in toluene (10 mL) was treated with
4-(N-pyrollyl)benzonitrile (0.30 g, 1.8 mmol). The reaction mixture
was stirred for 30 min before being filtered to separate a small
amount of solid material. The volatiles were removed under reduced
pressure, and the crude solid was crystallized from a mixture of
1,2-difluorobenzene and light petroleum at 2 °C, affording colorless
prisms of 3 (0.65 g, 0.88 mmol, 98%) suitable for X-ray analysis.
1H NMR (300 MHz, 293 K, C6D6): δ 6.91 (dd, 4H, J ) 7.0 and
1.6 Hz, H benzonitrile), 6.56 (t, 4H, J ) 2.1 Hz, H pyrrole), 6.43
(dd, 4H, J ) 7.0 and 1.6 Hz, H benzonitrile), 6.28 (t, 4H, J ) 2.1
Hz, Hpyrrole). 19F NMR (282 MHz, 293 K, C6D6): δ –118.3 to
–118.5 (m, 4 F, o-F), –157.4 (t, 2 F, J ) 19.6 Hz, p-F), –162.2 to
–162.4 (m, 4 F, m-F). Anal. Found: C, 54.94; H, 2.14; N, 7.91.
Calc for C34H16F10N4Zn: C, 55.49; H, 2.19; N, 7.61.
(C6F5)2Zn(NC5H4NC4H4)2 (6). 6 was prepared following the
general procedure, adding 4-(N-pyrrollyl)pyridine (0.29 g, 2 mmol).
The crude solid was crystallized from a mixture of dichloromethane
and petroleum ether (6:1) at –25 °C, affording crystals of 6 as
colorless blocks (0.65 g, 0.94 mmol, 94%) suitable for X-ray
1
analysis. H NMR (300 MHz, 293 K, C6D6): δ 8.08 (dd, 4H, J )
5.3, 1.6 Hz, H pyridine), 6.61 (t, 4H, J ) 2.2 Hz, H pyrrole), 6.33
(dd, 4H, J ) 5.3, 1.6 Hz, H pyridine), 6.23 (t, 4H, J) 2.2 Hz, H
pyrrole). 19F NMR (282 MHz, 293 K, C6D6): δ –115.9 to –116.2
(m, 4F, o-F), –157.4 (t, 2F, J ) 20 Hz, p-F), –161.7 to –162.0 (m,
4F, m-F). Anal. Found: C, 52.57; H, 2.40; N, 8.12. Calc for
C30H16F10N4Zn: C, 52.39; H, 2.34; N, 8.15.
(C6F5)2Zn(2,2′-bipy) (7). 7 was synthesized following the general
procedure, using 2,2-bipyridine (0.16 g, 1 mmol). The crude solid
was crystallized from toluene at –25 °C, affording crystals of 7
1
(0.49 g, 0.88 mmol, 88%). H NMR (300 MHz, 293 K, C6D6): δ
8.74 (d, 2H, 5.6 Hz), 6.73-6.76 (m, 2H), 6.48-6.44 (m, 2H). 19
F
NMR (282 MHz, 293 K, C6D6): δ –117.1 to –117.3 (m, 4F, o-F),
–157.9 (t, 2F, J ) 20 Hz, p-F), –162.0 to –162.2 (m, 4F, m-F).
Anal. Found: C, 47.17; H, 1.26; N, 5.13. Calc for C22H8F10N2Zn:
C, 47.55; H, 1.45; N, 5.04.
(C6F5)2Zn(NC5H5)2 (4). 4 was prepared utilizing the general
procedure, using pyridine (0.15 mL, 2 mmol). The crude solid was
crystallized from toluene at –25 °C, affording colorless block-like
crystals of 4 (0.51 g, 0.92 mmol, 92%) suitable for X-ray analysis.
1H NMR (300 MHz, 293 K, C6D6): δ 8.16 to 8.13 (m, 4H, o-H),
6.82 to 6.75 (m, 2H, p-H), 6.46 to 6.42 ppm (m, 4H, m-H). 19F
NMR (300 MHz, 293 K, C6D6): δ –116.4 to –116.5 (m, 4F, o-F),
–157.6 (t, 2F, J ) 19 Hz, p-F), –161.9 to –162.2 (m, 4F, m-F).
Anal. Found: C, 51.03; H, 1.41; N, 4.57. Calc for C22H10F10N2Zn:
C, 51.55; H, 1.66; N, 4.67.
Acknowledgment. We are grateful to the Engineering and
Physical Sciences Research Council for a studentship
(A.J.M.) and access to the National Crystallography Service.
E.M. thanks UEA for a Ph.D. scholarship and Oxford
Diffraction Ltd. for a CASE award.
Supporting Information Available: X-ray crystallographic data
in CIF format; ORTEP figures depicting the molecular structure of
2 · toluene, 3, and 5; an extended quantitative description of the
supramolecular architectures of compounds 1-6; diagrams illustrating
the degree of overlap in each occurrence of offset face-to-face aryl-aryl
and pentafluoroaryl-pentafluoroaryl interactions. This material is
available free of charge via the Internet at http://pubs.acs.org.
(C6F5)2Zn(NC5H4C6H5)2 (5). Using the general procedure,
(C6F5)2Zn(toluene) was treated with 4-(phenyl)pyridine (0.31 g, 2
mmol). The crude solid was crystallized from toluene at –25 °C,
affording crystals of 5 as colorless plates (0.62 g, 0.87 mmol, 87%)
1
suitable for X-ray analysis. H NMR (300 MHz, 293 K, C6D6): δ
OM701127P