The synthesis, molecular structures, and supramolecular architecture of amine adducts of bis(pentafluorophenyl)zinc

Article abstract of DOI:10.1021/om060319d

A series of eight adducts of the form (RR′R″N) ₂·Zn(C₆F₅)₂ have been prepared through treatment of the Lewis acid Zn(C₆F₅) ₂ with 2 equiv of the corresponding amine (R = ^tBu or CH₂Ph, R′ = R″ = H; R = R′ = Me or CH ₂Ph, R″ = H; R = Me, R′ = CH₂Ph, R″ = H; RR′ = cyclo-C₄H₈ or cyclo-C₅H ₁₀, R″ = H; R = R′ = Me, R″ = CH₂Ph). The solid-state structures of all eight compounds have been elucidated by single-crystal X-ray diffraction. In each case the geometry about the zinc atom is essentially tetrahedral. However, there is considerable variation in the supramolecular architectures in the solid state. A number of types of noncovalent interactions are observed including phenyl-pentafluorophenyl stacking, X-H...F-C contacts, and offset face-to-face contacts between pentafluorophenyl rings, giving rise to one-, two-, and three-dimensional supramolecular structures. In our examples we find that no one intermolecular interaction predominates.

Full text of DOI:10.1021/om060319d

Organometallics 2006, 25, 3837-3847
3837
The Synthesis, Molecular Structures, and Supramolecular
Architecture of Amine Adducts of Bis(pentafluorophenyl)zinc
Andrew J. Mountford,† Simon J. Lancaster,*,† Simon J. Coles,‡ Peter N. Horton,‡
David L. Hughes,† Michael B. Hursthouse,‡ and Mark E. Light‡
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 April 10, 2006
A series of eight adducts of the form (RR′R′′N)2‚Zn(C6F5)2 have been prepared through treatment of
t
the Lewis acid Zn(C6F5)2 with 2 equiv of the corresponding amine (R ) Bu or CH2Ph, R′ ) R′′ ) H;
R ) R′ ) Me or CH2Ph, R′′ ) H; R ) Me, R′ ) CH2Ph, R′′ ) H; RR′ ) cyclo-C4H8 or cyclo-C5H10,
R′′ ) H; R ) R′ ) Me, R′′ ) CH2Ph). The solid-state structures of all eight compounds have been
elucidated by single-crystal X-ray diffraction. In each case the geometry about the zinc atom is essentially
tetrahedral. However, there is considerable variation in the supramolecular architectures in the solid state.
A number of types of noncovalent interactions are observed including phenyl-pentafluorophenyl stacking,
X-H‚‚‚F-C contacts, and offset face-to-face contacts between pentafluorophenyl rings, giving rise to
one-, two-, and three-dimensional supramolecular structures. In our examples we find that no one
intermolecular interaction predominates.
(I-III in Chart 1). The first is perhaps the best known attraction
and results from the opposing quadrupoles of the perfluoro-
phenyl and phenyl groups.13 Since the original discovery by
Patrick and Prosser, it has been employed in a number of ele-
gant studies to influence supramolecular assembly.14-17 Though
not as numerous as the organic examples, solid-state structures
of organometallic compounds that exhibit aryl-perfluoroaryl
stacking have been reported.18,19
Introduction
The widely accepted definition of supramolecular chemistry
offered by Lehn describes “chemistry beyond the molecule,
based on organized entities of higher complexity that result from
association of two or more chemical species held together by
intermolecular forces”.1 The dative bond has long been the
intermolecular interaction of choice in supramolecular coordina-
tion chemistry.2 However, recently a number of research groups
have met with considerable success by utilizing supramolecular
synthons familiar to organic chemists to assemble supramol-
ecular organometallic structures without recourse to dative
bonds.3-11 A series of observations by research groups including
our own that metal-pentafluorophenyl fragments participate in
a variety of intermolecular interactions prompted us to inves-
tigate the supramolecular assembly of these organometallic
species through the attractive forces unique to such compounds.
Organofluorine has a special place in supramolecular chem-
istry.12 Our interest is primarily in three key intermolecular inter-
actions in which pentafluorophenyl groups participate: stacking
interactions with aryl groups; as hydrogen bond acceptors; and
in offset face-to-face arrangements with other perfluoroaryl rings
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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.
‡ University of Southhampton.
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10.1021/om060319d CCC: $33.50 © 2006 American Chemical Society
Publication on Web 07/07/2006

3838 Organometallics, Vol. 25, No. 16, 2006
Mountford et al.
Chart 1
and main group organometallics with perfluorophenyl sub-
stituents.24d,e,26
Despite their opposite quadrupole moment the offset-face-
to-face (off) interaction between perfluoroaromatic groups is
comparable to the often observed supramolecular motif in which
hydroaromatics form off pairs or stacks.27,28 Such off interactions
between pairs or stacks of C6F5 groups have been reported for
organometallic and coordination compounds with pentafluo-
rophenyl substituents.18c,29 Dance surveyed the crystallographic
structure database for the groups E(C6F5)3 and E(C6F5)4 (where
E is an element from groups 13-15) and found a number of
examples of both 4-fold and 6-fold perfluorophenyl “embraces”
comprised of both off and edge-to-face (ef) interactions.27
Bis(pentafluorophenyl)zinc was chosen for this investigation
because we reasoned that the one-to-one correspondence
between the number of C6F5 groups and Lewis acidic sites would
favor the formation of infinite supramolecular assemblies
involving interactions of types I-III. Supramolecular archi-
tectures constructed using the coordination chemistry of zinc
are well-known and have been considered for applications in
nonlinear optics30 and as gas storage materials.31,32 There is also
increasing interest in the combination of dative and other
noncovalent interactions.33 The supramolecular assembly of zinc
complexes solely by truly intermolecular interactions is a
somewhat less developed field.34 Here we report the synthesis,
Since the seminal papers by Dunitz20 and O’Hagan and
Howard,21 there has been increased interest in hydrogen bonding
to weak organofluorine acceptors.22,23 We and others have
recently reported intramolecular bifurcated N-H‚‚‚F‚‚‚H-N
hydrogen bonding in a number of neutral and anionic adducts
of B(C6F5)3.24,25 Intermolecular X-H‚‚‚F interactions have been
found in the solid-state structures of a number of transition
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Amine Adducts of Bis(pentafluorophenyl)zinc
Organometallics, Vol. 25, No. 16, 2006 3839
Scheme 1
Table 1. δ(NH) for the Free Amines and Zinc Adducts
δ(NH)/ppm
compound
free amine
adduct
∆δ (δH adduct - δH amine)
1
2
3
4
5
6
7
0.77
0.86
0.23
0.71
1.09
0.87
0.87
2.52
1.61
1.40
2.29
2.90
2.07
2.24
1.75
0.75
1.17
1.58
1.81
1.20
1.37
Figure 1. Molecular structure of 1. Thermal ellipsoids are shown
at the 50% probability level. H atoms on carbons have been omitted
for clarity.
molecular geometry, and intermolecular interactions of eight
amine adducts of Zn(C6F5)2.
petroleum over dichloromethane solutions of the crude products
and cooling to -25 °C overnight. For compounds 2, 4, 7, and
8, X-ray quality crystals were afforded by cooling concentrated
toluene solutions to -25 °C. The solid-state structures of 1-8
were determined by X-ray crystallography and are described
below. No evidence for the formation of more than one type of
crystalline product was observed in any of the samples
examined.
Compound 1 has the expected essentially tetrahedral geometry
about the zinc center (Figure 1). Selected bond lengths and
angles for this and other members of the series are collated in
Table 2. To date, there are only five structures containing the
Zn(C6F5)2 fragment in the Cambridge Structural Database36
including that of Zn(C6F5)2 itself.37 The Zn-C and Zn-N
bond lengths are very similar to these reported examples. In
all our compounds the single most significant distortion from
ideal tetrahedral geometry is the opening of the C-Zn-C angle.
For 1 the 123.05(7)° C1-Zn1-C7 bond angle is compensated
by an asymmetric arrangement, which leads to smaller
C7-Zn1-N2 (98.84(6)°) and N1-Zn1-N2 (101.26(6)°) angles.
This asymmetry appears to result from the supramolecular
structure adopted in the solid state, which is discussed below.
There are two long intramolecular N-H‚‚‚F-C contacts
(Table 3).
Results and Discussion
Treatment of Zn(C6F5)2 with 2 equiv of the amines tBuNH2,
PhCH2NH2, Me2NH, PhCH2(Me)NH, (PhCH2)2NH, cyclo-
C4H8NH, cyclo-C5H10NH, or PhCH2NMe2 in toluene results in
the formation of the adducts 1-8 in excellent yield (Scheme
1). The elemental analyses were all in good agreement with
the expected compositions.
1
The H and 19F NMR data indicated that adduct formation
had taken place. Addition of further amine to NMR samples
1
did not result in duplication of H NMR resonances but did
produce a chemical shift change, whereas cooling the sam-
ples to -80 °C led only to a slight broadening of the reso-
nances. These observations suggest that the thermodynamics
strongly favor the adduct formation in solution but that the amine
ligands are labile. Table 1 gives the δ(NH) resonances for the
free amines and corresponding adducts. Where present, the 1H
NMR resonance of the NH group is shifted significantly
downfield on adduct formation. It is not clear to what extent
this is the result of the electronic influence of adduct formation
or an indicator for intramolecular N-H‚‚‚F-C interaction in
solution.35
There is little variation between the 19F NMR spectra of the
adducts. The ortho-fluorine resonances are all within a 3 ppm
chemical shift range at -116.5 ((1.5) ppm, and the para- and
meta-fluorine resonances vary even less, -158 ((0.5) and -162
((0.5) ppm, respectively. We have previously reported on the
intramolecular N-H‚‚‚F-C hydrogen-bonding interactions of
a number of tris(pentafluorophenyl)borane adducts related to
1-7.24 The room-temperature 19F NMR spectra of secondary-
amine borane adducts exhibit a distinctive high-field shift for
the o-F’s engaged in bifurcated intramolecular hydrogen-
bonding interactions (C-F‚‚‚H‚‚‚F-C) between amino hydro-
gens and ortho-fluorine atoms.24d The 19F NMR spectra of 3-7
provide no such evidence for solution-phase N-H‚‚‚F-C
interactions even on cooling to -80 °C.
The molecular structure of compound 2 is depicted in Fig-
ure 2. Here the distortion from tetrahedral to accommodate the
C6F5 groups is more evenly distributed in a pseudosymmetric
arrangement, with two very similar angles for C15-Zn1-N2
(102.66(18)°) and C21-Zn1-N1 (102.68(17)°), while
N2-Zn1-N1 (110.09(17)°) is near ideal. The conformation
aligns the C6H5 (PhH) and C6F5 (PhF) rings parallel to one
another, but there is no overlap. The only intramolecular
N-H‚‚‚F-C contact of note is N1-H1c‚‚‚F5, at 2.46 Å.
At the molecular level the structure of the Me2NH adduct 3
(Figure 3), which has a 2-fold symmetry axis, is distinguished
(35) The protic amine adducts of B(C6F5)3 in which there is a strong
bifurcated hydrogen-bonding interaction exhibit substantial downfield shifts
δ(NH) versus the free amine: see ref 24d.
(36) Fletcher, D. A.; McMeeking, R. F.; Parkin, D. J. Chem. Inf. Comput.
Sci. 1996, 36, 746.
(37) (a) Sun, Y.; Piers, W. E.; Parvez, M. Can. J. Chem. 1998, 76, 513.
(b) Weidenbruch, M.; Herrndorf, M.; Scha¨fer, 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.
Intramolecular Interactions. Single crystals of 1, 3, 5, and
6 suitable for X-ray diffraction were obtained by layering light
(34) (a) De Cires-Mejias, C.; Tanase, S.; Reedjik, J.; Gonza´lez-V´ılchez,
F.; Vilaplana, R.; Mills, A. M.; Kooijman, H.; Spek, A. L. Inorg. Chim.
Acta 2004, 357, 1494. (b) Bareberio, G.; Bellusci, A.; Crispini, A.; Ghedini,
M.; Golemme, A.; Prus, P.; Pucci, D. Eur. J. Inorg. Chem. 2005, 181.

3840 Organometallics, Vol. 25, No. 16, 2006
Mountford et al.
Table 2. Bond Lengths and Angles (in Å and deg) about the Zinc Atom, with esd’s in Parentheses
compound
Zn-C
Zn-N
C-Zn-C
N-Zn-N
1
2
3
4
5
6
7
8
2.030(2), 2.041(2)
2.025(5), 2.020(5)
2.0374(16)
2.0294(19), 2.028(2)
2.0301(17), 2.0277(17)
2.028(2)
2.1121(14), 2.1331(15)
2.108(4), 2.106(4)
2.1147(14)
2.1038(17), 2.1133(16)
2.1874(14), 2.1504(14)
2.0972(18)
123.05(7)
122.48(19)
120.22(9)
120.91(8)
133.72(7)
125.02(12)
120.05(18)
117.93(9)
101.26(6)
110.09(17)
105.01(8)
101.11(6)
112.50(5)
100.69(10)
108.09(17)
110.14(7)
2.032(5), 2.044(5)
2.0442(16)
2.094(4), 2.102(4)
2.1626(13)
Table 3. Hydrogen-Fluorine Contacts (in Å and deg)a
D-H‚‚‚A
d(D-H)
d(H‚‚‚A)b
d(D‚‚‚A)
(DHA)
symmetry operation
1
N1-H1a‚‚‚F5
N2-H2b‚‚‚F6
N2-H2a‚‚‚F1
2
0.92
0.92
0.92
2.39
2.44
2.32
3.014(2)
3.070(2)
3.197(2)
125
125
159
* 1-x, 2-y, 1-z
N1-H1c‚‚‚F5
N1-H1c‚‚‚F5
N2-H2a‚‚‚F6
3
N1-H1‚‚‚F1
N1-H1‚‚‚F2
4
0.92
0.92
0.92
2.46
2.48
2.37
3.014(5)
3.380(5)
3.278(5)
119
166
171
* -x+1, -y, -z+2
* -x, -y, -z+1
0.93
0.93
2.42
2.41
3.0388(18)
3.0995(17)
124
131
-x, y, 1/2-z
* x+1/2, y-1/2, z
N1-H1N‚‚‚F3
5
0.83(2)
2.30(3)
3.119(2)
168(2)
* x+1, y, z
N1-H1‚‚‚F1
N2-H2‚‚‚F10
6
0.93
0.93
2.21
2.48
2.9402(17)
3.1266(17)
134
126
N1-H1‚‚‚F1
8
0.93
2.39
2.991(2)
122
C8-H8b‚‚‚F5
C7-H7a‚‚‚F2
0.98
0.98
2.31
2.46
3.164(2)
3.174(2)
145
129
* -x+1, y-1, -z+3/2
a An asterisk indicates an intermolecular interaction; esd’s are in parentheses. b Only those contacts less than 2.50 Å are listed.
Figure 3. Molecular structure of 3 with a 2-fold symmetry axis
approximately vertical in the plane of the page. Thermal ellipsoids
are shown at the 50% probability level. The methyl group hydrogen
atoms have been omitted for clarity.
Figure 2. Molecular structure of compound 2. Thermal ellipsoids
are shown at the 50% probability level. H atoms on carbons have
been omitted for clarity.
The molecular structure of 5 (Figure 5) exhibits the only
examples of intramolecular aryl-aryl interactions found in this
series. Each of the two PhF groups associates with a benzyl
substituent through a pairing interaction with one of the PhH
rings, and the remaining pair of PhH rings are overlapping in
an off fashion. The centroid‚‚‚centroid distances (3.60 and 3.56
Å) and centroid‚‚‚plane separations of 3.41 and 3.35 Å for the
first interaction and 3.57 and 3.46 Å for the second with
interplanar dihedral angles of 12.4° and 8.0° describe good
alignment of associated rings. As a consequence of accom-
modating these ring-pairing interactions, the greatest C-Zn-C
angle in the series is observed in compound 5 (133.72(7)°). In
addition to the intramolecular aryl-pairing interactions there is
only by a symmetry-related pair of intramolecular N-H‚‚‚F-C
contacts (Table 3).
The molecular structure of compound 4 (Figure 4) lacks
the precise symmetry of the closely related adducts 3 and 8.
Each benzyl group is orientated to reduce steric interaction with
the methyl substituent. The distortion from tetrahedral geometry
is largely confined to an opening of the C17-Zn1-C23
(120.91(8)°) and closing of the N1-Zn1-N2 angle (101.11-
(6)°). In this instance there are no noteworthy intramolecular
N-H‚‚‚F-C contacts.

Amine Adducts of Bis(pentafluorophenyl)zinc
Organometallics, Vol. 25, No. 16, 2006 3841
Figure 4. Molecular structure of compound 4. Thermal ellipsoids
are shown at the 50% probability level. H atoms on carbons have
been omitted for clarity.
Figure 6. Molecular structure of 6 viewed down the 2-fold
symmetry axis. Thermal ellipsoids are shown at the 50% probability
level. The hydrogens, except for the amine H atoms, have been
omitted for clarity.
Figure 5. Molecular structure of compound 5. Thermal ellipsoids
are shown at the 50% probability level. H atoms on carbon have
been omitted for clarity.
one medium length N1-H1‚‚‚F1 (2.21 Å) and one long
N2-H2‚‚‚F10 (2.48 Å) contact.
Figure 7. Molecular structure of 7 viewed down the pseudo-2-
fold symmetry axis. Thermal ellipsoids are shown at the 50%
probability level. The pyrrolidine group hydrogens have been
omitted for clarity.
The molecular structure of 6 (Figure 6) has 2-fold sym-
metry. The pyrrolidine group adopts an orientation to minimize
steric interactions with the C6F5 groups, which results in a
relatively large C-Zn-C (125.02(12)°) and a small N-Zn-N
(100.69(10)°) angle. There is a pair of symmetry-related, long,
intramolecular N-H‚‚‚F-C contacts (Table 3).
Compound 7 crystallizes with pseudo-2-fold symmetry but
with a quite different orientation of the piperidine group from
that of the pyrrolidine group in 6 (Figure 7). Consequently, the
C-Zn-C and N-Zn-N angles at 120.05(18)° and 108.09(17)°,
respectively, are much closer to the tetrahedral ideal. There are
no X-H‚‚‚F-C interactions substantially shorter than the van
der Waals’ radii in the solid-state structure of 7.
series. Although 8 has no amino hydrogens, each of the methyl
groups provides a hydrogen donor in either an intra- or an
intermolecular C-H‚‚‚F-C interaction. The intramolecular
interaction C8-H8b‚‚‚F5 (2.31 Å, C-H‚‚‚F 145°) is remi-
niscent of the C-H‚‚‚F-C contact we reported for the
borane adduct cyclo-C4H8N(H)‚B(C6F5)3 (2.20 Å, C-H‚‚‚F
151°).23a,b,f,24c
Intermolecular Interactions. The frequency, nature, and
extent of the intermolecular interactions (possible with pen-
tafluorophenyl groups and introduced above) vary greatly in
the solid-state structures of compounds 1-8. That of 6 is
distinguished by the absence of either off interactions between
PhF rings or short intermolecular N-H‚‚‚F distances. The
shortest contacts to neighboring molecules are between m-F
atoms and N-H groups but with H‚‚‚F distances corresponding
Compound 8 is the only adduct of a tertiary amine in this
study and crystallizes with 2-fold molecular symmetry (Fig-
ure 8). As a consequence of the match between the steric
bulk of the tertiary amine group and the pentafluorophenyl
groups, we see the least distorted tetrahedral geometry in the

3842 Organometallics, Vol. 25, No. 16, 2006
Mountford et al.
contacts in 3, only one of the amino-H’s in each molecule of 1
participates in an intermolecular interaction; this leads to pairing
of molecules by two such interactions about a center of
symmetry. Also, one of the two PhF groups interacts with that
of a second molecule (also related by a center of symmetry);
while the distance between the two parallel rings is 3.190 Å,
the degree of overlap is small. Together these interactions link
molecules in chains parallel to the b-axis.
The supramolecular architectures of 1, 3, and 7 are thus
shaped by H‚‚‚F and/or offset PhF‚‚‚PhF stacking interactions.
The introduction of benzyl substituents in adducts 2, 4, 5, and
8 provides for further potential intermolecular associations,
namely, the well-documented PhH‚‚‚PhF motif and off
PhH‚‚‚PhH stacking interactions.
In compound 8, in addition to the intramolecular
C-H‚‚‚F-C interaction as described above, the second methyl
group is engaged in an, albeit rather weak, C-H‚‚‚F-C
intermolecular interactions with a m-F on a neighboring
molecule (H‚‚‚F 2.46 Å, C-H‚‚‚F 129°). In this way each
molecule is associated with two of its neighbors, forming in-
finite chains (Figure 12) that are linked via offset homoaryl
stacking interactions of the pentafluorophenyl rings. We note
that no PhF‚‚‚PhH interactions are seen in the crystal structure
of 8. The off PhF‚‚‚PhF stacks afford one-dimensional chains
that zigzag in a direction parallel to the crystallographic c-axis
(Figure 13). The extent of the overlap of the pentafluorophenyl
rings and the subsequent centroid‚‚‚plane distance at 3.171Å
are similar to that in 1. There is the suggestion of a PhH‚‚‚PhH
offset stack in the crystal lattice of 8, but the centroid‚‚‚plane
distances at 3.706 and 4.089 Å are much greater than those
calculated by Dance (ca. 3.4 Å) for the binding pair (C6H6)2 in
an off motif, so we conclude there is no significant interaction.27
Figure 8. Molecular structure of compound 8. Thermal ellipsoids
are shown at the 50% probability level. H atoms have been omitted
for clarity.
to the sum of the van der Waals’ radii (2.55 Å). The inter-
molecular interactions in the remaining structures are discussed
below in order of increasing complexity.
Compound 7 has a supramolecular structure consisting of one-
dimensional chains running parallel to the b-axis (Figure 9).
Molecules are linked by off interactions between pairs of PhF
rings that are not symmetry-related. The centroid‚‚‚centroid
distance between adjacent PhF rings is 3.84 Å, the centroid‚‚‚
plane distances are ca. 3.4 Å, while the offset between centroids
is 1.88 Å.38
The supramolecular structure of compound 3 is depicted in
Figure 10. Each molecule engages in N-H‚‚‚F-C interactions
with four neighbors (two as donor molecules, two as acceptors).
Collectively they define a supramolecular structure consisting
of infinite sheets parallel to the a-b-plane. Neighboring sheets
are aligned to each other through off PhF‚‚‚PhF interactions
similar to those in 7; the interplanar distance in 3 is 3.299 Å
(the extent of overlap is illustrated in Figure 17c). The two sets
of interactions cooperate to give a three-dimensional network
in which every group bonded to zinc participates in a significant
intermolecular interaction.
The most readily apparent intermolecular interactions in 4
are H‚‚‚F contacts (2.30 Å, N-H‚‚‚F 168°) between NH and
p-F atoms linking molecules in chains parallel to the a-axis.
Less obvious is the association of adjacent molecules through
PhH‚‚‚PhH vertex-to-face (Vf) interactions. There are four distinct
Vf contacts: one PhH‚‚‚PhF ring with an H‚‚‚centroid distance
of 2.51 Å, two PhH‚‚‚PhH with H‚‚‚centroid distances of 2.71
and 2.99 Å, and one methylene‚‚‚PhH with an H‚‚‚centroid
distance of 2.80 Å; PhH‚‚‚PhH contacts thus link four phenyl
rings in a cyclic arrangement about a center of symmetry (Figure
14), and the methylene‚‚‚PhH contacts link pairs of benzyl
groups about another center of symmetry. This packing is
distinct from the 4-fold phenyl embrace described by Dance,
which associates pairs of adjacent molecules.39
The supramolecular structure of 1 (Figure 11) is assembled
from the same types of intermolecular interaction as 3 but has
no molecular symmetry and does not realize the bonding
potential of every group. While shorter (2.32 Å) than the
In the crystal lattice of 5, there are two types of stacked ring
systems, first a stack of four C6H5 rings and second a stack of
Figure 9. Packing diagram illustrating the one-dimensional chains in 7 formed by a PhF‚‚‚PhF stacking motif.

Amine Adducts of Bis(pentafluorophenyl)zinc
Organometallics, Vol. 25, No. 16, 2006 3843
Figure 10. Sheet of molecules of 3 connected through the illustrated N-H‚‚‚F interactions. The two ligands labeled b are of a neighboring
sheet and have parallel overlapping PhF‚‚‚PhF ring interactions with groups of the first sheet.
Figure 11. Molecules of 1 viewed down a cell diagonal. Molecules are connected in pairs by short N2-H2a‚‚‚F1′ interactions, and these
pairs are linked in chains by overlap of PhF rings.
Figure 12. Packing diagram illustrating the formation of a chain through C-H‚‚‚F-C interactions in compound 8.
eight rings with a sequence PhF‚‚‚PhH‚‚‚PhF‚‚‚PhH‚‚‚PhH‚‚‚
PhF‚‚‚PhH‚‚‚PhF. The angles between the normals in adjacent
rings vary from 0° (about a center of symmetry) to 24.1° in the
longer stack, and 0° (also about an inversion center) and 11.6°
in the shorter stack. In the off stacking of PhH‚‚‚PhH rings, in
(39) Dance, I.; Scudder, M. Chem. Eur. J. 1996, 2, 481. Density
functional calculations for the Vf and ef supraisomers of the gas-phase
benzene pair (C6H6)2 concluded that both structures exhibit near identical
pair-binding energies, with the ef structure only slightly less favorable: see
ref 27.
(38) There are two slightly different centroid (cg)‚‚‚plane distances (3.452
and 3.351 Å (cg (C21-C26)‚‚‚plane (C11-C16) and cg (C11-C16)‚‚‚
plane (C21-C26), respectively); the planes lie at an angle of 4.96° to each
other).

3844 Organometallics, Vol. 25, No. 16, 2006
Mountford et al.
Figure 13. Zigzag pattern afforded by overlapping PhF substituents in the crystal lattice of 8.
Figure 14. Packing diagram showing the edge-to-face and vertex-
to-face arrangements of the PhF rings in 4.
Figure 16. Packing in 2 showing columns of overlapping rings in
two directions.
and 2.75 Å in the central PhH‚‚‚PhH contact in the longer
stack. Laterally adjacent molecules are associated through the
PhH‚‚‚PhH interactions in a zigzag pattern, which combines with
the heteroaryl stacks affording infinite sheets (Figure 15). For
the heteroaryl stacks the intermolecular centroid‚‚‚centroid
separation (3.897 Å) is larger than the intramolecular value
(3.579 Å (av)). The remaining aryl rings form an off stack in
which the average centroid‚‚‚plane separation between intramo-
lecular homo-rings is 3.408 Å, while the intermolecular distance
between the two symmetry-related rings is only slightly greater
at 3.446 Å. The packing of 5 involves no intermolecular H‚‚‚F
contacts less than 2.50 Å.
The supramolecular structure of compound 2 is intriguing;
each aryl ring has one homo- (to a symmetry-related ring) and
one hetero-intermolecular off-type interaction. This overlap of
aryl rings gives rise to columns, which are stacked in parallel
arrays in layers in the crystallographic a-b-plane. In one layer
the columns are directed along the [-110] vector; in the next,
they are parallel to the [110] vector, at an angle of 100.1° to
the first. The columns are linked by the zinc coordination bonds
(Figure 16). Thus each molecule associates with two neighbors
via four PhF‚‚‚PhH interactions, facilitated by two pairs of
essentially parallel phenyl and pentafluorophenyl rings within
the molecular structure. Another four molecules share the homo-
Figure 15. View close to the crystallographic c-axis showing the
staggered aryl‚‚‚aryl and eclipsed pentafluoroaryl‚‚‚aryl stacking
interactions in the crystal lattice of 5.
both stacks, an H atom of the adjoining methylene group is
directed toward the center of the opposing PhH ring with
H‚‚‚centroid distances of 2.57 and 2.76 Å in the short stack

Amine Adducts of Bis(pentafluorophenyl)zinc
Organometallics, Vol. 25, No. 16, 2006 3845
Figure 17. Projection of overlapping rings onto the plane of the lower ring for the off pentafluorophenyl rings in 1 (a), 2 (b), 3 (c), 7 (d),
and 8 (e).
Table 4. Structural Parameters for the Homo- and Hetero-aryl Interactions in Compound 2
centroid‚‚‚centroid
distances (Å)
centroid‚‚‚plane
distances (Å)
angle (deg) between
the normals to these planes
plane 1
plane 2
C2-C7 (PhH)
C9-C14 (PhH)
C2-C7 (PhH)
C9-C14 (PhH)
C15-C20 (PhF)
C21-C26 (PhF)
C15-C20 (PhF)
C21-C26 (PhF)
C2-C7 (PhH)
C9-C14 (PhH)
C15-C20 (PhF)
C21-C26 (PhF)
3.68
3.73
3.95
3.86
3.54
3.51
3.40, 3.41
3.38, 3.47
3.56
3.50
3.38
3.00
3.98
0
0
0
3.37
0
24d
aryl interactions of each ring. The centroid‚‚‚centroid and
centroid‚‚‚plane distances for each of these homo- and hetero-
aryl interactions are collated in Table 4. In addition, there are
two weak intermolecular H‚‚‚F interactions.
of Al(C6F5)3
and are comparable to the H‚‚‚F distances
reported for cis-(NH3)2‚Pd(C6F5)2.26c
There are relatively few reports of off interactions between
pairs of metal-bonded C6F5 rings.29b,37a However, interrogation
of the CSD yielded many further examples of crystal structures
where such interactions are present but have not attracted
comment by the authors. We were therefore not surprised to
find off interactions in five of our eight adduct lattices. The
centroid-plane distances (1, 3.19; 2, 3.38 (av); 3, 3.30 (av); 7,
3.40 (av); and 8, 3.17 Å) are in line with those observed for the
base-free parent compound Zn(C6F5)2 (3.26 Å (av)) and the
dimeric aluminum complex [AlMe(C6F5)(µ-Me)2]2 (3.39 Å
(av)).29b,37a These values approach the centroid‚‚‚plane distance
for the off paired gas-phase dimer (C6F6)2, in which the closest
calculated contacts were 3.19 and 3.14 Å for C‚‚‚C and C‚‚‚F,
respectively.27 There is however, as illustrated in Figure 17,
considerable variation in the degree of overlap (extent of offset)
between instances of this interaction.
In principle, the PhH‚‚‚PhF interaction is the most favorable
supramolecular synthon for molecules containing a combination
of these two rings but no other functional groups. However, in
our study the hetero-aryl stacking motif is found only in 5.
Although we see intramolecular hetero-aryl pairing in 2, it does
not extend to intermolecular PhH‚‚‚PhF‚‚‚PhH‚‚‚PhF stacks. The
pattern in 5, of intramolecular pairs combining to give inter-
molecular stacks, is very similar to that observed for the imine
adduct of tris(pentafluorophenyl)borane, Z-Ph(H)CdN(CH2Ph)‚
B(C6F5)3 (IV), and the PhCH2NH2 adducts of M(C6F5)3 (M )
B, Al).19b,24d Like 5, in IV the intramolecular centroid‚‚‚centroid
Discussion
The effect of complexing a protic amine to the Lewis acidic
zinc center in Zn(C6F5)2 is to increase its potential as a
hydrogen-bond donor. However, with the nitrogen lone pair now
participating in a dative bond, the only possible hydrogen-bond
acceptors are organofluorines. Furthermore, the length of the
Zn-N bond (2.03 Å (av)) disfavors the formation of the
bifurcated intramolecular hydrogen-bonding arrangements pre-
dicted by Etter’s rules and consistently found in protic amine
adducts of B(C6F5)3 (B-N bond length 1.64 Å).40 While there
are a number of N-H‚‚‚F-C contacts well within the van der
Waals’ radii (Table 3), few approach the Dunitz criteria for
designation as hydrogen bonds.20 Although interactions of this
type are doubtless individually weak in the absence of stronger
competing packing forces, they may play a significant role in
the supramolecular assembly of crystal lattices.23 The best
example from this study is compound 3, in which infinite sheets
are constructed by each molecule donating two and accepting
two N-H‚‚‚F-C interactions. While significantly longer than
the intermolecular interaction in (H2O)‚Al(C6F5)3,26d they are
within the range we have observed for the related amine adducts
(40) Walker, D. A.; Woodman, T. J.; Hughes, D. L.; Bochmann, M.
Organometallics 2001, 20, 3772.

3846 Organometallics, Vol. 25, No. 16, 2006
Mountford et al.
distance (3.374 Å) is significantly shorter than the intermolecular
separation (3.910 Å). Heteroaryl pairing or stacking was not
found in 4 or 8, where the supramolecular architectures rely
upon homoaryl interactions. We ascribe the relative paucity of
hetero-aryl stacking in this series of solid-state structures to steric
hindrance from the metal preventing optimal PhH‚‚‚PhF overlap,
thus rendering other interactions more favorable.
Conclusion
Amine adducts of Zn(C6F5)2 exhibit a rich supramolec-
ular chemistry [X-H‚‚‚F-C, where X ) N or C, PhF‚‚‚PhF,
PhH‚‚‚PhF, and PhH‚‚‚PhH interactions]. However, within the
series of related compounds reported here there are no indica-
tions for a preferred supramolecular motif. The rationalization,
prediction, and ultimately direction of supramolecular structure
in perfluoroaryl zinc complexes will require systems that do
not exhibit such a finely balanced set of competing intermo-
lecular interactions. We are currently exploring the effectiveness
of targeted molecular modifications in favoring PhH‚‚‚PhF
pairing.
Experimental Section
General Procedures. All reactions were conducted under an
atmosphere of dry nitrogen using standard Schlenk line techniques.
All solvents were purified by distillation from molten sodium
(toluene), sodium/potassium alloy (light petroleum), or calcium
hydride (dichloromethane). NMR spectra were recorded at 300.1
(1H), 75.5 (13C), and 282.4 (19F) MHz and at 24 °C unless otherwise
stated. Elemental analyses were performed by the in-house service
at the University of East Anglia. Zn(C6F5)2‚toluene was prepared
according to the literature procedure.40 The amines were purchased
from Aldrich or Lancaster, dried over 4 Å molecular sieves, and
used without further purification.
Crystal Structure Analyses. Suitable crystals were selected, and
data for 1-6 and 8 were measured on a Bruker Nonius KappaCCD
area detector equipped with a Bruker Nonius FR591 rotating anode
(λMo-KR ) 0.71073 Å) driven by COLLECT41 and processed by
DENZO42 software at 120 K. The structures were determined in
SHELXS-97 and refined using SHELXL-97.43 The intensities for
a crystal of 7 were measured on a Rigaku/MSC diffractometer with
Mo radiation and a graphite monochromator at 140 K. Data were
processed using MSC TeXsan/Process software.44 Structure analysis
was by the SHELX-97 software. The large residual electron density
and high/hanging R factor indicate possible twinning in compound
2; a twin law could not be found, and it occurs with all crystals.
Crystal data and refinement results for all samples are collated
in Table 5.
Synthesis of Compounds 1-8. (tBuNH2)2‚Zn(C6F5)2 (1). A
sample of Zn(C6F5)2‚toluene (1.23 g, 2.5 mmol) was dissolved in
toluene (10 mL) at room temperature and treated with tert-
butylamine (0.37 g, 5.0 mmol). After 1 h the solvent was removed
under reduced pressure, affording a sticky residue, which was
washed with light petroleum. Colorless crystals were grown from
a light petroleum/dichloromethane mixture cooled to -25 °C
overnight (0.98 g, 72%). Anal. Found: C 43.90, H 3.85, N 5.05.
Calc for C20H22F10N2Zn: C 44.01, H 4.06, N 5.13. 1H NMR
(C6D6): δ 2.52 (s, 4H, NH2), 1.17 (s, 18H, CH3). 13C{1H} NMR
(41) Hooft, R. COLLECT: Data collection software; Nonius, B.V.: The
Netherlands, 1998.
(42) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307.
(43) Sheldrick, G. M. SHELX-97, Programs for crystal structure deter-
mination (SHELXS) and refinement (SHELXL); University of Go¨ttingen:
Germany, 1997.
(44) PROCESS, TeXsan, Single-Crystal Structure Analysis; Molecular
Structure Corporation: The Woodlands, TX, 1993.

Amine Adducts of Bis(pentafluorophenyl)zinc
Organometallics, Vol. 25, No. 16, 2006 3847
(C6D6): δ 51.4 ((CH3)3C), 31.0 (CH3). 19F NMR (C6D6): δ -118.0
(d, 4F, J ) 19.6 Hz, o-F), -157.9 (t, 2F, J ) 19.5 Hz, p-F), -161.9
(4F, m, m-F).
20.0 Hz, o-F), -157.0 (t, 2F, J ) 19.7 Hz, p-F), -161.2 (m, 4F,
m-F).
(cyclo-C4H8NH)2‚Zn(C6F5)2 (6). Compound 6 was isolated
following the procedure outlined for 1 by addition of pyrrolidine
(0.25 mL, 3.0 mmol) to Zn(C6F5)2‚toluene (0.74 g, 1.5 mmol).
Colorless crystals suitable for X-ray crystallography were obtained
by slow diffusion of light petroleum into a dichloromethane solution
at -25 °C (0.67 g, 82%). Anal. Found: C 44.21, H 3.23, N 5.23.
Calc for C20H18F10N2Zn: C 44.34, H 3.35, N 5.17. 1H NMR
(C6D6): δ 2.36 (br, 8H, C4H8), 1.13 (br, 8H, C4H8), 2.07 (m, 2H,
NH). 13C{1H} NMR (C6D6): δ 47.8, 25.0 (C4H8). 19F NMR
(C6D6): δ -116.9 (d, 4F, J ) 19.8 Hz, o-F), -157.9 (t, 2F, J )
19.8 Hz, p-F), -161.9 (m, 4F, m-F).
(PhCH2NH2)2‚Zn(C6F5)2 (2). Compound 2 was isolated using
the same general method described for 1; benzylamine (0.29 g,
2.7 mmol) was added to Zn(C6F5)2‚toluene (0.67 g, 1.4 mmol).
Colorless crystals were isolated from a saturated toluene solution
cooled to -25 °C (0.63 g, 75%). Anal. Found: C 50.49, H 3.29,
N 4.57. Calc for C26H18F10N2Zn: C 50.88, H 2.96, N 4.56. 1H NMR
(C6D6): δ 7.04-6.60 (m, 10H, C6H5), 3.07 (t, 4H, J ) 7.3 Hz,
CH2), 1.61 (br, 4H, NH2). 13C{1H} NMR (C6D6): δ 129.4, 129.1,
127.7, 127.5 (C6H5), 46.4 (CH2). 19F NMR (C6D6): δ -118.0 (d,
4F, J ) 19.5 Hz, o-F), -157.2 (t, 2F, J ) 18.0 Hz, p-F), -161.4
(m, 4F, m-F).
(Me2NH)2‚Zn(C6F5)2 (3). Compound 3 was prepared following
a procedure similar to that for 1 by addition of dimethylamine (4.4
mmol, 1.8 mL of a 2.41 M solution in toluene) to Zn(C6F5)2‚toluene
(1.09 g, 2.2 mmol). Recrystallization from a light petroleum/
dichloromethane mixture led to isolation of the product as colorless
crystals (1.20 g, 75%). Anal. Found: C 38.89, H 2.84, N 5.59.
Calc for C16H14F10NZn: C 39.25, H 2.88, N 5.72. 1H NMR
(C6D6): δ 1.58 (s, 12H, CH3), 1.40 (br, 2H, NH). 19F NMR
(C6D6): δ -117.4 (d, 4F, J ) 19.8 Hz, o-F), -157.3 (t, 2F, J )
19.8 Hz, p-F), -161.6 (m, 4F, m-F).
(cyclo-C5H10NH)2‚Zn(C6F5)2 (7). Compound 7 was prepared
according to the method described for 1 using piperidine (0.17 g,
2.0 mmol) and Zn(C6F5)2‚toluene (0.5 g, 1.0 mmol). Colorless
crystals were afforded by cooling a saturated toluene solution to
-25 °C overnight (0.53 g, 92%). Anal. Found: C 46.62, H 3.85,
N 4.89. Calc for C22H22F10N2Zn: C 46.38, H 3.89, N 4.92. 1H NMR
(C6D6): δ 3.05 (br, 2H, NH), 2.54 (br, 4H, C5H10), 1.72 (br, 8H,
C5H10), 1.48 (br, 8H, C5H10). 13C{1H} NMR (C6D6): δ 47.9, 27.5,
24.1 (C5H10). 19F NMR (C6D6): δ -118.2 (d, 4F, J ) 19.7 Hz,
o-F), -158.0 (t, 2F, J ) 19.3 Hz, p-F), -161.9 (m, 4F, m-F).
(PhCH2NMe2)2‚Zn(C6F5)2 (8). Compound 8 was prepared in a
manner analogous to that described for 1 using N-dimethylben-
zylamine (0.77 g, 5.6 mmol) and Zn(C6F5)2‚toluene (1.39 g, 2.8
mmol). Colorless crystals were obtained from a minimum volume
of toluene cooled to -25 °C overnight (1.74 g, 93%). Anal.
Found: C 53.33, H 3.86, N 4.37. Calc for C30H26F10N2Zn: C 53.79,
(PhCH2(Me)NH)2‚Zn(C6F5)2 (4). Compound 4 was prepared in
a fashion similar to 1 by addition of N-benzylmethylamine (0.12
g, 1.0 mmol) to Zn(C6F5)2‚toluene (0.24 g, 0.5 mmol). X-ray-quality
crystals were obtained from a light petroleum/dichloromethane
mixture on cooling to -25 °C overnight (0.25 g, 78%). Anal.
Found: C 55.81, H 4.27, N 5.86. Calc for C28H22F10N2Zn: C 52.40,
1
H 3.91, N 4.18. H NMR (C6D6): δ 7.27-7.11 (m, 10H, C6H5),
1
H 3.45, N 4.36. H NMR (C6D6): δ 7.32-7.07 (m, 10H, C6H5),
3.56 (s, 4H, CH2), 2.08 (s, 12H, CH3). 13C{1H} NMR (C6D6): δ
130.7, 129.0, 128.9 (C6H5), 63.7 (CH2), 45.1 (CH3). 19F NMR
(C6D6): δ -115.5 (d, 4H, J ) 18.8 Hz, o-F), -156.0 (t, 2H, J )
19.7 Hz, p-F), -161.2 (m, 4H, m-F).
3.86 (br, 2H, NH), 2.29 (br, 4H, CH2), 1.34 (s, 6H, CH2). 13C{1H}
NMR (C6D6): δ 129.6, 128.8, 126.2 (C6H5), 52.6 (CH2), 24.6 (CH3).
19F NMR (C6D6): δ -118.5 (t, 4F, J ) 19.9 Hz, o-F), -157.8 (t,
2F, J ) 19.8 Hz, p-F), -161.8 (m, 4F, m-F).
Acknowledgment. We are grateful to the Engineering and
Physical Sciences Research Council for a studentship (A.J.M.),
access to the National Crystallography Service, and the use of
the Chemical Database Service at Daresbury.
((PhCH2)2NH)2‚Zn(C6F5)2 (5). Compound 5 was prepared in a
manner analogous to that for 1 using N,N-dibenzylamine (1.07 g,
5.4 mmol) and Zn(C6F5)2‚toluene (1.34 g, 2.7 mmol), yielding
colorless crystals from a light petroleum/dichloromethane mixture
cooled to -25 °C (1.89 g, 88%). Anal. Found: C 60.42, H 3.59,
N 3.55. Calc for C40H30F10N2Zn: C 60.50, H 3.81, N 3.53. 1H NMR
(C6D6): δ 6.98-6.81 (m, 10H, C6H5), 3.40 (d, 4H, J ) 6.6 Hz,
CH2), 2.90 (m, 2H, NH). 13C{1H} NMR (C6D6): δ 137.0, 129.1,
129.0 (C6H5), 52.6 (CH2). 19F NMR (C6D6): δ -115.2 (d, 4F, J )
Supporting Information Available: CIF containing the crystal-
lographic information for compounds 1-8. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM060319D