A matter of greasy tails: Interdigitation of alkyl chains in free and coordinated 4′-(4-dodecyloxyphenyl)-4,2′:6′,4″- terpyridines

Article abstract of DOI:10.1016/j.inoche.2011.10.004

The syntheses and single crystal structures of 4′-(4- dodecyloxyphenyl)-4,2′:6′,4″-terpyridine, 2, and [Zn ₂(μ-OAc)₄(2)₂] are reported. Crystal packing is dominated by van der Waals interactions between fully extended alky

Full text of DOI:10.1016/j.inoche.2011.10.004

Inorganic Chemistry Communications 15 (2012) 113–116
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Inorganic Chemistry Communications
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A matter of greasy tails: Interdigitation of alkyl chains in free and
coordinated 4′-(4-dodecyloxyphenyl)-4,2′:6′,4″-terpyridines
⁎
⁎
Edwin C. Constable , Guoqi Zhang, Catherine E. Housecroft , Jennifer A. Zampese
Department of Chemistry, University of Basel, Spitalstrasse 51, CH 4056 Basel, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
The syntheses and single crystal structures of 4′-(4-dodecyloxyphenyl)-4,2′:6′,4″-terpyridine, 2, and [Zn2(μ-
OAc)4(2)2] are reported. Crystal packing is dominated by van der Waals interactions between fully extended
alkyl chains producing lamellar structures in both compounds, and appears to be the reason for the formation
of crystalline [Zn2(μ-OAc)4(2)2] rather than a one-dimensional coordination polymer as observed for related
compounds.
Received 29 August 2011
Accepted 5 October 2011
Available online 13 October 2011
Keywords:
Zinc
© 2011 Elsevier B.V. All rights reserved.
4,2′:6′,4"-terpyridine
Crystal structure
Hydrophobic tail
2,2′:6′,2″-Terpyridine (tpy) ligands substituted in the 4′-position with
long hydrophobic tails (e.g. 4-octyloxyphenyl, 4-octadecycloxyphenyl)
form self-organized monolayers on highly oriented pyrolytic graphite,
and there is a close relationship between the structures of the mono-
layers and the lamellar building blocks of the 3-dimensional crystal
lattice of the same compound [1]. Studies of metal complexes of these
and related ligands include an assessment of the effect of the hydropho-
bic tail on f-block metal-extracting properties of the bis(tpy) metal-
binding domain [2], and effects of the substituents on the emissive
properties of zinc(II) and cadmium(II) complexes [3–5]. In contrast to
the chelating tpy ligand, 4,2′:6′,4″-terpyridine presents a divergent N,
N′-donor set with the central nitrogen atom typically uncoordinated
[6]. Raston and coworkers have reported 4′-(4-octyloxyphenyl)-
4,2′:6′,4″-terpyridine, 1, [7] its incorporation into supramolecular
host-guest capsules [8,9] and the one-dimensional coordination poly-
mer [ZnCl2(1)]n[10]. Here we report the synthesis of the longer homo-
logue 4′-(4-dodecyloxyphenyl)-4,2′:6′,4″-terpyridine, 2, (Scheme 1)
and the formation of [Zn2(μ-OAc)4(2)2]. We assess the role of the
alkyl chains in the solid state structures of these complexes.
molecules (Fig. 1A) in the asymmetric unit. Structurally, these differ
only in the orientations of the aromatic rings. In molecule A, angles
between the least squares planes of the rings containing pairs of
atoms N1a/N2a, N2a/N3a and N2a/C19a are 11.20(8), 21.43(8) and
17.98(8)°. Corresponding angles for molecule B are 8.83(9), 3.92(9)
and 29.30(8)°. The extended conformations of the alkyl chains allow
efficient packing of the molecules into two-dimensional sheets
(Fig. 1B) with van der Waals forces between the interdigitated alkyl
chains dominating the packing as observed in related compounds
[1,7,15] where assembly of near-planar sheets is most prevalent for
the longest alkyl chains [16]. Stacking of the sheets is such that
there are face-to-face π-interactions between tpy domains of two ad-
jacent sheets. The next pair of sheets is slipped with respect to the
first pair so that the alkyl chains of one pair lie over the tpy domains
of the next. The overall structure is thereby a double layer assembly.
Preliminary STM data for 2 indicate that formation of well-ordered
c
Ligand 2 was prepared in 60% yield [11] from 4-acetylpyridine and
4-dodecyloxybenzaldehyde [12] using Kröhnke methodology [13].
The base peak in the electrospray mass spectrum (m/z 516.3) was
assigned to [M+Na]+, and the highest mass peak (m/z 1009.9) to
[2 M+Na]+. The 1H and 13 C NMR spectra were consistent with the
structure shown in Scheme 1 and was assigned [11] using COSY,
HMQC, HMBC and DEPT techniques. Structural analysis [14] revealed
2
N
A
3
2
3
a
3
b
O
N B
C
that
2 crystallizes in space group P–1 with two independent
N
2
⁎
Corresponding authors.
E-mail addresses: edwin.constable@unibas.ch (E.C. Constable),
Scheme 1. Structure of ligand 2, and labelling scheme for NMR spectroscopic
catherine.housecroft@unibas.ch (C.E. Housecroft).
assignments.
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.10.004

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E.C. Constable et al. / Inorganic Chemistry Communications 15 (2012) 113–116
Fig. 1. (A) Structure of one (molecule A) of two independent molecules of 2 (ellipsoids plotted at 30% probability, and H atoms omitted). Selected bond distances and angles: N1a–
C5A=1.332(2), N1a–C1a=1.333(2), N2a–C10a=1.3410(18), N2a–C6a=1.3427(18), N3a–C11a=1.333(2), N3a–C15a=1.338(2), C19a–O1a=1.3573(18), O1a–C22a=1.4348
(18) Å; C5a–N1a–C1a=115.15(14), C10a–N2a–C6A=117.68(12), C11a–N3a–C15a =115.88(13), C19a–O1a–C22a=120.07(12)°. Bond lengths and angles for the second inde-
pendent molecule (molecule B) are similar; see text for torsion angles. (B) Part of one sheet of molecules of 2; the unit cell is viewed down the b-axis.
monolayers on highly ordered pyrolytic graphite (HOPG) and we are
currently pursuing studies using cryo-STM techniques.
By comparison with the one-dimensional polymers [{Zn2(μ-OAc)4}
L]n obtained from reactions of Zn(OAc)2∙2H2O with 4′-phenyl-, 4′-(4-
bromophenyl)- or 4′-(4-methylthiophenyl)-4,2′:6′,4″-terpyridine [6,17],
it is reasonable to assume that reaction of Zn(OAc)2∙2H2O with 2 will
give a similar coordination polymer. Supporting this premise is the fact
that treatment of ZnCl2 with 4′-(4-octyloxyphenyl)-4,2′:6′,4″-terpyridine
Fig. 2. Structure of [Zn2(μ-OAc)4(2)2] with ellipsoids plotted at 30% probability level and H atoms omitted. Important bond parameters: Zn1–Zn1i =2.9031(11), Zn1–N1=2.0431
(16), Zn1–O200=2.0471(16), Zn1–O100=2.0559(14), Zn1 O201i =2.0268(16), Zn1–O101i =2.0301(15) Å; N1–Zn1–Zn1i =178.11(4), N1–Zn1–O100=99.98(6), N1–Zn1–
O200=98.72(7), O201i–Zn1–N1=100.45(7), O101i–Zn1–N1=99.13(6)o The second half of the molecule is generated by the symmetry code i=−x, –y, –z.

E.C. Constable et al. / Inorganic Chemistry Communications 15 (2012) 113–116
115
solutions of 2 and the complex both show three bands in the UV region
assigned to π*←π transitions. The approximate doubling of εmax is
consistent with the formulation [Zn2(μ-OAc)4(2)2] determined by
X-ray diffraction.
A yellow block of [Zn2(μ-OAc)4(2)2] was selected from the bulk
sample and the structural analysis [24] confirmed the discrete centro-
symmetric molecule depicted in Fig. 2. Whereas the non-coordination
of N2 was anticipated (see above), the fact that atom N3 does not
bind zinc(II) is unexpected. The geometry of the {Zn2(μ-OAc)4} node
forces the two donors N1 and N1i to be mutually trans. The terpyri-
dine domains are close to planar (angles between the least squares
planes of the rings with atoms N1/N2 and N2/N3=2.04(8) and
12.70(11)°, and the arene substituent is twisted 24.1(4)° out of the
plane of the central pyridine ring. The extended conformation of the
alkyl chains imparts a Z-shaped configuration on the molecule.
Molecules of [Zn2(μ-OAc)4(2)2] assemble into 2-dimensional
sheets. The packing forces are dominated by van der Waals contacts
between interdigitated alkyl tails (Fig. 3) and are augmented by
weak CH…N hydrogen bonds involving pendant pyridyl donor atom
N3 (N3…H4aiiC4ii =2.51, N3…C4ii =3.440(3) Å, N3…H4aii–
C4ii =165°, symmetry code ii=1−x, −y, −z). The {Zn2(μ-OAc)4}
nodes disrupt the planarity of the sheets, but the protruding methyl
groups are neatly accommodated in cavities in adjacent sheets form-
ing a ‘peg-and-hole’ assembly (Fig. 4A). Details of this structural motif
are shown in Fig. 4B. The cavity comprises the arene and two pyridine
rings of one [Zn2(μ-OAc)4(2)2] molecule, and the arene ring and
part of the alkyl chain of a second molecule in the same sheet. The
penetration of the Me group into the cavity results in significant, repul-
sive H…H contacts, and the 24.1(4)° twist of the arene ring noted above
is probably in response to these interactions. The dominant packing
interactions between sheets are van der Waals forces between adjacent
alkyl chains. In addition there are offset face-to-face π-interactions
between pyridine rings containing N2 and N1iii (symmetry
code iii=−1−x, −y, 1−z), one of which is visible in the centre of
Fig. 4B.
Fig. 3. Part of one sheet of molecules of [Zn2(μ-OAc)4(2)2]. The unit cell is viewed along
the b axis.
gives a helical, one-dimensional polymer [10] similar to those formed
when ZnCl2 reacts with other 4,2′:6′,4″-terpyridines [18–20]. Inspection
of the crystallographic data (accessed from the Cambridge Structural
Database [21]) for these structures [22] reveals that the principal effect
of the octyl chains is to alter the pitch of the helix. Compound 2 reacts
with two equivalents of Zn(OAc)2∙2H2O to give a yellow, crystalline
product [23]. Whereas the [{Zn2(μ-OAc)4}L]n polymers mentioned
above are poorly soluble in common organic solvents [6], the new
compound is soluble in CHCl3 and CH2Cl2. The highest mass (base)
peak (m/z 1295.9) in the electrospray mass spectrum was assigned
to [Zn2(2)4(OAc)3]+, and the isotope pattern matched that calculated.
Solution 1H and 13 C NMR spectra indicated the presence of one sym-
metrical ligand environment. Except for the appearance of a singlet at
δ 2.11 ppm arising from the acetate groups, the 1H NMR spectrum of
the complex is very similar to that of 2 (both in CDCl3). Only the signals
for protons HA2 and HA3 show any noticeable change on going from
ligand to complex (δ 8.76 to 8.83 ppm, and δ 8.05 to 8.13 ppm, respec-
tively). In the 13 C NMR spectrum, the signal for the C=O group was
assigned from a cross-peak in the HMBC spectrum to the signal for the
acetato methyl group. The electronic absorption spectra of CH2Cl2
Fig. 4. (A) Sheets of [Zn2(μ-OAc)4(2)2] molecules are locked together by a ‘peg-and-hole’ assembly. The {Zn2(μ-OAc)4} nodes are shown in green, and ligands 2 in red. (B) Detail of
the ‘peg-and-hole’ motif. The molecule coloured orange lies beneath the other two molecules which are in the same layer.

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E.C. Constable et al. / Inorganic Chemistry Communications 15 (2012) 113–116
130.0 (CC1), 128.5 (CC2), 121.4 (CA3), 118.6 (CB3), 115.5 (CC3), 68.5 (Ca), 32.1
(CCH2), 29.9 (CCH2), 29.85 (CCH2), 29.81 (CCH2), 29.79 (CCH2), 29.6 (CCH2), 29.55
The role of van der Waals interactions between extended alkyl
chains is an expected feature of the solid state structure of 2, and
the observed lamellar structure mirrors those of related compounds.
We suggest that the preference for the formation of crystalline
[Zn2(μ-OAc)4(2)2] over a one-dimensional coordination polymer is
driven by the dominance of van der Waals interactions between the
alkyl tails of the coordinated 4′-(4-dodecyloxyphenyl)-4,2′:6′,4″-
terpyridine ligands.
(CCH2), 29.4 (Cb), 26.2 (CCH2), 14.3 (Cc). UV/VIS λmax/nm (5.0×10–5 mol dm–3
,
CH2Cl2), 233 (ε/103 dm3 mol−1 cm−1 32.8), 272 (34.7), 305sh (22.4). IR (solid,
cm–1) 2913 s, 2849 m, 1591 s, 1471 m, 1390 m, 1368w, 1294w, 1173 s, 1073w,
1059w, 819 s, 719w, 693w, 662 s. ESI-MS (CH2Cl2/MeOH) m/z 1009.9 [2 M+Na]+
(calc. 1009.6), 516.3 [M+Na]+ (base peak, calc. 516.3). After recrystallization
from CH2Cl2/MeOH: found C 80.13, H 8.00, N 8.50; C33H39N3O requires C 80.29, H
7.96, N 8.51%.
[12] A. Brun, G. Etemad-Moghadam, Synthesis (2002) 1385.
[13] F. Kröhnke, Synthesis (1976) 1.
[14]
C33H39N3O, M=493.67, colourless block, triclinic, space group P–1, a=10.850(2),
b=14.523(3), c=18.610(4) Å, α=111.285(16), β=91.129(15), γ=96.378(17)°,
U=2709.8(10) Å3, Z=4, Dc=1.210 Mg m–3, μ(Mo-Kα)=0.073 mm−1, T=173 K,
80308 reflections collected, 10059 unique, Rint=0.0602. Refinement of 8563 reflec-
tions (669 parameters) with I >2σ (I) converged at final R1=0.0502 (R1 all
data=0.0610), wR2=0.1191 (wR2 all data=0.1252), gof=1.100.
Acknowledgements
We thank the Swiss National Science Foundation and the University
of Basel for support. GZ thanks the Novartis Foundation, formerly
Ciba-Geigy Jubilee Foundation, for support. Rémy Pawlak is thanked
for preliminary STM measurements.
[15] G.W.V. Cave, C.L. Raston, J. Chem. Soc., Perkin Trans. I (2001) 3258.
[16] Inspection of the structures and packing of 4′-(4-octyloxyphenyl)-4,2′:6′,4″-terpyr-
idine (CCDC refcode XAYPOC) and 4′-(4-butyloxyphenyl)-4,2′:6′,4″-terpyridine
(CCDC refcode ACUKAK) shows that the alkyl chains are not fully extended.
[17] E.C. Constable, G. Zhang, C.E. Housecroft, M. Neuburger, J.A. Zampese, CrystEng-
Comm 12 (2010) 2146.
Appendix A. Supplementary material
[18] M. Barquín, J. Cancela, M.J. González Garmendia, J. Quintanilla, U. Amador,
Polyhedron 17 (1998) 2373.
[19] L. Hou, D. Li, Inorg. Chem. Commun. 8 (2005) 190.
[20] X.-Z. Li, M. Li, Z. Li, J.-Z. Hou, X.-C. Huang, D. Li, Angew.Chem. Int. 47 (2008) 6371.
[21] I.J. Bruno, J.C. Cole, P.R. Edgington, M.K. Kessler, C.F. Macrae, P. McCabe, J. Pearson,
R. Taylor, Acta Crystallogr., Sect. B 58 (2002) 389.
[22] Conquest v. 1.13, CSD version 5.32 with updates May 2011: refcodes AJURIG,
FEPRUO, GAQYUS, LOCTED, NOGFIZ, NOGFOF.
CCDC 831693 and 831694 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.
cam.ac.uk).
[23] Solid Zn(OAc)2∙2H2O (21.8 mg, 0.10 mmol) was added to
a solution of 2
(24.7 mg, 0.05 mmol) in CH2Cl2/CH3OH (9 cm3, v/v 2:1). The reaction mixture
was stirred at room temperature for 30 min and the pale yellow solution was
then filtered. The filtrate was left to evaporate, and after 1 week, pale yellow
blocks had formed. The crystals were collected by filtration, washed with
MeOH, and dried in air. Yield: 24.5 mg, 72.6% (based on 2). m.p. 220–222 °C. 1H
NMR (500 MHz, CDCl3) δ / ppm 8.83 (d, J=5.5 Hz, 8H, HA2), 8.13 (d, J=6.0 Hz,
8H, HA3), 8.01 (s, 4H, HB3), 7.67 (d, J=8.7 Hz, 4H, HC2), 7.05 (d, J=8.7 Hz, 4H,
HC3), 4.03 (t, J=6.5 Hz, 4H, Ha), 2.11 (s, 6H, HMeCO2), 1.81 (m, 4H, Hb), 1.47 (m,
4H, HCH2), 1.40-1.18 (m, 32H, HCH2), 0.86 (t, J=6.9 Hz, 6H, Hc). 13 C NMR
(126 MHz, CDCl3) δ / ppm 180.2 (CC = O), 161.4 (CC4), 154.9 (CB2), 150.6 (CB4),
150.5 (CA2), 146.7 (CA4), 131.2 (CC1), 128.6 (CC2), 122.0 (CA3), 119.1 (CB3),
115.6 (CC3), 68.2 (Ca), 32.1 (CCH2), 29.9 (CCH2), 29.85 (CCH2), 29.82 (CCH2), 29.79
(CCH2), 29.6 (CCH2), 29.55 (CCH2), 29.4 (Cb), 26.2 (CCH2), 22.9 (CMeCO2), 14.3
(Cc). IR (solid, cm-1): 2922 m, 2848 m, 1635 s, 1605 m, 1596 m, 1519 m, 1423 s,
1398 m, 1296 m, 1240 m, 1180 m, 997w, 848 m, 829 s, 728w, 662 s. UV/VIS
References
[1] E.C. Constable, H.-J. Güntherodt, C.E. Housecroft, L. Merz, M. Neuburger, S. Schaffner,
Y. Tao, New J. Chem. 30 (2006) 1470.
[2] M.G.B. Drew, P.B. Iveson, M.J. Hudson, J.O. Liljenzin, L. Spjuth, P.-Y. Cordier, Å.
Enarsson, C. Hill, C. Madic, J. Chem. Soc., Dalton Trans. (2000) 821.
[3] X. Chen, Y. Ding, Y. Cheng, L. Wang, Synthetic Met. 160 (2010) 625.
[4] X. Chen, Q. Zhou, Y. Cheng, Y. Geng, D. Ma, Z. Xie, L. Wang, J. Lumin. 126 (2007) 81.
[5] S.-C. Yu, C.-C. Kwok, W.-K. Chan, C.-M. Che, Adv. Mater. 15 (2003) 1643.
[6] E.C. Constable, G. Zhang, E. Coronado, C.E. Housecroft, M. Neuburger, CrystEng-
Comm 12 (2010) 2139 and references therein.
[7] G.W.V. Cave, C.L. Raston, Chem. Commun. (2000) 2199.
[8] C.L. Raston, G.W.V. Cave, Chem. Eur. J. 10 (2004) 279.
[9] G.W.V. Cave, M.J. Hardie, B.A. Roberts, C.L. Raston, Eur. J. Org. Chem. (2001) 3227.
[10] G.W.V. Cave, C.L. Raston, J. Supramol. Chem. 2 (2002) 317.
[11] 4-Acetylpyridine (1.45 g, 8.00 mmol) was added to
λ
max/nm (5.0×10–5 mol dm–3, CH2Cl2), 231 (ε/103 dm3 mol−1 cm−1 59.6), 272
(59.1), 305sh (37.4). ESI-MS (CH2Cl2/MeOH) m/z 1295.9 [M – OAc]+ (base peak,
calc. 1295.5), 1110.0 [Zn(2)2(OAc)]+ (calc. 1109.6), 616.5 [Zn(2)(OAc)]+ (calc.
616.3). Found C 65.60, H 6.67, N 6.26; C37H45N3O5Zn requires C 65.63, H 6.70, N
6.21%.
a solution of 4-
dodecyloxybenzaldehyde (1.16 g, 4.00 mmol) in EtOH (20 cm3). A KOH pellet
(0.45 g, 12 mmol) was added to the solution, followed by aqueous NH3 (33%,
15 cm3). The reaction mixture was stirred at room temperature overnight, during
which time a white suspension formed. The white solid was collected by filtration,
washed well with H2O and EtOH, and dried in vacuo over P2O5. Yield: 1.19 g, 60%.
m.p. 143-145 °C. 1H NMR (500 MHz, CDCl3) δ / ppm 8.76 (d, J=6.0 Hz, 4H, HA2),
8.05 (d, J=6.0 Hz, 4H, HA3), 7.98 (s, 2H, HB3), 7.68 (d, J=8.7 Hz, 2H, HC2), 7.04 (d,
J=8.7 Hz, 2H, HC3), 4.02 (t, J=6.5 Hz, 2H, Ha), 1.81 (m, 2H, Hb), 1.46 (m, 2H,
HCH2), 1.40-1.18 (m, 16H, HCH2), 0.86 (t, J=6.8 Hz, 3H, Hc). 13 C NMR (126 MHz,
CDCl3) δ / ppm 160.8 (CC4), 155.4 (CB2), 150.9 (CB4), 150.8 (CA2), 146.4 (CA4),
[24]
C74H90N6O10Zn2, M=1354.30, yellow block, triclinic, space group P–1, a=10.753
(2), b=11.845(2), c=14.054(3) Å, α=82.14(3), β=75.51(3), γ=81.06(3)o,
U=1703.1(6) Å3, Z=1, Dc=1.320 Mg m–3, μ(Mo-Kα)=0.767 mm−1, T=173 K,
30240 reflections collected, 8163 unique, Rint=0.0826. Refinement of 7666 reflec-
tions (591 parameters) with I >2σ (I) converged at final R1=0.0418 (R1 all
data=0.0442), wR2=0.1151 (wR2 all data=0.1173), gof=1.049.