[PF6]2 (4′-PhStpy = 4′-phenylthio-2, 2′:6′,2″-terpyridine): A centrosymmetric embrace

Article abstract of DOI:10.1016/j.inoche.2012.03.003

The syntheses and characterizations of 4′-phenylthio-2,2′: 6′,2″-terpyridine (4′-PhStpy) and [Fe(4′-PhStpy) ₂][PF₆]₂ are reported. In the solid state, centrosymmetric pairs of [Fe(4′-PhStpy)₂]^{2 +} cations in [Fe(4′-PhStpy)₂][PF₆]₂·2MeCN associate through face-to-face π-stacking of pyridine rings, and twisting of the PhS substituents enables the cations to embrace one another, capturing four MeCN solvate molecules within the {Fe(4′-PhStpy)₂} ₂-motif.

Full text of DOI:10.1016/j.inoche.2012.03.003

Inorganic Chemistry Communications 20 (2012) 180–183
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Inorganic Chemistry Communications
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[Fe(4′-PhStpy)2][PF6]2 (4′-PhStpy = 4′-phenylthio-2,2′:6′,2″-terpyridine):
A centrosymmetric embrace
1
⁎
⁎
Yaqiu Tao , Edwin C. Constable , Catherine E. Housecroft , Markus Neuburger
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 characterizations of 4′-phenylthio-2,2′:6′,2″-terpyridine (4′-PhStpy) and [Fe(4′-PhStpy)2]
[PF6]2 are reported. In the solid state, centrosymmetric pairs of [Fe(4′-PhStpy)2]2+ cations in [Fe(4′-
PhStpy)2][PF6]2·2MeCN associate through face-to-face π-stacking of pyridine rings, and twisting of the PhS
substituents enables the cations to embrace one another, capturing four MeCN solvate molecules within
the {Fe(4′-PhStpy)2}2-motif.
Received 7 February 2012
Accepted 6 March 2012
Available online 15 March 2012
Keywords:
4′-Phenylthio-2,2′:6′,2″-terpyridine
© 2012 Elsevier B.V. All rights reserved.
Crystal structure
Iron
Host–guest
Complexes containing {M(tpy)2}n+ cores and featuring thiol-
terminated alkyl tethers are of interest in molecular electronics, the
soft sulfur donor facilitating anchoring of the complexes on to, for
example, gold or platinum surfaces [1–4]. In contrast, the coordina-
tion chemistry of 4′-RS-2,2′:6′,2″-terpyridine (4′-RStpy) ligands
containing thiol or thioether substituents with the sulfur atom directly
bonded to the tpy domain is still rather poorly explored. The synthetic
strategy of Potts allows access to thioether derivatives (e.g., 4′-MeStpy,
4′-EtStpy, 4′-nPrStpy) [5,6], the latter being easily oxidized to sulfones
[7]. For the 4′-thiol derivative 4′-HStpy, the thione tautomer is favored
both in solution and in the solid state, and deprotonation of the complex
[Fe(4′-HStpy)]2+ (isolated as its hexafluoridophosphate salt) results in
neutral [Fe(Stpy)2] [8]. Oxidative coupling of 4′-HStpy provides a facile
route to disulfide bridged tpySStpy ligands [9], and we have also de-
scribed analogous interconversions involving 2,2′:6′,2″-terpyridine-
6(1H)-thione [10]. Although 4′-RStpy (R = alkyl) ligands and their
complexes are documented, 4′-PhStpy (Scheme 1) has not been previ-
ously reported. There are, rather surprisingly, few structurally charac-
terized complexes of the type [M(4′-ArXtpy)2]n+ in which Ar is a
phenyl ring or part of a more extended aromatic system: Ar = Ph,
X = NH, M = Fe; Ar = 4-MeOC6H4, X = NH, M = Fe or Ru [11], Ar =
Ph, X = P(O)Ph, M = Ru [12].
and 13C NMR spectra [13] were consistent with the structure shown
in Scheme 1 and were assigned using HMQC and HMBC methods.
The structure of the compound (Fig. 1) was confirmed by single crys-
tal X-ray diffraction [14]. The tpy domain adopts the expected trans,
trans-configuration and is planar; angles between the least squares
planes of rings with atoms N1/N2 and N2/3 are 2.2(2) and 6.1(3)°.
The plane containing the phenyl substituent is twisted 64.3(3)°
out of the plane of the central pyridine ring. The similarity of the
two S\C bond distances (see Fig. 1 caption) and the value of
104.0(3)° for the C8\S1\C16 bond angle are consistent with an sp3
hybridized S atom and lack of any π-contribution to the C\S bond
adjacent to the arene ring. This contrasts with the typical observation
in 4′-ROtpy derivatives in which bond parameters in the solid
state support π-contribution to the Carene\O bond [15], consistent
with 2p-3p π-orbital overlap being less effective than a 2p–2p π-
interaction. Centrosymmetric pairs of molecules associate through
non-classical CH … N hydrogen bonds (C15H151 … N3i=2.67, C15 …
N3i=3.479(10)Å, C15–H151 … N3i=145°, symmetry code i=3−x,
1−y, −z) and engage in face-to-face π-stacked interactions, leading
to an assembly of separated tpy and SPh domains (Fig. 2).
Reaction of two equivalents of 4′-PhStpy with FeCl2·4H2O in
MeOH followed by anion exchange (by addition of aqueous NH4PF6)
leads to the formation of purple [Fe(4′-PhStpy)2][PF6]2 [17]. The
electrospray mass spectrum of the product exhibited peaks at m/z
Reaction of 4′-chloro-2,2′:6′,2″-terpyridine with thiophenol at
170 °C yielded 4′-PhStpy, after workup, in 82% yield [13]. The 1H
882.8 and 369.4 (base peak) assigned to [M\PF6]+ and [M\2PF6]2+
,
respectively. The isotope patterns were consistent with those calculated
for these ions. The 1H NMR spectrum of a CD3CN solution of the com-
plex demonstrated the presence of one ligand environment, consistent
with the formation of the homoleptic complex. Although solubility
differences did not permit the NMR spectra to be recorded in a common
⁎
Corresponding authors.
E-mail addresses: edwin.constable@unibas.ch (E.C. Constable),
catherine.housecroft@unibas.ch (C.E. Housecroft).
1
Current address: College of Materials Science and Engineering, Nanjing University
of Technology, No. 5 Xinmofan Road, Nanjing 210009, PR China.
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2012.03.003

Y. Tao et al. / Inorganic Chemistry Communications 20 (2012) 180–183
181
Fig. 3 shows the structure of the [Fe(4′-PhStpy)2]2+ cation in
[Fe(4′-PhStpy)2][PF6]2·2MeCN. The Fe2+ ion is in an octahedral envi-
ronment and bond lengths and angles (see figure caption) are unex-
ceptional. As in the free ligand, the bond angle at the sulfur atoms
and the C\S bond distances indicate that there is no π-contribution
to the Carene\S bonds. The phenyl rings attached to S1 and S2 are
twisted at 66.39(13) and 76.48(12)°, respectively, with respect to
the pyridine ring to which each SPh unit is attached. Centrosymmetric
pairs of cations engage in a face-to-face π-stacking of pyridine rings
containing atoms N3 and N3i (symmetry code i=2−x, −y, 1−z).
The separation of the planes of the rings is 3.58 Å and the centroid …
centroid distance is 3.82 Å making this an efficient interaction
[19]. This interaction is supplemented by edge-to-face contacts
(C13H131…centroid of ring containing N4i =2.54 Å). More extensive
π-interactions as commonly occur in the solid state structures
of complexes containing {M(tpy)2} units [20–24] are not operative.
Instead, the pendant SPh groups of each enantiomer embrace
one another (Fig. 4a) although there are no edge-to-face contacts
between phenyl rings. Each cavity revealed in Fig. 4a hosts two
Scheme 1. Structure of ligand 4′-PhStpy, and labeling scheme for NMR spectroscopic
assignments.
solvent, a comparison of the 1H NMR spectra of the free ligand and
[Fe(4′-PhStpy)2][PF6]2 revealed a significant shift to lower frequency
for the signal for protons HA6 (δ 8.58 to 7.12 ppm). This is diagnostic
of the formation of an {M(tpy)2} domain, with the HA6 protons on one
ligand lying over the ring current of the second tpy. The formation of
the complex was confirmed by single crystal X-ray diffraction [18].
MeCN molecules with short N…Hpyridine contacts (C12H121
…
N9ii =2.46 Å, C15H151…N10iii =2.49 Å, symmetry codes: ii=−1+
x, y; iii=3−x, −y, 1−z). The structural motifs shown in Fig. 4a
assemble into chains through S … H hydrogen bonds (C4H41 …
S1iv =2.85, C4 … S1iv =3.671(3)Å, C4H41…S1iv =139°, symmetry
code iv=1−x, 1−y, 1−z) as depicted in Fig. 4b.
Fig. 1. ORTEP [16] representation of the structure of 4′-PhStpy with ellipsoids (non H atoms) drawn at 30% probability. Selected bond parameters: C1\N1=1.318(7),
C5\N1=1.330(7), C6\N2=1.338(6), C10\N2=1.326(6), C11\N3=1.328(7), C15\N3=1.318(7), C8\S1=1.764(5), C16\S1=1.759(6)Å; C5\N1\C1=117.5(5),
C6\N2\C10=117.5(4), C11\N3\C15=118.0(5), C8\S1\C16=104.0(3)°.
Fig. 2. Packing of molecules of 4′-PhStpy showing π-stacked tpy domains interleaved by domains of SPh substituents.

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Y. Tao et al. / Inorganic Chemistry Communications 20 (2012) 180–183
Fig. 3. ORTEP [16] representation of the structure of the [Fe(4′-PhStpy)2]2+ cation in [Fe(4′-PhStpy)2][PF6]2·2MeCN (ellipsoids at 30% probability level). Selected bond parameters:
Fe1\N1=1.9614(18), Fe1\N2=1.8741(18), Fe1\N3=1.9653(19), Fe1\N4=1.977(2), Fe1\N5=1.8805(18), Fe1\N6=1.9724(19), S1\C8=1.762(2), S1\C31=1.765(3),
S2\C23=1.763(2), S2\C37=1.772(2)Å; N1\Fe1\N2=81.05(8), N2\Fe1\N3=81.50(8), N4\Fe1\N5=81.13(8), N5\Fe1\N6=81.15(8), C8\S1\C31=104.50(11),
C23\S2\C37=102.18(11)°.
Fig. 4. (a) The embrace of [Fe(4′-PhStpy)2]2+ enantiomers. (b) Association of pairs of cations through S … H interactions.
In conclusion, we have prepared and characterized 4′-PhStpy
which, despite its simplicity, is a new 2,2′:6′,2″-terpyridine deriva-
tive. The principal packing motif in [Fe(4′-PhStpy)2][PF6]2·2MeCN is
a {Fe(4′-PhStpy)2}2-unit in which centrosymmetric pairs of [Fe(4′-
PhStpy)2]2+ cations embrace one another. The motif is stabilized by
a face-to-face π-stacking interaction, and captures two MeCN mole-
cules in each of its two cavities.
Appendix A. Supplementary data
Supplementary data to this article can be found online at doi:10.
1016/j.inoche.2012.03.003.
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We thank the Swiss National Science Foundation and the Univer-
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1.18 mmol) were heated to 170 °C under N2 for 4 h. Addition of Et2O gave a
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2H, HA3), 8.07 (s, 2H, HB3), 7.94 (td, J=7.7, 1.8 Hz, 2H, HA4), 7.66 (m, 2H,
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triclinic, space group P−1, a=9.2931(6), b=15.9261(13), c=17.0422(6)Å,
α=75.124(4), β=80.979(5), γ=75.723(5)°, U=2350.7(3)Å3, Z=2, Dc =
1.569 mg m−3 μ(Mo-Kα)=0.570 mm−1
, , T=173 K. Total 89719 reflections,
11365 unique, Rint =0.095. Refinement against F of 643 parameters using 7286
reflections with I>2σ (I) converged at final R1=0.0449 (R1 all data=0.0743),
wR2=0.0414 (wR2 all data=0.0542), gof=1.1336.
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