The first complex of 4′-(4-methylthiophenyl)-2,2′:6′,2″-terpyridine - A model for terpylated self-assembled monolayers

Article abstract of DOI:10.1016/j.inoche.2008.01.030

The complex [Ru(2)₂][PF₆]₂ where 2 is 4′-(4-methylthiophenyl)-2,2′:6′,2″-terpyridine is the first example of a coordination compound of this ligand, and provides structural information relevant to understanding the interaction of self-assembled monolayers of related ligands with metal ions.

Full text of DOI:10.1016/j.inoche.2008.01.030

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Inorganic Chemistry Communications 11 (2008) 518–520
www.elsevier.com/locate/inoche
The first complex of 40-(4-methylthiophenyl)-2,20:60,200-terpyridine –
A model for terpylated self-assembled monolayers
Edwin C. Constable *, Catherine E. Housecroft *, Elaine Medlycott, Markus Neuburger,
´
Federica Reinders, Sebastien Reymann, Silvia Schaffner
Department of Chemistry, University of Basel, Spitalstrasse 51, CH 4056 Basel, Switzerland
Received 14 January 2008; accepted 31 January 2008
Available online 9 February 2008
Abstract
The complex [Ru(2)2][PF6]2 where 2 is 40-(4-methylthiophenyl)-2,20:60,200-terpyridine is the first example of a coordination compound
of this ligand, and provides structural information relevant to understanding the interaction of self-assembled monolayers of related
ligands with metal ions.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Terpyridine; Ruthenium; Thiophene; Monolayers; Crystal structure
A popular strategy for the construction of molecular-
based devices is the self-assembly of sulfur-containing mol-
ecules onto noble metal (particularly gold) surfaces [1,2].
The choice of sulfur-containing species allows a variety of
pendant functionality to be introduced and metal-binding
domains are of particular interest as they allow the intro-
duction of specific electronic and photophysical properties.
The {M(tpy)2}n+ motif (tpy = 2,20:60,200-terpyridine) is one
of the most popular metallosupramolecular strategies for
the specific introduction of metal centres into supramolec-
ular assemblies and devices and numerous examples of
structural motifs functionalised with tpy (terpylation) have
been reported [3–6]. We recently described the preparation
and coordination behaviour of 2,20:60,200-terpyridine-
40(10H)-thione (1) [7] and the formation of self-assembled
monolayers of 1 and the related compounds 2 and 3
(Scheme 1) on gold has been reported [8–12]. Although
the complexation of 3 on gold surfaces or gold nanoparti-
cles has been reported, no complexes of 2 or 3 have been
characterised. In this paper we report the preparation
and solid state structure of [Ru(2)2][PF6]2.
Ligand 2 was prepared in 53% yield as a pale yellow crys-
talline solid from a one-pot reaction of 2-acetylpyridine
with 4-methylthiobenzaldehyde in aqueous ethanolic
KOH at room temperature. In our hands, this procedure
is more reliable than the ‘‘green” version using PEG-300
as solvent [13]. The reaction of 2 with RuCl3 ꢀ 3H2O in eth-
anol gave the dark brown insoluble species [Ru(2)Cl3] in
55% yield. Subsequent reaction of [Ru(2)Cl3] with 2 gave
the desired complex [Ru(2)2][PF6]2 as a deep red solid in
17% yield after chromatographic purification and recrystal-
lisation [14]. The complex exhibits the typical properties of a
{Ru(tpy)2} species with the visible spectrum dominated by
an MLCT absorption at 494 nm. The complex is electro-
chemically active and exhibits a reversible (83 mV) ruthe-
nium(II/III) process (Fig. 1) at +0.87 V (versus Fc/Fc+)
in MeCN solution. Of interest for potential applications
of this type of complex in electrochemical devices, is the
observation of a second quasi-reversible process at
+1.08 V assigned to the oxidation of the thioether sulfur.
This second process is absent in the model complex [Ru(pt-
py)2][PF6]2 (ptpy = 40-phenyl-2,20:60,200-terpyridine) [15] and
is characteristic of the thioether-functionalised compounds.
The solid state structure of the complex [Ru(2)2]-
[PF6]2 ꢀ 0.34MeCN has been determined [16] and the
*
Corresponding authors.
E-mail address: catherine.housecroft@unibas.ch (C.E. Housecroft).
1387-7003/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2008.01.030

E.C. Constable et al. / Inorganic Chemistry Communications 11 (2008) 518–520
519
SMe
SH
S
N
H
N
3
N
2
N
N
N
N
N
N
1
Scheme 1. Structures of ligands 1 to 3.
observed. The thioethers are close to coplanar with the
directly bonded phenyl ring exhibiting CSCC torsion
angles of 6.9° (S1) and –16.6° (S2). The phenyl ring bearing
S1 is significantly twisted with respect to the pyridine ring
containing N5 (angle between least squares planes, 35.6°)
whereas the corresponding least squares planes angle for
the rings containing S2 and N2 is 2.0°, indicating a much
greater degree of conjugation. The second conformation
of the ring containing S2 is also essentially coplanar with
the pyridine ring containing N2. The C–S–C angles of
104.7(2) and 104.7(3)° indicate that the S atoms can be con-
sidered to be sp3 hybridized. In contrast, we have pointed
to the fact that in ROtpy ligands and their ruthenium(II)
complexes, the CtpyꢁO bond is typically shorter than the
OꢁR bond and the C–O–C bond angles are close to
120°, consistent with an sp2 hybridized O atom and a
p-contribution to the C–O bond adjacent to the central
pyridine ring of the tpy domain [21–23]. The C–S–C angles
in [Ru(2)2]2+ are close to the modelled Au–S–C angles
proposed for monolayers and mixed monolayers of 3 on
gold surfaces [11] and the metrical parameters of the
Fig. 1. Cyclic voltammogram of [Ru(2)2][PF6]2 (MeCN, 50 MV s–1 scan
rate, 0.1 M [nBu4N][PF6] supporting electrolyte).
structure of the cation is presented in Fig. 2. The 4-methyl-
thiophenyl ring bearing the substituent with S2 is disor-
dered and adopts two conformations with occupancies of
0.64 and 0.36 and only the higher occupancy conformation
is presented in Fig. 2. The principal difference between the
conformations lies in the orientation of the thioether
methyl group to the left or to the right. The {Ru(tpy)2}
sub-structure is unexceptional with all angles and distances
within the typical limits for [Ru(tpy)2]2+ [17–20]. The typi-
cal pattern of Ru–N bond lengths, with longer bonds to the
terminal and shorter to the central pyridine rings is
complex cation [Ru(2)2]2+ provide
a good starting
point for improved modelling of metallated SAMS incor-
porating 3.
Fig.
3
illustrates the packing of cations in
[Ru(2)2][PF6]2 ꢀ 0.34MeCN. The pair of symmetry related
Fig. 2. The [Ru(2)2]2+ cation present in [Ru(2)2][PF6]2 ꢀ 0.34MeCN. Hydrogen atoms, the anions and lattice acetonitrile molecule have been omitted for
clarity and only the major conformation of the 4-methylthiophenyl ring containing S2 is shown. Thermal ellipsoids are represented at 50% probability.
˚
Selected bond lengths (A) and angles (°): Ru1–N = 2.084(3), Ru1–N = 1.987(3), Ru1–N3 = 2.085(3), Ru1–N4 = 2.088(3), Ru1–N5 = 1.984(3),
Ru1–N6 = 2.071(2), S1–C37 = 1.771(5), C41–S2 = 1.782(5), S2–C44 = 1.799(6), N1–Ru1–N2 = 78.05(12), N1–Ru1–N3 = 156.58(13), N2–Ru1–
N3 = 78.54(11), N1–Ru1–N4 = 89.54(11), N2–Ru1–N4 = 104.51(11), N3–Ru1–N4 = 97.07(10), N1–Ru1–N5 = 101.11(12), N2–Ru1–N5 = 177.32(10),
N3–Ru1–N5 = 102.24(12), N4–Ru1–N5 = 77.98(11), N1–Ru1–N6 = 93.34(11), N2–Ru1–N6 = 98.45(10), N3–Ru1–N6 = 89.30(10), N4–Ru1–
N6 = 156.95(12), N5–Ru1–N6 = 79.03(10), C34–S1–C37 = 104.7(2), C41–S2–C44 = 104.7(3).

520
E.C. Constable et al. / Inorganic Chemistry Communications 11 (2008) 518–520
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Fig. 3. Packing of [Ru(2)2]2+ cations in [Ru(2)2][PF6]2 ꢀ 0.34MeCN.
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cations in the unit cell encapsulate one of the [PF6]ꢁ ions
1
(the P atom of which (P2) lies on the inversion centre /2,
1/2, 1/2), and there are no close contacts between these
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;
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˚
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ꢀ
orange plates, M = 1115.22, triclinic, space group P1, a = 9.3408(2),
˚
b = 12.7532(2), c = 19.0187(5) A, a = 96.199(2), b = 102.007(1),
3
c = 91.380(2), V = 2200.56(8) A , Z = 2, Dc = 1.683 Mg m–3, l(Mo-
˚
Acknowledgements
Ka) = 0.618 mmꢁ1, T = 173 K, 9697 reflections collected. Refinement
of 7098 reflections (672 parameters) with I > 1.0r(I) converged at final
R1 = 0.0590 (R1 all data = 0.0875), wR2 = 0.0637 (wR2 all
data = 0.0820), gof = 1.0356.
We thank the Swiss National Science Foundation, the
Swiss National Center of Competence in Research in
Nanoscale Science, the Swiss Energy Ministry (Project
NEFIOS), the European Union (Project HETEROMOL-
MAT) and the University of Basel for support.
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Appendix A. Supplementary material
CCDC 673952 contains the supplementary crystallo-
graphic data for [Ru(2)2][PF6]2 ꢀ 0.34MeCN. These data
can be obtained free of charge from The Cambridge Crys-
tallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.inoche.2008.01.030.
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