Isolated facial and meridional tris(bipyridine)Ru(ii) for STM studies on Au(111)

Article abstract of DOI:10.1039/c2cc37904j

Tripodal facial and meridional Ru(ii) complexes comprising three conjugated legs with acetyl-protected thiol end groups are designed, synthesized and isolated for investigation on a gold surface. Preliminary ultrahigh vacuum scanning tunnelling microscopy

Full text of DOI:10.1039/c2cc37904j

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Isolated facial and meridional tris(bipyridine)Ru(II) for
STM studies on Au(111)†
Cite this: Chem. Commun., 2013,
49, 1076
Alexandrina Schramm,a Christophe Stroh,*a Kerrin Dossel,ab Maya Lukas,ab
¨
Received 31st October 2012,
Accepted 12th December 2012
Olaf Fuhr,ab Hilbert v. Lohneysenabc and Marcel Mayor*abd
¨
DOI: 10.1039/c2cc37904j
www.rsc.org/chemcomm
Tripodal facial and meridional Ru(II) complexes comprising three con- trishomoleptic complexes: facial (fac) and meridional (mer) stereo-
jugated legs with acetyl-protected thiol end groups are designed, isomers, each of them appearing in a subset of D and L enantio-
synthesized and isolated for investigation on a gold surface. Preliminary mers.11–13 The isolation of the isomers is challenging and less well
ultrahigh vacuum scanning tunnelling microscopy (UHV STM) measure- established which prevents the widespread use of such compounds
ments of a monolayer of facial isomer deposited on Au(111) are and their applications in science.14 Few reports characterized and
presented.
compared the two geometrical isomers with each other,11 which is
of primary importance for the attachment and the geometrical
The class of Ru(II)–polypyridine complexes have been extensively control on the metallic surfaces. Furthermore, to our knowledge a
studied due to their cumulative charge-transfer characteristics, single isomer of the fac or mer complex on metal surfaces has not
chemical stability, photophysical and redox properties.1 They have been reported for STM studies so far.
found widespread applications in photoinduced electron-transfer
Our synthetic strategy is based on the functionalization of a
processes,2 artificial photosynthesis3 and dye-sensitized solar cells.4 conjugated 2,20-bipy ligand, equipped with thioacetyl protection
Moreover, all these properties make them attractive for nano- end-groups, which after coordination forms the two fac and mer
technology as potential photon-induced switches5 and for molecular isomers (Fig. 1). The different spatial geometry of the two isomers
machines and devices.6 To understand the specific way of surface provides distinguishable polarities between them which allows for
arrangement on the molecular level is a challenge which in the case separation by standard column chromatography.
of Ru–polypyridine complexes has been tackled by STM in few
reports only.7,8
Both isomers form rigid tripodal structures, where the conju-
gated ligands enclose the Ru(II) ion. In the fac species all three legs
For better stability and control of molecular spacing on the are prearranged on one side. In the mer isomer one ligand is turned
surface a few groups designed tripodal to multipodal structures to the opposite direction which defines a pronounced geometry
based on polypyridine systems.9 Recently we reported a differently difference to the facial arrangement. Of particular interest will be the
functionalized tripodal Ga(III) complex and its single molecule behaviour of the fac stereoisomer on a gold substrate. The three
detection on Au(111) by UHV STM.10 The structure was intended thiol groups define a plane and should in principle enable
to stand by itself upright on the surface, a structural concept
which also led to the design of a tris(bipyridine)Ru(II) complex
presented here.
In a pseudo-octahedral ligand field unsymmetrical bidentate
ligands, e.g. 2,20-bipyridine (bipy), adopt two geometric forms in
a Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT),
P.O. Box 3640, D-76021 Karlsruhe, Germany
b DFG Center for Functional Nanostructures (CFN), Karlsruhe Institute
of Technology (KIT), D-76049 Karlsruhe, Germany
c Physics Institute and Institute of Solid State Physics, Karlsruhe Institute
of Technology (KIT), P.O. Box 3640, D-76021 Karlsruhe, Germany
d Department of Chemistry, University of Basel, CH-4003 Basel, Switzerland.
E-mail: marcel.mayor@unibas.ch; Fax: +41 61 267 1016; Tel: +41 61 267 1006
† Electronic supplementary information (ESI) available: Experimental proce-
dures, compounds characterization and X-ray crystallographic data for the fac
isomer. CCDC 813446. For ESI and crystallographic data in CIF or other electronic
Fig. 1 Designed tripodal fac and mer [RuL3]2+
.
format see DOI: 10.1039/c2cc37904j
c
1076 Chem. Commun., 2013, 49, 1076--1078
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Scheme 1 Synthesis of fac and mer [RuL3](PF6)2. (i) PdCl2(PPh3)2, CuI, THF anh./
DIEA anh., N2, rt, 50%; (ii) (a) Ru(DMSO)4Cl2, EtOH, Ar, reflux; (b) KPF6 aq., MeCN, rt,
26% fac, 10% mer.
interactions of all three anchor groups with a plane substrate. In the
present case however, the three thiol groups remain protected Fig. 2 1H NMR spectra of: (a)
preventing the formation of direct gold–thiol bonds. In addition,
on an atomically flat substrate the match of distances and orienta-
L
in CDCl3; (b) fac [RuL3](PF6)2; (c) mer
[RuL3](PF6)2 in CD2Cl2; the assignment of protons is declared in Scheme 1 above.
tions between the thiol groups with the periodicity of the surface The photophysical investigation of both fac and mer isomers in
atoms might become important as well. Finally, the superior photo- MeCN indicates no significant difference between two structural
physical properties of Ru(II)tris(bipyridine) derivatives motivated the forms as reported for similar structures.11 There, transitions of the
investigation of the fac isomer on metallic surfaces.
ligand-centered (1LC) and metal-to-ligand-charge-transfer (1MLCT)
The thioacetyl terminated ligand L was obtained by a character dominate the spectrum. The features of ligand based
Sonogashira cross-coupling reaction (Scheme 1) between transitions are found at 325–335 nm (broad) which is very close to
5-ethynyl-2,20-bipyridine15 and 4-(thioacetyl)-iodobenzene.16 The the absorption of the free ligand. The 1MLCT based peak at 471 nm
reaction of 3 eq. of L with Ru(DMSO)4Cl217 resulted in both fac experiences a bathochromic shift in comparison to the reported
and mer [RuL3]2+ isomers. The isolation of each of the two values of the [Ru(bipy)3](PF6)2 (452 nm 1MLCT, 286 nm 1LC)19 which
isomers was achieved by silica flash column chromatography is caused by the extended conjugated p-electron system on the
3
eluted with an aqueous solution of KNO3. After separation, anion ligand.8 A strong MLCT emission band at 635 nm was observed
metathesis was carried out by application of excess of KPF6 aq. after excitation into the 471 nm 1MLCT maximum. The MLCT
The ligand L and its fac and mer Ru(II) complexes were fully character of the emissive species is supported by the emission–
characterized by NMR, electrospray ionization mass spectro- excitation spectrum (maximum peak at 471 nm) which complies
metry, IR, UV-vis spectroscopy and elemental analysis. The well with the UV-vis absorption spectrum.
molecular structure of the fac Ru(II) isomer was determined
by single crystal X-ray diffraction. All obtained data confirm the in Fig. 3.‡ The metal ion is coordinated by six nitrogen atoms of the
formation and separation of the two stereoisomers. three bidentate bipy ligands in a distorted octahedral arrangement.
The molecular structure of the L isomer of fac [RuL3]2+ is shown
1
As observed by H NMR (Fig. 2b) the fac complex exhibits in In the solid state the three bipy ligands are positioned in a pseudo
solution a C3-symmetry, expressed in a clearly resolved spectrum C3-symmetry around the central metal. The cis-arrangement of each
1
with all three legs being chemically equivalent. The mer isomer is of the three ligand legs confirms the H NMR studies described
not symmetric, which leads to a more complex spectrum (Fig. 2c), before. The values of the Ru–N bond lengths are between 2.058(6) Å
where the signals appear as broad complex multiplets. This results and 2.066(5) Å, and N–Ru–N angles are between 78.6(2)1 and
from the non-equivalent interaction of each proton with the electron 174.4(2)1, similar to that found in comparable [Ru(bipy)3]2+ struc-
density of the neighbouring ligands. Integration of these signals tures.12 The distance between the plane formed by three sulphur
corresponds to the formulated structure, considering accordant atoms and the upper 50-bipy carbon atoms is 12.6 Å, while that from
1
assignment to the fac isomer. Our H NMR shift resulting from the Ru ion to the same plane of sulphur atoms is 9.5 Å. The
the changes in electron densities after chelation of the ligands distances between the sulphur atoms are on the average 13 Å. The
(Fig. 2a) is comparable with reported values.13,18 A closer analysis complex crystallizes as a racemate in the triclinic space group P1
%
of the 1H NMR spectra, electrospray-ionisation mass spectra, IR and with two formula units and three additional solvate molecules
elemental analysis is discussed in the ESI.†
(MeCN) per unit cell. The unit cell contains the two enantiomers,
Additional evidence for the distinction of the two isomers can be one D and the other L, positioned head to head.† The crystal
gained from 13C NMR analyses.† The higher symmetric fac species packing presents intermolecular pꢀꢀꢀp stacking interactions (3.86 Å
resolves 18 carbon signals, while the mer-species exhibits several sets centroid–centroid) between the pyridine and phenyl rings of two very
of closely pitched signals representing the chemically similar but closely positioned neighbouring molecules.†
symmetrically different carbon atoms of the three ligand moieties.
Molecules of the fac isomer were deposited on Au(111) from
The UV-vis spectrum in MeCN of the ligand presents a broad MeCN solution (for details see ESI†). Fig. 4 shows an STM image of
absorption peak at 320 nm with a red-sided shoulder at 336 nm, such a sample at monolayer coverage. In contrast to the previously
which is usually observed for n–p* or p–p* transitions.† Excitation reported tripodal molecules,10 the fac Ru complex forms islands of
into these wavelengths yielded a strong fluorescence signal at 376 nm. dimers which exhibit a medium range order. The intermolecular
c
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Chem. Commun., 2013, 49, 1076--1078 1077

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Financial support from the KIT is gratefully acknowledged.
We thank Dr Frank Schramm and Matthias Fischer for provid-
ing chemical reagents.
Notes and references
‡ Crystal data: fac [RuL3](PF6)2ꢀ1.5(CH3CN), CCDC 813446,
%
C63H46.5F12N7.5O3P2RuS3, M = 1443.77, triclinic, space group P1, a =
13.081(5) Å, b = 15.083(6) Å, c = 18.470(8) Å, a = 95.06(3)1, b = 108.12(3)1,
g = 104.55(3)1, V = 3297(2) Å3, T = 143(2) K, Z = 2, m(MoKa) = 0.465 mmꢂ1
,
18 404 reflections measured, 10 343 unique (Rint = 0.0693); 7058 with I >
2s(I); 820 parameters, 2 restraints, GOF = 1.020; final R1 values for refl.
with I > 2s(I): R1 = 0.0775, wR(F2) = 0.1940; for all data: final R1 = 0.1148,
wR(F2) = 0.2169. Crystals of the fac isomer were grown by slow diffusion
of Et2O into a MeCN solution of the complex, yielding red needles
suitable for single crystal X-ray investigation. Two thioacetyl groups and
one hexafluorophosphate anion were refined disordered.
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1078 Chem. Commun., 2013, 49, 1076--1078
This journal is The Royal Society of Chemistry 2013