Modular synthesis of alkyne-substituted ruthenium polypyridyl complexes suitable for click coupling

Article abstract of DOI:10.1021/ic302827s

A modular synthetic method has been developed for the preparation of Ru polypyridyl complexes bearing a terminal alkyne. This method proceeds through a readily accessible intermediate with a silyl-protected alkyne and allows access to a variety of five- and six-coordinate Ru complexes. These complexes can be easily attached to azide-functionalized electrode surfaces with only slight perturbation of the redox properties of the parent complex.

Full text of DOI:10.1021/ic302827s

Communication
pubs.acs.org/IC
Modular Synthesis of Alkyne-Substituted Ruthenium Polypyridyl
Complexes Suitable for “Click” Coupling
James B. Gerken, Matthew L. Rigsby, Rose E. Ruther, Riviam J. Perez-Rodríguez, Ilia A. Guzei,
́
Robert J. Hamers,* and Shannon S. Stahl*
Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States
S Supporting Information
*
Scheme 2. Modular Synthesis of Alkyne-Functionalized
Ruthenium Polypyridyl Complexes
ABSTRACT: A modular synthetic method has been
developed for the preparation of Ru polypyridyl complexes
bearing a terminal alkyne. This method proceeds through a
readily accessible intermediate with a silyl-protected alkyne
and allows access to a variety of five- and six-coordinate Ru
complexes. These complexes can be easily attached to
azide-functionalized electrode surfaces with only slight
perturbation of the redox properties of the parent complex.
Initial efforts to prepare the target compounds sought to ligate
alkyne-functionalized tpy ligands to RuCl ; however, these
3
uthenium polypyridyl complexes can access multiple
11
efforts were unsuccessful. As an alternative, we identified a
R
oxidation states at reasonable potentials and have been
widely studied as (photo)electrochemical redox mediators and
II
synthetic route originating from the Ru precursor, cis-
12
Ru(DMSO) Cl (DMSO = dimethyl sulfoxide; Scheme 3).
1
−3
4
2
electrocatalysts.
A number of recent studies have targeted
covalent functionalization of electrode surfaces with these
a
Scheme 3. Synthesis of Common Intermediate 3
3
b,4
molecules.
In this context, we have employed Cu-catalyzed
5
azide−alkyne cycloaddition (i.e., “click” coupling ) to tether
II
Ru (tpy) (tpy = 2,2′;6′,2″-terpyridine) to boron-doped
2
diamond and tin oxide electrodes and investigated the
electrochemical and photochemical properties of these con-
6
jugates. These reactions involve coupling of a surface-bound
azide with a terminal alkyne appended to the molecular Ru
complex (Scheme 1). Several methods have been identified for
a
Conditions: (a) triisopropylsilyl acetylene, Pd(PPh ) Cl , CuI,
3
2
2
iPr NH, benzene; (b) cis-Ru(DMSO) Cl , CH Cl ; see the SI for
2
4
2
2
2
synthetic details.
Scheme 1. Attachment of an Alkyne-Substituted Catalyst to an
Azide-Modified Electrode
The reaction of 4′-bromo-2,2′:6′,2″-terpyridine (1) with
triisopropylsilyl (TIPS) acetylene under Sonogashira conditions
(
Pd, Cu catalysts; see the Supporting Information, SI) affords the
protected alkyne-substituted tpy ligand (2). The reaction of 2
II
with cis-Ru(DMSO) Cl affords the ligated Ru cis-dichloride
4
2
complex 3. Although 3 is a bench-stable solid, UV−visible
spectroscopy reveals that it undergoes photoisomerization in
solution to the more stable and less-reactive trans-dichloro
functionalizing surfaces with azide groups, including the direct
7
introduction of a surface-bound azide or attachment of an azide
8
tethered to the surface via an alkyl chain. The preparation of Ru
13
isomer (Figure 1).
Complex 3 is a common intermediate for further synthetic
polypyridyl complexes bearing a terminal alkyne, however, has
proven to be synthetically challenging because many routes to
14
1b,3c,9
elaboration. To demonstrate the versatility of complex 3 as a
synthetic intermediate, we sought complexes of 3 with a
bidentate bipyrimidine (bpm) ligand and a tridentate unsub-
stituted tpy ligand. The bpm complex represents a substituted
analogue of a molecular water oxidation catalyst reported by
coordination complexes of this type use RuCl3,
which is
chemically incompatible with alkynes. Here, we describe an
efficient synthetic route to alkyne-derivatized Ru polypyridyl
II
complexes, originating from a readily accessible Ru source
(
Scheme 2). By proceeding through a common alkyne-
3b
Meyer et al. Dehalogenation of 3 by AgNO , followed by
3
functionalized intermediate, this route allows for late-stage
installation of bidentate or tridentate ligands to produce
electronically modified Ru complexes suitable for use in “click”
coupling. In this manner, the problem of attaching m complexes
treatment with the appropriate bidentate or tridentate ligand,
Received: December 22, 2012
Published: March 4, 2013
10
to n surfaces would require m + n syntheses rather than m·n.
©
2013 American Chemical Society
2796
dx.doi.org/10.1021/ic302827s | Inorg. Chem. 2013, 52, 2796−2798

Inorganic Chemistry
Communication
a
Scheme 5. Click Coupling of 5bpm to an Aryl Azide
a
II
Conditions: p-tert-butylphenyl azide, [Cu (TBTA)]SO , sodium
4
ascorbate, DMSO; see the SI for full synthetic details.
Figure 1. UV−visible spectra revealing the cis-to-trans isomerization of
3
in solution by ambient-light illumination on the benchtop (33 μM,
CH CN; cf. NMR spectra in Figure S12 in the SI).
3
1
5,13
affords 4bpm and 4tpy (Scheme 4, step a).
Removal of the
TIPS group proved challenging because standard silyl depro-
a
Scheme 4. Synthesis and Elaboration of Alkynyl Complex 4
Figure 3. CVs of [Ru(tpy)(bpm)(OH )]2 (red, E1/2 = 1.17 and 1.51 V)
+
2
and triazole-modified complex 6 (blue, offset by −5 μA, E = 1.15 and
1
/2
1
.50 V) (10 mV/s scan rate, 50 mM in 0.1 M HNO ).
3
a
Conditions: (a) AgNO and then addition of a ligand; (b) AgF and
Having confirmed that triazole attachment to 5bpm is
3
then HClO ; see the SI for synthetic details.
possible, we investigated attachment of the modified water
oxidation catalyst 5bpm to a diamond surface bearing azides
tethered to the surface via an alkyl chain (Scheme S3 in the SI).
CVs obtained from the 5bpm-treated surfaces display a reversible
oxidation centered at 1.18 V vs NHE (Figure 4A). The
4
tection reagents are ineffective against this robust protecting
1
6
group. After screening various conditions (Table S1 in the SI),
including ones reported in the literature to be effective on similar
complexes, we found that AgF effects the desilylation in high
1
7
yield to afford 5bpm and 5tpy (Scheme 4, step b).
Spectroscopic and crystallographic characterization of 5bpm
and 5tpy confirms the presence of the deprotected alkyne
(Figure 2).
Figure 4. Electrochemical data from 5bpm-functionalized diamond
electrodes in 0.1 M aqueous HNO : (A) CV, 20 V/s scan rate (E
=
3
1/2
1
.18 V); (B) anodic peak current versus scan rate.
Figure 2. ORTEP plots of (ellipsoid at 40% probability) X-ray crystal
structures of 5bpm with a coordinated MeCN solvent molecule (left)
and 5tpy (right). Counterions and hydrogen atoms not shown.
disappearance of the broad, quasi-reversible feature seen at
higher potential in Figure 3 may reflect the higher scan rate
necessary to observe significant current from the surface-bound
complex. Peak currents display a linear scan rate dependence,
consistent with a surface-attached species (Figure 4B). Addi-
tionally, X-ray photoelectron spectroscopy (XPS) confirms the
presence of Ru on the functionalized surfaces (Figure S3A in the
With the deprotected alkyne-functionalized complexes in
hand, we explored their reactivity with azide coupling partners in
click reactions in order to probe the electrochemical properties
and robustness of the linkage. Of specific interest in this study
was the degree to which triazole attachment affects the redox
properties of the parent complex and whether the presence of the
labile solvent ligand in 5bpm would interfere with the “click”
reaction.
18
SI).
Preliminary studies suggest that the 5bpm-functionalized
diamond surface is not sufficiently stable to withstand sustained
electrolysis at potentials needed for electrocatalytic water
19−21
oxidation.
Nevertheless, the synthetic protocol elaborated
The reaction of 5bpm with 4-tert-butylphenyl azide under click
conditions affords the molecular aryltriazole complex 6 (Scheme
). Cyclic voltammograms (CVs) of 6 in 0.1 M aqueous HNO3
herein provides a valuable foundation for the use of click-
coupling methods to attach Ru polypyridyl derivatives to diverse
molecules and materials and to explore the photo- and
electrocatalytic chemistry of such conjugates. Specifically, the
reactions outlined in Schemes 3 and 4 allow rapid access to
alkyne-functionalized Ru complexes with a variety of redox
properties, and the triazole linkage arising from the click-
5
display redox features that closely match those of the
2
+
[
Ru(tpy)(bpm)(OH )] parent complex (Figure 3). These
2
results indicate that the triazole linker does not significantly
perturb the redox properties of parent molecules.
2
797
dx.doi.org/10.1021/ic302827s | Inorg. Chem. 2013, 52, 2796−2798

Inorganic Chemistry
Communication
coupling reaction has a minimal effect on the redox properties of
complexes to which it is attached.
133, 3696−3699. (c) Landis, E. C.; Hamers, R. J. Chem. Mater. 2009, 21,
7
24−730.
(8) (a) Strother, T.; Knickerbocker, T.; Russell, J. N., Jr.; Butler, J. E.;
Smith, L. M.; Hamers, R. J. Langmuir 2002, 18, 968−971. (b) Collman,
ASSOCIATED CONTENT
Supporting Information
X-ray crystallographic data in CIF format, alternate synthetic
routes, experimental procedures and details, crystallographic
data, and further characterization of derivatized surfaces. This
material is available free of charge via the Internet at http://pubs.
acs.org.
■
J. P.; Devaraj, N. K.; Chidsey, C. E. D. Langmuir 2004, 20, 1051−1053.
*
S
(c) Lummerstorfer, T.; Hoffmann, H. J. Phys. Chem. B 2004, 108, 3963−
3966. (d) Wang, M.; Das, M. R.; Li, M.; Boukherroub, R.; Szunerits, S. J.
Phys. Chem. C 2009, 113, 17082−17086. (e) Wang, X. Y.; Landis, E. C.;
Franking, R.; Hamers, R. J. Acc. Chem. Res. 2010, 43, 1205−1215.
(f) Nakazawa, J.; Smith, B. J.; Stack, T. D. P. J. Am. Chem. Soc. 2012, 134,
2
750−2759. (g) Yao, S. A.; Ruther, R. E.; Zhang, L.; Franking, R. A.;
Hamers, R. J.; Berry, J. F. J. Am. Chem. Soc. 2012, 134, 15632−15635.
9) Exceptions include syntheses of Ru(bpy) L complexes that
(
2
2
AUTHOR INFORMATION
Corresponding Author
E-mail: stahl@chem.wisc.edu (S.S.S.), rjhamers@wisc.edu
R.J.H.).
■
proceed via Ru(bpy) Cl , which is ill-suited for the preparation of
2
2
Ru(tpy) complexes. (a) Wang, H.-X.; Zhou, K.-G.; Xie, Y.-L.; Zeng, J.;
Chai, N.-N.; Li, J.; Zhang, H.-L. Chem. Commun. 2011, 47, 5747−5749.
(b) Baron, A.; Herrero, C.; Quaranta, A.; Charlot, M.-F.; Leibl, W.;
Vauzeilles, B.; Aukauloo, A. Inorg. Chem. 2012, 51, 5985−5987.
*
(
Notes
(10) Diverse strategies exist for functionalizing electrodes with
The authors declare no competing financial interest.
electroactive molecules; see refs 2, 3b, 4, and 6−9 and the SI for a
partial bibliography. Most of these approaches are specific to the
electrode surface that is employed, while an ideal linkage would allow
attachment of any desired catalyst to any surface.
ACKNOWLEDGMENTS
■
Dr. Elizabeth C. Landis provided the XPS data in Figure S5 in the
SI. The authors thank Dr. Charles G. Fry and Paul B. White for
(11) The initial route developed for preparation of the desired
1
5
compounds employs RuCl as a starting material, and the Ru center is
helpful advice on N NMR techniques. This work was
supported, in part, by National Science Foundation (NSF)
Grants CHE-0911543 (to R.J.H., for functionalization and
characterization of the diamond electrodes), DMR-0832760
3
II
reduced to Ru prior to incorporating the alkyne. This route proved to
be less effective than the route reported in the main text. This alternate
route is elaborated on in the SI (see Scheme S2 and associated text).
(
1
(
12) James, B. R.; Ochiai, E.; Rempel, G. L. Inorg. Nucl. Chem. Lett.
971, 7, 781−784.
13) The relative stability and reactivities of cis- and trans-dichloro
(
REU support for R.J.P.-R.), and CHE-0802907 (to S.S.S., for
the design and synthesis of the molecular catalysts). NMR facility
funding was supported under NSF Grants CHE-8813550, CHE-
Ru(tpy)(DMSO)Cl have been studied by Ziessel et al.: Ziessel, R.;
2
9
629688, and CHE-0342998 and National Institutes of Health
Grosshenny, V.; Hissler, M.; Stroh, C. Inorg. Chem. 2004, 43, 4262−
Grant 1S10RR004981-01.
4271.
(14) Generally, DMSO complexes of divalent iron-group dihalides are
versatile intermediates for further elaboration. These compounds have
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