Coordination chemistry of single-site catalyst precursors in reductively electropolymerized vinylbipyridine films

Article abstract of DOI:10.1021/ic302472r

Reductive electropolymerization of [Ru^II(PhTpy)(5,5′- dvbpy)(Cl)](PF₆) and [Ru^II(PhTpy)(5,5′-dvbpy)(MeCN)] (PF₆)₂ (PhTpy is 4′-phenyl-2,2′:6′, 2″-terpyridine; 5,5′-dvbpy is 5,5′-divinyl-2,2′- bipyridine) on glassy carbon electrodes gives well-defined films of poly{[Ru^II(PhTpy)(5,5′-dvbpy)(Cl)](PF₆)} (poly-1) or poly{[Ru^II(PhTpy)(5,5′-dvbpy)(MeCN)](PF₆) ₂} (poly-2). Oxidative cycling of poly-2 with added NO ₃^- results in the replacement of coordinated MeCN by NO₃^- to give poly{[Ru^II(PhTpy)(5,5′- dvbpy)(NO₃)]⁺}, and with 0.1 M HClO₄, replacement by H₂O occurs to give poly{[Ru^II(PhTpy)(5, 5′-dvbpy)(OH₂)]²⁺} (poly-OH₂). Although analogous aqua complexes (e.g., [Ru(tpy)(bpy)(OH₂)]²⁺) undergo rapid loss of H₂O to MeCN in solution, poly-OH₂ and poly-OH₂⁺ are substitutionally inert in MeCN. The substitution chemistry is reversible, with reductive scans of poly-1 or poly-OH₂ in MeCN resulting in poly-2, although with some loss of Faradaic response.

Full text of DOI:10.1021/ic302472r

Communication
pubs.acs.org/IC
Coordination Chemistry of Single-Site Catalyst Precursors in
Reductively Electropolymerized Vinylbipyridine Films
Daniel P. Harrison,† Alexander M. Lapides,‡ Robert A. Binstead,‡ Javier J. Concepcion,‡
Manuel A. Mendez,‡ Daniel A. Torelli,‡ Joseph L. Templeton,‡ and Thomas J. Meyer*,‡
́
†Department of Chemistry, Virginia Military Institute, Lexington, Virginia 24450, United States
‡Department of Chemistry, University of North Carolina at Chapel Hill, CB 3290, Chapel Hill, North Carolina 27599-3290, United
States
S
* Supporting Information
(5,5′-dvbpy)(MeCN)](PF6)2 (2), where PhTpy is 4′-phenyl-
ABSTRACT: Reductive electropolymerization of
2,2′:6′,2″-terpyridine and 5,5′-dvbpy is 5,5′-divinyl-2,2′-bipyr-
[RuII(PhTpy)(5,5′-dvbpy)(Cl)](PF6) and [RuII(PhTpy)-
idine (Figure 1), and the behavior of the resulting interfacial
(5,5′-dvbpy)(MeCN)](PF6)2 (PhTpy is 4′-phenyl-
2,2′:6′,2″-terpyridine; 5,5′-dvbpy is 5,5′-divinyl-2,2′-bipyr-
idine) on glassy carbon electrodes gives well-defined films
of poly{[RuII(PhTpy)(5,5′-dvbpy)(Cl)](PF6)} (poly-1)
or poly{[RuII(PhTpy)(5,5′-dvbpy)(MeCN)](PF6)2−}
(poly-2). Oxidative cycling of poly-2 with added NO3−
results in the replacement of coordinated MeCN by NO3
to give poly{[RuII(PhTpy)(5,5′-dvbpy)(NO3)]+}, and
with 0.1 M HClO4, replacement by H2O occurs to give
poly{[RuII(PhTpy)(5,5′-dvbpy)(OH2)]2+} (poly-OH2).
Figure 1. Structures of single-site ruthenium complex catalyst
precursors to poly-1 and poly-2.
Although analogous aqua complexes (e.g., [Ru(tpy)(bpy)-
(OH2)]2+) undergo rapid loss of H2O to MeCN in
+
solution, poly-OH2 and poly-OH2 are substitutionally
inert in MeCN. The substitution chemistry is reversible,
with reductive scans of poly-1 or poly-OH2 in MeCN
resulting in poly-2, although with some loss of Faradaic
response.
films, poly-1 and poly-2. Earlier strategies relied on multiple
polymerizable ligands and cross-linking, which limited the
generality of the coordination chemistry. We report here that
the doubly derivatized 5,5′-dvbpy ligand in these complexes is
sufficient to achieve stable interfacial film structures, as reported
earlier by Nie and co-workers.4
ell-established procedures are available for the electro-
polymerization of vinyl-1 and pyrrole-derivatized2 metal
complexes on a variety conducting substrates. The electron
transfer,1b,i photochemical,1c,e,j diffusional,2b and related proper-
ties of the resulting films, including electrocatalysis,1f−h,2a,d have
also been investigated. Electropolymerization offers significant
advantages over other approaches to modifying surfaces. With
multiple polymerizable functional groups, cross-linking and
formation of relatively high polymers lead to film formation by
physical adsorption to the electrode surface. The resulting
interfacial film structures are stable in a variety of media and over
an extended pH range in water. This is in contrast to surface
binding to oxides by carboxylate- or phosphonate-derivatized
complexes, the former of which are unstable on oxide surfaces in
water and the latter at elevated pHs.3
Electropolymerization of 1 and 2 to give poly-1 and poly-2 was
induced by controlled potential electrolysis or cyclic voltam-
metric (CV) scans at (or to) potentials sufficiently negative to
reduce the ligands and initiate polymerization (Figure S1,
Supporting Information, SI). Either technique produces surface
coverages (Γ in mol/cm2; see eqs S1 and S2, SI) that increase
linearly with the number of reductive scan cycles or with time
(Figures S2 and S3, SI). Electropolymerized films of poly-1
W
5
(RuIII/II: E1/2 = +0.56 V vs Ag/AgNO3 and −0.094 V vs
FeCp2+/0) and poly-2 (RuIII/II: E1/2 = +0.99 V vs Ag+/0) on 0.071
cm2 glassy carbon electrodes (GCEs) in 0.1 M [TBA]PF6/
MeCN ([TBA]PF6 is tetra-n-butylammonium hexafluorophos-
phate) display peak-to-peak separations (ΔEp) of 22 and 21 mV
(Figure S4, SI), respectively, at a scan rate, ν, of 100 mV/s. ΔEp
approaches 0 as the scan rate is decreased, as expected for a
surface wave (Figure S5, SI). UV−vis spectra of poly-1 and poly-
2 on semitransparent fluorine-doped tin oxide (FTO) surfaces
closely resemble those of 1 and 2 in solution (Figures S6 and S7,
SI).
A particular target for us is to design interfaces for
electrocatalytic and photoelectrocatalytic applications. In one
strategy, electropolymerization is used to form films from
monomer-based catalyst precursors; recent examples have
appeared based on oxidatively induced pyrrole polymer-
ization.2c,d Here we report our initial findings on reductive
electropolymerization of the catalyst precursor complexes
[RuII(PhTpy)(5,5′-dvbpy)(Cl)](PF6) (1) and [RuII(PhTpy)-
Received: November 12, 2012
Published: April 24, 2013
© 2013 American Chemical Society
4747
dx.doi.org/10.1021/ic302472r | Inorg. Chem. 2013, 52, 4747−4749

Inorganic Chemistry
Communication
Oxidatively Induced Ligand Substitution. Oxidatively
cycling poly-2 through the RuIII/II wave in a 1 mM solution of
[TBA]NO3 (in 0.1 M [TBA]PF6/MeCN) produces a new
surface couple at E1/2 = +0.68 V (Figure 2). Interconversion from
Figure 3. Formation of poly-H2O following oxidative scan cycles of
poly-2 (red arrows; Γinitial = 2.7 × 10−9 mol/cm2) in 0.1 M HClO4; GCE,
0.071 cm2, v = 100 mV/s.
scan cycles. This wave may arise from sites near the film−
solution interface that undergo substitution more rapidly than
sites in the film interior.
Figure 2. Oxidative CVs of poly-2 on a GCE in 1 mM [TBA]NO3 (ν =
250 mV/s), illustrating the loss of poly-2 (red arrows; Γ = 3.7 × 10−9
mol/cm2) and the appearance of poly-ONO2 (green arrows). The
shoulder at +0.55 V vs Ag/AgNO3 appears to be poly-OH2, arising from
trace water in the initial solution (see below).
3+
poly‐RuIIINCMe3+ + H2O → poly‐RuIIIOH2 + MeCN
(3)
Appearance of the aqua complex is significant given the known
pH-dependent proton-coupled electron-transfer (PCET) prop-
erties of [Ru(tpy)(bpy)(OH2)]2+ and the oxidative reactivity of
higher-oxidation-state RuIV(O) and RuV(O) forms.3,8 The film-
based redox chemistry is currently under investigation. In
MeCN, [Ru(tpy)(bpy)(OH2)]2+ undergoes substitution of H2O
by MeCN in minutes. By contrast, there is no evidence for poly-2
when poly-H2O is soaked in [TBA]PF6/MeCN for extended
periods or after oxidative cycling. NO3− is lost from poly-ONO2
to give poly-H2O upon oxidative cycling in 0.1 M HClO4 or upon
soaking in 0.1 M HClO4. It is noteworthy that the substitution
kinetics of MeCN in poly-2 for OH2 or NO3− are zero-order over
an extensive dynamic range consistent with a noncomplex rate-
limiting step, namely, diffusion into the film (Figures S8 and S11,
SI).
Reductively Induced Substitution. Ligand substitution is
also induced by reductive cycling following reduction at the
π*(polypyridyl) levels of the ligands. The results of three
reductive scan cycles of poly-1 at 100 mV/s in 0.1 M [TBA]PF6/
MeCN at 0 → −1.97 V under N2 are shown in Figure 4a.9 On the
first scan, a prewave appears at Ep,c = −1.27 V followed by surface
waves at E1/2 = −1.66 V (PhTpy reduction) and at E1/2 = −1.87 V
(5,5′-poly-vbpy reduction).
poly-2 to the new couple is complete after 20 scans from 0 to
+1.5 V (Figure S8, SI). A characteristic prewave appears at Ep,c
=
+0.80 V because of changes in the film structure arising from ion
transport.6 The negative shift of ΔE1/2 = −0.34 V is consistent
with oxidation to RuIII, followed by substitution of MeCN by
−
2+
+
NO3 to give poly-RuIIIONO2 (poly-ONO2 ; eqs 1 and 2).
UV−vis spectral data are consistent with this conclusion (Figure
S9A, SI). Oxidatively induced substitution of MeCN for NO3− in
Ru(tpy)(bpy)(NCMe)2+ also occurs in solution under the same
conditions.
−e−
poly‐RuIINCMe2+ ⎯⎯→⎯ poly‐RuIIINCMe3+
(1)
poly‐RuIIINCMe3+ + NO3−
2+
→ poly‐RuIIIONO2 + MeCN
(2)
2+
The poly-RuIIIONO2 couple was not present following a
single oxidative sweep of 0 → 1.5 V or after soaking of a GCE−
poly-2 electrode in [TBA]NO3 for 72 h. There was no sign of
coordination of HSO4 , ClO4 , or OTf− by oxidative cycling of
−
−
−
poly-2 under comparable conditions. Cl− is preferred over NO3
in the coordination spheres of both RuIII and RuII. There was no
evidence for substitution of Cl− for NO3− in poly-1. In MeCN 5
Following the first scan through both ligand-based reductions,
a new surface-based couple appears at Ep,c = −1.49 V, which
coincides with Ep,c for the first PhTpy-based reduction in poly-2.
A subsequent oxidative scan and the appearance of a wave at E1/2
= +1.02 V for the poly-RuIIINCMe3+/2+ couple reveals that
ligand-based reduction induces conversion of poly-1 into poly-2
(eqs 4 and 5; Figures 4b and S12 and S13, SI). UV−vis spectral
data corroborate these results (Figure S14, SI). The substitution
mechanism, following π*(PhTpy)/π*(5,5′-poly-vbpy) reduc-
tion, is presumably by thermal population of ruthenium-based
dσ* levels, which induces ligand labilization.
mM in Cl−, substitution of NO3 occurs, converting poly-
−
+
2+
ONO2 to poly-1 by the reaction, poly-RuIIIONO2 + Cl− →
−
poly-1 + NO3 with oxidative cycling with slow substitution
(hours) occurring without cycling.
Similarly, oxidative cycling of poly-2 in 0.1 M HClO4 between
0 and +1.5 V gives poly-RuIIIOH23+ (poly-OH2 ; eqs 1 and 3),
+
with E1/2 = +0.80 V compared to E1/2 = +0.79 V vs SCE for the
[Ru(bpy)(bpy)OH2]3+/2+ couple at pH 1 (Figure 3).7 UV−vis
spectral data corroborate these results (Figure S10, SI). The
reaction is complete after 20 cycles (Figure S11, SI). There was
no sign of aquation when poly-2 was soaked in 0.1 M HClO4 for
72 h. Careful inspection of the scan sequence in Figure 3 reveals
that a smaller wave at E1/2 ∼ 0.68 V appears during the first few
poly‐RuCl+ + 2e− + MeCN → poly‐RuNCMe0 + Cl−
(4)
4748
dx.doi.org/10.1021/ic302472r | Inorg. Chem. 2013, 52, 4747−4749

Inorganic Chemistry
Communication
ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed experimental procedures, synthetic procedures, NMR
spectra, Figures S1−S22, additional acknowledgments, and
equations. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: tjmeyer@unc.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
D.P.H. acknowledges support from the U.S. Department of
Energy Office of Science, Office of Basic Energy Sciences, under
Award DE-FG02-06ER15788 and the Virginia Military Institute.
A.M.L., R.A.B., and J.J.C. acknowledge support from the UNC
Energy Frontier Research Center (EFRC): Center for Solar
Fuels, an Energy Frontier Research Center funded by the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences, under Award DE-SC0001011.
Figure 4. (A) Reductive CVs of poly-1 under N2 (black). (B) Oxidative
CVs of a poly-1 electrode prior to reductive cycling (blue; Γ = 1.7 × 10−9
mol/cm2) and after reductive cycling (red). Both parts A and B were
obtained in fresh solutions of 0.1 M [TBA]PF6/MeCN after
electropolymerization on a 0.071 cm2 GCE.
poly‐RuNCMe0 − 2e− → poly‐RuNCMe2+
(5)
REFERENCES
■
Ligand-based reduction and substitution are accompanied by a
loss of Faradaic response, with Γ = 1.7 × 10−9 mol/cm2 for the
initial poly-1 RuIII/II wave at E1/2 = +0.56 V decreasing to Γ = 9.3
× 10−10 mol/cm2 for the poly-2 wave at E1/2 = +1.03 V. In
addition, a new, distorted prewave appears at Ep,a = +0.82 V
(Figure 4b). This observation points to a 46% decrease in the
redox response at the end of three reductive scan cycles. A related
response was observed for a thinner film of poly-1 with Γ = 4.5 ×
10−10 mol/cm2 before a reductive cycle and Γ = 3.2 × 10−10 mol/
cm2 for poly-2, a 29% loss. Reductive cycling of poly-ONO2 and
poly-H2O both result in poly-2 with comparable decreases in Γ
(Figures S15 and S16, SI). The loss mechanism is currently
under investigation. It is noteworthy that, after the initial
exchange occurs with a loss of electroactivity, further decreases
are greatly ameliorated upon additional reductive scan cycles
(Figure S13, SI).
Our results are important in revealing systematic and
synthetically exploitable features in the film-based coordination
chemistries of poly-1 and poly-2 with significant differences
between film and solution behavior. Polypyridyl complexes of
dπ6 RuII typically undergo slow loss of nitrile ligands. Nitrile
ligands are weak σ donors and derive coordinative stability from
dπ−π* back-bonding from RuII. With oxidation to RuIII, back-
bonding stabilization is no longer a factor, and nitriles become
good leaving groups. Nitrile labilization was exploited here to
convert poly-2 into poly-ONO2 and poly-OH2.
(1) (a) Calvert, J. M.; Schmehl, R. H.; Sullivan, B. P.; Facci, J. S.; Meyer,
T. J.; Murray, R. W. Inorg. Chem. 1983, 22, 2151−2162. (b) Gould, S.;
Gray, K. H.; Linton, R. W.; Meyer, T. J. Inorg. Chem. 1992, 31, 5521−
5525. (c) Devenney, M.; Worl, L. A.; Gould, S.; Guadalupe, A.; Sullivan,
B. P.; Caspar, J. V.; Leasure, R. L.; Gardner, J. R.; Meyer, T. J. J. Phys.
Chem. A 1997, 101, 4535−4540. (d) Moss, J. A.; Argazzi, R.; Bignozzi, C.
A.; Meyer, T. J. Inorg. Chem. 1997, 36, 762−763. (e) Moss, J. A.; Yang, J.
C.; Stipkala, J. M.; Wen, X.; Bignozzi, C. A.; Meyer, G. J.; Meyer, T. J.
Inorg. Chem. 2004, 43, 1784−1792. (f) O’Toole, T. R.; Margerum, L. D.;
Westmoreland, T. D.; Vining, W. J.; Murray, R. W.; Meyer, T. J. J. Chem.
Soc., Chem. Commun. 1985, 1416−1417. (g) Ramos Sende, J. A.; Arana,
C. R.; Hernandez, L.; Potts, K. T.; Keshevarz-K, M.; Abruna, H. D. Inorg.
Chem. 1995, 34, 3339−3348. (h) Cosnier, S.; Deronzier, A.; Moutet, J.-
C. J. Mol. Catal. 1988, 45, 381−391. (i) Abruna, H. D.; Denisevich, P.;
Umana, M.; Meyer, T. J.; Murray, R. W. J. Am. Chem. Soc. 1981, 103, 1−
5. (j) Yang, J.; Sykora, M.; Meyer, T. J. Inorg. Chem. 2005, 44, 3396−
3404.
(2) (a) Cosnier, S.; Deronzier, A.; Moutet, J.-C. J. Electroanal. Chem.
Interfacial Electrochem. 1986, 207, 315−321. (b) Deronzier, A.; Eloy, D.;
Jardon, P.; Martre, A.; Moutet, J.-C. J. Electroanal. Chem. 1998, 453,
179−185. (c) Mola, J.; Mas-Marza, E.; Sala, X.; Romero, I.; Rodríguez,
M.; Vinas, C.; Parella, T.; Llobet, A. Angew. Chem., Int. Ed. 2008, 47,
̃
5830−5832. (d) Cheung, K.-C.; Guo, P.; So, M.-H.; Zhou, Z.-Y.; Lee, L.
Y. S.; Wong, K.-Y. Inorg. Chem. 2012, 51, 6468−6475.
(3) (a) Concepcion, J. J.; Jurss, J. W.; Brennaman, M. K.; Hoertz, P. G.;
Patrocinio, A. O. v. T.; Murakami Iha, N. Y.; Templeton, J. L.; Meyer, T.
J. Acc. Chem. Res. 2009, 42, 1954−1965. (b) Chen, Z.; Concepcion, J. J.;
Jurss, J. W.; Meyer, T. J. J. Am. Chem. Soc. 2009, 131, 15580−15581.
(4) Nie, H.-J.; Shao, J.-Y.; Wu, J.; Yao, J.; Zhong, Y.-W. Organometallics
2012, 31, 6952−6959.
The film environment also plays an important role. Following
conversion of poly-2 into poly-OH2, there is no sign of
(5) Silver wire reference electrode solution: 0.01 M AgNO3, 0.1 M
[TBA]PF6, MeCN solution; −0.094 V vs FeCp2.
substitution of H2O for MeCN in poly-OH2 or poly-RuIIIOH2
3+
(6) Abruna, H. D. Coord. Chem. Rev. 1988, 86, 135−189.
̃
even over extended soaking or oxidative cycling periods in
MeCN. This is a potentially important observation for possible
applications in organic electrocatalysis based on RuO forms of
poly-OH2 with MeCN as the external solvent.9 Oxidatively
induced anation and aquation provide a basis for systematic
manipulation of the coordination environment at the redox-
active RuII sites in films. Ligand-based reduction offers a route to
loss of anions or water in MeCN to return the films to the initial
poly-2 state.
(7) Takeuchi, K. J.; Thompson, M. S.; Pipes, D. W.; Meyer, T. J. Inorg.
Chem. 1984, 23, 1845−1851.
(8) (a) Meyer, T. J.; Huynh, M. H. V. Inorg. Chem. 2003, 42, 8140−
8160. (b) Gagliardi, C. J.; Vannucci, A. K.; Concepcion, J. J.; Chen, Z.;
Meyer, T. J. Energy Environ. Sci. 2012, 5, 7704−7717.
(9) Vanucci, A. K.; Chen, Z.; Concepcion, J. J.; Meyer, T. J. ACS Catal.
2012, 2, 716−719.
4749
dx.doi.org/10.1021/ic302472r | Inorg. Chem. 2013, 52, 4747−4749