An anodic method for covalent attachment of molecules to electrodes through an ethynyl linkage

Article abstract of DOI:10.1021/ja312405h

Electroactive organometallic molecules have been covalently attached to electrode surfaces through an ethynyl linkage. The process takes advantage of ethynyl-based radicals generated by anodic oxidation of a lithio-activated terminal ethynyl group. Electrophores containing redox-active ferrocene, cymantrene, or cobaltocenium moieties have been deposited at the one-to-three monolayer level. Both metal-based and ligand-based chemical reactions have been carried out on the surface-modified systems.

Full text of DOI:10.1021/ja312405h

Communication
pubs.acs.org/JACS
An Anodic Method for Covalent Attachment of Molecules to
Electrodes through an Ethynyl Linkage
Matthew V. Sheridan, Kevin Lam, and William E. Geiger*
Department of Chemistry, University of Vermont, Burlington, Vermont 05405, United States
S
* Supporting Information
A terminal alkyne group, RCCH, has relatively high
ABSTRACT: Electroactive organometallic molecules
have been covalently attached to electrode surfaces
through an ethynyl linkage. The process takes advantage
of ethynyl-based radicals generated by anodic oxidation of
a lithio-activated terminal ethynyl group. Electrophores
containing redox-active ferrocene, cymantrene, or cobalto-
cenium moieties have been deposited at the one-to-three
monolayer level. Both metal-based and ligand-based
chemical reactions have been carried out on the surface-
modified systems.
acidity, allowing for facile generation of its lithio derivative,
RCCLi, under mild conditions. The deposition method
involves anodic oxidation of RCCLi, which is expected to
produce an alkynyl radical owing to rapid loss of lithium ion
from the radical cation (eq 1), followed by reaction of the
radical with an atom of the electrode (eq 2). In principle, the
oxidation may occur either at R or at the alkynyl group,
depending on the relative oxidizability of the two moieties
(Scheme 1). In compounds 1, 5, and 6, oxidation of R is clearly
here is intense interest in broadening the families of
T
electroactive molecular tags (“electrophores”) which can
be covalently attached to metal surfaces.1 Attachment of an
electrophore via an alkynyl linkage is particularly attractive
owing to the structural rigidity of a −CC− moiety and its
facility for efficient electron transfer.2 The literature on
electrografting from alkyne precursors is sparse. Halocarbon-
substituted alkynes have been deposited on glassy carbon
electrodes,3 and the anodic reactions of ethynyl Grignards at
surface-hydrogenated silicon have been reported to result in
polymer layers having mostly single and double C−C bonds.4 A
wide-ranging electrochemical method for the covalent grafting
of directly alkynyl-linked electrophores to surfaces has not been
described. We now report such a method, which is based on the
anodic oxidation of molecules having a lithio-activated ethynyl
group generated by standard (n-BuLi in THF) in situ lithiation
of a terminal ethyne. Among the electroactive organometallic
moieties successfully deposited at the one-to-several monolayer
level are those derived from ferrocene, cobaltocenium ion, and
MnCp(CO)3 (cymantrene, Cp = η5-C5H5). Alkynyl-substituted
amines and ethers have also been successfully deposited,
indicating the molecular versatility of an activated alkyne
approach. The ethynyl precursors for the electrografting
reactions are shown below as compounds 1 and 3−6. Strongly
bound modified surfaces have been prepared using a number of
different types of carbon and precious metal electrodes.
Scheme 1
the more facile process (path I in the scheme), the potential for
ferrocene being much lower than that usually encountered for
an alkyne.5 In these cases, production of the alkynyl radical is
likely to occur through an internal electron-transfer (ET)
process, with the electrophore acting as an ET mediator,
analogous to the approach employed by Hernandez-Munoz et
́
̃
al. for the oxidatively promoted loss of CO2 from ferrocenyl
carboxylates.6 In the case of compound 3, the more facile
oxidation is likely to occur directly at the ethynyl group (path II
in Scheme 1) owing to the very high potential required to
oxidize the cobaltocenium ion.5,7 Both approaches have been
used to produce modified electrodes.
Attachment of an ethynylferrocenyl moiety to a glassy carbon
electrode (GCE) follows path I. To an ∼100 mM solution of
ethynylferrocene (1) in THF was added an equimolar amount
of dilute (0.1 M) n-BuLi in THF to produce Fc-CCLi (2),8
where Fc = FeCp(η5-C5H4). After being stirred for only 1 min
at 0 °C, a portion of the “lithio solution” sufficient to make a
nominal 4 mM concentration of 2 was transferred to an
electrochemical cell containing 0.1 M [NBu4][PF6] in THF, at
which point cyclic voltammetry (CV) scans showed two major
Received: December 19, 2012
Published: February 5, 2013
© 2013 American Chemical Society
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anodic waves (gray insert of Figure 1). The irreversible first
wave (A, Epa = −0.10 V vs FcH)9 is assigned to the oxidation of
The question of whether this electrografting approach is
restricted to alkynyl groups directly bonded to cyclopentadienyl
rings was addressed by employing the lithio activation method
to the amine 5 and the ether 6. In the case of 5 a nominal
monolayer was obtained after three CV scans of the lithio
solution (Γ = 3.7 × 10−10 mol/cm2) and multilayer coverage16
has been obtained for 6 (Γ = 33 × 10−10 mol/cm2). Surface-
bound ligand reactivity was observed for electrodes modified by
5, for which the ferrocenyl potential was either E1/2 = 0.13 or
−0.06 V, depending on whether the electrode had been dipped
in acid or base,17 respectively (see Figure 2 and Scheme 2). The
Figure 1. Background-subtracted CV of a GCE modified by 2 in
CH2Cl2/0.1 M [NBu4][PF6], scan rate 0.4 V/s, Γ = 12.3 × 10−10 mol/
cm2. The insert shows CVs in THF/0.05 M [NBu4][B(C6F5)4] (scan
rate 0.1 V/s) of gray line, 4 mM 2; black line, 4 mM 1 and small
amount of decamethylferrocene.
Figure 2. CV of modified GCEs in CH2Cl2/0.1 M [NBu4][PF6] (scan
rate 0.4 V/s). Solid line: electrode modified by amine 5; dashed line:
modified electrode after treatment with 2 M H2SO4, now containing 5-
H+. The original solid line is regenerated after treatment of the 5-H+
electrode with 2 M NaOH.
lithioethynylferrocene, with wave B (E1/2 = 0.13 V) being
assigned to oxidation of the parent, 1.10 If a CV scan at 0.1 V/s
was carried out through wave A alone, followed by transfer of
the electrode to a fresh dichloromethane/0.1 M [NBu4][PF6]
solution, evidence of monolayer-level coverage was obtained.
Higher coverage levels (up to about three monolayers) were
obtained by scanning through both waves A and B (typically,
three times). However, it is important to note that no
deposition was obtained when scanning through the oxidation
of solutions of pure 1, thus confirming the critical role of lithio
activation of the ethynyl group. The electrode modifications are
highly repeatable and their CV behavior in a pure electrolyte
solution (Figure 1) is stereotypical11 of a surface-confined
reversible couple: peak currents are proportional to scan rate,
and there are only small differences in the Epa and Epc values,
the ko value being 15 s−1.12 The E1/2 value of 0.18 V vs FcH is
consistent with the one-electron oxidation of an ethynyl
ferrocene.13 Little if any loss of current is seen in multiple
scans, and the voltammetric performance did not degrade after
storage of the modified GCE for several weeks under ambient
conditions. Estimates of surface coverage (Γ) were obtained by
integration of the CV waves. Based on comparison with the
value of Γ = 4.5 × 10−10 mol/cm2 calculated for an idealized
ferrocene monolayer,14 surface coverages of 1−3 monolayers
are typically obtained.15 Similar high-quality surface-confined
waves were also obtained on edge and basalplane pyrolytic
graphite, platinum, and gold electrodes.
Scheme 2
positive shift of the ferrocenyl potential for the acid-treated
derivative has been verified by independent study of both 5 and
the ammonium salt, 5-H+, in homogeneous solution. This
potential shift results from the electronic influence of the
nitrogen atom on the oxidation at the iron center in 5. The
protonated amine is more electron deficient, resulting in a more
positive oxidation potential.
Metal-based chemical reactions have also been carried out on
the organometallic-modified surfaces. Electrodes which had
been modified by propargyl cymantrene (4) contain an anodic
peak in pure CH2Cl2/0.05 M [NBu4][PF6] at the potential (Epa
= 0.76 V) expected for a cymantrene derivative.18 It is well
known that, compared to their 18-electron counterparts, 17-
electron metal carbonyl complexes have a greatly increased
propensity for substitution of one or more COs by an electron-
donating ligand.19 To test this effect in the surface-bound
manganese tricarbonyl system 4, the modified electrode was
scanned anodically in an electrolyte solution containing
Ethynylferrocene-modified electrodes produced to date do
not exhibit significant “blockage” toward electron-transfer to
redox-active solution analytes (see CV for decamethylferrocene
at an ethynylferrocene-modified GCE, Figure S1).
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P(OMe)3 (Figure S2). A single scan past the oxidation of 4
resulted in loss of most of the cymantrene wave and
introduction of reversible waves for follow-up products at
E1/2 = 0.41 and −0.18 V. The new waves (see square wave
voltammogram in Figure 3, taken in pure electrolyte solution)
features in the range of −0.2 to 1.2 V (Figure S3). Scanning to
1.2 V resulted in an electrode with a surface wave at (E1/2
=
−1.31 V vs FcH, Figure 4) that is consistent with the one-
electron reduction of ethynylcobaltocenium to ethynylcobalto-
cene.22,23
Figure 4. CV of a GCE modified with 3 in THF/0.1 M [NBu4][PF6],
scan rate 0.4 V/s.
Figure 3. Square-wave voltammogram (200 Hz) of modified GCE in
CH2Cl2/0.05 M [NBu4][B(C6F5)4].
In summary, a route to covalently modified electrodes is
described based on the method of anodic oxidation of lithio-
activated24 ethynyl groups. Organometallic electrophores
varying in transition metal, ligand composition, and charge
have been attached at the one-to-several monolayer level to a
number of different electrode surfaces. The surface-bound
molecules undergo either metal-based or ligand-based reactions
which mimic the homogeneous reactions of their parent
molecules. Although the grafting process has conceptual
analogies in other radical-based electrode modification
procedures, including the aryldiazonium reduction1,25 and
carboxylate oxidation6 procedures, an ethynyl-based method
offers the unprecedented advantage of molecule-to-electrode
bonding through a structurally rigid π linkage. Future work will
concentrate on developing the surface chemistry of the
organometallic-modified electrodes and in expanding the
attachment method to include strictly organic electrophores.
are assigned to monosubstituted 7 and disubstituted 8,
respectively (see Scheme 3). Comparisons of the ligand
Scheme 3
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures for the electrode modifications, a
table of potentials relevant to this work, sample cyclic
voltammograms, and the preparation of new compounds 4, 5,
and 6. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
william.geiger@uvm.edu
■
substitution reactions with those observed in homogeneous
solution20 and at a directly cyclopentadienyl-bonded man-
ganese tricarbonyl electrode21 are underway.
Notes
The authors declare no competing financial interest.
Finally, we report a deposition ascribed to direct oxidation of
the lithio-activated ethynyl moiety (path II in Scheme 1). A
lithio solution of the ethynylcobaltocenium ion, 3, which is not
easily oxidizable at the metal center,7 has complex anodic
ACKNOWLEDGMENTS
This work was supported by the National Science Foundation
(CHE -0808909 and -1212339).
■
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alkynyl-linked metallocene monolayer. However, the location of the
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concentration-and counterion-dependent equilibrating systems, in-
cluding Li−C bonded species, ion pairs, aggregates, and solvates. See:
(a) Reich, H. J. J. Org. Chem. 2012, 77, 5471. (b) Bauer, W. In Lithium
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