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Phys. Chem. Chem. Phys.
, 2021,
23
, 9921–9929 |
9921
Cite this:
Phys. Chem. Chem. Phys.
,
2021,
23
, 9921
Design of robust 2,2
0
-bipyridine ligand linkers for
the stable immobilization of molecular catalysts
on silicon(111) surfaces
Samantha I. Johnson,
ab
James D. Blakemore,
c
Bruce S. Brunschwig,
d
Nathan S. Lewis,
de
Harry B. Gray,
e
William A. Goddard III
*
ae
and
Petter Persson
*
b
The attachment of the 2,2
0
-bipyridine (bpy) moieties to the surface of planar silicon(111) (photo)electrodes
was investigated using
ab initio
simulations performed on a new cluster model for methyl-terminated
silicon. Density functional theory (B3LYP) with implicit solvation techniques indicated that adventitious
chlorine atoms, when present in the organic linker backbone, led to instability at very negative potentials of
the surface-modified electrode. In prior experimental work, chlorine atoms were present as a trace surface
impurity due to required surface processing chemistry, and thus could plausibly result in the observed
surface instability of the linker. Free energy calculations for the Cl-atom release process with model silyl-
linker constructs revealed a modest barrier (14.9 kcal mol

1
) that decreased as the electrode potential
became more negative. A small library of new bpy-derived structures has additionally been explored
computationally to identify strategies that could minimize chlorine-induced linker instability. Structures with
fluorine substituents are predicted to be more stable than their chlorine analogues, whereas fully non-
halogenated structures are predicted to exhibit the highest stability. The behavior of a hydrogen-evolving
molecular catalyst Cp*Rh(bpy) (Cp* = pentamethylcyclopentadienyl) immobilized on a silicon(111) cluster
was explored theoretically to evaluate differences between the homogeneous and surface-attached
behavior of this species in a tautomerization reaction observed under reductive conditions for catalytic H
2
evolution. The calculated free energy difference between the tautomers is small, hence the results suggest
that use of reductively stable linkers can enable robust attachment of catalysts while maintaining chemical
behavior on the electrode similar to that exhibited in homogeneous solution.
Introduction
In artificial photosynthesis, sol
ar photons with the aid of suitable
light absorbers and electrocatalyst
s drive reactions that split water,
evolving dioxygen (O
2
).
1
The protons and electrons produced in
this process can form chemical fuels, either as H
2
or as other
energy-rich products arising, for example, from reduction of
carbon dioxide (CO
2
). Many technical challenges remain in success-
ful implementation of artificial photosynthesis, including facile
approaches to interface catalysts with light absorbers and other
device components. Molecular electrocatalysts can be selective,
are readily tuned, and have been successfully demonstrated
to produce solar fuels at reasonable rates.
2–5
Heterogeneous
catalytic systems
6,7
offer the advantages of simplified product
separation and longevity. Hence, immobilization of a homo-
geneous, molecular catalyst on the surface of an electrode can,
in principle, be beneficial,
8–14
while potentially avoiding pitfalls
such as catalyst deactivation by dimerization or aggregation,
15
or insolubility in the chosen solvent/electrolyte.
16
Extensive effort has focused on preparation of molecular
catalysts with reactive functionalities that enable polymerization
of catalysts onto, for example, conducting carbon electrodes.
17–19
Noncovalent interactions such as
p
p
interactions have also been
exploited to promote association between polycyclic aromatic
groups appended to a catalyst and the surfaces of graphitic
electrodes. For example, a pyrene-appended Re(CO)
3
(bpy)Cl
catalyst exhibited selective conversion of CO
2
to CO when the
complex was immobilized on a graphitic carbon electrode.
a
Materials Research Center, California Institute of Technology, Pasadena, CA,
91125, USA. E-mail: wag@wag.caltech.edu
b
Theoretical Chemistry Division, Chemistry Department, Lund University, Box 124,
SE-221 00 Lund, Sweden. E-mail: Petter.Persson@teokem.lu.se
c
Department of Chemistry, University of Kansas, Lawrence, Kansas, 66045, USA
d
Molecular Materials Resource Center, Beckman Institute, California Institute of
Technology, Pasadena, California 91125, USA
e
Division of Chemistry and Chemical Engineering, California Institute of
Technology, Pasadena, CA, 91125, USA
Electronic supplementary information
(ESI) available. See DOI: 10.1039/d1cp00545f
Received 5th February 2021,
Accepted 6th April 2021
DOI: 10.1039/d1cp00545f
rsc.li/pccp
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However, loss of catalytic activity attributed to loss of the catalyst
from the electrode was observed on a timescale of hours.
20
Similarly, a pyrene-appended iridium POCOP (POCOP =
k
3
-C
6
H
3
-
2,6-(OP
t
Bu
2
)
2
) catalyst was immobilized onto a carbon nanotube
gas diffusion electrode and used for formate production from CO
2
,
yielding a turnover number (TON) of
4
54 000 with little loss in
activity.
21
The performance of the surface-attached Ir POCOP system
can be attributed, in part, to insolubility of the organometallic
iridium species in the aqueous elec
trolyte used for electrocatalysis.
Covalent attachment of electrocatalysts to (photo)electrode
surfaces might therefore be expected to result in more stable
systems than those that only exploit noncovalent interactions.
Covalent attachment of catalysts to metallic or semiconducting
electrode surfaces exploits a variety of ligands and attachment
strategies.
22
Stable systems include attachment of a Co por-
phyrin to a conductive diamond electrode.
23
In this system, a
relatively long (
4
9CH
2
units) sp
3
-hybridized alkane group with
a terminal azide was coupled using a Cu
I
-catalyzed click reaction
(CuAAC) to a conductive diamond surface that contained a function-
alized alkyne. The catalyst was stable for at least 1000 electro-
chemical cycles and showed a turnover frequency (TOF) of
0.8 s

1
for reduction of CO
2
to CO.
23
In related work, Fe
4
N
clusters have been attached to glassy carbon electrodes using
similar click reactions, yielding stability for
4
100 h and improved
turnover numbers relative to the homogeneous system.
24
Other
carbon-bound catalyst systems include Co and Ni
25,26
catalysts
grafted onto glassy carbon surfaces, as well as Rh and Co
27,28
catalysts bound to graphite. Ni(P
2
N
2
)
2
hydrogen-evolution catalysts
have also been bound to glassy carbon electrodes, although due
to Ni–P bond cleavage the catalysts decompose in acidic
acetonitrile.
29
H
2
evolution catalysts have been attached to
semiconducting GaP electrodes,
30,31
and Re catalysts have been
bound to SiO
2
32
and TiO
2
.
15,33
To obtain Si surfaces modified with vinyl ferrocene, a chlori-
nated Si(111) surface was first reacted with vinyl-tagged ferrocene.
The remaining surface sites were then terminated with methyl
groups, using a methyl Grignard reagent.
34,35
These covalently
bound ferrocene systems exhibited stable redox cycling on both
planar silicon(111)
31,36
and Si microwire arrays.
37
Highlighting
the challenge of understanding chemical reactivity occurring on a
semiconductor surface, only small amounts of chlorine were
detected by X-ray photoelectron (XP) spectroscopy, and an infrared
vibrational stretch associated with formation of a C–C double bond
in the linker group was not observed.
38
Silicon(111) surfaces have
also been modified using UV-induced attachment of 4-vinyl-2,2
0
-
bipyridyl. Following attachment of
this organic group, the bipyridine
on the surface was readily metalated to form surface-attached
[Cp*Rh], [Cp*Ir], and [Ru(acac)
2
] complexes. X-ray photoelectron as
well as X-ray absorption spectro
scopic data on the immobilized
[Cp*Rh] complexes are consistent with formation of the desired
metal complexes. However, when the surface-modified electrode was
biased to the negative potentials required to reduce Rh(
III
)toRh(
I
)
(centered at
B

1V
vs.
ferrocenium/ferrocene), the complexes, and
substituted bipyridyl ligand, were lost from the electrode surface
within the timescale of three compl
ete cyclic voltammetric cycles.
36
Computational approaches have been explored to understand
the stabilities and activities of a variety of surface-immobilized
redox-active compounds on semiconductor surfaces. Extensive
work has been performed on photoanodes in dye-sensitized solar
cells (DSSC). Computation has also revealed important phenom-
ena and reactivity in molecular catalysis in particulate photo-
catalytic systems
33,39
as well as for immobilized molecular
catalysts deposited on Au clusters (for comparison of theoretical
structures to experimental data).
40
Typically, molecular calculations
are completed with a discrete treatment using Gaussian basis sets,
41
while surfaces are approached using a periodic formalism employ-
ing plane wave basis sets.
42,43
Surface attached molecular entities
are challenging because they require treatment at both length
scales. One way to bridge between these length scales is to use
a molecular cluster, a large molecule of units approximating the
surface.
44
The active molecule can then be appended onto this
cluster. This approach allows us to maximize the accuracy of the
properties of the molecular entities and has been used in
catalytic
33,40
and in DSSC literature.
45–47
We report herein a computational investigation of the 4-vinyl-
2,2
0
-bipyridine linker system attached to Si(111)
via
a photo-
initiated olefin immobilization. The results, which provide a
plausible failure mechanism supported by the available spectro-
scopic data, inform a predictive paradigm of alternative linker
and catalyst structures that could afford reductively stable
surface-immobilized catalysts on Si(111). The results indicate
that chlorine used as part of the surface-attachment process is
expected theoretically to deleteriously affect catalyst stability
when the electrode is biased to very negative electrode potentials.
We have additionally investigated theoretically the effect of
surface attachment on the behavior of surface-attached Rh-based
H
2
evolution catalysts. This work i
s relevant to the important role
played by metal-bpy groups in redox catalysis in conjunction
with the beneficial use of such systems when attached to
electrode surfaces.
48–51
Methods
Geometry optimizations were performed using the B3LYP
functional
52,53
with the Los Alamos small core potential on
transition metals.
54
Double-zeta basis sets were used on transition
metals and the 6-311G** basis set was used on organic atoms.
41,55
A Poisson–Boltzmann polarizable
continuum acetonitrile solvent
model was also used in geometry optimizations. Iso values of 0.005
were evaluated for spin density and orbital plots. Silicon clusters
were initially cut along the (111) plane using Crystal,
56
and were
then trimmed to minimize the number of doubly H-terminated
Si atoms. A similar method has been used previously for oxide
clusters.
57,58
The clusters were large enough to enable a full
ring of CH
3
–Si bonds around the primary bond, minimizing the
outward bending of neighboring CH
3
–Si units. This bending
has been observed in smaller clusters and affects the electronic
behavior of the system.
59
Due to their large size, the energies
for large clusters consisted only of electronic and solvation
components.
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Phys. Chem. Chem. Phys.
, 2021,
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, 9921–9929 |
9923
(CH
3
)
3
Si)
3
Si-Moieties were used for mechanistic assessment,
including transition state calculations for dissociation, because
frequency calculations were computationally intractable on
larger clusters. All (CH
3
)
3
Si)
3
Si-species were optimized using
the methods described above; however, full free energies were
calculated from single-point electronic energies, entropies, and
enthalpies, in accordance with previously used methods.
60,61
Single-point energies were calculated as follows:
G
=
E
M06
+
G
solv
+
E
ZPE
+
H
vib
+
H
TR

T
(
S
vib
+
S
elec
)
Zero-point energies,
E
ZPE
, vibrational enthalpies,
H
vib
, and
vibrational and electronic entropies,
S
vib
and
S
elec
respectively,
were obtained from frequency calculations. Translational and
rotational enthalpies were taken as 12/2
k
B
T
. Single-point electro-
nic energies were calculated with the M06 functional
62
and the
6-311G**++ basis set.
63,64
Frequency calculations provided the
entropic and enthalpic terms. Transition states were verified
using these frequency calculations and intrinsic reaction
coordinate calculations. All calculations were performed in
Jaguar,
59
except where otherwise noted.
Results and discussion
Experimental and computational work on H-terminated Si
surfaces has shown that photoexcitation of Si–H bond electrons
leads to H-atom loss, producing a dangling Si radical.
65
This
radical then reacts with vinyl-tagged molecules, affording
a radical on the
b
-carbon of the vinyl linker.
65,66
For a
Cl-terminated Si(111) surface, the analogous attachment mecha-
nism would yield a Cl on the
b
-carbon of the linker. For ease of
reference, the carbon of the linker furthest from the bpy moiety is
referred to as the
a
-carbon and the one closer to bpy is referred to
as the
b
-carbon, while the subscript Si is used to indicate attach-
ment to the Si cluster. Support for this process is evident by
comparing the calculated free
energies of the various bpy con-
formations attached to Si(111) clusters with chlorine (Fig. 1). In
1
Si
,
which provides the lowest energy conformation, the Cl is on the
b
-carbon. The energy of
2
Si
,withClonthe
a
-carbon, is higher by
11.9 kcal mol

1
, largely due to an unfavorable steric interaction
with neighboring methyl groups b
onded to surficial Si atoms. The
sp
2
analog of the linker shown in Fig. 1 as
3
Si
,inwhichHClis
lost in the linking process, is higher than the energy of
1
Si
by
7.1 kcal mol

1
(due to the lack of steric repulsion in
1
Si
).
To investigate the effect of chlorination on the behavior of
the surface-bound complex, large basis set calculations were
performed on bipyridine with Cl substituted on the linker
(Fig. 2). The molecules were singly reduced, in accord with
expectations for the surface chemistry under catalytic conditions.
The computations suggest that the reduced complexes
2
and
3
should remain intact, whereas the reduced complex
1
would
release a chloride ion under reaction conditions, yielding bpy
with a two-carbon radical linker. Spin density is useful in
qualitatively determining the ‘‘location’’ of radical population
upon reduction. The radical doublet is primarily centered on the
b
-carbon of the linker, as indicated by atomic charges and spin
populations (see Table 1). As shown in Fig. 3, the spin density of
the reduced complex extends from a pyridine ring into the linker.
The bpy ligand can be reversibly reduced in solution,
67
so it
should be able to host an extra electron without participation of
the linker. However, the linker is theoretically expected to
decompose upon reduction when Cl is initially on the
b
carbon
atom (complex
1
).
When the reduction process was evaluated computationally
with complex
1
attached to a Si cluster, the same decomposi-
tion process was observed, and chlorine dissociated. The spin
density plot of the attached complex (Fig. 4) was similar to that
of the molecular species (Fig. 3).
The molecular and cluster calculations collectively suggest a
decomposition pathway reliant on the presence of chlorine in the
system. During the attachment process, a chlorine radical,
formed by photoexcitation that cleaves the Si–Cl bond, is ejected.
Fig. 1
Relative energies (compared to
1
Si
) for various Cl binding motifs.
Fig. 2
Molecular complexes with chlorinated linkers.
Table 1
Mulliken charges and spin populations of the reduced bpy
complex, (
1
)
Atom
Atomic charges
Spin
Cl

0.889
0.020
b
-Carbon

0.365
0.693
a
-Carbon

0.105

0.056
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