Mechanistic Origin of Photoredox Catalysis Involving Iron(II) Polypyridyl Chromophores

Article abstract of DOI:10.1021/jacs.0c08389

Photoredox catalysis employing ruthenium- and iridium-based chromophores have been the subject of considerable research. However, the natural abundance of these elements are among the lowest on the periodic table, a fact that has led to an interest in developing chromophores based on earth-abundant transition metals that can perform the same function. There have been reports of using FeII-based polypyridyl complexes as photocatalysts, but there is limited mechanistic information pertaining to the nature of their reactivity in the context of photoredox chemistry. Herein, we report the results of bimolecular quenching studies between [Fe(tren(py)3)]2+ (where tren(py)3 = tris(2-pyridyl-methylimino-ethyl)amine) and a series of benzoquinoid acceptors. The data provide direct evidence of electron transfer involving the lowest-energy ligand-field excited state of the Fe(II)-based photosensitizer, definitively establishing that Fe(II) polypyridyl complexes can engage in photoinduced redox reactions but by a mechanism that is fundamentally different than the MLCT-based chemistry endemic to their second- and third-row congeners.

Full text of DOI:10.1021/jacs.0c08389

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Mechanistic Origin of Photoredox Catalysis Involving Iron(II)
Polypyridyl Chromophores
Matthew D. Woodhouse and James K. McCusker*
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ABSTRACT: Photoredox catalysis employing ruthenium- and iridium-based chromophores have been the subject of considerable
research. However, the natural abundance of these elements are among the lowest on the periodic table, a fact that has led to an
interest in developing chromophores based on earth-abundant transition metals that can perform the same function. There have
been reports of using FeII-based polypyridyl complexes as photocatalysts, but there is limited mechanistic information pertaining to
the nature of their reactivity in the context of photoredox chemistry. Herein, we report the results of bimolecular quenching studies
between [Fe(tren(py)3)]2+ (where tren(py)3 = tris(2-pyridyl-methylimino-ethyl)amine) and a series of benzoquinoid acceptors. The
data provide direct evidence of electron transfer involving the lowest-energy ligand-field excited state of the Fe(II)-based
photosensitizer, definitively establishing that Fe(II) polypyridyl complexes can engage in photoinduced redox reactions but by a
mechanism that is fundamentally different than the MLCT-based chemistry endemic to their second- and third-row congeners.
hotoredox catalysis is one of the most rapidly expanding
polypyridyls as photocatalysts.11 These latter compounds
represent an interesting case insofar as they are isoelectronic
with their ruthenium and iridium congeners, a fact that has led
to an assumption that their mode of action will mirror the
chemistry of their second- and third-row counterparts.
However, important differences in excited-state electronic
structure arise upon shifting to the first transition series,
differences that should significantly impact the nature of their
reactivity in the context of photoredox chemistry.12 While the
realization of a given chemical transformation can sometimes
represent the desired end point of some studies, understanding
the mechanism of photoredox reactions can lead to improve-
ments in catalytic efficiency as well as widening and/or
redefining the scope of a given reaction. We have therefore
undertaken a study to probe the mechanism of a photoredox
reaction involving a Fe(II) polypyridyl complex and a group of
organic electron acceptors in an effort to define the mode of
reactivity for this class of transition metal-based chromophores.
The structure and ground-state absorption spectrum of the
photosensitizertris(2-pyridylmethyliminoethyl)amine iron-
(II)are shown in Chart 1 and Figure 1, respectively. This
compound possesses a low-spin diamagnetic ground state;
excitation in the visible produces a 1MLCT state that converts
to the lowest-energy excited state of the compounda high-
spin metal-centered ligand-field state (5T2)in ∼200 fs that
persists for 55 ns prior to ground-state recovery.13 The
presence of this state is indicated by a loss of absorbance across
P
fields in chemistry.1 Driven by interest in developing new
approaches to known chemistry as well as the discovery of
previously unknown reactions, its transformational impact on
both academic and industrial research is undeniable.2 Central
to this work has been the use of transition-metal-based
compounds possessing excited states capable of engaging in
electron and, more recently, energy transfer reactions with
organic substrates. Notwithstanding efforts by a number of
groups extending this reactivity motif to organic chromo-
phores,3 photoredox catalysis has predominantly leveraged the
well-documented and well-understood photoreactivity endem-
ic to the charge-transfer excited states of second- and third-row
transition metal polypyridyl complexes such as tris(2,2′-
bipyridine)ruthenium(II) ([Ru(bpy)3]2+) and (fac-tris(2-
phenylpyridine)iridium(III) (fac-Ir(ppy)3).4 The metal-to-
ligand charge-transfer (MLCT) states accessed upon visible
light excitation of these types of chromophores possess
lifetimes on the order of microseconds and are capable of
storing on the order of 2 V of energy,5 allowing them to engage
in a wide range of chemical reactions reliant on electron and/
or energy transfer with a given substrate.
Despite all of the obvious advantages (and success) of
ruthenium- and iridium-based photocatalysts, the fact that
these elements are among the rarest in the earth’s crust6 raises
legitimate questions concerning cost and scalability of
processes dependent upon such chromophores. These issues,
coupled with the possibility of unlocking new chemistry, has
motivated interest in exploring the possibility of replacing
these compounds with chromophores based on earth-abundant
transition metals that can carry out analogous excited-state
reaction chemistry.7 Indeed, there have been multiple reports
of copper,8 molybdenum,9 and chromium10 photocatalysts
being used to sensitize photoredox reactions. There have also
been reports of photoredox reactions utilizing iron(II)
Received: August 4, 2020
https://dx.doi.org/10.1021/jacs.0c08389
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
© XXXX American Chemical Society
A

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Chart 1. Fe(II)-Based Photosensitizer ([Fe(tren(py)3)]2+)
and the Series of Benzoquinones Used for the Bimolecular
Quenching Experiments
reduction potentials to serve as electron acceptors in
bimolecular quenching studies (Chart 1), using time-resolved
absorption spectroscopy to probe the kinetics of the reaction
shown in eq 1.
a
−
[Fe(tren(py)3)]2+* + A → [Fe(tren(py)3)]3+ + A
(1)
Data acquired in acetone solution using DDQ as an acceptor
under pseudo-first-order conditions are shown in Figure 2a. It
5
can be seen that the measured lifetime of the T2 state of
[Fe(tren(py)3]2+ systematically decreases with increasing
concentration of DDQ, a clear indication of dynamic
quenching of the ligand-field excited state of the iron(II)
sensitizer. In contrast, analogous experiments using p-TCBQ,
which has a significantly more negative reduction potential
5
than DDQ, reveal an insensitivity of the T2-state lifetime to
a
DDQ = 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone, DCBQ = 2,3-
the presence of the quinone (Figure 2b), suggesting the
absence of any reaction between the two compounds. Similar
data acquired from studies on all five quinones shown in Chart
1 can be found in the Supporting Information and are
summarized in the form of the Stern−Volmer plot shown in
Figure 3.
Dicyano-1,4-benzo-quinone, o-TCBQ = 3,4,5,6-Tetrachloro-1,2-ben-
zoquinone, o-TBBQ = 3,4,5,6-Tetrabromo-1,2-benzoquinone, and p-
TCBQ = 2,3,5,6-Tetrachloro-1,4-benzoquinone.
As we have emphasized previously concerning mechanistic
studies of photoredox reactions,5a a Stern−Volmer plot serves
to establish that a reaction is occurring between the excited
state of a sensitizer and a quencher, but it does not address the
question of mechanism: energy transfer (both dipolar (i.e.,
̈
Forster) and exchange (Dexter)) as well as electron transfer
can all produce identical Stern−Volmer plots. Since [Fe(tren-
(py)3)]2+ is nonemissive, Forster transfer can be immediately
̈
ruled out. Dexter transfer does not require significant radiative
coupling between the excited state of the sensitizer and its
ground state; however, this mechanism can also be ruled out in
5
the present case because coupling of the T2 excited state of
[Fe(tren(py)3)]2+ to either a singlet or triplet excited state of
the quinone acceptor would not conserve angular momentum
for the overall process and is therefore forbidden.16
An electron transfer quenching mechanism is best
established via spectroscopic identification of the oxidized
donor and/or the reduced acceptor. In the present case,
overlapping features in the absorption profiles of the 5T2
excited state of [Fe(tren(py)3)]2+ (Figure 1, inset), [Fe(tren-
(py)3)]3+, and the semiquinone forms of the quenchers (Figure
S8) make it challenging to do this using electronic absorption
spectroscopy. We argue that the exclusion of energy transfer as
a viable mechanism combined with the clear correlation
between quenching dynamics and the reduction potential of
the quinones used in this study (Table 1) makes for a
compelling case that electron transfer is the operative
mechanism giving rise to the reaction dynamics reflected in
Figure 3.
Figure 1. Electronic absorption spectrum of [Fe(tren(py)3)]2+
acquired in CH3CN solution. The inset shows the compound’s
differential absorption spectrum following A1 → MLCT excitation
at 580 nm and is characteristic of the lowest-energy ligand-field
excited state of the compound.
1
1
the visible spectrum, as shown in the inset of Figure 1. Given
the fact that this ligand-field excited state is formed on a time
scale that is orders of magnitude faster than diffusion, the
catalytically relevant excited state for any bimolecular photo-
5
redox chemistry exhibited by this compound will be this T2
state.14
Given the difficulty with determining the zero-point energy
of the ligand-field excited state alluded to above, we can look to
the thermodynamic properties of the quinone acceptors,
specifically, the cutoff potential for reactivity, as a means of
estimating the effective redox potential for the reactive state of
the iron complex.17 The data in Table 1 clearly show that
quenching is suppressed in the case of o-TBBQ and p-TCBQ.
Assuming this break corresponds to a transition from an
exergonic to an endergonic reaction, we estimate an effective
Unlike ruthenium(II) and iridium(III) polypyridyl com-
plexes, the lowest-energy excited states of iron(II) polypyridyls
are typically nonemissive due to a lack of radiative coupling
between two electronic states that differ by two units of spin
1
5
(i.e., S = 0 and S = 2 for the A1 and T2 ground and lowest-
energy excited states of [Fe(tren(py)3)]2+, respectively).
Therefore, the zero-point energy (E00) of the photoreactive
excited state, which is typically obtained from an emission
spectrum and is needed to determine the redox potential
associated with the excited state based on the Rehm−Weller
formalism,15 cannot be directly measured in these cases. We
therefore chose a series of benzoquinones spanning a range of
5
excited-state oxidation potential of the T2 state of [Fe(tren-
(py)3)]2+ in acetone of ∼ −0.35
0.05 V versus the
ferrocene/ferrocenium couple; corresponding data acquired
in CH3CN solution afford a value of ∼ −0.25 0.05 V and are
B
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Figure 2. (a) Single-wavelength kinetics of transient absorption quenching experiments between [Fe(tren(py)3)]2+ and DDQ in acetone solution
with 0.1 M of TBAPF6 as supporting electrolyte. The data were acquired at 560 nm following excitation at 580 nm and are probing the loss of the
5
transient signal associated with the T2 ligand-field excited state of [Fe(tren(py)3)]2+ illustrated in the inset of Figure 1. (b) Corresponding data
from quenching experiments with [Fe(tren(py)3)]2+ and p-TCBQ, also in acetone solution with 0.1 M TBAPF6. The invariance of the time
5
constant indicates the absence of quenching of the T2 excited state under these conditions.
Figure 3. Stern−Volmer plots of bimolecular quenching studies between [Fe(tren(py)3)]2+ and the quinones indicated (see Chart 1). All studies
were carried out in acetone solution with 0.1 M TBAPF6. The data were acquired via nanosecond time-resolved absorption measurements
following 1A1
→
1MLCT excitation of the iron-based chromophore. Details concerning the analysis of the data, as well as corresponding
measurements carried out in CH3CN solution, can be found in the Supporting Information. Bimolecular quenching constants (kq) calculated from
the slopes of the lines shown are collected in Table 1.
transfer is the operative reaction mechanism in this chemistry,
as energy transfer processes are far less sensitive to variations in
solvent dielectric.
Table 1. Reduction Potentials and Quenching Rate
Constants for Quenchers in Acetone with 0.1 M TBAPF6
a b
,
Quencher
Ered (V)
kq (M−1 s−1
)
ΔG (V)
In summary, with this report we have provided the first
direct evidence supporting an electron transfer mechanism in a
photoredox reaction involving an iron(II)-based polypyridyl
chromophore. Based on the well-documented electronic
structure and dynamics of the iron complex used, this study
definitively establishes that the electron transfer chemistry
involves a metal-centered ligand-field excited state of the
chromophore. This makes it fundamentally different from the
charge transfer-based photoredox chemistry associated with
chromophores of the second and third transition series more
commonly employed in the photoredox catalysis community.
We believe this mechanistic distinction is important insofar as
the types of chemical modifications needed in order to alter the
DDQ
0.07
−0.15
−0.33
−0.35
−0.42
2.7 × 105
2.6 × 105
2.1 × 105
−
−0.41
−0.19
−0.01
0.01
DCBQ
o-TCBQ
o-TBBQ
p-TCBQ
−
0.08
b
a
Potentials are relative to the ferrocene/ferrocenium couple. The
Fe(II)/Fe(III) couple sits at +0.51 V and +0.55 V in acetone and
acetonitrile solutions, respectively (see Figures S4 and S7).
provided in the Supporting Information. The ca. 0.1 V shift in
the effective driving force for the reaction in response to the
change in solvent is yet another indication that electron
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Communication
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ligand-field in nature. More detailed mechanistic studies,
including a wider range of spectroscopic probes as well as
additional iron(II)-based sensitizers designed to expand the
scope of this chemistry, are currently underway.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c08389.
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Synthesis, characterization, cyclic voltammetry, spec-
troelectrochemistry, ground-state absorption, and tran-
sient absorption single wavelength data (PDF)
AUTHOR INFORMATION
Corresponding Author
■
(5) (a) Arias-Rotondo, D. M.; McCusker, J. K. The Photophysics of
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Bimolecular Electron Transfer Reactions of the Excited States of
Transition Metal Complexes. Top. Curr. Chem. 1978, 75, 1−64.
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Critical Resources for High Technology. U.S. Geological Survey, 2002.
https://pubs.usgs.gov/fs/2002/fs087-02/ (accessed 2020-05-27).
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Metals. J. Am. Chem. Soc. 2018, 140, 13522−13533. (b) Larsen, C. B.;
Wenger, O. S. Photoredox Catalysis with Metal Complexes Made
from Earth-Abundant Elements. Chem. - Eur. J. 2018, 24, 2039−2058.
(c) Wenger, O. S. Is Iron the New Ruthenium? Chem. - Eur. J. 2019,
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James K. McCusker − Department of Chemistry, Michigan
State University, East Lansing, Michigan 48824, United States;
orcid.org/0000-0002-5684-3117; Email: jkm@
chemistry.msu.edu
Author
Matthew D. Woodhouse − Department of Chemistry, Michigan
State University, East Lansing, Michigan 48824, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c08389
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(8) (a) Minozzi, C.; Caron, A.; Grenier-Petel, J. C.; Santandrea, J.;
Collins, S. K. Heteroleptic Copper(I)-Based Complexes for Photo-
catalysis: Combinatorial Assembly, Discovery, and Optimization.
Angew. Chem., Int. Ed. 2018, 57, 5477−5481. (b) Reiser, O. Shining
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Atom Transfer Radical Addition Reactions and Related Processes.
Acc. Chem. Res. 2016, 49, 1990−1996. (c) Hernandez-Perez, A. C.;
Collins, S. K. Heteroleptic Cu-Based Sensitizers in Photoredox
Catalysis. Acc. Chem. Res. 2016, 49, 1557−1565. (d) Knorn, M.;
Rawner, T.; Czerwieniec, R.; Reiser, O. [Copper(phenanthroline)-
(bisisonitrile)]+-Complexes for the Visible-Light-Mediated Atom
Transfer Radical Addition and Allylation. ACS Catal. 2015, 5,
5186−5193.
This work was generously supported through BioLEC
(Bioinspired Light-Escalated Chemistry), an Energy Frontier
Research Center funded by the Office of Basic Energy
Sciences, Office of Science, U.S. Department of Energy,
under Award number DE-SC0019370. M.D.W. would like to
thank Dr. Sara Adelman and Dr. Daniela Arias-Rotondo for
thoughtful advice.
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(14) It should be noted that the electronic structures and dynamics
of [Fe(tren(py)3)]2+ and compounds such as [Fe(bpy)3]2+ and
[Fe(phen)3]2+the latter two having been employed as photoredox
catalysts by several researchers (cf. ref 11)are identical with regard
to the ultrafast nature of MLCT-state deactivation (∼100 fs), the
identity of the ligand-field excited states that lie below the initially
formed MLCT state, as well as their relative energies. The only
significant difference is that the weaker ligand-field strength of the
tren(py)3 ligand lowers the energies of the ligand-field excited states.
5
This will attenuate the redox potential associated with the T2 state
and thereby affect the types of reactions the photocatalyst can engage
in, but not its innate reactivity. The increase in lifetime (55 ns versus
5
∼1 ns for the T2 state of [Fe(bpy)3]2+ in CH3CN) has the added
benefit of facilitating its study using the nanosecond time-scale
methods employed here.
(15) (a) Rehm, D.; Weller, A. Kinetik und Mechanismus der
̈
̈
Elektronubertragung bei der Fluoreszenzloschung in Acetonitril. Ber.
Bunsen-Ges. 1969, 73, 834. (b) Vlcek, A. A.; Dodsworth, E. S.; Pietro,
W. J.; Lever, A. B. P. Excited State Redox Potentials of Ruthenium
Diimine Complexes. Correlations with Ground State Redox Potentials
and Ligand Parameters. Inorg. Chem. 1995, 34, 1906−1913.
(c) Rehm, D.; Weller, A. Kinetics of Fluorescence Quenching by
Electron and H-Atom Transfer. Isr. J. Chem. 1970, 8, 259−271.
(16) The excited state of [Fe(tren(py)3)]2+ is a quintet (i.e., S = 2).
Since the ground states of the quinone and [Fe(tren(py)3)]2+ are both
diamagnetic (S = 0), only an excited state of the quinone having S = 2
could engage in energy transfer and yield a process for which ΔS = 0.
For additional details concerning the issue of angular momentum
conservation in energy transfer, see: Guo, D.; Knight, T. E.;
McCusker, J. K. Angular Momentum Conservation in Dipolar Energy
Transfer. Science 2011, 334, 1684−1687.
(17) The quenching dynamics depend upon the thermodynamics of
the reactant pair but are also a reflection of the kinetics. The latter are
affected by both the driving force of the reaction and the associated
reorganization energy. Conversion of high-spin Fe(II) to low-spin
Fe(III), which is the process that accompanies the quenching process
E
https://dx.doi.org/10.1021/jacs.0c08389
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX