Incorporation of redox-inactive cations promotes iron catalyzed aerobic C-H oxidation at mild potentials

Article abstract of DOI:10.1039/c7sc04486k

The synthesis and characterization of the Schiff base complexes Fe(ii) (2M) and Fe(iii)Cl (3M), where M is a K⁺ or Ba²⁺ ion incorporated into the ligand, are reported. The Fe(iii/ii) redox potentials are positively shifted by 440 mV

Full text of DOI:10.1039/c7sc04486k

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Incorporation of redox-inactive cations promotes
iron catalyzed aerobic C–H oxidation at mild
potentials†
Cite this: DOI: 10.1039/c7sc04486k
*
Teera Chantarojsiri, ‡ Joseph W. Ziller and Jenny Y. Yang
The synthesis and characterization of the Schiff base complexes Fe(II) (2M) and Fe(III)Cl (3M), where M is a K+
or Ba2+ ion incorporated into the ligand, are reported. The Fe(III/II) redox potentials are positively shifted by
440 mV (2K) and 640 mV (2Ba) compared to Fe(salen) (salen ¼ N,N0-bis(salicylidene)ethylenediamine), and
by 70 mV (3K) and 230 mV (3Ba) compared to Fe(Cl)(salen), which is likely due to an electrostatic effect
(electric field) from the cation. The catalytic activity of 3M towards the aerobic oxidation of allylic C–H
bonds was explored. Prior studies on iron salen complexes modified through conventional electron-
donating or withdrawing substituents found that only the most oxidizing derivatives were competent
catalysts. In contrast, the 3M complexes, which are significantly less oxidizing, are both active.
Mechanistic studies comparing 3M to Fe(salen) derivatives indicate that the proximal cation contributes
to the overall reactivity in the rate determining step. The cationic charge also inhibits oxidative
deactivation through formation of the corresponding Fe2-m-oxo complexes, which were isolated and
characterized. This study demonstrates how non-redox active Lewis acidic cations in the secondary
coordination sphere can be used to modify redox catalysts in order to operate at milder potentials with
a minimal impact on the reactivity, an effect that was unattainable by tuning the catalyst through
traditional substituent effects on the ligand.
Received 17th October 2017
Accepted 28th January 2018
DOI: 10.1039/c7sc04486k
rsc.li/chemical-science
a major challenge in the optimization of catalysts for both high
activity and energetic efficiency (low overpotential).
Introduction
Various strategies have been successfully employed to
improve redox reactivity at mild potentials, such as enzyme-
inspired cooperative or secondary interactions.8–36 An addi-
tional approach would utilize the secondary coordination
sphere to adjust the redox properties. In nature, only three
metal–ligand motifs are commonly used to mediate electron
transfer (iron–sulfur clusters, hemes, and cupredoxins), yet the
measured redox potentials of each cofactor span a wide range
depending on their microenvironment.37 Elegant studies
employing site-directed mutagenesis in articial metal-
loenzymes demonstrate how secondary coordination sphere
interactions, such as hydrogen-bonding or hydrophobicity,
Transition metal complexes are ubiquitous as redox catalysts
because they can both activate substrates and mediate electron
transfer through multiple accessible redox states. Catalytic
function is conventionally optimized through inductive effects
by modifying the supporting ligand with electron-withdrawing
or -donating substituents (Chart 1, le).1 Modifying ligands in
this fashion impacts both the electronic structure of the metal
as well as the redox properties, which can lead to an inverse
correlation between the activity and operation at mild redox
potentials. Scaling relationships of this type are observed for
many classes of homogeneous oxidation catalyst,2,3 as well as for
polymerization catalysts.4 The trade-off between catalytic
activity and function at mild potentials has also been found in
electrocatalysts for O2,5 CO2,6 and H+ (ref. 7) reduction, and is
Department of Chemistry, University of California, Irvine, 92697, USA. E-mail:
j.yang@uci.edu
† Electronic supplementary information (ESI) available: General experimental
conditions and synthesis, solid state structure images, crystal data and
renement, UV-visible spectra, EPR spectra, and oxidation products under
rigorously dry conditions. CCDC 1580343, 1580342, 1580341, 1580340, and
1580339. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c7sc04486k
‡ Present address: Department of Chemistry, Mahidol University, Bangkok, 10400
Thailand.
Chart 1
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adjust the redox potential.38–40 Local electric eld (electrostatic)
These results demonstrate that the installation of a proximal
effects have also been cited as a key component for the high cation can provide another “knob” to tune the redox reactivity.
catalytic activity found in enzymes41,42 and are emerging as an In contrast to adjusting the ligand eld strength through elec-
important tool for achieving selective or increased reactivity in tronic effects in the primary coordination sphere, modications
organic synthesis.43–59 However, there are fewer examples of to the secondary coordination sphere are effectively used to
applying electrostatic interactions to enable more efficient improve the redox activity at a milder potential.
redox catalysis.60–62
Transition metal complexes that contain crown ether-like
functionalities to encapsulate alkali or alkaline earth metal
ions have been reported for a variety of applications.63–92 We
Results and discussion
Synthesis and characterization
The preparation of the ligands 1M (M ¼ K+ or Ba2+, Scheme 1)
recently reported that the incorporation of non-redox active
cations in a Co(II) Schiff base complex leads to signicant
positive shis in the Co(II/I) redox potential,93 an effect that has
been observed in other synthetic systems.94–101 Notably, our
spectroscopic studies indicate that the shi in redox potential is
likely due to an electric eld potential from the cation, since the
electronic structure of the metal is not signicantly perturbed.93
We were interested in exploring how cation incorporation
would impact the redox reactivity compared to complexes tuned
through traditional inductive modications. To this end, we
examined the effect of proximal cations on iron catalysts for the
aerobic allylic C–H oxidation of cyclohexene. Prior studies on
iron Schiff base complexes have demonstrated a strong corre-
lation of the Fe(III/II) redox potential (Table 3) with the overall
catalytic activity.102 When inductive effects were used to modify
the iron Schiff base complexes, catalysis was only observed for
the tetranitro substituted complexes, which have potentials
positive of ꢀ0.27 V vs. [Fe(Cp)2]+/0. For these complexes, the
activity increases with the more oxidizing redox potentials.102
For this study we synthesized the iron complexes 3M, shown
in Chart 1 and Scheme 1, which have either a proximal M ¼ K+
or Ba2+ ion in the secondary coordination sphere. 3K and 3Ba
are both active catalysts despite having Fe(III/II) redox potentials
that would be insufficient for catalysis if they had been tuned by
installing electron withdrawing groups.
has previously been reported.93,102 The corresponding Fe(II) and
Fe(III)Cl complexes, 2M and 3M, were synthesized by metal-
lation of the ligand with Fe(OAc)2 in methanol (MeOH) and
anhydrous FeCl3 in ethanol (EtOH), respectively. The products
were recrystallized by the vapor diffusion of diethyl ether (Et2O)
into an acetonitrile (CH3CN) solution of the complex. 2M reacts
with air in MeOH or CH3CN to form the diiron m-oxo species
(4M), while the Fe(III)Cl complexes (3M) are air-stable. The 4M
complexes were isolated by recrystallization (vapor diffusion of
Et2O into concentrated CH3CN solution). The purity and
formulation of the complexes were conrmed using elemental
analysis and mass spectrometry. The corresponding complexes
Fe(salen) and Fe(Ph2salenCl4) (ligands for compound A and C,
respectively, in Table 3), which have a similar primary coordi-
nation environment but lack proximal cations, were synthesized
according to previously published procedures in order to
compare their reactivity with 3M, as they have similar redox
potentials.102
X-ray crystallography. Crystals of 2Ba, 3K, 3Ba, 4K, and 4Ba
suitable for single crystal X-ray analysis were grown under the
same conditions used for the recrystallization. The ORTEPs are
shown in Fig. 1 and the structural parameters are provided in
Table 1. The coordination geometries around the Fe(II) and
Fe(III) ions are distorted square pyramidal, with the s5 value for
all complexes between 0.11 and 0.36, (where s5 ¼ 0 for a square
pyramidal geometry and s5 ¼ 1 for a trigonal bipyramidal
geometry). These values are similar to those for the previously
reported Fe(Cl)(salen) and Fe(salen) complexes without the
crown-appendage.115–117 3M and 4M have lower s5 values for M ¼
Ba2+ compared to M ¼ K+. Although the ionic radii of K+ and
Ba2+ are similar, with values of 138 and 135 pm, respectively, the
Fe–Ba distance is longer than the corresponding Fe–K distance
in 3M. We attribute this difference to the stronger charge
repulsion between the Fe3+ ion and dicationic Ba2+.
Mechanistic studies with 3M and 2M indicate that the K+ or
Ba2+ ion facilitates the rate determining step. Aside from their
role in adjusting the redox potential, non-redox active Lewis
acidic metals are also known to play a role in the activation of
dioxygen and the corresponding oxidizing intermedi-
ates.101,103–114 Thus, the incorporation of non-redox active Lewis
metals in the secondary coordination sphere also provides
a route toward cooperative reactivity. Increasing the cationic
nature of the Fe intermediates also inhibits the major deacti-
vation pathway, which is formation of a diiron m-oxo complex.
Scheme 1
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Fig. 1 Solid-state structures of 2Ba, 3K, 3Ba, 4K, and 4Ba. Hydrogen atoms and non-coordinating anions are omitted for clarity. 4K contains two
outer sphere triflate ions. Ellipsoids are drawn at 50% probability.
Table 1 Structural parameters of 2Ba, 3K, 3Ba, 4K, and 4Ba, and the Fe(III/II) redox potentials vs. [Fe(Cp)2]+/0 in CH3CN for 2M, 3M, and selected
iron salen complexes lacking a proximal cation for comparison
Fe–Cl
distance (A)
˚
˚
˚
Compound
Fe–M distance (A)
s5
Fe–O distance (A)
E1/2 (FeIII/II) (V)
2K
2Ba
Fe(salen)
3K
—
—
0.11
—
0.36
0.13
0.20
0.30
0.18
0.27, 0.38
—
—
—
2.218
2.228
2.238
—
—
—
—
—
—
—
—
—
1.787
1.771
ꢀ0.29
3.6592(4)
—
3.6596(6)
3.8115(3)
ꢀ0.09
ꢀ0.73
ꢀ0.69
3Ba
ꢀ0.53
[Fe(Cl)(salen)] (A)102,115
—
ꢀ0.76 (ref. 102)
4Ka
4Ba
3.704(8), 3.751(6)
3.780
—
—
—
—
[Fe2(m-O)(salen)2]116
1.788, 1.785
a
The structure of 4K contains K+ that is disordered over 2 positions.
in Table 1. The previously measured potentials for Fe(salen),
Fe(salen)Cl, and Fe(III)Cl(Ph2salenCl4) are provided for
comparison to 2M and 3M.102
The standard potentials for the Fe(III/II) couple for 2K and
2Ba are 440 and 640 mV more positive than that for Fe(salen).
The standard potentials for 3K and 3Ba are 70 and 230 mV more
positive compared to that for Fe(salen)Cl. Moreover, the
potential for 3Ba approaches that for FeCl(Ph2salenCl4), the
derivative with four electron withdrawing chloride functional-
ities in the ligand backbone.
Electrochemistry. Cyclic voltammograms of the Fe(III/II)
redox couples for 2M and 3M (M ¼ K+ and Ba2+) in CH3CN are
shown in Fig. 2. All the Fe(III/II) redox couples are electro-
chemically reversible, and the standard potentials are reported
Catalytic oxidation activity
The aerobic oxidation activity of 3K and 3Ba with cyclohexene
was tested using a similar protocol to prior studies with iron
porphyrin and iron salen complexes.102 Oxygen-saturated
CH3CN solutions of cyclohexene and 3K or 3Ba were mixed
(benzene was added as an internal standard). The reaction
mixtures were kept under 1 atm of O2 for 24 hours before being
analyzed using H NMR spectroscopy to quantify the concen-
tration of the cyclohexene hydroperoxide, as well as the alcohol
and ketone products. Prior studies used gas chromatography
Fig. 2 Cyclic voltammograms of the Fe(III/II) redox couple for 2K, 2Ba,
3K, and 3Ba, in 0.2 M nBu4NPF6 in CH3CN with a glassy carbon disc as
the working electrode, glassy carbon rod as the counter electrode, and
Ag+/Ag in 0.2 M nBu4NPF6 in CH3CN solution as the pseudo reference
electrode, and ferrocene as an internal standard.
1
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Table 3 Comparison of the aerobic cyclohexene oxidation activity vs.
the Fe(III/II) redox potential for the iron Schiff base complexes. Entries
are sorted by the redox potential (low to high). Italicized entries: 1, 2,
and 5–8 are tuned through electronic induction (left structure), and
are reported from ref. 102. Bold entries 3 and 4 are tuned through
electrostatic effects (3M, right structure)
Fig. 3 Concentration of the products detected by 1H NMR after
24 hours for the 0.5 M solutions of cyclohexene under 1 atm of O2 and
the specified conditions. Conditions for each entry can be found in
Table 2.
Entry Compound
R
E1/2 Fe(III/II) vs. [FeCp2]+/0 TONc
1
A
B
H
ꢀ0.81
ꢀ0.75
0
0
2a
H
for the product analysis, so the more reactive hydroperoxide
intermediate was unobservable.
3b
4b
3K
3Ba
—
—
—
—
ꢀ0.71
ꢀ0.57
17
46
The overall activity and product distributions are shown in
Fig. 3 and Table 2. 3K and 3Ba provide total turnover numbers
of 17.4 and 46.6, respectively, for 2-cyclohexen-1-ol and 2-
cyclohexenone (Fig. 3 and Tables 2 and 3). The reactivity of
Fe(Cl)(salen) and Fe(Cl)(Ph2salenCl4) (compounds A and C in
Table 3), which have similar redox potentials to 3K and 3Ba,
respectively, was also examined and the compounds showed
negligible activity towards aerobic C–H allylic oxidation (entries
1 and 2, Fig. 3 and Table 2). The hydroperoxide intermediate is
observed with Fe(Cl)(salen), indicating that the latter is
unreactive towards promoting subsequent oxidation reactions,
a supposition supported in our mechanistic studies (vide infra).
The addition of external non-redox active metal salts has
enhanced the oxidation activity for some transition metal
catalysts, in some cases forming adducts in situ.101,103–114 For this
catalyst, the incorporation of cations into the ligand framework
is key for the reactivity; the addition of various Ba2+ salts to
Fe(Cl)salen or FeCl3 gave no detectable quantity of product,
although hydroperoxide was sometimes observed (entries 7, 8,
5a
C
D
Cl
ꢀ0.56
0
6a
Cl
ꢀ0.44
0
7a
E
F
NO2
ꢀ0.32
30
8a
NO2
ꢀ0.255
165
a
b
c
ref. 102. This work. Total turnover numbers for 2-cyclohexen-1-ol
and 2-cyclohexenone from a 0.5 M solution of cyclohexene aer 24
hours under 1 atm O2 with the respective iron compound.
and 9, Fig. 2 and Table 2). The absence of product under these
conditions indicates that there is no intermolecular role for the
non-redox active Lewis acidic metals in promoting catalysis.
Table 2 Concentration of the products, as quantified by 1H NMR spectroscopy, of a 0.5 M solution of cyclohexene after 24 hours under 1 atm O2
and the specified conditions
Cyclohexenol
(mM)
Cyclohexenone
(mM)
Total turnover
(alcohol/ketone)
Entry
Conditions
1
2
Fe(Cl)(salen) (A)
Fe(Cl)(Ph2salenCl4) (C)
1.1
0
0
0
0
0
3
4
5
6
7
8
9
10
11
12
13
14
3K
2.0
0.8
3.1
1.5
0.2
0
0
0
0
0
3.4
3.4
10.0
8.2
0
0
0
0
0
17.4
14.9
46.4
35.9
0
0
0
0
0
3K + TBACl
3Ba
3Ba + TBACl
Fe(Cl)(salen) (A) + Ba(18-crown-6)(OTf)2
Fe(Cl)(salen) (A) + Ba(OTf)2
FeCl3 + Ba(OTf)2
3K + BHT
3Ba + BHT
1Ba
0
0
0
0
0
0
No Fe control
TBACl
0
0
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The addition of excess tetrabutylammonium chloride reaction of the Fe(II) intermediate with oxygen (instead of
(TBACl) to 3M had a slight inhibitory effect (entries 4 and 6, a hydroperoxide) to form the inactive diiron(III) m-oxo complex
Fig. 2 and Table 2). When no iron complexes were present, with (step C in Scheme 2). As this is a redox reaction, it is also ex-
or without TBACl, or when only the ligand was present, no pected to exhibit a redox-dependent rate, whereupon the
catalytic activity was observed. Prior studies have indicated that complexes with lower Fe(III/II) redox potentials react more
catalysis occurs through a radical mechanism.102,118–120 We quickly with oxygen. For both the rate-limiting step and the
found that addition of the radical scavenger butylated hydrox- deactivation pathway, the overall reactivity should be more
ytoluene (BHT) inhibited all reactivity, supporting a similar favorable with higher Fe(III/II) standard potentials. These two
radical-based mechanism for 3K and 3Ba.
steps are consistent with the trend observed in previously re-
Water tolerance. A notable difference in the catalytic oxida- ported high-spin (S ¼ 5/2) iron complexes, in that only
tion activity of 3K and 3Ba compared to the prior electron-rich complexes with Fe(III/II) couples above ꢀ0.27 V are active
iron salens and porphyrins is a tolerance to water. Maximal (Scheme 1). Incorporation of the proximal cation in 3M results
activity with the previously reported catalysts was only achieved in catalytic activity at signicantly less oxidizing potentials.
when the solvent and oxygen source were rigorously dried. In
The reactivity of 3K and 3Ba in the rate-determining step and
contrast, the activity of 3K was only slightly diminished in the deactivation pathway was also investigated. For the former, the
presence of water, while the activity of 3Ba was unaffected. reactivity of 3K, 3Ba, and Fe(Cl)(Ph2salenCl4) (compound C in
Comparisons of the relevant turnover numbers are shown in Table 3) with tert-butyl hydroperoxide (tert-BuOOH) was moni-
Table S1 and Fig. S6.†
tored using electronic absorption spectroscopy. Tert-BuOOH
Mechanistic studies. Gray, Labinger and coworkers previ- was used as a substitute for cyclohexene hydroperoxide, since
ously proposed a Haber–Weiss radical-chain mechanism for the latter cannot be generated in reproducible concentrations.
catalysis by iron porphyrins (Scheme 2).118–120 The organic 3K and 3Ba showed spectroscopic changes upon the addition of
hydroperoxide is oxidized by the Fe(III) complex to generate tert-BuOOH,113 while no change was observed for Fe(Cl)(Ph2-
a hydroperoxide radical (B in Scheme 2). The hydroperoxide salenCl4) (Fig. S7†). We were unable to measure a rate constant
radical can react with the corresponding Fe(II) complex to for the reaction of 3M with tert-BuOOH, since the reduced Fe(II)
generate an alkoxy radical and hydroxide, resulting in the complex (2M) can react with another equivalent of tert-BuOOH
regeneration of the Fe(III) active catalyst. The organic radicals to regenerate 3M. However, it is evident that the 3M complexes
propagate and ultimately terminate to yield the ketone and are both reactive while Fe(Cl)(Ph2salenCl4), which has a similar
alcohol. Similar to the iron porphyrins, the reactivities of 3K redox potential to 3Ba, is unreactive. Incorporation of the cation
and 2Ba are also inhibited upon addition of BHT, indicating into the ligand framework could also play a role in facilitating
that they likely proceed through the same radical-based mech- intramolecular electron transfer,105,121,122 as Lewis acidic metal
anism. Additionally, the use of cyclohexenol as the substrate cations can bind or otherwise interact with the lone pairs on the
instead of cyclohexene, under the same conditions, led to no hydroperoxide.103
conversion to cyclohexanone, which is consistent with the
proposed mechanisms where cyclohexenol and cyclohexenone of deactivation through m-oxo formation by the Fe(II) interme-
result from different termination steps. diates. The rate of reaction of 2K and 2Ba with O2 was also
We also examined whether there was a difference in the rate
In the proposed mechanism, the total catalytic activity of the measured using electronic absorption spectroscopy. The diiron
iron macrocycles, represented by the turnover number, is m-oxo complexes were independently synthesized to determine
determined by two factors. The rst is the reduction of the Fe(III) their spectroscopic signatures. While the rate of oxidation was
Cl complex with the organic hydroperoxide (B in Scheme 2), difficult to determine because of the complicated kinetic prole
which is believed to be rate limiting since the hydroperoxide is (Fig. S8†), the half-lives for the oxidation of 2K and 2Ba are 18
observable in millimolar concentrations during catalysis. The seconds and 37 seconds at 0 ꢁC, respectively. The slower rate of
second is deactivation of the catalyst, which occurs though oxidation for 2Ba (and corresponding higher overall activity) is
consistent with the more positive Fe(III/II) redox potential.
Conclusion
For Fe salen complexes modied through traditional inductive
effects, only derivatives with four nitro groups and Fe(III/II)
potentials greater than ꢀ0.27 V were sufficiently oxidizing to
catalyze the aerobic oxidation of cyclohexene. However, the
incorporation of K+ and Ba2+ ions in the secondary coordination
sphere results in active catalysts despite having Fe(III/II) redox
potentials of ꢀ0.71 and ꢀ0.57 V, respectively. Mechanistic
studies indicate that incorporation of the Lewis acidic cation
promotes reactivity in the rate-determining step, oxidation of an
organic hydroperoxide, and inhibits deactivation through Fe2-m-
Scheme 2 The Haber–Weiss radical-chain mechanism of cyclo-
hexene hydroperoxide decomposition and deactivation by diiron m-
oxo formation.
oxo formation.
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For catalytic reactions in which substrate activation is rate
limiting, increasing the catalytic rate through inductive ligand
effects oen has the undesirable outcome of shiing the redox
potential to more extreme values. Overcoming these scaling
relationships requires utilizing alternative routes to facilitate
reactivity or non-inductive methods to modify redox properties.
The incorporation of a non-redox active proximal cation in the
secondary coordination sphere was successfully used in this
system to achieve catalytic activity at a less oxidizing potential.
15 S. M. M. Knapp, T. J. Sherbow, R. B. Yelle, L. N. Zakharov,
J. J. Juliette and D. R. Tyler, Organometallics, 2013, 32,
824–834.
16 M. Yoshida, M. Kondo, S. Torii, K. Sakai and S. Masaoka,
Angew. Chem., Int. Ed., 2015, 54, 7981–7984.
17 W. W. Kramer and C. C. L. McCrory, Chem. Sci., 2016, 7,
2506–2515.
18 C. L. Ford, Y. J. Park, E. M. Matson, Z. Gordon and
A. R. Fout, Science, 2016, 354, 741–743.
19 H. Sun, Y. Han, H. Lei, M. Chen and R. Cao, Chem.
Commun., 2017, 53, 6195–6198.
Conflicts of interest
˜
20 N. B. M. Posada, M. A. Guimaraes, D. S. Padilha,
J. A. L. C. Resende, R. B. Faria, M. Lanznaster,
R. S. Amado and M. Scarpellini, Polyhedron, 2018, 141,
30–36.
There are no conicts to declare.
Acknowledgements
21 J. Y. Yang and D. G. Nocera, J.Am. Chem. Soc., 2007, 129,
8192–8198.
This material is based upon work supported by the National
Science Foundation under Grant No. 1554744. We would like to
thank Professor A. S. Borovik and V. F. Oswald for helpful
discussions, assistance collecting spectroscopic data, and
instrumental support.
22 G. M. Jacobsen, J. Y. Yang, B. Twamley, A. D. Wilson,
R. M. Bullock, M. Rakowski DuBois and D. L. DuBois,
Energy Environ. Sci., 2008, 1, 167–174.
23 J. Y. Yang, R. M. Bullock, W. G. Dougherty, W. S. Kassel,
B. Twamley, D. L. DuBois and M. Rakowski DuBois,
Dalton Trans., 2010, 39, 3001–3010.
24 J. Y. Yang, R. M. Bullock, M. R. DuBois and D. L. DuBois,
MRS Bull., 2011, 36, 39–47.
Notes and references
1 C. Hansch, A. Leo and R. W. Ta, Chem. Rev., 1991, 91, 165–
195.
25 S. Samanta, K. Mittra, K. Sengupta, S. Chatterjee and A. Dey,
Inorg. Chem., 2013, 52, 1443–1453.
2 H. M. Neu, T. Yang, R. A. Baglia, T. H. Yosca, M. T. Green,
M. G. Quesne, S. P. de Visser and D. P. Goldberg, J. Am.
Chem. Soc., 2014, 136, 13845–13852.
3 W. Nam, Y.-M. Lee and S. Fukuzumi, Acc. Chem. Res., 2014,
47, 1146–1154.
4 L. Chiang, L. E. Allan, J. Alcantara, M. C. Wang, T. Storr and
M. P. Shaver, Dalton Trans., 2014, 43, 4295–4304.
5 M. L. Pegis, B. A. McKeown, N. Kumar, K. Lang,
D. J. Wasylenko, X. P. Zhang, S. Raugei and J. M. Mayer,
ACS Cent. Sci., 2016, 2, 850–856.
26 B. de Souza, G. L. Kre, T. Bortolotto, H. Terenzi,
A. J. Bortoluzzi, E. E. Castellano, R. A. Peralta,
J. B. Domingos and A. Neves, Inorg. Chem., 2013, 52,
3594–3596.
27 X. Zhang, Y. Zhu, X. Zheng, D. L. Phillips and C. Zhao, Inorg.
Chem., 2014, 53, 3354–3361.
28 J. T. Bays, N. Priyadarshani, M. S. Jeletic, E. B. Hulley,
D. L. Miller, J. C. Linehan and W. J. Shaw, ACS Catal.,
2014, 4, 3663–3670.
29 H. Yamagishi, S. Nabeya, T. Ikariya and S. Kuwata, Inorg.
Chem., 2015, 54, 11584–11586.
´
6 I. Azcarate, C. Costentin, M. Robert and J.-M. Saveant, J.
Phys. Chem. C, 2016, 120, 28951–28960.
30 E. W. Dahl, T. Louis-Goff and N. K. Szymczak, Chem.
Commun., 2017, 53, 2287–2289.
31 M. Li, X. Wang, Y. Luo and C. Chen, Angew. Chem., Int. Ed.,
2017, 56, 11604–11609.
32 A. Rana, B. Mondal, P. Sen, S. Dey and A. Dey, Inorg. Chem.,
2017, 56, 1783–1793.
7 A. J. P. Cardenas, B. Ginovska, N. Kumar, J. Hou, S. Raugei,
M. L. Helm, A. M. Appel, R. M. Bullock and M. O’Hagan,
Angew. Chem., Int. Ed., 2016, 55, 13509–13513.
8 J. J. Kiernicki, M. Zeller and N. K. Szymczak, J. Am. Chem.
Soc., 2017, 139, 18194–18197.
´
9 K.-N. T. Tseng, J. W. Kampf and N. K. Szymczak, J. Am.
Chem. Soc., 2016, 138, 10378–10381.
33 C. Costentin, S. Drouet, M. Robert and J.-M. Saveant,
Science, 2012, 338, 90–94.
10 D. K. Dogutan, R. McGuire and D. G. Nocera, J. Am. Chem.
Soc., 2011, 133, 9178–9180.
34 T. Zhang, C. Wang, S. Liu, J.-L. Wang and W. Lin, J. Am.
Chem. Soc., 2014, 136, 273–281.
11 M. Zhao, H.-B. Wang, L.-N. Ji and Z.-W. Mao, Chem. Soc.
Rev., 2013, 42, 8360–8375.
12 A. J. M. Miller, J. A. Labinger and J. E. Bercaw, J. Am. Chem.
Soc., 2008, 130, 11874–11875.
13 R. L. Shook, S. M. Peterson, J. Greaves, C. Moore,
A. L. Rheingold and A. S. Borovik, J. Am. Chem. Soc., 2011,
133, 5810–5817.
14 T. J. Schmeier, G. E. Dobereiner, R. H. Crabtree and
N. Hazari, J. Am. Chem. Soc., 2011, 133, 9274–9277.
35 R. Matheu, M. Z. Ertem, J. Benet-Buchholz, E. Coronado,
V. S. Batista, X. Sala and A. Llobet, J. Am. Chem. Soc.,
2015, 137, 10786–10795.
36 D. W. Shaffer, Y. Xie, D. J. Szalda and J. J. Concepcion, J. Am.
Chem. Soc., 2017, 139, 15347–15355.
37 J. Liu, S. Chakraborty, P. Hosseinzadeh, Y. Yu, S. Tian,
I. Petrik, A. Bhagi and Y. Lu, Chem. Rev., 2014, 114, 4366–
4469.
Chem. Sci.
This journal is © The Royal Society of Chemistry 2018

View Article Online
Edge Article
Chemical Science
38 F. Nastri, M. Chino, O. Maglio, A. Bhagi-Damodaran, Y. Lu
64 C. J. Baylies, T. Riis-Johannessen, L. P. Harding, J. C. Jeffery,
R. Moon, C. R. Rice and M. Whitehead, Angew. Chem., Int.
Ed., 2005, 44, 6909–6912.
and A. Lombardi, Chem. Soc. Rev., 2016, 45, 5020–5054.
´
39 P. Hosseinzadeh, N. M. Marshall, K. N. Chacon, Y. Yu,
M. J. Nilges, S. Y. New, S. A. Tashkov, N. J. Blackburn and
Y. Lu, Proc. Natl. Acad. Sci. U. S. A., 2016, 113, 262–267.
40 A.-F. Miller, Acc. Chem. Res., 2008, 41, 501–510.
41 A. Warshel, P. K. Sharma, M. Kato, Y. Xiang, H. Liu and
M. H. M. Olsson, Chem. Rev., 2006, 106, 3210–3235.
42 S. D. Fried, S. Bagchi and S. G. Boxer, Science, 2014, 346,
1510–1514.
65 A. Gund, B. K. Keppler and B. Nuber, Inorg. Chem., 1995, 34,
2788–2790.
66 S. K. Dutta, D. Gan and M. W. Perkovic, Eur. J. Inorg. Chem.,
2003, 2003, 2812–2819.
67 N. D. Lowe and C. D. Garner, J. Chem. Soc., Dalton Trans.,
1993, 2197–2207, DOI: 10.1039/DT9930002197.
68 T. Lazarides, A. Barbieri, C. Sabatini, F. Barigelletti,
H. Adams and M. D. Ward, Inorg. Chim. Acta, 2007, 360,
814–824.
43 R. Maji and S. E. Wheeler, J. Am. Chem. Soc., 2017, 139(36),
12441–12449.
`
44 B. Chattopadhyay, J. E. Dannatt, I. L. Andujar-De Sanctis,
K. A. Gore, R. E. Maleczka, D. A. Singleton and
M. R. Smith, J. Am. Chem. Soc., 2017, 139, 7864–7871.
45 J. M. Um, D. A. DiRocco, E. L. Noey, T. Rovis and K. N. Houk,
J. Am. Chem. Soc., 2011, 133, 11249–11254.
46 H. Yang and M. W. Wong, J. Am. Chem. Soc., 2013, 135,
5808–5818.
69 L. J. Charbonniere, R. F. Ziessel, C. A. Sams and
A. Harriman, Inorg. Chem., 2003, 42, 3466–3474.
70 S. Belanger, M. Gilbertson, D. I. Yoon, C. L. Stern, X. Dang
and J. T. Hupp, J. Chem. Soc., Dalton Trans., 1999, 3407–
3411, DOI: 10.1039/A906106A.
71 L. H. Uppadine, J. E. Redman, S. W. Dent, M. G. B. Drew and
P. D. Beer, Inorg. Chem., 2001, 40, 2860–2869.
72 M. Chiba, H.-B. Kim and N. Kitamura, Anal. Sci., 2002, 18,
461–466.
47 E. Lyngvi, J. W. Bode and F. Schoenebeck, Chem. Sci., 2012,
3, 2346–2350.
48 D. A. DiRocco, E. L. Noey, K. N. Houk and T. Rovis, Angew.
Chem., Int. Ed., 2012, 51, 2391–2394.
73 V. Wing-Wah Yam, C.-K. Li and C.-L. Chan, Angew. Chem.,
Int. Ed., 1998, 37, 2857–2859.
49 T. J. Seguin and S. E. Wheeler, ACS Catal., 2016, 6, 2681–
2688.
74 V. Wing-Wah Yam, Y.-L. Pui, W.-P. Li, K. Kam-Wing Lo and
K.-K. Cheung, J. Chem. Soc., Dalton Trans., 1998, 3615–3622,
DOI: 10.1039/A806243I.
´
50 Q. N. N. Nguyen, M. W. Lodewyk, S. Bezer, M. R. Gagne,
M. L. Waters and D. J. Tantillo, ACS Catal., 2015, 5, 1617–
1622.
51 K. Lee, D. L. Silverio, S. Torker, D. W. Robbins, F. Haeffner,
F. W. van der Mei and A. H. Hoveyda, Nat. Chem., 2016, 8,
768–777.
52 T. J. Seguin and S. E. Wheeler, Angew. Chem., Int. Ed., 2016,
55, 15889–15893.
53 A. C. Doney, B. J. Rooks, T. Lu and S. E. Wheeler, ACS Catal.,
2016, 6, 7948–7955.
54 C. R. Kennedy, J. A. Guidera and E. N. Jacobsen, ACS Cent.
Sci., 2016, 2, 416–423.
55 G. Xiao, G. A. Cintron-Rosado, D. A. Glazier, B.-m. Xi, C. Liu,
P. Liu and W. Tang, J. Am. Chem. Soc., 2017, 139, 4346–4349.
56 M. C. Holland, S. Paul, W. B. Schweizer, K. Bergander,
75 C.-K. Li, X.-X. Lu, K. M.-C. Wong, C.-L. Chan, N. Zhu and
V. W.-W. Yam, Inorg. Chem., 2004, 43, 7421–7430.
76 X.-X. Lu, C.-K. Li, E. C.-C. Cheng, N. Zhu and V. W.-W. Yam,
Inorg. Chem., 2004, 43, 2225–2227.
77 M. Delgado, J. M. Ziegler, T. Seda, L. N. Zakharov and
J. D. Gilbertson, Inorg. Chem., 2016, 55, 555–557.
78 M. R. Kita and A. J. M. Miller, J. Am. Chem. Soc., 2014, 136,
14519–14529.
79 Z. Cai, D. Xiao and L. H. Do, J. Am. Chem. Soc., 2015, 137,
15501–15510.
80 M. R. Kita and A. J. M. Miller, Angew. Chem., Int. Ed., 2017,
56, 5498–5502.
81 P. D. Beer, P. K. Hopkins and J. D. McKinney, Chem.
Commun., 1999, 1253–1254, DOI: 10.1039/A903440D.
˜
¨
C. Muck-Lichtenfeld, S. Lakhdar, H. Mayr and
82 S. Shrestha, C. Gimbert-Surinach, M. Bhadbhade and
R. Gilmour, Angew. Chem., Int. Ed., 2013, 52, 7967–7971.
57 A. L. Chan, J. Estrada, C. E. Kefalidis and V. Lavallo,
Organometallics, 2016, 35, 3257–3260.
58 J. B. Smith and A. J. M. Miller, Organometallics, 2015, 34,
4669–4677.
S. B. Colbran, Eur. J. Inorg. Chem., 2011, 2011, 4331–4337.
83 Z. Halime, M. Lachkar, N. Matsouki, G. Charalambidis,
M. di Vaira, A. G. Coutsolelos and B. Boitrel, Tetrahedron,
2006, 62, 3056–3064.
84 Y.-H. Kim and J.-I. Hong, Chem. Commun., 2002, 512–513,
DOI: 10.1039/B109596J.
59 C. C. Lu and J. C. Peters, J. Am. Chem. Soc., 2002, 124, 5272–
5273.
˚
¨
85 J. Martensson, K. Sandros and O. Wennerstrom, J. Phys.
´
60 I. Azcarate, C. Costentin, M. Robert and J.-M. Saveant, J. Am.
Org. Chem., 1994, 7, 534–544.
Chem. Soc., 2016, 138, 16639–16644.
61 H. Shao, S. K. Muduli, P. D. Tran and H. S. Soo, Chem.
Commun., 2016, 52, 2948–2951.
86 Y. Shen and B. P. Sullivan, Inorg. Chem., 1995, 34, 6235–
6236.
´
´
´
87 A. Gordillo, E. de Jesus and C. Lopez-Mardomingo, Org.
62 S. Sung, D. Kumar, M. Gil-Sepulcre and M. Nippe, J. Am.
Chem. Soc., 2017, 139, 13993–13996.
63 F. C. J. M. van Veggel, W. Verboom and D. N. Reinhoudt,
Chem. Rev., 1994, 94, 279–299.
Lett., 2006, 8, 3517–3520.
´
´
´
88 J. Fresneda, E. de Jesus, P. Gomez-Sal and C. Lopez
Mardomingo, Eur. J. Inorg. Chem., 2005, 2005, 1468–1476.
89 M. Sawamura, Y. Nakayama, W.-M. Tang and Y. Ito, J. Org.
Chem., 1996, 61, 9090–9096.
This journal is © The Royal Society of Chemistry 2018
Chem. Sci.

View Article Online
Chemical Science
Edge Article
90 C. D. Hall, G. J. Kirkovits and A. C. Hall, Chem. Commun., 106 C. G. Miller, S. W. Gordon-Wylie, C. P. Horwitz,
1999, 1897–1898, DOI: 10.1039/A904069B.
S. A. Strazisar, D. K. Peraino, G. R. Clark, S. T. Weintraub
´
´
´
91 S. Castro-Juiz, A. Fernandez, M. Lopez-Torres, D. Vazquez-
and T. J. Collins, J. Am. Chem. Soc., 1998, 120, 11540–11541.
´
´
´
Garcıa, A. J. Suarez, J. M. Vila and J. J. Fernandez, 107 H. Guo, Z. Chen, F. Mei, D. Zhu, H. Xiong and G. Yin,
Organometallics, 2009, 28, 6657–6665. Chem.–Asian J., 2013, 8, 888–891.
92 C. Nataro, H. M. Baseski, C. M. Thomas, B. J. Wiza and 108 L. Dong, Y. Wang, Y. Lv, Z. Chen, F. Mei, H. Xiong and
K. M. Rourke, Polyhedron, 2001, 20, 1023–1028. G. Yin, Inorg. Chem., 2013, 52, 5418–5427.
93 A. H. Reath, J. W. Ziller, C. Tsay, A. J. Ryan and J. Y. Yang, 109 Z. Zhang, K. L. Coats, Z. Chen, T. J. Hubin and G. Yin, Inorg.
Inorg. Chem., 2017, 56, 3713–3718. Chem., 2014, 53, 11937–11947.
94 R. C. Cammarota and C. C. Lu, J. Am. Chem. Soc., 2015, 137, 110 S. Qin, L. Dong, Z. Chen, S. Zhang and G. Yin, Dalton Trans.,
12486–12489. 2015, 44, 17508–17515.
95 E. Y. Tsui and T. Agapie, Proc. Natl. Acad. Sci. U. S. A., 2013, 111 S. Zhang, Z. Chen, S. Qin, C. Lou, A. M. Senan, R.-Z. Liao
110, 10084–10088. and G. Yin, Org. Biomol. Chem., 2016, 14, 4146–4157.
96 E. Y. Tsui, R. Tran, J. Yano and T. Agapie, Nat. Chem., 2013, 112 C. Choe, L. Yang, Z. Lv, W. Mo, Z. Chen, G. Li and G. Yin,
5, 293–299. Dalton Trans., 2015, 44, 9182–9192.
97 P.-H. Lin, M. K. Takase and T. Agapie, Inorg. Chem., 2015, 113 J. Park, Y. Morimoto, Y.-M. Lee, W. Nam and S. Fukuzumi,
54, 59–64. J. Am. Chem. Soc., 2011, 133, 5236–5239.
98 D. E. Herbert, D. Lionetti, J. Rittle and T. Agapie, J. Am. 114 Y. J. Park, J. W. Ziller and A. S. Borovik, J. Am. Chem. Soc.,
Chem. Soc., 2013, 135, 19075–19078. 2011, 133, 9258–9261.
99 C. P. Horwitz and Y. Ciringh, Inorg. Chim. Acta, 1994, 225, 115 M. Gerloch, J. Lewis, F. E. Mabbs and A. Richards, Nature,
191–200. 1966, 212, 809–810.
100 H. M. Neu, J. Jung, R. A. Baglia, M. A. Siegler, K. Ohkubo, 116 M. Gerloch and F. E. Mabbs, J. Chem. Soc. A, 1967, 1900–
S. Fukuzumi and D. P. Goldberg, J. Am. Chem. Soc., 2015,
137, 4614–4617.
101 S. Hong, Y.-M. Lee, M. Sankaralingam, A. K. Vardhaman,
1908, DOI: 10.1039/J19670001900.
117 A. T. M. P. Coggon, F. E. Mabbs and V. N. McLachlan, J.
Chem. Soc. A, 1971, 1014–1019, DOI: 10.1039/J19710001014.
¨
Y. J. Park, K.-B. Cho, T. Ogura, R. Sarangi, S. Fukuzumi 118 A. Bottcher, E. R. Birnbaum, M. W. Day, H. B. Gray,
and W. Nam, J. Am. Chem. Soc., 2016, 138, 8523–8532.
M. W. Grinstaff and J. A. Labinger, J. Mol. Catal. A: Chem.,
1997, 117, 229–242.
¨
102 A. Bottcher, M. W. Grinstaff, J. A. Labinger and H. B. Gray, J.
Mol. Catal. A: Chem., 1996, 113, 191–200.
103 S. Fukuzumi and K. Ohkubo, Chem.–Eur. J., 2000, 6, 4532–
4535.
119 M. Grinstaff, M. Hill, J. Labinger and H. Gray, Science, 1994,
264, 1311–1313.
120 J. A. Labinger, Catal. Lett., 1994, 26, 95–99.
104 S.-M. Yiu, W.-L. Man and T.-C. Lau, J. Am. Chem. Soc., 2008, 121 S. Bang, Y.-M. Lee, S. Hong, K.-B. Cho, Y. Nishida, M. S. Seo,
130, 10821–10827.
R. Sarangi, S. Fukuzumi and W. Nam, Nat. Chem., 2014, 6,
105 Y. Morimoto, H. Kotani, J. Park, Y.-M. Lee, W. Nam and
S. Fukuzumi, J. Am. Chem. Soc., 2011, 133, 403–405.
934.
122 K. D. Karlin, Nat. Chem., 2010, 2, 711.
Chem. Sci.
This journal is © The Royal Society of Chemistry 2018