C-H activation and nucleophilic substitution in a photochemically generated high valent iron complex

Article abstract of DOI:10.1039/c7sc05378a

The photochemical oxidation of a (TAML)Fe^III complex 1 using visible light generated Ru(bpy)₃³⁺ produces valence tautomers (TAML)Fe^IV (1⁺) and (TAML⁺)Fe^III (1-TAML⁺), depending on the exogenous anions. The presence of labile Cl^- or Br^- results in a ligand-based oxidation and stabilisation of a radical-cationic (TAML⁺)Fe^III complex, which subsequently leads to unprecedented C-H activation followed by nucleophilic substitution on the TAML aryl ring. In contrast, exogenous cyanide culminates in metal-based oxidation, yielding the first example of a crystallographically characterised S = 1 [(TAML)Fe^IV(CN)₂]^2- species. This is a rare report of an anion-dependent valence tautomerisation in photochemically accessed high valent (TAML)Fe systems with potential applications in the oxidation of pollutants, hydrocarbons, and water. Furthermore, the nucleophilic aromatic halogenation reaction mediated by (TAML⁺)Fe^III represents a novel domain for high-valent metal reactivity and highlights the possible intramolecular ligand or substrate modification pathways under highly oxidising conditions. Our findings therefore shine light on high-valent metal oxidants based on TAMLs and other potential non-innocent ligands and open new avenues for oxidation catalyst design.

Full text of DOI:10.1039/c7sc05378a

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Chemical Science
ARTICLE
C-H activation and nucleophilic substitution in a photochemically
generated high valent iron complex
Received 00th January 20xx,
Accepted 00th January 20xx
Jia Hui Lim,ab Xenia Engelmann,c Sacha Corby,bd Rakesh Ganguly,b Kallol Ray,*c and Han Sen Soo*bef
DOI: 10.1039/x0xx00000x
3+
The photochemical oxidation of a (TAML)FeIII complex 1 using visible light generated Ru(bpy)3 produces valence
tautomers (TAML)FeIV (1+) and (TAML+)FeIII (1-TAML+), depending on the exogenous anions. The presence of labile Cl– or
Br– results in a ligand-based oxidation and stabilisation of a radical-cationic (TAML+)FeIII complex, which subsequently
leads to unprecedented C-H activation followed by nucleophilic substitution on the TAML aryl ring. In contrast, exogenous
cyanide culminates in metal-based oxidation, yielding the first example of a crystallographically characterised S = 1
[(TAML)FeIV(CN)2]2- species. This is a rare report of an anion-dependent valence tautomerisation in photochemically
accessed high valent (TAML)Fe systems with potential applications in the oxidation of pollutants, hydrocarbons, and water.
Furthermore, the nucleophilic aromatic halogenation reaction mediated by (TAML+)FeIII represents a novel domain for
high-valent metal reactivity and highlights the possible intramolecular ligand or substrate modification pathways under
highly oxidising conditions. Our findings therefore shine light on high-valent metal oxidants based on TAML and other
potential non-innocent ligands and open new avenues for oxidative catalyst design.
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facilitates the stabilisation of a formal FeV centre in Compound
I, thereby contributing to its high electrophilic reactivity.
Consistent with the observed high reactivity of the
Introduction
High-valent iron oxo cofactors are key intermediates in
carrying out important metabolic reactions such as
hydroxylation, epoxidation, and electrophilic aromatic
(Porp

+)FeIV(O) core in biological oxidation reactions,
+)MnIV(OH) species showed a five-fold higher
a
bioinspired (tpfc

electrophilic reactivity compared to its redox-tautomeric form,
(tpfc)MnV(O) (tpfc = 5,10,15-tris(pentafluorophenyl)corrole).12
Similarly, Goldberg and co-workers reported an enhanced rate
of electrophilic C-H cleavage in the Lewis acid (LA)-induced
substitution.1-7
Particularly,
the
heme-containing
metalloenzyme, cytochrome P450, has generated enormous
scientific interest. P450 can utilise O2 to transform into a highly
reactive oxo iron(IV)-porphyrin radical-cation, (Porp
+)FeIV(O)
MnIV(O-LA)(TBP8Cz+
butylphenyl)corrolazinato3−) complex.13,14
)
(TBP8Cz
=
octakis(p-tert-
(Compound I; Porp = porphyrin), capable of driving challenging
biological oxidations.8,9 Efforts to understand Compound I
have thus inspired many metalloporphyrin and porphyrinoid
Concomitantly, non-heme iron complexes bearing ligands
such as tetraamido macrocyclic ligands (TAML),15 tripodal
tris(carbene),16,17 tetracarbene macrocycle,18 N4Py,19,20 and
tetramethyl cyclam (TMC)21 have also been investigated as
bioinspired and biomimetic counterparts.22 These complexes
have provided seminal insights to pivotal biochemical
transformations using oxidants such as O2 and H2O2.1,23,24 In
particular, the (TAML)Fe system developed by Collins and
coworkers are such potent oxidants that they have been used
for pulp bleaching, degradation of recalcitrant pollutants,
water oxidation, and even the destruction of explosives in
water.15,25-28
model compounds.10 Through these studies, (Porp +)FeIV(O) is

now recognised as the reactive species governing the rate-
limiting hydrogen abstraction step to give (Porp)FeIV(OH)
(Compound II).8,11
The redox non-innocent nature of the porphyrin possibly
a.Energy Research Institute@NTU (ERI@N), Nanyang Technological University,
Interdisciplinary Graduate School, Research Techno Plaza, Singapore 63755
b.Division of Chemistry and Biological Chemistry, School of Physical and
Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link,
Singapore 637371. E-mail: hansen@ntu.edu.sg
c. Humboldt-Universität zu Berlin, Institut für Chemie, Brook-Taylor-Stra
Berlin, Germany. E-mail: kallol.ray@chemie.hu-berlin.de

e 2, 12489
In our continuous efforts to develop (photo)chemical
solutions for energy research and environmental applications
by using only earth-abundant elements in bioinspired
d.Imperial College London, Department of Chemistry, South Kensington Campus,
London, SW7 2AZ, United Kingdom
e. Singapore-Berkeley Research Initiative for Sustainable Energy, 1 Create Way,
Singapore 138602
systems,29-32 we explored if (TAML)Fe complex
1 can be
activated for novel oxidative chemistry using light as the
energy source instead of O2 or H2O2. Nam and Fukuzumi have
demonstrated that high valent (N4Py)FeIV(O) intermediates
can be generated by 2e- oxidation via irradiating (N4Py)FeII in
f. Solar Fuels Laboratory, Nanyang Technological University, 50 Nanyang Avenue,
Singapore 639798
†
Electronic Supplementary Information (ESI) available: Detailed synthetic
procedures and characterisation. CCDC 1535587. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/x0xx00000x
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the presence of Ru(bpy)32+ and sacrificial oxidants.33 They have (Cl+) mechanism at high-valent metal centres. Our work
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DOI: 10.1039/C7SC05378A
also applied this concept in the asymmetric epoxidation of highlights photochemically accessible high valent (TAML)Fe
terminal olefins via photocatalytic oxidation of the chiral systems with potential applications in the oxidation of
manganese catalyst, (R,R-BQCN)MnII(OTf)2 (BQCN = N,N- pollutants, hydrocarbons, and water, and provides valuable
dimethyl-N,N-bis(8-quinolyl)cyclohexane-diamine;
OTf
=
insights for potential side-reactions with exogenous
CF3SO3).34 In addition, Panda et al. have shown photochemical compounds under highly oxidising conditions. This may open
water oxidation by biuret-modified (TAML)FeIII catalysts with new avenues for C-H activation reactions and oxidative
2+
Ru(bpy)3 as the photosensitiser, via the intermediacy of catalyst design based on other redox non-innocent ligands.
(TAML)FeV(O).35 Herein, we describe the photochemical
generation of bona fide high valent (TAML)Fe using a visible
Results and discussion
light driven chromophore.
Through a series of transient absorption spectroscopic
(TAS) and electrochemical measurements, we show that
Spectroscopic and electrochemical characterisation of 1
We prepared
as previously reported.37 The zero-field
Mössbauer spectrum of consists of a major doublet (88%,
isomer shift δ = 0.195 mmꢀs−1, quadrupole splitting ΔEQ = 3.949
mmꢀs−1) and a minor doublet (12%, δ = 0.208 mmꢀs−1, ΔEQ
3.158 mmꢀs−1, Fig. S2c in ESI†). Both species are consistent with
intermediate spin FeIII in S = 3/2 ground states. The doublet
with larger ΔEQ is tentatively assigned as the five coordinate
[(TAML)FeIII(H2O)]- complex and the one with lower ΔEQ
corresponds to the six coordinate [(TAML)FeIII(H2O)2]-. In
solution, the [(TAML)FeIII(H2O)2]- complex represents the major
component as evident from the Mössbauer spectrum of 57Fe-
1
2+
2+
photoexcited Ru(bpy)3 can oxidise
1. Ru(bpy)3 and a
1
sacrificial oxidant are used as the established photosensitiser
system in our proof of concept, although the photosensitiser
can be replaced with non noble-metal chromophores and O2 in
our idealised future rendition.29,32,36 Interestingly, in the
presence of labile chloride or bromide exogenous anions, the
=
(photo)chemically oxidised solution of
equilibrium mixture of the (TAML)FeIV 1+) and (TAML
TAML+
intermediates. In contrast, the presence of
1
comprises an
(

+)FeIII (1-
)
exogenous cyanide culminates in a predominantly metal-based
oxidation, yielding solely 1+ (Fig. 1).
Extensive mechanistic studies and spectroscopic
measurements of the (TAML)FeIV and (TAML +)FeIII

labelled
ESI†). This mixed solvent combination was chosen to
accommodate the disparate solubilities of (soluble in ACN)
1 in a 1:1 water:acetonitrile (ACN) mixture (Fig. S2a in
1
intermediates reveal that the latter reacts via an unusual C-H
activation pathway involving nucleophilic halogen
and sodium persulfate (Na2S2O8, soluble in water).
We verified that the axial ligand(s) contained water
molecules with an isotope-labelling experiment. Infrared (IR)
a
substitution on the TAML aryl ring (Fig. 1). Notably, this
nucleophilic aromatic halogenation reaction mediated by
spectroscopic measurement of 1, after dissolution and heating
(TAML
reactivity and contrasts sharply with the typical halogenation
reactions occurring either via a radical (Cl ) or an electrophilic

+)FeIII represents a novel domain for high-valent metal
at 50 °C in D2O for 15 minutes and then drying in vacuo,
reveals new IR bands at 2382 and 2445 cm-1 (Fig. S2f in ESI†).
These bands are slightly redshifted from those of D2O (Fig. S2g
in ESI†). We attribute these new bands to the exchange of H by
D in the axially bound water molecule(s). Poor electron
paramagnetic resonance (EPR) signal to noise ratios were
observed for the FeIII in 571 alone, possibly due to aggregation,
as observed by Sullivan et al.38 Thus, no EPR spectroscopic data
are presented here. We conducted cyclic voltammetry (CV)

experiments (Fig. S3 in ESI†) and verified that the oxidation of
3+
1
by Ru(bpy)3
(ERu(III)/Ru(II) ≈
+0.85 V)39 should be
thermodynamically feasible, in agreement with the redox
potential reported previously in the CV of 1 38
.
Transient absorption measurements
Fig.
1 Reaction scheme featuring the reactivity of (photo)chemically oxidised 1,
depending on the exogenous anions. The presence of exogenous CN- stabilises the high
valent FeIV complex (top), whereas Cl– or Br– results in a ligand-based oxidation and
stabilisation of a radical-cationic (TAML+)FeIII complex, which subsequently leads to C-
H activation and nucleophilic substitution on the TAML aryl ring (bottom).
In light of these electrochemical studies, we performed
transient absorption spectroscopic (TAS) experiments to verify
if
1
can be photochemically oxidised under partially
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Table 1. Summary of the lifetimes from the fits to the kinetics data in Fig. S5-8 in ESI†.
Lifetimes recorded at different probing wavelengths (ns)
Entry
365 nm a
470 nm a
730 nm a
600 nm b
1c
2d
3e
4f
660 ± 20
540 ± 20
390 ± 20
620 ± 20
530 ± 20
380 ± 20
640 ± 20
570 ± 20
400 ± 20
680 ± 30
530 ± 20
390 ± 20
1= 360 ± 20
2= 3010 ± 150
1= 390 ± 20
2= 2840 ± 110
1= 370 ± 220
2= 3020 ± 120
360 ± 20
a
b
c
d
e
Transient absorption signal. Transient emission signal. Ru(bpy)3Cl2 (0.020 mM) only. Ru(bpy)3Cl2 (0.020 mM) + 1 (0.20 mM). Ru(bpy)3Cl2 (0.020 mM) +
Co(NH3)5Cl3 (10 mM). f Ru(bpy)3Cl2 (0.020 mM) + 1 (0.20 mM) + Co(NH3)5Cl3 (10 mM). c-f More details can be found in Table S1 in ESI†.
aqueous conditions, while concurrently characterising the transfer to
1
and oxidative quenching by [Co(NH3)5Cl]2+ (Fig. S8
absorption features of the oxidised (TAML)Fe intermediates. in ESI†). Concomitantly, a transient oxidised (TAML)Fe species
We probed the characteristic wavelengths (at 365, 470, and with an absorption band around 365 nm and broad absorption
730 nm in absorbance mode, and 600 nm in emission mode) of between 450-850 nm (Fig. 2b) was observed. A second new,
the photochemically excited Ru(bpy)32+* to observe the fate of longer-lived time constant,
Ru(bpy)3 upon irradiation with 450 nm laser pulses. Some entry 4) was determined from the transient absorption signals

2, of around 3010 ns (Table 1
2+
salient results are summarised in Fig. 2 and Table 1, with other at 365 nm (Fig. 2c), which we attribute to electron transfer
3+
details in the ESI† Table S1, and Fig. S4-10.
In the presence of
photoluminescence at 600 nm of Ru(bpy)32+* decayed with the timescales after the RuIII has transformed back to the
from
1
to Ru(bpy)33+. Ru(bpy)3 can oxidise
1
, which then
1
and [Co(NH3)5Cl]2+, the absorbs at 365 nm (blue, inset of Fig. 2c) at longer
s
2+
3+
shortest lifetime,
1 (Table 1 entry 4), due to both energy
Ru(bpy)3 ground state. The oxidation of 1 by Ru(bpy)3
occurs with a near diffusion-controlled bimolecular rate
constant of 3.9 x 109 M-1s-1 (Fig. S10b in ESI†), likely due to
attraction between the cationic Ru(bpy)33+ and the anionic
1.
To verify these experiments, we repeated TAS studies with
longer timescales up to 4 seconds. From the signals at 600 and
830 nm, the oxidised
ESI†). We also added
generated Ru(bpy)33+ to
spectra of the oxidised species. The intermediate (Fig. 2d, blue
line) has a
max at 606 nm (ε606 ≈ 1200 dm3 mol-1 cm-1) and
828 nm (ε828 ≈ 1700 dm3 mol-1 cm-1), with an estimated
lifetime, 828, of 27 ± 4 mins at room temperature (Fig. S12a in
ESI†), suggesting that the stable steady-state intermediate is
indeed the oxidised
1
did not decay in 4 seconds (Fig. S11 in
equivalent of photochemically
to independently obtain steady-state
1
1


1
.
C-H activation and nucleophilic substitution in (photo)chemically
oxidised 1
Interestingly, negative-ion mode electrospray ionisation-mass
3+
spectrometry (ESI-MS) of the reaction mixture of Ru(bpy)3
and
ratios of m/z = 451.9977 and 485.9579 (34 and 68 mass units
higher than ), which could have arisen from the nucleophilic
aromatic substitution of Cl (from Ru(bpy)3Cl2) into the TAML
ligand of . We tentatively propose that the photochemical
oxidation of is predominantly ligand-based, leading to
1 revealed high resolution ion peaks at mass-to-charge
Fig. 2. Spectroscopic characterisation data of the reaction between photochemically
generated Ru(bpy)3 and 1. TAS of Ru(bpy)3Cl2, Co(NH3)5Cl3, and 1, featuring spectral
3+
changes over time after laser excitation: (a) 0 – 10 s, and (b) 0 - 10 s with expanded
vertical scale. The squares (■) highlight absorption features of the transient oxidised 1.
(c) TAS signal of Ru(bpy)3Cl2 and 1 (red), and of Ru(bpy)3Cl2, Co(NH3)5Cl3, and 1 (blue),
probed at 365 nm. The long-timescale (0 - 10 s) TAS signal is in the inset. The fits to
1
1
3+
1
the kinetics are the lines in black. (d) Steady-state spectra of 1 oxidised by Ru(bpy)3
stabilisation of a radical-cationic (TAML (1-TAML
+)FeIII +),
(blue). The asterisks (*) at 606 and 828 nm are characteristic bands of the oxidised 1.
similar to CeIV oxidation in other TAML Fe systems.40,41 The
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oxidation of the TAML ligand results in the formation of a photoelectron spectroscopy (XPS) measureVmieewnAtrtsicle Oanllsinoe
DOI: 10.1039/C7SC05378A
radical-cation (akin to Compound I in P450) that increases its substantiate the presence of aryl bromides, since the binding
susceptibility to nucleophilic attack, leading to aromatic energy corresponds to Br bound to carbon instead of anionic
substitution in the ligand by Cl–. We propose that our Br- coordinated to Fe (Fig. 3b).43 To monitor the progress of
observations arose from the use of an outer-sphere oxidant, the substitution reactions, aliquots were withdrawn at specific
Ru(bpy)33+, as compared to the more commonly reported time intervals, quenched with Na2S2O3 solutions, deligated in
inner-sphere oxidants such as (organic) peroxides.
To evaluate this hypothesis, we chemically oxidised
pH 7 phosphate buffer solutions, and then analysed by 1H NMR
in the spectroscopy. Within 15 minutes, the signals corresponding to
1
presence of several anions to independently characterise the TAML disappeared (grey dashed line), accompanied with the
products. (ArMe)3N+ (ArMe = 4-methylphenyl) has been emergence of a set of new signals (orange dashed lines), which
chosen because it is an outer-sphere oxidant (E0’ = 0.40 V vs we propose to be mostly TAML(Br)2 (Fig. 3c). We assigned the
1
Fc+/Fc) suitable for only one electron oxidation of 1 42
.
In major H NMR signals (labelled as HA, HB, and HC) to those of
addition, unlike the 1H NMR spectroscopic signals of 2,2’- one TAML(Br)2 isomer, while the remaining signals belong to
bipyridyl from Ru(bpy)32+, those of (ArMe)3N do not overlap the minor isomers of TAML(Br)2 with Br substituted at other
with the signals of the TAML ligand, therefore enabling the positions of the aryl ring (Fig. 3c). We managed to isolate 1-
1
substituted TAML to be monitored by H NMR spectroscopy TAML(Br)2 by reverse-phase C18 column chromatography (Fig.
after deligation. Among the common anions inert towards S14b in ESI†), but were not successful at growing single
(ArMe)3N
Negative-ion mode ESI-MS data of the reaction of mixture substitution of Cl- proceeded more sluggishly, as evident from
oxidised by (ArMe)3N+ (4 equivalents) in the presence of the longer reaction time and the moderate conversions of 1-

+, we screened Cl–, Br–, and CN–.
crystals of this complex. On the other hand, aromatic
of
1
Br- (10 equivalents) show masses and isotopic distributions TAML to 1-TAML(Cl) and 1-TAML(Cl)2, based on the ESI-MS
corresponding to those of 1-TAML(Br) and 1-TAML(Br)2 (Fig. data. As a result, the NMR signals resemble TAML and the
3a), where one and two bromides have substituted for minor products cannot be unambiguously assigned (Fig. S16 in
hydrogens on the aromatic ring in
1
, respectively. X-ray
ESI†).
Having ascertained that substitution of Cl– or Br– into the
TAML ligand had occurred upon oxidation of , we proceeded
1
to investigate the mechanism. In the presence of 10
equivalents of Cl– or Br–, steady-state UV-visible spectral
changes were observed (Fig. S17a in ESI†). CV measurements
of
1
in the presence of 10 equivalents of Cl– or Br– also
revealed a small anodic shift in the first redox wave from Br– to
Cl– (Fig. S17c), suggesting that the Cl- interacts more strongly
with
1
compared to Br-. We propose that these changes arise
from the exchange of the solvent molecule at the axial position
with a halide, thus forming 1(X) (X = Cl or Br). Subsequently,
the oxidation of 1(X) should give 1+(X) (left, Fig. 4). Apart from
our proposed nucleophilic aromatic substitution on 1(X)-
TAML
+ (top, Fig. 4), we also considered the possibility that the
coordinated halide on 1+(X) may be electrophilic.
Intermolecular (middle, Fig. 4) or intramolecular (bottom, Fig.
4) electrophilic aromatic substitution could then result in the
1(X)-TAML(X) products in the presence of excess halides.
Fig. 3. Characterisation data of 1-TAML(Br)n generated from the reaction containing 1,
(ArMe)3N+ (4 eq), and tetraethylammonium bromide (TEABr, 10 eq). (a) Negative-ion
ESI-MS of the crude reaction mixture 30 minutes after addition of the (ArMe)3N+
oxidant. The assigned intermediates are labelled. (b) Br 3d XPS data of 1-TAML(Br)n.
The fit (red, binding energy
= 70.7 eV) represents the component completely
attributable to aryl bromide. (c) 1H NMR spectra of deligated aliquots of the reaction
mixture, withdrawn at time intervals 1 (red), 3 (blue), and 15 (green) minutes after
addition of the (ArMe)3N+ oxidant. The spectrum of TAML before oxidation is shown in
black. The dashed lines highlight the characteristic 1H NMR signals from TAML (grey)
and the proposed TAML(Br)2 (orange). The 1H NMR signals (labelled as HA, HB, and HC)
are proposed to arise from the major TAML(Br)2 isomeric product, while the remaining
signals (*) likely originate from another isomer of TAML(Br)2 with Br substitution at a
different position of the aryl ring.
Fig. 4 Possible mechanisms that could lead to the 1(X)-TAML(X) products following the
oxidation of 1(X) in the presence of excess halides. The compounds marked in red arise
from a second molecule of X- or 1, instead of the original 1(X) and its oxidised
intermediates.
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electrophilic aromatic substitution should be independent of
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-
the Br- concentration. The plot of kobs against Br concentration
DOI: 10.1039/C7SC05378A
reveals a positive linear correlation, giving a second-order rate
constant k of 9.6 x 10-2 M-1 s-1 for the aromatic substitution of
oxidised 1(Br) (Fig. 5b). In a separate experiment, analysis of
ESI-MS data from the reaction of 10 equivalents of Br- with
oxidised 1(Cl) (generated in situ by adding 2 equivalents of
(ArMe)3N+ to a solution 1 equivalent of Cl- and
1) revealed
predominantly 1-TAML(Br)n products being formed instead of
the 1-TAML(Cl)n products expected for an intramolecular
chloride migration (Fig. S17g in ESI†). Patently, the
subsequently added Br- has substituted on the aromatic ring in
Fig. 5. Kinetics experiments performed to probe the mechanism for the formation of 1-
TAML(X). Plot of kobs against the concentration of (a) DMOB and (b) Br–. The linear fit of
the data in (b) gives a second-order rate constant, k, for the aromatic substitution of
oxidised 1(Br).
1
ahead of the already coordinated Cl-. These results indicate
that the conventional electrophilic halide aromatic substitution
pathways are not operating for our system, and instead
support our proposed atypical mechanism, where 1(X)-TAML+
is attacked by an external anion to form the 1-TAML(X)
products.
To probe if an intermolecular mechanism is involved, 1,4-
dimethoxybenzene (DMOB) has been chosen as an electron-
rich aromatic substrate, with straightforward H NMR signals
1
(singlets), to facilitate monitoring of the substitution reaction
progress. In the presence of DMOB (10 equivalents), Br- (10
equivalents), and 1,1,2,2-tetrachloroethane as the internal
Mössbauer spectroscopic analysis on 57Fe-enriched
1 ( )
571
was carried out to obtain further insights into the oxidation
process. Without exogenous anions present, the oxidation of
571 gives a new species (δ = 0.153 mmꢀs−1, ΔEQ = 3.843 mmꢀs−1,
Fig. 6a; blue component), typical of S = 3/2 (TAML)FeIII
complexes (Table 2 entry 4). This clearly supports ligand-based
standard, we oxidised
1
with (ArMe)3N+ (2 equivalents).
Comparison of the 1H NMR spectroscopic and gas
chromatography-mass spectrometric (GC-MS) data of the
reaction mixtures before and after oxidation revealed that
DMOB remained unreacted, and no signals corresponding to
brominated DMOB were detected (Fig. S17e). Similar to
previous experiments in the absence of DMOB, the ESI-MS
data of the oxidised reaction mixture shows 1-TAML(Br)n (Fig.
S17f in ESI†), indicating that the bromide substitution reaction
occurs exclusively at the TAML ligand despite the presence of
the electron-rich DMOB. Furthermore, we conducted kinetics
measurements to evaluate the rate dependence on the
concentration of DMOB. We prepared a solution of 1(Br)
oxidation of 571, forming 571-TAML +. In the presence of labile

Br–, the Mössbauer spectrum of the oxidised product (1 min
after oxidation) shows a mixture of species comprising of 63%
of 571(Br)-TAML+ and 29 % of a new species (δ = -0.161
mmꢀs−1, ΔEQ = 3.232 mmꢀs−1; Table 2 entry 7), which we assign
to the formation of S = 1 (TAML)57FeIV(Br) (571+(Br), red
component in Fig. 6b). The major 571(Br)-TAML+ component
undergoes nucleophilic attack by Br– to eventually form
(generated in situ by mixing
1
with 10 equivalents of Br- for 5
minutes) and added 2 equivalents of (ArMe)3N+ with varying
concentrations of DMOB. By monitoring the decay of the UV-
visible spectral signals of the oxidised 1(Br) over time at the
absorption maximum of 893 nm (Fig. S17d), we fit the kinetics
data (inset of Fig. S17d) to obtain the corresponding pseudo
first-order rate constant, kobs. The plot of kobs against
concentration of DMOB does not appear to have a significant
positive correlation (Fig. 5a), suggesting that DMOB does not
compete with the reaction between the oxidised 1(Br) and
exogenous Br-. These results suggest that an intermolecular
electrophilic aromatic substitution mechanism is not
operational.
To explore the feasibility of an intramolecular electrophilic
aromatic substitution mechanism, we conducted kinetics
measurements with varying concentrations of exogenous
bromide. We reacted 1(Br) (generated in situ by mixing
>10 equivalents of Br- for 5 minutes) with 2 equivalents of
(ArMe)3N
+, with varying concentrations of Br-, and obtained
the corresponding kobs using the kinetics data collected by UV-
1 with
Fig. 6. Mössbauer spectra of (a) 571 and Ru(bpy)33+, (b) 57
1
, Ru(bpy)33+, and Br-, (c)
571, Ru(bpy)33+, and Cl-, (d) 571+(CN)2. All Ru(bpy)3 oxidant solutions were
generated photochemically from Ru(bpy)3(PF6)2 precursors to minimise
interference from the exogenous anions. Samples (a) to (c) were taken 1 minute
3+

visible spectroscopy. If the reaction occurs due to nucleophilic after the addition of the oxidant. The black lines are the simulations representing
>95% of the absorption. Shaded areas are the spectral contributions from 571-
TAML+ (blue) and 571+ (red). A high-spin FeIII impurity (5-10%) that we attribute
to a deligated by-product, is observed in all cases.
attack of Br- on the 1(Br)-TAML+ radical cation as we
proposed, the kobs should increase with increasing Br-
concentration. Conversely, the kobs from an intramolecular
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DOI: 10.1039/C7SC05378A
Table 2. Summary of the Mössbauer results from Fig. 6 and Fig. S2 in ESI†.
Entry
Reaction Mixture
EQ (mm s-1)
(mm s-1)
Percentagea (%)
Assigned Fe species
3.071
3.850
0.182
0.184
63
34
1
2
3
4
5
6
7
8
9
571b
FeIII
FeIII
3.071
3.844
0.186
0.186
73
27
571c
3.158
3.949
0.208
0.195
12
88
1d
FeIII
571 + RuIII(bpy)33+ (1 eq) b
3.843
0.153
93
FeIII-TAML+
3.123
3.542
-0.075
0.105
71
29
FeIV(Cl)
FeIII(Cl)-TAML+
571 + RuIII(bpy)33+ (4 eq) + Cl- (10 eq), 1 min b
571 + RuIII(bpy)33+ (4 eq) + Cl- (10 eq), 30 mins b
571 + RuIII(bpy)33+ (4 eq) + Br- (10 eq), 1 min b
571 + RuIII(bpy)33+ (4 eq) + Br- (10 eq), 90 mins b
3.123
3.547
-0.103
0.183
56
40
FeIV(Cl)
FeIII-TAML(Cl)n
3.232
3.580
-0.161
0.152
29
63
FeIV(Br)
FeIII(Br)-TAML+
4.000
2.977
0.125
85
80
FeIII-TAML(Br)2
FeIV(CN)2
571+(CN)2
-0.141
c
a
There is a small contribution (the remainder of the sample) from 57Fe species with an unusual EQ (0.3 – 0.7 mm s-1) and  (0.2 – 0.5 mm s-1), which we believe to
b
c
arise from deligated (TAML)Fe (unshaded region of the Mössbauer spectra). Samples were dissolved in 1:1 water:acetonitrile. Samples were dissolved in
acetonitrile. d Sample was measured as a solid.
571-TAML(Br)2 (δ = 0.125 mmꢀs−1, ΔEQ =4.000 mmꢀs−1; Table 2
entry 8 and Fig. S2e in ESI†) as the sole product in 90 mins the UV-visible measurements of
(bottom, Fig. 7). Similar valence tautomerisation is observed new absorption band with max = 440 nm (green, Fig. S17b in
between 571(Cl)-TAML + and 571+(Cl) in the presence of Cl- (Fig. ESI†), which is not observed in the case of Cl- or Br-. The first
6c), but the equilibrium markedly shifts to the FeIV 571+ form redox wave in the CV of
in the presence of 10 equivalents of
CN- becomes more positive compared to the CV of
alone,
and
in the presence of Br- and Cl- (Fig. S17c, blue). We
Intriguingly, in the presence of excess (10 equivalents) CN-,
1
reveal a slow formation of a

1
(Table 2 entry 5).
1
1
attribute this observation to the formation of 1(CN)2 (top
centre, Fig. 7). Oxidation of 1(CN)2 appeared to solely stabilise
( ,
the one electron-oxidised 1+ as [(TAML)FeIV(CN)2]2- 1+(CN)2
top right, Fig. 7). Negative-ion mode cold-spray ionisation-
mass spectrometry (CSI-MS) conducted on the crude reaction
mixture at -40 °C revealed that majority of the FeIV complex
was the mono-anionic 1+(CN), while a small proportion was
identified as the di-anionic complex paired with K+, K[1+(CN)2
]
(Fig. 8a). Using infrared (IR) spectroscopy (Fig. 8b), we
observed a blue-shift in the C≡N stretch on going from KCN
(2076 cm-1) to [FeIII-CN] (2098 cm-1) to [FeIV-CN] (2127 cm-1),
consistent with CN- being coordinated to an increasingly Lewis
acidic Fe metal centre, with the highest oxidation state FeIV
being most electrophilic. Raman spectroscopic measurements
did not give any observable signals for the C≡N vibration.
We
isolated
the
CN--stabilised
FeIV
complex
(PPh4)2[1+(CN)2] and grew single crystals of the dark blue
complex for X-ray crystallography (Fig. 8c). The data was
refined to give a disorder model with the TAML ligand
occupying two positions, although there was no disorder in the
Fe-(CN)2 core (Fig. S19a in ESI†). Other crystallographic
Fig. 7. Proposed mechanism for the oxidation of 1, followed by valence tautomerisation
between 1(X)-TAML+ and FeIV intermediates, depending on the exogenous anion.
6 | Chem. Sci., 2018, 00, 1-3
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EDGE ARTICLE
parameters are in Tables S2-6 in ESI†. Mössbauer dependent on the nature of the exogeneous VaienwiAortnicsle. OTnlhinee
measurements were performed to confirm the oxidation state presence of labile Cl– or Br– results in a liDgOanI:d10-b.1a0s3e9/dC7oSxCid05a3t7io8nA
+
of iron in 1+(CN)2. A major doublet (δ = -0.141 mmꢀs−1, ΔEQ
=
and stabilisation of a radical-cationic 1-TAML
•
complex. In
2.977 mmꢀs−1, Fig. 6d; red component) is consistent with an S = contrast, exogenous cyanide culminates in metal-based
1 FeIV centre (Table 2, entry 9). To the best of our knowledge, oxidation to form 1+
this is the first crystal structure of a high valent cyano FeIV
Notably, although valence tautomerisation is observed in
TAML complex, although Collins and coworkers had some synthetic porphyrinoid counterparts,11,12,14 anion-
spectroscopically characterised square pyramidal dependent valence tautomerisation in non-heme iron systems
[FeIV(DCB)(CN)]- mono-cyanide before.41,44 Notably, our finding are very uncommon.40,41 The reported radical-cations often
adds to
unique collection of high-valent cyano FeIV show enhanced substrate C-H activation behavior,47 similar to
complexes,45,46 with ours being the first, di-anionic, octahedral reports on cytochrome P450, and recently on MnIV(O-
.
a
a
(TAML)FeIV dicyanide complex to be characterised by X-ray LA)(TBP8Cz +) (LA = Lewis acid, TBP8Cz = octakis(p-tert-

crystallography. We also characterised the purified 1+(CN)2 butylphenyl)corrolazinato3−) as well.14 However, we show that
using UV-visible spectroscopy (Fig. 8d). Careful scrutiny of the radical-cations on sterically unprotected ligands, such as, 1-
oxidised products of
Br-, CN-), therefore, suggests valence tautomerisation between by nucleophilic halogen substitution on the TAML aryl ring. The
1+ and 1-TAML + (Fig. 7), governed by the exogenous anion.
nucleophilic nature of this halogenation reaction contrasts
From a broader perspective, our findings highlight some of with the more commonly reported mechanisms involving
1 
in the presence of different anions (Cl-, TAML +, can undergo unprecedented C-H activation followed

the important aspects for developing photodriven biomimetic either a halogen radical (X
oxidation catalysts and an uncommon mechanism for high- (X+).40,41,44
valent iron reactivity. Currently, the most common oxidants to
) or an electrophilic halogen
access high-valent iron species are inner-sphere reagents like
peroxides or hypervalent iodine reagents. We report here the
generation of (TAML)FeIV 1+) and (TAML•+)FeIII 1-TAML•+), by
( (
Conclusions
In summary, the present work highlights the significance of
redox non-innocent ligands under highly oxidising conditions,
and the possible unexpected reactivity the complexes may
have with the exogenous compounds, when designing new
catalysts for oxidation reactions. These insights should be
valuable in future studies and improvements to the designs of
biomimetic high valent catalysts. Ligand non-innocence could
be an advantage to achieve enhanced oxidation reactions, but
the present work highlights the possible alternative reactivity
that may be operative for systems involving radical-cations on
sterically unprotected ligands. Overcoming the C-H activation
followed by nucleophilic halide substitution on the aryl ring
will be required to facilitate high turnovers for photocatalytic
oxidation reactions involving TAML-based transition metal
complexes. Nonetheless, similar nucleophilic reactions could
arise in other complexes with redox non-innocent ligands.
Whether such a nucleophilic aromatic halogenation reaction
mediated by 1-TAML+ can also be extended to redox non-
innocent ligands such as porphyrinoids is now an intriguing
question, which will be investigated in subsequent studies in
our laboratories. Moreover, our work also offers a novel
approach for intramolecular C-H activation reactions with bona
fide high-valent Fe complexes.
oxidising
1 in presence of photochemically produced
Ru(bpy)33+. The relative proportion of the two tautomers is
Fig. 8. Characterisation data of the CN--stabilised 1+ product generated from the
reaction 1 + 5 eq. KCN + 1.05 eq. (ArMe)3N+. (a) Negative-ion CSI-MS and the assigned
intermediates in the crude reaction mixture containing the dianionic (TAML)FeIV
complex [1+(CN)2]2-. (b) Infrared spectra of KCN (black, vmax/cm-1 2076 [KCN]); 1 + 5 eq.
KCN (red, vmax/cm-1 2098 [FeIII(CN)]); and 1 + 5 eq. KCN + 1.05 eq. (ArMe)3N+ (blue,
vmax/cm-1 2127 [FeIV(CN)]). (c) X-ray crystal structure of the dianionic FeIV TAML complex
[(1+(CN)2](PPh4)2], supporting an assignment of the FeIV valency by charge balance.
Atom colours: grey for carbon, blue for nitrogen, red for oxygen, yellow for
phosphorus, and orange for iron. The hydrogen atoms have been omitted for clarity.
The ellipsoids are drawn at 50% probability. Selected bond lengths (Å): Fe1-C20
2.011(6), Fe1-C21 1.993(6), C20-N5 1.137(7), C21-N6 1.141(7). (d) UV-visible spectrum
of a 0.62 mM purified 1+(CN)2 solution. max(DCM)/nm 352 (ε/dm3 mol-1 cm-1 1800), 452
(550), 596 (540), 639 (540), 728 (290), and 855 (160).
Experimental
General information
Chemicals were purchased from Sigma-Aldrich, Alfa-Aesar, and
Tokyo Chemical Industry (TCI). Unless otherwise stated, the
commercial reagents were used as purchased. All reactions
involving air- or moisture-sensitive compounds were
performed by standard Schlenk techniques in oven-dried
reaction vessels under a nitrogen atmosphere. All UV-visible
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absorption measurements were carried out on a Shimadzu UV- Each Mössbauer sample was prepared by adding appropriate
View Article Online
DOI: 10.1039/C7SC05378A
3600 spectrophotometer, except for kinetics measurements, equivalents of the reagent(s) to a 2 mM 1:1 ultrapure water:
where an Agilent Cary 8454 spectrophotometer was used acetonitrile (ACN) solution of 571. [RuIII(bpy)3(PF6)2]+ (in the
instead. Deuterated solvents were purchased from Cambridge absence of chloride) was generated photochemically in the
1
Isotope Laboratories and were used as received. The H and presence of Na2S2O8 using the same procedure described in
13C NMR spectroscopic measurements were recorded on a the section “Oxidation of
1 Using Independently Generated
1
Bruker AV-500 (500 MHz) NMR spectrometer. The H and 13C Ru(bpy)33+”. LiCl and LiBr were used as the Cl- and Br- sources
NMR spectra are reported in parts per million (ppm) downfield respectively, and were dissolved together with 571 before
referenced to the residual protons of the deuterated solvents.
addition of the oxidant. Upon adding the reagent(s) to the 571
High-resolution mass spectra (HR-MS) were obtained with a Q- solution, the mixture was shaken vigorously. Subsequently, a
TOF Premier LC HR mass spectrometer. Cold-spray ionisation- ~0.5 mL aliquot was withdrawn and added to a pre-cooled
mass spectrometry (CSI-MS, negative ion mode) was Mössbauer cup. The half-filled Mössbauer cup was frozen in
performed using an AccuTOF (JEOL) mass spectrometer liquid nitrogen and Mössbauer spectroscopic measurements
equipped with a CSI source. The following conditions were were conducted on each sample. The Mössbauer data and fits
used: needle voltage = -2.0 kV; orifice 1 voltage = 0 V; ring lens are shown in Fig. 6 and Fig. S2 in ESI†, and their quadrupole
voltage = -50 V; and spray temperature = −40 °C. X-ray splittings (
photoelectron spectroscopy (XPS) data were acquired using a 2.
Phoibos 100 spectrometer and a Mg X-ray source (SPECS,
EQ) and isomer shifts () are summarised in Table
Germany). XPS samples were prepared inside a glovebox with Cyclic voltammetry
a nitrogen atmosphere and SPI double-sided adhesive carbon
Cyclic voltammetric measurements were conducted using a
Biologic SP-300 potentiostat with 0.10 M n-Bu4NPF6 in
anhydrous ACN. A standard three-electrode electrochemical
cell was used with a glassy-carbon working electrode (3 mm in
diameter from BAS), a Pt wire as the counter electrode, and
another Pt wire as the pseudo-reference electrode. The cyclic
tape was used to hold the sample on the sample plate. The
XPS data were calibrated based on the C 1s position at 284.6
eV and processed using the programme CasaXPS. Elemental
analyses were performed with an Elementar vario MICRO cube
analyser. Infrared spectroscopic measurements were carried
out on a Bruker VERTEX 80 spectrophotometer.
voltamogram of
1 at various scan ranges can be found in Fig.
Mössbauer spectra in the absence of magnetic fields were
each recorded on a SEECO MS6 spectrometer that consisted of
the following instruments: a JANIS CCS-850 cryostat, including
a CTI-CRYOGENICS closed-cycle 10 K refrigerator, and a CTI-
CRYOGENICS 8200 helium compressor. The cold head and
sample mounts are equipped with calibrated DT-670-Cu-1.4L
silicon diode temperature probes and heaters. The
temperatures are controlled by a LAKESHORE 335 temperature
controller. Spectra are recorded using a LND-45431 Kr gas
proportional counter with a beryllium window connected to a
SEECO W204 -ray spectrometer that includes a high voltage
supply, a 10 bit and 5 μs ADC, and two single channel
analysers. Motor control and recording of spectra is managed
by a W304 resonant -ray spectrometer. For the reported
spectra, a RIVERTEC MCO7.114 source (57Co in Rh matrix) with
an activity of about 1 GBq was used. The spectra were
recorded at 17 K and the data were accumulated for about 1
week for each sample. Mössbauer data were processed and
simulated using the WMOSS4 programme (www.wmoss.org).
Isomeric shifts are referenced to α-iron at room temperature.
S3 in ESI†.
Nanosecond transient absorption spectroscopic measurements
The transient absorption spectroscopic (TAS) and transient
emission spectroscopic (TES) measurements were performed
using an Edinburgh Instruments model LP920 transient
absorption spectrometer equipped with a pulsed Xe probe
lamp in conjunction with a Nd:YAG laser (Continuum model
Surelite II-10) as the excitation source. The laser pulse width is
5-8 ns and the repetition rate is 10 Hz. During TAS
measurements, the pulses were synchronised with the LP920
system at a frequency of 1 Hz. The pulse energy used was
between 4-5 mJ/pulse. In all our transient absorption spectra,
the detected intensity of the transmitted signals are presented
as a logarithm of the ratio of the light intensity from the probe
beam after laser excitation to the intensity before laser
excitation
absorption (positive
(negative OD) of the transient photoexcited species relative
(

OD). Hence,
OD refers to the increased

OD) or reduced absorption/emission

to the ground state.
Pentaamminechlorocobalt(III) chloride (Co(NH3)5Cl3) was
added to the sample solution as
a solid. Tris(2,2'-
Synthesis of 1
bipyridyl)dichlororuthenium(II) hexahydrate (Ru(bpy)3Cl2) was
prepared as a 1.0 mM stock solution in a volumetric flask. The
stock solution was stored in a refrigerator for a maximum
The procedures were adopted and modified from both Sullivan
et al. and Ellis et al.38,48 The modified experimental procedures
and the characterisation data of the synthetic intermediates
can be found in the ESI† (Fig. S1, and Fig. S20-23).
period of one week. The Na salt of
1 was purified by reverse-
phase column chromatography and used in all transient
absorption measurements. Similarly, a 1.0 mM stock solution
Mössbauer spectroscopy
of
1 was prepared. A stock solution of 1 was freshly prepared
every day, since its stability in ACN is moderate. The quality of
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1
may affect the absolute lifetimes of the transient absorption The same instrumental setup and sample preparations as
View Article Online
DOI: 10.1039/C7SC05378A
measurements. Therefore, the same batch of
throughout the whole set of experiments.
1
was used described in the section “Nanosecond Transient Absorption
Spectroscopic Measurements” were used. Instead of the
The samples were added in the solid form or as stock Xe900 xenon lamp, a tungsten halogen lamp (50 W, 12 V) was
solutions into a 3.5 mL quartz cuvette (LATECH™, Model: Q- employed as the probe beam because it would be more stable
204). Ultrapure water (H2O, Milli-Q Advantage A10, TOC ≤ 5 over a longer timescale.
ppb) and ACN (HPLC grade) were added to make a final volume
of 3.0 mL with a ratio of H2O:ACN = 3:1. The cuvette was then Oxidation of 1 using independently generated Ru(bpy)3
3+
fitted with a rubber septum and sealed using Parafilm M®. The
3+
Ru(bpy)3
was independently generated using sodium
cuvette was sonicated to ensure that the solution was
homogeneous. Then with a needle fitted through the rubber
septum as a gas outlet, argon (Ar) gas was bubbled through
the solution for 5 minutes and its transient absorption
properties were measured. The detailed composition of each
set of experiment is depicted in Table S1 in ESI†.
Each set of data was fitted to a single exponential or
biexponential function to obtain the time constant(s) for the
transient absorption signal according to the following two
equations respectively:
persulfate (Na2S2O8) for stoichiometric oxidation reactions of
the Na salt of
1. Co(NH3)5Cl3 has not been used as the
sacrificial electron acceptor because both CoII and CoIII are
2+
coloured and will interfere with the signals from Ru(bpy)3
, 1,
and the product(s) formed. In addition, after Co(NH3)5Cl3 was
reduced, a precipitate that was likely to be the poorly soluble
Co(OH)2 formed, which would further complicate UV-visible
spectroscopic measurements.
The experiment was carried out in a dark room. Stock
solutions of
as those described above in the section “Nanosecond
Transient Absorption Spectroscopic Measurements”.
1 and Ru(bpy)3Cl2 were prepared in the same way
푦 = 푦0 + 퐴1푒−(푡−휏 )/휏
0
1
−(푡−휏0)/휏2
푦 = 푦0 + 퐴1푒−(푡−휏 )/휏 + 퐴2푒
0
1
Ru(bpy)3Cl2 (2.5 mL of a 1.0 mM stock solution) was added to a
Schlenk flask connected to a N2 inlet, following which H2O (2.5
mL) was added. Na2S2O8 (6.0 mg) was then added to the
solution, giving a composition of [Ru(bpy)3Cl2] = 0.50 mM and
[Na2S2O8] = 5.0 mM. The solution was bubbled with N2 for 10
The parameters 푦0 휏0, 퐴푛, and 휏푛 were determined by a least-
,
squares fitting procedure in Origin. The term 푦0 corresponds
to the vertical intercept at long lifetimes and indicates whether
the signal decays to a ‘permanent’ bleach (negative y0) or
absorption (positive y0). The term 휏0 is the delay time of the
excitation pulse from the start of the probe measurement
during each photoexcitation cycle. 퐴푛 is the change in optical
minutes and then irradiated with blue LEDs (450 nm,
∼1 W)
for 10 minutes. The solution turned from yellow (Ru(bpy)32+) to
green (Ru(bpy)33+). Aliquots (0.20 mL, 1 equivalent or 0.40 mL,
2 equivalents) of the solution were drawn using a micropipette
density after irradiation for the
푛th exponential term and 휏푛 is
and added to a 1 mL volumetric flask. The Na salt of
1 (0.10
the corresponding time constant.
mL) in a 1.0 mM stock solution was measured using a
The errors corresponding to the fits of the transient signal
lifetimes were determined via the principle of error
propagation by calculating the root-mean-square deviation
from the sum of squares of the uncertainties in each measured
value. For each time-resolved measurement, the associated
uncertainties included the laser pulse duration, spectrometer
time-resolution, mass of samples, and volumes of samples.
Hence, the error of the transient signal lifetime,was
calculated according to the following equation:
3+
micropipette and added into the Ru(bpy)3 solution. The
solution was diluted with appropriate volumes of ACN and H2O
to make up a total volume of 1 mL with a ratio of H2O:ACN =
1:1, giving the final concentrations as [Ru(bpy)33+] = 0.10 mM
(1 equivalent) or 0.20 mM (2 equivalents), and [
The cuvette was then placed in
1
a
] = 0.10 mM.
UV-visible
spectrophotometer and its spectrum was recorded at regular
time intervals.
Synthesis of tri(p-tolyl)amine ((ArMe)3N)
2
2
2
2
훿휏a
훿휏b
훿푚k
푚k
훿푉
n
√
훿휏 = 휏 ⋅
(
) + (
) + (
) + (
)
The synthesis was modified from that described by Goodbrand
and Hu (Fig. S13 in ESI†).49 Compound p-toluidine (2.14 g, 20.0
mmol), 1-iodo-4-methylbenzene (9.16 g, 42.0 mmol),
potassium hydroxide (8.75 g, 150 mmol), anhydrous copper(I)
chloride (77 mg, 0.78 mmol), and 1,10-phenanthroline (140
mg, 0.80 mmol) were suspended in anhydrous toluene (20
mL). Three freeze-pump-thaw cycles were applied to remove
dissolved O2 from the suspension. The mixture was then
sealed and heated at 120 °C for 72 hours, and completion was
confirmed by TLC. The reaction mixture was diluted with water
(100 mL) and extracted with 3 x 150 mL hexane. The organic
fractions were combined and rinsed with saturated NaHCO3
solution (100 mL). The combined organic fractions were dried
over anhydrous Na2CO3 and filtered. The hexane was removed
휏a
휏b
푉
n
where = transient signal lifetime; a = uncertainty of the
laser pulse duration; b = uncertainty of the spectrometer
time-resolution;
component, where k represents the different dissolved
mk =
uncertainty of the mass of kth
constituents such as Ru(bpy)3Cl2,
uncertainty of the nth volume measurement, where
represents the n different volume measurements including
stock solution and sample preparations.
1
, and Co(NH3)5Cl3;

Vn
=
n
Transient absorption measurements at four-seconds timescale
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by rotary evaporation to yield a dark brown precipitate. The The same experimental procedures as those for Br-
View ArticlewOnelirnee
precipitate was purified using flash column chromatography employed, with TEABr being replaced byDtOetI:r1a0e.1t0h3y9la/Cm7SmCo05n3iu78mA
and eluted with hexane:DCM = 9:1. Upon solvent removal by chloride (TEACl). HR-MS (ESI-, m/z) calculated for
rotary evaporation, colourless crystals of (ArMe)3N (4.14 g, C19H13ClFeN4O4 (1-TAML(Cl)) [M - H]- m/z = 451.9975, found
1
72%) were obtained. H NMR (DMSO–d6, 500 MHz, Fig. S24 in 451.9977; and C19H12Cl2FeN4O4 (1-TAML(Cl)2) [M - H]- m/z =
ESI†): δ = 2.24 (s, 9 H, Me), 6.84 (d, J = 13.5 Hz, 6 H, ArH), 7.06 485.9585, found 485.9579. XPS (Cl 2p) measurements revealed
(d, J = 13.5 Hz, 6 H, ArH). The data concurs with that reported two sets of signals: one set has a 2p3/2 binding energy = 197.8
previously.49
eV (grey) and a 2p1/2 binding energy = 199.3 eV (orange),
matching that of inorganic Cl-, while the second set has a 2p3/2
binding energy = 200.4 eV (blue) and a 2p1/2 binding energy =
201.9 eV (green) (Fig. S16b in ESI†). The new set of signals
does not correspond to that of inorganic Cl- but falls within the
range of aryl chlorides, giving additional evidence for our
proposed formulation of 1-TAML(Cl) and 1-TAML(Cl)2
products.
Synthesis of tri(p-tolyl)aminium radical-cation ((ArMe)3N+)
This reaction was performed without Schlenk techniques.
Compound (ArMe)3N (75 mg, 0.26 mmol) and PbO2 (155 mg,
0.65 mmol) were suspended in ACN (3.0 mL). Hydrofluoroboric
acid (HBF4, min. 40%, 57 mL, 0.26 mmol) was added to the
suspension, giving a dark blue suspension. The mixture was
filtered and the dark blue filtrate containing (ArMe)3N+ was
used in situ for subsequent reactions.
Oxidation of 1 in the presence of cyanide anions
This modified reaction was performed without any Schlenk
t
techniques. The Na salt of
1
(30 mg, 0.065 mmol), BuOK (8.0
Oxidation of 1 in the presence of bromide anion
mg, 0.072 mmol), and potassium cyanide (KCN, 21 mg, 0.32
mmol) were suspended in ACN (AR grade, 5 mL) and sonicated
for about 30 seconds until the red solution turned into a green
suspension. The suspension was cooled to 0 °C with stirring.
(ArMe)3N+ (1.05 equivalents) generated in situ was added
dropwise to the suspension, resulting in a black solution
This reaction was performed without any Schlenk techniques.
t
The Na salt of
1
(30 mg, 0.065 mmol), BuOK (31 mg, 0.27
mmol), and tetraethylammonium bromide (TEABr, 140 mg,
0.65 mmol) were suspended in ACN (30 mL) and sonicated for
about 30 seconds until the red solution turned into a green
suspension. The (ArMe)3N+ (4 equivalents) generated in situ
was added dropwise to the suspension, resulting in a black
solution. Within 30 minutes, the black solution turned brown,
and then red, accompanied by the formation of a white
precipitate. Upon completion of the reaction, as monitored by
negative-ion mode ESI-MS, the suspension was filtered and the
red filtrate was diluted with diethyl ether (Et2O, 50 mL), giving
an orange precipitate. The precipitate was filtered and washed
with Et2O (10 mL) and water (5 mL). The residue was dissolved
in methanol (MeOH, 5 mL) and filtered. The solvent of the
filtrate was removed by rotary evaporation to give the crude
TEA[1-TAML(Br)2] product. C18 reverse-phase preparative
column chromatography (Merck LiChroprep ®RP-18, 40 – 63
mm) was utilised to further purify the crude product. HR-MS
(moderately stable at room temperature, lifetime, 837nm, ≈ 4.4
± 0.7 hours, Fig. S18a-b in ESI†). After one minute of stirring,
the solvent was removed on a Schlenk line at 0 °C. Once dried,
the resulting black-grey solid (stable at room temperature
under inert gases), was transferred into a glovebox for
subsequent workup. In a glovebox, the solid was suspended in
Et2O (20 mL) and filtered. The black residue was washed with
Et2O (2 x 10 mL) and dichloromethane (DCM, 10 mL). The black
residue was then dissolved in ACN (4 mL) and filtered.
Tetraphenylphosphonium chloride (PPh4Cl, 50 mg, 0.13 mmol)
was added to the black filtrate, giving a dark blue mixture. The
solvent was then evaporated in vacuo. The dried residue was
suspended in DCM (5 mL) and filtered to remove KCl. The dark
blue filtrate was dried in vacuo. The residue was dissolved in
ACN (0.5 mL), and recrystallised via vapor diffusion of Et2O (5
mL) into the ACN solution at -30 °C. After five days, the
supernatant was decanted to reveal black crystals of
(PPh4)2[1+(CN)2] (20 mg, 27 %). Found: C, 71.1; H, 4.9; N, 7.1.
Calc. for C69H56FeN6O5P2: C, 71.0; H, 4.8; N, 7.2%.
(ESI-, m/z) calculated for C19H1279Br81BrFeN4O4 (1-TAML(Br)2
)
[M - H]- m/z = 575.8554, found 575.8566. The XPS (Br 3d)
binding energy = 70.7 eV, corresponded to Ar-Br and not
inorganic Br- (Fig. 3b). Despite many attempts through
different solvent combinations, different counter cations
(PPh4+ and PPN+), and different temperatures, single crystals of
1-TAML(Br)2 could not be obtained. To gather further
structural information of 1-TAML(Br)2, purified 1-TAML(Br)2
was suspended in 1:9 MeOH: pH 5 acetate buffer solution to
induce deligation of 1-TAML(Br)2 to give TAML(Br)2, which was
diamagnetic and therefore could be measured by 1H NMR
Deligation of 1-TAML(X)n for 1H NMR measurements
The same procedures as those described above in the sections
“Oxidation of
1 in the presence of bromide/chloride anions”
were employed. At specific time intervals, an aliquot (5 mL)
was withdrawn, followed by the addition of pH 7 phosphate
buffer solution (0.2 M, 1 mL) and aqueous sodium thiosulfate
(Na2S2O3, 0.2 M, 1 mL) to the aliquot, resulting in an
instantaneous change in the colour of the mixture from
black/dark brown (depending on the reaction progress) to
orange. The solvent was evaporated in vacuo. The dried
residue was suspended in a pH 7 buffer solution (0.2 M, 1 mL)
and was allowed to stir for 12 hours, during which the
1
spectroscopy. The H NMR spectrum reveals a major product
of the symmetrical TAML(Br)2, and other signals, which we
believe to be asymmetrically bromide-substituted TAML(Br)2
or decomposed TAML(Br)2 during acid hydrolysis (Fig. S15 in
ESI†).
Oxidation of 1 in the presence of chloride anions
10 | Chem. Sci., 2018, 00, 1-3
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S. Shaik, H. Hirao and D. Kumar, Acc. Chem. Res., 2007, 40
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3
4
5
,
suspension changed from orange to beige. The suspension was
filtered and the residue was washed with Et2O (2 mL) to
remove most of the N(ArMe)3. After removing the Et2O by
filtration, the residue was suspended in DMSO-d6. The
suspension was filtered and one drop of concentrated
hydrochloric acid (HCl, 12 N) was added to the filtrate to
ensure that the TAML and the other products are not in their
deprotonated forms. The solution was then characterised by
1H NMR spectroscopy.
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Oxidation of 1(Br) in the presence of DMOB
Compound 1,4-dimethoxybenzene (DMOB, 5.0 mg, 36
the Na salt of (1.7 mg, 3.6 mol), and TEABr (7.6 mg, 36
mol), were dissolved in CD3CN (0.50 mL). The 1H NMR
spectrum of the solution was recorded. Compound (ArMe)3N+
mol),
1

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Kinetics measurements of the reaction of oxidised 1(Br) with
varying concentrations of bromide anions
A solution of the Na salt of
in ACN (2.0 mL) was allowed to stir for 5 minutes to form 1(Br)
(ArMe)3N+ (40
L, 20 mM) was added to the solution and the
1 (0.20 mM) and TEABr (2 - 14 mM)
.

UV-visible spectrum of the reaction mixture was recorded at 5-
second intervals. Each kinetics data set was fitted to a single
exponential function and the time constant was determined by
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constant, k, for the aromatic substitution of the oxidised 1(Br)
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Conflict of interest
23 A. D. Ryabov, in Adv. Inorg. Chem., eds. E. Rudi van and D. H.
Colin, Academic Press, 2013, vol. 65, pp. 117-163.
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There are no conflicts of interest to declare.
Acknowledgements
HSS is supported by the Nanyang Assistant Professorship and
and MOE Tier 1 grants (M4011611 and M4011791). HSS also
gratefully acknowledges the Agency for Science, Technology
and Research (A*STAR), AME IRG grants A1783c0002 and
A1783c0003, for funding this research. The authors
acknowledge support from the Solar Fuels Lab at NTU and the
SinBeRISE CREATE. JHL would like to thank Mr. Zonghan Hong
(NTU) for the XPS measurements, and Ms. Yijie Yang (NTU) for
the Raman spectroscopic measurements. KR is funded by
UniCat and the Heisenberg Professorship of DFG.
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Table of contents
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DOI: 10.1039/C7SC05378A
(Photo)chemical oxidation of a (TAML)FeIII complex by outer-
sphere oxidants result in valence tautomerisation and C-H
activation governed by exogenous anions.
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