A nonheme peroxo-diiron(iii) complex exhibiting both nucleophilic and electrophilic oxidation of organic substrates

Article abstract of DOI:10.1039/d1dt01502h

The complex [FeIII2(μ-O₂)(L₃)₄(S)₂]⁴⁺(L₃= 2-(4-thiazolyl)benzimidazole, S = solvent) forms upon reaction of [Fe^II(L₃)₂] with H₂O₂and is a functional model of peroxo-diiron intermediates invoked during the catalytic cycle of oxidoreductases. The spectroscopic properties of the complex are in line with those of complexes formed with N-donor ligands. [FeIII2(μ-O₂)(L₃)₄(S)₂]⁴⁺shows both nucleophilic (aldehydes) and electrophilic (phenol,N,N-dimethylanilines) oxidative reactivity and unusually also electron transfer oxidation.

Full text of DOI:10.1039/d1dt01502h

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A nonheme peroxo-diiron(III) complex exhibiting
both nucleophilic and electrophilic oxidation of
organic substrates†
Cite this: Dalton Trans., 2021, 50,
7181
Received 9th May 2021,
Accepted 14th May 2021
c
Patrik Török,a Duenpen Unjaroen,b Flóra Viktória Csendes,a Michel Giorgi,
a
Wesley R. Browne *b and József Kaizer
*
DOI: 10.1039/d1dt01502h
rsc.li/dalton
The complex [Fe2III(μ-O2)(L3)4(S)2]4+ (L3 = 2-(4-thiazolyl)benzimida- peroxodiiron(III) species is capable of direct electrophilic and/
zole, S = solvent) forms upon reaction of [FeII(L3)2] with H2O2 and is or nucleophilic reactions.13 We reported the first examples for
a functional model of peroxo-diiron intermediates invoked during the reactivity of peroxodiiron(III) species in H2O2 disproportio-
the catalytic cycle of oxidoreductases. The spectroscopic properties nation,13a RNR-R2 type phenol oxidation and as mimics for
of the complex are in line with those of complexes formed with ADO reactivity towards aldehydes.13a,c Ambiphilic behaviour in
N-donor ligands. [FeI2II(μ-O2)(L3)4(S)2]4+ shows both nucleophilic these complexes was rationalised by the involvement of two
(aldehydes) and electrophilic (phenol, N,N-dimethylanilines) oxi- distinct oxidants, namely functional models of the nucleophi-
dative reactivity and unusually also electron transfer oxidation.
lic P, and the electrophilic Q species.
The spectroscopic characterization of complex with [Fe2III
(μ-O2)(L1,2)4(S)2]4+ cores (S = solvent), as synthetic models of
non-heme iron oxygenases, generated from the trishomoleptic
complexes [FeII(L1)3](CF3SO3)2 (1) (L1 = 2-(2′-pyridyl)benzimida-
zole) and [FeII(L2)3](CF3SO3)2 (2) (L2 = 2-(2′-pyridyl)-N-methyl-
benzimidazole) was reported earlier (Scheme 1).12c
Dinuclear nonheme iron enzymes, including soluble methane
monooxygenase (sMMO),1 toluene monooxygenases (TMO),2
steraoyl-ACP Δ9-desaturase (Δ9D),3 ribonucleotide reductases
(RNR-R2),4 cyanobacterial aldehyde deformylase oxygenase
(cADO),5 arylamine N-oxygenase (CmII),6 p-aminobenzoate oxy-
genase (AurF),7 and deoxyhypusine hydroxylase (hDOHH)8 are
responsible for a broad range of oxidative reactions such as
hydrogen atom transfer (HAT), oxygen atom transfer (OAT) and
C–C bond cleavage. For such enzymes, catalytic (µ-1,2-peroxo)
diiron(III) (P) intermediates have been postulated as key inter-
mediates during the formation of mixed valent iron(III)iron(IV)
(X) or diiron(IV) (Q) intermediates to react via electrophilic HAT
and OAT reactions. P intermediates show nucleophilic reactiv-
ity in deformylation of aldehydes.9
Here we report the formation of the peroxo-diiron(III)
species [FeI2II(μ-O2)(L3)4(S)2]4+ (5) from the N-heterocyclic
ligand, 2-(4-thiazolyl)benzimidazole ligand (L3) (Scheme 1).
This complex shows electrophilic and nucleophilic reactivity in
the oxidation of O–H bonds (H2O2, phenols), aldehyde defor-
mylation, and oxidative N-demethylation of DMA via electro-
philic C–H activation. The broad range of reactivity makes the
complex a good functional model for diiron oxidoreductase
enzymes.
Several (μ-oxo)(μ-1,2-peroxo)diferric,10 (μ-OH or μ-OR)(μ-1,2-
peroxo)diferric,11 and (μ-1,2-peroxo)diferric species have been
characterized12 spectroscopically (resonance Raman, UV-vis
absorption, Mössbauer, etc.) as structural models, however
only a few functional models have been reported where the
aResearch Group of Bioorganic and Biocoordination Chemistry, University of
Pannonia, H-8200 Veszprém, Hungary. E-mail: kaizer@almos.uni-pannon.hu
bStratingh Institute for Chemistry, University of Groningen, Nijenborgh 4,
9747 AG Groningen, The Netherlands. E-mail: w.r.browne@rug.nl
cAix-Marseille Université, FR1739, Spectropole, Campus St Jérome, Avenue Escadrille
Normandie-Niemen, 13397 Marseille Cedex 20, France
†Electronic supplementary information (ESI) available: Additional spectroscopic
and kinetic data, synthetic procedures and characterisation. CCDC 2053805. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d1dt01502h
Scheme 1 Structures of ligands and complexes discussed in the text.
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The hetero-bidentate 2-(4-thiazolyl)benzimidazole ligand characteristic absorption bands can be ascribed to the charge
2−
(L3) reacts spontaneously with Fe(II) salts to form thermo- transfer between Fe(III) and the O2 ligand (Fig. 2, inset).
dynamically stable yellow [FeII(L3)3]2+ (5) complex (Scheme 1), Similarly, the resonance Raman spectrum of 6 obtained with
which was characterized by UV-visible spectroscopy, electro- λexc = 785 nm shows features at 877 and 463 cm−1, which can
spray mass spectroscopy (ESI-MS) (Fig. S1 and S2†), 1H NMR be assigned to ν(O–O) and ν(Fe–O) stretching modes, respect-
spectroscopy Fig. S3†), and X-ray single crystal analysis of ively (Fig. 2). The formation of a Fe–O–Fe bridge in the per-
[FeII(L3)3](ClO4)2 (5) (Fig. 1, Fig. S4, Tables S1 and S2†)). The oxide complex is possible and cannot be excluded; however, in
Fe–N bond distances of 5 are longer than 2.1 Å and are typical the absence of observable bands for such an Fe–O–Fe motif in
of high-spin Fe(II) complexes in contrast to the analogous low- the resonance Raman spectra at 785 nm (ref. 14) and the corre-
spin Fe(II) complex 2 in which the Fe–N bond distances are lations drawn earlier by Que and co-workers between O–O
2.0 Å (Fig. S4†).
stretching frequency and Fe–Fe separation, the detailed struc-
Raman spectroscopy in the solid state and solution show ture of the complex cannot be assigned and is therefore indi-
that the structure is retained in solution, i.e. that unbound cated as [FeI2II(μ-O2)(L1,2,3)2(S)2]4+.
ligand is not present (Fig. S5†). However, 1H NMR spec-
The partial order in H2O2 (1st) and complex 5 (1st) was
troscopy (Fig. S3†) indicates that more than one complex is determined with a second order rate constant k = 6.54 0.28
present in solution, with distinct sets of signals corresponding M−1 s−1 at 293 K (Table S3†) that is close to that for complexes
to several coordination isomers, e.g., mer and fac. In contrast 1 and 2 (5.38 M−1 s−1 and 6.6 M−1 s−1 at 293 K, respectively).12c
to the low spin complexes 1 and 2,12c in solution, 5 does not The half-lives (t1/2) for complexes 3, 4 and 6 are 1200 s, 4740 s,
show absorption in the visible region, which is consistent with and 400 s at 288 K, respectively, demonstrating that complex 6
a high-spin electronic configuration of the latter (Fig. 2). These is more reactive (kdecay = 0.95 × 10−3 s−1) compared to complex
data highlight the sensitivity of the spin-state to the heteroaro- 3 and 4 (kdecay = 1.066 × 10−4
matic unit in the ligand.
s
−1) (Fig. S6 and Table S5†), and
the large ΔH‡ of 84(6) kJ mol−1 with a small ΔS‡ of −2(21) J
In acetonitrile, a green (μ-1,2-peroxo)diiron(III) intermediate mol−1 K−1 (Fig. S7†) suggests a unimolecular decay process.
(λmax = 705 nm, ε = 1200 M−1 cm−1) forms upon addition of Overall, the change from a pyridyl to a thiazolyl moiety in the
H2O2 to 5 at room temperature, that is similar to those formed ligand is sufficient to switch spin state in the iron(II) state and
from 1, 2 (λmax = 720 nm (ε = 1360 M−1 cm−1) and λmax
=
while it does not affect rate of formation of the biomimetically
685 nm (ε = 1400 M−1 cm−1), respectively12c), Fig. 2. These relevant diiron(III) complexes, it does affect the stability of the
species. The presence of excess ligand does not affect the rate
of formation of the peroxide bridged species and has only a
modest effect on its rate of self-decay (Fig. S6†) Hence, it is of
interest whether the reduced stability is translated into
increased reactivity in the oxidation of organic substrates.
The series of complexes [FeI2II(μ-O2)(L2)2(CH3CN)2]4+,12c
[FeI2II(μ-O2)(L3)2(CH3CN)2]4+
and
[FeI2II(μ-O)(μ-O2)(Py)2-
10c
indH)2(CH3CN)2]2+
‡
allows their reactivity towards benz-
aldehyde to be compared.13b,c
The rate of the decomposition of 6 (loss of absorbance at
705 nm) in CH3CN in the presence of substrates was compared
with that for 3 and 4 (Scheme 2). The second-order rate con-
stants in the oxidation of benzaldehyde (BA) with 6 is 2.86 M−1
s−1 at 288 K (Fig. 3, Table S6†), which is three times that for 4
(0.934 M−1 s−1) but identical to 3 (2.39 M−1 s−1), Fig. S8†).
The activity of 6 in nucleophilic and electrophilic oxidation
reactions at 278 K was explored with benzaldehyde (BA), phe-
nylacetaldehyde (PAA), and with 4-Me-DMA, 2,6-DTBP, respect-
ively. The nucleophilic Baeyer–Villiger reactions with BA and
PAA resulted in the formation of benzoic acid and benz-
aldehyde (∼85% and ∼65%, respectively based on 6), and the
second order rate constants (k2) were 0.61 M−1 s−1 for BA and
0.022 M−1 s−1 for PAA at 288 K (Fig. 3, Table S7†). H-atom
abstraction from the 2,6-DTBP, a model electrophilic substrate,
resulted in the formation of 3,3′,5,5′-tetra-tert-butyl-4,4′-diphe-
noquinone (∼80%), with a k2 = 0.008 M−1 s−1 (Fig. 4,
Table S8†). Notably, 6 reacted with 2,6-DTBP an order of mag-
nitude more slowly than does [FeI2II(μ-O)(μ-O2)(Py)2-
indH)2(CH3CN)2]2+ ‡ with a k2 = 0.028 M−1 s−1 at 278 K.13c
Fig. 1 X-ray structure of 5. Thermal ellipsoids are plotted at 50% prob-
ability level.
Fig. 2 (Left) UV-vis absorption spectra of FeII(L1,2,3)3]2+ (1, 2, 5) in MeCN
and their corresponding complexes [FeI2II(μ-O2)(L1,2,3)2(S)2]4+ (3, 4 and 6)
generated by addition of 4 equiv. H2O2 at 25 °C (inset). (Right) Raman
spectrum of 6 at 785 nm generated from 5 (10 mM) by addition of 2
equiv. H2O2, with solvent signals subtracted.
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Fig. 4 Reactions of [FeI2II(μ-O2)(L3)2(S)2]4+ (6) (S
= solvent) with
4-methyl-N,N-dimethylaniline (DMA). A) Time course of the decay of 6
monitored at 705 nm with DMA. B) Plot of k’obs (= kobs − kselfdecay) versus
[substrate] for reactions of 6 with DMA at 278 K. C) Time course of the
decay of 6 monitored at 705 nm with 2,6-di-tert-butylphenol (2,6-
DTBP). D) Plot of k’obs (= kobs − kselfdecay) versus [substrate] for reactions
of 6 with 2,6-DTBP at 278 K.
of 6 toward the electrophilic and nucleophilic model com-
pounds shows the order BA > 4Me-DMA > PAA > 2,6-DTBP
(Fig. 5).
Complex 6 is able to oxidize the N,N-dimethylanilines
under mild anaerobic conditions with formation of MA with
CH2O (80% yield w.r.t. 6) as the main product (see ESI†).
Hammett plot analysis shows that the rate constant for the
N-demethylation of DMA by 6 is sensitive to changes in the
electronic properties of the DMA (Fig. 6A), with a ρ value that
is identical to that for the [FeIV(N4Py)(O)]2+/DMA system (ρ =
−2.1), where electron transfer (ET) and concomitant proton
transfer (PT) was proposed (Fig. 6B, Tables S10 and S11†).16
The correlation of log k2 values with the one-electron oxidation
potentials of DMAs (Eo°x) is good, and the slope (−3.13) is com-
parable to those obtained for the [FeIV(N4Py)(O)]2+/DMA
(−3.3), [FeIV(TMC)(O)]2+/DMA (−4.0) and [FeIV(TPFPP)(O)]2+/
DMA (−5.0) systems (Fig. 6C). ‡ 16 The magnitude of these
values is also in a good agreement with the ET-PT mechanism
proposed elsewhere and together these data provide strong evi-
dence for the electrophilic feature of the active oxidant.
Scheme 2 Reactions discussed in text.
Fig. 3 Reaction of [FeI2II(μ-O2)(L3)2(S)2] (6) (S = solvent) with benz-
aldehyde. UV-vis absorption spectrum over time (inset absorbance at
705 nm over time) following addition of (Upper) benzaldehyde and
(Lower) phenylacetaldehyde to in situ generated 6 and corresponding
plots of k’obs (= kobs − kselfdecay) versus [aldehyde] at 278 K.
Additionally, H-atom abstraction from 4-Me-DMA pro-
ceeded at a similar rate with a second-order rate constant k2 =
0.008 M−1 s−1 at 278 K is noticeably faster than with
[FeIV(N4Py*)(O)]2+ ‡/4Me-DMA system (9.8 0.4) × 10−2 M−1 s−1
Fig. 5 Decay of absorbance of [FeI2II(μ-O2)(L3)2(S)2] (6) (S = solvent) at
705 nm in CH3CN upon and in the presence of 4Me-DMA, 2,6-DTBP or
at 298 K) reported recently (Table S9†).15 The relative reactivity PAA at 278 K with 2nd order rate constants for comparison.
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to W.R.B.), the Hungarian National Research Fund (OTKA
K108489 to JK), GINOP-2.3.2-15-2016-00049 and the European
Research Council (279549 to WRB) are gratefully acknowledged.
Notes and references
‡((Py)2-indH = 1,3-bis(2′-pyridylimino)-isoindoline, N4Py = 1,1-di(pyridin-2-yl)-
N,N-bis(pyridin-2-ylmethyl)methanamine,
TPFPP
=
5,10,15,20-tetrakis-
pentafluorophenylporphyrin,
tetraazacyclotetradecane.
TMC
= 1,4,8,11-tetramethyl-1,4,8,11-
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suggesting a nucleophilic attack of the peroxide on the alde-
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values were obtained for mononuclear peroxo complexes,16
but they are slightly lower than those with [FeI2II(μ-O2)
(L2)2(CH3CN)2]4+
(+0.67)13b
and
[FeI2II(μ-O)(μ-O2)(Py)2-
indH)2(CH3CN)2]2+ (+0.48)13c complexes. However, the strength
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We have shown the complex [FeI2II(μ-O2)(L3)4(CH3CN)2]4+ 6,
where L3 is not pyridine based, forms rapidly with H2O2 as
terminal oxidant. The peroxy species serves as a functional
model of the peroxo-diiron intermediates considered central to
the catalytic cycle of oxidoreductases, in particular in showing
both nucleophilic and electrophilic oxidation of substrates.
The data show that a move towards ligand sets that are closer
in electronic character to those available in metalloenzymes is
key to emulating their reactivity.
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Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
Financial support from The Netherlands Ministry of
Education, Culture and Science (Gravity Program 024.001.035
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