Catalytic oxyfunctionalization of saturated hydrocarbons by non-heme oxo-bridged diiron(III) complexes: role of acetic acid on oxidation reaction

Article abstract of DOI:10.1007/s11243-020-00412-w

Oxo-bridged diiron(III) complexes [Fe₂O(L¹)₂(H₂O)₂](ClO₄)₄ (1) and [Fe₂O(L²)₂(H₂O)₂](ClO₄)₄ (2), where L¹ and L² are tetradentate N-donor N,N′-bis(2-pyridylmethyl)-1,2-cyclohexanediamine and N,N′-bis(2-pyridylmethyl)ethane-1,2-diamine respectively, have been isolated as synthetic models of non-heme iron oxygenases and characterized by physicochemical and spectroscopic methods. Both the complexes have been studied as catalysts for the oxyfunctionalization of saturated hydrocarbons using green hydrogen peroxide (H₂O₂) as oxidant under mild conditions. The selectivity (A/K) and regioselectivity (3°/2°) in oxidative C–H functionalization of alkanes suggests the involvement of metal-based intermediate in the oxygenation reaction. The catalytic efficiency is found to be strongly dependent on the presence of acetic acid. Remarkable increase in conversion and selectivity favoring the formation of alcohols in the oxidation of cyclohexane and cyclooctane and exclusive hydroxylation of adamantane with drastic enhancement of regioselectivity has been achieved by the addition of acetic acid in the presence of H₂O₂.

Full text of DOI:10.1007/s11243-020-00412-w

Transition Metal Chemistry
https://doi.org/10.1007/s11243-020-00412-w
Catalytic oxyfunctionalization of saturated hydrocarbons
by non‑heme oxo‑bridged diiron(III) complexes: role of acetic acid
on oxidation reaction
Uday Sankar Agarwalla1
Received: 21 May 2020 / Accepted: 23 June 2020
©
Springer Nature Switzerland AG 2020
Abstract
1
2
1
2
Oxo-bridged diiron(III) complexes [Fe O(L ) (H O) ](ClO ) (1) and [Fe O(L ) (H O) ](ClO ) (2), where L and L are
2
2
2
2
4 4
2
2
2
2
4 4
tetradentate N-donor N,N′-bis(2-pyridylmethyl)-1,2-cyclohexanediamine and N,N′-bis(2-pyridylmethyl)ethane-1,2-diamine
respectively, have been isolated as synthetic models of non-heme iron oxygenases and characterized by physicochemical
and spectroscopic methods. Both the complexes have been studied as catalysts for the oxyfunctionalization of saturated
hydrocarbons using green hydrogen peroxide (H O ) as oxidant under mild conditions. The selectivity (A/K) and regiose-
2
2
lectivity (3°/2°) in oxidative C–H functionalization of alkanes suggests the involvement of metal-based intermediate in the
oxygenation reaction. The catalytic eꢀciency is found to be strongly dependent on the presence of acetic acid. Remarkable
increase in conversion and selectivity favoring the formation of alcohols in the oxidation of cyclohexane and cyclooctane
and exclusive hydroxylation of adamantane with drastic enhancement of regioselectivity has been achieved by the addition
of acetic acid in the presence of H O .
2
2
Introduction
(tpa = tris-(2-pyridylmethyl)amine, X = Br, Cl, H O) [15,
2
4
+
1
6]; [Fe O(H O) (tmima) ] (tmima = tris[(l-methyl-
2 2 2 2
Selective oxyfunctionalization of saturated hydrocarbons
using iron complexes as catalysts under mild and environ-
mental friendly conditions is of enormous importance in
homogeneous catalysis and synthetic chemistry [1–5]. Non-
heme iron enzymes have attracted a great deal of interest to
carry out selective oxidative oxygen transfer into the C–H
bond of a variety of hydrocarbons [6, 7]. Presence of oxo-
bridged diiron centers in a variety of metalloenzymes like
methane monooxygenases [8–10], ribonucleotide reductase
imidazol-2-yl)methyl]amine) [17]; [Fe O(mebpa) Cl ]
2 2 2
(ClO ) (mepba = N(2-methoxyethyl)- N,N-bis(pyrid-
4
2
2-ylmethyl)amine) [18]; [Fe O(phen) (H O) ](ClO )
2
4
2
2
4 4
(phen=1,10-phenanthroline) [19]; [Fe O(L) (H O) ](ClO )
2
4
2
2
4 4
(L = L-(-),4,5-pinenebipyridine) [20]. Among the other
active systems of non-heme diiron complexes containing
Fe–O–Fe unit reported so far, [Fe O(bbp) (MeOH) (NO ) ]
2
2
2
3 2
(NO ) (bbp = 2,6-bis(Nmethyl benzimidazol-2-yl)pyri-
3
2
dine) [21]; [Fe O(tpoen) ](ClO ) (tpoen = N(2-pyridyl-
2
2
4 4
[
11, 12] and purple acid phosphatase [13, 14] has directed
methoxyethyl)- N,N-bis(2-pyridylmethyl)amine) [22] and
m+
′
much focus in the chemistry of oxo-bridged diiron(III) com-
plexes. Over the past few years, a large number of μ-oxo-
bridged diiron complexes have been synthesized as structural
models and their catalytic activities in the selective function-
alization of saturated hydrocarbons using environmentally
green oxidant have been widely investigated. These studies
[Fe O(L) X ] (L=bipyridine, 4,4 -(Me) -bipy, X=H O,
2 4 n 2 2
Cl) [19, 23] are noteworthy.
In our earlier studies, we have reported that non-
heme mononuclear iron(III) complexes of N,N′-bis(2-
pyridylmethyl)-diamine framework have shown moderate
catalytic activity with low selectivity toward oxidation of
hydrocarbons in the presence of H O as oxidant [24]. The
m+
include the following model complexes: [Fe O(tpa) X ]
2
2
2
2
2
outcome of previous studies suggested that the iron(III)-
chloride bond is too strong, which makes the mononuclear
iron complexes poor catalysts [25]. From the synthetic
models reported so far it has also been observed that the
catalytic activity of iron complexes in terms of product
yield and selectivity requires the presence of exchangeable
*
Uday Sankar Agarwalla
udaygrwlla@gmail.com
1
Department of Chemistry, P. D. Women’s College,
Jalpaiguri, West Bengal 735101, India
Vol.:(0
1
123456789)
3

Transition Metal Chemistry
coordination sites in the metal center [25, 26]. This obser-
vation encouraged us to design iron complexes with the
same ligand framework in order to enhance their catalytic
eꢀciency toward hydroxylation of saturated C–H bond.
In the present study, we have synthesized μ-oxo-bridged
diiron(III) complexes containing exchangeable water mol-
The brown mass obtained was puriꢂed by recrystallization
from acetonitrile.
Yield: 0.51 g (44%). Anal. Calcd. for C H Cl Fe N O :
36
52
4
2
8
19
C, 37.4; H, 4.5; N, 9.7. Found: C, 37.2; H, 4.3; N, 9.6%. IR
−
1
(KBr pellet): selected peaks ν (cm ):798 for Fe–O–Fe
,
asym.
−
−1
1096 for ClO ion. Uv–vis (in CH CN) λ /nm (ɛ, M
4
3
max
1
−1
ecule in each iron center, [Fe O(L ) (H O) ](ClO ) (1)
cm ): 254 (24,150), 327 (9630), 489 (850).
2
2
2
2
4 4
2
1
and [Fe O(L ) (H O) ](ClO ) (2), where L =N,N′-bis(2-
2
2
2
2
4 4
2
pyridylmethyl)-1,2-cyclohexanediamine and L = NN,N′-
bis(2-pyridylmethyl) ethane-1,2-diamine, in the hope that
they can serve as an eꢀcient catalyst for the oxyfunctionali-
zation of saturated hydrocarbons. The catalytic properties of
the complexes have been studied in the oxidation of alkanes
with green and benign H O as the terminal oxidant under
2
Synthesis of [Fe O(L ) (H O) ](ClO ) (2)
2
2
2
2
4 4
Complex 2 was prepared by the same procedure employed
2
1
for complex 1, using L (0.242 g, 1 mmol) in place of L as
ligand.
2
2
mild conditions. The eꢁect of acetic acid on the selectivity
of oxidation products in the hydrocarbon oxygenation has
also been reported.
Yield: 0.50 g (48%). Anal. Calcd. for C H Cl Fe N O :
28
40
4
2
8
19
C, 32.2; H, 3.9; N, 10.7. Found: C, 32.4; H, 3.7; N, 10.6%.
−
1
IR (KBr pellet): selected peaks ν (cm ): 802 for Fe–O-
−
Fe
, 1093 for ClO ion. Uv–vis (in CH CN) λ /nm
asym.
4
3
max
−
1
−1
(
ɛ, M cm ): 254 (36,500), 324 (18,600), 489 (1500).
Experimental
Instruments and reagents
Catalytic oxidation reaction
The electronic spectra were recorded in acetonitrile on
a JASCO V-530 spectrophotometer in the 200-800 nm
region. The infrared spectra were performed from KBr
disk on a JASCO 5300 FT-IR spectrophotometer in the
In a typical run, 7 mM of oxidant H O was added to a small
2
2
screw capped vials ꢂtted with PTFE septa containing 2 ml
of acetonitrile solution (or 1:1 mixture of acetonitrile and
dichloromethane in case of adamantane oxidation) of cata-
lyst (0.7 mM) and substrate (700 mM) in a catalyst: oxidant:
substrate molar ratio of 1:10:1000. After 3 h of stirring at
room temperature (1 h when the catalytic oxidation was car-
ried out in the presence of 50 μM AcOH), a standard solu-
tion of internal standard, pentaꢃuoroiodobenzene (PFIB),
was added to the reaction mixture. The analysis was done
by gas chromatographic analysis after injecting an aliquot
(1 μl) from the reaction vial into preheated GC. The conver-
sion of alkanes into the oxygenated products was identiꢂed
and quantiꢂed by a comparison of the GC retention times
of the authentic reference sample and the reaction sample.
−
1
4
000–400 cm range. Heraeus Carlo Erba 1108 elemental
analyzer was used to collect Elemental microanalyses (C,
H and N) data. The product analyses were done by Perkin
Elmer Clarus-500 Gas Chromatograph with FID (Elite-I,
Polysiloxane, 15-meter column).
All chemicals were purchased from Sigma-Aldrich and
used as received. Acetonitrile and dichloromethane used for
catalytic experiments were distilled under argon and stored
over 4 Å molecular sieves. The active oxygen content of
H O (as ~ 30% solution in water) was determined iodo-
2
2
metrically prior to use.
Synthesis of the ligands
1
2
The ligands L and L were synthesized and characterized
using a previously reported procedure [27].
Results and discussion
Synthesis
Synthesis of the catalysts
1
Oxo-bridged diiron(III) complexes (1 and 2) containing L
1
2
Synthesis of [Fe O(L ) (H O) ](ClO ) (1)
and L ligand framework were obtained readily by stirring
2
2
2
2
4 4
a reaction mixture of ligand with an equimolar amount of
Fe(ClO ) .10H O in methanol at room temperature. The
A solution of Fe(ClO ) .10H O (0.540 g, 1 mmol) in metha-
4
3
2
4 3
2
1
nol (5 ml) was added to a stirred solution of L (0.296 g,
resultant brownish yellow precipitate was recrystallized
from acetonitrile and was characterized by spectral meth-
ods and elemental analysis. The proposed structure of the
complexes 1 and 2 is shown in Scheme 1.
1
1
mmol) in methanol (5 ml). The mixture was stirred for
h, then the precipitate was ꢂltered oꢁ, washed twice with
small quantities of diethyl ether and dried under vacuum.
1
3

Transition Metal Chemistry
(
17%) than those found for complex 1, A/K ratio remains
the same. Similar results are obtained when the catalytic
reaction is performed under nitrogen atmosphere. Under
the similar reaction conditions, cyclooctane was oxidized to
the corresponding alcohol and ketone with selectivity (A/K
2
.3 for complex 1 and 2.4 for complex 2) higher than that
found for cyclohexane oxidation, the amount of oxidized
product formed being almost same (Table 1, entries 3,4).
Control experiments performed in the absence of catalyst
fail to aꢁect any oxidative transformation of alkanes in the
presence of H O revealing that complexes 1 and 2 have
n
Scheme 1 Proposed structure of the complexes [Fe O(L ) (H O) ]
2
2
2
2
(
ClO ) (n=1 and 2)
4 4
2
2
signiꢂcant contribution in the oxidation reaction. The cata-
Characterization
lytic activity with Fe(ClO ) .10H O instead of diiron com-
4
3
2
plex has also been tested in alkane oxidation and very little
catalytic activities were observed under our experimental
conditions. Further, no products are observed in the reaction
of cyclohexane and catalyst in the absence of H O .
Complexes 1 and 2 display electronic absorption spectra in
the region 250-500 nm in acetonitrile. An intense band at
2
54 nm obtained in both the complexes is attributed to the
2
2
intra-ligand π→π* transition within the pyridine ring [22,
The behavior of present catalytic system toward adaman-
tane oxidation has also been investigated. The use of ada-
mantane as substrate resulted in oxidation at both second-
ary and tertiary carbon centers, aꢁording the hydroxylation
products 1-adamantanol and 2-adamantanol along with very
little amount of subsequent oxidation product 2-adaman-
tanone (Scheme 2).
2
8]. The electronic spectrum of 1 shows absorption bands
at 327 nm and 489 nm which are characteristic of oxo-to-
iron(III) LMCT transitions [18, 29]. Complex 2 shows simi-
lar absorption pattern with bands at 324 nm and 489 nm.
The less intense band at 489 nm in both the complexes is
6
4
4
assigned to the A →( A , E) transition and is the signa-
1
1
ture of the Fe-(μ-O)-Fe core, suggesting that the dinuclear
The regioselectivity is parameterized as a 3°/2° ratio
derived from the amount of 1-adamantanol (1-ol) divided
by the amount of 2-adamantanol (2-ol) and 2-adamantanone
(2-one) and multiplied by 3 to correct the threefold higher
number of secondary C–H bonds.
structure is maintained in solution [30].
The presence of the Fe–O–Fe motif in the diiron(III)
complexes 1 and 2 is further conꢂrmed by the vibrational
−1
spectroscopy. In the IR spectrum, peak at 798 cm for com-
−
1
plex 1 and at 802 cm for complex 2 can be assigned to
Fe–O–Fe asymmetric stretching mode on the basis of the
literature reports [31, 32]. Both the complexes also exhibit
A high selectivity of the catalytic oxidizing system for
the tertiary C–H bonds versus secondary ones has been
observed in the oxidation of adamantane with H O , as
2
2
−
1
intense bands around 1100 cm , indicating the presence of
perchlorate anion [32].
indicated by 3°/2° ratio (Table 1, entries 5, 6). Complex
1 showed much higher eꢀciency in adamantane oxidation
relative to complex 2. A total conversion of 33% with 3°/2°
ratio of 11.1 was obtained by 1/H O system. On the other
The spectral and elemental analyses data (experimental
section) of the complexes 1 and 2 are in agreement with the
proposed structures shown in Scheme 1.
2
2
hand, oxo-bridged dinuclear complex 2 displayed an activity
with 24% conversion with 3°/2° ratio of 11.4.
Oxyfunctionalization of saturated hydrocarbons
by diiron complexes
It is interesting to compare the catalytic behavior of the
synthesized non-heme diiron(III) complexes with those of
reported catalysts toward oxidation of saturated hydrocar-
bons. The selectivity for cyclohexane oxidation (A/K) and
regioselectivity (3°/2°) in the case of adamantane oxidation
has been found to be higher in comparison to Gif family of
catalysts [33]. Catalysts such as [Fe O(OAc) (bpy) ]Cl [34],
The catalytic properties of the synthesized diiron(III) com-
plexes 1 and 2 toward alkane functionalization has been
explored at room temperature in the presence of environ-
mentally friendly H O as oxidant. The product distributions
2
2
2
2
2
2
for cyclohexane, cyclooctane and adamantane oxidation
[Fe O(OAc) (tmima) ](ClO ) [17] and [Fe O(bpy) (H O) ]
2 2 2 4 3 2 4 2 2
have been compiled Table 1.
(ClO ) [35] exhibit A/K values around 1. With adaman-
4
4
The present catalytic system catalyzed the oxidation
of cyclohexane affording cyclohexanol (CyOH, A) and
cyclohexanone (CyO, K). In the case of complex 1 as cata-
lyst, a combined yield of 22% based on oxidant has been
achieved after 3 h of reaction with an A/K ratio of 1.8. The
conversion with 2 under similar reaction conditions is lower
tane as substrate, the selectivity for the oxidation at the
tertiary position (3°/2°) higher than the published results
with diiron complexes has also been observed [22, 34–36].
The selectivity with A/K ratio of 1.8-2.4 in the oxidation of
cyclohexane and cyclooctane and regioselectivity with 3°/2°
ratio of 11.1–11.4 in adamantane oxidation catalyzed by the
1
3

Transition Metal Chemistry
Table 1 The results of oxidation of alkanes by diiron(III) complexes 1 and 2 with H O
2
2
Entry
Substrate
Catalyst
Conversion (%)
Product yield (%)a
Remarksb
Cyclohexane oxidation
1
Cyclohexanol
14
Cyclohexanone
A/K
1.8
1
22
8
2
2
1
17
23
11
6
7
1.8
A/K
2.3
Cyclooctane oxidation
3
Cyclooctanol
16
Cyclooctanone
2-one
4
2
1
17
33
12
1-ol
26
5
2
2.4
°
°
Adamantane oxidation
5
2-ol
3 /2
5
11.1
6
2
24
19
3
2
11.4
Reaction conditions: Alkane (700 mM), catalyst (0.7 mM), H O (7 mM), CH CN (2 ml), RT, 3 h
2
2
3
a
Yields are determined by GC analysis with internal standard based on the oxidant used
b
°
°
A/K=alcohol/ketone, 3 /2 =(1-ol×3)/(2-ol+2-one)
synthesized oxo-bridged non-heme diiron complexes with
H O convey a hint of the presence of metal-based oxidant
Cyclohexane oxidation catalyzed by 1/H O in the pres-
2
2
ence of acetic acid provided 26% of CyOH and 7% of CyO
with an A/K ratio of 3.7. However, complex 2 as catalyst
aꢁorded lower yield of oxygenates with 24% conversion
based on oxidant (Table 2, entry 2). Similarly, oxyfunction-
alization of cyclooctane by oxo-bridged diiron(III) catalysts
produced cyclooctanol with yields 32% and 24% along with
minor amount of cyclooctanone (5–7%) while the corre-
sponding A/K ratio was found to be 4.6 and 4.8 respectively
2
2
Scheme 2 Possible products obtained in the oxidation of adamantane
(
Table 2, entries 3–4). In both the cases, amount of ketone
formed remains the same as that obtained in the absence
of AcOH. On the other hand, exclusive hydroxylation has
been achieved with H O in the presence of acetic acid and
[
37, 38]. Also no change in the product selectivity and yield
2
2
was observed upon performing the hydroxylation reaction
under nitrogen atmosphere, which ruled out the formation
of cyclohexylperoxide species. This is also supported the
involvement of metal-based oxidant in the catalytic oxyfunc-
tionlization of saturated hydrocarbons [39].
not even a trace amount of subsequent oxidation product
2
-adamantanone was detected (Table 2, entries 5 and 6). A
total conversion of 48% for 1 and 35% for 2 by H O /AcOH
2
2
°
°
with 3 /2 of 17.6–18.0 was obtained.
The data presented in Table 2 indicated that the use of
AcOH as an additive in non-heme diiron(III) catalyzed
alkane oxidation resulted in an increase in both catalytic
activity and selectivity toward hydroxylation as compared
with the reactivity in the absence of acetic acid (Table 1).
Moreover, addition of AcOH to 1 or 2 in the presence of
H O also resulted in an increase in the oxidation rate;
Role of acetic acid in the catalytic oxidation
of alkanes
The catalytic activities of the diiron(III) complexes 1 and 2
toward oxidation of saturated hydrocarbons have also been
studied with H O in the presence of acetic acid (AcOH).
2
2
2
2
the reaction time reduced to 1 h as compared to 3 h in the
absence of AcOH. Here also, the involvement of metal-
based oxidants in the C–H hydroxylation of cyclohexane,
Both the complexes displayed eꢀcient hydroxylation of
cyclohexane, cyclooctane and adamantane with an excel-
lent selectivity in the presence of H O /AcOH. The obtained
2
2
catalytic results are summarized in Table 2.
1
3

Transition Metal Chemistry
Table 2 Product distributions for the oxidation of alkanes with H O in the presence of AcOH
2
2
Entry
Substrate
Catalyst
Conversion (%)
Product Yield (%)a
Remarksb
Cyclohexane oxidation
1
Cyclohexanol
26
Cyclohexanone
A/K
3.7
1
33
7
2
2
1
24
39
19
5
7
3.8
A/K
4.6
Cyclooctane oxidation
3
Cyclooctanol
32
Cyclooctanone
4
2
1
29
48
24
1-ol
41
5
4.8
3 /2
17.6
°
°
Adamantane oxidation
5
2-ol
2-one
–
7
6
2
35
30
5
–
18.0
Reaction conditions: Alkane (700 mM), catalyst (0.7 mM), H O (7 mM), AcOH (50 μM),CH CN (2 ml), RT, 1 h
2
2
3
a
Yields are determined by GC analysis with internal standard based on the oxidant used
b
°
°
A/K=alcohol/ketone, 3 /2 =(1-ol x 3)/(2-ol+2-one)
cyclooctane and adamantane has been suggested as indicated
oxidation of cyclohexane and cyclooctane provide mainly
alcohol along with very little amount of ketone with H O /
°
°
by A/K and 3 /2 ratio.
2
2
To explore the eꢁect of AcOH in alkane oxidation by
H O , it has been proposed that in the first step of the
AcOH. On the other hand, it is remarkable that only the
hydroxylation products 1-adamantanol and 2-adamantanol
with no subsequent oxidation product 2-adamantanone have
been detected in the presence of acetic acid during adaman-
tane oxidation. Both the product yield and selectivity toward
hydroxylation of saturated hydrocarbons have been drasti-
cally improved by the synthesized non-heme oxo-bridged
diiron complexes with the exchangeable coordination sites
using H O as oxidant in the presence of acetic acid. Thus,
2
2
reaction, the binding of the oxidant H O to the dinuclear
2
2
III
complex results in the formation of Fe –OOH intermedi-
ate, which play a key role in the catalytic cycle of alkane
hydroxylation. On the basis of literature reports [40, 41], it
has been suggested that the addition of AcOH promoted the
heterolytic cleavage of the O–O bond to generate high-valent
iron-oxo species (active oxidizing intermediate), which on
subsequent reaction with alkane results in hydroxylation.
The involvement of this high-valent iron-oxo species leads
to hydrocarbon oxidation with significant chemo- and
regioselectivity.
2
2
the addition of acetic acid in the presence of H O can be
2
2
a good strategy to design eꢀcient and synthetically useful
systems for hydroxylation of saturated hydrocarbons. Fur-
ther studies on detailed mechanism of the reactions are cur-
rently in progress. This work also explores the opportunity
to investigate the eꢁect of acetic acid in the presence of
H O toward oleꢂn epoxidation catalyzed by non-heme dii-
Conclusion
2
2
ron complexes.
In summary, μ-oxo-bridged diiron(III) complexes (1 and 2)
containing tetradentate N,N′-bis(2-pyridylmethyl)-diamine
ligand framework have been synthesized and character-
ized. The synthesized complexes eꢀciently catalyze the
oxidation of alkanes at room temperature with environmen-
tally friendly H O as oxidant. The catalytic experimental
Acknowledgements This work was ꢂnancially supported by the Uni-
versity Grants Commission [No. F.PSW-197/15-16 (ERO)]. The author
is grateful to Dr. P. Bandyapadhyay for his assistance and kind coopera-
tion. The author also expresses his sincere thanks to the Department of
Chemistry, University of North Bengal for providing instrumentation
facilities.
2
2
results (A/K and 3°/2° ratio) suggest that the oxidation of
saturated hydrocarbons by H O occurs via a mechanism
2
2
References
involving metal-based oxidant. The preference for hydroxy-
lation toward hydrocarbon oxidation has been ampliꢂed in
the presence of acetic acid. It has been found that catalytic
1
. Feig AL, Lippard SJ (1994) Chem Rev 94:759–805
1
3

Transition Metal Chemistry
2
3
4
5
6
.
.
.
.
.
Sun CL, Li BJ, Shi ZJ (2011) Chem Rev 111:1293–1314
26. Costas M, Chen K, Que L Jr (2000) Coord Chem Rev
200:517–544
Newhouse T, Baran PS (2011) Angew Chem Int Ed 50:3362–3374
Shang R, Ilies L, Nakamura E (2017) Chem Rev 117:9086–9139
Kal S, Xu S, Que L Jr (2020) Angew Chem Int Ed 59:7332–7349
Costas M, Mehn MP, Jensen MP, Que L (2004) Chem Rev
27. He Y, Gorden JD, Goldsmith CR (2011) Inorg Chem
50:12651–12660
28. Esmelindro MC, Oestreicher EG, Alvarez HM, Dariva C, Egues
SMS, Fernandes C, Bortoluzzi AJ, Drago V, Antunes OAC (2005)
J Inorg Biochem 99:2054–2061
1
04:939–986
7
8
.
.
Jasniewski AJ, Que L Jr (2018) Chem Rev 118:2554–2592
Merkx M, Kopp DA, Sazinsky MH, Blazyk JL, Muller J, Lippard
SJ (2001) Angew Chem Int Ed 40:2782–2807
29. Hazell A, Jensen KB, MacKenzie CJ, Toftlund H (1994) Inorg
Chem 33:3127–3134
9
.
Banerjee R, Jones JC, Lipscomb JD (2019) Annu Rev Biochem
30. Reem RC, McCormick JM, Richardson DE, Devlin FJ, Ste-
phens PJ, Musselman RL, Solomon EI (1989) J Am Chem Soc
111:4688–4704
8
8:409–431
1
0. Wang VC, Maji S, Chen PPY, Lee HK, Yu SSF, Chan SI (2017)
Chem Rev 117:8574–8621
31. Sun H, Wang M, Li F, Li P, Zhao Z, Sun L (2008) Appl Organo-
metal Chem 22:573–576
1
1
1. Jordan A, Reichard P (1998) Annu Rev Biochem 67:71–98
2. Logan DT, Sxu XD, Aberg A, Rengnstrom K, Hajdu J, Eklund H,
Nordlund P (1996) Structure 4:1053–1064
32. Do LH, Xue G, Que L Jr, Lippard SJ (2012) Inorg Chem
51:2393–2402
1
1
1
1
1
1
1
2
2
2
2
3. Guddat LW, McAlpine AS, Hume D, Hamilton S, de Jersey J,
Martin JL (1999) Structure 7:757–767
33. Barton DHR, Doller M (1992) Acc Chem Res 25:504–512
34. Fish RH, Konings MS, Oberhausen KJ, Fong RH, Yu WM, Chris-
tou G, Vincent JB, Coggin DK, Buchanan RM (1991) Inorg Chem
30:3002–3006
4. Schenk G, Mitić N, Hanson GR, Comba P (2013) Coord Chem
Rev 257:473–482
5. Kim J, Harrison RG, Kim C, Que L (1996) J Am Chem Soc
35. Menage S, Vincent JM, Lambeaux C, Fontecave M (1996) J Mol
Catal A: Chem 113:61–75
1
18:4373–4379
6. Leising RA, Kim J, Perez MA, Que L Jr (1993) J Am Chem Soc
36. Kojima T, Leising RA, Yan S, Que L Jr (1993) J Am Chem Soc
115:11328–11335
1
15:9524–9530
7. Buchanan RM, Chen S, Richardson JF, Bressan M, Forti L, Mor-
villo A, Fish RH (1994) Inorg Chem 33:3208–3209
37. Kryatov SV, Rybak-Akimova EV, Schindler S (2005) Chem Rev
105:2175–2226
8. Tanase S, Foltz C, Gelder R, Hage R, Bouwman E, Reedijk J
38. van den Berg TA, de Boer JW, Browne WR, Roelfes G, Feringa
BL (2004) Chem Commun 2550–2551
(
2005) J Mol Catal A: Chem 225:161–167
9. Menage S, Vincent JM, Lambeaux C, Chottard G, Grand A,
Fontecave M (1993) Inorg Chem 32:4766–4773
39. Visvaganesan K, Suresh E, Palaniandavar M (2009) Dalton Trans
3814–3823
0. Duboc-Toia C, Menage S, Ho RYN, Que L Jr, Lambeaux C,
Fontecave M (1999) Inorg Chem 38:1261–1268
40. Que L Jr, Tolman WB (2008) Nature 455:333–340
41. Balleste RM (2007) Que Jr L 129:15964–15972
1. Wang X, Wang S, Li L, Sundberg EB, Gacho GP (2003) Inorg
Chem 42:7799–7808
Publisher’s Note Springer Nature remains neutral with regard to
2. Li F, Wang M, Ma C, Gao A, Chen H, Sun L (2006) Dalton Trans,
jurisdictional claims in published maps and institutional aꢀliations.
2
427–2434
3. Menage S, Wilkinson EC, Que L Jr, Fontecave M (1995) Angew
Chem Int Edn Engl 34:203–205
2
2
4. Agarwalla US (2020) Rasayan J Chem 13:960–967
5. Meckmouche Y, Menage S, Toia-Duboc C, Fontecave M, Galey
JB, Lebrun C, Pecaut J (2001) Angew Chem Int Ed 40:949–952
1
3