Iron(III) Complexes with Cross-Bridged Cyclams: Synthesis and Use in Alcohol and Water Oxidation Catalysis

Article abstract of DOI:10.1002/ejic.201800451

A class of iron(III) complexes containing diversely substituted cross-bridged cyclams (1-R,R′: 1-Me,Me, 1-Me,Et, 1-Et,Et, 1-Me,Bn, 1-Bn,Bn) has been prepared and characterized. (1-R,R′) complexes catalyze the oxidation of alcohols to ketones in green cond

Full text of DOI:10.1002/ejic.201800451

A Journal of
Accepted Article
Title: Iron(III) complexes with cross-bridged cyclams: synthesis and
use in alcohol and water oxidation catalysis
Authors: Alfonso Annunziata, Roberto Esposito, Giordano Gatto,
Maria Elena Cucciolito, Angela Tuzi, Alceo Macchioni, and
Francesco Ruffo
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800451
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10.1002/ejic.201800451
European Journal of Inorganic Chemistry
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Iron(III) complexes with cross-bridged cyclams: synthesis and
use in alcohol and water oxidation catalysis
Alfonso Annunziata,[a] Roberto Esposito,[a] Giordano Gatto,[b] Maria Elena Cucciolito,[a] Angela Tuzi,[a]
Alceo Macchioni*[b] and Francesco Ruffo*[a]
Abstract:
A
class of iron(III) complexes containing diversely
adjacent nitrogen donors with different R groups, giving either
symmetrical or unsymmetrical N,N’-disubstituted ligands. The
corresponding complexes display kinetic and thermodynamic
stability, due to the all-tertiary nature of the N-functions which
substituted cross-bridged cyclams (1-R,R’: 1-Me,Me, 1-Me,Et, 1-
Et,Et, 1-Me,Bn, 1-Bn,Bn) has been prepared and characterized. (1-
R,R’) complexes catalyze the oxidation of alcohols to ketones in
green conditions, by using t-butylhydroperoxide as the oxidant in
aqueous solutions. They also exhibit catalytic activity in water
oxidation (WO) to molecular oxygen driven by NaIO4, and the results
were found to be dependent on the steric hindrance of the R-
substituents. Best compromise in terms of accessibility and
protection of the metal center was obtained with 1-Me,Et (TOF= 1.4
min‒1 and TON= 111).
prevents ligand oxidation, while the presence of the
R
substituents inhibits undesired dimerization. The macrocyclic
nature also obstacles metal oxide/hydroxide precipitation in
oxidative conditions and consequent complex destruction. These
features render the complexes available to different applications,
such as in redox catalysis, biomimicry and pharmaceutical
chemistry.
Cross bridged cyclam ligands have been first investigated by
Weisman, who reported a detailed synthesis of the dimethyl
species L-Me,Me.[1-3] Busch and co-workers used both the
dimethyl and dibenzyl L-Bn,Bn ligands to coordinate mid-late
transition metals,[4-6] getting complexes useful for different
applications: manganese compounds were largely discussed in
patent literature[7-13] as bleach catalysts,[14-16] while copper
complexes found application in radiopharmaceutical and medical
fields.[17-20]
Introduction
Macrocyclic polyamines deserve great interest as ligands for
transition metal ions. Cross-bridged cyclams (L-R,R’ in Figure
1a) represent an excellent instance of the class,[1-19] as their
cleft-like structure allows the lone pairs of the four nitrogen
atoms to converge, thus becoming suitable to chelate a metal
ion (Figure 1b).
Less investigated are iron complexes of cross-bridged cyclams,
which are fairly represented only in the oxidation state (II),[4,5,21-
23]
while [FeCl2(L-Me,Me)]PF6 represents to date the sole
example reported so far of a fully characterized iron(III)
derivative.[6,24]
This is in contrast with the prominent involvement of iron in a
great number of natural heme and non-heme enzymes, which
have a crucial role in biological processes, mainly oxidation
transformations. Considering non-heme catalysts, iron
complexes bearing robust N-macrocyclic donor have been
widely investigated for oxidation of organic substrates, using
peroxides, peracids and molecular oxygen as external
oxidant.[25] Costas,[26-33] Lanzalunga[34-35] and their co-workers
developed various catalytic systems based on iron tetradentate
complexes for the oxidation of alcohols and alkanes, the
epoxidation and the dihydroxylation of olefins.
Figure 1. (a) Generic structure of cross-bridged cyclam. (b) Coordination to
metals.
The synthetic strategy allows to easily functionalize two non-
Iron complexes with nitrogen supporting ligands are also having
some success as catalysts for water oxidation (WOCs),[24,36-42]
a
reaction extremely important to provide electrons and protons for
the generation of renewable fuels, although their performances
are still considerably lower than those obtained with noble-metal
complexes.[43-48]
Given these premises, we have focused our attention on the
synthesis of a library of iron(III) complexes containing cross-
bridged cyclams with different substituents (1-R,R’ in Figure 2).
[a]
[b]
A. Annunziata, R. Esposito, M. E. Cucciolito, A. Tuzi, F. Ruffo
Dipartimento di Scienze Chimiche
Università di Napoli Federico II and CIRCC
Complesso Universitario di Monte S. Angelo,
via Cintia 21 – 80126 Napoli, Italy
E-mail: ruffo@unina.it
G. Gatto, A. Macchioni
Department of Chemistry, Biology and Biochemistry
University of Perugia and CIRCC
Via Elce di Sotto, 8 – 06123 Perugia, Italy
E-mail: alceo.macchioni@unipg.it
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201800451
European Journal of Inorganic Chemistry
FULL PAPER
monoalkylated product 4-Bn within three days. The collected
precipitate was then methylated with an excess of MeI. The
dipositive salts 3-R,R and 5-R,R’ were hydrogenated to the
desired ligands L-R,R’ with sodium borohydride following
literature procedures (iv). Both ligands and pro-ligands were
characterized by NMR spectroscopy, and the data were in full
agreement with those reported in literature.
Synthesis and characterization of the iron complexes. The
synthesis of the complexes (Scheme 2) was inspired by
literature.[6,16]
Figure 2. The structure of complexes of type 1-R,R’.
The complexes have been characterized by spectroscopic
measurements, and the X-ray diffractometric structure of the
dibenzyl derivative 1-Bn,Bn has been solved. The catalytic
properties of the compounds have been verified against the
oxidation of alcohols using green conditions, such as aqueous
and benign oxidizing systems, i.e. hydrogen peroxide and t-butyl
hydroperoxide. The ability of the same species to promote the
oxidative splitting of water has also been assessed. The results
are competitive with those already reported in the literature and
have allowed disclosing some dependence of catalytic activity
on the nature of the R substituents.
Scheme 2. Synthesis of the complexes 1-R,R’.
Results and Discussion
Anhydrous FeCl2 was reacted with the appropriate ligand in dry
dimethylacetamide (DMAC) at 100°C under an inert atmosphere
(step i). Within three hours the corresponding Fe(II) species
precipitated as a dark solid, which was immediately oxidized
using molecular bromine in dry methanol, in the presence of an
excess of ammonium hexafluorophosphate. All the iron(III)
complexes precipitated as bright orange microcrystalline solids.
Their spectroscopic characterization was performed using UV-
vis and FT-IR techniques. The electronic spectra were in
agreement with that of the known 1-Me,Me,[6,24] showing two
main CT bands (Figure 3).
Synthesis of the Ligands. The strategy for the synthesis of the
ligands, thoroughly discussed in other works,[1-3,16,18] is briefly
summarized in Scheme 1. Condensation of commercial cyclam
with aqueous glyoxal in acetonitrile gave intermediate 2 (step i).
20000
1-Bn,Bn
1-Me,Me
18000
1-Me,Bn
1-Et,Et
16000
14000
12000
10000
8000
6000
4000
2000
0
1-Me,Et
Scheme 1. Synthesis of the ligands L-R,R’.
Subsequent regioselective N-alkylation (ii) led to both mono- and
di-alkylated products (3-R,R and 4-R in Scheme 1), depending
on the experimental conditions. A great excess of alkyl iodides
RI (R= Me, Et) determined the precipitation of the di-iodide salts
3-Me,Me and 3-Et,Et as microcrystalline solids (ii). The mother
liquor of the latter species contained substantial amount of the
mono-alkylated product 4-Et, and addition of excess MeI
allowed to obtain the mixed salt 5-Me,Et (iii). Addition of only
one equivalent of BnBr to an acetonitrile solution of 2 yielded the
250
300
350
400
 (nm)
450
500
550
600
Figure 3. UV-vis absorption spectra of the complexes.
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10.1002/ejic.201800451
European Journal of Inorganic Chemistry
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The solid state FT-IR spectra showed a band typical of non-
coordinated hexafluorophosphate ion at 840 cm-1. The molar
conductivities of the complexes in acetonitrile (see experimental)
were comprised between 120 and 140 S cm2 mol‒1. These
values are consistent with those expected for 1:1 electrolytes,[6]
which also indicates that in this solvent the complexes retain the
same structure found in the solid state for 1-Bn,Bn.
Measurements of magnetic moments confirmed the presence of
five unpaired electrons.
of the tetradentate ligand completing its coordination sphere with
two chlorine atoms in a pseudo-octahedral geometry. The N
atoms of the ligand occupy two axial and two cis-equatorial sites
of the distorted octahedral, the chlorine atoms occupy the two
remaining cis-equatorial sites. A lengthening of Fe-N distances
suggests the high-spin configuration of Fe(III) ion. Neutrality of
-
compound is assured by PF6 as counter ion, which locates
close to the ethylene bridge, consistently with what generally
found in solution for other complexes by means of 19F,1H-
HOESY NMR studies.[50-52] The two benzyl groups attached at
Structure description of 1-Bn,Bn. Crystals suitable for X-Ray
analysis of 1-Bn,Bn were obtained by slow evaporation of an
acetonitrile/water solution at room temperature. The molecular
structure is reported in Figure 4, crystal data and structure
refinement details are reported in the Experimental Section.
Compound crystallizes in the orthorombic P212121 space group
N2 and N4 extend axially in agreement with a non-
crystallographic C2 symmetry of the molecule, leaving
unrestrained the equatorial plane. The small dimension of Fe(III)
ion allows a good engulfement of the metal in the cavity of the
ligand and a fairly regular octahedral geometry with Nax-Fe-Nax
angle very close to 180° is found. The value of Nax-Fe(III)-Nax
angle is 172.2(3)°, as compared with literature data of the
analogous Fe(II) complex,[5] in which the axial angle Nax-Fe(II)-
Nax is 147.71(8)° and the enhanced protrusion of Fe(II) out of the
cavity is evident (Figure 5).
-
with one Fe(III) mononuclear complex and one PF6 anion
contained in the asymmetric unit. All bond distances and angles
fall in the normal range, bond parameters of the metal
coordination sphere are listed in the caption to Figure 4.
Figure 5. Superimposition of 1-Bn,Bn (element color) with literature data[5]
(orange) of the analogous Fe(II) complex showing the better engulfement of
Fe(III) in the ligand cavity.
Figure 4. ORTEP view of 1-Bn,Bn with thermal ellipsoids drawn at 30%
probability level. Selected bond distances and angles: Fe1-N1 = 2.186(8) ,
Fe1-N2 = 2.263(9)2.253(8), Fe1-N3 = 2.193(8), Fe1-N4 = 2.256(9), Fe1-Cl1 =
2.279(3), Fe1-Cl2 = 2.271(3) Å; N2-Fe1-N4 = 172.2(3), N1-Fe1-N2 = 94.4(3),
N1-Fe1-N3 = 80.6(3), N1-Fe1-N4 = 79.5(3), N2-Fe1-N3 = 79.8(3), N3-Fe1-N4
= 94.3(3), Cl1-Fe1-Cl2 = 99.28(12)°.
Alcohol Oxidations. The catalytic activity of 1-R,R’ complexes
toward the oxidation of alcohols to ketones was explored. The
only previous result concerning a complex of this library deals
with 1-Me,Me, by using sodium periodate as the oxidant.[24]
Aiming at improving the atom-economy, within this study the
oxidations were conducted under more convenient conditions,
i.e. using hydrogen peroxide or t-butylhydroperoxide, in aqueous
systems. All the reactions were carried out in D2O containing
CD3CN as co-solvent at 27°C. After 18 hours, the reaction
mixture was extracted with CDCl3 containing a known amount of
tetrachloroethane as internal standard, and both the organic
phase and the residual solvent were analysed through NMR
spectroscopy. This choice allowed conversion, selectivity and
The compound belongs to the class of mononuclear cross-
bridged cyclam complexes, where a high conformational rigidity
of the tetraazamacrocycle exists. As a result, the ligand
conformation is scarcely influenced by the nature of the metal
contained in the cavity of the ligand or by the presence of
substituents at the cyclam scaffold unity. As expected and
already found in cross-bridged cyclam complexes,[16,49] in 1-
Bn,Bn the Fe(III) ion is hexa-coordinated. It occupies the cavity
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10.1002/ejic.201800451
European Journal of Inorganic Chemistry
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yielding to be quantified, by simple integration of appropriate
regions of the NMR spectra.
Preliminary tests were performed using cyclohexanol, hydrogen
peroxide and 1-Me,Me as substrate, oxidant and catalyst,
respectively. An almost complete conversion of cyclohexanol
could be obtained with 5 or 10 equivalents of H2O2, but with a
very poor selectivity, probably due to overoxidation of the
substrate. This was also confirmed by the presence of signals in
the NMR spectrum in the range over 4 ppm, which were
compatible with those of -caprolactone.
The unimpressive results obtained with hydrogen peroxide
induced a change of the external oxidant, and t-butylperoxide
was selected, due to its environmental compatibility. Preliminary
tests were again performed by using 1-Me,Me, and the results
are reported in Table 1.
Table 2. Oxidation of cyclohexanol to cyclohexanone by using 1-R,R with t-
BuOOHa
Entry
Catalyst
Yield
(selectivity)
(%)
Conversion
(%)
1
2
3
4
5
1-Me,Me
1-Et,Et
76
68
67
69
57
55 (72)
45 (66)
43 (67)
40 (58)
45 (79)
1-Me,Et
1-Bn,Me
1-Bn,Bn
Table 1. Oxidation of cyclohexanol to cyclohexanone by using 1-Me,Me with t-
BuOOHa
[a] Reaction conditions = cyclohexanol: 0.2 mmol; D2O/CD3CN: 1.2 mL (3:1,
v/v); t-BuOOH (0.5 mmol) water solution (70% w/w); T: 27°C; reaction time:
18h.
Entry
%
catalyst
t-BuOOH/sub.
(mol/mol)
Conversion
(%)
Yield
(selectivity)
(%)
(mol/mol)
On these grounds, catalyst 1-Me,Me was selected for a
screening of oxidation of various alcohols (Table 3).
2.5
2.5
5
0
0
1
2
3
4
5
0
1
1
2
2
76
88
74
90
55 (72)
51 (58)
48 (65)
57 (63)
Table 3. Oxidation of alcohols with 1-Me,Me and t-BuOOHa
En.
Substrate
D2O/
Conv.
(%)
Product
Yield
(select.)
(%)
CD3CN
(v/v)
2.5
5
1
2
3:1
3.1
76
34
55 (72)
26 (76)
[a] Reaction conditions = cyclohexanol: 0.2 mmol; D2O/CD3CN: 1.2 mL (3:1, v/v);
t-BuOOH water solution (70% w/w); T: 27°C; reaction time: 18h.
3
4
5
2:1
3:1
2:1
66
67
84
41 (62)
66 (98)
83 (99)
The outcomes show a clear improvement with respect to
hydrogen peroxide: selectivity and yields were more favorable
even at the most convenient conditions of entry 2, that implies
1% mol/mol of catalyst and 2.5 equivalents of oxidant. Therefore,
these options were adopted to compare the other catalysts, as
summarized in Table 2.
All the catalysts gave acceptable results, with conversions
slowly decreasing as the steric hindrance of the R substituents
increased. This trend can be interpreted by invoking the oxygen
transfer mechanism proposed in the case of the oxidation with
periodate:[24] formation of the active species [Fev(L-
R,R)(O)(OH)]2+ would be inhibited as the steric bulk increases,
also in line with the behavior of the corresponding Mn
complexes.[16]
6
7
2:1
2:1
99
81
99 (99)
78 (96)
[a] Reaction conditions = alcohol: 0.2 mmol; catalyst: 1% mol/mol;
D2O/CD3CN: 1.2 mL; t-BuOOH (0.5 mmol) water solution (70% w/w); T:
27°C; reaction time: 18h
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10.1002/ejic.201800451
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The v/v ratio between D2O and CD3CN was slightly adjusted for
phenyl-substituted substrates to favor their dissolution in a
homogeneous system. Aromatic alcohols gave in all cases good
conversions and yields (entry 5-7), while linear and cyclic
aliphatic alcohols were more resistant to oxidation. The low
conversion of menthol (entry 2) is plausibly due to the steric
hindrance around the alcoholic function, which reduces the
catalytic activity.[34] However, the results are competitive with
those reported by using the heavier sodium periodate as
oxidant,[24] and confirm the robustness and versatility of this kind
of catalysts.
The conversion versus time trend (Figure 6) for the oxidation of
diphenylmethanol was determined (entry 6). The reaction is
competitive with that reported using the same catalyst and
sodium periodate as oxidant.[24] A conversion of 96% was
obtained within 4 hours, whereas a maximum of 93% of
conversion was reached in 22 hours with NaIO4. The high
activity of the complex can be assessed by the value of initial
TOF (4.6 min-1, calculated considering the five-minute
experimental data), apparently reached without any induction
time.
process occurs in homogeneous phase with a mechanism
-
involving an initial O-atom transfer from IO4 to iron generating
FeV=O species, which subsequently undergoes
a water
nucleophilic attack, ultimately leading to the liberation of O2.[24]
Furthermore, O2 evolution was not observed when CAN was
used as sacrificial oxidant.[24] For those reasons, WO
experiments with 1-R,R’ were performed using exclusively
NaIO4 as SO, assuming that catalytic and mechanistic features
deduced for 1-Me,Me[24] can be reasonably translated to other 1-
R,R’ complexes.
Results are reported in Table 4. 1-R,R’ complexes exhibit
performances that well-compare with those of other previously
reported Fe WOCs.[24,37,38] In particular, 1-Me,Me shows a TOF
value equal to 1.5 min-1 (in unbuffered H2O), 1.0 min-1 (pH 4 by
acetate buffer), and 1.5 min-1 (pH 4 by phosphate buffer) (Table
4, entries 1, and 5) and a maximum TON= 111 (Table 4, entry 7).
TOF values are in nice agreement with those reported by Lau
and co-workers[24] (for example, TOF = 0.8 min-1, pH 4 by
acetate buffer), whereas they found substantially higher TON
values.[24] For instance, they found a TON = 224 at pH 4 by
acetate buffer whereas,[24] under identical conditions, we
obtained a TON = 100 (Table 4, entry 3).
Only the complex cis-[Fe(OTf)2(mcp)] (mcp = N,N’-dimethyl-
N,N’-bis(2-pyridyl-methyl)-cyclohexane-1,2-diamine), reported by
Lloret-Fillol et al., exhibits a significant higher activity reaching a
TOF = 3.7 min-1 with a TON > 1050, when tested under the
same conditions in unbuffered water.[37]
Owing to the fact that performances of 1-Me,Me in buffered
solution were similar to those in pure H2O, it was decided to test
other 1-R,R’ complexes in water. From Table 4 and Figure 7 it is
evident that one Me group has to be present in order to observe
some catalytic activity. This can be appreciated by comparing
the catalytic activity of 1-Me,Me with that of 1-Et,Et. The former
exhibits a TOF = 1.5 min-1 and a TON = 77 (entry 1 in Table 4),
whereas 1-Et,Et is able to complete only 16 cycles with a TOF =
0.03 min‒1 (entry 8 in Table 4). Keeping constant one Me group
and increasing the steric hindrance of the other group causes a
small decrease of activity: 1-Me,Me (TOF = 1.5 min‒1, entry 1 in
Table 4) = 1-Me,Et (TOF = 1.4 min-1, entry 8 in Table 4) > 1-
Me,Bn (TOF = 0.8 min‒1, entry 12 in Table 4). In terms of TON
(Figure 7), 1-Me,Et (TON = 111, entry 7 in Table 4) exhibits the
best performance, followed by 1-Me,Bn (TON= 81, entry 11 in
Table 4) and 1-Me,Me (TON = 77, entry 1 in Table 4), likely
because it has right compromise between a relatively high TOF
and steric protection of the active site against degradation.
Figure 6. Conversion versus time trend for the oxidation of diphenylmethanol.
Water oxidation. The activity of 1-R,R’ complexes toward WO
driven by NaIO4 was evaluated under conditions similar to those
utilized by Lau and co-workers for 1-Me,Me,[24] aiming at
understanding the dependence of catalytic activity on the nature
of R-substituents, which was found to be a key factor in other
WOCs.[53] Clark Electrode (CE) and differential manometry
(Man) techniques were exploited to detect O2 evolution in
solution and in the gas phase, respectively. In all cases, the
yield in O2 was much lower with respect to that expected based
on the amount of NaIO4 used (< 5%, Table 4). Kinetics were
slow but reproducible leading to a 10% error in TOF, TON and
yield of O2, as deduced by duplicating them under identical
conditions.
Table 4. Water oxidation driven by NaIO4.
Entry
Tech.
[cat]
(µM)
[NaIO4]
(mM)
TOF
(min-1)
TON
Yield O2
(%)
1-Me,Me
1.5 ±
0.15
0.7 ±
0.07
1.5 ±
0.15
1a
Man
CE
12.5
12.5
125
125
77 ± 7
7 ± 1
0.15 ±
0.02
2a
Elegant and exhaustive studies by Lau and co-workers on the
application of 1-Me,Me as WOC suggest that the catalytic
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100 ±
10
2.0 ±
0.2
3b
Man
CE
12.5
12.5
5.0
125
125
20
1.0 ± 0.1
0.60 ±
0.06
0.10 ±
0.01
5.0 ±
0.5
4b
5a
6c
5 ± 1
1.5 ±
0.14
101 ±
10
Man
Man
1.3 ±
0.13
3.0 ±
0.3
5.0
20
61 ± 6
1-Me,Et
1.4 ±
0.14
111 ±
10
2.2 ±
0.22
7a
Man
CE
12.5
12.5
125
125
0.40 ±
0.04
7.1 ±
0.7
0.14 ±
0.02
8a
1-Et,Et
0.03 ±
0.01
0.32 ±
0.04
9a
Man
CE
12.5
12.5
125
125
16 ± 2
0.10 ±
0.01
1.4 ±
0.1
0.06 ±
0.01
10a
1-Me,Bn
0.80 ±
0.08
1.6 ±
0.16
11a
Man
CE
12.5
12.5
125
125
81 ± 8
0.60 ±
0.06
5.1 ±
0.5
0.10 ±
0.01
12a
[a] In H2O. [b] In acetate (0.1 M) buffer at pH = 4. [c] In phosphate (0.2 M)
buffer at pH = 7.
The activity is apparently reduced when O2 evolution was
followed by CE instead of Man, due to the formation of
undetectable oxygen bubbles (compare entries 1 with 2, 3 with 4,
8 with 9, 10 with 11 and 12 with 13), as previously observed.[54]
Activity was little affected by a change of pH (compare entries
1/3 and 5/6 in Table 4).
Figure 7. TON and TOF versus time for WO driven by NaIO4 (125 mM) in H2O
(25°C) catalyzed by 1-R,R’ complexes (12.5 µM).
Interestingly, the TOF versus t trends, obtained by the derivative
of the TON versus t trends, indicate that the maximum activity is
immediately reached without any appreciable induction time
(Figure 7), in sharp contrast with the behavior of Ir WOCs,
studied under the same experimental conditions and elaborating
kinetic data with exactly the same methodologies.[55] This
suggests that the starting 1-R,R’ complex is the real molecular
active species that is likely not very stable under the strong
oxidative conditions, thus leading to moderate TON values.
Conclusions
This study aimed at preparing and characterizing a class of
iron(III) complexes containing cross-bridged cyclams as ligands.
The presence of an N4-macrocycle with distinctive
characteristics was exploited to verify the ability of the
complexes to catalyze the oxidation of alcohols to ketones in
aqueous and green systems. The results obtained with t-
butylhydroperoxide confirm the ability of this class of ligands to
confer robustness and reliability to the corresponding metal
complexes, which enable good catalytic activity in WO. The
performances in WO driven by NaIO4 were found to be
dependent on the steric hindrance of the R-substituents. An
increase of steric hindrance causes two conflicting effects. It
worsens the accessibility at the metal center of reagents (lower
TOF) but also kinetically protects the catalyst from deactivation
(higher TON). Best compromise is obtained with 1-Me,Et
complex, which exhibits a TOF = 1.4 min-1 and TON= 111. The
set of results justifies the increased interest that today the iron
complexes arouse in catalysis, in both homogeneous[56-58] and
heterogeneous[59] conditions.
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10.1002/ejic.201800451
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directly into the measuring reactor and dissolved in 4.5-4.9 mL of pure or
buffered water. The same amount of water was loaded into the reference
reactor. Both reactors were equipped with a side arm for the connection
to the manometer and with a septum to seal the system, which was kept
at a constant temperature (T= 25°C). When equilibration was reached,
the measurement was started by injecting 100–500 µL of water and
catalyst solution in the reference and measuring reactor, respectively.
The concentration of the stock solution of catalyst was adjusted,
depending on the desired final concentration of the catalyst.
Experimental Section
General: All experiments were performed under dry argon using
standard Schlenk techniques. The compounds were purchased from
Sigma-Aldrich and used as received without further purification. Ligands
L-R,R’ were synthesized according to a literature procedure.[1-3,16,18] IR
and UV spectra were recorded with a spectrometer Jasco UV-560 and
Jasco FT/IR-430. Mass spectra were recorded on a Q ToF Agilent 6540,
LC system 1290 Infinity series, Column Acquity UPLC BEH C18 1.7 µm
2.1x100 mm. The spectra exhibited peaks at m/z according to [MCl2(L-
R,R’)]+ for all four complexes (see Supporting Information). Conductivity
measurements were performed on a Hanna Instrument model HI 8820 N.
The measurement of magnetic susceptibility was performed in
acetonitrile by the Evans method[60] (see Supporting Information).
Methanol and diethyl ether were freshly distilled under argon from
magnesium and Na, respectively.
X-ray diffraction analysis. Single crystals of 1-Bn,Bn suitable for X-ray
structural analysis were obtained by slow evaporation of
acetonitrile/water solution at room temperature. One selected crystal was
mounted in flowing N2 at 173
K on a Bruker-Nonius KappaCCD
diffractometer equipped with Oxford Cryostream 700 apparatus, using
graphite monochromated MoK radiation ( = 0.71073 Å, CCD rotation
images, thick slices,  and  scans to fill asymmetric unit). Reduction of
data and semiempirical absorption correction were done using SADABS
program.[61] The structure was solved by direct methods (SIR97
program[62]) and anisotropically refined by the full matrix least-squares
method on F2 against all independent measured reflections using
SHELXL-2016/6[63] with the aid of the program WinGX.[64] All hydrogen
atoms were generated stereochemically and refined according to the
riding model with C–H distances in the range 0.95–0.99 Å and with
Uiso(H) equal to 1.2Ueq of the carrier atom. The figures were generated
using ORTEP-3[65] and Mercury CSD 3.9[66] programs.
Synthesis of 1-R,R’ complexes. A solution of L-R,R’ ligand (1.8 mmol)
in DMAC (3 mL) was added to FeCl2 (0.23 g, 1.8 mmol). After stirring the
mixture at 100°C for 3 h, the solvent was removed by decantation, and
the residue was washed with DMAC (2 x 3 mL), dry diethyl ether (3 x 5
mL), and then dried in vacuo.
A
solution of ammonium
hexafluorophosphate (0.98 g, 6.0 mmol) and bromine (4-5 drops) in
methanol (15 mL) was added to the dark residue. After stirring the
resulting orange suspension at room temperature for 2h, the yellow-
orange precipitate was filtered, washed with methanol (2 x 5mL) and
diethyl ether (2 x 5 mL), and dried in vacuo.
Crystallographic data of 1-Bn,Bn. C26H38FeCl2N4PF6, M= 678.32,
orthorhombic, P212121, a= 8.502(3) Å, b= 15.763(3) Å, c= 21.886(5) Å,
α= β= γ= 90°, V= 2933.1(13) Å3, T = 173(2) K, Z = 4, Dcalcd = 1.536
Mg/m3, crystal size 0.50 × 0.08 × 0.05 mm3, F(000) = 1404, absorption
1-Me,Et. Yield: 40%. C15H32Cl2F6FeN4P (540.2): calcd. C 33.35, H 5.97,
N 10.37; found C, 33.51, H, 5.88, N 10.50. UV/Vis (MeCN): λ (ε, Lmol‒
1cm‒1)= 372 (1721) nm, 265 (17568) nm. FTIR (nujol)= 840 cm‒1 (PF6‒).
 (10‒3 M, MeCN)= 146 S cm2 mol‒1. s= 5.8 BM. 1-Et,Et. Yield: 35%.
C16H34Cl2F6FeN4P (554.2): calcd. C 34.68, H 6.18, N 10.11; found C
coefficient 0.814 mm−1
, reflections collected 18998, independent
34.85, H 6.07, N 10.36. UV/Vis (MeCN): λ (ε, Lmol‒1cm‒1)= 378 (2197)
reflections 5566 [Rint = 0.1335], final R indices [I >2σ(I)] R1 = 0.0688, wR2
= 0.1452, R indices (all data) R1 = 0.162, wR2 = 0.1835, largest diff. peak
and hole 0.730 and -0.695 eA-3.
‒
nm, 269 (17469) nm. FTIR (nujol)= 840 cm‒1 (PF6 ).  (10‒3 M, MeCN)=
135 S cm2 mol‒1. s= 5.5 BM. 1-Me,Bn. Yield: 47%. C20H34Cl2F6FeN4P
(602.2): calcd. C 39.89, H 5.69, N 9.30; found C 39.66, H 5.80, N 9.22.
UV/Vis (MeCN): λ (ε, Lmol‒1cm‒1)= 378 (3708) nm, 287 (9505) nm. FTIR
All crystallographic data were deposited at Cambridge Crystallographic
Data Centre with assigned number CCDC 1834734. These data can be
obtained free of charge from www.ccdc.cam.ac.uk/data_request/cif.
‒
(nujol)= 840 cm‒1 (PF6 ).  (10‒3 M, MeCN)= 142 S cm2 mol‒1. s= 5.8
BM. 1-Bn,Bn. Yield: 34%. C26H38Cl2F6FeN4P (678.3): calcd. C 46.04, H
5.65, N 8.26; found C 45.83, H 5.79, N 8.09. UV/Vis (MeCN): λ (ε, Lmol‒
1cm‒1)= 385 (3892) nm, 270 (16529) nm. FTIR (nujol) = 840 cm‒1 (PF6‒).
 (10‒3 M, MeCN)= 139 S cm2 mol‒1. s= 6.1 BM.
Acknowledgements
Catalytic oxidation of alcohols. The appropriate amount of H2O2 or t-
BuOOH (see Tables 1-4) was added to a solution of 1-R,R’ (1-4 mol%)
and alcohol (0.2 mmol) in CD3CN/D2O (1.2 mL). The mixture was stirred
at 27°C for 18 h. CDCl3 (2 mL) and tetrachloroethane, as internal
standard, were added. The organic phase was separated and another
extraction with CDCl3 (2 mL) was performed. The solution was dried on
sodium sulphate and analyzed by NMR spectroscopy.
The authors wish to thank the Italian Ministry of Education,
University and Research (MIUR) for financial support (PRIN
project: Towards a Sustainable Chemistry: Design of Innovative
Metal-Ligand Systems for Catalysis and Energy Applications,
Award Number 20154X9ATP). The University of Perugia and
MIUR acknowledged for financial support to the project AMIS,
through the program “Dipartimenti di Eccellenza - 2018-2022”.
The authors also thank Dr. Simone Moretti and Prof. Gabriele
Cruciani for their help in recording MS spectra.
Water Oxidation. Clark Electrode (CE). All measures were performed
using HANNA HI 4421 instrument. NaIO4 powder was added directly into
the reaction vessel and the proper amount of pure or buffered water was
added in order to reach the final reaction concentration. A proper amount
of a stock solution of catalyst was injected in order to obtain the desired
concentration. The reaction temperature was maintained at 25°C.
Keywords: iron • cyclam • oxidation • alcohols • water oxidation
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Entry for the Table of Contents
FULL PAPER
Iron(III) complexes containing
Key Topic*
diversely substituted cross-bridged
cyclams catalyze the oxidation of
alcohols to ketones in green
Alfonso Annunziata, Roberto Esposito,
Giordano Gatto, Maria Elena Cucciolito,
Angela Tuzi, Alceo Macchioni*,
Francesco Ruffo*
conditions, by using t-
butylhydroperoxide as the oxidant in
aqueous solutions. They also exhibit
catalytic activity in water oxidation
(WO) to molecular oxygen driven by
NaIO4, and the results depend on the
steric hindrance of the R-substituents.
Page No. – Page No.
Iron(III) complexes with cross-bridged
cyclams: synthesis and use in
alcohol and water oxidation catalysis
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