Catalytic Selective Oxidation of Primary and Secondary Alcohols Using Nonheme [Iron(III)(Pyridine-Containing Ligand)] Complexes

Article abstract of DOI:10.1002/ejoc.202001201

The selective oxidation of different primary and secondary alcohols to carbonyl compounds by hydrogen peroxide was found to be catalyzed in conversion ranging from good to excellent by an iron(III) complex of a pyridine-containing macrocyclic ligand (Pc-L), without the need of any additive. The choice of the counteranion (Cl, Br, OTf) appeared to be of fundamental importance and the best results in terms of selectivity (up to 99 %) and conversion (up to 98 %) were obtained using the well-characterized [Fe(III)(Br)₂(Pc-L)]Br complex, 4c. Magnetic moments in solid-state, also confirmed in solution ([D₆]DMSO) by Evans NMR method, were calculated and point out to an iron metal center in the high spin state of 5/2. The crystal structure shows that the iron(III) center is coordinated by the four nitrogen atoms of the macrocycle and two bromide anions to form a distorted octahedral coordination environment. The catalytic oxidation of benzyl alcohol in acetonitrile was found occurring with better conversions and selectivities than in other solvents. The reaction proved to be quite general, tolerating aromatic and aliphatic alcohols, although very low yields were obtained for terminal aliphatic alcohols. Preliminary mechanistic studies are in agreement with a catalytic cycle promoted by a high-spin iron complex.

Full text of DOI:10.1002/ejoc.202001201

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Catalytic selective oxidation of primary and secondary alcohols
using nonheme [Iron(III)(Pyridine-Containing Ligand)] complexes
Authors: Nicola Panza, Armando di Biase, Silvia Rizzato, Emma Gallo,
Giorgio Tseberlidis, and Alessandro Caselli
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202001201
Link to VoR: https://doi.org/10.1002/ejoc.202001201
01/2020

10.1002/ejoc.202001201
European Journal of Organic Chemistry
Catalytic selective oxidation of primary and secondary alcohols using
nonheme [Iron(III)(Pyridine-Containing Ligand)] complexes
Nicola Panza,a Armando di Biase,a Silvia Rizzato,a Emma Gallo,a Giorgio Tseberlidisa,b and
Alessandro Casellia*
a Department of Chemistry, Università degli Studi di Milano and CNR-SCITEC, via Golgi 19 – 20133
Milano, Italy. b Department of Materials Science and Solar Energy Research Center (MIB-SOLAR),
University of Milano-Bicocca, Via Cozzi 55, 20125 Milano, Italy. Email:
alessandro.caselli@unimi.it; Corresponding author homepage url: http://users.unimi.it/acaselli/
ABSTRACT:
The selective oxidation of different primary and secondary alcohols to carbonyl compounds by
hydrogen peroxide was found to be catalysed in conversion ranging from good to excellent by an
iron(III) complex of a pyridine-containing macrocyclic ligand (Pc-L), without the need of any
additive. The choice of the counteranion (Cl, Br, OTf) appeared to be of fundamental importance and
the best results in terms of selectivity (up to 99%) and conversion (up to 98%) were obtained using
the well characterised [Fe(III)(Br)2(Pc-L)]Br complex, 4c. Magnetic moments in solid state, also
confirmed in solution (d6-DMSO) by Evans NMR method, were calculated and point out to an iron
metal centre in the high spin state of 5/2. The crystal structure shows that the iron(III) centre is
coordinated by the four nitrogen atoms of the macrocycle and two bromide anions to form a distorted
octahedral coordination environment. The catalytic oxidation of benzyl alcohol in acetonitrile was
found occurring with better conversions and selectivities than in other solvents. The reaction proved
to be quite general, tolerating aromatic and aliphatic alcohols, although very low yields were obtained
for terminal aliphatic alcohols. Preliminary mechanistic studies are in agreement with a catalytic cycle
promoted by a high-spin iron complex.
1
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10.1002/ejoc.202001201
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INTRODUCTION
The oxidation of alcohols to the corresponding carbonyl compounds is an important chemical process
in industrial synthesis and academic research. Compared to the alcohols, in fact, carbonyl compounds
show a wider reactivity as precursors and intermediates for many bioactive compounds.[1] Both
laboratory-scale syntheses and industrial oxidation processes in the past heavily relied on the use of
highly toxic metal-oxo reagents (i.e. KMnO4) and metal oxide surfaces.[2] Despite the fact that a large
number of oxidation reactions have been reported in the last decades,[3] the demand for more selective,
efficient, eco-friendly oxidation methods is still increasing. In light of the rising interest in green
chemistry principles, a tremendous effort is devoted to develop environmentally benign catalysts and
bioinspired oxidation reactions. Several different strategies and efficient methods have been reported
for the selective oxidation of primary and secondary alcohols to aldehydes and ketones.[4] The
reagents of choice as green oxidants are by far O2 and H2O2 since they possess the highest oxygen
content.[5] For instance, in the last five years, significant advances have been achieved in the oxidation
of benzyl alcohols to benzaldehydes, using homogeneous iron(II),[6] iron(III)[7] and copper(II)[7a]
complexes, metallacarboranes,[7d] supported nanoparticles,[8] MOF,[9] and substoichiometric organic
reagents.[10]
In particular, the use of iron,[11] the most abundant transition metal on earth, and its complexes in
organic synthesis, biochemistry and also in industrial application is of particular interest.[12] The
impressive boost that this area has received in the last few years is witnessed by several timely
reviews.[13]
Natural mononuclear nonheme iron enzymes are involved in metabolically vital oxidative
transformations, catalysing a wide range of chemical reactions with high efficiency,[14] and the search
of synthetic models mimicking Nature is constantly pursued.[15] In the case of nonheme iron enzymes
and their models, key intermediates are high-valent iron-oxo intermediates, such as iron(III)-super-
oxo, iron(III)-peroxo, iron(III)-hydroperoxo, and iron(IV)- or iron(V)-oxo species.[14a, 16] While
iron(III)-superoxo species have been recognised as active oxidants in the C−H bond activation and
oxygen atom transfer (OAT) reactions although the intermediates are only able to activate weak C-H
bonds in hydrocarbons, the reactivities of nonheme iron(III)−hydroperoxo species are still matter of
debate and subject of in depth experimental and theoretical investigations.[17] On the other hand,
nonheme iron(IV)-oxo species are competent oxidants capable of abstracting hydrogen atom in C−H
bond activation reactions.[15e]
Amongst nonheme catalysts, iron complexes of polyamine and aminopyridine ligands surely occupy
a prominent role.[18] In particular, iron(II) complexes of tetradentate N4 donor-ligands,
2
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European Journal of Organic Chemistry
[Fe(CF3SO3)2(PyNMe3)],[19] have proven to be easily oxidised by H2O2 to yield a hydroperoxo
iron(III) species which upon the acid triggered O-O bond cleavage is active in the stereospecific
hydroxylation of strong C-H bonds.[20]
Our group has been interested since several years in the synthesis and study of the coordination
behaviour of a series of pyridine-based 12-membered tetraaza-macrocyclic ligands (Pc-L),[21] and we
have reported the catalytic activity of their copper(I)[22] and silver(I)[23] complexes in C-C and C-X
bond formation reactions. Last year, we reported the synthesis and characterization of robust
[iron(III)(Pyridine-Containing Ligand)] complexes, [Fe(III)(Pc-L)], highly active and selective for
the alkene oxidation using hydrogen peroxide as the terminal oxidant, in the absence of any
additive.[24] Depending on the anion, X-, of the iron(III) metal complex employed as a catalyst, we
observed the selective formation of epoxide (X = Cl), or a clean dihydroxylation reaction (X = OTf)
(Figure 1a). –the rationale for this reversal of selectivity could be traced to the influence of the
counteranion on the spin state of the iron centre (low spin for X = Cl, high spin for X = OTf) although
a possible role of hidden HOTf in the opening of the epoxide cannot be ruled out. Intrigued by this
results, we were interested in further exploring the reactivity of these iron(III)(Pc-L) complexes in
other oxidation reactions. We report here our findings in the selective oxidation of alcohols by using
H2O2, in the absence of any acid co-catalyst Figure 1b).
Figure 1. a) Epoxidation or syn-dihydroxylation reactions catalysed by [FeIII(X)2(Pc-L)]X complexes; b) selective
oxidation of primary and secondary alcohols reported in this work.
RESULTS AND DISCUSSION
Preparation of the iron complexes. As we recently reported,[24] macrocycle 1 was obtained in a
useful overall yield by treatment of N-tritosyl-diethylenetriamine with pyridine-2,6-
3
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European Journal of Organic Chemistry
diylbis(methylene) dimethanesulfonate, followed by the hydrolysis in HBr of the Ts protecting
groups. The sp3 nitrogen atoms of the bromohydrated salt, 2·HBr, have been directly benzylated in
the presence of excess of DIPEA (DIPEA = N,N-diisopropylethylamine) as a base (see Supporting
Information for details), Scheme 1. Metal complexes 3a-b were obtained by mixing an acetonitrile
solution of the selected ligand with an acetonitrile solution of iron(III) chloride hexahydrate or
iron(III) triflate at room temperature in a 1:1 ligand to metal ratio. Both complexes showed a good
solubility in polar non protic solvents (acetone, CH3CN, etc.).
Better yields of the free base ligand 2 were obtained when using concentrated sulphuric acid followed
by treatment with NaOH in the deprotection step. As already reported,[19a] with this method, less
reproducible yields ranging from 60 to 80% are obtained, but the product can be isolated in high
purity after a simple extraction with organic solvents. Metal complexes 4a-c were isolated using the
same procedure described above, albeit in slightly lower yields. These complexes are characterised
by a lower solubility in non-protic solvents but are highly soluble in water.
The structures of complexes 4a-c were characterised by means of mass and elemental analysis as
detailed in the Supporting Information. Elemental analysis for the iron chloride complex 4a·H2O is
consistent with the presence of a lattice molecule of water, not observed in the case of complexes 4b-
c. Based on the ionization observed, together with our previous findings for complexes 3a,b, we
propose the structure depicted in Scheme 1, with the iron placed in an octahedral environment and
two X groups directly bound to the metal (X = Cl, Br or OTf). Magnetic moments, eff, of 4.67 B
and of 5.42 B were measured by the Evans’ method for complexes 4a·H2O and 4c, respectively.[25]
These room temperature magnetic moments in solid state were also confirmed in solution (d6-DMSO)
by Evans NMR method,[26] where eff, of 5.37 B (4a·H2O) and of 5.36 B (4c) were calculated and
seems to point out to an iron metal centre in the high spin state of 5/2.[27] This values have to be
compared with those of complexes 3a·H2O and 3b, that at room temperature possess lower magnetic
moments, in agreement with a mixture of low spin and high spin states as already reported by us.[24]
Detailed EPR studies showed that for complex 3a·H2O both low (S = 1/2) and high (S = 5/2) spin
iron(III) species are present at RT, while lowering the T, the populated spin state 5/2 increases.
4
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European Journal of Organic Chemistry
Scheme 1. Synthetic route used to obtain ligands 1 and 2 and their corresponding ferric complexes 3 and 4. Ts= tosyl; Ms
= mesyl.
Single crystals suitable for X-ray crystallography were grown by slow evaporation from a water
solution of complex 4c. Figure 2 shows the chemical structure of the complex.
Figure 2. Molecular structure of the complex [Fe(III)(Br)2(Pc-L)]Br (50% probability thermal ellipsoids). The water
molecule has been omitted for clarity. Selected bond lengths (Å): Fe – N1 2.102(2), Fe – N2 2.180(2), Fe – N3 2.169(2),
Fe – N4 2.188(2), Fe – Br1 2.4219(5), Fe – Br2 2.4353(5). Full crystal structure data of 4c are reported in the Supporting
Information.
The iron(III) centre is coordinated by the four nitrogen atoms of the macrocycle and two bromide
anions to form a distorted octahedral coordination environment. As shown, the macrocycle ligand L
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European Journal of Organic Chemistry
chelates the metal centre with the four nitrogen atoms in a tetradentate cis- fashion, leaving two cis
position free for two bromine.[28] Among the Fe-N distances the Fe-N1 bond length is the shortest
one, indicating that the strongest metal-ligand interaction occurs with the pyridinic nitrogen.[24] All
the structural parameters are in good agreement with those previously reported in the literature for
similar complexes.[24, 28][29] A comparison with the analogous chloride complex [Fe(III)(Cl)2(Pc-
L)]Cl (CSD refcode JIKPAX)[28b] reveals similar values of the geometrical parameters (Fe-N bond
lengths and Nax-Fe-Nax , Neq-Fe-Neq bond angles: 2.182, 2.182 and 2.161 Å for the amino nitrogens
and 2.098 Å for the pyridinic nitrogen, 146.68°, 86.62° in JIKPAX; Fe-N values range from 2.102 to
2.188 Å , see figure 2 caption, Nax-Fe-Nax , Neq-Fe-Neq bond angles: 147.63° , 85.63° for the complex
under study).
Iron-catalysed alcohol oxidations. At the outset, oxidation of benzyl alcohol 5a by H2O2 served as
benchmark for the optimization of the reaction conditions (see Table 1). This reaction has been widely
studied in recent years, but good results with high selectivities were obtained mainly with
heterogeneous catalysts.[8, 30] The lack of good homogeneous catalysts capable of yielding selectively
benzaldehyde in such a reaction prompted us to test our iron(III) complexes 3a-b, choosing as a
starting point the conditions very close to what we have already recently reported for the
epoxidation/dihydroxylation reaction of alkenes.
Reactions were performed by adding the catalyst and the benzyl alcohol to the solvent and then the
oxidant (H2O2, 30%) by slow addition with syringe pump or by repeated single additions. The
reactions were followed by 1H NMR adding CH2Br2 as internal standard;[31] GC (dodecane as internal
standard) confirmed conversion of the starting benzyl alcohol and selectivities in 6a and 7a. Results
of the optimisation of the reaction conditions are reported in Table S1 in the supporting information.
Although complex 3b seems to be much more active as catalyst and higher conversion were always
observed, better selectivities were obtained at 0 °C using catalyst 3a·2H2O (5 mol%) and a 1:1 ratio
between the oxidant and the substrate, although under those conditions a very low conversion was
observed (see Table S1, Supporting Information). Under those conditions, blank control experiments
in the absence of any catalyst gave no conversion of the starting aldehyde.
Oxidations by hydrogen peroxide catalysed by Fenton’s reagent (ferrous ion) have received
considerable scrutiny since last century,[32] while the use of ferric salts have been given considerably
less attention. However, iron(III) chloride in the absence of a ligand can catalyse the oxidation of
benzylic primary aromatic alcohols to aldehydes by using a three-fold excess of H2O2 using water as
solvent at room temperature, albeit in very modest yields and with concomitant over-oxidation to
benzoic acids.[7c] Martin and Garrone have reported since 2003, that FeBr3 can catalyse efficiently
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the selective oxidation of benzylic alcohols by H2O2 under solvent free conditions.[33] A large excess
amount of hydrogen peroxide was needed because of its fast decomposition in the presence of the
ferric ion. To the best of our knowledge, ferric salts have been since then neglected in such oxidation
reaction, until recent reports on the use of Fe(OTs)3 for the selective oxidation of benzyl alcohol[7b]
and on the efficient use of Fe(OTf)3 as the catalyst for the solvent free oxidation of cyclohexane.[34]
We were thus interested in comparing the results of the use of simple iron(III) salts under our
conditions (Table 1). All the tested ferric salts catalysed a fast decomposition of H2O2, but if a large
excess of the oxidant was used, 20-fold excess with respect to the alcohol, a good conversion could
be achieved. However, only using FeBr3 an acceptable selectivity was observed and benzaldehyde
was obtained as the major product (83% selectivity, entry 3, Table 1). If the addition of the oxidant
was slow enough, its quantity can be reduced and sustainable conversions, 76%, and selectivity, 83%,
were obtained also when using a 4:1 H2O2/alcohol ratio (entry 4, Table 1). However, it should be
pointed out that both FeCl3 and Fe(OTf)3 behave very similarly as metal complexes 3a·2H2O and 3b
respectively, despite their lower solubility in the reaction medium (compare for example entry 1,
Table 1 with entry 5, Table S1).
Table 1. Oxidation reaction of benzyl alcohol to benzaldehyde with H2O2 as oxidant catalysed by
ferric salts, [Fe(III)X3][a]
Entry
Catalyst eq H2O2 Conv (%)[b] 6a Select (%)[b] 7a Select (%)[b]
1
2
20
20
20
4
98
72
84
76
6
31
21
5
FeCl3
31
83
83
Fe(OTf)3
3
FeBr3
4[c]
4
[a] Reactions were performed with [FeIII] (2.5 x 10-2 mmol) in the CH3CN (10 mL) at a cat/alcohol ratio of 1:20 at 30 °C;
H2O2 (30% sol; equiv. as reported) was added in a single addition, unless otherwise stated, [b] Conversions and
selectivities were calculated after 24 h by 1H NMR adding CH2Br2 as the internal standard to the crude reaction mixture
and confirmed by GC (dodecane as the internal standard, [c] H2O2 (30% sol) was added dropwise by a syringe pump in
2 h.
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It is known that the presence of free N-H bonds might render polyazamacrocyclic ligands sensitive
to strong oxidants and affect the complex stability, but it is also reported that N-alkylation of the
ligand can cause a change in the spin state configuration at iron, resulting in a lower catalytic
activity.[35] We were thus interested in studying the catalytic activity of our iron(III) complexes 4a-c,
that in the solid state were shown to possess a high-spin iron centre. Interestingly, complexes 4a·H2O
and 4b, which showed a sluggish solubility in CH3CN compares quite well with their respective salts,
and in particular for complex 4a·H2O a slightly better selectivity in benzaldehyde was observed
(compare entry 1, Table 2 with entry 1, Table 1). On the other hand, complex 4c, under those
conditions was less active and less selective than ferric bromide alone when a 20-fold excess of the
oxidant was added (compare entry 7, Table 2 with entry 3, Table 1). Instead, if the oxidant was added
in small portions, the catalytic system was much more selective and, for instance, if 1 eq of hydrogen
peroxide was added every 15 minutes to the reaction mixture, a reasonable selectivity of 73% with a
conversion of 76% in just 3h was observed (entry 10, Table 2). A better selectivity (>98%) was
obtained when using complex 4c as catalyst by slowly adding 2 eq of the oxidant by syringe pump in
2 h and allowing the reaction mixture to stir at 30 °C for further 22 h (entry 12, Table 2).
Finally, the best compromise between conversion (up to 96%) and selectivity (90%) was obtained by
the slow addition of 4 eq of oxidant and allowing the reaction to stir at 30 °C for 24 h (Entry 14, Table
2).
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Table 2. Optimization of [Fe(III)(X)2(Pc-L)]X, 4a-c, catalysed alcohol oxidation[a]
Conv (%)[b] 6a Select (%)[b]
7a Select (%)[b]
Entry Catalyst
Solvent
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
AcOEt
eq H2O2 time (h)
95
33
46
86
69
85
66
30
60
78
29
50
45
96
58
87
49
55
35
<10
98
76
1
20
4
24
1
11
55
52
19
36
26
55
57
72
76
93
98
87
90
88
92
82
99
86
90
<1
89
27
3
2[c]
4a·H2O
3[c]
4
8
2
7
20
4
24
1
20
32
39
11
-
5[c]
4b
6[c]
7
8
2
20
4
24
1
8[c]
9[c]
8
2
2
10[c]
11[d]
12[d]
13[d]
12
2
3
3
2
3
2
24
2
2
4
2
14[d]
4
24
2
10
2
4c
15[d]
8
16[d]
17[d,e]
18[d]
19[d]
20[d]
21[d]
22[d,f]
8
24
24
24
24
24
24
2
2
4
2
4
1
acetone
t-amylalcohol
water
4
-
4
-
4
-
CH3CN
4
7
[a] Reactions were performed with [FeIII] (2.5 x 10-2 mmol) in the solvent (10 mL) at a cat/alcohol ratio of 1:20 at 30 °C;
H2O2 (30% sol; equiv. as reported) was added in a single addition, unless otherwise stated, [b] Conversions and
1
selectivities were calculated by H NMR adding CH2Br2 as the internal standard to the crude reaction mixture and
confirmed by GC (dodecane as the internal standard, [c] 1 equiv. of H2O2 was added every 15 min to the reaction mixture,
[d] H2O2 (30% sol) was added dropwise by syringe pump in 2 h, [e] Cat/alcohol ratio of 1:40, [f] T = 60 °C.
Under those conditions, even if the catalyst amount was halved (2.5 mol%), a high selectivity in
benzaldehyde was maintained (88%, entry 17, Table 2). It should be pointed out that under identical
conditions pure FeBr3 was almost inactive (< 24% conversion) and less selective (< 75%).
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We next checked the effect of the solvent, and higher selectivities (> 99%) were obtained only in
AcOEt (Entry 18, Table 2). The lower conversion observed when using acetone as solvent could be
accounted to its ability to trap free hydroxyl radicals that might, in this case, be involved in the
reaction.[36]
As expected, at higher temperatures, 60 °C, good conversions were observed in just 2 h, and still a
reasonable selectivity (89 %) was kept (see Table S2, Supporting Information).
Reaction scope. We decided to explore the scope of the reaction by using the optimised conditions
(entry 14, Table 2) that gave the best compromise between quantitative conversions while keeping
high selectivity. Results are summarised in Table 3, where conversion of the starting alcohol is
reported in brackets.
Oxidation of para substituted benzyl alcohols proceeded smoothly and high conversions with good
selectivities were observed especially with electron withdrawing substituents (products 6b and 6c,
Table 3). Slightly lower conversions and selectivities were obtained instead when electron donating
substituents were presents (products 6d and 6e, Table 3; vide infra for a comparative Hammett
analysis). Reasonable selectivities were retained also in case of 2-substituted benzyl alcohols, with
lower conversions (products 6f and 6g, Table 3).
Benzylic secondary alcohols gave excellent results and products 6h and 6g were obtained in 83% and
71% yield respectively. Given the fact that selectivity in the case of secondary benzyl alcohol
oxidation is not an issue, product 6h could be selectively obtained in 71% yield in just 2h by slowly
adding the oxidant at 90 °C (see Table S2, Supporting Information).
A lower efficacy in the oxidation of both primary and secondary aliphatic alcohols stands out from
data reported in Table 3. As a matter of fact, conversions of both nonyl alcohol, 5k, and 1-penhyl-2-
propanol, 5l, hardly exceeds 15%, although in both cases reasonable selectivities were observed (67%
for product 6k and 99% for 6l). Cyclic secondary alcohols were oxidised more efficiently and ketones
6m-o were obtained in good to excellent selectivities. Interestingly, cyclobutanol, 5p, was mainly
converted into cyclobutanone 6p (78% selectivity), but from GC-MS analysis we were able to detect
4-hydroxybutyraldehyde, 8, and -butyrolactone as major by-products (see later for mechanistic
considerations).
In order to further test the selectivity of our system, several alcohols having both –CH2OH and C=C
units were considered. Since we have already studied the reactivity of alkenes to give epoxide or diols
in the presence of Fe(III)Pc-L complexes using hydrogen peroxide as oxidant,[24] it was interesting to
evaluate the possibility of competitive reactions. When cinnamyl alcohol, 5r, was used as substrate,
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we observed a good conversion (60%), but a poor selectivity towards the expected cinnamaldehyde,
6r (17%). Interestingly, the low yield in 6r was accompanied by the formation of benzaldehyde as
major byproduct (25%), which is known to occur through a disruptive oxidation of the starting
unsaturated alcohol via the formation of epoxides as reaction intermediates.[37] The complete absence
of products derived from the epoxidation reaction of the double bond was observed also in the case
of 2-cyclohexenol and geraniol.
Table 3. Scope of alcohol oxidations catalysed by complex 4c.[a]
[a] Reactions were performed with [FeIII] (2.5 x 10-2 mmol) in CH3CN (10 mL) at a cat/alcohol/H2O2 ratio of 1:20:80;
H2O2 (30% sol) was added by syringe pump in 2 h than allowing the reaction mixture to stir at 30 °C for further 22 h.
Conversions of the starting alcohol (in brackets) and selectivities were calculated by GC and GC-MS (dodecane as the
internal standard, n.d. = not detected), [b] When the same reaction was repeated at 90 °C a 81% conversion was observed
in just 2 h and 6h was obtained with a 88% selectivity, [c] The presence of ring opened product was detected by GC-MS
and 8 was obtained as major by-product, [d] Benzaldehyde 6a was obtained as a major product in 25% selectivity.
Finally, we also checked the possibility to selectively oxidise 1,2-diols, whose chemo- and/or
regioselective oxidation has still proven to be challenging.[38] When hydrobenzoin was treated under
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our optimised conditions, the diketone 6u was formed with a 46% selectivity, accompanied by the
formation of benzaldehyde (18%) as major by product, along with traces of other overoxidation
products such as benzoic acid. Dodecane-1,2-diol, 5v, was not efficiently converted into the
corresponding hydroxymethyl ketone 6v, which was observed by GC-MS as one of the products
formed along with high quantities of undecanal as major byproduct, 6w.
Discussion of the proposed reaction mechanism. In order to gain better insights in the reaction
mechanism, UV spectra of complex 4c were recorded both in CH3CN (Figure S4a) and in water
(Figure S4b) before and after the addition of H2O2. Samples with different concentration of the
complex were used to estimate the ε value of the most relevant absorption bands. In CH3CN, two
bands at 255 nm and 394 nm respectively accounted to MLCT and solvent interaction, with ε values
(L mol-1 cm-1) of ε255nm= 2719 and ε394nm=1154 whilst in water the only relevant band observed was
found at 253 nm with ε253nm= 7087. After addition of small amount of hydrogen peroxide to a 10-4 M
solution of the complex in acetonitrile, we could observe the disappearance of the band at 394 nm
and the formation of an intense band at 272 nm, which may imply a modification in the coordination
sphere of the metal and the formation of an iron(III)-hydroperoxo intermediate with long life time
(Figure S5a). On the other hand, we were not able to detect any formation of significant bands in the
range between 500-700 nm accountable to an iron(IV)-oxo species.[39] When the experiment was
repeated on a more concentrated solution of the complex (2.5*10-3 M) in water, we detected a small
band at 540 nm that might be attributed to a possible iron(IV)-oxo intermediate (Figure S5b). This
band disappeared upon the addition of more equivalents of hydrogen peroxide, due to the reaction of
the oxo complex with hydrogen peroxide itself to yield oxygen and water.
The chemistry of nonheme iron complexes in oxidation reactions has been widely studied and it is
commonly accepted that the true active species in such transformation are high-valent iron oxo
complexes.[36] The formation of such species derives from the homolytic or heterolytic cleavage of
O-O bond in Fe(III)OOH generated by the interaction of iron with hydrogen peroxide. The iron(III)
hydroperoxo is known to be itself a sluggish oxidant. The reactivity of these iron complexes is
strongly influenced by various condition including the solvent, the substrate and oxidant
concentrations and the pH.[6a]
In our case, to shed light in the reactivity of our system, we performed a series of experiment in order
to assess the different possible reaction path followed by our catalyst. First, we evaluated the influence
of EWG and EDG on the reaction kinetics using a series of p-substituted benzyl alcohols under the
optimised reaction conditions described above. Plotting the krel of the reactions (krel = kX/kH) and the
Hammett parameter we obtained a  value of −0.42 (Figure 3). The low value of this parameter
indicates that the reaction is not strongly influenced by EW and ED ability of the substituents and
12
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10.1002/ejoc.202001201
European Journal of Organic Chemistry
that the transition state does not involve a strong electrophilic species. The value is comparable with
the synthetic Fe(IV)(O)−porphyrin•+ ( = −0.39).[40]
Figure 3. Plot of the krel of the reactions (krel = kX/kH) against the Hammett parameter for the reaction of benzyl alcohols
5a-d catalysed by complex 4c.
The kinetic constant (kD) for the oxidation of ,-d2 benzyl alcohol was calculated (see Supporting
Information) and a C-H kinetic isotope effect (KIE - kH/kD) of 3.52 was found. This relatively low
value at 30 °C is typical for other oxo species and seems not to involve any tunnelling effect, which
might become predominant at lower temperatures. The rate determining step can be considered the
αC-H hydrogen atom abstraction of alcohols.[41] The high valent iron species responsible for this
transformation can be nevertheless formed by two substantial mechanisms: i) a 1e- mechanism in
which O-O of iron(III) hydroperoxo undergoes homolytic cleavage to form Fe(IV)(O) + ∙OH or ii) a
2e- mechanism in which the same species undergoes heterolytic cleavage to form Fe(V)(O) and OH-
(Scheme 2). In our case, it seems that the bond dissociation energy (BDE) of the C-H hydrogen
atom cannot be neglected. In fact, the conversion of benzylic alcohols (BDE ~80 Kcal/mol) are much
higher than the aliphatic ones (BDE ~90+ Kcal/mol).[41] This might be more predominant in a 2e-
mechanism, where instead a 1e- mechanism would produce a strong hydroxyl radical that would
discriminate less between different C-H bonds. Unfortunately, most of the substrates are not
mechanism sensitive to this, because for example benzyl alcohol can be oxidised to benzaldehyde by
gem-diol formation and dehydration ( typical for hydroxyl radicals) and by oxidation of the carbon
radical and proton transfer to the solvent cage (typical of 2e- mechanism).[41]
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10.1002/ejoc.202001201
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More significant in this respect is the careful observation of the product distribution in the oxidation
of cyclobutanol (Scheme 2). In this case in fact, only a single step, 2e- process will lead to the expected
ketone 6p leaving the 4-membered ring intact.[42] When we carried out this reaction in our optimised
condition, as reported above a 78% selectivity for the cyclobutanone 6p was observed which can be
fully consistent with a 2e- process. Nevertheless, some ring-opening products were detected by GC-
MS and thus we cannot exclude the participation of different active species in the oxidation reaction.
Scheme 2. Possible reaction mechanism and product distribution in the oxidation of cyclobutanol.
Finally, we carried out benzyl alcohol oxidation in presence of a radical scavenger (BHT = butyl-
hydroxytoluene) and the observed conversion of the substrate was lower and this might be consistent
with a radical mechanism (see Table S3, Supporting Information). A similar observation was made
also when we performed the reaction in acetone as solvent (see above).[36] Taking in consideration
the long reaction time needed for our system to reach a good conversion of the substrate, we cannot
exclude the interaction of carbon centre radicals deriving from the hydrogen atom abstraction (HAA)
with molecular oxygen to form organic peroxides and hydroperoxides which are known to be slowly
converted into alcohol and aldehyde/ketone (Scheme 3).
Scheme 3. a) Possible formation of organic peroxides/hydroperoxides and b) their decomposition pathways.
14
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10.1002/ejoc.202001201
European Journal of Organic Chemistry
This very complex behaviour is not unusual for iron systems in oxidation reaction. The presence of
single electron processes is typical for high-spin iron complexes and this might lead to a plethora of
possible reaction mechanism. Our catalyst presents two cis-labile positions which might be
responsible for this behaviour because once the Fe(III)(OOH) is formed, it can both undergo the well-
defined homolytic cleavage to Fe(IV)(O) or it can be coordinated in a 2 fashion and undergo
heterolytic cleavage to Fe(V)(O)(OH) (Figure 4).
Figure 4. Two possible coordination geometries (1 and 2) of iron-bound OOH species.
The simultaneous occurrence of both 1e- and 2e- processes might also be the reason for the trace
presence of brominated products detected by GC-MS analysis (see Supporting information). If a 2e-
mechanism reduces iron(IV) to iron(II), we formally produce a free bromide atom which might
compete with other species to product formation. In fact, we also observed the formation of
brominated acetonitrile when CH3CN was used as the solvent and it is well documented that it could
be a promoter for 1e- processes.[36] After 24 h of reaction, the metal complex could only be recovered
as a flocculent brown powder that was not active anymore as catalyst in the oxidation reaction. This
might be attributed to either the decomplexation of iron or the formation of other inactive dimer
species. Finally, an interesting finding was the peculiar reactivity of vicinal diols as substrates: in
addition to keto-alcohol and di-keto products, we found the C-C bond cleavage products, which
usually are produced by strong oxidizing agents.
Conclusions
In summary, the well-defined [Fe(III)(Br)2(Pc-L)]Br complex, 4c, served as an excellent catalyst for
primary and secondary alcohols oxidation to carbonyl compounds using hydrogen peroxide as the
sole terminal oxidant under mild reaction conditions. In particular, aromatic benzyl alcohols were
converted to benzaldehydes in good to excellent selectivities (up to 99%) and conversions (up to
98%). The effect of different anions coordinated to the iron centre were investigated and the non-
innocent behaviour of the bromine atom was shown. Characterisation of the most active catalyst
precursor was performed by several different techniques and by resolution of the X-ray structure.
UV-Vis studies were performed to monitor the changes of the pre-catalyst in presence of the oxidant
15
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10.1002/ejoc.202001201
European Journal of Organic Chemistry
and the formation of a long life iron(III)-hydroperoxo intermediate could be inferred. Hammett
correlation with different p-substituted benzyl alcohols pointed out to the C-H abstraction as the rate
determining step. However, a quite low KIE at 30 °C and the observation that “radical clock”
cyclobutanone as a substrate are consistent with the occurrence of both 1e- and 2e- oxidation
processes.
EXPERIMENTAL SECTION
General experimental details. All the reactions that involved the use of reagents sensitive to oxygen
or to hydrolysis were carried out under an inert atmosphere. The glassware was previously dried in
an oven at 110 °C and was set with cycles of vacuum and nitrogen. All chemicals and solvents were
1
commercially available and used as received except where specified. H NMR analyses were
performed with 300 or 400 MHz spectrometers at room temperature. The coupling constants (J) are
expressed in hertz (Hz), and the chemical shifts (δ) in ppm. Low resolution MS spectra were recorded
with instruments equipped with electron ionization (EI), electronspray ionization (ESI)/ion trap
(using a syringe pump device to directly inject sample solutions), or fast atom bombardment (FAB)
sources. The values are expressed as mass−charge ratio and the relative intensities of the most
significant peaks are shown in brackets. Synthesis of the ligands and iron complexes is described in
the experimental section. GC experiments were conducted on a Shimadzu GC-2010 Pro instrument
using an SBL™-5ms fused silica capillary column (10 m x 0.1 mm x 0.1 µm film thickness, Sigma-
Aldrich – Supelco). Detailed instrument parameters and all methods used are reported in the
Supporting Information.
General procedure for the synthesis of Fe(III) complexes.
To a stirring solution of ligand 2 (0.48 mmol) in acetonitrile (10 mL) at 40°C, the appropriate iron
salt was added (0.48 mmol) and the reaction mixture was left stirring for 4 h. An immediate colour
change was observed and, in the case of complexes 4a·H2O and 4c, the precipitation of a solid was
noticed. Complexes 4a·H2O and 4c were collected by filtration and washed with diethyl ether, then
dried under vacuum to obtain a yellow and a red powder, respectively. For complex 4b, the solvent
was evaporated under vacuum and the residue was washed several times with n-hexane to yield a
brown powder.
Yields: 4a·H2O
MS (ESI)
66% [iron source: FeCl3∙6H2O]
m/z (%) = calcd for C11H18Cl3FeN4: 385.01, found: 332.06 (100) [M+ -Cl]
C11H20Cl3FeN4O Calcd: C 34.18, H 5.22, N 14.50; found: C 34.72, H 4.90, N
Elem. An.
14.20
16
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10.1002/ejoc.202001201
European Journal of Organic Chemistry
4b
MS (ESI)
84% [iron source: Fe(OTf)3 anhydrous]
m/z (%) = calcd for C14H18 F9FeN4O9S3: 708.94, found: 411 (100), [M+-
2OTf]
Elem. An.
C14H18F9FeN4O9S3 Calcd: C 23.71, H 2.56, N 7.90; found: C 24.17, H 2.95,
N 7.52
4c
61% [iron source: FeBr3 anhydrous]
MS (ESI)
Elem. An.
m/z (%) = calcd for C11H18Br3FeN4: 500.84, found: 421,95 (40) [M+-Br]
C11H18Br3FeN4 Calcd: C 26.33, H 3.62, N 11.16; found: C 26.38, H 3.66, N
11.30
General catalytic procedure
The catalyst (0.025 mmol) and the substrate (0.5 mmol) were dissolved in acetonitrile (10.0 mL) at
30°C. H2O2 (30%, 2.0 mmol) was added in 2 hours by using a syringe pump and the mixture was
stirred for 24 h. After this period, 1 mL of the reaction mixture was collected, then a standard aliquot
of dodecane was added and the solution was diluted to 10 mL and analysed by GC. To selected
reactions, the remaining reaction mixture was filtered through a PTFE 0.20 m filter to remove the
catalyst and concentrated in vacuum. The so obtained crudes were analysed by NMR (CH2Br2 as
internal standard) to identify the products and to confirm the GC values.
Crystal structure determination
Suitable block red crystals of the compound [Fe(III)(Br)2(Pc-L)]Br·½H2O for X-ray analysis were
isolated by slow evaporation from an aqueous solution of complex 4c at room temperature.
The single crystal X-ray diffraction experiment was performed on a Bruker Smart APEX II CCD
diffractometer with graphite monochromated Mo-K radiation ( = 0.71073 Å). Data were collected
at 180 K in the -scan mode within the range 3.5° < 2 < 53.0°. The frames were integrated and
corrected for Lorentz-polarization effects with the Bruker SAINT software package.[43] The intensity
data were then corrected for absorption by using SADABS.[44] No decay correction was applied. The
structure was solved by direct methods (SIR-97)[45] and refined by iterative cycles of full-matrix least-
squares on Fo2 and F synthesis with SHELXL-97[46] within the WinGX interface.[47] Compound
(4c) crystallises in the monoclinic space group P2/c (n° 13) , Z = 4. All atoms are in general position
except for the water oxygen atom, which is located on the 2-fold axes. Hydrogen atoms of the ligand
were placed in geometrically calculated positions and then refined using a riding model based on the
positions of the parent atoms with Uiso = 1.2 Ueq(C). The hydrogen atom of water molecule (H1W)
was located from a difference Fourier map and refined isotropically. A restrain was applied to the
17
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10.1002/ejoc.202001201
European Journal of Organic Chemistry
H...H distance in order to obtain a reasonable value for the H-O-H angle. All non-hydrogen atoms
were refined with anisotropic displacement parameters.
Full crystallographic data have been deposited with the Cambridge Crystallographic Data Centre
(CCDC No. 2016034). A copy of the data can be obtained free of charge on application to CCDC, 12
Union Road, Cambridge CB2 IEZ, UK (Fax: +44 1223 336 033; e-mail: deposit@ccdc.cam.ac.uk).
ASSOCIATED CONTENT
Supporting Information
Detailed experimental procedures, text, figures and tables reporting NMR spectra of products and
characterisation of the iron complexes. General catalytic procedures and chromatograms of selected
reactions including general GC methods used, GC-MS spectra of certain reactions. Kinetic studies on
p-substituted benzyl alcohols, Hammett plot and KIE determination procedure. Uv-Vis and IR spectra
of the complexes. More detailed crystal structure determination for compound 4c and detailed study
on its geometry. This material is available free of charge via the Internet.
ACKNOWLEDGEMENT
We thank the MUR-Italy (Ph.D. fellowship to N. P.) for financial support. Margherita Orazi is gratefully
acknowledged for precious help in the synthesis of iron(III) complexes.
KEYWORDS
Alcohol oxidation; Hydrogen peroxide; Macrocycles; Nonheme iron complexes; Pyridine containing
macrocyclic ligands.
REFERENCES
[1]
[2]
M. H. S. A. Hamid, P. A. Slatford, J. M. J. Williams, Adv. Synth. Catal. 2007, 349, 1555-1575.
W. J. Mijs, C. R. H. I. De Jonge, Editors, Organic Syntheses by Oxidation with Metal Compounds,
Plenum, 1986.
[3]
[4]
[5]
J.-E. Baeckvall, Editor, Modern Oxidation Methods, 2nd Completely Revised, Wiley-VCH Verlag
GmbH & Co. KGaA, 2010.
a) P. G. Mingalev, A. Y. Olenin, G. V. Lisichkin, Pet. Chem. 2019, 59, 1-10; b) F. Minisci, F.
Recupero, M. Rodino, M. Sala, A. Schneider, Org. Process Res. Dev. 2003, 7, 794-798.
a) G. Strukul, A. Scarso, in Liquid Phase Oxidation Via Heterogeneous Catalysis: Organic Synthesis
and Industrial Applications (Eds.: M. G. Clerici, O. A. Kholdeeva), 2013, pp. 1-20; b) J. Piera, J. E.
Bäckvall, Angew. Chem. Int. Ed. 2008, 47, 3506-3523.
[6]
a) B. Das, M. J. Baruah, M. Sharma, B. Sarma, G. V. Karunakar, L. Satyanarayana, S. Roy, P. K.
Bhattacharyya, K. K. Borah, K. K. Bania, Appl. Catal. A: Gen. 2020, 589, 117292; b) E. Farnetti, C.
Crotti, E. Zangrando, Inorg. Chim. Acta 2020, 502, 119318; c) A. Guðmundsson, K. E. Schlipköter,
J.-E. Bäckvall, Angew. Chem. Int. Ed. 2020, 59, 5403-5406; d) J. E. Chàvez, C. Crotti, E. Zangrando,
E. Farnetti, J. Mol. Catal. A: Chem. 2016, 421, 189-195.
18
This article is protected by copyright. All rights reserved.

10.1002/ejoc.202001201
European Journal of Organic Chemistry
[7]
a) A. F. Vailati, R. D. Huelsmann, E. Martendal, A. J. Bortoluzzi, F. R. Xavier, R. A. Peralta, New J.
Chem. 2020, 44, 2514-2526; b) Y. Zhao, C. Yu, S. Wu, W. Zhang, W. Xue, Z. Zeng, Catal. Lett. 2018,
148, 3082-3092; c) Q. Yan, Y. C. Fang, Y. X. Jia, X. H. Duan, New J. Chem. 2017, 41, 2372-2377; d)
I. Guerrero, Z. Kelemen, C. Viñas, I. Romero, F. Teixidor, Chem. Eur. J. 2020, 26, 5027-5036.
E. Prathibha, R. Rangasamy, A. Sridhar, K. Lakshmi, ChemistrySelect 2020, 5, 988-993.
C.-W. Ding, W. Luo, J.-Y. Zhou, X.-J. Ma, G.-H. Chen, X.-P. Zhou, D. Li, ACS Appl. Mater. Interf.
2019, 11, 45621-45628.
[8]
[9]
[10]
[11]
[12]
[13]
L. Jieun, L. Jong Chan, Lett. Org. Chem. 2018, 15, 895-898.
A. Furstner, ACS Cent. Sci. 2016, 2, 778-789.
I. Bauer, H.-J. Knölker, Chem. Rev. 2015, 115, 3170-3387.
a) R. J. M. K. Gebbink, M. E. Moret, Non-Noble Metal Catalysis: Molecular Approaches and
Reactions, Wiley, 2018; b) S. M. Hoelzl, P. J. Altmann, J. W. Kueck, F. E. Kuehn, Coord. Chem. Rev.
2017, 352, 517-536; c) I. Gamba, Z. Codola, J. Lloret-Fillol, M. Costas, Coord. Chem. Rev. 2017, 334,
2-24; d) K. Gopalaiah, Chem. Rev. 2013, 113, 3248-3296; e) T. Wai-Shan, G. Q. Chen, Y. Liu, C. Y.
Zhou, C. M. Che, Pure Appl. Chem. 2012, 84, 1685-1704; f) E. P. Talsi, K. P. Bryliakov, Coord.
Chem. Rev. 2012, 256, 1418-1434; g) M. Costas, Coord. Chem. Rev. 2011, 255, 2912-2932; h) S.
Enthaler, K. Junge, M. Beller, Angew. Chem. Int. Ed. 2008, 47, 3317-3321; i) A. Correa, O. Garcia
Mancheno, C. Bolm, Chem. Soc. Rev. 2008, 37, 1108-1117; l) W. Nam, Acc. Chem. Res. 2007, 40,
522-531; n) E. Rose, B. Andrioletti, S. Zrig, M. Quelquejeu-Etheve, Chem. Soc. Rev. 2005, 34, 573-
583.
[14]
[15]
a) M. Guo, T. Corona, K. Ray, W. Nam, ACS Cent. Sci. 2019, 5, 13-28; b) M. L. Neidig, E. I. Solomon,
Chem. Commun. 2005, 5843-5863.
a) L. Vicens, M. Costas, Dalton Trans. 2018, 47, 1755-1763; b) O. Cusso, M. W. Giuliano, X. Ribas,
S. J. Miller, M. Costas, Chem. Sci. 2017, 8, 3660-3667; c) G. Olivo, O. Cusso, M. Borrell, M. Costas,
J. Biol. Inorg. Chem. 2017, 22, 425-452; d) G. Olivo, O. Cusso, M. Costas, Chem. Asian J. 2016, 11,
3148-3158; e) W. Nam, Y.-M. Lee, S. Fukuzumi, Acc. Chem. Res. 2014, 47, 1146-1154; f) J. Yoon,
S. A. Wilson, Y. K. Jang, M. S. Seo, K. Nehru, B. Hedman, K. O. Hodgson, E. Bill, E. I. Solomon,
W. Nam, Angew. Chem. Int. Ed. 2009, 48, 1257-1260; g) M. Costas, M. P. Mehn, M. P. Jensen, L.
Que, Jr., Chem Rev 2004, 104, 939-986.
[16]
S. H. Bae, X.-X. Li, M. S. Seo, Y.-M. Lee, S. Fukuzumi, W. Nam, J. Am. Chem. Soc. 2019, 141, 7675-
7679.
[17]
[18]
W. Nam, Acc. Chem. Res. 2015, 48, 2415-2423.
a) K. P. Bryliakov, E. P. Talsi, Coord. Chem. Rev. 2014, 276, 73-96; b) C.-L. Sun, B.-J. Li, Z.-J. Shi,
Chem. Rev. 2011, 111, 1293-1314.
[19]
a) J. Serrano-Plana, A. Aguinaco, R. Belda, E. García-España, M. G. Basallote, A. Company, M.
Costas, Angew. Chem. Int. Ed. 2016, 55, 6310-6314; b) J. Serrano-Plana, W. N. Oloo, L. Acosta-
Rueda, K. K. Meier, B. Verdejo, E. García-España, M. G. Basallote, E. Münck, L. Que, A. Company,
M. Costas, J. Am. Chem. Soc. 2015, 137, 15833-15842.
[20]
J. Serrano-Plana, F. Acuna-Pares, V. Dantignana, W. N. Oloo, E. Castillo, A. Draksharapu, C. J.
Whiteoak, V. Martin-Diaconescu, M. G. Basallote, J. M. Luis, L. Que, Jr., M. Costas, A. Company,
Chem. Eur. J. 2018, 24, 5331-5340.
[21]
[22]
G. Tseberlidis, D. Intrieri, A. Caselli, Eur. J. Inorg. Chem. 2017, 2017, 3589-3603.
a) G. Tseberlidis, A. Caselli, R. Vicente, J. Organomet. Chem. 2017, 835, 1-5; b) B. Castano, E. Gallo,
D. J. Cole-Hamilton, V. Dal Santo, R. Psaro, A. Caselli, Green Chem. 2014, 16, 3202-3209; c) B.
Castano, S. Guidone, E. Gallo, F. Ragaini, N. Casati, P. Macchi, M. Sisti, A. Caselli, Dalton Trans.
2013, 42, 2451-2462; d) B. Castano, P. Zardi, Y. C. Honemann, A. Galarneau, E. Gallo, R. Psaro, A.
Caselli, V. Dal Santo, RSC Adv. 2013, 3, 22199-22205; e) B. Castano, T. Pedrazzini, M. Sisti, E. Gallo,
F. Ragaini, N. Casati, A. Caselli, Appl. Organomet. Chem. 2011, 25, 824-829; f) A. Caselli, F. Cesana,
E. Gallo, N. Casati, P. Macchi, M. Sisti, G. Celentano, S. Cenini, Dalton Trans. 2008, 4202-4205; g)
E. Brambilla, V. Pirovano, A. Caselli, A. Colombo, C. Dragonetti, M. Giannangeli, E. Rossi,
ChemCatChem 2020, doi: 10.1002/cctc.202000887.
[23]
a) D. Garanzini, V. Pirovano, I. Menghi, G. Celentano, S. Rizzato, E. Rossi, A. Caselli, G. Abbiati,
Eur. J. Org. Chem. 2020, 2020, 3660-3670; b) V. Pirovano, G. Hamdan, D. Garanzini, E. Brambilla,
E. Rossi, A. Caselli, G. Abbiati, Eur. J. Org. Chem. 2020, 2020, 2592-2599; c) G. Tseberlidis, M.
Dell'Acqua, D. Valcarenghi, E. Gallo, E. Rossi, G. Abbiati, A. Caselli, RSC Adv. 2016, 6, 97404-
97419; d) T. Pedrazzini, P. Pirovano, M. Dell'Acqua, F. Ragaini, P. Illiano, P. Macchi, G. Abbiati, A.
19
This article is protected by copyright. All rights reserved.

10.1002/ejoc.202001201
European Journal of Organic Chemistry
Caselli, Eur. J. Inorg. Chem. 2015, 2015, 5089-5098; e) M. Dell’Acqua, B. Castano, C. Cecchini, T.
Pedrazzini, V. Pirovano, E. Rossi, A. Caselli, G. Abbiati, J. Org. Chem. 2014, 79, 3494-3505; f) M.
Trose, M. Dell’Acqua, T. Pedrazzini, V. Pirovano, E. Gallo, E. Rossi, A. Caselli, G. Abbiati, J. Org.
Chem. 2014, 79, 7311-7320.
[24]
[25]
[26]
G. Tseberlidis, L. Demonti, V. Pirovano, M. Scavini, S. Cappelli, S. Rizzato, R. Vicente, A. Caselli,
ChemCatChem 2019, 11, 4907-4915.
The complex 4b could not be characterised by this method due to its highly hygroscopic behavior,
which made impossible to weight and fill the sample tube.
F. Della Monica, A. Buonerba, V. Paradiso, S. Milione, A. Grassi, C. Capacchione, Adv. Synth. Catal.
2019, 361, 283-288.
[27]
[28]
M. D. Timken, D. N. Hendrickson, E. Sinn, Inorg. Chem. 1985, 24, 3947-3955.
a) S. M. Brewer, P. M. Palacios, H. M. Johnston, B. S. Pierce, K. N. Green, Inorg. Chim. Acta 2018,
478, 139-147; b) S. M. Brewer, K. R. Wilson, D. G. Jones, E. W. Reinheimer, S. J. Archibald, T. J.
Prior, M. A. Ayala, A. L. Foster, T. J. Hubin, K. N. Green, Inorg. Chem. 2018, 57, 8890-8902.
a) N. W. Alcock, D. H. Busch, C. Y. Liu, CSD Commun., 2007, CCDC 639154; b) N. W. Alcock, D.
H. Busch, C. Y. Liu, CSD Commun., 2007, CCDC 639153.
A. Shaabani, R. Mohammadian, H. Farhid, M. Karimi Alavijeh, M. M. Amini, Catal. Lett. 2019, 149,
1237-1249.
Excitation Sculpting technique was used to suppress the signal of the non-deuterated solvent: T. L.
Hwang, A. J. Shaka, J. Magn. Reson., Ser. A 1995, 112, 275-279.
[29]
[30]
[31]
[32]
[33]
[34]
C. Walling, Acc. Chem. Res. 1998, 31, 155-157.
S. E. Martın, A. Garrone, Tetrahedron Lett. 2003, 44, 549-552.
́
P.-A. Payard, Y.-T. Zheng, W.-J. Zhou, L. Khrouz, L. Bonneviot, R. Wischert, L. Grimaud, M. Pera-
Titus, Eur. J. Org. Chem. 2020, 2020, 3552-3559.
[35]
[36]
[37]
W. Ye, R. J. Staples, E. V. Rybak-Akimova, J. Inorg. Biochem. 2012, 115, 1-12.
G. Roelfes, M. Lubben, R. Hage, J. L. Que, B. L. Feringa, Chem. Eur. J. 2000, 6, 2152-2159.
Y. Wang, P. Prinsen, F. Mangin, A. Yepez, A. Pineda, E. Rodríguez-Castellón, M. R. Hasan Shah
Gilani, G. Xu, C. Len, R. Luque, Mol. Catal. 2019, 474, 110409.
N. N. H. M. Eisink, A. J. Minnaard, M. D. Witte, Synthesis 2017, 49, 822-829.
J. B. Gordon, A. C. Vilbert, I. M. DiMucci, S. N. MacMillan, K. M. Lancaster, P. Moënne-Loccoz, D.
P. Goldberg, J. Am. Chem. Soc. 2019, 141, 17533-17547.
[38]
[39]
[40]
N. Y. Oh, Y. Suh, M. J. Park, M. S. Seo, J. Kim, W. Nam, Angew. Chem. Int. Ed. 2005, 44, 4235-
4239.
[41]
[42]
[43]
[44]
[45]
M. Ghosh, Y. L. K. Nikhil, B. B. Dhar, S. Sen Gupta, Inorg. Chem. 2015, 54, 11792-11798.
O. Pestovsky, A. Bakac, J. Am. Chem. Soc. 2004, 126, 13757-13764.
Bruker AXS Inc., 2007.
Bruker AXS Inc., 2001.
A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, M. C. Burla, M. Polidori, G. & Camalli, J.
Appl. Crystallogr. 1994, 27, 435.
[46]
[47]
G. M. Sheldrick, Acta Crystallogr. Sect. C Cryst. Struct. Commun. Found. Crystallogr. 2015, 71, 3–
8.
L. J. Farrugia, J. Appl. Crystallogr. 1999, 32, 837-838.
20
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