Sulphur-sulphur, sulphur-selenium, selenium-selenium and selenium-carbon bond activation using Fe3(CO)12: An unexpected formation of an Fe2(CO)6 complex containing a μ2,κ3-C,O,Se-ligand

Article abstract of DOI:10.1039/c9nj02642h

Three diiron hexacarbonyl complexes containing dithiolato (5), diselenolato (6), and selenolato-thiolato ligands (7), respectively, have been prepared as [FeFe]-hydrogenase mimics. Treatment of Fe₃(CO)₁₂ with one equivalent of the corresponding 5-membered heterocycles 1, 3 and 4 in toluene at reflux afforded the corresponding complexes 5-7. The reaction of 5,5-bis(bromomethyl)-2,2-dimethyl-1,3-dioxane with in situ generated Na₂Se₂ results in the formation of 8,8-dimethyl-7,9-dioxa-2,3-diselenaspiro[4.5]decane (1) and traces of 7,7-dimethyl-6,8-dioxa-2-selenaspiro[3.4]nonane (2). Alternatively, 5,5-bis(bromomethyl)-2,2-dimethyl-1,3-dioxane reacts with in situ generated Na₂Se yielding compound 2 in 26% yield. When Fe₃(CO)₁₂ reacts under reflux with the selenaspiro compound 2 in toluene, the unique diiron complex, [Fe₂(CO)₆{μ²,κ³-Se,C,O-SeCH₂C₇H₁₂O₂}] (8), is obtained as a result of an initial selenium-carbon bond activation. Compounds 5, 6, 7, and 8 were characterized by IR, ¹H, ¹³C{¹H}, and ⁷⁷Se{¹H} NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray single-crystal structure analysis. The chiral complex 8 shows a coordination of the O atom at the dioxane ring to one Fe atom and the O-CH^- carbanionic group to the other Fe atom. Furthermore, we investigated the redox properties and the catalytic behaviour of complexes 5-8 in the presence of AcOH as a source of protons. The reduction of complexes 5-7 is accompanied by a chemical process resulting in an overall two-electron transfer at their primary reduction wave. This observation is consistent with an ECE reduction (E = electrochemical process, C = chemical process), while each reduction event in the case of complex 8 involves simple transfer of one electron. Moreover, high level DFT calculations were performed on neutral 8 and its reduction products 8^- and 8^2-.

Full text of DOI:10.1039/c9nj02642h

View Article Online
View Journal
NJC
New Journal of Chemistry
Accepted Manuscript
A journal for new directions in chemistry
This article can be cited before page numbers have been issued, to do this please use: R. Trautwein, H.
Abul-Futouh, H. Görls, W. Imhof, L. Almazahreh and W. Weigand, New J. Chem., 2019, DOI:
10.1039/C9NJ02642H.
Volume 42
This is an Accepted Manuscript, which has been through the
Number
6
21 March 2018
Pages 3963-4776
Royal Society of Chemistry peer review process and has been
accepted for publication.
NJC
New Journal of Chemistry
rsc.li/njc
A journal for new directions in chemistry
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 1144-2546
PAPER
Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
prepared via a microwave-assisted hydrothermal route using
H2O2 and KMnO4 as oxidizing agents
rsc.li/njc

Page 1 of 35
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
7
8
Sulphur-Sulphur, Sulphur-Selenium, Selenium-Selenium and Selenium-
Carbon Bond Activation Using Fe3(CO)12: An Unexpected Formation of a
Fe2(CO)6 Complex Containing a μ2,κ3-C,O,Se-Ligand
9
Ralf Trautwein,a Hassan Abul-Futouh,b Helmar Görls,a Wolfgang Imhof,c* Laith R.
Almazahrehd*, Wolfgang Weiganda*
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
a
Institut für Anorganische und Analytische Chemie, Friedrich-Schiller-Universität Jena,
Humboldt Str. 8, 07743 Jena, Germany. E-Mail: wolfgang.weigand@uni-jena.de
b
Department of Pharmacy, Al-Zaytoonah University of Jordan, P.O. Box 130, Amman 11733,
Jordan
c Institut für Integrierte Naturwissenschaften, Universität Koblenz-Landau, Universitätsstr. 1, D-
56070 Koblenz, Germany
d
ERCOSPLAN Ingenieurbüro Anlagentechnik GmbH, Arnstädter Straße 28, 99096 Erfurt,
Germany. (The current address), E-mail: laithmazahreh81@gmail.com
Abstract
Three diiron hexacarbonyl complexes containing dithiolato (5), diselenolato (6), and selenolato-
thiolato ligands (7), respectively have been prepared as [FeFe]-hydrogenase mimics. Treatment
of Fe3(CO)12 with one equivalent of the corresponding 5-membered heterocycles 1, 3 and 4 in
toluene at reflux afforded the corresponding complexes 5-7. The reaction of 5,5-
bis(bromomethyl)-2,2-dimethyl-1,3-dioxane with an in situ generated Na2Se2 results in the
formation of 8,8-dimethyl-7,9-dioxa-2,3-diselenaspiro[4.5]decane (1) and traces of 7,7-dimethyl-
6,8-dioxa-2-selenaspiro[3.4]nonane (2). Alternatively, 5,5-bis(bromomethyl)-2,2-dimethyl-1,3-
dioxane reacts with an in situ generated Na2Se yielding compound 2 in 26% yield. When
Fe3(CO)12 reacts under reflux with the selenaspiro compound 2 in toluene, the unique diiron
complex [Fe2(CO)6{(μ2,κ3-Se,C,O-SeCH2C7H12O2}] (8) is obtained as a result of an initial

New Journal of Chemistry
Page 2 of 35
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
7
8
9
1
selenium-carbon bond activation. Compounds 5, 6, 7, and 8 were characterized by IR, H,
13C{1H}, and 77Se{1H} NMR spectroscopy, mass spectrometry, elemental analysis, and X-ray
single-crystal structure analysis. The chiral complex 8 shows a coordination of the O atom at the
dioxane ring to one Fe atom and the O-CH- carbanionic group to the other Fe atom. Furthermore,
we investigated the redox properties and the catalytic behaviour of complexes 5-8 in the presence
of AcOH as a source of protons. The reduction of complexes 5-7 is accompanied by a chemical
process resulting in an overall two-electron transfer at their primary reduction wave. This
observation is consistent with an ECE reduction (E = electrochemical process, C = chemical
process), while each reduction event in the case of complex 8 involves simple transfer of one
electron. Moreover, high level DFT calculations were performed on neutral 8 and its reduction
products 8- and 82-.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60

Page 3 of 35
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
Introduction
5
6
7
8
9
The global demand for energy is of continuous increase in step with economic development and
population growth while the limited quantities of fossil fuel have begun to diminish. Since
several years an ongoing challenge for scientists and engineers has been the development of
technologies for power generation from renewable resources. Hydrogen, which can be produced
via water electrolysis, is an eco-friendly energy carrier owing to its clean cold combustion in fuel
cells.1-4a Moreover, hydrogen is an important starting material for the synthesis of fertilizers
(e.g., potassium nitrate or ammonium nitrate production), where ammonia is produced via the
Haber-Bosch process.4b Therefore, many studies have entirely devoted to development of an
economically feasible production of hydrogen via proton reduction, which requires an
electrocatalyst as an alternative to the platinum.5 In fact, the process of proton reduction into
hydrogen occurs in nature through enzymes known as [FeFe]-hydrogenases, which efficiently
catalyse this process (ca. 104 turnover per s).4,6 Crystallographic and spectroscopic studies have
shown that the active site of these enzymes contains an organometallic cluster, the H-cluster, that
consists of two Fe atoms making Fe-Fe bond, dithiolato bridge between these Fe atoms forming
a butterfly structure of the [2Fe2S] core, two unusual types of ligands (CO, CN-) in the
coordination sphere of each Fe atom and a [4Fe4S] cluster bonded to one Fe atom (Figure 1,
left).7 The bridgehead of the dithiolato linker has been recently proven to be a NH group.8 The
activity of the H-cluster in catalyzing the reduction of protons is attributed to three functional
moieties: (i) a redox functionality ([4Fe4S] cluster), (ii) a pendant base in the dithiolate bridge,
playing the role of proton relay to the iron core and (iii) a vacant site at the Fe atom located
under the amine group where the H+/H2 conversion occurs as shown in Figure 1 (left).9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60

New Journal of Chemistry
Page 4 of 35
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 1. The active site of the [FeFe]-Hydrogenase (H-cluster) (left) and the model complexes
used in this study (right).
With these functional moieties in mind, a remarkable number of diiron dithiolato complexes
mimicking the butterfly [2Fe2S] core of the H-cluster has been synthesized and tested as
electrocatalysts.5,10 Moreover, modifications have been performed by introducing the heavier
selenium or tellurium atoms instead of sulfur.11 Nonetheless, these mimics catalyse the reduction
of protons in non-aqueous solvents with low turnover numbers and large overpotentials
compared to the H-cluster.5a
Although the reversible H+/H2 conversion is apparently a simple reaction, the mechanism of
proton reduction catalysed by the H-cluster mimics may follow different pathways. The catalytic
pathway may be initiated with either protonation or reduction depending on the structural
features of the catalyst or the strength of the acid used as a proton source.5a,12,13 These structural
features include the basicity of the diiron units, the nature of the dithiolato ligand and the
presence of an internal basic site. Indeed, studying the protonation properties as well as the redox
features of any complex is essential for obtaining a clear picture of its mechanistic pathway for
H2 production from protons.
One focus of our group is an investigation of the impact of the dithiolato ligand on the physical,
redox and electrocatalytical properties of the diiron core of the [FeFe]-hydrogenase H-cluster
models. Moreover, we have incorporated selenium atoms into artificial mimics of the H-cluster
since the presence of selenium enhances the catalytic activity of the enzymes.14,15 Furthermore,

Page 5 of 35
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
H-cluster model complexes featuring [2Fe2Se] cluster reveal less negative reduction potentials
compared with those featuring [2Fe2S] core.16,17
7
8
9
Herein, we report syntheses of H-cluster model complexes 5, 618 and 7 (Figure 1) obtained by
activation of Se-Se, Se-S and S-S bonds in dioxane-containing molecules 1, 3 and 419,
respectively. Moreover, we describe the reaction of 7,7-dimethyl-6,8-dioxa-2-
selenaspiro[3.5]nonane (2) with Fe3(CO)12 that is initiated with Se-C bond activation affording
an unexpected formation of complex 8 (Figure 1). In this report, we compare the molecular
structures of these complexes and illustrate their structure-property relationship in terms of redox
reactions as well as electrocatalysis of H2 production using acetic acid, AcOH, as a proton
source.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Results and discussion
Synthesis and characterization of 8,8-dimethyl-7,9-dioxa-2,3-diselenaspiro[4.5]decane (1)
and 7,7-dimethyl-6,8-dioxa-2-selenaspiro[3.4]nonane (2)
The reaction of 5,5-bis(bromomethyl)-2,2-dimethyl-1,3-dioxane with an in situ generated Na2Se2
results in the formation of 8,8-dimethyl-7,9-dioxa-2,3-diselenaspiro[4.5]decane (1) and traces of
7,7-dimethyl-6,8-dioxa-2-selenaspiro[3.4]nonane (2) as shown in Scheme 1. Alternatively, 5,5-
bis(bromomethyl)-2,2-dimethyl-1,3-dioxane reacts with an in situ generated Na2Se leading to
generation of compound 2 in 26% yield (Scheme 1). Compounds 1 and 2, have been
1
1
characterized by H, 13C{1H} and H77Se-HMBC NMR spectroscopic techniques, mass
spectrometry and elemental analysis as well as X-ray crystallography for compound 2.

New Journal of Chemistry
Page 6 of 35
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
Me Me
Me Me
O
O
O
O
Me Me
+
7
O
O
8
9
Se Se
Se
Na2Se2
Na2Se
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2:3
1:2
(29%)
(traces)
1
2
Br Br
EtOH
reflux 1.5h
NaBH4 + Se
Me Me
O
O
Se
(26%)
2
Scheme 1. Synthesis of 8,8-dimethyl-7,9-dioxa-2,3-diselenaspiro[4.5]decane (1) and 7,7-
dimethyl-6,8-dioxa-2-selenaspiro[3.4]nonane (2).
1
The H NMR spectrum of compound 1 exhibits singlets at 3.81, 3.23 and 1.41 ppm for the
hydrogen atoms in CH2O, CH2Se and CH3 moieties, respectively. The singlets due to the
hydrogen atoms of CH2O (3.80 ppm), CH2Se (2.84 ppm) and CH3 (1.35 ppm) moieties in
compound 2 are slightly shifted upfield in comparison to their corresponding hydrogen atoms in
compound 1. The CH2Se singlets in compounds 1 and 2 show 77Se-satellites with coupling
constant 2JH-Se = 15.43 Hz (for compound 1) and 8.70 Hz (for compound 2).
The C{1H} NMR spectrum of compound 1 displays singlets for the carbon atoms in OCCH3
13
(98.77 ppm), CCH2O (67.10 ppm), CCH2O (51.71 ppm), CH2Se (36.17 ppm) and CH3 (23.53
ppm) moieties that are slightly shifted downfield with respect to their corresponding carbon
atoms in compound 2; OCCH3 (97.89 ppm), CCH2O (67.06 ppm), CCH2O (42.22 ppm), CH3
(23.50 ppm) and CH2Se (17.10 ppm). The CH2Se singlets in compounds 1 and 2 show 77Se-
1
satellites with coupling constant JC-Se = 68.44 Hz (for compound 1) and 12 Hz (for compound
2). In the 1H77Se HMBC NMR spectrum of compound 1, a singlet is observed at 276.40 ppm that
is significantly shifted downfield compared to that detected in the spectrum of compound 2
(144.48 ppm). While attempts to crystallize compound 1 has led only to a structural motif (not
shown), suitable crystals for molecular structure determination has been obtained for compound
2 by sublimation method (Figure S1 in the Supporting Information).

Page 7 of 35
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
Synthesis and characterization of 8,8-dimethyl-7,9-dioxa-2-selena-3-thiaspiro[4.5]decane
(3)
7
8
9
8,8-Dimethyl-7,9-dioxa-2-selena-3-thiaspiro[4.5]decane (3) has been synthesized following a
standard multistep procedure described previously (Scheme 2).20-23 The reaction between 5,5-
bis(bromomethyl)-2,2-dimethyl-1,3-dioxane and potassium thioacetate (KSAc) in equimolar
amounts results in formation of 5-bromomethyl-5-acetylthiomethyl-2,2-dimethyl-1,3-dioxane
(A).23 Subsequently, the resulting product A is reacted with potassium selenocyanate to afford 5-
selenocyanatomethyl-5-acetylthiomethyl-2,2-dimethyl-1,3-dioxane (B).24 In the final step,
compound B is reacted with NaBH4 followed by oxidative ring closure using elemental iodine to
give compound 3 in 51% yield. Compounds A, B and 3 have been characterized by 1H, 13C{1H}
and 1H77Se HMBC NMR spectroscopic techniques, mass spectrometry and elemental analysis.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Me Me
KSAc
K+
Me Me
Me Me
Me Me
S
O
O
O
O
O
O
O
O
O-
NaBH4
I2
KSeCN
Acetone
THF
Br Br
SAc Br
SAc SeCN
S
Se
A
B
3
(26%)
(38%)
(51%)
Scheme 2. Synthesis of 8,8-dimethyl-7,9-dioxa-2-selena-3-thiaspiro[4.5]decane (3).
The synthetic procedures and characterizations of A and B are described in the Supporting
Information. The hydrogen atoms of the CH2O moiety in compound 3 resonate as multiplet in
1
the range 3.75-3.82 ppm in the H NMR spectrum whereas singlets were detected for the
hydrogen atoms of CH2Se and CH2S at 3.15 ppm and 3.14 ppm, respectively. The signal due to
2
1
the CH2Se shows 77Se-satellites with coupling constant JH-Se = 15.14 Hz. Moreover, the H
NMR spectrum confirms the chemical nonequivalency of the two CH3 groups in compound 3
13
where two singlets are detected at 1.42 ppm and 1.40 ppm. In the C{1H} NMR spectrum of
compound 3, the carbon atoms of OCCH3, CCH2O and CCH2O resonate as singlets at 98.62
ppm, 66.12 ppm and 51.08 ppm, respectively. Further singlets are detected for the CH2S and
CH2Se moieties at 45.67 ppm and 34.96 ppm, respectively. The singlet observed at 34.96 ppm
77
1
features Se-satellites with coupling constant JC-Se = 66.77 Hz. The carbon atoms of the two

New Journal of Chemistry
Page 8 of 35
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
1
nonequivalent CH3 groups result in two singlets at 24.44 ppm and 22.72 ppm. The H77Se
HMBC NMR spectrum of compound 3 displays a singlet at 410.13 ppm owing to the CH2Se
moiety.
7
8
9
Synthesis of the diiron complexes 5-8.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Treatment of Fe3(CO)12 with 8,8-dimethyl-7,9-dioxa-2,3-dithiaspiro[4.5]decane, 419, or
compound 1 and 3 in toluene under reflux affords the diiron complexes [Fe2(CO)6{μ-
(S(CH2)2S)C(C5H10O2)}] (5), [Fe2(CO)6{μ-(Se(CH2)2Se)C(C5H10O2)}] (6) and [Fe2(CO)6{μ-
(Se(CH2)2S)C(C5H10O2)}] (7), respectively (Scheme 3). Complex 6 was previously synthesized
by reacting Fe3(CO)12 with 2,2-dimethyl-5,5-bis(selenocyanatomethyl)-1,3-dioxane.18 When
Fe3(CO)12 is refluxed with compound 2 in toluene, a unique diiron product 8 is obtained. In the
dashed box in Scheme 4, a mechanism is proposed for the formation of the chiral complex 8. As
shown in Scheme 4, formation of complex 8 is initiated by activation of the Se-C bond in 2
through oxidative addition to Fe(0) carbonyl species. Indeed, examples for the activation of C-X
(X = S or Se)25-27 by iron carbonyl complexes has been reported by us and others. This oxidative
addition step would afford an intermediate in which the Se atom bridges the two Fe atoms and
the CH2 group is involved in a 3c-2e bond; Fe-CH2-Fe. One O atom in the dioxane ring
coordinates then to one Fe atom, leading to localization of the two bonding electrons of the Fe-
CH2-Fe core between the other Fe atom and the CH2 carbon atom. Subsequently, scission of the
Fe-CH2 bonding takes place that is successively followed by a transfer of proton from the α-
-
carbon of the dioxane ring to the carbanionic C-CH2 group. In the final step, complex 8 is
formed upon coordination of the O-CH- carbanionic group with the Fe atom. Indeed, an
alternative mechanism (Scheme S1) might involve homolytic Fe-CH2 bond scission leading to a
radical intermediate in which the hydrogen atom of the O-CH2 group is transferred to the radical
•
CH2 substituent. Subsequently, cyclization takes place through formation of Fe-CH bond to
afford complex 8.
Complexes 5-8 have been characterized by 1H, 13C{1H} and 1H77Se HMBC NMR spectroscopic
techniques, mass spectrometry and elemental analysis as well as X-ray crystallography.

Page 9 of 35
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
Me Me
O
O
Me Me
5
6
7
S
(57%)
(54%)
(59%)
X
X
X
X'
X'
=
=
=
=
6
7
8
9
O
O
Fe3(CO)12
Se
X
X'
Toluene
reflux 2h
OC
OC
X'
= S;
= Se
CO
CO
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
X
X'
Fe Fe
1 3
, , and
4
CO CO
5-7
Scheme 3. Synthetic pathway of obtaining complexes 5-7.

New Journal of Chemistry
Page 10 of 35
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
H
Me
O
Me
O
H
O
Me
C
H
6
7
8
9
H
Fe3(CO)12
Me
C
O
Toluene
reflux 2h
CH2
Se
(OC)3Fe
Fe(CO)3
Se
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
8
2
(42%)
O
O
Me
Me
Me
O
Me
CH2H
CH
O
Se
(OC)3Fe
Fe(CO)3
Se
CH
(OC)3Fe
Fe(CO)3
O
O
O
O
Me
Me
Me
Me
H
CH2
CH2
H
CH2
Se
C
Se
(OC)3Fe
Fe(CO)3
(OC)3Fe
Fe(CO)3
Scheme 4. Synthesis of complex 8. A tentative mechanism for the formation of complex 8 is
shown between brackets.
In the 1H NMR spectra of complexes 5 and 6, the hydrogen atoms of CH2O resonate as singlets
at 3.53 ppm (for 5) and 3.50 ppm (for 6) whereas the spectrum of complex 7 displays a multiplet
1
(3.48-3.56 ppm) for the hydrogen atoms of CH2O. Furthermore, the H NMR spectrum of
complex 5 shows a singlet (2.23 ppm) due to the hydrogen atoms of the CH2S moiety that is
slightly shifted downfield in comparison to the singlet (2.16 ppm) of the CH2S hydrogen atoms
in the spectrum of complex 7. The hydrogen atoms of the CH2Se moiety appear in the 1H NMR
spectrum of complex 6 as singlet at 2.25 ppm. In comparison, the 1H NMR spectrum of complex
7 exhibits a singlet, with Se-satellites (2JH-Se = 17.5Hz), for the hydrogen atoms of the CH2Se
77

Page 11 of 35
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
moiety that is slightly shifted downfield, 2.31 ppm. The hydrogen atoms of the CH3 groups in
1
complexes 5-7 resonate with the same chemical shift of 1.31 ppm in the H NMR spectra. The
13C{1H} NMR spectra exhibit singlets at 207.31 ppm, 208.20 ppm and 207.75 ppm for the
7
8
9
carbonyl carbon atoms CO of complexes 5, 6 and 7, respectively. The quaternary carbon atoms
13
of the OCCH3 moiety resonate as singlets in the C{1H} NMR spectra at 98.84 ppm (for 5),
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
98.87 ppm (for 6) and 98.88 ppm (for 7). The C{1H} NMR spectra of these complexes also
13
show singlets at 68.09 ppm (for 5), 66.95 ppm (for 6) and 67.59 ppm (for 7) due to the carbon
atoms in CH2O moieties. The quaternary carbon atoms of the CCH2O moieties appear at 35.79
ppm (for 5), 35.25 ppm (for 6) and 35.61 ppm (for 7). Moreover, singlets due to the CH2S carbon
atoms in complexes 5 and 7 appear at 26.75 ppm and 28.10 ppm, respectively. In comparison to
these CH2S singlets, the CH2Se carbon atoms are shifted upfield, 17.94 ppm (for 6) and 16.91
ppm (for 7). These CH2Se singlets features 77Se-satellites with coupling constant 1JC-Se = 85.5 Hz
(for 6) and 83.1 Hz (for 7). The CH3 groups in complex 7 are chemically non-equivalent
resulting in two singlets in the 13C{1H} NMR spectrum at 24.00 ppm and 23.34 ppm. For
complex 5 or 6, one singlet is detected at 23.34 ppm or 23.33 ppm, respectively, due to their CH3
carbon atoms. The H77Se HMBC NMR spectrum of complex 6 shows a singlet at 69.73 ppm
1
due to the selenium atom of CH2Se, which is shifted downfield in comparison to that singlet
(56.01 ppm) in the spectrum of complex 7.
1
In the H NMR spectrum of complex 8, a singlet is detected at 5.30 ppm and assigned for the
1
hydrogen atom of the CHFe core. The H NMR spectrum of complex 8 also shows the splitting
pattern of an AB spin system at 3.38 ppm and 2.74 ppm for the diastereotopic methylene
hydrogen atoms of the CH2Se moiety. Moreover, the methylene hydrogen atoms in the dioxane
ring resonate also as doublets at 3.45 ppm and 3.30 ppm. The observed geminal coupling
constants in all case are comparable; 2JH-H = 11.3-11.5 Hz. In addition, the 1H NMR spectrum of
complex 8 displays two singlets at 1.25 ppm and 1.20 ppm for the diastereotopic OCCH3 methyl
groups as well as a singlet at 0.59 ppm for the methyl group of the CH2CCH3 moiety. In the
13C{1H} NMR spectrum of complex 8, the carbonyl carbon atoms resonate as singlets at 215.52
ppm, 212.19 ppm and 209.88 ppm. Furthermore, additional singlets are detected at 107.53 ppm,
102.30 ppm, 67.28 ppm and 52.91 ppm for the carbon atoms of CHFe, OCCH3, CH2O and
CCHFe moieties, respectively. The carbon atom of the CH2Se moiety resonates as a singlet at
32.80 ppm with 77Se-satellite, where 1JC-Se = 59.0 Hz. The carbon atoms of the methyl groups in

New Journal of Chemistry
Page 12 of 35
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
the OCCH3 moieties resonate as singlets at 28.72 ppm and 18.43 ppm while the methyl group in
CH2CCH3 core leads to a singlet at 21.92 ppm. The 1H77Se HMBC NMR spectrum of complex 8
shows a singlet at 31.49 ppm owing to the selenium atom of the CH2Se core.
7
8
9
The IR spectra of the complexes 5-8 exhibit absorption bands for the terminal CO ligands (Table
1). It is evident from these data that the ν(CO) wavenumbers are shifted to lower values upon
systematic replacement of the S atoms by Se ongoing from 5 through 7 to 6. This finding can be
rationalized in terms of increasing back-donation to CO due to the stronger σ-donor capability of
Se compared to S. Obviously, the electron density at the diiron core of complex 8 is the highest
among the complexes owing to the presence of the carbanionic ligand coordinated at one Fe
centre.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 1. Wavenumbers of the carbonyl ligands, ν(CO) in cm-1, in the IR spectra of complexes 5-
8 (ATR, neat solid).
Complex
ν(CO), cm-1
5
6
7
8
2073 (s), 2029 (s), 2015 (s), 1997(s), 1981 (s), 1971 (s), 1960 (bs)
2064 (s), 2021 (s), 1999 (s), 1979 (s), 1970 (s), 1948 (bs)
2069 (s), 2025 (s), 2004 (s), 1982 (s), 1972 (s), 1950 (bs)
2059 (s), 2008 (s), 1978 (s), 1956 (s), 1931 (bs)
Molecular Structures
Suitable crystals for single-crystal X-ray analysis were accessible by slow evaporation of a
saturated solution of complexes 5-8 in hexane at 7 °C. The molecular structures of complexes 5,
6 and 8 are shown in Figure 2. Indeed, the (S, Se) position in 7 is superimposed, where the (S,
Se) is disordered between the locations of the chalcogen atoms (Figure S2, supporting
information). In complexes 5-7, each iron atom of the Fe-Fe bond coordinates to three terminal
CO ligands in facial fashion as well as two chalcogen (X = S and/or Se) atoms that bridge both
iron atoms. The bicyclic [2Fe2X] structure in these complexes reveals a butterfly conformation.

Page 13 of 35
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
In a similar way, all CO ligands in complex 8 are terminally bound in a facial manner at the iron
centres. Additionally, the two iron sites are bridged by one Se atom as well as the dioxane ring
through a carbanionic and O donor sites.
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. Molecular structures (50% probability) of one isomer of complex 5 and complexes 6
and 8. Hydrogen atoms are omitted for clarity.
Table 2 display some structural features of these complexes. The Fe-Fe distance increases
slightly as the size of the bridging chalcogen atoms increases on going from 5 to 6. Replacing
one bridging Se atom in 6 by O,C-donor sites of the dioxane ring in complex 8 explains the
longer Fe-Fe distance in the latter complex compared to the former. The Fe-Se distances in
complexes 6 and 8 are comparable. In a similar way, complex 5 shows slight difference in the
Fe-S distances. The Fe-CCarbanion distance in complex 8 (2.050(2) Å) is comparable to that in
structurally related complexes.25 Interestingly, the torsion angle in the symmetrically
independent molecules 5A (17.50°) and 5B (30.27°) even though these two molecules have
similar flap angles; 141.0° (for 5A) and 141.8° (for 5B). Only complex 6 shows an almost
eclipsed conformation of its two Fe(CO)3 units as indicated by the torsion angle, Cap-Fe-Fe-Cap,
of 1.04°.

New Journal of Chemistry
Page 14 of 35
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
Table 2. Selected bond lengths [Å] and angles [°] in complexes 5, 6 and 8.
5
6
7
8
5(A)
2.4982(4)
2.2575(5)
―
5(B)
2.5034(4)
2.2579(5)
―
6
2.5432(4)
2.3789(3)
―
8
Fe-Fe [Å]
2.5623(4)
2.3631(4)
2.050(2)
9
Fe-S/Se [Å]
Fe-CCarbanion [Å]
Fe-O [Å]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
―
―
―
2.0579(14)
Cap-Fe-Fe-Cap
[°][a]
17.50
141.0
30.27
141.8
1.04
20.47
Flap angle, α
131.6
―
[°][b]
[a] Dihedral twist angle (torsion angle) of the apical carbonyl carbon atoms across the Fe-Fe bond. [b] Refers to angle between
plane made from bridgehead carbons-elbow carbons and the plane made from the elbow carbons-S/Se.
Redox Properties
Table 3 summarizes the redox potentials (referenced to ferrocenium/ferrocene couple; Fc+/Fc) of
complexes 5-8 in MeCN-[n-Bu4N][BF4] obtained by cyclic voltammetry. The cyclic
voltammetry of complexes 5-7 shows one reduction event at half-wave potentials E1/2 of -1.51, -
1.48 and -1.49 V, respectively as shown in Figure 3a-c. In comparison, two reduction events are
observed in the cyclic voltammogram of complex 8 at E1/2 of -1.13 V and -1.77 V (Figure 3d).
Table 3. Summary of the redox features of complexes 5-8 in MeCN-[n-Bu4N][BF4] (0.1 M)
measured at 0.2 V∙s-1 using glassy carbon disk (diameter = 1.6 mm). Potentials E are given in
volts (V) and referenced to Fc+/Fc.
1 [a]
1[b]
2
2 [b]
1 [c]
2 [d]
3
Complex
Ered
E1/2
Ered
−
E1/2
Eox
Eox
Eox
1
1
1
1
5
6
7
-1.54 (Epc1), -1.48 (Epa
-1.51 (Epc1), -1.46 (Epa
-1.51 (Epc1), -1.47 (Epa
)
)
)
)
-1.51
-1.48
-1.49
-1.13
−
+0.22
+0.17
+0.19
+0.03
+0.73 +0.79[e]
−
−
−
+0.65
+0.69
+0.52
−
−
−
−
1
8
-1.21 (Epc1), -1.06 (Epa
-1.81 (Epc1), -1.73 (Epa
)
-1.77
1
2
[a] Ered and Ered are the potentials for the first and the second reductions, where Epc and Epa are the cathodic and anodic scan
peak potentials. [b] E1/21 and E1/22 are the half-wave potentials for the first and second reduction events, respectively. [c] Eox is
1
the potential of a very weak oxidation peak attributed to an initial oxidation to a rotated structure as discussed previously for
complexes structurally related to those of 5-7.[5b,17] [d] Eox2 is the potential of the primary oxidation peak. The oxidation of only

Page 15 of 35
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
complex 6 (Epa = +0.65 V) is characterized by a small cathodic event at Epc = +0.45 V observed in the return sweep of the cyclic
voltammogram. [e] This oxidation event is characterized by a small cathodic event at Epc = +0.72 V observed in the return sweep
of the cyclic voltammogram. Increasing the scan rate results in an overlap of Eox2 and Eox3 of complex 5.
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3. Cyclic voltammetry of 1.0 mM a) complex 5, b) complex 6, c) complex 7 and d)
complex 8 in MeCN-[n-Bu4N][PF4] (0.1 M) solutions at different scan rates using glassy carbon
disk (d = 1.6 mm). The arrows indicate the scan direction. The potentials E are given in V and
referenced to the Fc+/Fc couple.
The oxidation of complexes 5-8 shows a very weak anodic wave Eox1 in the vicinity of +0.17 to
+0.22 V (for complexes 5-7) and at +0.03 V (for complex 8) while the primary oxidation events
2
3
occur at more positive potentials Eox and Eox as given in Table 3. The primary oxidation
2
potential Eox shows an anodic shift on going from complex 8 to complex 6 to complex 7 to

New Journal of Chemistry
Page 16 of 35
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
7
8
9
complex 5, which is consistent with the order of electron richness of the diiron core of these
2
complexes indicated by their relative ν(CO) wavenumbers (Table 1). While Eox values and the
ν(CO) wavenumbers are directly proportional to the electron density at the diiron carbonyl core,
other factors may tune the reduction potentials.5b,17 In spite of the fact that complex 8 features the
1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
highest electron richness at the diiron core, the first reduction potential of this complex E1/2 is
the least negative in Table 3. The observed trend in the reduction potentials could be related to
the number of polarizable atoms coordinated at the diiron site, such that the soft S- and Se-donor
atoms are more capable of moderating the increase in the electron density upon reduction than
the hard oxygen- or the carbanionic-donor sites in complex 8.
According to our previous photoelectron spectroscopic studies and calculations of ionization
energies of complexes structurally related to those of 5-7,5b,17 the primary oxidation event of 5, 6
or 7 at Eox2 could be attributed to an initial oxidation forming a cationic species with all-terminal
CO ligands similar to the structure of their neutral state. Also similar to these studies, the very
weak oxidation events of 5-7 that are observed in the vicinity of +0.17 to +0.22 V could arise
from an oxidation leading to a cationic species featuring one inverted Fe(CO)3 unit and a
bridging CO ligand.
An insight on the reduction mechanism at a given potential can be obtained by comparing the
value of the current function ipc / c∙v1/2 (ipc = cathodic peak current, c = concentration and v =
scan rate) of that reduction at various scan rates.5b,28-30 The current function is given by Randles-
Sevcik equation:29,30
ipc / c∙v1/2 = (2.69 × 105)·A·D1/2·n3/2
where A, D and n are the surface area of the electrode, the diffusion coefficient and the number
of electrons responsible for the reduction event, respectively. In principle, in the absence of
chemical complications the current function ipc / c∙v1/2 of a reduction peak should remain constant
at all scan rates because (2.69 × 105)·A·D1/2·n3/2 is constant. Nonetheless, a chemical process
may accompany the electron transfer in an ECE mechanism of reduction (E = electrochemical
process, C = chemical process) making the second electron reduction thermodynamically
favoured over the first electron reduction; i.e. potential inversion. This intervening chemical
process will be prevented at fast scan rates since there would be no enough time for the chemical
process to take place and hence the second electron transfer will not occur. Simply stated,

Page 17 of 35
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
7
8
9
performing the voltammetric experiment at fast scan rates prevents the intervening chemical
process and alters the redox mechanism from ECE (n = 2) to a simple E process (n = 1).
Therefore, a decrease of ipc / c∙v1/2 at fast scan rates suggests an ECE mechanism while a constant
value of ipc / c∙v1/2 at slow and fast scan rates is indicative of an uncomplicated one-electron
transfer. Whereas the current function of the reduction of complexes 5-7 decreases towards a
constant value as the scan rate increases, both reduction events of complex 8 show no
dependency of their current function on the scan rate (Figure 4). These observations are
consistent with an ECE reduction in the case of complexes 5-7 while each reduction event in the
case of complex 8 involves simple transfer of one electron. As shown in Figure 4, the scan rate
dependence of the current function of the reduction of complexes 5-7 is similar to that in the case
of [Fe2(CO)6(bdt)]31 (bdt = μ-benzenedithiolato), which is known to undergo two-electron
reduction with potential inversion. The chemical process intervening in the reduction of
complexes 5-7 might involve a core reorganization such as formation of a dianionic species
featuring semi-bridging CO ligand in a similar manner described for [Fe2(CO)6(bdt)].31
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4. Scan rate dependence of the current function of the reduction events of complexes 5-8
in comparison to that of the reduction of [Fe2(CO)6{bdt}]31 under the same conditions.
Electrocatalysis

New Journal of Chemistry
Page 18 of 35
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
The electrocatalytic proton reduction by complexes 5-8 using acetic acid (AcOH, pKaMeCN
=
22.6)32 as a source of protons is shown in Figure 5. It is evident from Figure 5 that complexes 5-
7 exhibit two potential-dependent catalytic processes in the vicinities of -1.9 to -2.1 V (for
process I) and -2.1 to -2.3 V (for process II), respectively. In comparison, only one catalytic
process is observed at -1.84 V by the cyclic voltammetry of complex 8.
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The electrocatalytic reduction of AcOH by complexes 5-7 is similar to that in the case of their
analogues complexes [Fe2(CO)6{μ-(XCH2)2CMe2}] (X = S or Se).28 In the presence of various
equivalents of AcOH, the current of the two-electron reduction waves of complexes 5-7, that is
observed in the absence of acid, remains unaffected, but the reduction peaks show small anodic
shifts of 10-30 mV. Protonation of the dianionic species of complexes 5-7 accounts for these
Figure 5. Cyclic voltammetry of 1.0 mM a) complex 5, b) complex 6, c) complex 7 and d)
complex 8 in MeCN-[n-Bu4N][PF4] (0.1 M) solutions at 0.2 V s-1 in the presence of different

Page 19 of 35
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
concentration of AcOH using glassy carbon disk (d = 1.6 mm). The arrows indicate the scan
direction. The potentials E are given in V and referenced to the Fc+/Fc couple.
7
8
9
anodic shifts.31,33 Reduction of the resulting protonated species HA- (A = 5-7) that is followed by
a protonation step would lead to H2 release from an H2A- species in process I (Scheme 5, green
arrow). As shown in Scheme 5, the catalytic process I of complexes 5-7 levels off at higher acid
concentrations while process II dominates upon a further reduction step affording H2A2- species.
In the case of complex 8, neither the current nor the potential of its first one-electron reduction is
affected by the presence of AcOH. Furthermore, the first reduction wave of complex 8 remains
reversible at all acid concentrations (Figure S3, see Supporting Information). These observations
indicate that the monoanionic species 8- is not basic enough to undergo protonation. As evident
from Figure 5d, the catalytic process is triggered upon reduction of 8- affording the more basic
species 82- that undergoes protonation to give 8H-. A second protonation step would then lead to
formation of H2 as shown in Scheme 5 (blue arrows).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
H2
process I
[H2A- ]
[A- ]
e-
e-
e-
[H2A2-
]
H+
process II
H2
[A2-
]
[A] = complexes 5-8
[A]
Blue Arrows: Complex 8
Green Arrows: Complexes 5-7
[HA2-
]
H+
H2
[HA- ]
[H2A ]
e-
H+
Scheme 5. Tentative mechanism for the proton reduction cycle catalyzed by complexes 5-8 in
the presence of AcOH.

New Journal of Chemistry
Page 20 of 35
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
DFT Investigations
5
6
7
8
Cyclovoltammetric measurements have revealed that catalytic hydrogen production occurs from
dianionic species derived from 8 upon two reduction steps. In order to get some insight into this
catalytic process high level DFT calculations were performed on neutral 8 and its reduction
products 8- and 82-. In addition, corresponding calculations have been performed for diiron
carbonyl complexes produced from 82- by the addition of one or two protons. Calculations were
performed applying the B3LYP/6-311++G(d,p) functional and basis set as it is implemented in
Gaussian09.34,35,36 Moreover, relativistic ECPs of the Stuttgart-Dresden groups were used for
iron and selenium atoms.36 Besides, a continuum solvent model for acetonitrile was applied to
the calculations since it has been shown that this is crucial to obtain results that correspond to the
experimental outcome of electrochemical investigations.37 As the addition of protons represent
bimolecular reactions, thermal and entropic corrections were considered. All energy values Ecorr
as well as the results of frequency calculations are summarized in Table S1 (Supporting
Information). Vibrational analysis for all calculated molecules show that they represent
minimum structures on the hypersurface (NImag = 0).
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60

Page 21 of 35
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 6. Calculated molecular structures of 8, 8- and 82- (upper row from left to right) as well as
the protonated species H8-_1, H8-_2 and H28 (lower row from left to right).
The molecular structure of 8 was calculated starting with the geometry of the molecule as it has
been the result of an X-ray structural analysis, which remains almost unchanged in the
theoretical investigation. 8 is a diiron hexacarbonyl compound with a bridging selenium atom
and another organic ligand coordinating via formally anionic C1 and one of the ether oxygen
atoms of the cyclic acetal. By the two latter interactions the organic ligand also adopts a position
bridging both iron atoms. The iron selenium bond toward the iron that is coordinated by the
acetal oxygen atom next to three CO ligands is about 6 pm longer than the other iron selenium
distance. If 8 is reduced, the anionic complex 8- is formed upon a single electron transfer. The
most significant structural effects caused by the enhanced number of electrons is the elongation
of the iron iron distance by more than 40 pm as well as the breakage of the bonding interaction
between Fe2 and O1. In addition, the bridging position of the selenium atom now is almost
symmetrical due to an elongation of the Fe1-Se bond. Moreover, also the interaction between C1

New Journal of Chemistry
Page 22 of 35
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
7
8
9
and Fe1 obviously is slightly weakened upon reduction of 8 since this bond distance also is
elongated. Interestingly, upon a second electron transfer the iron iron distance in the resulting
dianion 82- is not further elongated compared to 8- but is calculated to be more than 10 pm
shorter. Concurrently, the selenium bridge gets unsymmetrical again, although in contrast to the
starting compound 8 in 82- Fe2-Se is significantly shorter than Fe1-Se. On the other hand, the
bond between Fe1 and C1 is further elongated compared to the situation in 8-. There are no semi-
bridging CO ligands observed in the calculated structure of 82-. All Fe-C-O bond angles are
above 170° and the distances between CO carbon atoms and the second iron atom are well above
300 pm. Nevertheless, it is noticeable, that Fe-C-O bond angles in 82- in average are only slightly
above 170° whereas for all other compounds they are generally observed between 175° and 180°.
So the second reduction step seems to lead to a situation in which -backbonding into * orbitals
of coordinated CO ligands is quite effective leading to a slight bending of all CO ligands rather
than to another shortening of the Fe-Fe distance.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Table 4. Selected bond lengths and distances [pm] of the calculated compounds
8
8-
82-
H8-_1
277.2
241.9
241.0
212.5
-
H8-_2
270.8
245.5
245.4
211.3
-
H28
285.9
244.0
244.0
211.0
-
Fe1-Fe2
Fe1-Se
Fe2-Se
Fe1-C1
Fe2-O1
Fe1-H2
Fe2-H1
Fe2-H2
257.8
241.1
247.0
206.3
211.0
-
299.0
286.2
245.3
246.4
247.5
240.8
209.8
212.4
-
-
-
-
-
-
-
-
-
167.5
-
167.6
151.3
166.2
-
152.5
-
-
165.1

Page 23 of 35
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
7
8
9
If a proton is added to 82- the number of electrons in the protonated species is identical to the
number of electrons in 82- which means that H8-_1 and H8-_2 both are monoanions. It was one
of the goals of this theoretical investigation to see at which iron atom the first protonation will
take place. So we started the geometry optimization from three different situations: two starting
geometries with hydride ligands attached either to Fe1 or Fe2 and a situation with a bridging
hydride. By this methodology we were able to observe two minimum structures on the
hypersurface. In H8-_1 a hydride ligand coordinates to Fe2 whereas starting from a hydride at
Fe1 or with a bridging hydride in both cases ends up in the same minimum structure H8-_2 with
the hydride adopting a symmetrical bridging position. The bonding situation in H8-_2 is
stabilized relative to the one in H8-_1 by 29.6 kJ/mol. In both isomers the position of the
selenium atom is almost perfectly symmetrical although the iron selenium bonds in H8-_2 are
slightly longer than the respective bond lengths in the other isomer. In both compounds the iron
iron distance is significantly shorter than in 12- with the effect being even more pronounced in
H8-_2 with the bridging hydride ligand. The interaction between Fe1 and C1 is almost not
affected by protonation of 82-. If a second protonation occurs the resulting compound is a neutral
species. We were only able to identify one single minimum on the hypersurface because
independently of the position of the second hydrogen atom in the starting geometry, optimization
of the structure always leads to H28. This is true for the second hydrogen atom being placed as
1-hydride ligand at Fe1 or as a bridging ligand in starting geometries produced from H8-_1 or
as a 1-hydride ligand at Fe2 in starting geometries produced from H8-_2. Iron hydrogen
distances in all protonated species are almost identical for 1-hydrides on one side and for
bridging hydrides on the other side. Nevertheless, there is one significant difference in the
coordination of the bridging hydride in H8-_2 compared to H28. The former shows a typical
butterfly arrangement with an angle of 57.9° between the planes Fe1-Fe2-Se and Fe1-Fe2-H2,
whereas in the latter the corresponding angle measures to 29.15° which means that the
arrangement of these 4 atoms in H28 is closer to planarity than in H8-_2. This also leads ton
elongation of the iron iron distance which in H28 is 15 pm longer than in H8-_2. In addition, in
H28 the selenium bridge is perfectly symmetrical and Fe-Se bond lengths as well as the Fe1-C1
interaction are observed with very similar values compared to the situation in H8-_2.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60

New Journal of Chemistry
Page 24 of 35
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
Conclusion
5
6
7
8
Activation of the chalcogen-chalcogen bonds in compounds 1, 3 or 4 by Fe3(CO)12 result in the
formation of complexes 6, 7 or 5, respectively (Scheme 3). Unexpectedly, refluxing compound 2
with Fe3(CO)12 starts with an activation of the Se-C bond, but this oxidative addition is followed
by rearrangement steps leading to unexpected formation of complex 8 (Scheme 4). The electron
density at the diiron core of the complexes in this study decreases on going from 8 to 6 to 7 to 5
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2
as evident from their ν(CO) wavenumbers (Table 1) as well as the oxidation potentials Eox
(Table 3). This trend reflects the σ-donor ability of the ligands in these complexes, where the
ligand set in complex 8 inductively increases the electron richness of the diiron units compared
to the ligands in complexes 5-7. Despite this order of electron density at the diiron core of these
complexes, their reduction potentials E1/2 (Table 3) follow an opposite trend owing to the
polarizability of the donor atoms coordinated at the Fe atom. The reduction of complexes 5-7 is
accompanied by a chemical process resulting in an overall two-electron transfer at their primary
1
reduction wave, which afford their dianionic form at E1/2 shown in Table 3. Uncommonly, the
cyclic voltammetry of complex 8 reveals reversibility of two well-separated cathodic events for
8/8- and 8-/82- redox couples having E1/2 of -1.13 V and -1.77 V, respectively. The
electrocatalytic reduction of protons, using AcOH as a proton source, requires an initial
reduction of the complexes. The dianionic species of complexes 5-7 undergoes protonation in the
presence of AcOH, but further reduction is required for catalysis. In the case of complex 8,
catalysis takes place once the dianionic species 82- is formed leading to H2 formation at potential
lower than that required in the case of complexes 5-7.
Experimental
Materials and techniques.
All reactions were performed using standard Schlenk and vacuum-line techniques under an inert
gas (nitrogen). The 1H, 13C{1H}, 77Se{1H} and 1H 77Se HMBC NMR spectra were recorded with
a Bruker Avance 400 or 600 MHz spectrometer. Chemical shifts are given in parts per million
1
with references to internal SiMe4 (1H, 13C) or to Me2Se (77Se, H 77Se HMBC). The mass
spectrum was recorded with Finnigan MAT SSQ 710 instrument. The IR spectra were recorded
with a Bruker Equinox 55 spectrometer equipped with an ATR unit. Elemental analysis was

Page 25 of 35
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
7
8
9
performed with a Leco CHNS-932 apparatus. TLC was performed by using Merck TLC
aluminum sheets (Silica gel 60 F254). Solvents from Fisher Scientific and other chemicals from
Acros and Aldrich were used without further purification. All solvents were dried and distilled
prior to use according to standard methods. The compound 8,8-dimethyl-7,9-dioxa-2,3-
dithiaspiro[4.5]decane, 4, was synthesized according to the known literature methods.19
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Electrochemistry
Corrections for the iR drop were performed for all experiments. Cyclic voltammetric
measurements were conducted in three-electrode technique [glassy carbon disk (diameter
= 1.6 mm) as working electrode, Ag/Ag+ in MeCN as reference electrode, Pt wire as counter
electrode] using a Reference 600 Potentiostat (Gamry Instruments). All experiments were
performed in MeCN solutions (concentration of the complexes 1.0 mM) containing 0.1 M [n-
Bu4N][BF4] at room temperature. The solutions were purged with N2 and a stream of it was
maintained over the solutions during the measurements. The vitreous carbon disk was polished
on a felt tissue with alumina before each measurement. All potential values reported in this paper
are referenced to the potential of the ferrocenium/ferrocene (Fc+/Fc) couple.
Theoretical Calculations
Full geometry optimizations (i. e. without symmetry constraints) were carried out with the
GAUSSIAN09 program package using throughout the hybrid Hartree-Fock-DFT approach
(B3LYP/6-311++G(d,p)).34,35,36 The B3LYP functional has previously been found to be of
suitable theoretical level for the study of iron and ruthenium carbonyl complexes.38 For iron and
selenium atoms we used a relativistic ECP of the Stuttgart-Dresden group (SDD).36 Stationary
points were rigorously characterized as minima according to the number of imaginary modes by
applying a second-order derivative calculation (vibrational analysis). Zero point energy (ZPE)
corrections have been applied. Solvent effects were addressed by performing a polarizable
continuum model calculation for CH3CN using the CPCM model.37
Crystal structure determination
The intensity data for the compounds were collected on a Nonius KappaCCD diffractometer
using graphite-monochromated Mo-K radiation. Data were corrected for Lorentz and
polarization effects; absorption was considered on a semi-empirical basis using multiple-scans.39-

New Journal of Chemistry
Page 26 of 35
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
41
The structures were solved by direct methods (SHELXS)41 and refined by full-matrix least
squares techniques against Fo2 (SHELXL-97).42 The hydrogen atoms of 7 were included at
calculated positions with fixed thermal parameters. All other hydrogen atoms were located by
difference Fourier synthesis and refined isotropically. All non-disordered, non-hydrogen atoms
were refined anisotropically.42 In the crystal of complex 7, the sulfur and selenium atoms can
occupy the same lattice sites. As a result, the diffraction pattern is a superposition of four
compounds or isomers: the pure S-S complex and / or the pure Se-Se complex; as well as the S-
Se and / or Se-S complex. This is associated with a disorder. Although the disorder of S and Se
could be resolved, the s.u. of the sulfur and selenium distances are so large that these are not
discussed in this paper (e.g., Fe1-S (1) 2.211 (1) or Fe1-Se1 2.398 (15)Å). Crystallographic data
as well as structure solution and refinement details are summarized in Table S2 in the Supporting
Information. MERCURY43 was used for structure representations.
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Synthesis of 8,8-dimethyl-7,9-dioxa-2,3-diselenaspiro[4.5]decane (1)
162 mg (4.28 mmol) of NaBH4 and 473 mg (6.00 mmol) of selenium were initially introduced
into a Schlenk flask, and the mixture was cooled down to 0 °C before the addition of 20 mL
ethanol. After the initial reaction had subsided, the mixture was stirred and heated at reflux for 2
h with N2 passing into the liquid in order to dissolve the Se and eject H2Se. The resulting deep-
o
red ethanolic solution of Na2Se2 was cooled down again to 0 C before the addition of the 5,5-
bis(bromomethyl)-2,2-dimethyl-1,3-dioxane (604 mg, 2.00 mmol) dissolved in 15 mL ethanol.
The ice-bath was removed, and the mixture stirred for 24 h at room temperature. Water was
added, extracted three times with CH2Cl2, the combined organic phases were washed with water,
dried over Na2SO4 and the solvent was removed in vacuo. After purification by column
chromatography (eluent: CH2Cl2 / hexane 3: 1) it was possible to isolate a red-brown solid
which, however, contained in a small amount the corresponding compound 2. Suitable crystals
for single-crystal X-ray analysis could be obtained by slowly evaporating a solution of the
compound in hexane. Yield: 29% (173 mg, 0.58mmol). Elemental analysis for C8H14O2Se2:
calc.: C 32.02; H 4.70 found.: C 32.50; H 4.74. 1H NMR (400 MHz, CDCl3, 23°C): δ = 3.81 (s,
2
4H, CH2O), 3.23 (s, with 77Se satellite, JH-Se = 15.43 Hz, 4H, CH2Se), 1.41 (s, 6H, CH3).
13C{1H} NMR (101 MHz, CDCl3, 23°C): δ = 98.8 (s, OCCH3), 67.1 (s, CCH2O), 51.7 (s,

Page 27 of 35
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
77
1
1
CCH2O), 36.8 (s, with Se satellite, JC-Se = 68.44 Hz, CH2Se), 23.5 (s, CH3). H77Se HMBC
NMR (600 MHz, 114 MHz, CDCl3, 23°C): δ = 276.40 (s, CH2Se). DEI-MS: m/z = 302 [M]+.
7
8
Synthesis of 7,7-Dimethyl-6,8-dioxa-2-selenaspiro[3.4]nonane (2)
9
400 mg (10.57 mmol) of NaBH4 and 473 mg (5.99 mmol) of elemental selenium were initially
introduced into a Schlenk flask, and the mixture was cooled down to 0 ° C before the addition of
20 mL ethanol. After the initial reaction had subsided, the mixture was stirred and heated at
reflux for 2 h with N2 passing into the liquid in order to dissolve the Se and eject H2Se. The
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
o
resulting deep-red ethanolic solution of Na2Se2 was cooled down again to 0 C before the
addition of the 5,5-bis(bromomethyl)-2,2-dimethyl-1,3-dioxane (2.50 g, 8.28 mmol) dissolved in
20 ml of ethanol. After the reaction mixture had slowly warmed to room temperature, stirring
was continued overnight and then refluxed for a further two hours. The bulk of the ethanol was
removed in vacuo, water was added to the remaining solution and extracted three times with
CH2Cl2. After washing the combined organic phases with water, drying over Na2SO4 and
removal of the solvent in vacuo, the residue thus obtained was purified by column
chromatography (eluent: CH2Cl2 / hexane 3: 1). The desired product was obtained as a white
solid, and suitable crystals for single-crystal X-ray diffraction analysis were obtained by
sublimation under normal pressure at 60 ° C. Yield: 26% (483 mg, 2.18 mmol). Elemental
1
analysis for C8H14O2Se: calc: C 43.45; H 6.38 found: C 43.66; H 6.29. H NMR (600 MHz,
CDCl3, 23°C): δ = 3.80 (s, 4H, CH2O), 2.84 (s, with 77Se satellite, 2JH-Se = 8.70 Hz, 4H, CH2Se),
1.5 (s, 6H, CH3). 13C{1H} NMR (151 MHz, CDCl3, 25°C): δ = 97.9 (s, OCCH3), 69.1 (s, OCH2),
1
1
42,2 (s, CCH2O), 23.5 (s, CH3), 17.1 (s, with 77Se satellite JC-Se = 12 Hz, CH2Se). H77Se
HMBC NMR (600 MHz, 114 MHz, CDCl3, 24°C): δ = 144.48 (s, CH2Se). DEI-MS: m/z = 207
[M-CH3]+.
Synthesis of 8,8-Dimethyl-7,9-dioxa-2-selena-3-thiaspiro[4.5]decane (3)
In a Schlenk falsk, 75 mg (0.23 mmol) of 5-selenocyanatomethyl-5-acetylthiomethyl-2,2-
dimethyl-1,3-dioxane which was prepared following the same procedure reported for similar
compounds23 were placed under nitrogen atmosphere and dissolved in 10 ml of ethanol. After
addition of 86 mg (2.27 mmol) NaBH4, the reaction mixture was stirred overnight at room
temperature and washed with 20 mL water, 3 mL Glacial acetic acid and 20 mL CHCl3 was
added. The above solution together with a solution of I2 dissolved in CHCl3 were added dropwise

New Journal of Chemistry
Page 28 of 35
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
7
8
9
and simultaneously to a solution made of 40 mL of CHCl3 and 10 mL of triethylamine under a
nitrogen atmosphere. Upon complete addition, the reaction mixture was washed with water,
dried with Na2SO4 and the solvent removed in vacuo. The resulting residue was purified by
column chromatography (eluent: hexane / CH2Cl2 4:1) to give an orange solid. Yield: 51% (30
mg, 0.12 mmol). Elemental analysis for C5H10O2SSe: Calc.: C 28.17; H 4.73; S 15.04. found: C
28.41; H 4.81; S 15.22. 1H NMR (400 MHz, acetone-d6, 24°C): δ = 4.09 (t, 2H, 3JH-H = 5.36 Hz,
OH), 3.61-3.71 (m, 4H, CH2OH), 3.23 (s, with 77Se satellite 2JH-Se = 15.26 Hz, 2H, CH2Se), 3.10
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
13
(s, 2H, CH2S). C{1H} NMR (101 MHz, acetone-d6, 24°C): δ = 64.64 (s, CH2OH), 59.56 (s,
1
1
CCH2OH), 45.11 (s, CH2S), 35.10 (s, with 77Se satellite JC-Se = 65.27 Hz, CH2Se). H77Se
HMBC NMR (400 MHz, 76 MHz, CDCl3, 24°C): δ = 412.95 (s, CH2Se). DEI-MS: m/z = 253
[M]+.
General procedure for the synthesis of the diiron complexes 5-8.
A solution of Fe3(CO)12 (200 mg, 0.40 mmol) and compounds 1-4, respectively, (0.40 mmol) in
toluene (30 mL) was heated at reflux for 2 h under N2. The green solution turned deep-red and
the solvent was removed under reduced pressure. The residue was purified by column
chromatography using CH2Cl2-hexane mixture (2: 3) to isolate a red solid. Suitable crystals for
single-crystal X-ray analysis were accessible by slow evaporation of a solution of the compound
in hexane at 7 °C.
[Fe2(CO)6{μ-(S(CH2)2S)C(C5H10O2)}] (5). Yield: 57% (110 mg, 226 mmol). Elemental analysis
1
for C14H14Fe2O8S2: Calc.: C 34.59; H 2.90; S 13.19 found: C 34.83; H 2.92; S 13.25. H NMR
(400 MHz, CDCl3, 23°C): δ = 3.53 (s, 4H, CH2O), 2.23 (s, 4H, CH2S), 1.31 (s, 6H, CH3).
13C{1H} NMR (101 MHz, CDCl3, 23°C): δ = 207.3 (s, CO), 98.8 (s, OCCH3), 68.1 (s, CCH2O),
35.8 (s, CCH2O), 26.8 (s, CH2S), 23.3 (s, CH3). IR (νCO, cm-1): 2073 (s), 2029 (s), 2015 (s),
1997(s), 1981 (s), 1971 (s), 1960 (bs). DEI-MS: m/z = 486 [M]+, 458 [M-CO]+, 430 [M-2CO]+,
402 [M-3CO]+, 374 [M-4CO]+, 346 [M-5CO]+, 318 [M-6CO]+.
[Fe2(CO)6{μ-(Se(CH2)2Se)C(C5H10O2)}] (6). Yield: 54% (125 mg, 0.22 mmol). Elemental
analysis for C14H14Fe2O8Se2: Calc.: C 29.0; H 2.43 found.: C 29.49; H 2.44. 1H NMR (400 MHz,
13
CDCl3, 25°C): δ = 3.50 (s, 4H, CH2O), 2.25 (s, 4H, CH2Se), 1.31 (s, 6H, CH3). C{1H} NMR
(101 MHz, CDCl3, 25°C): δ = 208.2 (s, CO), 98.9 (s; OCCH3), 66.9 (s, CCH2O), 35.3 (s,
77
1
1
CCH2O), 23.3 (s, CH3), 17.9 (s, with Se satellite JC-Se = 85.53 Hz, CH2Se). H77Se HMBC

Page 29 of 35
New Journal of Chemistry
View Article Online
DOI: 10.1039/C9NJ02642H
1
2
3
4
5
6
NMR (400 MHz, 76 MHz, CDCl3, 25°C): δ = 69.73 (s, CH2Se). IR: (νCO, cm-1): 2064 (s), 2021
(s), 1999 (s), 1979 (s), 1970 (s), 1948 (bs). DEI-MS: m/z = 582 [M]+, 554 [M-CO]+, 526 [M-
2CO]+, 498 [M-3CO]+, 470 [M-4CO]+, 442 [M-5CO]+, 414 [M-6CO]+.
7
8
9
[Fe2(CO)6{μ-(Se(CH2)2S)C(C5H10O2)} (7). Yield: 59% (126 mg, 0.236 mmol). Elemental
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
analysis for C14H14Fe2O8SSe: Calc: C 31.55; H 2.65; S 6.02 found.: C 31.70; H 2.70; S 6.00. 1H
NMR (400 MHz, CDCl3, 24°C): δ = 3.48-3.56 (m, 4H, CH2O), 2.31 (s, with 77Se satellite 2JH-Se
=
17.48 Hz, 2H, CH2Se), 2.16 (s, 2H, CH2S), 1.31 (s, 6H, CH3). 13C{1H} NMR (101 MHz, CDCl3,
25°C): δ = 207.8 (s, CO), 98.9 (s, OCCH3), 67.6 (s, CCH2O), 35.6 (s, CCH2O), 28.1 (s, CH2S),
24.0 (s, CH3), 23.3 (s, CH3), 16.9 (s, with 77Se satellite 1JC-Se = 83.12 Hz, CH2Se). 1H77Se HMBC
NMR (400 MHz, 76 MHz, CDCl3, 24°C): δ = 56.01 (s, CH2Se). IR (νCO, cm-1): 2069 (s), 2025
(s), 2004 (s), 1982 (s), 1972 (s), 1950 (bs). DEI-MS: m/z = 534 [M]+, 506 [M-CO]+, 478 [M-
2CO]+, 450 [M-3CO]+, 422 [M-4CO]+, 394 [M-5CO]+, 366 [M-6CO]+.
[Fe2(CO)6{(μ2,κ3-Se,C,O-SeCH2C7H12O2}] (8). Yield: 42% (84 mg, 0.168 mmol). Elemental
analysis for C14H14Fe2O8Se: Calc.: C 33.57; H 2.82 found: C 33.50; H 2.66. 1H NMR (600 MHz,
2
2
CDCl3, 24°C): δ = 5.30 (s, 1H, CHFe), 3.45 (d, JH-H = 11.46 Hz, 1H, CH2O), 3.38 (d, JH-H
=
2
2
11.31 Hz. 1H, CH2Se), 3.30 (d, JH-H = 11.39 Hz, 1H, CH2O), 2.74 (d, JH-H = 11.42 Hz, 1H,
CH2Se), 1.25 (s, 3H, OCCH3), 1.20 (s, 3H, OCCH3), 0.59 (s, 3H, CH2CCH3). 13C{1H} NMR
(151 MHz, CDCl3, 25°C): δ = 215.5 (s, CO), 212.2 (s, CO), 209.9 (s, CO), 107.5 (s, CHFe),
102.3 (s, OCCH3), 67.3 (s, CH2O), 52.9 (s, CCHFe), 32.8 (s, with 77Se satellite 1JC-Se = 59.04 Hz.
1
CH2Se), 28.7 (s, OCCH3), 21.9 (s, CH2CCH3), 18.4 (s, OCCH3). H77Se HMBC NMR (600
MHz, 114 MHz, CDCl3, 24°C): δ = 31.49 (s, CH2Se). IR (νCO, cm-1): 2059 (s), 2008 (s), 1978
(s), 1956 (s), 1931 (bs). DEI-MS: m/z = 502 [M]+, 474 [M-CO]+, 446 [M-2CO]+, 418 [M-3CO]+,
388 [M-3CO-2CH3]+, 360 [M-4CO-2CH3]+, 332 [M-5CO-2CH3]+, 304 [M-6CO-2CH3]+.
Acknowledgment
H. Abul-Futouh thanks the deanship of research, Al-Zaytoonah university of Jordan for financial
support (Grant No. 18/2018-2019).