A cis-dioxomolybdenum Schiff base complex: Synthesis, crystal structure and catalytic activity in oxidation of olefins using hydrogen peroxide

Article abstract of DOI:10.1007/s11243-015-9985-3

A cis-dioxomolybdenum(VI) Schiff base complex [MoO₂L] has been synthesized and structurally characterized. Homogeneous oxidation of olefins using the complex as a catalyst with hydrogen peroxide as oxidant in ethanol solution showed high catalytic activity for this complex. Under reflux conditions, the oxidation of cyclooctene, styrene, α-methyl styrene, 4-methyl styrene and 1-methyl cyclohexene with hydrogen peroxide gave the corresponding epoxides as the sole products.

Full text of DOI:10.1007/s11243-015-9985-3

Transition Met Chem (2015) 40:891–897
DOI 10.1007/s11243-015-9985-3
A cis-dioxomolybdenum Schiff base complex: synthesis, crystal
structure and catalytic activity in oxidation of olefins using
hydrogen peroxide
Nasim Tadayonpour1 Saeed Rayati2 Moayad Hossaini Sadr1 Karim Zare1
Andrzej Wojtczak3
•
•
•
•
Received: 12 July 2015 / Accepted: 14 September 2015 / Published online: 22 September 2015
Ó Springer International Publishing Switzerland 2015
Abstract A cis-dioxomolybdenum(VI) Schiff base com-
plex [MoO2L] has been synthesized and structurally char-
acterized. Homogeneous oxidation of olefins using the
complex as a catalyst with hydrogen peroxide as oxidant in
ethanol solution showed high catalytic activity for this
complex. Under reflux conditions, the oxidation of
cyclooctene, styrene, a-methyl styrene, 4-methyl styrene
and 1-methyl cyclohexene with hydrogen peroxide gave
the corresponding epoxides as the sole products.
of toxic waste products. Hydrogen peroxide constitutes a
potentially green and environmentally friendly oxidant
because it releases only water as by-product [2]. Thus,
much effort has been put into the development of ideal
conditions for its use in oxidation reactions [3–7].
In the present research, a tetradentate Schiff base ligand
and its cis-dioxomolybdenum(VI) complex have been
prepared and characterized. The use of this complex as a
catalyst for oxidation of various olefins with hydrogen
peroxide has been explored.
Introduction
Experimental
Oxidations are among the most important reactions in
the conversion of crude oil to fine chemicals, in the
introduction of functional groups into hydrocarbons and
in synthetic transformations in organic chemistry [1]. In
the context of green chemistry, it is of importance to
develop oxidation reactions which produce the minimum
Instruments and reagents
2,20-Dimethylpropylenediamine, 5-bromo-2-hydroxyben-
zaldehyde, molybdenyl acetylacetonate and hydrogen per-
oxide (27 % solution in water) were used as received from
commercial suppliers. Solvents were dried and distilled by
standard methods before use. Other commercially available
chemicals were purchased from Merck or Aldrich.
Infrared spectra were recorded as KBr pellets using a
Unicam Matson 1000 FT-IR spectrometer. The electronic
absorption spectra were recorded on a single-beam spec-
trophotometer (Cam Spec-M330). The oxidation products
were analyzed with a gas chromatograph (Shimadzu, GC-
14B) equipped with an SAB-5 capillary column (phenyl
methyl siloxane 30 m 9 320 mm 9 0.25 mm) and a flame
ionization detector.
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-015-9985-3) contains supplementary
material, which is available to authorized users.
& Saeed Rayati
rayati@kntu.ac.ir; srayati@yahoo.com
1
Department of Chemistry, Science and Research Branch,
Islamic Azad University, Tehran, Iran
X-ray data were collected with an Oxford Sapphire CCD
˚
2
diffractometer using Mo Ka radiation (k = 0.71073 A), at
Department of Chemistry, K.N. Toosi University of
Technology, P.O. Box 16315-1618, Tehran 15418, Iran
293(2) K, by x–2h method. The complex crystallizes in
the triclinic space group P-1. The structure was solved by
direct methods and refined with the full-matrix least-
3
Faculty of Chemistry, N. Copernicus University, Gagarina 7,
´
87-100 Torun, Poland
123

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Transition Met Chem (2015) 40:891–897
Fig. 1 Crystal structure of the
molybdenum complex
squares method on F2 with use of the SHELX97 [8] pro-
gram package. An analytical absorption correction was
applied (RED171 package of programs) [9], with maxi-
mum and minimum transmission of 0.4439 and 0.1698,
respectively. No extinction correction was applied. Posi-
tions of all hydrogen atoms were found from the difference
electron density maps, and subsequently hydrogen atoms
were constrained in the refinement with the appropriate
AFIX option in SHELXL-97 [8] to maintain the idealized
geometry.
O
Br
O
N
O
Br
Mo
O
N
Me
Me
Scheme 1 Dioxomolybdenum Schiff base complex
Preparation of the Schiff base (H2L)
Table 1 UV–Vis data for the complex
kmax (nm) (e, M-1 cm-1
)
Assignment
The Schiff base was prepared by standard methods [10–
12]. In a typical procedure, a mixture of 2,20-dimethyl-
propylenediamine (0.102 g, 1 mmol) in absolute ethanol
(15 mL) was added to a solution of 5-bromo-2-hydroxy-
benzaldehyde (0.402 g, 2 mmol) in ethanol (15 mL) while
stirring and the mixture was refluxed for 1 h. The yellow
precipitate was filtered off, washed with diethyl ether
(3 9 5 mL) and dried in air.
224 (27,064)
296 (16,442)
408 (8076)
p ? p*
n ? p*
LMCT
General catalytic oxidation procedure
Catalytic experiments were carried out in a 10-mL glass
flask fitted with a water condenser. In a typical procedure,
0.4 mmol cyclooctene, 0.04 mmol MoO2L and 2 mL H2O2
were added to 3 mL of ethanol. The reaction mixture was
refluxed for 4 h. After each interval of 1 h, an aliquot was
taken from the reaction mixture and analyzed by gas
chromatography. The oxidation products were identified by
comparison with authentic samples (retention times in GC).
Preparation of the complex
The molybdenum complex (Fig. 1) was prepared according
to the literature [12], as follows: to a solution of H2L
(0.468 g, 1 mmol) in ethanol (20 mL), a solution of
molybdenyl acetylacetonate (0.326 g, 1 mmol) in ethanol
(10 mL) was added, and the reaction mixture was refluxed
for 1 h. The light orange solution was concentrated to yield
an orange powder. The products was filtered off, washed
with ethanol and dried in air. Mo(O)2L: Yield: 83 %,
0.493 g, D.p.: [ 295, Selected FT-IR data, m (cm-1): 2922
(w, C–H), 1614 (s, C=N), 1520 (s, C=C), 826 and 900 (m,
Results and discussion
Spectroscopic studies
1
Mo–O). H NMR (d): 0.75, 1.17 (s, 6H, NCH2C(CH3)2-
CH2N), 3.38–4.63 (4H, NCH2C(CH3)2CH2N), 6.54–7.60
(m, 6H, ArH), 8.04–8.12 (s, 2H, N=CH).Anal. Calc. for C19
H18Br2MoN2O4 (594.11): C, 38.4; H, 3.1; N, 4.7. Found:
C, 38.6; H, 3.1; N, 4.8 %.
Comparing the IR spectra of H2L with the molybde-
num(VI) complex (Scheme 1), the band for the free Schiff
base at 1626 cm-1 can be assigned to the characteristic
123

Transition Met Chem (2015) 40:891–897
893
Table 2 Crystal data and
structure refinement for MoO2L
Empirical formula
C19 H18Br2MoN2O4
594.11
Formula weight
Temperature (K)
293(2)
˚
Wavelength (A)
0.71073
Crystal system, space group
Triclinic, P-1
a = 10.3363(10)
b = 11.2107(11)
c = 11.2413(12)
a = 99.012(9)
b = 111.276(9)
c = 113.847(10)
1037.50(18)
2, 1.902
˚
Unit cell dimensions (A, °)
3
˚
Volume (A )
Z, Calculated density (Mg/m3)
Absorption coefficient (mm-1
F(000)
)
4.511
580
Crystal size (mm)
0.61 9 0.28 9 0.22
2.30 to 27.96
Theta range for data collection (°)
Limiting indices
-11 B h B 13, -14 B k B 13, -13 B l B 14
5809/4149 [R(int) = 0.0550]
25.00 94.3 %
Reflections collected/unique
Completeness to theta
Absorption correction
Max. and min. transmission
Refinement method
Analytical
0.4439 and 0.1698
Full-matrix least-squares on F2
Data/restraints/parameters
Goodness of fit on F2
Final R indices [I [ 2sigma(I)]
R indices (all data)
4149/0/253
1.067
R1 = 0.0864, wR2 = 0.1986
R1 = 0.1520, wR2 = 0.2669
1.302 and -1.274
-3
)
˚
Largest diff. peak and hole (eA
Table 3 Selected bond lengths
˚
MoO2L
[A] and angles [°] for MoO2L
Mo1–O2 1.714(10) Br1–C4
1.926(14) O3–Mo1–O4
90.3(4) O1–Mo1–N1 86.7(4)
96.9(4) O3–Mo1–N1 81.5(4)
86.2(4) O4–Mo1–N1 79.2(3)
Mo1–O1 1.720(9)
Mo1–O3 1.916(9)
Mo1–O4 2.109(9)
O2–Mo1–O1 104.1(5)
O2–Mo1–O3 103.5(4)
O2–Mo1–N2
O1–Mo1–N2
O1–Mo1–O3
99.6(4)
88.6(4)
O3–Mo1–N2 156.6(4 N2–Mo1–N1 76.2(4)
O4–Mo1–N2 78.7(4)
O2–Mo1–N1 166.9(4)
Mo1–N2 2.155(10) O2–Mo1–O4
Mo1–N1 2.334(10) O1–Mo1–O4 161.4(4)
X-ray crystal structure of the complex
stretching vibration of C=N imine mode. In the molybde-
num(VI) complex this band is shifted to lower wave
number by 12 cm-1, being deserved at 1614 cm-1. This
indicates involvement of the azomethine nitrogen in
coordination [13]. The bands at region of 826 and
900 cm-1 in the spectrum of the complex can be assigned
to the symmetric and asymmetric stretching vibrations of
the cis-MoO2 fragment.
The X-ray experimental data and structure refinement for
the complex are summarized in Table 2. Selected bond
lengths and angles are presented in Table 3. The asym-
metric unit consists of the cis-dioxomolybdenum(VI)
complex, with the Schiff base ligand being tetradentate.
The complex is similar to the molybdenum complexes
reported previously [14]. The coordination sphere of Mo is
a distorted octahedron. Two oxo ligands positioned cis in
the coordination sphere form short bonds, with Mo1–O1
Table 1 provides electronic spectral data for the com-
plex. The electronic absorption spectrum obtained in
methanol showed three bands, assigned to p ? p*,
n ? p* and LMCT transitions, respectively.
˚
and Mo1–O2 distances of 1.720(9) and 1.714(10) A,
123

894
Transition Met Chem (2015) 40:891–897
oxo ligands and assisted by electrostatic repulsion. The
angles involving both the oxo and phenolate oxygens differ
significantly from the ideal 90°/180° (Table 3), possibly
also because of electrostatic repulsion. The smallest angle
between the cis-positioned donor atoms is N2–Mo1–N1 at
76.2(4)°. This also results in values of the C–N–Mo angles
involving the carbon atoms of the central bridge (C8 and
C10) which are significantly smaller than those involving
the imine C7 or C11 values.
Table 4 Catalytic oxidation of cyclooctene at room temperature
No.
Catalyst
Oxidant
Conversion (%)
1
2
3
None
H2O2
None
H2O2
0
0
MoO2L
MoO2L
100
Reaction conditions: catalyst (0.04 mmol), alkene (0.4 mmol), H2O2
(25 mmol). Reaction time 4 h
The geometry of the Schiff base ligand is typical for
such complexes. The imine bond lengths, C7–N1 1.284(15)
100
˚
and C11–N2 1.302(19) A, are similar to those reported
Ethanol
Methanol
Chloroform
80
60
40
20
0
previously [14] for other Schiff base ligands. The phenolate
O–C bonds are O3–C1 1.358(15) and O4–C17
˚
1.314(16) A. The Br1–C4 and Br2–C14 distances of
˚
1.926(14) and 1.931(13) A are typical for such bonds.
Further analysis revealed the effect of bromine substituents
on the geometry of the Schiff base phenyl rings. The bonds
involving C4 and C14 (Fig. 1) are among the shortest C–C
bonds in these rings. Also C3–C4–C5 and C13–C14–C15
angles both of 123.9(13)° are significantly larger than those
in the rest of the rings.
0
1
2
3
4
Time (h)
Fig. 2 Effect of various solvents on the epoxidation of cyclooctene
with H2O2 catalyzed by MoO2L. Reaction conditions: cyclooctene
(0.4 mmol), catalyst (0.04 mmol), solvent (3 mL), H2O2 (2 mL);
reflux
The central bridge of the Schiff base ligand is defined by
torsion angles C7–N1–C8–C9 -115.5(13), N1–C8–C9–
C10 -45.8(16), C8–C9–C10–N2 -25.0(16) and C9–C10–
N2–C11 -94.4(14)°, which are different from those
reported for the dimethoxy analog MoO2{salnptn(3-
OMe)2} [14]. The torsion angles N1–C7–C6–C1 and N2–
C11–C12–C17 of 10.0(19) and -20(2), respectively,
indicate significant differences in the positions of the
phenyl ring relative to the central N1–Mo1–N2 moiety.
The dihedral angle between the best planes of the two
phenyl rings in the ligand is 69.5(8)°. As a result, the six-
membered chelate rings Mo1–O3–C1–C6–C7–N1, Mo1–
O4–C17–C12–C11–N2 and Mo1–N1–C8–C9–C10–N2
reveal conformations of twist boat, screw boat and twist
boat, respectively. The steric strain within the complex
results in significant displacement of the central Mo1 from
the best planes of the phenyl rings, being 0.609(1) and
100
0.02 mmol
0.04 mmol
0.06 mmol
80
60
40
20
0
1
2
3
4
Time (h)
Fig. 3 Influence of catalyst concentration on the oxidation of
cyclooctene. Reaction conditions: cyclooctene (0.4 mmol), ethanol
(3 mL), H2O2 (2 mL); reflux
˚
-0.971(2) A for the C1–C6 and C12–C17 moieties,
respectively. The phenolic O atoms of the Schiff base
ligand form two longer bonds to Mo, with distances of
˚
1.916(9) and 2.109(9) A for Mo1–O3 and Mo1–O4,
respectively.
The pairs of torsion angles N1–C8–C9–C18 -164.0(12)
and N1–C8–C9–C19 76.2(15), and C18–C9–C10–N2
90.9(14) and C19–C9–C10–N2 -147.0(13) indicate that
the position of the gem-dimethyl group at C9 is not sym-
metric relative to the central bridge, resulting in an
respectively. The longest bonds are formed by the N atoms
of the Schiff base, the Mo1–N2 and Mo1–N1 distances
˚
being 2.155(10) and 2.334(10) A, respectively. The dif-
˚
ferences observed for these pairs of bonds can be attributed
to the trans effects of the oxo ligands positioned opposite
to O4 and N1 in the coordination sphere. The O2–Mo1–O1
angle between the oxo ligands is 104.1(5)°, larger than the
values reported previously for similar cis-dioxomolybde-
num complexes reported previously [14]. This might be
related to the relatively short Mo–O bonds formed by the
intramolecular C18…C11 contact of 3.44(3) A but a lack
of such contact for the C19 methyl group.
For the suggested mechanism of catalytic olefin epoxi-
dation, access to the oxo ligands followed by formation of
a hepta-coordinated Mo intermediate seems to be important
[15–17]. The analysis of the intramolecular contacts reveal
that O1 forms such contacts with C1, C10 and H10A, the
123

Transition Met Chem (2015) 40:891–897
895
corresponding distances being 3.342(14), 2.928(18) and
˚
2.37 A. The space around O2 seems to be less crowded,
of blank experiments (Table 4) revealed that the presence
of both catalyst and oxidant is essential for an effective
catalytic reaction.
with interactions to C11, C12 and C17 of 3.29(2), 3.22 and
˚
3.026(18) A, only the latter being slightly shorter than the
In order to optimize the suitable reaction conditions, the
effects of various reaction parameters were studied. The
influence of different solvents on the oxidation of
cyclooctene was studied with the results presented in
Fig. 2. The highest conversion (up to 100 %) was obtained
in ethanol possibly due to the higher boiling point of this
solvent compared to the others.
sum of the vdW radii. Also O2 is involved in a non-clas-
sical C10–H10B…O2[1 - x,2 - y,1 - z] intermolecular
H-bond, with the H…O and C…O distances being 2.57 and
˚
3.245(16) A, respectively. These interactions around the
O2 oxo ligand probably make it more suitable for
involvement in the catalytic cycle.
Analysis of the crystal packing revealed a C5–H5A…p
interaction involving the C12–C17[1 - x,2 - y,-z] phe-
The effect of different catalyst concentrations was also
explored (Fig. 3). The reaction required 0.04 mmol of
catalyst for complete conversion in 4 h. A decrease in the
yield of epoxide with 0.06 mmol of the catalyst was
observed. This may be due to higher degradation of the
oxidant in this case [1].
˚
nyl ring, with a C…H distance of 2.90 A. There is also a
C7–H7A…O4[1 - x,2 - y,-z] interaction, with a C…O
˚
distance of 3.284(19) A.
Homogeneous catalytic epoxidation of olefins
In order to establish the general applicability of this
catalytic system, under the optimized conditions, different
olefins were oxidized in the presence of MoO2L as catalyst
with the results presented in Table 5. Oxidation of several
The catalytic performance of MoO2L was investigated for
the epoxidation of olefins with hydrogen peroxide. A series
Conversion %a
100
Product
Selectivity
(epoxide) %
100
Table 5 Epoxidation of olefins
using H2O2 catalyzed by
MoO2L in ethanol
Entry
1
Alkene
O
2
96
100
100
34b
25c
O
3
4
100
100
O
O
O
5
6
87
Cl
Cl
21d
O
100
Me
Me
29e
43f
O
7
8
60
MeO
Me
MeO
Me
O
100
9
71
87
100
100
O
10
5
O
5
The molar ratio of alkene/catalyst/H2O2 was 0.4:0.04:2 m, reaction time 4 h
a
Conversions and selectivities were determined by GC based on the starting alkene
b
Benzaldehyde is the by-product
c
4-Chlorobenzaldehyde is the by-product
d
4-Methylbenzaldehyde is the by-product
e
4-Methoxybenzaldehyde is the by-product
f
Acetophenone is the by-product
123

896
Transition Met Chem (2015) 40:891–897
Scheme 2 Proposed catalytic
cycle
O
Br
O
N
O
Br
Mo
O
H2O2
H2O
N
Me
Me
OH
H
O
O
O
Br
O
N
Br
Mo
O
H
H
O
N
O
Br
O
N
O
Br
Mo
O
Me
Me
N
Me
Me
O
H
o
H
O
O
Br
O
O
Br
Mo
O
N
N
Me
Me
Acknowledgments The financial support of this work by Science
and research branch, Islamic Azad University and K.N. Toosi
University of Technology research council is acknowledged.
of these substrates gave the corresponding epoxide as the
sole product, whereas the oxidations of various substituted
styrenes also gave some of the corresponding carbonyl
compounds as by-products (Table 5, entries 4–8).
A mechanism for these reactions is proposed in
Scheme 2. In this mechanism, hydrogen peroxide is acti-
vated by coordination to the molybdenum center [18, 19].
Subsequent nucleophilic attack of the substrate on the
peroxide oxygen gave the epoxide product.
References
1. Sheldon RA, Kochi JK (1981) Metal-catalyzed oxidation of
organic compounds. Academic Press, New York
2. Goti A, Cardona F (2008) In: Tundo P, Esposito V (eds) Green
chemical reactions. Springer, Berlin
3. McAteer B, Beattie N, Richens DT (2013) Catalytic oxidation of
cyclohexene by aqueous iron(III)/H2O2 in mildly acidic solution:
epoxidation versus allylic oxidation. Inorg Chem Commun
35:284–289
Conclusions
4. Yoo S-K, Han JH, Lee SJ, Ryu JY, Kim C, Jin SW, Kim Y, Nam
W (2003) Conversion of olefins into trans-diols or trans-diol
mono-ethers by using iron porphyrin(III) complex and H2O2.
Inorg Chem Commun 6:1148–1151
The distorted octahedral cis-dioxomolybdenum Schiff base
complex reported in this paper acts as catalyst for green
oxidation of various olefins with hydrogen peroxide as
primary oxidant. All of the investigated substrates gave the
epoxide, with high selectivities in some cases but much
lower selectivities in others.
´
5. Ziolek M, Sobczak I, Decyk P, Sobanska K, Pietrzy KP, Sojka Z
(2015) Search for reactive intermediates in catalytic oxidation
with hydrogen peroxide over amorphous niobium(V) and tanta-
lum(V) oxides. Appl Catal B Environ 164:288–296
6. Islam SM, Mobarok M, Mondal P, Roy AS, Hossain D, Salam N,
Mondal S (2012) Use of immobilized transition metal complexes
as recyclable catalysts for oxidation reactions with hydrogen
peroxide as oxidant. Trans Metal Chem 37:97–107
Supplementary material
7. Monfared HH, Abbasi V, Rezaei A, Ghorbanloo M, Aghaei A
(2012) A heterogenized vanadium oxo-aroylhydrazone catalyst
for efficient and selective oxidation of hydrocarbons with
hydrogen peroxide. Trans Metal Chem 37:85–92
8. Sheldrick GM (2008) A short history of SHELX. Acta Cryst A
64:112–122
CCDC 1403403 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
123

Transition Met Chem (2015) 40:891–897
897
9. Crys Alis CCD171 and RED171 package of programs, Oxford
Diffraction (2000)
catalytic performance for homogeneous oxidation of olefins.
Inorg Chim Acta 386:27–35
10. Heshmatpour F, Rayati S, Hajiabbas MA, Neumu¨ller B (2011)
Synthesis, characterization, and catalytic performance in oxida-
tion of styrene and a-methyl styrene of a nickel(II) complex
derived from a Schiff base ligand. Z Anorg Allg Chem
637:1224–1228
11. Heshmatpour F, Rayati S, Hajiabbas MA, Abdolalian P, Neu-
muller B (2012) Copper(II) Schiff base complexes derived from
2,20-dimethyl-propandiamine: synthesis, characterization and
catalytic performance in oxidation of styrene and cyclooctene.
Polyhedron 31:443–450
15. Rayati S, Ghaemi A, Sadeghzadeh N (2010) Electronic effects of
substituents on the oxidation potentials of vanadyl complexes
with tetradentate Schiff base ligands derived from 1,2-propy-
lenediamine. Catal Commun 11:792–796
16. Rayati S, Koliaei M, Ashouri F, Mohebbi S, Wojtczak A,
Kozakiewicz A (2008) Oxovanadium (IV) Schiff base complexes
derived from 2,20-dimethylpropandiamine: a homogeneous cata-
lyst for cyclooctene and styrene oxidation. Appl Catal A Gen
346:65–71
´
17. Sobczak JM, Ziołkowski JJ (2003) Molybdenum complex-catal-
12. Rayati S, Abdolalian P (2013) Heterogenization of a molybde-
num Schiff base complex as a magnetic nanocatalyst: an eco-
friendly, efficient, selective and recyclable nanocatalyst for the
oxidation of alkenes. C R Chim 16:814–820
13. Maiti SK, Abdul Malik KM, Bhattacharyya R (2004) Oxoperoxo-
molybdenum and -tungsten (VI) complexes: their synthesis,
structure and catalytic uses in the peroxidic oxidation of alcohols
to aldehydes and ketones. Inorg Chem Commun 7:823–828
14. Rayati S, Rafiee N, Wojtczak A (2012) cis-Dioxo-molybde-
num(VI) Schiff base complexes: synthesis, crystal structure and
ysed epoxidation of unsaturated fatty acids by organic
hydroperoxides. Appl Catal A Gen 248:261–268
18. Farias M, Martinelli M, Bottega DP (2010) Epoxidation of soy-
bean oil using a homogeneous catalytic system based on a
molybdenum (VI) complex. Appl Catal A Gen 384:213–219
19. Wang G, Chen G, Luck RL, Wang Z, Mu Z, Evans DG (2004)
Duan X New molybdenum(VI) catalysts for the epoxidation of
cyclohexene: synthesis, reactivity and crystal structures. Inorg
Chim Acta 357:3223–3229
123