Iron(ii) coordination pyrazole complexes with aromatic sulfonate ligands: The role of ether

Article abstract of DOI:10.1039/d0nj02823a

Four novel heteroleptic mononuclear Fe(ii) coordination complexes including 3,5-dimethyl-1-(2′-pyridyl)-pyrazole (DMPP) and aromatic sulfonate ligands were synthesized and characterized using single crystal-XRD, SQUID magnetometry and 57Fe M?ssbauer spectroscopy. The crystal structures of the complexes revealed the unusual coordination of sulfonate groups to Fe(ii) ions, leading to an Fe-N4O2 coordination environment. This specific molecular layout is presumably caused by diethyl ether, which was vapour diffused into the complexation medium to obtain crystalline complexes. Alongside yielding single crystals, diethyl ether also altered the self-assembly process of a usual FeN6 homoleptic coordination environment. All the complexes remain paramagnetic at all temperatures as suggested by magnetic susceptibility measurements. All are stable in air except [Fe(DMPP)2(tos)2] which got partially oxidized to Fe(iii) over time as suggested by 57Fe M?ssbauer spectroscopy. This journal is

Full text of DOI:10.1039/d0nj02823a

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Iron(II) coordination pyrazole complexes with
aromatic sulfonate ligands: the role of ether†
Cite this:DOI: 10.1039/d0nj02823a
b
c
Varun Kumar,a Mohamed El-Massaoudi,b Smaail Radi,
Kristof Van Hecke,
a
Aurelian Rotarud and Yann Garcia
*
Four novel heteroleptic mononuclear Fe(II) coordination complexes including 3,5-dimethyl-1-(20-
pyridyl)-pyrazole (DMPP) and aromatic sulfonate ligands were synthesized and characterized using single
crystal-XRD, SQUID magnetometry and 57Fe Mossbauer spectroscopy. The crystal structures of the
¨
complexes revealed the unusual coordination of sulfonate groups to Fe(II) ions, leading to an Fe–N4O2
coordination environment. This specific molecular layout is presumably caused by diethyl ether, which
was vapour diffused into the complexation medium to obtain crystalline complexes. Alongside yielding
single crystals, diethyl ether also altered the self-assembly process of a usual FeN6 homoleptic coordina-
tion environment. All the complexes remain paramagnetic at all temperatures as suggested by magnetic
susceptibility measurements. All are stable in air except [Fe(DMPP)2(tos)2] which got partially oxidized to
Received 3rd June 2020,
Accepted 21st July 2020
DOI: 10.1039/d0nj02823a
Fe(III) over time as suggested by 57Fe Mossbauer spectroscopy.
rsc.li/njc
¨
of the coordination complexes of a pyrazole-dicarboxylate acid
ligand and investigated their supramolecular structures.14
1. Introduction
Coordination compounds represent a remarkable class of hybrid We have also shown the effect of the aliphatic backbone of a
materials because they can combine the functionalities of both bis-pyrazole–bis-carboxylate ligand on the supramolecular struc-
metal cations and organic molecules/anions operating as ligands tures of their coordination complexes and polymers.15 The effect
into a single compound. Besides the intrinsic properties of the of anions and hydrogen bonding on the supramolecular struc-
involved molecular precursors, they may even exhibit new proper- tural diversities of coordination complexes obtained from a
ties, thanks to the ligand field, which causes a disturbance in the novel bis-pyrazole ligand was also reported.16 Medicinal proper-
electronic configuration of the metal ions which in turn leads to ties were also investigated for new pyrazole–acetamide ligands
intriguing phenomena such as spin crossover1 (SCO), charge and complexes showing remarkable antioxidant activity.17 More
transfers2 etc. Coordination compounds based on azole ligands recently we have reported the crystal structure–bioactivity corre-
and transition metals are of particular interest because of their lation of three mononuclear coordination complexes of a
use in various potential applications involving thermochromism,3 pyrazolyl-benzimidazole ligand.18
photoluminescence,3,4 sensors,5 vapochromism,6,7 gas storage,8
Our interest lies in one such chelating bidentate azole ligand
molecular magnetism,9 metallic Hg storage,10 catalysis,11 named 3,5-dimethyl-1-(20-pyridyl)pyrazole (DMPP), which has
nanomedicine12 and many others.13 Among azole molecules, been used in a couple of applications. For instance, Takahira
pyrazoles are particularly useful in coordination chemistry. For et al. synthesized a deep blue colour emitting phosphorescent
instance, our group has recently reported the crystal structures Ir(III) complex,19 whereas Das et al. not only explored the fluores-
cence properties of DMPP but also of their Cd(II) complexes.20 The
a Institute of Condensed Matter and Nanosciences, Molecular Chemistry, Materials
DMPP molecule also possesses weak cytotoxic properties against
cancer cell lines, which gets enhanced by 10–20 fold when
complexed to Cu(II) ions.21 On the other hand, Baker et al.
successfully prepared Fe(II) DMPP mononuclear complexes and
studied their thermally induced spin crossover (SCO) behaviour.22
Fe(II) coordination compounds are indeed particularly attrac-
tive primarily because of their SCO behaviour,1 which may
find applications in photomagnetism,23 data storage,24 charge
transport,25 sensors26 etc.
´
and Catalysis (IMCN/MOST), Universite catholique de Louvain, Place Louis
Pasteur 1, 1348 Louvain-la-Neuve, Belgium. E-mail: yann.garcia@uclouvain.be
b
´
´
´
LCAE, Departement de Chimie, Faculte des Sciences, Universite Mohamed I,
BP 524, 60000 Oujda, Morocco
c XStruct, Department of Chemistry, Ghent University, Krijgslaan 281-S3,
9000 Ghent, Belgium
d Department of Electrical Engineering and Computer Science & Research Center
MANSiD, ‘‘Stefan cel Mare’’ University, 13 Universitatii St., 720229 Suceava,
Romania
† Electronic supplementary information (ESI) available. CCDC 2000062–2000065.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/d0nj02823a
These enticing applications motivated us to design Fe(II)
coordination complexes with DMPP in pursuit of the synthesis
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Scheme 1 Molecular representation of the ligands and coordination complexes.
of new SCO materials. The novelty in our work lies in employing reported procedures.34–36 C, H, and N elemental analyses were
aromatic phenylsulfonate anions to counterbalance Fe(II) ions, a performed at MEDAC Ltd, UK. Infrared spectra were recorded on a
strategy which has been seldom used in the synthesis of mono- Shimadzu FTIR-8400S instrument using KBr pellets. High resolu-
nuclear SCO complexes.27 The inclusion of sulfonate anions is not tion mass spectral data were recorded on a Q-Exactive mass
only expected to influence the SCO temperature but also to intro- spectrometer from Thermo Fisher Scientific in the electrospray
duce additional functionalities into the coordination compounds.28 ionization mode. Powder X-ray Diffraction (PXRD) experiments
In this context, we employed four different aromatic sulfonate were carried out on a D8 Advanced diffractometer from Bruker. The
anions, ps (phenylsulfonate), tos (tosylate), nitps (3-nitrophenyl- diffractogram was recorded using CuKa radiation (l = 1.5406 Å)
sulfonate), and psca (p-sulfocinnamic acid), as shown in and a linkeye XE-T detector (Bruker) in the 5–801 (2y) range with an
Scheme 1. Though all anions possessing oxygen rich functional increment of 0.00151 and an integration time of 0.15 s. Single crystal
groups are quite useful in the sense of hydrogen bonding and X-ray diffraction data for 1, 2, and 3 were collected at 150 K on a
cooperativity,29 the psca anion is especially noteworthy as it is a Mar345 image plate detector using MoKa radiation (l = 0.71073 Å)
derivative of a photo-responsive molecule: trans-cinnamic acid. generated by a Rigaku Ultra X18S rotating anode with Xenocs Fox3D
psca is therefore expected to undergo a [2+2] photodimerization mirrors. Single crystal-XRD data for 4 were collected at 100 K on a
upon irradiation with UV-light following Schmidt’s topo- Rigaku Oxford Diffraction SuperNova, Dual Source, Cu at zero,
chemical rules of cycloaddition.30,31 In addition, any coordination diffractometer, equipped with an Atlas CCD detector, using o scans
compound or metal salt including the psca anion may also be and MoKa radiation (l = 0.71073 Å). Magnetic susceptibility
photo-responsive.32 The present account is devoted to the synthesis measurements for all the complexes were carried out on a MPMS3
and study of four novel Fe(II) complexes, whose formulae are given SQUID magnetometer (Quantum Design Inc.) in the DC mode,
in Scheme 1. The complexes have been characterized by single under a DC magnetic field of 1000 Oe, with a temperature rate of
crystal X-ray diffraction, elemental analyses, SQUID magnetometry 2 K minꢀ1. Magnetic data were corrected for both sample holder
and 57Fe Mossbauer spectroscopy.
¨
complex diamagnetic contributions. 57Fe Mossbauer spectra were
¨
recorded at room temperature in the transmission geometry mode
¨
with a constant acceleration mode conventional Wissel Mossbauer
spectrometer equipped with a 57Co(Rh) radioactive source, a
Reuter Stokes proportional counter detector and a CMCA-550
2. Experimental
2.1 Materials and characterization techniques
¨
multichannel analyzer. All isomer shifts in Mossbauer spectra
All the chemicals were bought from usual chemical companies are given with respect to a-Fe.
and used as received without further purification. 3,5-Dimethyl-
2.2 Synthesis
1-(20-pyridyl)pyrazole (DMPP) was prepared by following a previously
reported procedure.33 Similarly, the Fe(II) salts [Fe(H2O)6]X2
2.2.1 Synthesis of [Fe(H2O)6](psca)2. 5.0 g of finely ground
with X = tos, nitps and tos, were prepared by following the trans-cinnamic acid was added in small amounts to fuming
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sulfuric acid (8 mL, 20%) kept in an ice-water bath. After described in ref. 37, FeCl2ꢁ4H2O was added in excess to the
complete addition, the reaction mixture was vigorously stirred diluted sulfonation mixture. The targeted Fe(II) salt starts to
at room temperature for 30 min. Thereafter, the thick brown precipitate out as a slightly off-white colour powder within
reaction mixture was heated in an oil bath set at 60 1C for 2 h. five min of stirring. It was filtered out and recrystallized from
The dark brown solution was then cooled down to room distilled water. Addition of FeCl2ꢁ4H2O not only provided
temperature and added to ice water (30 mL). To this solution, [Fe(H2O)6](psca)2 but also neutralized sulfuric acid to form
an excess of FeCl2ꢁ4H2O (B5.0 g) was then added. The resulting FeSO4. The advantage of using an excess of FeCl2ꢁ4H2O is that
reaction mixture was stirred until a nearly white precipitate it saturates the aqueous solution by FeSO4 and unreacted FeCl2ꢁ
started to separate from the reaction mixture. The stirring was 4H2O which quickly precipitates out [Fe(H2O)6](psca)2 because of
continued for 20 more min. The precipitate was filtered and the relative higher solubility of FeCl2 and FeSO4 as compared
recrystallized from distilled water to obtain a pure white to the organic Fe salt. The formation of [Fe(H2O)6](psca)2 was
product. Yield: 57% (6.25 g). FTIR (KBr, cmꢀ1): 1695 cmꢀ1 confirmed by Fourier transform infrared spectroscopy and
(CQO), 1631 (CQC), 1314 (asym. SQO), 1126 (sym. SQO), elemental analysis.
807 (S–O). Elemental analysis calculated for FeC18O18H30S2: C,
33.04; H, 4.62; S, 9.80. Found: C, 33.04; H, 4.60; S, 9.79.
For the synthesis of [Fe(DMPP)3]2+ coordination complexes,
the molar ratio of Fe–salts and bidentate DMPP was kept at
2.2.2 Synthesis of Fe(II) complexes (1–4). 3 mmol of DMPP 1 : 3. Upon mixing both precursors, the colour of the reaction
was dissolved in hot methanol (5 mL). To this solution was mixture turned brownish yellow immediately and continued to
added a 1 mmol methanolic solution of [Fe(H2O)6]X2 with darken with time. The reaction mixture was stirred for 30 min
X = ps, tos, nitps, psca. The colour of the reaction mixture at 50 1C to ensure to completion of the reaction. Thereafter, the
turned brownish yellow immediately upon addition. The solvent was allowed to evaporate slowly to obtain the required
reaction mixture was stirred at 50 1C for 30 min. Thereafter, product as a powder or preferably as single crystals. However,
diethyl ether was allowed to vapour diffuse into the brownish upon solvent evaporation, a dark brown oily substance was
yellow reaction mixture. Within a period of 3–5 days, yellow obtained for the four targeted coordination compounds. The
crystals of complexes were obtained: blocks for [Fe(DMPP)2(ps)2] reason that the coordination compound could not precipitate
(1). Yield 0.34 g (48%). Anal. calc. for FeC32H32N6O6S2: C, 53.63; as a solid phase may be attributed to the hydrophilic nature of
H, 4.50; N, 11.73. Found: C, 53.63; H, 4.48; N, 11.70; needles for the organic sulfonate anions which are highly polar and are
[Fe(DMPP)2(tos)2] (2). Yield: 0.34 g (46%). Anal. calc. for prone to absorb water molecules from the atmosphere. To avoid
FeC34H36N6O6S2: C, 54.84; H, 4.87; N, 11.29. Found: C, 54.71; the absorption of atmospheric moisture into the complexes,
H, 4.70; N, 11.28; plates for [Fe(DMPP)2(nitps)2] (3). Yield: 0.3 g Et2O was vapour diffused into the reaction mixture inside a
(38%). Anal. calc. for FeC32H30N8O10S2: C, 47.65; H, 3.75; N, closed glassware. In a span of 3–5 days, it yielded yellow colour
13.89. Found: C, 47.59; H, 3.80; N, 13.73; rectangular prisms for crystals of different shapes. The crystals were not hygroscopic
[Fe(DMPP)2(psca)2] (4). Yield: 0.48 g (56%). Anal. calc. for and found to be stable under ambient conditions. The elemental
FeC38H36N6O10S2: C, 53.28; H, 4.24; N, 9.81. Found: C, 53.06; analysis indicated the formation of heteroleptic complexes
H, 4.57; N, 9.76. Although no solvent was detected in the comprised of two DMPP and two sulfonate anion molecules,
elemental analysis of 4, one molecule of Et2O was observed in operating as ligands. Elemental analysis also suggested
the crystal structure. This situation may be due to the high the absence of any solvent molecules of crystallization in the
degree of volatility of Et2O from our complex when sending the complexes. Presumably, Et2O altered the self-assembly process
crystals for chemical analyses.
of [Fe(DMPP)3]2+ coordination complexes and resulted in a more
2.2.3. Controlled precipitation of 4. The same procedure stable [Fe(DMPP)2X2] (X = ps, tos, nitps, psca) coordination
was followed as above for 4 until the completion of the reaction, environment. High resolution mass spectrometry in the reaction
i.e. after stirring the reaction mixture at 50 1C for 30 min. mixture containing [Fe(H2O)6](psca)2 and three molecules
Subsequently, 10 mL of diethyl ether was added to the reaction of DMPP in MeOH revealed a distinct peak at m/z = 287.61
mixture, which immediately led to a yellow precipitate. It was which corresponds to [Fe(DMPP)3]2+ species (Fig. 1). The peak at
stirred for 2 min, and then filtered and dried in air. Yield: 0.69 g m/z = 629.13 which is assigned to [Fe(DMPP)2(psca)]+ is a
(81%). The sample was then recorded by X-ray powder diffrac- fragmented species of [Fe(DMPP)3](psca)2 and should not be
tion to compare with single crystal X-ray analysis of 4.
mistaken for the formation of 4. Such a [Fe(Ligand)2(Anion)] peak
was earlier observed in the mass spectra of another mononuclear
complex, [Fe(L1)3]Cl2 where L1 = 5-benzylamido-2,20-bipyridine.38
To further reinforce our finding that the heteroleptic layout is
caused by Et2O, controlled precipitation of the reaction mixture
of 4 was carried out using Et2O. The resulting precipitate was
3. Results and discussion
3.1 Synthesis and crystallization
The Fe(II) salts including sulfonate counteranions were prepared then analysed by powder-XRD. The obtained PXRD pattern was
according to a previously reported procedure yielding single compared with the simulated pattern from the single crystal
crystals,34–36 except [Fe(H2O)6](psca)2 which was synthesized structure of 4 (see Fig. S2, ESI†). Though the peaks of the
in situ by carrying out sulfonation of trans-cinnamic acid.37 After powder patterns differ in intensity, their 2y values match very
the completion of the reaction, instead of separating out psca as well with each other, which infers that the precipitate of 4
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Diffraction,39 by applying the implemented absorption correc-
tion. The structures were solved by Direct Methods, SHELXT40
and refined by full-matrix least squares minimization on F2
using SHELXL2014/7.41 For complex 4, the Olex242 software was
used as a graphical user interface. Non-hydrogen atoms were
refined anisotropically and hydrogen atoms were placed in
the calculated positions and refined in the riding mode with
isotropic temperature factors fixed at 1.2 times Ueq of the
parent atoms (1.5 times for methyl and hydroxyl groups).
ꢀ
A minor disorder was observed for the oxygen atoms of an SO3
group in 2, giving a refined occupancy ratio of approximately
60/40. For the structure of 3, the selected crystal was found to be
twinned, the twin matrix was found by TWINROTMAT (Platon)
and an HKLF 5 type SHELXL file was subsequently written.
Refinement cycles were performed against these HKLF 5 reflec-
tion data. Additional solvent accessible voids (9% of the unit cell)
were treated by the SQUEEZE procedure in Platon,43 accounting
for the diffused solvent contribution inside these cavities. The
crystallographic and refinement data for all the complexes are
summarized in Table S1 (ESI†). CCDC 2000062, 2000063, 2000064
and 2000065 for 1, 2, 3 and 4 respectively.†
3.2.1 Crystal structure of 1. 1 crystallizes in the P21 space
group. The asymmetric unit is comprised of one Fe(II) ion as the
metal centre, two molecules of DMPP and two molecules of the
anionic ligand ps as shown in Fig. 2(a). DMPP is coordinated to
Fe(II) through each nitrogen atom of pyridine and pyrazole,
whereas ps is coordinated through oxygen atoms of sulfonate
Fig. 1 Mass spectrum of the reaction mixture containing [Fe(H2O)6](psca)2
and three moles of DMPP in MeOH.
obtained from controlled precipitation has the same chemical
formula as crystals of 4 i.e., [Fe(DMPP)2( psca)2]. The whole
process of expected and obtained products is summed up in
Scheme 2. The reproducibility in the formulae of the four
complexes thus validates the reproducibility of the employed
synthesis procedure.
3.2 Single crystal X-ray diffraction
The data integration and reduction for compounds 1–4 were groups, thus making an FeN4O2 coordination environment.
performed with the CrysAlisPro software from Rigaku Oxford The Fe(II) ions exhibit a distorted octahedral geometry as none
Scheme 2 General synthesis scheme of coordination complexes, showing the expected and obtained products. X = ps, tos, nitps and psca.
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of the bond angles [+O46–Fe1–O16 = 92.3(1)1, +O16–Fe1–N2 = ligands. The anionic tos ligands are coordinated to Fe(II) ions in
94.3(1)1, +N2–Fe1–N9 = 73.9(1)1, +N9–Fe1–N32 = 86.1(1)1, a cis manner. The Fe–N bond lengths which are of the order
+N32–Fe1–N39 = 72.7(1)1, +N39–Fe1–O46 = 92.2(1)1] of the of B2.2 Å confirm the the HS state of 2.44
octahedron are right angles. One of the apical positions of the
The crystal packing of 2 is shown in Fig. 3(b) along the b-axis.
octahedron is occupied by a pyridine ring from DMPP and The shortest distance between two Fe(II) ions is 9.058(9) Å
the other is occupied by a sulfonate group from one of the as depicted in Fig. 3(b). Intra- and intermolecular (only non-
anionic ligands ps. The anionic ligands ps are found to be classical) hydrogen bonds are observed between tos –SO3 oxygen
coordinated to Fe(II) ions in a cis manner with their aromatic atoms and –CH and –CH3 hydrogen atoms of tos and DMPP
rings positioned away from each other. Moreover, the pyridine (with C(H)ꢁ ꢁ ꢁO distances in the range of 2.844(15)–3.404(14) Å).
and pyrazole rings of DMPP do not lie in one plane but are All the D–H–A parameters such as distances, angles and types of
slightly twisted. The Fe–N bond lengths which are of the order interactions are summed up in Table S2 (ESI†). Intramolecular
of 2.1–2.2 Å indicate 1 to be a HS complex.44 The crystal packing pꢀp interactions are observed between the aromatic rings of tos
of 1 is shown in Fig. 2(b). When viewed down the b-axis, the and DMPP molecules. Only a pair of tos and DMPP molecules
molecules seem to arrange themselves in a style as to be on one side of Fe gets involved in intramolecular aromatic
forming steps. In the crystal packing, the Fe(II) ions of two interaction. The angles between the tos ring plane and DMPP
differently oriented molecules within the unit cell are 11.001(7) Å ring plane are found to be 5.32(17)1 (pyridine) and 12.9(19)1
apart, while the distance between the Fe(II) ions of the same (pyrazole). The ring centroid to centroid distances are 3.903(2) Å
oriented molecules is 8.702(6) Å, which is also the shortest and 4.286(2) Å. Since the angle between the tos ring and
among all the Fe(II) ions. Non-classical multiple intra-, and pyridine ring plane is very small, they can be considered as
intermolecular hydrogen bonds are observed between oxygen nearly parallel. This implies that the aromatic interaction
atoms of sulfonate groups and neighbouring hydrogen atoms between the tos ring and pyridine ring is mostly the slipped p
of aromatic rings with C(H)ꢁ ꢁ ꢁO distances in the range of stacking type, whereas between the tos ring and pyrazole ring, it
2.902(5)–3.427(6) Å. All the D–H–A parameters such as dis- is the intermediate of slipped p stacking and edge on C–Hꢁ ꢁ ꢁp
tances, angles and types of interactions are summed up in interaction with slipped p stacking dominating the aromatic
Table S2 (ESI†). Intramolecular p–p interactions are observed interaction.45 The aromatic interaction parameters in 2 are
between the aromatic rings of ps and DMPP molecules. A pair given in Table S3 (ESI†).
of DMPP and ps molecules on each side of Fe gets involved in
3.2.3 Crystal structure of 3. 3 crystallizes in the C2/c space
p–p interactions. We denote these pairs of DMPP and ps as pair-1 group. The asymmetric unit of 3 is shown in Fig. 4(a). It is
and pair-2 (see Fig. S1, ESI†). ps rings are positioned in the middle comprised of an Fe(II) ion as the metal centre coordinated to
of DMPP molecules so as to optimize the p–p repulsions and p–s two DMPP and two nitps molecules. DMPP coordinates to Fe(II)
attractions among the aromatic rings.45 In pair-1, the angles through the nitrogen atoms of pyridine and pyrazole rings whereas
between the ps ring plane and DMPP ring plane are 11.6(2)1 nitps coordinates through the oxygen atoms of sulfonate groups,
(pyridine) and 11.9(2)1 (pyrazole), whereas in pair-2, the angles thus providing a N4O2 coordination environment for the Fe(II)
between the planes are 15.9(2)1 (pyridine) and 14.0(2)1 (pyrazole). metal centres. The mononuclear complex 3 has a distorted octa-
The shortest ring centroid to centroid distances are found to be hedral geometry with all the bond angles [+O46–Fe1–O16 =
3.987(2) Å and 3.928(2) Å in pair-1 and pair-2, respectively. Since 84.92(9)1, +O46–Fe1–N32
= 86.72(9)1, +N32–Fe1–N39 =
the planes of the aromatic rings are not parallel, the aromatic 73.02(9)1, +N9–Fe1–N39 = 93.0(1)1, +N2–Fe1–N9 = 73.6(1)1,
interaction is an intermediate of slipped p stacking and edge on +N2–Fe1–O16 = 87.96(9)1] of the octahedron being off right
C–Hꢁꢁꢁp interaction with slipped p stacking dominating the angle. One of the apical positions of the octahedron is occupied
aromatic interaction. The aromatic interaction parameters in 1 by a pyrazole ring from DMPP and the other one is occupied by
are compiled and given in Table S3 (ESI†).
a sulfonate group from one of the anionic nitps ligands. The
3.2.2 Crystal structure of 2. 2 crystallizes in the P21/n space Fe–N bond lengths are in the order of B2.2 Å, which indicates 3
group. The asymmetric unit is comprised of an Fe(II) ion as the to be a HS compound.44 The anionic ligands nitps are found to
metal centre coordinated to two bidentate DMPP ligand mole- be coordinating to Fe(II) ions in a cis manner with their aromatic
cules and two monodentate tos ligand molecules as shown in rings positioned away from each other. Intra- and intermolecular
Fig. 3(a). DMPP coordinates to Fe(II) through the nitrogen (only non-classical) hydrogen bonds are observed between nitps
atoms of pyridine and pyrazole rings whereas tos coordinates –SO3 and –NO2 oxygen atoms and –CH hydrogen atoms of nitps
through the oxygen atoms of sulfonate groups, thus leading and DMPP (with C(H)ꢁ ꢁ ꢁO distances in the range of 2.852(4)–
to an FeN4O2 coordination environment. The mononuclear 3.328(5) Å). All the D–H–A parameters in 3 such as distances,
complex 2 crystallizes in a distorted octahedral geometry with angles and types of interactions are summed up in Table S2
all bond angles [+O16–Fe1–O46 = 96.2(5)1, +O16–Fe1–N9 = (ESI†). Aromatic interaction is observed in both the pairs of
96.9(5)1, +N2–Fe1–N9 = 73.0(1)1, +N39–Fe1–N2 = 91.1(1)1, DMPP and nitps molecules on each side of Fe(II). We denote
+N32–Fe1–N39 = 73.5(1)1, +N32–Fe1–O46 = 94.7(1)1] of the these pairs as pair-1 and pair-2. The nitps ring is not located in
octahedron being off right angle. One of the apical positions is the middle of DMPP like 1 and 2 rather it is more shifted towards
occupied by a pyridine ring from DMPP and the other one is the pyrazole ring. This shifting of nitps is attributed to the –NO2
occupied by a sulfonate group from one of the anionic tos group which withdraws the p electron density from the phenyl
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Fig. 3 (a) Asymmetric unit of 2, showing thermal displacement ellipsoids
at the 30% probability level and atꢀom labelling scheme of the non C and H
atoms. The disorder of the SO3 group is shown in yellow; (b) crystal
packing, viewed down the b-axis, with Feꢁ ꢁ ꢁFe distances (Å) indicated and
hydrogen atoms omitted for clarity.
Fig. 2 (a) Asymmetric unit of 1, showing thermal displacement ellipsoids
at the 30% probability level and atom labelling scheme of the non C and H
atoms. (b) Crystal packing viewed down the b-axis, with Feꢁ ꢁ ꢁFe distances
(Å) indicated and hydrogen atoms omitted for clarity.
Interestingly, p–p interactions between non coordinated
phenylsulfonate units were also found in the crystal structure
of [Cu(hyptrz)3](4-chloro-3-nitro-phenylsulfonate)2ꢁH2O (hyptrz =
ring and reduces the p–p repulsions45 between the pyrazole and 4-30-hydroxypropyl-1,2,4-triazole).46 Fig. 4(b) also presents unit
phenyl ring, thus allowing the phenyl ring to be positioned over cell packing of 3. The shortest distance between two metal
the pyrazole ring. The pyridine centroid and phenyl centroid in centres is found to be 8.101(7) Å while the longest distance is
pair-1 and pair-2 are 4.497(2) Å and 4.602(2) Å apart, respec- 13.134(1) Å. Most interestingly, 3 represents the first iron complex
tively. The closest carbon to carbon distance is however of the incorporating the nitps anion for which the crystal structure was
order of 3.4 Å, allowing weak slipped p stacking between determined. This discovery is interesting given the outstanding
pyridine and phenyl rings. On the other hand, the pyrazole SCO properties developed by iron complexes including this
centroid and phenyl centroid are 3.807(2) Å and 3.767(2) Å apart anion.29,34,47
in both pairs, respectively. In pair-1, the angles between the
3.2.4 Crystal structure of 4. 4 crystallizes in the P21/n space
nitps ring plane and DMPP rings planes are 1.35(17)1 (pyridine) group. The asymmetric unit of 4 comprises one Fe(II) ion as the
and 7.54(19)1 (pyrazole), whereas in pair-2, the angles between metal centre, two molecules of DMPP, both as bidentate
the planes are 3.96(17)1 (pyridine) and 5.12(18)1 (pyrazole). ligands, two molecules of psca as anionic ligands, and one
Since the planes containing nitps molecules and aromatic rings solvent molecule of Et2O as shown in Fig. 5(a). DMPP is
of DMPP molecules are nearly planar, the aromatic interaction coordinated to Fe(II) through both nitrogen atoms of pyridine
can be considered as slipped p stacking interaction. The dis- and pyrazole whereas psca is coordinated through the oxygen
placement angle, i.e. ring normal (pyrazole) to centroid– atoms of sulfonate groups, thus making an FeN4O2 coordina-
centroid vector angle is found to be 18.81 and 21.21 for pair-1 tion environment. The Fe(II) ions exhibit a distorted octahedral
and pair-2 respectively. Besides intramolecular slipped p stacking, geometry as none of the bond angles [+O11–Fe1–O16 =
an intermolecular aromatic interaction is also observed between 92.57(9)1, +O16–Fe1–N1 = 93.0(1)1, +N1–Fe1–N3 = 74.4(1)1,
the nitps molecules as marked in Fig. 4(b). The planes containing +N3–Fe1–N6 = 85.9(1)1, +N6–Fe1–N4 = 73.8(1)1, +N4–Fe1–
the nitps molecules are at an angle of 15.59(16)1 with the O11 = 91.42(9)] of the octahedron are right angles. One of the
centroids of the ring being 3.871(2) Å apart. These values suggest apical positions of the octahedron is occupied by a pyrazole
that the intermolecular aromatic interaction between nitps mole- ring from DMPP and the other one is occupied by a sulfonate
cules is a combination of slipped p stacking and C–Hꢁꢁꢁp group from one of the anionic psca ligands. The Fe–N bond
interaction.45 All the aromatic interaction parameters in 3 are lengths which are of the order of 2.1–2.2 Å indicate 1 to be a HS
given in Table S3 (ESI†).
complex.44 Interestingly, no Et2O molecules were observed in
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Fig. 4 (a) Asymmetric unit of 3, showing thermal displacement ellipsoids
at the 30% probability level and atom labelling scheme of the non C,
H atoms; (b) crystal packing, viewed down the b-axis, with Feꢁ ꢁ ꢁFe
distances (Å), selected C–Hꢁ ꢁ ꢁO and p–p stacking interactions and
hydrogen atoms omitted for clarity.
Fig. 5 (a) Asymmetric unit of 4, showing thermal displacement ellipsoids
at the 30% probability level and atom labelling scheme of the non C, H
atoms; and (b) crystal packing, viewed down the a-axis, with Feꢁ ꢁ ꢁFe
distances (Å) and hydrogen atoms omitted for clarity.
the elemental analysis of 4, which could presumably be attri-
buted to the high volatility of Et2O due to which it might have
been evaporated taking into account the time between CHN parallel as the angle between their planes is not very large,
analysis and the synthesis itself (ca. 2–3 months). The anionic thus the aromatic interaction in pair-1 can be considered as a
psca ligands are found to be coordinated to Fe(II) ions in a cis slipped p stacking type while the displacement angle (ring
manner with their aromatic rings pointing away in opposite normal to centroid–centroid vector) between the rings is 12.71.
directions. The unit cell of 4 is shown in Fig. 5(b). The shortest In pair-2, the planes of the phenyl ring and pyridine ring are not
distance between the two Fe(II) ions is found to be 10.016(7) Å parallel but lie at an angle of 14.24(16)1. Thus in pair-2 the
whereas the largest one is 17.905(9) Å. In the crystal packing, aromatic interaction is an intermediate type between slipped
classic intermolecular hydrogen bonds are observed between p stacking and C–Hꢁ ꢁ ꢁp interaction with slipped p stacking
psca –SO3 oxygen atoms and psca carboxyl –OH groups (with dominating the interaction.45 The aromatic interaction para-
O(H)ꢁ ꢁ ꢁO distances of 2.729(3) and 2.714(3) Å), linking the meters in 4 are given in Table S3 (ESI†).
complexes together in 1D chains along the [100] direction
The solvent molecule Et2O in the crystal lattice is not
(Fig. 6). An intramolecular aromatic interaction is observed involved in H-bonding, which might be the reason that it
between phenyl and pyridine rings on both sides of Fe. We evaporated so quickly. Besides non-covalent interactions, the
represent them as pair-1 and pair-2. The phenyl ring of psca is [2+2] potential photodimerization of psca molecules was con-
more inclined towards the pyridine ring to minimize the p–p sidered too. The vinyl carbons which are supposed to undergo
repulsions. As a result, the distance between the pyrazole ring [2+2] photocycloaddition are labelled as C28, C27, C36, and C37
of DMPP and the phenyl ring of psca is too large to observe any in Fig. 6. The average distance between the two vinyl bonds is
aromatic interaction. On the other hand, the distances between 4.6 Å which is beyond the maximum allowed distance (4.2 Å)
phenyl centroids and pyridine centroids are 3.539(19) Å and according to the Schmidt’s topochemical postulates.31 Also, the
3.791(2) Å, for pair-1 and pair-2, respectively. The planes of angles +C27–C28–C36 a +C36–C37–C27 infer that the two
pyridine and psca rings are at an angle of 6.03(16)1 in pair-1. vinyl bonds are not parallel to each other (Fig. 6). Hence, psca
The pyridine and psca rings of pair-1 can be treated as nearly molecules in 4 unfortunately do not satisfy the criteria to
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Fig. 6 1-D chains formed by hydrogen bonding along the c-axis in the structure of 4, with p–p interactions indicated.
undergo the [2+2] photodimerization reaction. This situation is is observed. This decrease is associated with zero-field splitting
associated with the coordination of psca which prevents a effects of HS Fe(II) ions, as a result of octahedral distortions.48
suitable photo-responsive arrangement.
As the complexes are paramagnetic, a wꢀ1 vs. T plot was fitted
according to the Curie–Weiss law recalled below in eqn (1).49
3.3 SQUID magnetometry
1
T
C
y
C
¼
ꢀ
(1)
Magnetic susceptibility measurements of 1–4 were recorded
over the temperature range of 300–10 K. Fig. 7 collectively
w
presents the molar magnetic susceptibility, w, with temperature where C and y are the Curie and Weiss constants, respectively.
for all complexes. The room temperature wT values are in The values presented in Table S4 (ESI†) are within errors and
the range of 3.0–3.5 cm3 molꢀ1 K which is typical for HS confirm the absence of any magnetic interactions between Fe(II)
mononuclear Fe(II) complexes. This situation is maintained ions, as expected from the large distances between magnetic
on cooling down to 30 K, after which a slight decrease of wT centres. The same conclusions were drawn for a magnetic study
Fig. 7 wT vs. T, and wꢀ1 vs. T plots of 1–4. The red line represents a Curie–Weiss law fit of the magnetic data.
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Table 1 57Fe Mossbauer parameters of 1–4 at 298 K. Isomer shifts (d) are
given with respect to metallic a-Fe. G/2 = half width of the lines
of coordination compounds having an FeN4O2 coordination
environment with two aromatic sulfonate anions coordinated
to Fe(II) ions.50
¨
d
DEQ
G/2
Fraction
Compound
Sites
(mm sꢀ1) (mm sꢀ1) (mm sꢀ1) (%)
57
¨
Fe Mossbauer spectroscopy
3.4
1
2
Fe(II) HS 1.20(1)
Fe(II) HS 1.09(1)
Fe(III) HS 0.14(2)
Fe(II) HS 1.21(1)
Fe(II) HS 1.20(1)
2.66(1)
2.25(1)
0.51(5)
2.91(1)
2.50(1)
3.47
0.27(1)
0.13(1)
0.16(3)
0.16(5)
0.21(1)
0.27
100
72
28
100
100
100
57
¨
Fe Mossbauer spectra of all coordination complexes are gathered
in Fig. 8. The spectra were fitted to the sum of Lorentzian lines by a
3
4
¨
least-squares refinement using Recoil 1.05 Mossbauer Analysis
Software (Table 1).51 A red doublet is detected in each spectrum, [Fe(pyridine)4(tos)2]53 Fe(II) HS 1.08
centred around an isomer shift of d = 1.1–1.2 mm sꢀ1 with a
quadrupole splitting in the range DEQ = 2–3 mm sꢀ1, which is
¨
¨
characteristic of Fe(II) HS ions. Mossbauer spectra confirm that quadrupole splitting in Mossbauer spectra. Indeed, for ferrous
complexes 1, 3, and 4 exist as whole Fe(II) HS compounds which is compounds, the higher the distortion degree, the lower the
well supported by susceptibility measurements and single crystal quadrupole splitting. Indeed, this situation results from the
X-ray diffraction. However, an additional yellow doublet was presence of a lattice contribution to the electric field gradient
observed in the spectrum of 2 centred at d = 0.14 mm sꢀ1 with a which is negative compared to the valence contribution.52 If we
quadrupole splitting of DEQ = 0.51 mm sꢀ1 which corresponds to compare the quadrupole splitting of Fe(II) complexes in Table 1, we
HS Fe(III) ions.28 The absence of any Fe(III) HS in SQUID measure- find that 2 has the least value of DEQ suggesting that 2 has the
ment leads to the conclusion that 2 was synthesized as an Fe(II) HS most distorted octahedral geometry. The distortion is the conse-
compound but got partially oxidized to Fe(III) over a period of time quence of the appearance of oxidized iron species, which are
¨
needed to record the Mossbauer spectrum, which was performed located nearby Fe(II) ions.
¨
No Mossbauer data are available for comparison on Fe(II)
under ambient air conditions. It is intriguing that only 2 got
partially oxidized with time and not the other complexes. The coordination complexes with two bidentate diimine ligands
reason may be attributed to the degree of distortion of Fe(II) and two aromatic sulfonate ligands. [Fe(pyridine)4(tos)2] is
octahedral geometries. This degree can be probed through the however the only closest coordination complex akin to our
Fig. 8 Room temperature 57Fe Mossbauer spectra of 1–4.
¨
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synthesized complexes holding an FeN4O2 coordination sphere References
and two aromatic sulfonate ligands coordinated to Fe(II).53 The
isomer shift is found to be of the same order but a significant
difference is observed in quadrupole splitting (Table 1).
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Four novel mononuclear Fe(II) coordination complexes
including phenylsulfonate ligands were successfully synthe-
sized. Single crystals could be isolated by vapour diffusing
diethyl ether into the crystallization mixture, though this
solvent was only observed as occluded species in the crystal
structure of 4. This solvent interfered with the self-assembly of
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complexes53–57 All synthesized complexes display an FeN4O2
core, which has been found in several mononuclear spin cross-
over complexes.58 Nevertheless, all of them display a perma-
nent high-spin state, because the sulfonate coordination
prevents any thermally induced spin crossover behaviour from
occurring. The fact that we have obtained the same coordina-
tion environment for all the complexes suggests that the
synthetic approach deployed here is reliable and could be
used to design new heteroleptic coordination complexes, in
the near future.
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There are no conflicts to declare.
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through the grant No. PN-III-P4-ID-PCCF2016-0175. This work
was supported by the WBI-COP22 Morocco project as well as
the PPR2-MESRSFC-CNRST-P10 project (Morocco) and FNRS
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