The vital role of ditopic: N - N bridging ligands with different lengths in the formation of new binuclear dioxomolybdenum(vi) complexes: Synthesis, crystal structures, supramolecular framework and protein binding studies

Article abstract of DOI:10.1039/d0nj03702h

A new series of binuclear dioxomolybdenum(vi) complexes 1-4 of general formula [(MoO2L)2(N-N)] with an ONS donor Schiff base ligand (H2L = S-benzyl-β-N-(5-bromo-2-hydroxyphenyl)methylenedithiocarbazate) and bridging auxiliary ligands having different leng

Full text of DOI:10.1039/d0nj03702h

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The vital role of ditopic N–N bridging ligands
with different lengths in the formation of new
binuclear dioxomolybdenum(VI) complexes:
synthesis, crystal structures, supramolecular
framework and protein binding studies†
a
Cite this:DOI: 10.1039/d0nj03702h
c
Debanjana Biswal,
Malini Roy,a Nikhil Ranjan Pramanik,*b Suvendu Paul,
Michael G. B. Drewd and Syamal Chakrabarti
*
a
A new series of binuclear dioxomolybdenum(VI) complexes 1–4 of general formula [(MoO2L)2(N–N)] with an
ONS donor Schiff base ligand (H2L = S-benzyl-b-N-(5-bromo-2-hydroxyphenyl)methylenedithiocarbazate)
and bridging auxiliary ligands having different lengths and flexibilities [N–N donor 4,40-bipyridine (4,40-bipy),
1,2-bis(4-pyridyl)ethene (bpe), 1,3-bis(4-pyridyl)propane (tmp) and 1,4-bis[(1H-imidazole-1-yl)methyl]benzene
(bix)] have been synthesized. The complexes have been thoroughly characterized by elemental analyses, IR,
1H NMR and UV-Vis spectroscopy, cyclic voltammetry and thermal analyses. The molecular structures of all
four complexes 1–4 have been determined by single-crystal X-ray diffraction techniques. Crystal structures of
binuclear molybdenum complexes having bridging 1,3-bis(4-pyridyl)propane and 1,4-bis[(1H-imidazole-1-yl)-
methyl]benzene ligands are reported for the first time. Each molybdenum site in the binuclear complexes
adopts a distorted octahedral geometry. The auxiliary spacers (N–N) act as bis-monodentate ligands occupy-
ing one axial position of each molybdenum centre thereby connecting two molybdenum(VI) atoms. The influ-
ence of auxiliary ligands on structural and supramolecular features of the complexes has been studied.
Hirshfeld surface and fingerprint plots uncover the central role of different intermolecular interactions and
their comparative dimension to build up the crystal architectures of the molybdenum complexes. Supportive
DFT calculations regarding non-covalent interactions and molecular orbitals have been carried out to support
the experimental data. Apart from these interesting structural features, the effects of binding of the ligand
H2L and complexes 1–4 with bovine serum albumin (BSA) have been explored using absorption and
steady-state fluorescence titration measurements. Molecular docking studies have also been carried out to
further deepen the understanding of interaction patterns and binding modes of the ligand and binuclear
dioxomolybdenum(VI) complexes with BSA.
Received 22nd July 2020,
Accepted 14th September 2020
DOI: 10.1039/d0nj03702h
rsc.li/njc
more environment friendly compared to other transition metals.1
Characteristic features of molybdenum are its ability to form
Introduction
Coordination chemistry of molybdenum is one of the most oxo-anions and a wide range of oxidation states (ꢀ2 to +6)2 of
fascinating areas of chemical research and will continue to be relatively similar stability. The important oxidation states of
so in the years to come. Molybdenum is much less toxic and molybdenum are +IV, +V and +VI since biological systems work
a Department of Chemistry, University College of Science, 92, Acharya Prafulla Chandra Road, Kolkata 700009, West Bengal, India. E-mail: schakrabarti2014@gmail.com;
Fax: +91-033-2337-4782, +91-033-2351-9755; Tel: +91-033-2350-8386, +91-033-2337-4389
b Department of Chemistry, Bidhannagar College, EB-2, Sector1, Salt Lake, Kolkata 700064, India. E-mail: nr_pramanik@yahoo.co.in
c Department of Chemistry, University of Kalyani, Kalyani, Nadia, West Bengal 741235, India
d Department of Chemistry, The University of Reading, Whiteknights, Reading RG6 6AD, UK
† Electronic supplementary information (ESI) available: Supplementary figures S1 and S2 shows IR and UV spectra of the complexes 1–4 respectively. Fig. S3–S8 contain
the CV diagrams, Hirshfeld surface plots, frontier orbitals, TG-DT curves, FRET and molecular docking images of the complexes. Tables S1–S4 contains fingerprint
plots of the complexes. Table S5 presents Hirshfeld surface volume and surface area of the complexes 1–4. The FRET and molecular docking parameters are given in
Tables S6 and S7 respectively. CCDC 1974269–1974272 contain the supplementary crystallographic data for the complexes 1–4, respectively. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/d0nj03702h
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within the range of the hydrogen and oxygen electrodes, a span
Serum protein is an important blood plasma protein mainly
of about 1.4 volts. Other oxidation states are not stable in this involved in transportation of compounds which are otherwise
range and are short-lived intermediates in catalysis reactions. insoluble in plasma.18 Consistent with this biological role,
Coordination chemists have been fascinated by the bioinorganic albumin has a high affinity for a variety of compounds. Bovine
chemistry of molybdenum which has lead to extensive research. serum albumin (BSA) shares a high degree of homology with
Molybdenum participates in a number of enzymatic reactions.3 human serum albumin (HSA) and was considered to have a
Particularly, the modeling of the active centre of molybdo- structure similar to that of HSA, with only minor differences.
enzymes through innovative efforts in inorganic synthesis is a The capability of BSA to reversibly bind with drug molecules
very interesting area of research.
has led to the development of this fascinating research area of
Schiff base complexes of transition metal ions are of great protein interactions. Some Schiff bases and their corresponding
interest because of the multitude of industrial, biological and metal complexes have shown good protein binding and anti-
pharmaceutical applications.4,5 Transition metal complexes of tumor activities against animal tumors.19,20 The nature of the
hydrazones, semicarbazones, and thiosemicarbazones6 attract metal21 and ligand22 is expected to play an important role in the
special attention due to their important biological applications, binding of metal complexes to biomolecules such as proteins.
such as antibacterial, antioxidant, antiproliferative, antimalarial, Structure activity relationship (SAR) of binuclear complexes with
antitrypanosomal, anticonvulsant and antitumor, etc.7
bridging ligands of different lengths and flexibility has been
The importance of weak non-covalent interactions in crystal investigated for their biological activities.23 Binuclear complexes
engineering is highlighted in several studies.8 Hirshfeld surface are found to have many advantages in comparison with mono-
analysis is an important approach for the analysis of inter- nuclear complexes.24 Rajkovica et al. has recently reported the
´
molecular interactions and packing in crystals. Wang et al.9 BSA-binding studies of dinuclear palladium(II) complexes with
admirably utilized the Hirshfeld surface plots to depict the role different pyridine-based bridging ligands.25 The study showed
of various intermolecular interactions underneath the formation that the nature of pyridine-based bridging ligand in the dinuclear
of polymorphs of trinuclear Cu(I) pyrazolate complex. Recently, palladium(II) complexes has an influence on the BSA binding
Tao et al.10 employed this tool to explore the role of different affinity. The complexes with flexible –CH2–CH2– group in the
intermolecular interactions to facilitate the formation of crystals. bridging ligands exhibited lower binding affinity whereas rigid
Hydrogen bonding plays a significant role in construction of –CHQCH– group in bridging ligands caused planarity and lower
supramolecular assemblies with differing dimensional nature flexibility of the complexes leading to greater binding affinity. Size
and strength which have garnered widespread interest due to of the co-ligands are also found to influence biological activities of
their potential applications in various fields such as selective gas- transition metal complexes.26
adsorption,11 separation,12 drug delivery13 and catalysis.14 Supra-
Considering the possible future directions which may expand
molecular frameworks which involve organic bridging ligands,15 the scope of molybdenum chemistry, studies of its supra-
acting as spacers, have provided an interesting area of research. molecular chemistry and protein interactions are discussed in
Structurally diverse supramolecular lanthanide coordination this present work. Herein we report the synthesis of binuclear
polymers with flexible glutaric acid as a linker have been reported dioxomolybdenum(VI) complexes 1–4 of general formula
by Sheikh et al.16 Neutral O/N-donor ligands are often used as [(MoO2L)2(N–N)] with an ONS donor Schiff base ligand (H2L =
auxiliary ligands due to their affinities to transition metal ions S-benzyl-b-N-(5-bromo-2-hydroxyphenyl)methylenedithiocarbazate)
and their conformations as pillar-like connectors.17 Bis-(pyridyl) and auxiliary bridging ligands [N–N donor 4,40-bipy, 1,2-bis(4-py)-
and bis-(imidazole) molecules have specific coordination directivity ethane (bpe), 1,3-bis(4-py)propane (tmp) and 1,4-bis(1H-imidazole-
and hence can function as an excellent class of auxiliary bridging 1-ylmethyl)benzene (bix)]. 1,4-Bis(1H-imidazole-1-ylmethyl)benzene
ligands. In some cases, the framework may generate spacious (bix) in combination with molybdenum is perhaps the first report.
cavities, voids and channels. The crystal structures have the normal All the synthesized complexes have been characterized by IR,
tendency to maximize the packing density. As a result, the voids 1H NMR and UV-Vis spectroscopy. Their electrochemical behavior
are usually filled by guest solvent molecules or self-folding which has been investigated by cyclic voltammetric studies. Numerous
is a common phenomenon observed in porous supramolecular mononuclear dioxomolybdenum(VI) complexes have been reported
frameworks.
but the crystal structures of only a few binuclear Mo(VI) complexes
Design of protein targeting compounds is of significant interest with ONS donor ligands have been determined so far.27–30
because specific proteins are an important biological target for Fortunately, we succeeded in obtaining the crystal structures
medicinal drugs. Drug–protein interactions play an important role of all the four binuclear complexes by single-crystal X-ray
in the distribution and transport of drug molecules in biological diffraction. This is the first report of crystal structures of
systems. Understanding the molecular basis of these interactions binuclear molybdenum complexes having bridging 1,3-bis(4-py)-
is important in the rational design of new and more efficient propane (tmp) and 1,4-bis(1H-imidazole-1-ylmethyl)benzene
therapeutic agents that can recognize and bind to specific biological (bix) ligands. Although molybdenum complexes are of great
targets for improving drug activity.18 Interactions at the molecular importance, few articles portray the potential of molybdenum
level are often monitored using spectroscopic techniques. complexes in crystal engineering.7,31 The interplay between
Absorption and fluorescence spectroscopy are valuable techniques hydrogen bonding, halogen bonding, C–Hꢁ ꢁ ꢁp and pꢁ ꢁꢁp stacking
to study the interaction of compounds with proteins.
interactions is studied. A very convenient technique, Hirshfeld
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surface analysis, has been exploited to understand the role of 1,3-bis(4-pyridine)propane] were commercially purchased. 1,4-Bis-
different intermolecular interactions in the construction of crystal [(1H-imidazol-1-yl)methyl]benzene was prepared by the following
architectures of the complexes 1–4. In addition, the thermal method:
analyses and framework stabilities of the complexes have been
investigated. Supportive DFT calculations on the complexes have
been carried out in order to obtain deeper insight into the non-
covalent interactions and electronic environment around the
1,4-Bis[(1H-imidazol-1-yl)methyl]benzene. The compound was
prepared by refluxing a solution of 3.40 g (50 mmol) of
imidazole and 4.37 g (25 mmol) of a,a0-dichloro-p-xylene in 50 mL
of methanol for 18 hours. A colorless solution was obtained which
molybdenum centres. Absorption and steady-state fluorescence
on evaporation resulted in a yellow syrup. The yellow syrup obtained
titration studies have been employed to determine the Stern–
was dissolved in 100 mL of aqueous K2CO3 (6.91 g, 50 mmol)
Volmer constants, binding modes and binding constants of the
ligand and complexes 1–4 with BSA. Molecular docking studies are
also carried out to decipher the binding mechanism.
solution.32 The solution on standing overnight yielded white
crystals of 1,4-bis(imz-1-ylmethyl)benzene (Scheme 2). The crystals
were then filtered off and dried in vacuo over anhydrous CaCl2,
Yield B 78%, M.P. 132 1C.
Scheme 3 shows the distance (calculated from experimental
single-crystal X-ray diffraction study) between the N–N donor
atoms of the auxiliary bridging ligands (N–N).
Experimental section
Materials
All reagents and chemicals were procured from commercial sources
(Sigma Aldrich, USA) and used without further purification.
Synthesis of the binuclear cis-dioxomolybdenum(VI) complexes
[(MoO2L)2(N–N)] (1–4). Initially, the mononuclear [MoO2L] complex
was prepared by refluxing [MoO2(acac)2] and the ligand (H2L) in
chloroform for 2 h (Scheme 4).
Syntheses
Synthesis of S-benzyl-b-N-(5-bromo-2-hydroxyphenyl)methylene-
The binuclear complexes (1–4) were prepared by refluxing a
dithiocarbazate (H2L). The ONS donor Schiff base ligand H2L was solution of the mononuclear [MoO2L] complex and the respective
prepared by a previously reported method7b (Scheme 1). The ligand auxiliary bridging ligand N–N [4,40-bipy, bpe, tmp and bix] in 2 : 1
was satisfactorily characterized by elemental analyses, mass spectro- molar proportion in dry CHCl3 (20 mL) for 3 h (Scheme 5). The
metry, IR and 1H NMR spectroscopy.
orange solutions were filtered and the filtrates were kept at low
Synthesis of the auxiliary bridging ligands (N–N). The auxiliary temperature. Shiny orange single crystals of the complexes 1–4
bridging ligands [4,40-bipyridine, 1,2-bis(4-pyridine)ethene and suitable for X-ray diffraction analysis were obtained after 1–2 days.
Scheme 1 Reaction diagram for isolation of the ligand (H2L).
Scheme 2 Reaction diagram for isolation of the auxiliary bridging ligand 1,4-bis[(1H-imidazol-1-yl)methyl]benzene (bix).
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1
(7420); H NMR (CDCl3, d/ppm):
8.74 s (2H), (aromatic
protons) 6.85–7.60 m (24H), (–S–CH2–) 4.38 s (4H), (–CH2–CH2–CH2–)
2.61–2.66 t (6H).
[(MoO2L)2(bix)] (4). Yield B 70%; Anal. calcd for C44H36
S4N8O6Br2Mo2 (%): C, 42.14; H, 2.87; N, 8.94; Mo, 15.32, found:
C, 42.24; H, 2.74; N, 9.01; Mo, 15.27; IR (KBr Pellet), cmꢀ1
-
:
n(CQN) 1594 (vs), n(MoQO) 930 (vs), 899 (vs), n(Mo–N) 657 (m),
n
(Mo–S) 437 (m); UV-Vis (CH2Cl2) [lmax/nm (e/dm3 molꢀ1 cmꢀ1)]:
1
409 (3265), 324 (11626); H NMR (CDCl3, d/ppm):
8.74 s (2H), (aromatic protons) 6.90–7.58 m (25H),
(–S–CH2–) 4.42 s (4H), (–CH2–) 5.02 s (4H).
Physical measurements
Elemental analyses were performed on a PerkinElmer 240 C, H,
N analyzer. IR spectra were recorded as KBr pellets on a
PerkinElmer model 883 infrared spectrophotometer. Electronic
spectra were recorded using a HITACHI U-3501 UV-Vis recording
spectrophotometer. Electrochemical data were collected on a
Sycopel model AEW2 1820 F/S instrument at 298 K using a Pt
working electrode, Pt auxiliary electrode and SCE reference elec-
trode. Cyclic voltammograms were recorded in DMF containing
0.1 M TBAP as supporting electrolyte. PXRD data were recorded
using powder X-ray diffractometer (XRD; X’Pert PRO, PANalytical)
with Cu-Ka radiation in the range of 51 o 2y o 501. Thermal
analyses were carried out in a NETZSCH STA 449 F3 Jupiter thermal
Analyzer in a dynamic atmosphere of N2 (flow rate = 30 cm3 minꢀ1).
Scheme 3 N–N donor auxiliary ligands showing the distance between
the donor atoms.
[(MoO2L)2(4,40-bipy)]ꢁ2CHCl3 (1). Yield B 75%; Anal. calcd
for C42H32S4N6O6Br2Cl6Mo2 (%): C, 35.76; H, 2.27; N, 5.96;
Mo, 13.62, found: C, 35.87; H, 2.24; N, 5.98; Mo, 13.70; IR (KBr
Pellet), cmꢀ1: n(CQN) 1594 (vs), n(MoQO) 931 (vs), 900 (vs), n(Mo–N)
624 (m), n(Mo–S) 440 (m); UV-Vis (CH2Cl2) [lmax/nm
1
(e/dm3 molꢀ1 cmꢀ1)]: 407 (1500), 318 (3950); H NMR (CDCl3,
Crystallographic measurements
d/ppm): 8.82 s (2H), (aromatic protons) 6.93–7.66 m (24H),
(–S–CH2–) 4.40 s (4H).
The crystallographic data for complexes 1, 3 and 4 were
collected on a Bruker APEX-II diffractometer with CCD-area
detector and for complex 2 were collected using the Oxford
Diffraction X-Calibur CCD System. All data sets were obtained
with graphite-monochromated Mo-Ka (l = 0.71073 Å) radiation.
Unit-cell dimensions and intensity data for 1, 3 and 4 were
measured at 296(2) K and for 2 at 150(2) K.
For 1, 3 and 4 the crystals were positioned at 60 mm from
the CCD and 360 frames were measured with counting times of
10 s. For 2, the crystal was positioned at 50 mm from the CCD
and 321 frames were measured with counting times of 10 s. Data
[(MoO2L)2(bpe)]ꢁ2CHCl3 (2). Yield B 72%; Anal. calcd for
C44H34S4N6O6Br2Cl6Mo2 (%): C, 36.81; H, 2.30; N, 5.85; Mo, 13.38,
found: C, 36.47; H, 2.28; N, 5.72; Mo, 13.36; IR (KBr Pellet), cmꢀ1
:
n
(CQN) 1592 (vs), n(MoQO) 925 (vs), 900 (vs), n(Mo–N) 658 (m), n(Mo–S)
433 (m); UV-Vis (CH2Cl2) [lmax/nm (e/dm3 molꢀ1 cmꢀ1)]: 406
(1660), 291 (6330); 1H NMR (CDCl3, d/ppm): 8.73 s (2H), (aromatic
protons) 6.97–7.99 m (24H), (–S–CH2–) 4.42 s (4H), (–CHQCH–)
7.27 s (2H).
[(MoO2L)2(tmp)] (3). Yield B 72%; Anal. calcd for C44H36- analyses for 1, 3 and 4 were carried out with the SAINTprogram33
S4N6O6Br2Cl5Mo2 (%): C, 37.66; H, 2.56; N, 5.99; Mo, 13.69, found: and for 2 with CrysAlis.34 The crystal structures were solved by
C, 37.54; H, 2.49; N, 6.05; Mo, 13.48; IR (KBr Pellet), cmꢀ1: n(CQN) direct methods using SHELXS-9735 and refined by full-matrix
1592 (vs), n(MoQO) 928 (vs), 902 (vs), n(Mo–N) 656 (m), n(Mo–S) 436 (m); least-squares based on F2 with anisotropic displacement para-
UV-Vis (CH2Cl2) [lmax/nm (e/dm3 molꢀ1 cmꢀ1)]: 412 (2380), 323 meters of non-hydrogen atoms using SHELXL-16/6.36
Scheme 4 Reaction diagram for synthesis of the mononuclear [MoO2L] complex.
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Scheme 5 Reaction diagram for the synthesis of binuclear [(MoO2L)2(N–N)] complexes.
The non-hydrogen atoms were refined with anisotropic surrounding the asymmetric unit of the crystal is the authority to
thermal parameters. The hydrogen atoms bonded to carbon calculate the distances. The distances, de and di is defined as the
were included in geometric positions and given thermal para- closest distance from the point on the isosurface to the nearest
meters equivalent to 1.2 times those of the atom to which they nucleus of atom outside and inside the surface respectively.43,45 In
were attached. In complex 2 the solvent CHCl3 is disordered. addition, the normalised contact distance (dnorm) is well defined by
Two chlorines are fixed; the other is disordered over two sites as the following equation:
is the carbon atom. Refinement with C–Cl distance constraints
di ꢀ rvi dW de ꢀ rvedW
was carried out with occupation factors of x and 1ꢀx, x refining
to 0.55(2). In complex 3 it was not possible to successfully refine
a solvent model for the residual electron density and therefore
the SQUEEZE program in PLATON was used.37 Details of the
crystallographic data are summarized in Table 2 with dimensions
in the metal coordination spheres compared in Tables 3 and 4. All
four structures show imposed crystallographic symmetry, a centre
of symmetry for complexes 1, 2 and 4 and a two-fold axis for
complex 3. Absorption corrections for 1, 3 and 4 were carried
out with SADABS38 and for 2 with ABSPACK.39 The MERCURY40
and DIAMOND41 programs are used for the presentation of the
structures.
dnorm
¼
þ
rvi dW
rvedW
where, rvidw and rvedw are the van der Waals radii of the atoms.
The 3D surface plot is mapped with dnorm that generates the red,
white and blue colour codes. The red, white and blue colours
designate distances (dnorm) shorter, equal and greater than the
sum of the van der Waals radius, respectively.
DFT calculations
Single point calculations were carried out using the Gaussian
03 program.46 Geometry optimizations used the B3LYP density
functional with basis sets LANL2DZ for Mo, Br, 6-31G for
C,H,N,O and 6-31+G* for S. Starting models for optimizations
were taken from the crystal structures.
Methodology of Hirshfeld surface analysis
Crystal Explorer 17.542 was thoroughly employed to generate 3D
Hirshfeld surfaces43 and 2D fingerprint plots44 from single-
crystal XRD structures of complexes to portray unambiguous insight
Protein binding studies
Absorption study of compounds with BSA protein. Absorption
of intermolecular interactions within the crystal architecture. It is spectra were recorded using a HITACHI U-3501 UV-Vis spectro-
well accepted that Hirshfeld surface analysis is a convenient and photometer. BSA was purchased from Sigma-Aldrich. Stock
user-friendly tool to explore the intermolecular interactions within solution of BSA (10ꢀ5 M) was prepared in phosphate buffer
and between different asymmetric units of a crystal.42–45 The surface (pH = 7.0) and stored in the dark at 4 1C for further use.
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The stock solution of the ligand (H2L) and binuclear dioxomolyb- complex and the respective auxiliary bridging ligand N–N [4,40-
denum(VI) complexes 1–4 were prepared by dissolving them in bipy, 1,2-bis(4-py)ethene, 1,3-bis(4-py)propane and 1,4-bis(imz-
EtOH for required concentrations (10ꢀ4 M). 2.0 mL of BSA 1-ylmethyl)benzene] in 2 : 1 molar proportion using dry CHCl3
solution (10ꢀ5 M) was titrated by successive additions of the as the solvent (Scheme 5). All the orange crystalline binuclear
compounds (0–20 mL) using a micropipette at room temperature. dioxomolybdenum(VI) complexes 1–4 are air stable in the solid
Each solution was made thoroughly homogeneous before record- state and readily soluble in alcohol, CH2Cl2, CH3CN, DMF and
ing the spectra at room temperature.
DMSO but are insoluble in water. The complexes are diamagnetic
at room temperature as is expected for d0 Mo(VI) centres.52 Molar
conductivity data in 10ꢀ3 M CH2Cl2 solution indicate that they are
non-electrolytes. All the complexes were satisfactorily characterized
by elemental analyses, IR, 1H NMR, electronic spectra, cyclic
voltammetry and single-crystal X-ray diffraction.
Fluorescence quenching study of compounds with BSA protein
Emission spectra were recorded on a PerkinElmer LS55 Fluores-
cence Spectrometer. The binding of the ligand (H2L) and binuclear
dioxomolybdenum(VI) complexes 1–4 with bovine serum albumin
(BSA) was studied using fluorescence quenching experiment
recorded in the range 290–500 nm at a fixed excitation wavelength
corresponding to bovine serum albumin (BSA) at 295 nm while the
emission at 358 nm at room temperature. The excitation and
emission slit widths and scan rates were constantly maintained for
all the experiments. Samples were carefully degassed using pure
nitrogen gas for 20 min by using quartz cells with high vacuum
Teflon stopcocks. 2.0 mL of BSA solution (10ꢀ5 M) was titrated by
successive additions of 10ꢀ4 M complexes (0–20 mL).47 Each
solution was made thoroughly homogeneous before recording
the spectra at room temperature. In order to determine the
quenching property, the fluorescence decay data were analyzed
via the Stern–Volmer equation:48
IR spectra
Characteristic IR bands of the binuclear dioxomolybdenum(VI)
complexes 1–4 are given in the Experimental section and the
spectra are presented in Fig. S1 (ESI†). The strong n(CQN)
band53,54 at 1615 cmꢀ1 of the free ligand (H2L) is red shifted
to the region 1594–1592 cmꢀ1 in the complexes indicating the
coordination of azomethine nitrogen to the metal ion. The
strong n(CQS) band observed at1324 cmꢀ1 region for the ligand,
which disappeared on complex formation indicated the coordination
of thioenolate sulfur to the [MoO2]2+ centre. Bands around 658–
55
624 cmꢀ1 for the complexes are assigned to n(Mo–N)
.
Medium
intensity bands for the complexes around 440–433 cmꢀ1 are assigned
to n(Mo–S).
56 The complexes exhibit two very strong n(MoQO) bands in
the region 931–899 cmꢀ1, characteristic of the anti-symmetric and
F0/F = 1 + KSV [Q]
symmetric stretching vibrations of the cis-[MoO2]2+ moiety.57,58
where, F0 and F are the steady-state fluorescence intensities of
BSA in the absence and the presence of ligand or complex,
respectively. KSV is the Stern–Volmer quenching constant and
[Q] is the concentration of quencher, i.e., ligand or complex.
The binding constants (Kb) and number of binding sites (n)
were calculated using the following equation:49
1H NMR spectra
1H NMR data for the binuclear dioxomolybdenum(VI) complexes
1–4 in CDCl3 are summarized in the Experimental section.
The relatively broad signals at d 11.28 ppm and d 10.25 ppm
1
in the H NMR spectra corresponding to O–H and N–H of the
log [(F0 – F)/F] = log Kb+ n log [Q]
ligand H2L are found to disappear in the complexes 1–4 as a
result of complexation.
where, Kb is the binding constant and n is the number of
The signals at d 7.91 ppm of the ligand H2L have shifted
downfield to the region d 8.73–8.82 ppm in the complexes
indicating coordination from the azomethine nitrogen in the
complexes. This feature is indicative of a decrease in the electron
density caused by electron withdrawal by the molybdenum.
The methylene protons of the ligand H2L in the range d
4.60 ppm remain practically unaffected in the complexes
(d 4.38–4.42 ppm) indicating the non-involvement of the S-benzyl
sulfur in coordination. In the complexes, aromatic protons appear
as multiplets within the range d 6.85–7.99 ppm.
In complexes 2 and 4 the two ethene (–CHQCH–) and four
methylene (–CH2–) protons exhibit their chemical shifts at 7.27
and 5.02 ppm respectively. In complex 3, a triplet in the region
d 2.61–2.66 ppm is observed for the six protons corresponding to
the propane (–CH2–CH2–CH2–) moiety of the auxiliary bridging
ligand 1,3-bis(4-pyridine)propane.
binding sites per albumin.
Molecular docking
The 3D structures of binuclear dioxomolybdenum(VI) com-
plexes 1–4 were generated using the CIFs of the X-ray crystal
structures. The CIFs were converted to the PDB format using
MERCURY.40 Docking was performed by the PatchDock
molecular-docking program50 using the PDB files of complexes
1–4 against BSA protein (obtained from the protein data bank;
PDB id: 3V03). The data obtained from docking were visualized
using PyMOL.51
Results and discussion
Synthesis
Initially, the mononuclear [MoO2L] complex was prepared by
refluxing [MoO2(acac)2] and the ligand (H2L) in dry chloroform.
Electronic spectra
The binuclear complexes 1–4 of general formula [(MoO2L)2(N–N)] Electronic spectra of the binuclear dioxomolybdenum(VI) com-
were prepared by the reaction of the mononuclear [MoO2L] plexes 1–4 were recorded in dry CH2Cl2. The spectral data are
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listed in the Experimental section and the spectra are shown in
Electrochemical behaviour of all the synthesized complexes
Fig. S2 (ESI†). All the complexes display the lowest energy reveal that the reduction of binuclear [(MoO2L)2(N–N)] com-
absorption maxima in the 412–406 nm range which are plexes (1–4) in aprotic solvents is generally irreversible.
assigned59 to S(pp) - Mo(dp) LMCT transition involving the
Description of the crystal structures of binuclear complexes 1–4
promotion of an electron from the filled HOMO of the ligand to
the empty LUMO of molybdenum. Other LMCT bands which
may be assigned to the nitrogen to molybdenum and oxygen to
molybdenum charge transfer transitions are observed in the
324–291 nm range.59–62
The molecular structures and the atom numbering schemes of
the binuclear dioxomolybdenum(VI) complexes 1–4 are shown
in Fig. 2–5. Crystallographic data, relevant bond distances and
bond angles are listed in Tables 2–4. The crystal data reveals
that the orange coloured complexes 1 and 3 crystallized in
the monoclinic space groups P21/c and P2/c, respectively and
Electrochemistry
Cyclic voltammograms of the binuclear dioxomolybdenum(VI)
complexes 1–4 at a platinum electrode were recorded in dry
degassed DMF containing 0.1 M TBAP as the supporting
electrolyte over a potential range of 0.0 to ꢂ1.5 V. Cyclic
voltammetric results of all the binuclear dioxomolybdenum(VI)
complexes are given in Table 1 and the cyclic voltammograms
are shown in Fig. 1 and Fig. S3 (ESI†).
Complexes 1–4 show two irreversible one electron two step
reductions in the potential range ꢀ0.45 to ꢀ0.52 V and ꢀ1.07 to
ꢀ1.14 V corresponding to MoVI–MoVI/MoV–MoV and MoV–MoV/
MoIV–MoIV couples.27,63,64 On scan reversal, one irreversible
two-electron one step oxidative response is located in the +0.85
to +0.94 V range which corresponds to a MoIV–MoIV/MoVI–MoVI
couple for complexes 1–4.
%
complexes 2 and 4 crystallized in the triclinic space group P1.
The binuclear complexes 1–4 contain crystallographic centres of
inversion at the middle of the auxiliary bridging ligands [4,40-bipy,
bpe, tmp and bix]. In the crystal structures of complexes 1 and 2,
one solvent chloroform molecule is found in the asymmetric unit
but in 2 it is disordered with two overlapping molecules given
population parameters x and 1 ꢀ x with x refined to values close to
0.5. It is likely that complex 3 also contains a disordered chloroform
Table 1 Cyclic voltammetric resultsa (V vs. SCE) for the binuclear com-
plexes 1–4 at 298 K
Potential (V)
Complex
Epc
Epa
1
2
3
4
ꢀ0.52, ꢀ1.07
ꢀ0.49, ꢀ1.12
ꢀ0.51, ꢀ1.14
ꢀ0.45, ꢀ1.12
+0.85
+0.94
+0.86
+0.92
a
Fig. 2 Centrosymmetric structure of complex [(MoO2L)2(4,40-bipy)](1)
(thermal displacement ellipsoids drawn at 50% probability).
Solvent: DMF (dry, degassed); supporting electrolyte: 0.1 M TBAP;
solution strength: 10ꢀ3 M; working electrode: platinum; reference
electrode: SCE; scan rate: 100 mV sꢀ1
.
Fig. 1 Cyclic voltammogram of complex [(MoO2L)2(bipy)] (1) in DMF at
Fig. 3 Centrosymmetric structure of complex [(MoO2L)2(bpe)] (2) (thermal
298 K.
displacement ellipsoids drawn at 50% probability).
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density. In all the complexes each Mo(1) is bonded to two oxo-
oxygens O(2) and O(3), phenolate oxygen O(1), thioenolate sulfur
S(1), azomethine nitrogen N(1) of the ligand and N(3) of the auxiliary
bridging ligand giving rise to a distorted octahedral coordination
environment around the molybdenum centre (Fig. 6). The ligand is
active in its enolate form and behaves in a dianionic tridentate
manner. Three donor atoms O(1), N(1), S(1) of the ligand and one
terminal oxygen O(3) occupy the equatorial plane.60 The dianionic
tridentate ligand forms one five-membered and another six-
membered chelating rings around the [MoO2]2+ centre with bite
angles of N(1)–Mo(1)–S(1) in the range 75.8(1)1–76.8(1)1 and
O(1)–Mo(1)–N(1) in the range 81.5(2)1–82.8(2)1. The oxo-oxygen
O(2) occupies an axial position in the coordination sphere of
Mo(1). The other axial position is occupied by N(3) of the auxiliary
spacer ligand. In the complexes 1–4, the molybdenum(VI) centre
is displaced by 0.328, 0.325, 0.327 and 0.287 Å respectively from
the median plane described by O(1), O(3), S(1) and N(1) in the
direction of the terminal oxygen O(3).
Fig. 4 Molecular structure of complex [(MoO2L)2(tmp)] (3) which has
crystallographic C2 symmetry (thermal displacement ellipsoids drawn at
50% probability).
Due to the trans influence of O(2), the Mo(1)–N(3) bond
distances [2.374(3)–2.480(5) Å] are considerably longer than the
other Mo(1)–N(1) bonds [2.271(5)–2.290(4) Å], which indicates
that the nitrogen atom N(3) of the auxiliary bridging ligand is
comparatively weakly bonded to the [MoO2]2+ centre pointing to
the possibility of ligand exchange reaction at this point. As the
ligand is coordinated in its thiolate form, the C(8)–S(1) bond is
expected to assume a single bond character but bond distances
are in the range 1.730(8)–1.751(8) Å, distances which lie between
the expected lengths of C–S bond and CQS bond and may be
attributed to electron delocalization in the coordinated ligand.65,66
The adjacent C(8)–N(2) bond lengths are in the range 1.273(9)–
1.284(5) Å and are closer to the CQN distance than the normal C–N
Fig. 5 Centrosymmetric structure of complex [(MoO2L)2(bix)] (4) (thermal
displacement ellipsoids drawn at 50% probability).
molecule but it could not be refined successfully and the SQUEEZE distance. The N(1)–N(2) bond distances in the range 1.389(7)–
option was therefore applied to describe the residual electron 1.425(7) Å reveal their predominant single bond character.
Table 2 Crystallographic data and details of refinement for the binuclear dioxomolybdenum(VI) complexes 1–4
Complex 1
Complex 2
Complex 3
Complex 4
Chemical formula
Formula weight (M)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a (1)
b (1)
C42H32S4N6O6Br2Cl6Mo2 C44H34S4N6O6Br2Cl6Mo2 C43H36S4N6O6Br2Mo2 C44H36S4N8O6Br2Mo2
1409.38
Monoclinic
P21/c
1435.46
Triclinic
P1
1212,62
Monoclinic
P2/c
12.318(6)
6.690(3)
31.384(14)
90
1252.75
Triclinic
P1
%
%
7.710(6)
26.796(2)
12.966(10)
90
98.888(3)
90
11.064(10)
11.107(14)
13.136(13)
69.636(10)
65.582(9)
66.708(10)
1317.0(2)
1
7.845(6)
10.096(8)
15.635(12)
80.063(8)
84.016(8)
76.674(9)
1184.2(16)
1
100.576(2)
90
2542.6(2)
2
g (1)
V (Å3)
2646.7(4)
2
Z
Temperature (K)
296(2)
150(2)
296(2)
296(2)
Crystal size
0.16 ꢃ 0.14 ꢃ 0.12
0.24 ꢃ 0.05 ꢃ 0.05
0.26 ꢃ 0.15 ꢃ 0.15
0.16 ꢃ 0.14 ꢃ 0.12
DC (g cmꢀ3
)
1.769
1.810
1.584
1.757
m (Mo Ka) (mmꢀ1
F(000)
)
2.495
1388
0.998
R1 = 0.0631
wR2 = 0.1792
0.1310, 0.2156
2.508
708
0.994
R1 = 0.0695
wR2 = 0.1052
0.1168, 0.1238
1.165, ꢀ1.247
2.278
1386
1.542
R1 = 0.0385
wR2 = 0.0872
0.0580, 0.0946
0.0695, ꢀ0.465
2.450
622
1.013
R1 = 0.0355
wR2 = 0.0804
0.0499, 0.0872
0.428, ꢀ0.451
Goodness of fit on F2
a
R1,wR2 [I 4 2s(1)]
R1,wR2 (all data)
Residual electron density (max, min) e Aꢀ3 0.687, ꢀ0.835
aR = S||F0| ꢀ |Fc||/S|F0|; wR(F2) = [Sw(|F0|2 ꢀ |Fc|2)2/Sw|F0|4]1/2
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Table 3 Observed (X-ray) and calculated (DFT) bond distances of binuclear complexes 1–4
Complex 1
Complex 2
Complex 3
Complex 4
Bond distances (Å)
Exp.
Calc.
Exp.
Calc.
Exp.
Calc.
Exp.
Calc.
Mo(1)–O(1)
Mo(1)–O(2)
Mo(1)–O(3)
Mo(1)–N(1)
Mo(1)–N(3)
Mo(1)–S(1)
C(8)–S(1)
1.927(6)
1.695(6)
1.689(5)
2.271(5)
2.469(6)
2.430(2)
1.730(8)
1.280(1)
1.425(7)
1.977
1.737
1.745
2.324
2.507
2.476
1.762
1.310
1.410
1.940(4)
1.693(3)
1.707(6)
2.285(4)
2.430(4)
2.446(2)
1.746(6)
1.297(6)
1.401(6)
1.978
1.740
1.745
2.327
2.492
2.477
1.761
1.309
1.408
1.937(6)
1.690(6)
1.699(3)
2.284(2)
2.481(3)
2.422(1)
1.746(3)
1.280(4)
1.403(3)
1.977
1.738
1.745
2.328
2.503
2.475
1.761
1.310
1.411
1.927(3)
1.687(3)
1.693(3)
2.290(4)
2.374(3)
2.430(2)
1.742(5)
1.284(5)
1.405(5)
1.977
1.744
1.760
2.333
2.391
2.501
1.754
1.311
1.411
C(8)–N(2)
N(1)–N(2)
Table 4 Observed (X-ray) and calculated (DFT) bond angles of binuclear complexes 1–4
Complex 1
Complex 2
Complex 3
Complex 4
Bond angles (1)
Exp.
Calc.
Exp.
Calc.
Exp.
Calc.
Exp.
Calc.
O(2)–Mo(1)–O(3)
O(3)–Mo(1)–O(1)
O(3)–Mo(1)–N(1)
O(3)–Mo(1)–S(1)
O(3)–Mo(1)–N(3)
O(2)–Mo(1)–O(1)
O(2)–Mo(1)–N(1)
O(2)–Mo(1)–S(1)
O(2)–Mo(1)–N(3)
O(1)–Mo(1)–N(1)
O(1)–Mo(1)–S(1)
O(1)–Mo(1)–N(3)
S(1)–Mo(1)–N(3)
N(1)–Mo(1)–S(1)
N(1)–Mo(1)–N(3)
104.9(3)
104.0(2)
161.2(2)
91.2(2)
83.3(2)
99.9(2)
91.3(2)
98.5(2)
171.8(2)
82.0(2)
152.1(2)
77.1(2)
81.6(1)
76.8(1)
80.7(2)
106.2
107.5
160.4
90.4
81.3
98.2
90.3
99.9
172.5
80.0
149.8
77.9
81.1
76.0
82.7
106.7(2)
104.4(2)
159.7(2)
90.6(2)
83.1(2)
99.4(2)
90.6(2)
97.2(2)
170.2(2)
82.6(2)
153.0(1)
78.4(2)
81.6(1)
76.0(1)
79.7(2)
106.1
108.1
160.6
90.1
81.8
97.9
89.7
99.9
172.0
80.0
149.8
77.9
81.3
75.9
82.9
105.7(1)
103.8(1)
161.6(1)
92.5(1)
83.0(1)
99.3(1)
90.6(1)
99.0(1)
171.4(1)
81.5(1)
151.3(1)
78.1(1)
80.6(1)
76.3(1)
80.9(2)
105.9
107.1
161.5
91.0
81.5
98.5
89.4
99.4
172.5
80.1
149.8
78.2
80.9
76.0
83.5
106.1(1)
106.6(1)
160.0(1)
90.2(1)
86.3(1)
97.6(1)
90.0(1)
96.1(1)
167.5(1)
82.3(1)
154.4(1)
78.9(1)
83.11(8)
76.17(8)
77.7(1)
106.2
107.7
161.3
91.1
83.6
97.6
88.9
97.2
170.0
80.4
151.6
80.8
80.6
75.8
81.0
With a view to obtain further information on the influence of
the nature and length of spacer on the structure of complexes, a
flexible auxiliary ligand, 1,4-bis(1H-imidazole-1-ylmethyl)benzene,
was used which led to the formation of complex 4. Unlike the rigid
auxiliary ligands, the flexibility and conformational freedom of
1,4-bis(1H-imidazole-1-ylmethyl)benzene allows it to coordinate
to the molybdenum centres in two possible modes, gauche-
gauche (cisoid) and trans-gauche (transoid) conformations on
the basis of relative orientations of the two imidazole rings
(Fig. 7). However, in the solid-state, the transoid conformation
Fig. 6 The distorted coordination octahedra MoSN2O3 in complexes 1–4.
The coordination mode of the Schiff base ligand allows the predominated with the distance between the two donor atoms
inclusion of the auxiliary ligands. The auxiliary ligands act in a N–N being 11.40(3) Å (Scheme 3). The 1,4-bis(1H-imidazole-1-yl-
bis-monodentate manner occupying one axial position and methyl)benzene ligand contains a rigid spacer of phenyl ring
connecting two molybdenum(VI) atoms and can be broadly and two freely rotating methylimidazole arms. The two parallel
classified into two categories - (i) N–N donor bis-(pyridyl) [ rigid
4,40-bipy, 1,2-bis(4-py)ethane and flexible 1,3-bis(4-py)propane]
and (ii) flexible N–N donor bis-(imidazole) [1,4-bis(1H-imidazole-
1-ylmethyl)benzene].
In complex 1, 4,40-bipy has N–N distance of 7.10(3) Å
(Scheme 3). Lengthening the separation between the two pyridyl
rings by introducing –CHQCH– and –CH2–CH2–CH2– groups
yielded the complexes 2 and 3.
The 1,3-bis(4-py)propane auxiliary ligand coordinates to the
two molybdenum centres in a cisoid conformation, spanning
8.673(1) Å (Scheme 3), to form the binuclear complex 3. The –CH2–
Fig. 7 (a) Gauche–gauche (cisoid) and (b) trans–gauche (transoid) con-
formations of 1,4-bis(imz-1-ylmethyl)benzene.
CH2–CH2– group imparts slight flexibility to the auxiliary ligand.
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Table 5 Geometry of the non-covalent interactions in the crystal structures of the binuclear complexes 1–4
Complexes
D–H
A
Hꢁ ꢁ ꢁA (Å)
Dꢁ ꢁ ꢁA (Å)
D–Hꢁ ꢁ ꢁA (1)
Symmetry code
1
C(7)–H(7)
C(5)–H(5)
C(13)–H(13)
C(1a)–H(1a)
C(1a)–H(1b)
pb
C(14)–H(14)
C(21)–H(21A)
C(9)–H(9A)
C(9)–H(9B)
C(19)–H(19B)
pa
O(2)
O(2)
O(3)
O(3)
O(3)
pc
2.56
2.96
2.73
2.34
2.26
4.27
2.91
3.46
3.05
3.20
3.52
4.55
3.409(9)
3.727(9)
3.343(9)
3.239(9)
3.239(9)
—
3.708(8)
4.033
3.845(5)
3.933
161
141
125
151
177
—
145
120
140
134
135
—
x,3/2 ꢀ y,zꢀ1/2
x,3/2 ꢀ y,z ꢀ 1/2
x,ꢀ1 + y,z
2
x,ꢀ1 + y,x
x, ꢀ1 + y,z
1 ꢀ x,ꢀy,1 ꢀ z
3
4
pa
1 + x,y,z
pb
1 ꢀ x,y,1/2 ꢀ z
1 ꢀ x,2 ꢀ y,1 ꢀ z
x,1 + y,z
Br(1)
pd
pb
4.264
—
x,ꢀ1 + y,z
pa
1 ꢀ x,1 ꢀ y,1 ꢀ z
pa = C(1)–C(6), pb = C(10)–C(15). pc = N(3), C(16)–C(20), pd = N(3)C(16)N(4)C(17)C(18).
imidazole rings are almost perpendicular to the phenyl ring
with an intersection angle of 89.7(2)1. In all the complexes the
open sites are trans to each other.
Intermolecular non-covalent interactions have been investi-
gated to study the different crystal packing67 and supramolecular
framework formations in the complexes 1–4. The aromatic nature
of both the Schiff base ligand and auxiliary bridging ligands
(consisting of salicylaldehyde/benzyl/pyridine/imidazole rings)
provide potential supramolecular recognition sites for pꢁ ꢁ ꢁp
Fig. 9 (a) Packing diagram of complex 2 showing the non-covalent inter-
actions (along c-axis), (b) space-filling representation illustrating the for-
mation of supramolecular pillared-layer structure with cavities in complex 2.
stacking, C–Hꢁ ꢁ ꢁp interaction and also act as hydrogen bond
donors and acceptors to assemble supramolecular frameworks.
For all the complexes 1–4 under investigation, the details of the
supramolecular C–Hꢁ ꢁ ꢁN/O/S/Br/p and pꢁ ꢁ ꢁp interactions are
listed in Table 5. The non-covalent interactions guide the
organization of molecules to supramolecules. In all the packing
diagrams (Fig. 8–11) of the complexes, the non-bonding hydro-
gen atoms are omitted for clarity.
[(MoO2L)2(4,4’-bipy)]ꢁ2CHCl3 (1). In complex 1, there are two
types of hydrogen bonds C(7)–H(7)ꢁ ꢁ ꢁO(2) and C(5)–H(5)ꢁ ꢁ ꢁO(2)
(blue and red dotted lines respectively) [Fig. 8(a)]. These
C–Hꢁ ꢁ ꢁO hydrogen bonds assemble together four binuclear
complexes to form the rectangular cavities which entrap the
solvent CHCl3 molecules. The 3D supramolecular framework is
best portrayed as honey-comb disposition [Fig. 8(b)].
[(MoO2L)2(bpe)]ꢁ2CHCl3 (2). In complex 2, pillared-layer
architecture is formed along the b-axis in which the 1,2-bis(4-
py)ethene spacer acts as the pillar. C(13)–H(13)ꢁ ꢁ ꢁO(3) hydrogen
Fig. 10 (a) Packing diagram of complex 3 showing the non-covalent
interactions (along b-axis), (b) space-filling representation illustrating the
formation of rhombus-shaped cavities.
Fig. 8 (a) Packing diagram of complex 1 showing all the non-covalent
interactions (along a-axis), (b) space-filling representation illustrating the
formation of honey-comb architecture in complex 1.
Fig. 11 (a) Packing diagram of complex 4 showing all the non-covalent
interactions (along a-axis), (b) space-filling representation showing the
formation of cavities.
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bonding interaction (red dotted lines) exists between the oxo- spacer length, flexibility and conformational freedom endows
oxygen O(3) (cis to 1,2-bis(4-py)ethene) and the benzyl ring different capacities of spatial extension. These features enable
hydrogen of the adjacent complex. The pillared-layers are the complexes to display diversified supramolecular architectures
connected along a-axis via pꢁ ꢁ ꢁp stacking interactions (magenta with different cavity size. Initially, with increase in spacer length
dotted lines) between the aromatic benzyl ring and the pyridine an increase in the dimensions of the cavities is observed. With
ring of 1,2-bis(4-py)ethane [Fig. 9(a)]. In three dimensions, the further elongation, as in 1,3-bis(4-py)propane and 1,4-bis(1H-
pillared-layer architecture forms cavities which entrap the imidazole-1-ylmethyl)benzene, the 3D supramolecular network
solvent CHCl3 molecules [Fig. 9(b)] with the help of C(1a)– tends to fold itself due to increase in flexibility and conformational
H(1a)ꢁ ꢁ ꢁO(3) hydrogen bonds (blue dotted lines).
freedom resulting in smaller cavities (Table 6).
[(MoO2L)2(tmp)] (3). In complex 3, a concerted set of inter-
A notable salient feature of the complexes is their ability to
molecular aromatic C–Hꢁ ꢁ ꢁp interactions (red and blue dotted store solvent in the crystal structure. Hence, the host–guest
lines) associate two binuclear complexes which lead to the architectures of the complexes are likely to be beneficial for
formation of rhombus-shaped cavities. Both the benzyl and future design of supramolecular hosts for the storage68 or
salicylaldehyde rings take part in the C–Hꢁ ꢁ ꢁp interactions. sensing69 of solvents by means of crystalline materials.
Fig. 10 illustrates the formation of rhombus-shaped cavities
in the crystal lattice. Due to the presence of a flexible auxiliary
bridging ligand, comparatively smaller sized cavities are
formed which are unable to host the solvent molecules.
[(MoO2L)2(bix)] (4). Unlike the previous complexes 1 and 2,
herein complex 4, a flexible spacer, 1,4-bis(imz-1-ylmethyl)-
benzene, is involved which adopts a transoid conformation in
the complex as seen from the solid state crystal structure. The
bis-(imz) ligand does not influence the basic coordination
pattern, but leads to an increase in aromatic non-covalent inter-
actions. The aromatic imidazole ring of the spacer and the benzyl
ring of the ligand are involved in C–Hꢁꢁꢁp interactions (red and
green dotted lines). The methylene protons (–CH2–) of both the
spacer and the ligand act as proton donors. pꢁꢁꢁp stacking inter-
actions (magenta dotted lines) also exist between the aromatic
salicylaldehyde rings. Hence, the presence of a flexible auxiliary
bridging ligand results in self-folding of the binuclear complex
leading to the formation of comparatively smaller cavities
(Fig. 11), too small to be occupied by the solvent chloroform.
Despite a common metal centre and primary ligand, distinct
differences in the supramolecular architectures can be attributed
mainly to the ditopic auxiliary bridging ligands. The auxiliary
ligands play an intrinsic structural role within the network and
are also sufficiently large to encourage the engineering of porous
network structures. Hence, the most intriguing feature of the
supramolecular architectures is the formation of cavities in
most of the complexes. The dimensions of the cavities are
calculated using MERCURY40 and are given in Table 6. The
cavities are basically sustained by hydrogen bonding and
aromatic–aromatic interactions, thus, giving rise to host–guest
type of architectures.
Hirshfeld surface analysis
Hirshfeld surface analysis is a latest and convenient technique
to portray different types of intermolecular interactions (from
typical hydrogen bonding to C–Hꢁ ꢁ ꢁp, pꢁ ꢁ ꢁp, Hꢁ ꢁ ꢁH interactions
and even halogen bonding) that play a crucial role in 3D
architecture of a crystal.70 Contribution of the various inter-
molecular interactions are tabulated in Table 7. The Hirshfeld
surface plot and fingerprint plot of complex 4 and contribution
of intermolecular interactions of complexes 1–4 are presented
in Fig. 12. Besides, Hirshfeld surface plot and fingerprint plot
of the other complexes 1–3 along with the individual fingerprint
plots of complex 4 are presented in Fig. S4 and Tables S1–S4
(ESI†), respectively. The Hirshfeld surface volume and surface
area are presented in Table S5 (ESI†) which indicates that
complex 3 has the highest surface volume. It can be clearly noted
from Fig. 12(c) that C-all interaction, Hꢁ ꢁ ꢁH and C–Hꢁ ꢁꢁp inter-
actions hold a significant role in the construction of the crystal
lattice and the white portions within the Hirshfeld surface plots
majorly denote these type of interactions. Moreover, the spike
indicated by a green circle within the fingerprint plot [Fig. 12(b)]
of complex 4, clearly reveals the abundance of Hꢁ ꢁ ꢁH interactions.
It is also observed that Hꢁ ꢁ ꢁH and C–Hꢁ ꢁ ꢁp interactions have a
dominating contribution in case of complexes 3 and 4 in
Table 7 Percentage surface area of interaction in complexes 1–4
% Surface area of interaction
Inside Outside Complex 1 Complex 2 Complex 3 Complex 4
Mo
H
H
C
All
All
H
All
C
0.0
50.7
18.0
18.5
4.9
0.0
52.8
17.0
18.5
3.0
0.0
56.6
32.3
14.3
0.5
0.0
54.1
27.3
16.4
3.6
Although the auxiliary ligands have the same number of
nitrogen donor atoms (ditopic coordination mode), change of
C
C
N
N
H
All
H
7.2
1.9
1.3
11.0
2.4
1.4
10.6
1.7
0.6
10.4
1.7
1.2
Table 6 Dimensions of cavities formed in the 3D supramolecular frame-
work of the binuclear complexes 1–4
O
O
S
S
Br
Br
Br
All
H
All
H
All
H
Br
11.1
10.1
8.6
3.6
9.2
6.7
0.0
10.7
9.4
7.9
6.5
7.7
6.1
0.0
10.4
9.3
7.2
7.7
7.8
5.8
0.0
11.3
8.4
8.1
7.2
8.4
6.5
0.0
Complex
Dimensions of cavities (Å2)
1
2
3
4
19.076 ꢃ 7.793
14.237 ꢃ 11.108
10.650 ꢃ 9.860
9.882 ꢃ 6.440
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Fig. 12 (a) 3D Hirshfeld surface mapped with dnorm of complex 4; (b) corresponding fingerprint plot (all in-all out) of complex 4; (c) percentage of
contribution of various intermolecular interactions in complexes 1–4.
comparison to complexes 1 and 2. On the other hand, in case of majorly by C–HꢁꢁꢁO hydrogen bonding, HꢁꢁꢁH and C–Hꢁꢁꢁp inter-
hydrogen bonding interaction, it is observed that the C–Hꢁ ꢁ ꢁN actions and weak C–HꢁꢁꢁBr and C–HꢁꢁꢁS van der Waals forces.
hydrogen bonding bears an insignificant role whereas C–Hꢁ ꢁ ꢁO
DFT calculations
hydrogen bonding holds a momentous role to build up the
crystal architectures (Table 7). The C–Hꢁ ꢁ ꢁO hydrogen bonding The four structures of complexes 1–4 were optimized till con-
interactions are stronger as a result of the higher electronegativity vergence. The resulting dimensions in the metal coordination
of oxygen compared to nitrogen. The spikes indicated by red sphere are compared with the experimental data in Tables 3
circles within the fingerprint plot [Fig. 12(b)] well illustrates the and 4. In general, bond lengths around the metal atom are
existence of C–Hꢁꢁ ꢁO hydrogen bonds. Furthermore, the red slightly longer in the calculated than in the experimental
points on the 3D Hirshfeld surface clearly delineate these strong structures, though the general features of the structure are
hydrogen bonding interactions.
maintained. However, some significant differences are found
The contribution of C–Hꢁ ꢁ ꢁO hydrogen bonding is more in the conformation of the bridging auxiliary ligands which
prominent in complexes 1 and 2 in comparison to complexes emphasize that the structures in the crystals are affected by
3 and 4 (Table 7). In addition, C–Hꢁ ꢁ ꢁS and C–Hꢁ ꢁ ꢁBr van der packing effects. In particular, differences are found in the
Waals interactions hold a vital role in all the four complexes 1–4, orientation of the auxiliary bridging ligands bonded to the metal.
but there are no Brꢁ ꢁ ꢁBr, Brꢁꢁ ꢁO or Brꢁꢁ ꢁN halogen bonding This effect is quantified by the O(3)–Mo(1)–N(3)–C torsion angles
interactions (Fig. 12(c) and Table 7). The most amazing part is which change from ꢀ43.9(4), 36.4(5), 9.9(4), 38.9(5)1 to 14.1,
that individual fingerprint plots denote all the intermolecular ꢀ13.6, 3.9, 4.51 in complexes 1–4, respectively. Other differences
interactions more evidently (Tables S1–S4, ESI†) and these inter- are found in the conformations of the auxiliary bridging ligands.
actions occupy major part of the asymmetric units of the crystal. In complex 1, the angle between the two phenyl rings is changed
These intermolecular forces play an important role in stabilization from 0.01 in the centrosymmetric crystal structure to ꢀ35.21. In
of the crystal architecture. Thus, the Hirshfeld analysis reveals that complex 2 the spacer has become planar as the C(19)–C(18)–
the crystal structures of all the complexes (1–4) are stabilized C(21)–C(21)$1 and C(18)–C(21)–C(21)$1–C(18)$1 torsion angles in
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the linker have changed from 157.91, 1801 to 179.51, 180.01. In at lmax B 300 nm which does not overlap with the tryptophan
complex 3 there are only minor variations as unique torsion absorbance at lmax = 280 nm and negligible fluorescence in EtOH
angles around the two-fold axis in the bridging auxiliary ligands medium, avoiding interference with the fluorescence of BSA protein.
have changed from ꢀ133.2(4)1, 71.2(4)1 to ꢀ116.91, 63.41. In
Absorption titration study of ligand (H2L) and complexes 1–4
with BSA protein
complex 4 the torsion angles of the bridging auxiliary ligands
of 136.8(4)1, ꢀ64.6(4)1 have changed to ꢀ99.01, 57.21.
The Frontier orbitals of the four complexes were calculated The absorbance of BSA at 280 nm increased remarkably with
and are shown in the Fig. S5 (ESI†).
incremental addition of the compounds (0–20 mL) accompanied
In complex 1, the frontier HOMOs show no contribution with hypsochromic shifts of B2–3 nm (Fig. 13). A reasonable
from the metals nor indeed from the bridging ligand. Instead explanation for the gradual enhancement of absorbance and
they are concentrated upon parts of the ligands. By contrast the shift in wavelength of BSA is a definite interaction between the
four lowest LUMOs have significant contributions from the compounds and BSA with the formation of non-fluorescent
metal and the ligands L, while LUMO+4 is concentrated upon ground-state complexes and is indicative of a static interaction.
the bridging ligand. In 2 the frontier orbitals follow the same Dynamic interaction only affects the excited state of fluoro-
pattern, but in the LUMOs there are significant contributions phores and does not alter the absorption spectrum. The hypso-
from the bridging ligand as well as the metal.
chromic shift in absorption spectra also suggests that the
microenvironment of the fluorophore is changed resulting in
the alteration of the energy gap between the ground and excited
Thermogravimetric analysis and framework stability
Thermogravimetric analyses of complexes 1–4 were conducted states of the fluorophore.
in the temperature range 30–700 1C with a 10 1C minꢀ1 interval
Quenching of intrinsic fluorescence of BSA protein by the
ligand H2L and complexes 1–4
in nitrogen atmosphere to assess their framework stabilities.
The thermal studies reveal that these complexes start to decom-
pose in the temperature range of 175–210 1C. Thermograms of Among the three fluorophores in BSA namely, tryptophan,
the complexes are shown in Fig. S6 (ESI†). tyrosine and phenyl alanine, the tryptophan (Trp) residue is
Complexes 1–3 decompose in a single step supported by primarily responsible for the intrinsic fluorescence of BSA.47
prominent and sharp exothermic peaks in the temperature Fluorescence quenching involves the decrease of the quantum yield
range of B175–210 1C with mass loses of 61.39%, 57.34% of the fluorophore induced by the quencher molecule. The process
and 49.23%, respectively. The mass loses can be justified by of quenching requires molecular contact between the fluorophore
considering the concomitant release of the S-benzyl moieties and the quencher leading to change in protein conformation,
from the primary ligand, auxiliary ligand, bromosalicyladehyde subunit associations, substrate binding or denaturation. Fluores-
moieties and the solvent CHCl3 molecules (if any) [calcd cence quenching can occur through either static or dynamic modes.
62.91% (1), 55.47% (2), 48.71% (3)].
Static quenching refers to the formation of a fluorophore-quencher
In complex 4 the first step of mass loss (expt. 17.16%) occurs ground state complex whereas dynamic quenching results from
due to the release of the auxiliary ligand (calcd 18.99%) at collisional encounters, between the fluorophore and the quencher
B198 1C followed by a second mass loss of 27.11% at B400 1C during the transient existence of the excited state.47
corroborating the loss of carbon disulphide and S-benzyl
moieties (calcd 26.66%) from both the primary ligands.
The fluorescence spectrum of BSA exhibits strong emission
maximum at 358 nm, when excited at 295 nm in an aqueous
The TG/DTA measurements reveal that the bis-(pyridyl) buffer solution.73 Fig. 14 depicts a concentration dependent
complexes 1–3 have very similar thermal decomposition trends progressive decrease in fluorescence intensity of BSA in a
whereas the bis-(imz) based complex 4 exhibited slightly different consistent manner upon complexation with the ligand H2L
decomposition patterns.
and complexes 1–4.
Upon protein unfolding, lmax usually shifts from shorter to
longer wavelength. BSA gets unfolded with the increasing
Protein binding studies
The transportation of metallodrugs through the bloodstream to concentration of the complex forming channels or pathways
cells and tissues is carried out by various proteins.71 Thus, the for the complex to reach the buried tryptophan which lead to
interaction of potential drug molecules with serum albumins changes in the fluorophore environment.74 When the molar
(SAs) is often studied by employing spectroscopic techniques. concentration of the complex was 1 mM, the emission maximum
BSA is relatively less expensive and also structurally analogous for native BSA shifted from 358 nm to 363 nm, in the presence of
to human serum albumins (HSA).72 The interactions of the the ligand H2L and complexes 1–4. This shift can be attributed to
ligand H2L and binuclear dioxomolybdenum(VI) complexes 1–4 tryptophan being buried in a hydrophobic environment.
with BSA have been monitored by absorption and fluorescence
No further change in the microenvironment of the trypto-
quenching experiments. An attempt has been made to under- phan residues occurs corresponding to the protein saturation point
stand the effect of the nature of different auxiliary bridging and formation of free molybdenum(VI) complex. Considerable
ligands (in the same metal and primary ligand environment) on fluorescence sustained even at high complex concentration
BSA binding propensity. The ligand H2L and complexes 1–4 indicates that complexes 2–4 quench the fluorescence of BSA
possess very attractive photophysical properties, such as, absorption with less effectiveness than the ligand H2L and complex 1.
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Fig. 13 Absorption spectra of 10ꢀ5 M BSA with increasing amount of 10ꢀ4 M (0–20 mL) (a) ligand H2L (b) complex 1, (c) complex 2, (d) complex 3 and
(e) complex 4.
A quantitative estimate of the extent of quenching was F0/F versus [Q] plot (Fig. 15). The high quenching ability of the
obtained by the Stern–Volmer equation. The Stern–Volmer ligand and complexes 1–4 is clearly reflected by their high KSV
quenching constants (KSV) were obtained from the slope of values presented in Table 8. The nature of F0/F versus [Q] curves
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Fig. 14 Fluorescence quenching spectra of 10ꢀ5 M BSA with increasing amount of 10ꢀ4 M (0–20 mL) (a) ligand H2L, (b) complex 1, (c) complex 2, (d)
complex 3 and (e) complex 4 upon excitation at 295 nm.
is found to be linear, thus supporting a static quenching complexes with BSA are presented in Table 8. The sufficiently high
mechanism involving the formation of fluorophore-quencher Kb values indicate the possibility for reversible binding and release of
ground state complex.
the compounds from BSA. The n values for the ligand and complexes
Further, the plot of log [(F0 ꢀ F)/F] versus log [Q] (Fig. 16) are approximately equal to unity at room temperature which
predicted the number of binding sites per albumin (n) from the indicates the availability of only one binding site per albumin.
slope and the binding constant (Kb) was obtained from the intercept.
The high BSA binding propensities can be attributed to the
The KSV, Kb and n values for the interaction of the ligand and the bromo substitution75 in the primary ligand which increases the
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obtained. The bulky non-linear complex 4 exhibits weak interaction
due to its lesser ability to approach the binding sites of BSA. This is
most likely the reason for the observed lower Kb value for complex 4.
Hence, the synthesized ligand and complexes are anticipated
to be potential candidates for future applications as therapeutic
drugs and these investigations would stimulate further study of
the biological activities of binuclear dioxomolybdenum(VI) com-
plexes. As far as our knowledge, this is the first study of protein
interaction with binuclear dioxomolybdenum(VI) complexes.
Fluorescence resonance energy transfer (FRET) from BSA to
ligand and its dioxomolybdenum complexes
Fluorescence resonance energy transfer (FRET) is a well known
technique useful for measuring the distance between donor and
acceptor molecules. FRET occurs between a donor molecule in
the excited state and an acceptor molecule in the ground state.
The donor molecules emit a shorter wavelength that overlap
with absorption spectrum of the acceptor. The efficiency of
FRET depends upon the (i) extent of spectral overlap of the
emission spectrum of the donor with the absorption spectrum
of the acceptor, (ii) distance between the donor and acceptor
Fig. 15 Stern–Volmer plots of the fluorescence titrations of the ligand
(H2L) and complexes (1–4) with BSA.
Table 8 Stern–Volmer constant (KSV), protein binding constant (Kb) and
number of binding sites (n) for the compounds
Compound
KSV (Mꢀ1
)
Kb (Mꢀ1
)
n
H2L
1
2
3
4
2.14 ꢃ 106
1.73 ꢃ 106
1.46 ꢃ 106
1.20 ꢃ 106
0.62 ꢃ 106
3.25 ꢃ 105
2.00 ꢃ 105
1.83 ꢃ 105
1.43 ꢃ 105
0.95 ꢃ 105
1.25
1.31
¨
must be within the specified FOrster distance of 2–8 nm and (iii)
1.36 relative orientation of the donor and acceptor transition dipoles.
1.37
1.39
In the presence of ligand and complexes, the fluorescence of
BSA is quenched due to the energy transfer from the tryptophan
of BSA to the ligand and complexes. From Fig. 17 and Fig. S7
(ESI†) it is clear that there is appreciable overlap between the
tryptophan emission band of BSA and the absorption band of
the ligand and complexes. The energy transfer efficiency (E) has
been calculated using the equation,
R06
R60 þ r6
F
E ¼
¼ 1 ꢀ
F0
and the values are given in the Table S6 (ESI†).
Here, F and F0 are the fluorescence intensity of BSA after and
before the addition of the compounds respectively; r is the
distance between donor and acceptor during the energy trans-
fer and R0 is the critical distance when the transfer efficiency is
50%. The value of R0 is calculated by the equation,
R0 = 8.79 ꢃ 10ꢀ25 k2 nꢀ4 j J
where, k2 is the orientation factor related to the geometry of the
donor–acceptor dipole, n is the refractive index of the medium,
j is the quantum yield of BSA and J is the spectral overlap
Fig. 16 Plot of log [(F0 ꢀ F)/F] vs. log [compound] for the fluorescence
titrations of the ligand (H2L) and complexes (1–4) with BSA.
overall lipophilicity of the compounds. The more pronounced integral between the donor emission and acceptor absorption.
binding efficiency of the ligand H2L as compared to the com-
Using k2 = 2/3, n = 1.34 and j = 0.15, the calculated values of J,
plexes can be explained by an increase in steric requirement R0 and r are given in Table S6 (ESI†). It is evident from the
of the complexes due to involvement of additional bridging calculated values that the donor to acceptor distance is in the
auxiliary ligands.
range of 2.25–3.40 nm. This clearly indicates that energy transfer
Close examination reveals that the value of the binding constant takes place from tryptophan residue of BSA to the compounds.
(Kb) varies significantly for the four complexes (1–4) having a
common metal centre and primary ligand, hence, indicating the
importance of the auxiliary bridging ligands. From the Kb values the
Molecular docking of dioxomolybdenum(VI) complexes 1–4
with BSA
binding constant follows the order: H2L 4 1 4 2 4 3 4 4.
Docking studies of compounds with proteins are able to clarify
As the length of the auxiliary bridging ligand increases, bulkiness the specific environment of the protein bound metal centre
of the complex increases which explains the sequence of Kb values showing the various albumin binding groups participating in
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Fig. 17 Overlap of emission spectra of BSA and absorption spectra of (a) ligand (H2L) and (b) complex 3.
the interaction. Bovine serum albumin comprises of three homo- acid residues (Glu-186, Glu-399, Gln-430, Gln-521, Arg-144,
logous domains [I (residues 1–183), II (residues 184–376) and III Arg-427, Arg-458, Pro-516, Thr-190, His-145, Asp-108, Ser-428)
(residues 377–583)] and each domain consists of two sub-domains and hydrophobic Ile-455 amino acid residue. Complex 4 also
(A and B).76–78 The primary sites of drug binding to BSA are located interacts extensively with both hydrophobic (Leu-115, Ile-181,
in the hydrophobic cavities in subdomains IIA and IIIA.79 The Tyr-160) and polar (Ser-109, Arg-185, Arg-458, Asp-111, Pro-110,
interactive forces between drugs and biomolecules may include Pro-117, Glu-424, Lys-116) amino acid residues.
hydrophobic, electrostatic, van der Waals, hydrogen bonding
interactions.80
Moreover, it was found that all the complexes interact most
favourably within the binding pocket near Tyr (147, 160, 451)
The lowest energy docked images of the predicted binding residues leading to fluorescence quenching of BSA.81
model for the four complexes located within the hydrophobic
cavity of BSA are shown in Fig. 18 and Fig. S8 (ESI†). The
Conclusions
docking of BSA with the synthesized complexes 1–4 exhibits
well established hydrogen and halogen bonds with the amino
A series of new binuclear dioxomolybdenum(VI) complexes 1–4
acid residues (Table S7, ESI†).
having a common ONS donor Schiff base primary ligand
Some of the amino acid residues in the neighborhood of
environment but different N–N donor auxiliary bridging ligands
complex 1 are hydrophobic (Leu-112, Leu-115, Tyr-147) and
of varying length have been synthesized. All the complexes have
most are polar (Lys-114, Arg-144, Arg-185, Arg-458, Ser-192,
been characterized by various spectroscopic techniques (IR,
His-145, Thr-518, Asp-108). Similarly, the vicinity of complex 2
1H NMR, UV-Vis), cyclic voltammetry, single-crystal X-ray dif-
shows the presence of both hydrophobic (Ala-193, Leu-115,
fraction and thermal studies.
Leu-178, Ile-181, Tyr-451) and mostly polar (Glu-186, Arg-185,
The auxiliary ligands are able to generate interesting 3D
Arg-435, Pro-117, Thr-190, Thr-518, Lys-431, Ser-428) amino
supramolecular structures with unique network architectures of
acid residues, which indicates that the mode of binding is
molybdenum complexes. Several examples of synergy between the
governed by significant contributions from both hydrophobic
non-covalent interactions have been found; more specifically,
forces and hydrogen bonding interactions. Complex 3 exhibits
extensive hydrogen bonding interactions with the polar amino
between pꢁ ꢁ ꢁp stacking and C–Hꢁ ꢁ ꢁp interactions and also
between hydrogen bonds and C–Hꢁ ꢁ ꢁp interactions. The
features of the auxiliary ligands, such as, length, rigidity and
flexibility are important factors in modulating the aesthetically
appealing supramolecular frameworks of the complexes. The
presence of cavities in the structures offers an opportunity for
these complexes to be used in various applications such as gas
storage and guest transport. Since, the potential of molyb-
denum complexes in crystal engineering has scarcely been
demonstrated so far, our results may offer new insights into
crystal engineering. The visually appealing Hirshfeld surface
analysis helped us to venture into all close contacts between
molecules in the crystals and reveals that the crystal structures
of all the complexes (1–4) are stabilized majorly by C–Hꢁ ꢁ ꢁO
Fig. 18 Molecular docking image of complex 1 with BSA, residues sur-
rounding the binding sites are highlighted.
hydrogen bonding, Hꢁ ꢁ ꢁH and C–Hꢁ ꢁ ꢁp interactions. The DFT
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6 R. Eswaran, R. Bertani, P. Sgarbossa, N. Karuppannan and
B. Nattamai S. P., J. Inorg. Biochem., 2016, 155, 1–8.
7 (a) D. Biswal, N. R. Pramanik, S. Chakrabarti, N. Chakraborty,
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9 X.-L. Wang, J. Zheng, M. Li, S. W. Ng, S. L.-F. Chan and D. Li,
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calculations of the complexes aided in obtaining a deeper insight
into the non-covalent interactions and electronic environment
around the molybdenum centres.
The ligand H2L and complexes 1–4 show significant protein
binding propensity to BSA. The binding ability of the complexes
with BSA decreases with increase in the length of the auxiliary
bridging ligands. There is a strong fluorescence energy transfer
due to occurrence of FRET between tryptophan residue of BSA
and the compounds. The docking results show that the mode of
binding between binuclear Mo(VI) complexes is governed by the
significant contribution from both hydrophobic forces and
hydrogen bonding interactions. The results are of significance
towards further designing and developing new therapeutic
agents for certain diseases.
Conflicts of interest
There are no conflicts to declare.
10 Q. Tao, Q.-Q. Hao, A. P. Voronin, X.-L. Dai, Y. Huang,
G. L. Perlovich, T.-B. Lu and J.-M. Chen, Cryst. Growth
Des., 2019, 19, 5636–5647.
Acknowledgements
D. Biswal and M. Roy acknowledge UGC for their Senior
Research Fellowships. DST-FIST is acknowledged for providing
Single-crystal XRD at the Department of Chemistry, University
of Calcutta. The authors thank the University of Reading and
the EPSRC (U.K.) for funds for the Oxford Diffraction diffracto-
meter and Prof. Pramit Chowdhury (IIT Delhi) for the thermal
analysis. The authors also thank Ms Moumita Mukherjee for
her sustained interest in the protein interaction study.
11 (a) J. Kim, H. Furukawa, N. Ko, Y. B. Go, N. Aratani, S. B.
Choi, E. Choi, A. O. Yazaydin, R. Q. Snurr, M. O’Keeffe and
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