The role of a weakly coordinating thioether group in ligation controlled molecular self-assemblies and their inter-conversions in Ni(ii) complexes of l-methionine derived ligand

Article abstract of DOI:10.1039/c9nj02573a

A series of Ni(ii) complexes of an l-methionine derived reduced Schiff base ligand has been synthesized and their interconversions are investigated for the first time with the help of a variety of physico-chemical techniques. The different reactant/reagent conditions lead to mono- (1), di- (2a/2b) and tri-nuclear (3a/3b) Ni(ii) complexes and they are unequivocally characterized by single crystal X-ray crystallography. Alkali ions K⁺/Na⁺ can serve as efficient templates for the conversion of 1 to tri-nuclear complexes 3a/3b which are thermally stable at increased temperature of 70 °C, but amenable to dissociation to 1 in the presence of strongly interacting 18-crown-6-ether. 2a gets transformed to 2b in DMF, in which the essential structural change is the incorporation of one DMF molecule to the coordination sphere of one of the Ni(ii) centers with concomitant release of the coordinating thioether arm of the ligand. 2a and 2b can also be converted to a self-assembled complex (3a/3b) by alkali metal salt addition at ambient temperature. Warming the solution to 70 °C disassembles the tri-nuclear complexes with a perceptible colour change and this cycle can be repeated several times. Additional evidence for the inter-conversion and retention of all the complex species in solution is clearly established by ESI-mass and UV-vis spectroscopy. Variable temperature magnetic analysis shows weak ferromagnetic and antiferromagnetic coupling for binuclear and self-assembled complexes, respectively.

Full text of DOI:10.1039/c9nj02573a

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Mutreja, G. Sharma, S. Kumar, A. Ali, S. K. Mehta, P. Venugopalan, R. Kataria and S. C. Sahoo, New J.
Chem., 2019, DOI: 10.1039/C9NJ02573A.
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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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DOI: 10.1039/C9NJ02573A
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The Role of Weakly Coordinating Thioether Group in the Ligation
Controlled Molecular Self-Assemblies and Their Inter-Conversions
in Ni(II) Complexes of L-methionine Derived Ligand
Received 00th January 20xx,
Accepted 00th January 20xx
c
Rakesh Kumara, Vishal Mutrejaa, Gyaneshwar Sharmab, Sanjeev Kumarb, Anzar Ali , S. K. Mehtaa, P.
DOI: 10.1039/x0xx00000x
Venugopalana, Ramesh Kataria*a, Subash Chandra Sahoo*a
www.rsc.org/
A series of Ni(II) complexes of L-methionine derived reduced Schiff base ligand has been synthesized and their
interconversions are investigated for the first time with the help of a variety of physico-chemical techniques. The different
reactant/reagent conditions lead to mono- (1), di- (2a/2b) and tri-nuclear (3a/3b) Ni(II) complexes and they are
unequivocally characterized by single crystal X-ray crystallography. Alkali ions K+/Na+ can serve as efficient templates for the
conversion of 1 to tri-nuclear complex 3a/3b which are thermally stable at elevated temperature of 70 °C, but amenable to
dissociation to 1 in the presence of strongly interacting 18-crown-6-ether. 2a gets transformed to 2b in DMF, in which the
essential structural change is the incorporation of one DMF molecule to the coordination sphere of one of the Ni(II) centers
with concomitant release of the coordinating thioether arm of the ligand. 2a and 2b can also be converted to self-assembled
complex (3a/3b) by alkali metal salt addition at ambient temperature. Warming the solution to 70 °C dis-assembles the tri-
nuclear complexes with a perceptible color change and this cycle can be repeated several times. Additional evidence for the
inter-conversion and retention of all the complex species in solution is clearly established by ESI-mass and UV-Vis
spectroscopy. Variable temperature magnetic analysis shows weak ferromagnetic and antiferromagnetic coupling for
binuclear and self-assembly complexes respectively.
The other choice, flexible ligands, usually give rise to inter-
convertible architectures and the stability of one over the other
is largely determined by internal and external factors such as
Introduction
Generating desired molecular associations through self-
assembly is an important endeavor, but, still many challenges
remain as these associations are intimately linked with size,
shape, spatial orientation and complimenting association-ability
of the interacting species, which are often difficult to control in
unison. Among many, Coordination-Driven Self-assembly (CDS)
has become a well-known approach for constructing functional
molecules owing to their potential applications in material
science1, catalysts2, sensor design3 and biology.4 The inbuilt
rigid-nature of specially designed coordinating ligands has been
advantageously exploited by Raymond and Fujita et.al.5 for
developing wide varieties of discrete coordination self-
assemblies; nevertheless, the rigidity itself can become another
controlling or restricting factor in the self-assembly process.
choice of ligands,6 metal ions,7 solvents,8 heat,9 light,10
mechanical energy,11 and counter ions.12 Thus, due to this
multi-variable influence, they are far less exploited in designing
CDSs.13 Though nature favors flexibility over rigidity14, and thus
it is possible to generate novel supramolecular architectures
using flexible ligands, the real challenge stems due to the
possibility of multiple reaction pathways, uncontrolled ligand
binding, conformational uncertainty and numerous product
formation. Even with such challenges, there are leaps and
bounds in generating new and novel CDSs using flexible
ligands.15
In this context, amino acids and amino acid derived reduced
Schiff bases need special mention due to their inherent
chirality, ease of functionalization and flexible metal binding
nature. Among these, metal complexes of amino acid derived
reduced Schiff bases (notably that derived from salicylaldehyde
with different amino acids) have generated considerable
interest and a variety of architectures including chiral capsules,
cages, channels and cavities with various sizes and shapes were
realized16 and even important applications were also
reported.17 Extensive investigations on such complexes have
shown that, in general, mononuclear18 or binuclear19 complexes
are formed and only a handful of polynuclear species are
a. Department of Chemistry, Panjab University, Sector-14, Chandigarh, India,
160014.
b. Department of Applied Science, Punjab Engineering College, Sector 12,
Chandigarh, India, 160012.
c. Department of Physical Sciences, Indian Institute of Science Education and
Research Mohali, Knowledge city, Sector 81, SAS Nagar, Mohali, PO 140306,
Punjab, India.
†Electronic Supplementary Information (ESI) available: Crystallographic data,
selected bond lengths and angles, FT-IR spectra, Mass spectra, TGA curves, PXRD,
UV-Vis spectra, kinetic plots, magnetic analysis data, movies. CCDC: 1836126-
1836130.See DOI: 10.1039/x0xx00000x
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isolated.20 That is, extensive self-assembly is restricted in these (monomer, dimer(s), self-assembly compounds etc) which
complexes, probably due to the variable coordination themselves may further get transformed to others, based on
View Article Online
DOI: 10.1039/C9NJ02573A
possibility of the side arm of the amino acid (e.g., none in the reagent\reaction conditions. Feasibility and evidence of
valine and leucine as they have aliphatic side arm, but possible such transformations among Ni(II) centered reduced Schiff
in glutamic acid, histidine etc); indeed it can become a deciding bases are not reported so far. Further, in favorable conditions,
factor in forming mono- , bi- or poly- nuclear self-assembly, it may be possible to isolate and study the individual
based on these reduced Schiff bases. In a nut shell, it may be architectures through various physicochemical methods
possible to tune the formation of different types of (including detailed structural and packing analysis employing
architectures by suitably varying the type of the side arm and single crystal X-ray crystallography). Such solid state analysis
thus it’s coordinating ability. Also, a fine blend of these two can give insight into the interaction of methionine with metal
factors, that is, inherent flexibility and variable coordinating centers, and may serve as model system to study methionine-
ability of a suitable amino acid derived ligand can make it sulfoxidereductase-A (MsrA), a well-known antioxidant enzyme
possible to assemble and disassemble the polynuclear which plays a key role in removing heavy metal ions from
structures by employing suitable metal centers and other tissues.22
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reaction conditions. It may also be possible to repeat this
Thus, herein we present a comprehensive report of the
association-dissociation cycle many times and study the synthesis, structural characterization of mono-, di- and self-
spectral and structural consequences associated with these assembly of Ni(II) complexes of L-methionine derived reduced
transformations; such studies would be highly rewarding.
Schiff base-ligand (H2L) and their inter conversions. The
The importance of L-methionine-salicylaldehyde derived reagent/reaction conditions (including temperature) dictate the
reduced Schiff base ligand stems here; the methyl-thio moiety ligating/not-ligating behavior of weakly coordinating S atom
as the side arm is quite flexible and the S-atom of the thio-ether which has been proved conclusively by X-ray crystallography. As
moiety may or may not involve in metal-ligand coordination. hypothesized (Figure 1), the make-break cycle of the self-
Indeed, occurrence of this either way depends on the overall assembly is controlled by the alkali metals (K+/Na+) and their
stability-demand of the ensuing architecture; an important absence make the weakly coordinating sulfur to become more
concept worth investigating. Even though some methionine or dominant, leading to binuclear complex formation. The various
methionine derived reduced Schiff base metal complexes have inter-conversions and transformations among the complexes
been reported,21 a very few solid state evidence is available in are established through solution kinetic analysis and other
which the sulphur atom acts as a variable ligating site for physico-chemical methods. Magnetic interaction of binuclear
binding to the metal ions, especially to Ni(II) centers. In this and self-assembly complexes were investigated by variable
backdrop, we anticipated a possible cyclic sequence of chemical temperature magnetic moment measurements.
events which is shown as a cartoon in Figure 1.
Experimental Section
Materials and Methods: Solvents used were purified prior to
use by following standard literature procedure.23
Salicylaldehyde, sodium perchlorate and Ni(II) perchlorate
hexahydrate were purchased from sigma-Aldrich. All other
chemicals such as L-methionine, lithium hydroxide, sodium
hydroxide, sodium borohydride, nickel(II) acetate tetrahydrate
and nickel(II) nitrate hexahydrate, and ethanolamine were
purchased from Avra, India, and were used as received. The IR
spectra were recorded on Thermo Scientific (NICOLET IS50) FT-
1
IR spectrophotometer in the range 4000-400 cm-1. The H NMR
spectrum was recorded on Bruker 400 MHz Avance-ІІ
spectrometer using TMS as an internal reference at 25 °C in
CD3OD. The electronic spectra were recorded using a JASCO V-
530 UV/Vis spectrophotometer. Field and temperature
dependent magnetic measurements were performed using
VSM (9T-Quantum Design) and Isothermal magnetic data were
collected in the temperature range 2K<T<10K. Solution
Figure 1. (A) Cartoon representation of diverse assembly-disassembly
electrical conductivity was measured with Pico Conductivity
processes that may be envisioned in L-methionine derived reduced
meter (Model CN04091201, Lab. India) and the instrument was
Schiff base metal complexes depending upon on reaction/reagent
calibrated with standard solution. Elemental analyses were
conditions.
performed on Thermo Scientific Flash 2000 Organic Elemental
Analyser instrument. Electrospray ionization mass (ESI-MS)
spectra were recorded on a XEVO G2-XS QTOF and WATERS Q-
conditions, reversible self-assembly/disassembly processes can
TOF MICROMASS mass spectrometers. Powder X-ray was done
be initiated, generating diverse molecular structures
Essentially, depending upon the reagents and reaction
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on X’Pert Pro Panalytical’s instrument (λ= 1.5406 Å, Kappa, solution was filtered off and the filtrate was VkieewpAtrticfloe rOnslinloew
Anode–Copper). Thermal analysis was carried out using TGA evaporation. Rod shaped green crystalsDwOeI:r1e0o.10b3t9a/iCn9eNdJw02i5th73inA a
instrument with an SDTA sensor from Mettler Toledo upon week. Yield 65 %. Anal.Calcd for [Ni(C12NO3SH16)2]·CH3OH: C,
loading over an alumina pan and heating from 30 °C to 1000 °C 50.09; H, 6.06; N, 4.68; found C, 50.62; H, 6.18; N, 4.84. IR (cm-
at a heating rate of 10 °C min-1 under N2 atmosphere. Caution! 1): υ(COO)asym1622 (s), υ(COO)sym1499 (s), υ(phenolic CO) 1251
Perchlorate salts used as starting materials are potentially (s). ΛΜ (ohm-1 cm2 mol-1): (MeOH) 0.3.
dangerous as explosives and should only be handled in small
quantities.
[HOCH2CH2NH3][Ni2(C12NO3SH15)2(CH3COO)].H2O (2a): To a
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Single-Crystal XRD Studies: All the crystals except 3a, were methanolic solution (15 ml) of ligand (H2L) (0.5 g, 1.58 mmol),
mounted on Hampton cryoloops. All geometric and intensity and nickel(II) acetate tetrahydrate (0.39 g, 1.58 mmol), were
data for the crystals were collected using a Super-Nova (Mo) X- mixed and stirred for 30 min. A methanolic solution (2 ml) of
ray diffractometer equipped with a micro-focus sealed X-ray ethanolamine (0.095 ml, 1.58 mmol) was added to the above
tube Mo-Kα (λ=0.71073 Å) X-ray source, and HyPix3000 complex mixture and thereafter a deep green coloured solution
detector of with increasing ω (width of 0.3 per frame) at a scan was obtained. The solution was stirred with heating for
speed of either
5
or 10 s/frame. For complex 3a,
a
approximately 3 hrs at 40-50 °C. The solution was then filtered
microfocused selaed tube supernova Cu x-ray wa used with a and the filtrate was kept for slow evaporation. Rod shaped
EosS2 type detector. The CrysAlisPro software was used for green crystals were obtained after 6-7 days. Yield 60%.
data acquisition, and data extraction. Using Olex224a, the Anal.Calcd for [HOCH2CH2NH3][Ni2(C12NO3SH15)2(CH3COO)].H2O:
structure was solved with the SIR200424b structure solution C, 43.94; H, 5.93; N, 5.49; found C, 45.82; H, 6.08; N, 5.79. IR
program using Direct Methods and refined with the ShelXL24c (cm-1): υ(COO)asym1603 (s), υ(COO)sym 1478 (s), υ(phenolic CO)
refinement package using Least Squares minimisation. All non- 1287 (s), υ(N-H) 3267(w). ΛΜ (ohm-1 cm2 mol-1): (MeOH) 65.
hydrogen atoms were refined with anisotropic thermal
parameters. Few of the guest solvent molecules (water and [HOCH2CH2NH3][Ni2(C12NO3SH15)2(CH3COO)(DMF)]·2H2O (2b):
methanol) in the lattice were found to be highly disordered and This was prepared following the method, stirring time
the dispersed electron densities were removed by solvent mask described for 2a except the solvent medium is a mixture of
using Olex2. Hence few molecular compositions obtained from methanol and DMF (3:1). Rod shaped green crystals were
elemental analysis and crystallography may not match. Detailed obtained after 7-8 days. Yield 68%. Anal.Calcd for
crystallographic data and structural refinement parameters are [C2H8NO][Ni2(C12NO3SH15)2(CH3COO)(C3H7NO)]·2H2O: C, 43.47;
summarized in Table S1-3. Hydrogen bond geometric H, 6.36; N, 6.55; found C, 43.71; H, 5.98; N, 6.41. IR (cm-1):
parameters are listed in Table S4. CCDC 1836126-1836127 and υ(CO, DMF)1650(s), υ(COO)asym1585 (s), υ(COO)sym 1483 (s),
1836129-1836130 contains the supplementary crystallographic υ(phenolic CO) 1250 (s), υ(N-H) 3276(w). ΛΜ (ohm-1 cm2 mol-1):
data. These data can be obtained free of charge from The (MeOH) 68.
Cambridge Crystallographic Data Centre.
K[Ni(C12NO3SH16)2]3.2DMF·NO3.H2O (3a): To
a methanolic
Synthesis:
(S)-2-(2hydroxylbenzyl)-amino-4-(methylthio)- solution of ligand (H2L) (0.5 g, 1.58 mmol), and KOH (0.09 g,
butanoicacid (H2L). A mixture of L-methionine (1g, 6.70 mmol) 1.58 mmol) was mixed with slight heating to obtain a clear
and LiOH.H2O (0.28 g, 6.70 mmol) was taken in methanol solution. Methanolic solution of nickel(II) nitrate hexahydrate
(MeOH) (100 ml) and kept for 30 min string to obtain a desired (0.23 g, 0.079 mmol) was added to above clear solution. Colour
cleared solution. A yellow coloured solution was developed of solution immediately changed from green to blue with slight
soon after adding salicyaldehyde (0.71 ml, 6.70 mmol) drop blue colour precipitation. Addition of 2 ml DMF dissolved the
wise to above reaction mixture. The solution was stirred for 3 entire precipitate and resulting a cleared solution. The solution
hrs at 50-60 °C. The resulting mixture was reduced with NaBH4 was stirred at 50-60 °C for 3 hrs followed by overnight at room
(0.25g, 6.70 mmol) by adding portion wise in an ice-cold temperature. The solution was filtered and the filtrate was kept
condition. After string for 3 hr, the whole mixture was treated for slow evaporation. Blue coloured crystals were obtained
for acidification using acetic acid at pH 6-7. A white precipitates after two weeks. Yield 60 %. Anal.Calcd for
were thus obtained, filtered out, thoroughly washed with water [K{Ni(C12H16O3NS)2}3]NO3·2C3H7NO·H2O: C, 48.00; H, 5.79; N,
followed by methanol and dried in desiccator. IR (cm-1): ʋ(C-N) 6.46; found C, 48.18; H, 5.87; N, 5.85. IR (cm-1): υ(COO)asym1659
1
1256; ʋ(C=O) 1595; ʋ(-NH-) 3033. H NMR (CD3OD+LiOH.H2O, (s), υ(COO)sym 1557 (s), 1407 (s), υ(phenolic CO) 1249 (s). ΛΜ
400 MHz ppm): -CH (aromatic) (7.03, m, 2H), -CH (aromatic) (ohm-1 cm2 mol-1): (MeOH) 115.
(6.71, m, 2H), -CH2 (3.30, m, 2H), -CH (3.18, t, 1H), -CH2 (2.56, Alternatively, 3a can be prepared by simple mixing complex 1(1
m, 2H), -CH3 (1.87, s, 3H), -CH2 (1.82, m, 2H).
eq.) with KNO3 (0.3eq) in methanol-DMF mixture and allowing
the mixture for slow evaporation to desired crystal of 3a within
[Ni(C12NO3SH16)2].CH3OH (1): To a methanolic solution (15 ml) a week.
of ligand (H2L) (0.5 g, 1.58 mmol), and nickel(II) acetate
tetrahydrate (0.39 g, 1.58 mmol) were mixed under string to Na[Ni(C12NO3SH16)2]3.2DMF·H2O·ClO4 (3b): This was prepared
result a green coloured solution. Thereafter, the resulting following the method, stirring time described for 3a except
solution was heated for approximately 3-4 hrs at 40-50 °C. The NaOH and Ni(ClO4)2 were replaced with KOH and Ni(NO3)2
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respectively. Blue coloured crystals were obtained after two the two metal centers within the first coordinatVioienw AsrptichleeOrnelinaere
weeks. Yield 60 %.Anal.Calcd
for nearly same (Table S2).The bond leDnOgtI:h10.f1o0r39/NC9i-NOJc0a2rb5o7x3ylAate
[Na{Ni(C12H16O3NS)2}3]ClO4·2C3H7NO·H2O: C, 47.10; H, 5.68; N, 2.053(6) and 2.102(6) Å; Ni-Namine, 2.049(7), and 2.077(8) Å; Ni-
5.64; found C, 47.05; H, 5.48; N, 5.60. IR (cm-1): υ(COO)asym1657 Ophenolate, 2.046(7), 2.025(6), 2.030(6) and 2.026(7) Å; Ni-Oacetate
(s), υ(COO)sym 1558 (s), 1405 (s), υ(phenolic CO) 1249 (s). ΛΜ 2.051(7) and 2.047(7) Å and Ni-Smethionine 2.453(3) and 2.483(3)
Å are found in the normal range as reported for the similar
complexes.26b In [Ni2(L)2(OAc)]¯, both carboxylate pairs are
Alternatively, 3b can be prepared by mixing complex 1 (1 eq.) oriented towards each other, flanked on both sides by the
with NaClO4 (0.3 eq) in methanol-DMF mixture and allowing the aromatic rings, thus forming a bowl like structure with a guest
mixture for slow evaporation to desired crystal of 3b within a binding pocket. The pocket provides a three point interactions
,
.
(ohm-1 cm2 mol-1): (MeOH) 110.
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week.
for hydrogen bonding, through both carboxylate O-atoms
(Figure S2). A lattice water molecule also helps in stabilizing the
ammonium ion of ethanol amine via additional H-bonds. The
Results and Discussion
crystal packing viewed down c-axis shows
a triangular
arrangement of three complex molecules and the lattice water
molecules are clustered together through H-bonds (Figure S2).
Crystal Structures
The sequence of self-assembly/disassociation events envisaged
in Figure 1 and realized by altering the reaction/reagent
conditions (Scheme-1) clearly shows a fine blend of flexibility
and weakly coordination ability of the side arm of L-methionine
that gives diverse architectures such as 1, 2a, 2b and 3a. The
unequivocal confirmation about the binding/free nature of the
side arm can be gathered through single crystal X-ray
crystallography. It can also further illustrate how the overall
architecture is stabilized though diverse Ni(II)-ligand
coordination along with lattice stabilizing weak interactions. A
succinct description of these features is given below.
Crystal structure of 1: Complex 1 crystallizes in trigonal
crystal system with space group of P321 (Table S1). In complex
1, the asymmetric unit consists of one Ni(II) center, surrounded
by two ligands (HL) in a distorted octahedral and cis-
conformation (Figure 2A and S1A SI). The Ni–Namino and four Ni–
O bond distances (Table S2) are comparable with other
reported structures.16b,25 The bond angles around the Ni(II)
center are in the ranges of 87.20-79.77° (cis-angle) and 166.50-
179.1° (trans-angle). One methanol molecule found to be highly
disordered and the dispersed electron densities were removed
by solvent mask using Olex2. The crystal lattice of 1 is stabilized
by two perpendicular chains of H-bonds (along a- and b-axis)
between carboxylate O-atoms and the phenolic group with a
distance of 2.440(2) Å (Table S4). As a result of 3-fold
symmetry, the lattice arrangement of the complex 1 results to a
beautiful triangular ‘propeller’ like self-assembly along ac-plane
(Figure S1B) in which the wings of the propeller protrudes to
the concavity of the propeller (see the region marked as C1, C2,
C3 in Figure S1B. Notably, both the thioether side arms are free
and do not participate in metal-ligand coordination.
2a also crystallizes in chiral space groups of P31 (Table S1).
The asymmetric unit contains one [Ni2(L)2(OAc)]¯unit, one
protonated ethanolamine and one water molecule as solvent of
crystallization. The [Ni2(L)2(OAc)]¯unit has two octahedral metal
centers bridged by two phenolate oxygen atoms (one from
each ligand) and one acetate (syn-syn-µ2-ƞ1:ƞ1) group. The rest
three positions are occupied by -N, -O and -S atoms from
amine, carboxylate and thioether unit of each ligand
respectively (Figure 2B). The anionic part of the complex is
stabilized by the ammonium group of ethanolamine via
Hydrogen bonds (Figure S2A). The bond lengths and angles of
Figure 2. (A) Ball and stick model of complex 1 showing cis arrangement
of ligand in the bis-complex. (B) Ball and stick model of complex 2a
showing that methionine ‘S’ is coordinated to Ni(II) centre and the
ammonium ions (ethanolamine) is stabilized by H-bonding interactions.
In 2a, the H-atoms and solvent (water) molecules are removed for
clarity. (Ni, green; O, red; N, blue; C, dark gray; S, yellow; H, turquoise.
The colour codes are the same as this, hereafter.
The two noteworthy features of this structure are the
involvement of an acetate ion as a bridging ligand for the two
Ni(II) centers and the involvement of the thioether arm in the
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coordination; both features are absent in 1. It is not that
straight forward to ascertain when this weakly coordination As a result, one ligand behaves as tetradentate while the other
thioether group would participate in coordination or not. is tridentate. Thus, this proves that the thioether side arm is
Probably a subtle combination of stereo-electronic factors flexible in coordination and can act as a ‘middle runner’.
along with steric demand dictates the role of the side-arm in However, why this did not happen to Ni2 is enigmatic,
deciding the final structure. This uncertainty is further probably, as mentioned previously; a combination of various
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DOI: 10.1039/C9NJ02573A
manifested in the crystal structure of 2b.
factors may become responsible for the final structure.
9
Complex 2b is crystallized in chiral space group of P43 of Nevertheless, the same reduced Schiff base behaving
tetragonal system (Table S1). Interestingly, crystallization in differently during the identical reaction conditions can be quite
DMF leads to its incorporation in the coordination sphere of rare and interesting. The crystal packing of this complex is
one of the Ni(II) centers, Ni(1) (Ni1-O9, 2.085 Å) relieving and dictated by combinations of various interactions and it is similar
making the coordinated thioether side arm free. However, this to 2a where the two lattice water molecules are trapped
incorporation does not happen to Ni(2), thus making the two through strong H-bonds between the acetate group of one
Ni(II) centers dissimilar in coordination environment (Figure 3A molecule and ethanol-ammonium ion/carboxylate group of the
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and –S3).
other. The packing of the complex along ac-plane forms a
lamellar like arrangement (Figure S3).
The self-assembly complex 3a crystallizes in the space group
P21, with total four molecules per asymmetric unit (Table S1).
Few lattice DMF molecules and side arms of the ligand were
found to be disordered and modeled based on the available
election density. The detail bond lengths and angles of the
metal centers are shown in Table S3. As shown in Scheme-1, 3a
can be obtained either from 2a or 2b, upon addition of MX, but,
in the solution, drastic reorientation of the coordination of the
side arm is required, as both the side arms are free in 3a. This
also shows the flexible role of the thioether group in realizing
the SA-complex. Three monomeric units are generated in-situ
and they together have self-assembled around metal ion (K+)
resembling a star-shaped architecture (Figure 3B and Figure S4,
SI). Notably, all the six highly coordinating carboxylate oxygen
atoms from the ligand get themselves involved in the self-
assembly formation (see Figure 1, left bottom, for the
diagrammatic representation). Same SA-complex can also be
generated from 1 where the cis arrangement of the ligands
facilitates the self-assembly formation by aligning their
carboxylate oxygen atoms towards central K+ ion. The cationic
self-assembly is neutralized by one nitrate anion and a
multitude of hydrogen bonds involving the occluded solvent
molecules (DMF, H2O) along with nitrate anion stabilize overall
crystal packing. Two water molecules found to be highly
disordered and the dispersed electron densities were removed
by solvent mask using Olex2. The overall assembly down the c-
axis shows a zig-zag arrangement where lattice solvents and
anions appear as if they are embedded within the voids formed
through the self-assembly of the complex molecules (Figure
S4B, SI).
Characterizations:
Spectroscopic properties and solution identity: The IR spectra
of Salmet, 1, 2a-2b and 3a-3b are given in Figure S5‒10, SI. The
complexes show respective characteristic peaks depending
upon the presence of metal bound carboxylates, counter
anions, amino alcohols which further confirms the nature of
ligand and their coordination modes. νasym(COO–) and νsym(COO)
are the most characteristic vibrations and include a sharp and
strong peak at 1621/1499 (1), 1603/1477 (2a), 1658/1482 (2b)
and 1658/1557 (3a).The sharp peak at 3267 (2a), and 3276 (2b)
elucidate the primary ammonium group.
Figure 3. (A) Ball and stick model of complex 2b in which one
methionine ‘S’ is coordinated and other one is free, along with the
ammonium ions stabilized by H-bonding inside the bowl shaped cavity
of the anionic complex. Solvent molecules are omitted for clarity and
hydrogen bonds are highlighted as dashed lines. (B) The stick model of
3a showing the binding of ligand O-atoms to cation K+ that is
reminiscent of the association of the six oxygen atoms of 18-crown-6 to
¯
the metal ion. The multiple H-bonds of the counter anion (NO3 ) is also
shown. Solvent molecules are omitted for clarity.
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The UV-Vis spectra of 1, 2a, 2b, 3a, and 3b were measured from Assembly, Disassembly and interconversions:
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200 to 1600 nm. Due to moderate solubility of the complexes in The cartoon of our hypothesis regardiDnOgI:t1h0e.10p39la/Cu9sNibJl0e25in73tAer-
methanol, most of the spectral studies were performed either conversions of L-methionine derived reduced Schiff base
in pure dimethylformamide (DMF) or in methanol-DMF complexes (Figure 1) takes the precise form as shown in
mixture. In DMF, both 1 and 2a showed three broad Scheme-1. In this scheme, the exact chemical nature of the
absorptions bands around 379, 636, and 1025 nm. However, complexes, that were isolated and studied, is numbered and
two differences in peaks distinguish 2a from 1, the distinct peak the conditions leading to their formation are also recorded.
at ∼379 is only a shoulder in 2a and there is also a significant Complex 1 can be readily synthesized by mixing Salmet ligand
red shift of ~11 nm in 2a at 1025 nm region (Figure S11, Table (H2L), and Ni(acetate)2 in methanol in 2:1 stoichiometric ratio.
S5). For complex 3a and 3b, there are three strong bands The acetate ion in the above reaction serves as a self-acting
around 388, 554, 851 nm and a broad band at 1181 nm along base to deprotonate the ligand which is essential for the
with a shoulder like peak at ∼755 nm (Figure S12). Notably, formation of 1. Our attempt to synthesize 1 with other Ni(II)
there is a blue shift of nearly 14 nm and 61 nm for the two salts such as Ni(NO3)2, Ni(ClO4)2, and an external alkali base
higher wavelength absorptions in 3b when compared with 3a (such as KOH/NaOH) never resulted in 1, but, strikingly, 3a/3b
(Figure S12 and Table S5). For 2b the optical spectra found to were readily formed which are alkali metal bound self-
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be almost identical that of 2a in DMF (Table S5).
assembly complexes derived from 1 (SA-complexes, hereafter).
This shows the subtle role of the acetate ion in stabilizing 1,
ESI-MS as supplementary evidence for SA-Complex formation
beyond its basic character. Interestingly, in a parallel
The identity of complexes in solution was further evaluated by experiment, in which complex 1 (in MeOH-DMF) was directly
ESI-MS (Figure S13-19). The m/z at 567.34, and 781.56 mixed with 1/3 equivalent of alkali salt MX (where M= K+, Na+,
-
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correspond to [Ni(HL)2] and [Ni2(L)2 + amine + acetate +2 water] X= NO3 , ClO4 ) resulted in the formation of same SA-complexes,
units and confirms the formation of complex 1 and 2a 3a/3b. These two observations (i.e., KOH/NaOH driven or MX
respectively (Figure S13, S14-15). The complex 2b shows almost driven synthesis of 3a/3b) clearly indicate that; a) sole presence
similar ESI-MS signature to 2a, which reveals the binuclear of K+ or Na+ ions in the solution medium is the key driving force
complexes have identical solution signature (Figure S16). for the self-assembly formation and b), it is independent of the
Finally, the ESI-MS of complex 3a and 3b show the molecular source from which K+/Na+ available (i.e., base or salt).
ion peak m/z of 1740.3 and 1723.3 correspond to the Additionally; c), 3a/3b is thermodynamically more stable in
K[Ni3(HL)6]+ and Na[Ni3(HL)6]+ unit respectively (Figure S17, solution than complex 1. As described in the structural
S18). The conversion of complex 1 to 3a was monitored by description section, (vide infra) the crystallographic evidence of
comparing ESI-MS of 1 before and after addition of KNO3 to it. the cis arrangement of the ligands in 1 makes the topology of
The appearance of distinct m/z of 1740.3 in the ESI-MS the complex to be in a perfect fashion for the spontaneous self-
spectrum of 1 after the addition of KNO3 clearly points towards assembly (3a & 3b) formation.
the formation of SA-complex (Figure S19; this peak is absent
with pristine 1).
The solution conductance measurement in methanol and DMF
clearly indicated that the complexes are 1:1 electrolytes
(Experimental Section) except complex 1, which is neutral.
PXRD and thermal behavior The experimental PXRD patterns
matched well with the simulated ones, confirming the bulk
phase purity of all the complexes (Figure S20‒24). The thermal
stability of the complexes was investigated by TGA analysis
(Figure S25‒29). All these complexes showed multi-step
decomposition pattern as these complexes consist of different
coordinating ligands, counter ions and different types of
solvents of crystallization (water, DMF and methanol) besides
the presence of K+/Na+ in 3a and 3b. Due to this multi-variant
nature of constituents, the weight loss steps are not that well
resolved, however, up to ~80 °C all the complexes are stable
except 2a which shows continues weight loss starting from a
low temperature of ~35°C (Figure S26). From 80 to 390°C, there
is continuous weight loss owing to the decomposition of the
complexes with concomitant loss of ligands and counter ions.
At around 400°C all the organic parts of the complex would
have lost and then the decomposition becomes gradual and
Scheme 1. Scheme of synthesis for 1, 2a, 2b, 3a and 3b complexes and
their inter-conversions. M′X stands for alkali salts where M′= K+(3a) or
Na+(3b) and X = NO3¯ or ClO4¯.
finally results in the formation of metal oxides nearly at a
temperature of 800°C.
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Hence, two possible reasons for the above conversion may be correspond to different fragmented/associated units (see
(i) formation of six extras bonds (K-Oave=2.67 Å) in 3a, without Figure S19). A similar type of observations was reported by
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DOI: 10.1039/C9NJ02573A
any cost, definitely a thermodynamic favorable pathway. (ii) in Dubey et al. based on leucine derived Ni(II) self-assembly
both the complex 1 and 3a, the ligand are in cis fashion around complexes.25b Thus, it is possible to recognize and differentiate
the Ni(II) centers, as a result no additional rearrangement the K+/Na+ driven SA-complex formation by different physico-
within the monomer required prior to 3a conversion. The chemical methods, but the one based on simple UV-Vis
conversion of 1 to 3a can be demonstrated quantitatively by a measurements would be the cheap and fast one.
titration experiment of 1 with KNO3 (Figure 4) which shows the
gradual color change from green (characteristic of 1) to blue
(characteristic of SA-complex), which is also recorded as a
movie (movie_S1, supporting information (SI)). Once the
required molar equivalent of KNO3 is added, further excess
addition has no effect and the spectra remains unchanged for
several days confirming the stability of the SA-complex in the
solution. Indeed, after about a week, shining crystals appeared
in the flask which were confirmed to be as that of 3a from unit
cell comparison with authentic 3a and FTIR matching. To check
the stability of SA-complex at high temperature, we kept blue
solution of 3a at 70 °C for two hours. It did not show any color
change establishing that SA-complex is quite stable, once it is
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self-assembled. Very similar events (formation of SA-complex
and stability at 70 °C) also holds good for conversion of 1 to 3b
with addition of NaClO4 (Figure S30).
The optical signatures of the SA-complexes (3a and 3b) are
quite unique and different from mononuclear complex 1 and
thus aid as a quick method to check their formation. In DMF, 1
shows three broad absorptions bands at ~379, 636, 1025 nm
varying amount potassium nitrate (30 mM per each addition) in DMF.
(Figure S11, SI) whereas 3a and 3b show three strong bands
Figure 4. UV-visible spectra (titration) of 1 solution (2 mL, 70 mM) with
Colour change of
1 (green) to 3a (blue) shown as inset.
around 387, 553, 855 nm, and a broad band at 1181 nm (Figure
S12, table S5) which are in agreement with the octahedral Ni(II)
environment.27 There are few multiple overlap transitions in
the region of 3000-1600 nm which are due to lower than
octahedral symmetry, which generally lifts the degeneracy of
the usual three transitions observed for Ni(II) systems.27 A small
sharp absorption maxima at ∼755 nm arising from the
forbidden singlet state is also seen in many other six
coordinated Ni(II) complexes.25 There is an additional spectral
feature that needs a special mention here. The spectral pattern
of 3a and 3b looks very similar, indicating their structural
similarity in solution, which is of course anticipated. However, a
closer look at the spectra shows two marked dissimilarities of
about 14 nm and 60 nm blue-shift in 3b, in comparison with 3a.
This occurs for the two lower bands at 836.8 and 1120.4 nm
respectively (Figure S12, and Table S5). These shifts can be
correlated to the difference in charge/radius ratio of the two
ions (K+, Na+) that bind with the already Ni-coordinated
carboxylates. This difference causes minor perturbation in the
ligand strengths around the Ni(II), and causes the blue shift.
Spectroscopically, this is highly advantageous as it gives a
simple but elegant way to distinguish between K+ and Na+
bound self-assembly process in this family of complexes.
Additional evidence for the conversion of 1 to 3a was further
gathered by comparing ESI-MS of pristine 1 and that after
adding KNO3. The distinct m/z of 1740.3 (absent in the ESI-MS
spectrum of 1 before KNO3 addition), appeared as soon as KNO3
solution is added, confirming the formation of K-bound self-
assembly (3a) along with several other minor peaks that
Binding constant of SA-complex (3a) was determined as per the
reported procedure28 and found to be 6.633×10-4 mM-3L3.
(Figure S31-S37, see supporting information for details). As
mentioned earlier, the dissociation of SA-complexes 3a/3b is
not easy and could not be realized even at elevated
temperatures; hence the disassembly has to be achieved by
forceful means and we resorted to the well proven ability of
crown ethers to capture K+ ions in solution (18-crown-6 has
best affinity (106 M-1) for potassium ions.29 As anticipated,
disassembly of 3a to 1 readily took place in the presence of the
18-crown-6 and it was quantitatively monitored with UV-visible
titration experiment of 3a with the crown ether as the titrant
(Figure S36). There is increase in absorbance at λmax = 633 nm
corresponding to 1 and decrease in absorbance at λmax = 851
nm corresponding to 3a upon addition of crown ether (0.2 M)
to solution of 3a (0.2 M), which indicates the disassembly of 3a
to 1. This conversion occurs due to the loss of potassium
species from the self-assembly due to higher binding affinity of
crown ether towards potassium ion. The above transformation
is also visually perceptible as a distinct color change from blue
to green and it is presented as a movie (movie_S2) in SI.
Our previous experience26 shows that, when NiX2 (X=
-
-
CH3COO-, ClO4 , NO3 ) reacts with ligand H2L (L=Histidine derived
Salhis ligand) in the presence of alcohol amines in alkaline
medium (KOH/NaOH), a binuclear complex similar to 2(a/b)
(Scheme-1) is invariably formed. Surprisingly, similar reactions
with the present H2L (L=Methionine derived Salmet ligand)
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always ended with SA-complexes and mononuclear complex 1; activities such as that in methionine-sulfoxidereductase-A
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the binuclear complex 2(a/b) was never formed. Two crucial (MsrA)22 and model studies may show further insights.
DOI: 10.1039/C9NJ02573A
understandings gathered from the synthesis of 1 and 3a (viz., i)
Our observation of spontaneous self-assembly of 1 to 3a/3b
acetate ion is a must, ii) should avoid alkali ions) advocated us prompted us to examine whether the dinuclear complex 2a and
to perform the reaction restricting to Ni(acetate)2 only as the 2b also possess similar propensity to form SA-complexes upon
metal source and to completely avoid the use of KOH/NaOH as addition of K/Na salts. Keeping the stoichiometry of nickel:
base. This way, the reaction was successful; we could isolate ligand in 2a (M:L= 1:1) and in SA-complex (M:L=1:2) in mind,
the expected binuclear Ni(II)-Salmet complexes. 2a was formed when two additional molar equivalent of ligand and 1/3 molar
when pure methanol was used as the solvent, but MeOH-DMF equivalent of KNO3 were mixed with one equivalent of 2a,
mixture as the reaction medium resulted in 2b. Both complexes there was an instant color change (green to blue, visually
could be isolated and detailed packing analysis could be done perceptible), which clearly indicated the formation of 3a. The
with the aid of X-ray crystallography. In this reaction, the resulting blue solution was stable at room temperature without
acetate ion takes the role of a self-acting base and, notably, any change for several days and finally got crystallized as 3a
also acts as a ligand by coordinating to the metal center and, (confirmed by unit cell checking and FTIR). Our curiosity to
eventually a bowl shaped binuclear complexes with outward check the stability of the SA-complex in this reaction prompted
opening capable of binding the ethanolamine (Scheme-1) is us to heat the solution and, interestingly, the color of the blue
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formed.
solution (typical color of SA-complex) became green (typical
The remarkable difference between 2a and 2b is the release color dinuclear complex) at about 65 °C (Figure S39). This can
of one of the methionine side arms as ‘free’ in 2b and the be attributed to the dissociation of the self-assembly and
incorporation of one DMF molecule into the Ni(II) coordination reverting back to 2a which is more stable at elevated
sphere. In this way, the two Ni(II) centers become distinctly temperatures. After auto-cooling, the green solution again
different and 2b becomes unique in such a way that it has both comes back to its original blue color within 15 minutes or
coordinating and free S-bearing side arms of L-methionine in cooling with ice-cold water brings the color back to blue in 1-2
the same molecule. Indeed, there are only very few compounds minutes. This process can be repeated for several times
in which methionine ‘S’ is coordinated to Ni(II) centers30 and 2b (movie_S3, SI). Same sequence of events can be repeated with
becomes the first structure with both coordinated and free side dinuclear complex 2b also. It is pertinent to mention that
arm in the same molecule. Owing to the various important role disassembly of 3a to 1 by heating was not possible (see the
of L-methionine in biology, the present solid state binuclear second paragraph of this section) where the SA-assembly
systems demonstrate that it is possible for S atoms to solution is devoid of any ethanolamine. In its absence, 3a/3b
coordinate or stay ‘free’ depending upon the overall reaction does not go back to 1 even at 70 °C indicating that K+/Na+
environment and this may be significant in understanding bound SA-complex is quite stable in the absence of
methionine-S metal interactions in enzymes or metallo-protein ethanolamine. External addition of ethanolamine to 3a solution
related studies.
and heating produced the color change from blue to green,
A comparison of various product formation of two other showing the requirement of ethanolamine as well as heat
salicylaldehyde derived reduced Schiff bases (Salhis and Salleu towards dissociation of 3a to 2a. Another reason for the above
where histidine and leucine are the respective amino acids) conversion may be due to easy deprotonation of phenol groups
with Ni(II) centers26a,31a and that of Salmet (present study) of the ligand in presence of ethanolamine (base) and heat, a
clearly points out the importance of the flexibility and must structural requisite for the formation of 2a.
coordinating ability of the R group (side arm) of the amino acid
in stabilizing various architectures (both in solution and in solid
The presence of multiple magnetically active Ni(II) centers
state). A perusal to Figure S38, where a schematic comparison held together by Salmet and other ligands in the di- and SA-
is presented, clearly shows that Salmet ligand is capable of complexes can give valuable information about magnetic
generating all the three architectures (i.e., monomer, dinuclear interactions and it can augment the structural observations
and SA-complexes) whereas restrictions are encountered with attained through X-ray crystallography. Results from such
Salhis (no SA-complex) and Salleu (no dinuclear complex studies are appended below.
formation). A plausible rationale can be advanced for this
structural variance; Salhis is endowed with strong binding unit Magnetism-structure relationship
(imidazole ‘N’) in the side arm whereas Salleu has only a passive
The magnetic properties of 2a, 3a and 3b were investigated
non-coordinating aliphatic isobutyl side chain. On the other
through variable-temperature susceptibility measurements in
hand, Salmet is a ‘middle-runner’, it has a weaker binding site
the temperature range of 20 to 300 K with an applied magnetic
field of 1000 Oe (Due to identical Ni(II) centers, 2b is not
studied). The plotted χ×T as a function of temperature shows a
quite different signature for 2a and 3a-3b (Figure 5). Isothermal
(thioether moiety) as the side arm, which may or may not
participate in metal coordination depending upon
presence/absence of other donors and reaction conditions. In
effect, a sort of tunability has got inbuilt in the ligand, thus
magnetization measurement shows non-linearity and sensitivity
leading to all the three products. This sort of inherent flexible
increases abruptly with lowering the temperature (Figure 6),
nature (i.e., coordinate or stay free) of the S-methyl thioether
while saturation in magnetization increases monotonically with
group in methionine may have a role in observed biological
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the decrease in temperature. The observed magnetization vs Figure 6. Isothermal magnetization of specimen Vi2ewa Aratictle Ovanrlinioeus
temperature data fitted with Curie-Wiess Law [χ=C/(T-)], temperatures (2K, 4K, 6K, 8K and 10K) and field ranges from -7T to +7T.
where C and  are Curie constant and Wiess constant, As temperature is lowered nonlinearity of MH curve arises. At lowest
respectively] where the positive Curie temperature found for temperature, 2K, MH curve gets saturated in high field region. MH
2a (+12.5K), and negative for 3b (-9.5K) and 3a (-11K) curve of 2K and 4K are having almost equivalent magnitude of
DOI: 10.1039/C9NJ02573A
confirming the ferromagnetism character for 2a and
magnetization. This thermally unusual behaviour can be assigned as
non-conventional magnetic interaction and may be appreciated as
geometrical frustration. Inset 3a and 3b are showing ascending branch
of MH curve of specimen 3a and 3b, respectively. Sensitivity of MH for
3a and 3b are different from geometrically frustrated 2a..
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Further, via Arrott plot (M2 vs H/M), a linear and positive
intercept (y-axis) resulted for 2a further confirms weak
ferromagnetic interaction and negative intercept confirms
antiferromagnetic nature of 3a and 3b (Figure S40).
The temperature dependence of magnetic entropy (-ΔS_M)
at various strength of magnetic field using Maxwell’s equation
[휟푺푴 = ∫푯풅푴풅푯 ―∆푺(푻,푯)
.
] were calculated. The result
풅푻
ퟎ
showed a broad anomaly at low temperature with small
magnitude of 0.5J/kg-K (Figure S41). It conforms that weak
antiferromagnetic interaction is mediated through the ligand in
3a and 3b. Anomaly in -∆S(T,H) of 2a continuously shift towards
higher temperature. 2a exhibits reasonably large -∆S=14J/kg-K
for 7T magnetic field and validates evolution of field induced
ferromagnetic phase.
Figure 5. Behaviour of χ×T is quite different for binuclear (2a) and self-
assembly (3a, 3b). For 3a and 3b, χ×T decreases on lowering
temperature of specimen, which is signature of antiferromagnetic
interaction. 2a shows broad anomaly at low temperature. This suggests
a change in nature of magnetic interaction and upturn anomaly is
inherent characteristics of geometrical frustration among nearest
neighbour of magnetic lattices of 2a specimen.
Conclusions
The present study convincingly demonstrates for the first time
that it is possible to construct a variety of L-methionine derived
reduced Schiff base Ni(II) complex architectures, furthermore,
they get transformed to one another depending upon the
reagent/reaction conditions. The mononuclear complex 1 can
be transformed to different dinuclear complexes (2a/2b) and all
of these can further reorganize and self-assemble around alkali
metal (K+/Na+) ions to form tri-nuclear SA-complexes (3a/3b).
The unequivocal structural identity of the mono-, di- and tri-
nuclear complexes has been established by single crystal X-ray
crystallography which shows how the weakly coordinating thio-
ether side chain of the amino acid influences the observed
molecular architectures (either by coordinating with Ni(II)
center or by staying free). Notably, the cis-configuration of the
ligands in 1 facilitates the instant self-assembly formation with
the help of alkali metal ions (K+, Na+) and its thermal
decomposition back to the mononuclear complex 1 cannot be
realized by heating (70 °C) showing its high stability. But, more
compelling 18-crown-6-ether can pull back the templating K+ to
its core that results the disassembly of 3a and regeneration of
1. Additional supporting evidences for the formation of these
complexes are gathered through ESI-MS investigations, UV-Vis
titration experiments and variable temperature magnetic
measurements. The inter-conversion among di-trinuclear
complexes is thermally facile and visually perceptible with
associated color change. In the presence of DMF, the ‘flexible
side arm’ of the ligand may give way to DMF for coordination
easing it to be free, thus generating 2b from 2a.The facile
antiferromagnetic for 3a and 3b.32 To explore the magnetism-
structure relationship further, the plotted of χ×T and
temperature were explored and it is observed that for 3a and
3b, χ×T decreases with lowering temperature further confirms
antiferromagnetic interaction in the self-assembly systems and
structural transformation from 2a to 3a favors a change in spin-
spin interaction.33
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formation and easy transformations of mononuclear/poly-
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variable ligation controlled functions of biological systems.
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Conflicts of interest
“There are no conflicts to declare”.
Keywords: Self-assembly• Ni(II) complexes• Amino acid side
chain• interconversions •Reduced Schiff base ligands.
Acknowledgements
R. K. thanks the UGC, India for fellowship, S.C.S. acknowledge
UGC-BSR for financial, DST-FIST for single crystal facility, DST-
PURSE and CAS for departmental support. V. M. is thankful to
SERB for National Post-Doctoral Fellowship (File No:
PDF/2017/002750).
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The table of contents entry:
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A series of Ni(II) complexes of a L-methionine derived reduced Schiff base ligand (Salmet) has been synthesized,
characterized with the help of a variety of physico-chemical techniques including X-ray crystallography. Alkali ions
K+/Na+ can serve as efficient templates for the conversion of monomers to multi-nuclear complexes. Different reactant/
reagent conditions lead to interconversions among these complexes which further can be established by ESI-mass and
UV-Vis spectroscopy. Variable temperature magnetic analysis shows weak ferromagnetic and antiferromagnetic
coupling for binuclear and self-assembly complexes respectively.
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TOC figure: