Population density at variable temperature in [Fe(L3)2]Cl·3H2O (7).
Abstract
The fabulous advancement of a large section of modern coordination chemistry depends upon different kinds of strategically designed and functionally tuned ligand systems; Schiff base ligands play a pivotal role among them. Such Schiff bases become more motivating when they are designed to be synthesized using very simple organic molecules. This paper reviews our work on a family of three functionally different types of Schiff base ligands, derived from diacetylmonoxime, which have been employed to synthesize mononuclear metal complexes with various binding modes of ligands and topologies around the metal centers. Such Schiff base ligands have been synthesized by reacting diacetylmonoxime with diethylenetriamine, 1,3-diaminopropane-2-ol, and morpholine N-thiohydrazide. The synthesized Schiff bases and the metal complexes of such “privileged ligands” show many interesting supramolecular coordination architectures involving different weak forces, e.g., H-bonding, C–H···π interactions, etc.
Keywords
- Schiff base
- diacetylmonoxime
- crystal structure
- weak force interactions
- semiconducting behavior
- optical properties
1. Introduction
The synthesis and characterization of metal complexes of Schiff bases have been started since 1865. But the importance of Schiff base ligands in several fields compelled us to consider it as a “privileged ligand” even in recent days [1, 2]. Currently, there has been considerable interest in the chemistry of Schiff base metal complexes, primarily because of their tremendous biochemical activity, viz., antibacterial [3, 4], antimalarial [5, 6], antiviral [7, 8], and antitumor activities [9, 10, 11]. Besides, some transition metal Schiff base complexes were found to be efficient catalysts in organic synthesis [12, 13, 14, 15, 16, 17, 18]. Such types of Schiff base metal complexes are also very interesting for opening of a new pathway in crystal engineering [19, 20]. Various kinds of supramolecules of diverse fascinating structures are being synthesized by different Schiff bases [20, 21]. Different types of weak force interactions (e.g., H-bonding, π···π, C–H···π, etc.) are responsible for construction of such new metal organic frameworks (MOF) [22, 23, 24]. Multinuclear metal complexes of Schiff base are of real importance on the field of magnetochemistry [25, 26, 27, 28]. Various ferro- and antiferro-type magnetic interactions are responsible for the generation of different magnetic materials. Besides, the solid-state properties, e.g., variable temperature conductivity, and optical properties of such complexes are also producing very interesting results which are extremely important in materials chemistry. Very recently the comparison of different observed physical properties with the theoretically predicted values is being done by the use of DFT approach [29, 30, 31, 32].
In the above context, the development of new pathways for the synthesis of new Schiff base ligands and their metal complexes is of immense significance. The strategic pathway becomes more important when the Schiff base ligands and their corresponding metal complexes are produced in a controlled approach fulfilling the main objectives of the synthesis. We have chosen easily available, exceptionally economical, and full of exciting properties organic molecule, diacetylmonoxime, as our precursor molecule for the synthesis of many new Schiff base ligands by reacting it with different molecular amine systems. One of the main advantages of such Schiff base ligands is the change in their ligational behavior depending on the metallic systems and the stoichiometry.
In this review, we have selected only three Schiff base ligands (Figure 1) derived from diacetylmonoxime and three different amine systems. Though a huge number of metal complexes have been synthesized and characterized using such ligand systems, only the structure of the ligands and metal complexes for which single crystal or PXRD have been determined, are discussed in this mini-review including the weak force interactions depicted therein. Some of the solid-state properties, viz., electrical and optical properties of such complexes, are also discussed to enlighten their fascinating material properties.
2. Synthesis of the ligands
2.1 Mono-imine Schiff base (L1) ligand
The stoichiometrically controlled condensation reaction of diacetylmonoxime (dam) (1.01 g, 10 mmol) and diethylenetriamine (dien) (1.04 g, 10 mmol) in 1:1 molar ratio in methanol (15 ml) on constant stirring for 45 min at room temperature and then refluxing for 2 h on water bath (Figure 2) afforded the monocondensed amine-imine-oxime Schiff base ligand 3-((2-((2-aminoethyl)-amino)ethyl)imino)butan-2-one oxime (
2.2 Di-imine Schiff base (H 2 L 2 ) ligand
The condensation reaction of 1,3-diaminopropane-2-ol (0.45 g, 5 mmol) (dapol) with diacetylmonoxime (1.01 g, 10 mmol) in 1:2 molar ratio in methanol (25 ml) under gentle reflux for 2 h yielded the tetradentate bicondensed di-imine Schiff base 3,3′-((2-hydroxypropane-1,3-diyl)bis(azanylylidene))bis(butan-2-one) dioxime (
2.3 Thio-hydrazone Schiff base (H 2 L 3 ) ligand
The condensation reaction of diacetylmonoxime (1.01 g, 10 mmol) with morpholine N-thiohydrazide (mth) (1.6 g, 10 mmol) in 1:1 molar ratio in ethanol (30 ml) on refluxing for 2 h afforded a gummy mass with very low yield. The isolation of solid ligand in pure form with high yield is still a challenge. Considering the low yield of the ligand, all the complexation reactions with this ligand were carried out under in situ condition. However, the molecular thiol form of the ligand
A slight modification of synthetic procedure by continuing the refluxing procedure for 16 h with few drops of water in 1:1 molar ratio in ethanol (50 ml) (shown in the left part of Figure 4) yielded a light yellow crystalline solid compound (yield 45%). X-ray diffraction study of single crystal along with other analytical data of this compound inferred a zwitterionic structure of a nitrogen–sulfur heterocyclic compound, N-(3,4-dimethyl-1,2,5-thiadiazole-2-ium-2-yl)morphine-4-carbathio-ate (abbreviated as
The hydrogen atom attached with N atom of hydrazide group can undergo thione-thiol tautomerism (Figure 5). Thus NNS coordination mode is facilitated during the formation of complexes [34].
2.3.1 Crystal structure of L 4
The molecular structure of
3. Synthesis of metal complexes
3.1 Metal complexes with mono-imine Schiff base (L1) ligand
With this new neutral N4 donor ligand system, a crystalline Ni(II) complex has been synthesized. Two routes of synthesis of the nickel complex (
3.2 Metal complex with di-imine Schiff base (H2L2) ligand
The di-imine Schiff base ligand (H2L2) was employed for the synthesis of a oxovanadium complex, the PXRD of which have also been determined. Reflux of equimolecular mixture of
3.3 Metal complexes with thio-hydrazone Schiff base (H2L3) ligand
The thio-hydrazone Schiff base ligand (H2L3) is very interesting, and the thiol form of the Schiff base is always observed for the binding purposes (see Figure 5) with the metal systems during complexation. Most interestingly, this ligand is found to show two types of binding modes (Figure 9), one is observed through N,S donor atoms and another one through N,N,S donor atoms. An efficient control over the ligand for binding through either N,S or N,N,S mode has been achieved though specific choice of metal systems [34, 35, 36, 37, 38].
An organometallic complex, [PhHg(HL3)] (
The Cd(II), Cr(III), and Fe(III) complexes, i.e., [Cd(HL3)2)] (
Another zinc(II) complex (
The complexes of Ni(II) (
4. Crystal structures, PXRD, and some interesting properties of the metal complexes
4.1 Crystal structure and catalytic properties of nickel(II) complex with the mono-imine Schiff base ligand (L1)
The neutral monomeric complex [Ni(L1)(NCS)2] (
The coordination environment around the nickel(II) ion is surrounded by N6 fashion (four N from ligand and two N from thiocyanate ions) tending towards distorted octahedral geometry. The Ni2+ center is not lying exactly within the equatorial plane of N4 moiety, and unequal axial and equatorial bond distances (2.112 Å and 2.072 Å, respectively) confirm the distortion. The non-coordinated O–H groups on the ligand L1 are engaged in H-bonding interactions with thiocyanate S atoms (Figure 6) which lead to 1D supramolecular sheet-like arrangement (Figure 13). These H-bonding interactions lead to O···S separations of 3.132 Å and play prominent role in crystal packing.
4.1.1 Catalytic activity of complex 1
Analytical grade reagents and freshly distilled solvents, viz., water, acetonitrile, methanol, and dichloromethane, were used to check the catalytic activity. The oxidation reaction was carried out in liquid phase under vigorous stirring in two-necked round bottom flask fitted with a water condenser and placed in an oil bath at 60°C. Substrate (5 mmol) was taken in 10 ml solvent(s) for different sets of reactions along with 5 mg catalyst, to which 10 mmol of
The [Ni(L1)(NCS)2] (
4.2 PXRD structure and solid-state properties of oxovanadium(IV) complex with the di-imine Schiff base ligand (H2L2)
Despite our repeated attempts and best effort, the single crystal of the oxovanadium(IV) complex with the ligand H2L2 could not be grown, and it led us to carry out the powder X-ray diffraction (PXRD) study to characterize the oxovanadium(IV) complex
Other solid-state properties, viz., electrical, optical, and thermal properties of the complex [VO(L2)], have also been studied [24]. The complex is electrically an insulator at room temperature; however, the conductivity is increased as the temperature increases from 330 K, indicating the semiconducting nature of the complex. It behaves as an n-type semiconductor, and the semiconducting behavior of the oxovanadium(IV) complex with the dibasic Schiff base ligand was substantiated by the extended conjugated chemical structure. The said properties are discussed in detail in the following sections.
4.3 Crystal structures and properties of metal complexes with thio-hydrazone Schiff base ligand (H2L3)
4.3.1 Crystal structure of organometallic phenylmercury(II) complex (3 )
The organometallic phenylmercury(II) compound
The Hg(II) atom remains 0.027(1) Å above the plane. Due to the contribution of electron flow from mercury to the π* orbitals of the phenyl group, the Hg–C bond distance is found shorter than that in the analogous methylmercury(II) compound, where no such electron drifting is observed. The C–S bond gets partial double bond character in the complex, similar to related thiosemicarbazonates of methylmercury(II) and dimethylthallium(III). It is interesting to note that there is no intermolecular π–π interaction between the phenyl rings. But a weak interaction between C(8)–H(8A) and a π group (phenyl ring) links the two phenylmercury molecules into a supramolecular dimer having a C–H π synthon (Figure 16) having characteristic H···Cg distance 2.84 Å, where Cg is the midpoint of the phenyl ring.
4.3.2 Crystal structure of the zinc(II) complex (4 )
The X-ray crystal structure shows that due to constrained ligand structure, the [Zn(HL3)(OAc)(H2O)].H2O complex
Due to intramolecular hydrogen bonding, the Zn–N(3) (azomethine) distance is slightly shorter than Zn–N(4) (oxime) distance. Here the H(1) of coordinated water molecule is hydrogen bonded to morpholinic oxygen O(1), while H(2) is hydrogen bonded to acetate oxygen O(3), and such H-bonding forms the 1D supramolecular framework diagonal to the
4.3.3 Crystal structure of the cadmium(II) complex (5 )
The cadmium(II) complex [Cd(HL3)2] (
Complex
One of the most interesting parts of the structure is that the crystal structure possesses a remarkably short intermolecular C(sp3)···O(sp3) contact [C9···O4* 2.958(3) Å]. An interesting packing force using an uncommon C(sp3)···O(sp3) interaction has been reported. The three H atoms (H9A, H9B, H9C) of the methyl group of the propylidene moiety form a triangular plane which is supported by the angle ∠C8–C9···O4* of 172.0°. The O4 atom of the oxime is directed towards the center of the plane formed by three H atoms (H9A, H9B, H9C) of the methyl group. The C9–H9A···O4*, C9–H9B···O4*, and C9–H9C···O4* angles are 89.0°, 83.8°, and 97.6°, respectively, which are less than the generally accepted ∠C–H···O angles (110°), and such short bond angles might be responsible for the greater C···O attraction. Thus, the cooperative effect of the three individual interactions acts on the face of the plane of the three methyl hydrogens of the C9 atom due to their high acidic character. This very unusual and nonconventional interaction was termed as a CH3···O interaction and proved to be a good supramolecular synthon. The trifurcated H-bonding interactions are shown in (Figure 21).
4.3.4 Crystal structure of the chromium(III) complex (6 )
The X-Ray single-crystal analysis revealed that complex [Cr(L3)2]Cl·3H2O (
4.3.5 Crystal structure of the iron(III) complex (7 )
The X-ray structure of [Fe(L3)2]Cl·3H2O (
The most interesting part of the crystalline structure of complex [Cr(L3)2]Cl·3H2O (
The 1D chains are arranged in parallel direction to form a supramolecular host having channels along the
It is proven by the crystal structure analysis that there are three crystals of water molecules per formula unit of
4.3.5.1 Magnetic property and Mössbauer spectroscopy of complex (7 )
The variable temperature (2.5–300 K) magnetic moment study shows the temperature dependence of the magnetic susceptibility. The χm values at 2.5 and 300 K are 0.33 and 0.005 cm 3 mol−1, while the μeff values are 2.61 and 3.46 B.M., respectively. The detailed study shows that the magnetic moment value consists of a superimposition of both the low-spin and high-spin states. At very low temperature, the 1-D supramolecular species which is formed by strong intermolecular C–H···O interactions and the cooperative interactions with the “water-chloride” cluster between mononuclear spin crossover (SCO) sites stabilize the low-spin state, and thus the high-spin contribution decreases to 21%, and the low-spin contribution increases to 79%. Thus, such variable temperature magnetic behavior may be due to a continuous S = 1/2 to 5/2 spin crossover phenomenon of iron centers [36].
The Mössbauer spectroscopic study also supports that a spin crossover phenomenon exists in the iron(III) complex (
Temperature | Spin state | Occupancy |
---|---|---|
300 K | Low spin (1/2) | 49% |
High spin (5/2) | 51% | |
20 K | Low spin (1/2) | 77% |
High spin (5/2) | 23% |
4.3.6 PXRD, SEM, and EDX studies of complexes (8 )–(11 )
Suitable single crystals of complexes (
Cell parameters | [Zn(L3)2]. 2H2O (8) | [Ni(L3)(OAc)] (9) | [Co(L3)(OAc)]. H2O (10) | [Cu(L3)(OAc)]. H2O (11) |
---|---|---|---|---|
System | Triclinic | Monoclinic | Monoclinic | Monoclinic |
V (Å3) | 1294.88 | 810.3 | 1127.7 | 1044.17 |
a (Å) | 10.297368 | 6.364172 | 19.600876 | 18.953438 |
b (Å) | 11.32531 | 27.497931 | 5.53422 | 6.365518 |
c (Å) | 12.345947 | 4.686936 | 12.32786 | 8.729238 |
α | 111.516869 | 90 | 90 | 90 |
β | 103.288712 | 98.92 | 122.51 | 97.5 |
γ | 91.155464 | 90 | 90 | 90 |
The SEM investigation of all the above complexes, the ground powders, and the fracture surfaces indicates that the grain size distribution is not uniform, and submicron grains (finely ground powder) as well as grains (fracture surfaces) even above 20
The formation of metal–ligand complexes and the presence of metal along with C and S within the metal complexes have been substantiated by the EDX analysis.
4.4 Electrical conductivity
To explore the utility of the metal complexes as functional materials, the electrical conductivity study was performed, and it shows the semiconducting nature of the complexes [33, 37, 38].
The samples for the measurement of electrical conductivity were prepared from the complexes in the form of tablets of approximately thickness ∼0.1 cm at a pressure of ca. 1 × 108 Pascal. These tablets were placed between two copper electrodes covered with silver paste, and contacts of the prepared tablets were to be Ohmic or not. A two-probe method was used to investigate the electrical conductivities of the complex tablets by measuring the current through the probes with a high impedance electrometer (Keithley 6514) upon application of a DC voltage current supplied by a programmable source of voltage (Keithley 230). The conductivities were calculated by using the general equation of σ = (I/Vc)(d/a), where (I) is the current in ampere, Vc the potential drop across the sample of cross-sectional area (a), and is the thickness (d).
Variation of electrical conductivity of a compound behaving like semiconductor with temperature can be obtained by the Arrhenius equation:
where σ is the electrical conductivity, σo denotes the pre-exponential factor,
If the graph obtained is linear (i.e., fitted with one straight line), then it may be concluded that no molecular rearrangement occurred during heating and the compound will have only one
Conduction corresponding to the region I is attributed to the intermolecular conduction via weak force interactions between the molecules. The charge carriers hop near Fermi level within the localized state. Delocalized π-electrons are mainly responsible for this conduction, whereas conduction corresponding to the region II is attributed to intramolecular conduction between the metal center and the ligand center within a metal complex. This conduction occurs due to tunneling of electrons between equivalent HOMO and LUMO of the ligand and metal ion, respectively. Such tunneling of electrons through the intermolecular potential barrier is reinforced through π–π stacking and extensive H-bonding [24]. Depending on the availability of π-electrons, the compound behaves like n-type semiconductor.
From the Arrhenius plots (Figure 27), the electronic parameters, i.e., activation energy of electrical conduction (Ea) and the energy gap for directly allowed transitions of metal complexes (
Complex | Ea1 (eV) | Ea2 (eV) | Egd (eV) |
---|---|---|---|
(Lower temp) | (Higher temp) | ||
[VOL2] ( | 0.48 | 1.18 | 3.45 |
[Zn(L3)2].2H2O ( | 0.59 | 4.2 | 2.52 |
[Ni(L3)(OAc)] ( | 0.54 | — | 2.75 |
[Co(L3)(OAc)].H2O ( | 0.97 | 0.76 | 2.37 |
[Cu(L3)(OAc)].H2O ( | 0.34 | 2.14 | 1.58 |
It is also very clear from the Arrhenius plots that the conductivity of metal complexes generally increases with increase in temperature. At room temperature they behave as an insulator, while at higher temperature the semiconducting nature of complexes is observed.
4.5 Optical properties
Optical absorption spectra was taken by using a UV–VIS spectrophotometer (Perkin Elmer Lambda 2S/45 Double Beam) and measured as function of wavelength in the wavelength range 190–1100 nm.
The energy band gaps and the nature of the optical transitions involved in the metal complex framework systems have been practically determined by the fundamental absorption edge analysis of the recorded optical transitions using the theory of Mott and Davis [39]. It is also observed that the semiconducting behavior of a material increases with rise in temperature which may also damage the actual molecular structure of the material. Hence, Tauc method is used to calculate the energy band gap through optical absorption properties [40].
Utilizing the relation between the optical linear absorption coefficient (α) with photon energy (hν), the energy band gap (Eg) between the top of the valence band and bottom of the conduction band can be determined using equation (Eq. (2)):
where A is a constant characteristic parameter of the respective transition independent of ν.
The values of n depend on the kind of optical transitions. For directly allowed, directly forbidden, indirectly allowed, and indirectly forbidden transitions, the values of n are ½, 3/2, 2, and 3, respectively. Thus the energy band gap for directly allowed (Egd) and indirectly allowed (Egi) transitions can be determined by relating Eq. (2) as follows:
and
where Egd and Egi are direct and indirect energy gaps, respectively.
To calculate the direct and indirect energy band gap, we need to plot a curve of (αhν)2 against f(hν) and (αhν)1/2 against f(hν) and then by the extrapolation of the most linear part of the curve to zero.
The satisfactory graphs were obtained for the metal complexes (
5. Conclusion
In this review, the synthesis, crystal structure, and solid-state properties of three Schiff base ligands derived from diacetylmonoxime with diethylenetriamine, 1,3-diaminopropane-2-ol, and morpholine N-thiohydrazide and their metal complexes have been vividly discussed. A zwitterionic nitrogen–sulfur heterocyclic compound with nonbonded S···S interaction has also been reported to be formed by the reaction of diacetylmonoxime with morpholine N-thiohydrazide under long refluxing (16 h) condition in ethanol. The single X-ray crystal structures have shown many beautiful weak force interactions including a CH3···O trifurcated interface communication. Wherever the single-crystal structures could not be grown, the PXRD study has enlightened their structural features. The electrical and optical properties also explored the semiconducting nature of some of the metal complexes. It is also observed that the electron transport process gets influenced by the supramolecular frameworks of the metal complexes.
Acknowledgments
One of the authors (S.S.) is thankful to the UGC (ERO), Kolkata, for financial grants (MRP) to carry out a part of this work and also to Prof. Y. Aydogdu, Department of Physics, Gazi University, and Dr. S. Biswas, our lab-mate for some useful discussion.
References
- 1.
Yoon TP, Jacobsen EN. Privileged chiral catalysts. Science. 2003; 299 :1691-1693. DOI: 10.1126/science.1083622 - 2.
Slassi S, Aarjane M, Yamni K, Amine A. Synthesis, crystal structure, DFT calculations, Hirshfeld surfaces, and antibacterial activities of Schiff base based on imidazole. Journal of Molecular Structure. 2019; 1197 :547-554. DOI: 10.1016/j.molstruc.2019.07.071 - 3.
Qian HY, Sun N. Synthesis and crystal structures of manganese (III) complexes derived from bis-Schiff bases with antibacterial activity. Transition Metal Chemistry. 2019; 44 :501-506. DOI: 10.1007/s11243-018-00296-x - 4.
Sarkar S, Dey K. A series of transition and non-transition metal complexes from a N4O2 hexadentate Schiff base ligand: Synthesis, spectroscopic characterization and efficient antimicrobial activities. Spectrochimica Acta Part A. 2010; 77 :740-748. DOI: 10.1016/j.saa.2010.06.041 - 5.
Sharma M, Chauhan K, Srivastava RK, Singh SV, Srivastava K, Saxena JK, et al. Design and synthesis of a new class of 4-aminoquinolinyl-and 9-anilinoacridinyl Schiff base hydrazones as potent antimalarial agents. Chemical Biology & Drug Design. 2014; 84 :175-181. DOI: 10.1111/cbdd.12289 - 6.
Ziegler J, Schuerle T, Pasierb L, Kelly C, Elamin A, Cole KA, et al. The propionate of heme binds N4O2 Schiff base antimalarial drug complexes. Inorganic Chemistry. 2000; 39 :3731-3733. DOI: 10.1021/ic000295h - 7.
Chang EL, Simmers C, Knight DA. Cobalt complexes as antiviral and antibacterial agents. Pharmaceuticals. 2010; 3 :1711-1728. DOI: 10.3390/ph3061711 - 8.
Chen Y, Li P, Su S, Chen M, He J, Liu L, et al. Synthesis and antibacterial and antiviral activities of myricetin derivatives containing a 1, 2, 4-triazole Schiff base. RSC Advances. 2019; 9 :23045-23052. DOI: 10.1039/C9RA05139B - 9.
Venkateswarlu K, Ganji N, Daravath S, Kanneboina K, Rangan K. Crystal structure, DNA interactions, antioxidant and antitumor activity of thermally stable Cu (II), Ni (II) and Co (III) complexes of an N, O donor Schiff base ligand. Polyhedron. 2019; 171 :86-97. DOI: 10.1016/j.poly.2019.06.048 - 10.
da Silveira VC, Luz JS, Oliveira CC, Graziani I, Ciriolo MR, da Costa Ferreira AM. Double-strand DNA cleavage induced by oxindole-Schiff base copper (II) complexes with potential antitumor activity. Journal of Inorganic Biochemistry. 2008; 102 :1090-1103. DOI: 10.1016/j.jinorgbio.2007.12.033 - 11.
Das U, Pattanayak P, Santra MK, Chattopadhyay S. Synthesis of new oxido-vanadium complexes: Catalytic properties and cytotoxicity. Journal of Chemical Research. 2018; 42 :57-62. DOI: 10.3184/174751918X15168821806597 - 12.
Cozzi PG. Metal–Salen Schiff base complexes in catalysis: Practical aspects. Chemical Society Reviews. 2004; 33 :410-421. DOI: 10.1039/B307853C - 13.
Gupta K, Sutar AK. Catalytic activities of Schiff base transition metal complexes. Coordination Chemistry Reviews. 2008; 252 :1420-1450. DOI: 10.1016/j.ccr.2007.09.005 - 14.
Pouralimardan O, Chamayou AC, Janiak C, Hosseini-Monfared H. Hydrazone Schiff base-manganese (II) complexes: Synthesis, crystal structure and catalytic reactivity. Inorganica Chimica Acta. 2007; 360 :1599-1608. DOI: 10.1016/j.ica.2006.08.056 - 15.
Bagherzadeh M, Mahmoudi H, Ataie S, Hafezi-Kahnamouei M, Shahrokhian S, Bellachioma G, et al. Synthesis, characterization, and comparison of two new copper (II) complexes containing Schiff-base and diazo ligands as new catalysts in CuAAC reaction. Inorganica Chimica Acta. 2019; 492 :213-220. DOI: 10.1016/j.ica.2019.04.036 - 16.
Mondal P, Parua SP, Pattanayak P, Das U, Chattopadhyay S. Synthesis and structure of copper (II) complexes: Potential cyanide sensor and oxidase model. Journal of Chemical Sciences. 2016; 128 :803-813. DOI: 10.1007/s12039-016-1063-7 - 17.
Ghosh K, Banerjee A, Bauzá A, Frontera A, Chattopadhyay S. One pot synthesis of two cobalt (III) Schiff base complexes with chelating pyridyltetrazolate and exploration of their bio-relevant catalytic activities. RSC Advances. 2018; 8 :28216-28237. DOI: 10.1039/C8RA03035A - 18.
Sedighipoor M, Kianfar AH, Mohammadnezhad G, Görls H, Plass W. Unsymmetrical palladium (II) N, N, O, O-Schiff base complexes: Efficient catalysts for Suzuki coupling reactions. Inorganica Chimica Acta. 2018; 476 :20-26. DOI: 10.1016/j.ica.2018.02.007 - 19.
Andruh M, Branzea DG, Gheorghe R, Madalan AM. Crystal engineering of hybrid inorganic–organic systems based upon complexes with dissymmetric compartmental ligands. CrystEngComm. 2009; 11 :2571-2584. DOI: 10.1039/B909476H - 20.
Ganguly R, Sreenivasulu B, Vittal JJ. Amino acid-containing reduced Schiff bases as the building blocks for metallasupramolecular structures. Coordination Chemistry Reviews. 2008; 252 :1027-1050. DOI: 10.1016/j.ccr.2008.01.005 - 21.
Liu X, Hamon JR. Recent developments in penta-, hexa-and heptadentate Schiff base ligands and their metal complexes. Coordination Chemistry Reviews. 2019; 389 :94-118. DOI: 10.1016/j.ccr.2019.03.010 - 22.
Furukawa H, Cordova KE, O’Keeffe M, Yaghi OM. The chemistry and applications of metal-organic frameworks. Science. 2013; 341 :1230444(1-12). DOI: 10.1126/science.1230444 - 23.
Li C, Tang H, Fang Y, Xiao Z, Wang K, Wu X, et al. Bottom-up assembly of a highly efficient metal–organic framework for cooperative catalysis. Inorganic Chemistry. 2018; 57 :13912-13919. DOI: 10.1021/acs.inorgchem.8b02434 - 24.
Xiong G, Chen XL, You LX, Ren BY, Ding F, Dragutan I, et al. La-metal-organic framework incorporating Fe3O4 nanoparticles, post-synthetically modified with Schiff base and Pd. A highly active, magnetically recoverable, recyclable catalyst for CC cross-couplings at low Pd loadings. Journal of Catalysis. 2018; 361 :116-125. DOI: 10.1016/j.jcat.2018.02.026 - 25.
Ross TM, Neville SM, Innes DS, Turner DR, Moubaraki B, Murray KS. Spin crossover in iron (III) Schiff-base 1-D chain complexes. Dalton Transactions. 2010; 39 :149-159. DOI: 10.1039/B913234A - 26.
Yang H, Liu SS, Meng YS, Zhang YQ, Pu L, Yu XQ. Magnetic properties and theoretical calculations of mononuclear lanthanide complexes with a Schiff base coordinated to Ln (III) ion in a monodentate coordination mode. Inorganica Chimica Acta. 2019; 494 :8-12. DOI: 10.1016/j.ica.2019.04.051 - 27.
Miyasaka H, Clérac R, Wernsdorfer W, Lecren L, Bonhomme C, Sugiura KI, et al. A dimeric manganese (III) tetradentate Schiff base complex as a single-molecule magnet. Angewandte Chemie International Edition. 2004; 43 :2801-2805. DOI: 10.1002/anie.200353563 - 28.
Nandy M, Shit S, Rosair G, Gómez-García C. Synthesis, characterization and magnetic studies of a tetranuclear manganese (II/IV) compound incorporating an amino-alcohol derived Schiff base. Magnetochemistry. 2018; 4 :57-67. DOI: 10.3390/magnetochemistry4040057 - 29.
Sarkar S, Nag SK, Chattopadhyay AP, Dey K, Islam SM, Sarkar A, et al. Synthesis, structure and catalytic activities of nickel (II) complexes bearing N4 tetradentate Schiff base ligand. Journal of Molecular Structure. 2018; 1160 :9-19. DOI: 10.1016/j.molstruc.2018.01.035 - 30.
Sarkar S, Biswas S, Liao MS, Kar T, Aydogdu Y, Dagdelen F, et al. An attempt towards coordination supramolecularity from Mn (II), Ni (II) and Cd (II) with a new hexadentate [N4O2] symmetrical Schiff base ligand: Syntheses, crystal structures, electrical conductivity and optical properties. Polyhedron. 2008; 27 :3359-3370. DOI: 10.1016/j.poly.2008.07.034 - 31.
Costes JP, Duhayon C, Vendier L, Mota AJ. Reactions of a series of ZnL, CuL and NiL Schiff base and non-Schiff base complexes with MCl2 salts (M = Cu, Ni, Mn): Syntheses, structures, magnetic properties and DFT calculations. New Journal of Chemistry. 2018; 42 :3683-3691. DOI: 10.1039/C7NJ04347C - 32.
Majumdar D, Das D, Sreejith SS, Das S, Biswas JK, Mondal M, et al. Dicyanamide-interlaced assembly of Zn (II)-Schiff-base complexes derived from salicylaldimino type compartmental ligands: Syntheses, crystal structures, FMO, ESP, TD-DFT, fluorescence lifetime, in vitro antibacterial and anti-biofilm properties. Inorganica Chimica Acta. 2019; 489 :244-254. DOI: 10.1016/j.ica.2019.02.022 - 33.
Sarkar S, Aydogdu Y, Dagdelen F, Bhaumik BB, Dey K. X-ray diffraction studies, thermal, electrical and optical properties of oxovanadium (IV) complexes with quadridentate Schiff bases. Materials Chemistry and Physics. 2004; 88 :357-363. DOI: 10.1016/j.matchemphys.2004.08.001 - 34.
Biswas S, Yap GP, Dey K. Reaction of diacetylmonoxime with morpholine N-thiohydrazide in the absence and in presence of a metal ion: Facile synthesis of a thiadiazole derivative with non-bonded S⋯S interaction. Polyhedron. 2009; 28 :3094-3100. DOI: 10.1016/j.poly.2009.06.091 - 35.
Biswas S, Sarkar S, Steele IM, Sarkar S, Mostafa G, Bhaumik BB, et al. Two-dimensional supramolecular assembly of phenylmercury (II) and cadmium (II) complexes with a tridentate thiohydrazone NNS donor ligand: Synthesis, coordination behavior and crystal structure. Polyhedron. 2007; 26 :5061-5068. DOI: 10.1016/j.poly.2007.07.027 - 36.
Saha R, Biswas S, Steele IM, Dey K, Mostafa G. A supramolecular spin crossover Fe (III) complex and its Cr (III) isomer: Stabilization of water–chloride cluster within supramolecular host. Dalton Transactions. 2011; 40 :3166-3175. DOI: 10.1039/C0DT01256D - 37.
Dagdelen F, Aydogdu Y, Dey K, Biswas S. Synthesis, characterization and solid-state properties of [Zn (Hdmmthiol)2].2H2O complex. The European Physical Journal Plus. 2016; 131 :143-150. DOI: 10.1140/epjp/i2016-16143-2 - 38.
Biswas S, Dagdelen F, Aydogdu Y, Dey K. Structural, electrical and optical properties of metal complexes of NNS donor ligand. Materials Chemistry and Physics. 2011; 129 :1121-1125. DOI: 10.1016/j.matchemphys.2011.05.071 - 39.
Mott NF, Davis EA. Electronic Processes in Non-crystalline Materials. Oxford: Clarendon Press; 1971. DOI: 10.1002/crat.19720070420 - 40.
Tauc J, Grigorovici R, Vancu A. Optical properties and electronic structure of amorphous germanium. Physica Status Solidi B. 1966; 15 :627-637. DOI: 10.1002/pssb.19660150224