In Partial Fulfillment of the Requirements for the Degree of

The crystal structure of the complex showed the formation of one-dimensional (1D) helical channels with a diameter of 7–9 Å in the crystal lattice. Each of the complexes exhibits a strong phenolate to Fe(III) charge transfer transition in the 520–585 nm region with a moderately high extinction coefficient.

Figure 1. The histidine-derived ligand (S-H 2 Salhis)

Purpose of the present investigation

Metal-ligand macrocycles

Self-assembled molecular square

Self-assembly chiral molecular square

Depending on the choice of spacer molecules, for example 4,4'-bipyridine in 7, the diameter of the channel can also be adjusted. On the other hand, the use of different spacer molecules at the binding site 9 and 10 within the square was included.

Figure 1.2 Self-assembled chiral and achiral molecular square

Molecular capsules

Metal directed self-assembly of cage compounds
Tetrahedral metal-ligand cage
Glycoluril-derived molecular capsules
Cyclophane-derived molecular capsules
Resorcin[4]are ne-derived metal-based molecular capsules

In the absence of guests, the anthracene-bridged ligand with TiIV or GaIII subunit resulted in the formation of the M2L3 helices as the most stable structure. Furthermore, the stability of the cage and binding of the guest molecules could be varied by simply changing the metal oxidation state.

Figure 1.3 Metal cages or molecular capsules

Complexes based on tris(pyridine) and tris(primidine) ligands

Enantiomeric separation using chiral metal center

Similar experiment was performed with the (S) isomer and the color of the solution did not change.

Cavity with Schiff bases

Definition of the problem

Objectives of the thesis

In this chapter, we discussed the reasons for our choice of ligand and presented the synthesis and characterization of the ligands. The conformational preference of the ligands in solution was identified by analyzing the 1H NMR spectra of the ligands.

Choice of ligand

Non-planarity of the ligand indicates that in an octahedral coordination with a metal ion, two coordination sites in the cis position will remain vacant. The synthetic procedures for the ligands (Scheme 2.I) are easy and can be synthesized in bulk.

Literature on the metal complexes of the ligands chosen in this work

Although the synthesis of the ligand N-(2-hydroxybenzyl)-L-histidine and its pharmaceutical activities have been investigated long ago, very few studies have been done on the complexation of these reduced Schiff bases with various metal centers. This led to our concern to investigate the underexpected complexing ability of N-(2-hydroxybenzyl)-L-histidine and N-(2-hydroxybenzyl)-L-methionine ligands with different metal centers and their self-assembly behavior in different environments .

Experimental Section .1 Solvents and Reagents

Measurements

The ESI capillary was set at 3.5 kV and the cone voltage was 40 V unless otherwise stated. The optical rotations of the methanolic solutions were measured on the Perkin-Elmer 343 or the Rudolf Autopol III polarimeter.

Syntheses of ligands

The yellow-colored solution was treated with sodium borohydride (0.248 g, 6.71 mmol) under continuous stirring, after which the solution became colorless (UV-visible in MeOH, 276 nm). 2-Hydroxy-3-methylbenzaldehyde was synthesized from o-cresol according to the literature procedure. Because the o-product was steam volatile, it was distilled from the mixture along with the water, while the p-product remained in the original mixture.

Syntheses and Selected Properties

The yellow solution was treated with sodium borohydride (0.137 gm, 3.69 mmol) with constant stirring, immediately the solution became colorless (UV-visible in MeOH, 276 nm). The ligand was precipitated as a white solid, the solution was filtered and the residue was thoroughly washed with water.

Tautomeric equilibrium of keto-enol form

Experimental Section .1 Solvents and Reagents

Measurements
X-ray Data Collection, Structure Solution and Refinement

Solid state magnetic susceptibility of the complexes at room temperature was recorded using Sherwood Scientific balance MSB−1. Intensity data, collected using ω−2θ scanning mode, were corrected for Lorentz – polarization effects and for absorption.[72] Structures were solved by the combination of Direct method and Fourier techniques and refined by the full-matrix least-squares method using SHELX system of programs.[73] The intensity data for 1 were collected using a Bruker SMART APEX CCD diffractometer, equipped with a fine focus 1.75 kW sealed tube Mo-Kα X-ray source, with increasing ω (width of 0.3° per frame) at a scanning speed of 3 s /frame.

Table 3.A Selected crystallographic data for the complexes

Syntheses of Cu(II) complexes

Rotary evaporation was used to reduce the volume of the solution to ~5 ml, the solution was left for slow evaporation at room temperature.

Results and Discussion

Syntheses and Selected Properties
Assembly and disassembly of the molecular capsule

The crystallization of complex 1 from pyridine and diethyl ether yielded deep green crystals of ([Cu8(S-Salhis)8(Py)10]·Py·3MeOH·(C2H5)2O). The elemental analysis of the dried crystals reproducibly corresponded to the formula [Cu8(S-Salhis)8(py)4] · 8H2O, instead of [[Cu8(S-Salhis)8(py)10] · py · 3MeOH · (diethyl ether) found during crystal structure analysis performed on crystals with mother liquor (pyridine and diethyl ether) in a closed tube.

Table 3.B Microanalytical Data a of the Cu(II) complexes

Assembly and disassembly of the molecular capsule

In addition to the asymmetric carbon in the ligand, the coordination of the amine nitrogen (N3a to N3h for Cu1 to Cu8) to the metal center gives rise to an asymmetric secondary nitrogen atom, which has the R absolute conformation. All the potential H-bond donors (amine, imidazole, imidazole in histidine fragment) and acceptors (phenolate oxygen, two carboxylate oxygen) in each of the ligand and the 15 water molecules in the unit cell participate in inter-molecular H-bonding formation that forms a giant interwoven H-bonded network in the cryst. TGA of complex 2 showed the loss of 8.43 % of total weight in the 30-150 ºC temperature ranges, which corresponds to the loss of two water molecules (expected 9.04 % weight loss).

Table 3.E Selected bond distances (Å) and bond angles ( °) of complex 2

Absorption Spectra

Redox stability of the cage

Addition of NaBH4 in different proportions (1eq, 2eq, 5eq /Cu) immediately decolorizes the solution, which, if kept open, changed back to the original green color. The ligand field transition of 1 disappears after addition of NaBH4 (1 eq /Cu) and after ~10 min ligand field transition reappears at 651 nm with a 35 nm blue shift compared to 1. We crystallized the green complex from the reaction mixture and it was identical to 1. Similar experiments were performed with 1 in an EPR tube. per Cu) of NaBH4 (addition at room temp., EPR at 77K).

Figure 3.11 UV-visible spectrum of complex 1 ( ___ ) and complex 2 (----) in MeOH Figure 3.10 UV-visible spectrum of complex 1 in MeOH (…….), in pyridine (___)

Magnetism

Channels in membrane-bound potassium channels [100] or water-permeable aquaporins [101-103] are formed from multiple peptide helices in an approximately C4 symmetric manner to form narrow channels (2.8-20 Å in diameter) as a passage for K + ion or water molecules selectively. 129] Using two enantiomers of the ligand, [(R)-H2Salhis] and [(S)-H2Salhis], the helicity of the channels was also found to be reversed. In addition, we were able to remove water from the channels and iodine was inserted into the channels, which was confirmed by the X-ray structure.

Table 3.I Variable Temperature Magnetic Susceptibility data for the complex 1 Temperature / K χ M / dimer × 10 3

Experimental Section

X-ray Data Collection, Structure Solution and Refinement

Apart from the channels present in nature, materials with channels of different pore diameters have been synthesized because of their expected use as molecular sieves, sensors, ion exchangers and catalysts.[107-111] Recently, porous channels have been used as a template to synthesize nano . fibers[112] and one-dimensional array of oxygen molecules.[113] Several reports have therefore recently appeared on the synthesis of materials with channels, helical structures with chiral or achiral ligand metal complex.[114-128] Despite this, literature on the formation of microporous helical channels with water molecules within is scarce. The crystals of the complexes have one-dimensional microporous helical channels filled with easily removable water molecules. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors are based on F, with F set to zero for negative F2.

Table 4.A Crystal data and structure refinement details

Syntheses of Fe(III) complexes

Single crystals of [Fe 2 ( µµ -OH)( µµ -OAc)(S-L) 2 ]· H 2 O (2)
Single crystals of [Fe 2 ( µµ -OH)( µµ -OAc)( S-L) 2 ]· H 2 O· I 2 (3)

The setup was left in this manner for five days to saturate the crystals with iodine and the resulting crystals were confined in a glass capillary for X-ray diffraction studies. The I2 content of the crystals was measured using titration (described below) and added to the weight loss in TGA. After applying vacuum followed by heating to produce I2, we were able to obtain the crystals that diffracted and the I2 content of the crystals was found (repeatedly) to be 12% using the same procedure.

Results and Discussion

Synthesis and Selected Properties
Structures of 1 and 4
Types of water in 1 and 4 from TGA
Determination of pKa of bridging hydroxide from UV-visible spectra

Furthermore, the complexes show the appearance of two bands at 1544 and 1452 cm−1 with Äν 92 cm−1, attributed to the νasym and νsym stretching states of the acetate anion coordinated to the FeIII centers in the bridge form.[76] IR shows no stretches for NO3−. For complex 1, the lattice diagram viewed perpendicular to the 001 plane shows hexagonal channels filled with water from one end to the other of the crystals (Figure 4.3). Each of the water sites in channel 1 forms a helix from one end of the crystal to the other end.

Table 4.B Microanalytical Data a of Fe(III) complexes

Complex 1 (as well as 4) have high-spin Fe(III) center which is not expected to show any ligand field transitions (both Laporte and Spin forbidden). The Fe(III)

The helical arrangement of the polar cavity thus organizes host molecules spirally using several weak interactions. The intactness of I2 and 1 in crystals of 3 was confirmed by their visible spectra in CCl4 and water, respectively. The I2 content of 3 is estimated at (experimental part) 16%, in contrast to the calculated value of 26% for [Fe2(OAc)(OH)(S-Salhis)2]·H2O·I2. Considering that I2 molecules are weakly bound and repeated washings can flush some of the I2 molecules out of the channel, we assume that in the channels about half of the iodine sites are filled with I2.

Protonation deprotonation equilibrium of 1 in water

Experimental Section .1 Solvents and Reagents

Details of the solvent purification, analytical measurements and starting materials have already been discussed in Chapter 2 and Chapter 3. The crystals tend to release solvent molecules easily and were therefore confined in a capillary tube for X-ray data collection. Crystallographic data, along with details of data collection and structure refinement, are listed in Table 5.A.

The volume of the reaction mixture was reduced by rotary evaporation to -10 mL and stored in the refrigerator. After one day, blue rod-shaped crystals formed which were filtered and washed with cold methanol. The reaction mixture was made alkaline using NH 4 OH and allowed to stand on the steam bath for 20-30 minutes, after which red precipitates precipitated.

Results and Discussion

Synthesis and Selected Properties
Crystal structure of complex Na[Ni 2 (S-Salhis) 2 (OAc)]· 6MeOH (1)
UV-visible studies

The magnetic moment at room temperature of the complex 1 was found to be 3.07 per Ni atom. An ORTEP[75] view of the complex 1 is shown in Figure 5.2, together with the numbering scheme. Similar triple-bridged Ni(II) dimers have been reported with comparably higher asymmetry. The acetato group bridges symmetrically with the two TH-149_994501.

Figure 5.1 TGA and DTA plot of complex 1

Experimental Section .1 Solvents and Reagents

Measurements

The color and conductivity of the solutions of binuclear Fe(III) complexes change with addition of amines. The molar conductance of the complexes in methanol shows 1:1 electrolyte properties, but addition of amines to the methanolic solution of the complexes increases molar conductance which behaves like 1:2 electrolyte. The temperature of the NMR probe was determined[157] using CH3OH proton signals using Equation 3.

Syntheses of Fe(III) complexes

After stirring for 30 min, the color of the solution changed from brown to purple with an undissolved white suspension. This solid contains excess KNO3 that was removed by dissolving the solid in pyridine and subsequent filtration of undissolved KNO3. The reaction mixture was stirred for 30 min resulting in a brown to purple color change of the solution with an undissolved white suspension.

Results and Discussion

UV-visible spectrum
Proposed structures of the complexes
Magnetism
Addition of external ligand and its effect on LMCT
Addition of amine and its effect on Conductance
The process involved for the increase of conductance

This may be due to the fact that some of the solvent molecules may have been lost near the 120-150 ° C region. Another possibility is that the basicity of the external ligand may affect the color change. Complex 3 in dichloromethane shows no saturation behavior in either color or conductance change and the nature of the graphs (Figure 6.8 D and D') suggests the presence of equilibrium.

Figure 6.2 TGA and DTA plot of complex 2 Figure 6.1 TGA and DTA plot of complex 1

The process involved for the increase of conductance

Use as sensor

The fact that with the addition of amines, the color and conductivity of the solution changes significantly led us to think about its possible use as a colorimetric and/or conductometric sensor for amines. Thus, to test the suitability of our complex, we soaked a Whatman® filter paper in the CH2Cl2 solution of complex 3 and dried it. The lack of structural characterization of the complexes prevented us from understanding the complete mechanism of color and conductivity change.

Experimental Section

K[Fe(S-Salmet) (C 6 H 4 O 2 )]· 1.5H 2 O (2)

The reaction mixture was stirred for 30 minutes, as a result some white suspension of KNO3 was observed. After adding catechol solution to the reaction mixture, there was no observable change in the color of the solution. However, upon addition of KOH (0.098 g, 1.75 mmol) in methanol under N 2 atmosphere to the reaction mixture, the color of the solution changed to blue from the initial blue-violet.

Results and Discussion

Temperature dependent magnetic measurements
Electron Paramagnetic Resonance (EPR)
UV-visible Spectrum

The decrease in intensity and the change in nature of the absorption ~ 463nm in 2 compared to 1 may be due to the absence of the histidineàFe(III) charge transfer contribution. The low magnetic coupling constants from temperature-dependent magnetic susceptibility measurements support the mononuclear formulation of the complexes. Donor atoms and spectroscopic properties of the complexes have similarities with those of some oxygenase enzyme active sites.

In order to synthesize the cavity for a specific purpose in the future, we needed to understand the coordination chemistry of the complexes and the structural factors that lead to the formation of the cavity. Furthermore, since the donor groups in the ligands used also closely represent amino acid residues in metalloproteins, some of the complexes may be used as structural model complexes of the active site in the future.