Molecule to Supramolecule: Effect of Assembly on the Molecular Properties of H-bonded Chiral Assembly of Coordination Complexes with Amino Acid Derivatives

I also express my sincere thanks to all faculty members, Department of Chemistry, IIT Guwahati for their help and encouragement. I am grateful to the Institute, Indian Institute of Technology Guwahati for providing me with the latest infrastructure and facilities for advanced research. Consequently, we synthesized different types of monomers as well as their conversion to polynuclear composition in the presence of alkali metal salts.

This chapter summarizes the literature on the different types of self-assemblies based on hydrogen-bonded, Resorcin[4]arene-derived, multi-dentate heterocyclic ligands, pH-dependent or cation-directed assemblies and finally assemblies with amino acid derivative ligands that are the objective of the thesis. The six Hbonds between phenolic oxygen and carboxylate oxygen from the adjacent [MII(HLL-leu)2] units (Figure 3) play the key role in stabilizing the assembly. Electrospray ionization (ESI) Mass spectra of the assemblies in MeOH demonstrated the preservation of assemblies in solution. Overall, the structural characterization of the assemblies showed that six L-leucine-derived tridentate ligands, three Ni(II) ions and one K+ or Na+ self-assemble into a fairly large (~1 nm) assembly with binding site for one alkali metal. ion and three sites for oxo anion (Figure 3).

Noticeable in this transformation is the trans orientation of the ligands in the plane of the monomer to the cis orientation in the multinuclear assembly that is vital for the binding of the alkali metal ion (Figure 5). A) ORTEP diagram of the monomer with thermal ellipsoids set at the 30% probability level, and (B) A space-filling model of the C2 symmetric cavity in the monomer. A pictorial representation of the coordination around the alkali metal ion and the subsequent transformations of the assemblies where (A) Monomer [Cu(HLL-leu)2(CH3CN)], (B) Assembly {[Cs{Cu(HLL-.

Chapter IV – Effect of amino acid and Cs + coordination on the assembly

Non-Covalent or Hydrogen-Bonded Assemblies
Assembly based on Resorcin[4]arene-derivative
Assemblies based on multi-dentate heterocyclic ligand and metal coordination
pH dependent and cation directed assemblies
Metallasupramolecular structures with amino acid derivative ligands
Experimental section .1 Materials and Methods
Syntheses and characterization

X-ray Data Collection, Structure Solution and Refinement

Results and discussion

Synthesis and solid state structures
Absorption spectra in DMF
Dissociation in MeOH and effect of [18]-crown-6 addition

Syntheses

Synthesis and selected properties
Structural analyses of the monomeric complexes
Pyridine vs imidazole in the axial site
Mononuclear complex with tyrosine derivative
Monomer to assembly formation
Absorption and EPR spectral characteristics of complexes

Syntheses

Ni(II) assemblies with different amino acid derivative
Assembly formation with Cesium salt
Monomer to assembly formation and their relative stability

Synthesis

Structure of [KNi 2 Cu(HL L -leu ) 6 ]ClO 4 (1)
Mass spectral study
Further analysis on Ni(II) and Cu(II) ratio
UV-visible absorption spectra
EPR spectral study

Syntheses .1 HP L -leu (1)

HP D -Leu (2)
HP L -meth (3)
HP L -tyr (4)

Hydrogel formation and property
a Fluorescence spectroscopy
b Powder diffraction studies
c Scanning electron microscopy
Organogel formation and properties
a SEM and TEM images of organogel

Mechanism of formation
Syntheses

The mass spectrum of 2 shows a strong increase in the intensity of the molecular ion peak after the addition of excess NaNO3 (Figure 2.5b). The greater dissociation of the Cu(II) complexes compared to the Ni(II) complexes may be due to the greater lability of the Cu(II) complexes. The visible diffuse reflectance spectra of the solid-state complexes were too noisy for comparison.

ESI mass spectra of 2 (Figure 2.5) indicated partial dissociation of the assembly into Na+ and [Ni(HLL-leu)2] units in MeOH. In the assembly, three [MII(HLL-leu)2] units encapsulate alkali metal ions via six oxygen donors forming a cryptate-like adduct.6 The assembly is homochiral with three potential chiral recognition sites on the surface (three pairs of NH between the isopropyl groups of L-leucine), two of which are occupied by bridging oxo anions in the present series of complexes (Figure 2.2). Two of the acetonitriles are arranged symmetrically around 1 and show a weak CH…N interaction (C15…N3, 3.342 Å).9 The other two acetonitriles are distributed over four half-occupancy positions.

The EPR spectral characteristics of the complexes are shown in Figure 3.15, and the data at 77 K are shown in Table 3.5. These H bonds are at the short end of the 2.5–3.0 Å range usual for O…O H bond distances (Ophenol…Ocarboxy ~2.48 Å).7 The cis orientation of the carboxylates allowed the the monomeric units to bind K+, similar orientation of. The EPR spectral characteristics of complexes 3 and 4 are shown in Figure 4.9 and the data in methanol at 77 K are shown in Table 4.4 Complexes 3 and 4 show a typical square-pyramidal EPR spectra as evidenced by their A║ values ~179G and g values.14.

IR spectral analysis shows that one broad peak at 1622 and a peak at 1368 cm-1 were identified as asymmetric and symmetric carboxylate regions originating from the ligand, respectively.1 The complex also showed regions at 1084 cm-1 as result of perchlorate anion.1 The elemental analysis could correspond to the formulation of the complex as [KNi2Cu(HLL-. leu)6]ClO412H2OCH3OHCH3CN. Solid state room temperature magnetic moment is 2.72 µB/metal, which is higher than the corresponding all Cu(II) analogue, but lower than all Ni(II) analogue.2 The composition is highly selective towards Ni(II) and Cu(II ) ratios 2:1, because we tried to make composition in reverse ratio, but selectively, it self-assembles into three copper-containing composition as confirmed by ESI- Mass spectrometric analysis. These H bonds are on the short side of the 2.5–3.0 Å range usual for O…O H bond distances (Table 5.2).4 The cis orientation of carboxylates allowed the monomeric units to bind K+ , similar orientation of amines provides a H-bonding site for perchlorate anion making the assembly, capable of binding both anion and cation of a salt within the same assembly in the solid state as before (Figure 5.1).

Solution EPR spectroscopic data for complex 1 at 77 K are consistent with tetragonally distorted octahedral Cu(II) structures of the complexes (Figure 5.8A, B Table 5.7).8 Glass solution spectra of the three Cu(II)-containing complexes indicate monomeric . Compared to the ligands in the previous chapters, the phenol part of the ligand was replaced by pyrene.

X-ray Structural features of [Cu(P L -leu ) 2 (H 2 O)].H 2 O(1) and [Cu(P D-
X-ray Structural features of [Cu(P L -meth ) 2 (H 2 O)]·H 2 O (3)
X-ray Structural features of [Zn(P L -leu ) 2 (H 2 O)]·H 2 O (5) and [Zn(P L -

Formation of Channel and refilling with iodine
Absorption Spectra and EPR spectral characteristics

Carboxylate stretches were observed at and at ~1380 cm-1 for assym and νsym, respectively.3 Elemental analyzes support the formulation of the complexes. The non-electrolytic nature of the complexes was confirmed by measuring the conductivity in MeOH.4 The magnetic moment of complexes 1, 2, 3 and 4 at room temperature is 1.84 μB, respectively, closer to the spin-only value of 1.73 for Cu(II) expected for mononuclear complexes.5 The presence of water of crystallization in 1 and 5 was confirmed by thermogravimetric analysis (TGA) (Figure 7.10). The ORTEP image of one of the monomer units, selected bond lengths and angles for both complexes are given in Figures 7.1, 7.2 and Table 7.2.

The crystal packing shows a CH pyrene ring-to-ring  cloud distance of ~3.6 Å and a CH.. angle of 166.15°, a pyrene ring-to-ring distance of 3.725 Å (Figure 7.3) are within the normal range of interaction distances of Å and CH .. bond length is at higher values of the accepted distance range (2.52.9 Å).12 In general, a structural feature is a clear separation of hydrophilic and hydrophobic parts within the crystal lattice. The coordination geometry around the copper center in 3 is a slightly distorted square pyramidal geometry with four in-plane, trans N2O2 coordination from the ligand, similar to 1 and 2, and the remainder of the fifth axial coordination occupied by a water molecule. Zn(II) in 5 coordinated with four in-plane coordinations from the ligand and a fifth coordination filled with a water molecule.

The ZnO1/N1 bond distances are in a comparable range as found in square-pyramidal Zn(II) complexes.10e Other structural parameters, features are the same as 1 and 2, except for the shorter H-bond distance between O1..O5 2.736 Å (Table 7.2). The dimensions of the channels (a cuboidal space of about 4.8 x 11.4 x 12.4 Å3) are larger than those previously observed in an iron(III) complex with an L-histidine-derived ligand. 7b Thus, we decided to choose complex 5 for loading with molecular iodine because the crystals of 5 were transparent, bright yellow, and strongly fluorescent blue under UV lamp. The distances between iodine and the water of crystallization solvent oxygen (I2O6 3.359 Å) indicate that one end of I2 is within the weak interaction distances of the solvent of crystallization (O6) of one molecule (Figure 7.7B).

Due to the non-planarity of the ligand, the sites on the metal ion remain coordinatively unsaturated, leading to polynuclear bridged species. The assembly proceeded together not only with the coordination of alkali ions to the coordinated carboxylate ligands, but also with six H-bonds between the phenol and the uncoordinated carboxylate oxygen atoms. The crystal analysis presented many obstacles, as it was not possible to determine the Cu/Ni ratio from structural data alone and we had to resort to many different analytical procedures to reach a reasonable conclusion.

In Chapters 6 and 7 we used a different ligand system by replacing the phenolic part of the ligand with pyrene. The formation of the empty channels itself remained a challenge since in most cases the crystals lose their crystallinity during the removal of the solvent from the interior of the channels; recharge channels are something few could do until now. The chiral and ubiquitous nature of amino acids as well as transition metal/alkali ions in the natural system provides further impetus to explore the coordination chemistry of amino acid-based ligands of this type, as some of the observations may provide clues to what is probably already happening. in natural systems.

Conferences and Symposia

Sodium and potassium ion-directed self-assembled multinuclear assembly of divalent nickel or copper and L-leucine-derived ligand”. Effect of metal coordination and intramolecular H-bonding on phenolic proton acidity in a series of structurally characterized octahedral Ni(II) complexes of L-histidine derivative.' Fluorescent gels of pyrenemethylamino acids and their dependence on amino acid side chain, LiOH, chirality, and solvent”.