Cu(II) and Ni(II) complexes of n (2 hydroxybenzyl) amino acid ligands synthesis, structures, properties and catecholase activity 4

Chapter 4 NiII Helical Staircase Coordination Polymer Encapsulating Helical Water Molecules... 1D hydrogen bonded helical water chains Among various assembly modes of water molecules,

Chapter 4

Ni(II) Helical Staircase Coordination Polymer Encapsulating Helical Water

Molecules

4-1 Introduction

Currently, in view of the idea that the constrained microenvironment of organic and metal-organic host lattices are excellent solid-state media to isolate and analyze different hydrogen bonded water clusters,1 there is a surge of interest in applying the principles of supramolecular chemistry.2 Consequently, supramolecular chemistry is now at a phase of understanding of various hydrogen-bonded water clusters in the form of tetramers,3 hexamers,4 octamers,5 decamers6 and dodecamers,6c and (H2O)15(CH3OH)3 clusters7 in diverse environments of various crystal hosts Zeolite-like 3D network structures with chiral channels filled with highly ordered water molecules are well known.8 Recent reports by Infantes and Motherwell,9 and Gillon

et al.10 illustrated an extensive survey on the patterns of water clusters in several varieties of hydrate structures obtained from CSD

1D hydrogen bonded helical water chains

Among various assembly modes of water molecules, 1D hydrogen bonded water chains have drawn a great deal of attention because of their intriguing hydrogen bonding features among themselves as well as with the host molecules.11 In this connection, particularly, the hydrogen bonded 1D helical water chains occupy a special place due to their crucial role in the fundamental biological processes such as transport of water, protons and ions (Figure 4-1) For example, the selective transport

of water across cell involves the hydrogen bonded assembly of single H-bonded helical chains of water molecules in the constricted pore of the aquaporin-1.12 These 1D water chains appear to be stabilized by strong H-bonding between neighboring water molecules along the chain as well as H-bonding between water molecules and donor-acceptor groups associated with channels

Figure 4-1 Schematic representation of water transport in aquaporin proteins.11c

While such 1D helical water chains are prevalent in biological systems, it is highly difficult to construct them in the synthetic hosts by design because the structural constraints required in stabilizing the 1D water chains are yet to be fully understood Such water chains could model the biological systems for the transport of water or ions across the membrane proteins with aquapores However, some amount of success has been achieved while generating helical 1D water chains in synthetic hosts

Chakravarty et al reported a hydrogen-bonded helical dicopper(II) complex as

supramolecular host anchored by hydrogen bonding to alternate water molecules (Figure 4-2) that were assembled as a single-stranded, both right- and left-handed, helical chain.13

Figure 4-2 1D helical water chain constructed by alternate water molecules

anchoring the supramolecular Cu(II) complex.13

Hong et al.14 reported a left handed 1D helical water chains (Figure 4-3) encapsulated in a chiral 3D hydrogen bonded supramolecular network structure in a

dicopper(II) complex of a Schiff base derived from L-histidine

In a recent report, Nangia et al.15 observed infinite 1D helical chain of water molecules (Figure 4-4) in nanoporous channels of organic hexahosts, (Cl-PHG.(H2O)3) and (Br-PHG.(H2O)3) It has been shown that the weak halogen···halogen interactions directed the handedness of the water helices surrounding the Cl-PHG and Br- PHG hosts

PHG.(H2O)3(disordered protons are shown) (right) along with the spiral assembly of host molecules (green, blue) around the right-handed water helix in Br–PHG.(H2O)3 15

4-2 Aim of the current investigation

Inspired by the fascinating structural features of helices demonstrating the cooperative self-assembly, recognition and their remarkable functions such as chemical transport and screening activities of membrane channels in biological

systems, the helicity has been successfully introduced into artificial systems by the chemists in the field of metalla-supramolecular chemistry.16-18 It is also explored recently that the transport of water or protons across the cell involves highly mobile hydrogen-bonded water molecules assembling into a single helical chain at the positively charged constricted pore of the membrane channel protein aquaporin-1.19 While 1D water chains are more predominant in biology to play crucial role in stabilizing the native conformation of biopolymers, such helical water chains are extremely rare in synthetic crystal hosts.11, 13-15

It is well known that a chiral ligand can often lead to the formation of helical structure.16b The presence of one or more non-chelating side arms in a chiral ligand may provide the possibility for selective and complementary aggregation of the metal complexes Among various ligands designed and their Cu(II) and Ni(II) complexes explored in Chapter 3, only the Ni(II) complex of the chiral ligand H3Sglu, has been found to generate spiral coordination polymer

4-3 Results and Discussion

The ligand, H3Sglu has been prepared according to the same procedure as described

in Chapter 3 As the ligand is found to be freely soluble in water, aqueous solution of

H3Sglu has been employed for the complexation with Nickel The Ni(II) complex, [(H2O)2⊂{Ni(HSglu)(H2O)2}]⋅H2O IV-1 has been synthesized by the reaction of

aqueous H3Sglu with aqueous nickel nitrate hexahydrate in 1:1 stoichiometry During

the slow diffusion of the reactants, greenish rod-like single crystals of IV-1 were

obtained after one week from the clear reaction mixture on slow evaporation

4-3-1 Crystal Structure of [(H 2 O) 2 ⊂{Ni(HSglu)(H 2 O) 2 }] ⋅ H 2 O, IV-1

IV-1 crystallized with two independent molecules in the asymmetric unit as shown

in Figure 4-5 Each Ni(II)unit has octahedral geometry with dianionic HSglu2- ligand coordinated through phenolic oxygen atom (Ni(1)-O(1), 2.089(4) Å and Ni(2)-O(6), 2.101(3) Å) and secondary amine N atom (Ni(1)-N(1), 2.084(4) Å; Ni(2)-N(2), 2.082(4) Å) and the α-carboxylate oxygen atom (Ni(1)-O(2), 2.047(4) Å and Ni(2)-

O(7), 2.042(4) Å) in a facial manner, two aqua ligands and another carboxylate

oxygen from the neighboring molecule Selected bond lengths and bond angles are given in Table 4-1

Figure 4-5 A view of the asymmetric unit of IV-1

Table 4-1 Selected bond lengths and bond angles in IV-1

O(5)a-Ni(2)-O(13) 84.5(2) O(5)a-Ni(2)-N(2) 173.7(2)

O(5)a-Ni(2)-O(6) 88.7(2) C(12)-O(5)-Ni(2)b 129.1(5)

C(12A)-O(5)-Ni(2)b 125.0(6) O(12)-Ni(1)-N(1) 97.3(2)

O(2)-Ni(1)-N(1) 80.3(2) O(9)-Ni(1)-N(1) 173.6(2) O(12)-Ni(1)-O(1) 88.1(1)

Symmetry transformations used to generate equivalent atoms: a: -x+1,y-1/2,-z+1; b: -

x+1,y+1/2,-z+1

The intermolecular connectivity via second carboxylate O atom generates a handed helical staircase-like coordination polymeric architecture with a pesueo-41

left-screw axis In this helical staircase, the aqua ligands trans to phenolic oxygen atoms

(namely O(11) and O(13)) are pointing inside the tube normal to the helical axis The

N-H and O-H protons are hydrogen-bonded to the carboxylate oxygen atoms complementing along the surface of the helical staircase as shown in Figure 4-6 The hydrogen-bond parameters are given in Table 4-2

view of the staircase polymer without helical water chain

In this square shaped cavity the dimensions are 7.65 and 7.53 Å (based on Ni···Ni distances) Of the six lattice water molecules present in the asymmetric unit, four have been found inside the helical pore and two outside Of these, two water molecules O(15) and O(16) are hydrogen-bonded to produce 1D helical polymer with a pseudo-

41 screw axis (Figure 4-7) This helical water chain, as a pole of the helical staircase, also supports and stabilizes the orientation of helical staircase by maintaining the hydrogen bonding with aqua ligands The other two water molecules O(17) and O(18) are found to propagate the hydrogen bonding both with the helical water chain and aqua ligands and it appears that their hydrogen bonding tendency would have

facilitated the positioning and orientation of water molecules forming the helical chain

Figure 4-7 (Left) Top view of IV-1 showing water filled helical channel (Right)

Hydrogen bonded helical water chain with space filling model

Table 4-2 Hydrogen bond lengths (Å) and bond angles (º) parameters in IV-1

* The hydrogen atoms have been placed in the calculated positions

The total potential solvent area in the lattice including the helical and the lattice

water molecules was found to be 405.1 Å3 (22.7% of the unit cell.20 All the tubular

coordination polymers are aligned in b-axis (Figure 4-8) and two more water

molecules (O(19) and disordered O(20) occupy the empty space in the lattice outside

the helical cavity

Figure 4-8 Packing of the staircase polymer IV-1 viewed along b axis showing chiral

channel The water molecules in the channels are omitted for clarity

As in the majority of the supramolecular syntheses, self-assembly of metal ions and ligands resulted in the formation of single, double, triple and quadruply stranded helical structures.17 However, helical chain inside a helical structure is very rare Unlike a water helix inside a hydrogen-bonded helical supramolecular host,13 the

structure of IV-1 has a hydrogen-bonded helix inside a helical 1D coordination

polymer Highly ordered stream of helical water molecules inside another helical polymer seems to be striking and has unique structural feature among those existing porous helical structures17c, 21-23 and other patterns of the water structures observed in diverse environments of both inorganic5-6 and organic3-4, 11 hosts and two dimensional supramolecular (H2O)12 rings.6c Whereas designing chiral materials from achiral molecular compounds presents a promising theme in materials science, using simple and available chiral precursor as an alternative remains another practical approach

The structure of IV-1 exemplifies the feasibility of such an approach.24

At this point, it is important to highlight that the same HSglu2- anion has displayed

a completely different coordination environment and connectivity, when the metal ion

is changed from Ni(II) to Cu(II), resulting in 1D zigzag coordination polymeric structure in [Cu(HSglu)(H2O)].H2O, III-2 as described in the previous chapter This

variation from helical staircase coordination polymeric structure in six- coordinated Ni(II) complex to 1D zigzag coordination polymeric structure in five- coordinated Cu(II) complex displayed by the same HSglu2- anion demonstrates that the overall topology depends on the nature of the metal ion and the coordination geometry at the metal centers

4-4 Physicochemical Studies

4-4-1 IR spectra

The X-ray crystal structure, IV-1 contains both aqua ligands and lattice water

molecules and the IR absorption bands observed between 3300 and 3450 cm-1 also suggest their presence25a which has been further supported by the weight loss observed in TG analysis The ν(N-H) band has been shifted from 2960 cm-1 for the free H3Sglu ligand to 2746 cm-1 for the complex indicating the complexation The asymmetric νas(COO-) and symmetric νs(COO-) stretching vibrations of carboxylate

in the free ligand have been observed at 1673 and 1388 cm-1 respectively For the

complex IV-1 the νas(COO-) and νs(COO-) stretching frequencies are observed at

1623 and 1348 cm-1 respectively.25b The difference (Δν > 200) between νas(COO-) and νs(COO-) indicates the terminal or monodentate coordination mode of carboxylate group.25c The stretching frequencies characteristic of phenolic C-O in the ligand and complexes are observed in the range of 1253 cm-1 The assigned IR stretching

frequencies here are in agreement with the available literature for the related Ni(II) complexes. 26

4-4-2 Electronic spectra

Electronic spectrum of IV-1 recorded as nujol mull transmittance displayed the

medium intensity d-d bands typical of octahedral Ni(II) at 642 and 730-737 nm while

the CT band corresponding to phenolate-to-Ni(II) transition was observed in the range

of 350-354 nm The d-d bands at 642 nm can be assignable to the spin allowed 3A2g

(F) Æ 3T1g transitions where as the shoulder at around 737 nm originates from the spin forbidden 3A2g Æ 1Eg transitions frequently observed in Ni(II) octahedral complexes. 26-27

4-4-3 Thermogravimetric studies

The TG analysis of IV-1 reveals that the weight loss occurs in the temperature

range 26-232 °C as shown in the Fig 4-9 The total weight loss observed (21.6%) agrees with the calculated value (22.5%) for the loss of five water molecules per Ni(II) ion The single crystal crumbles upon removal of water molecules or cooled to -50°C Our earlier attempts to collect X-ray data at low temperature failed due to this phenomenon

Figure 4-9 TGA of IV-1

The effect of thermal dehydration on the single crystals of IV-1 is shown in Figure

4-10 The structure is not expected to be robust when dehydrated due to the fact that these coordination polymers are not supported by strong non-covalent interactions (Figure 4-8)

Figure 4-10 (Left) Single crystals of IV-1 at RT before heating Single crystals of

IV-1 after heating to 150 ºC (Right)

The structure of the left-handed helical coordination polymer IV-1 encapsulating

the hydrogen-bonded helical stream of water molecules illustrates another novel cooperative assembly and recognition of water molecules in the inorganic crystal host These results may exemplify the maxim that the structural constraints operating on

orientation of water by its surrounding and vice versa can be very significant This

captivating structural feature of IV-1 displaying the helical chain of water molecules

supporting the metal coordination helical staircase brings to light yet another fascinating model for the water chains in membrane aquaporin proteins for the transport of water or protons and it appears to be extremely rare among metal coordination polymers until the present investigation.28

4-6 Experimental

4-6-1 Synthesis of ligand

H3Sglu ligand has been synthesized according to the procedure described in Chapter 3

4-6-2 Synthesis of complex

[(H 2 O) 2 ⊂{Ni(HSglu)(H 2 O) 2 }] ⋅ H 2 O, IV-1

A clear solution of H3Sglu (0.25 g, 1.0 mmol) in of water (2.5 mL) was allowed to diffuse slowly into a clear aqueous solution (2.5 mL) of nickel(II) nitrate hexahydrate (0.29 g, 1.0 mmol) The greenish rod-like crystals suitable for X-ray diffraction were obtained after a week from the clear reaction mixture on slow evaporation Yield: 0.28 g (70%) Anal Calcd for C12H23NO10Ni: C, 36.0; H, 5.8; N, 3.5; H2O, 22.5 Found: C, 36.2; H, 5.6; N, 3.7; H2O, 21.6 (from TG)

4-6-3 X-ray crystallography

The solid state structure of IV-1 has been determined by single crystal X-ray

crystallographic technique The details of crystal data and structure refinement parameters are shown in Table 4-3