Cu(II) and Ni(II) complexes of n (2 hydroxybenzyl) amino acid ligands synthesis, structures, properties and catecholase activity 3

This chapter presents the synthesis and characterization of III-1 to III-7, structural studies showing 1D and/or 2D hydrogen bonded supramolecular structures in III-2, III-3, III-5, III-

Chapter 3

Cu(II) and Ni(II) Complexes of reduced Schiff base Ligands containing Additional Functional Groups in the Amino Acid side

chain

Chapter 3

3-1 Introduction

Having a simple amino acid side chain in the reduced Schiff base ligand made this

to be an effective tridentate ligand and the carbonyl oxygen is able to bind intermolecularly to the neighboring metal ions to produce interesting coordination polymeric structures.1a Presence of additional reactive functional groups on the amino acid side chain of the ligand that are available to bind metal ions intermolecularly will

be more interesting Indeed, Cu(II) complexes of the H2Shis ligand form cyclic trimer Two such trimers form molecular capsule hosting pyridine molecules.1b Hence, we wish to investigate the Cu(II) and Ni(II) complexes of the ligand system having additional donor atoms available to coordinate neighboring metal ions or otherwise, these functional groups can also be used to form interesting hydrogen bonded structures in the solid state For this purpose, naturally occurring amino acids such as

aspartic acid and glutamic acid having additional carboxylate donor group,

L-methionine having –SCH3, and L-aspargine with –CONH2 in their side arms have been utilized to synthesize the corresponding ligands- H3Sas (N -(2-hydroxybenzyl)-

L-aspartic acid), H3Sglu (N -(2-hydroxybenzyl)-L-glutamic acid), H3MeSglu

(N-(2-hydroxy-5-methylbenzyl)-L-glutamic acid), H2Smet (N

-(2-hydroxybenzyl)-L-methionine) and H2Sapg (N -(2-hydroxybenzyl)-L-aspargine) These ligands (Figure

3-1) have been successfully employed for complexation with Cu(II) and Ni(II) ions and the complexes [Cu(HSas)(H2O)].2H2O, III-1; [Cu(HSglu)(H2O)].H2O, III-2;

[Cu(HMeSglu)(H2O)].2H2O, III-3; [Cu2(Smet)2], III-4; [Ni(HSas)(H2O)], III-5;

[Ni2(Smet)2(H2O)2], III-6 and [Ni(HSapg)2], III-7 have been obtained This chapter presents the synthesis and characterization of III-1 to III-7, structural studies showing 1D and/or 2D hydrogen bonded supramolecular structures in III-2, III-3, III-5, III-6 and 3D hydrogen bonded supramolecular structure in III-7

Figure 3-1 Ligands employed for complexation

3-2 Results and discussion

The ligands H3Sas, H3Sglu, H3MeSglu, H2Smet and H2Sapg and their complexes

III-1 to III-7 have been synthesized and isolated, following the procedures described

in the experimental section, in moderate to good yields ranging from 60-82% Single crystals were grown during the synthesis of the complexes by diffusion method The

solid state structures of III-2, III-3, III-5, III-6 and III-7 were determined by the

single crystal X-ray crystallographic studies The structural characterization indicated

III-2, III-3 and III-5 are 1D coordination polymers while III-7 has hydrogen bonded

2D structure comprising mononuclear building blocks However, the ligand H2Smet

generated dinuclear copper(II) and nickel(II) complexes, III-4 and III-6 respectively Our attempts to get suitable single crystals of III-1 and III-4 (structure refinement in

III-4 was not satisfactory, see Appendix) were unsuccessful All the ligands and

complexes were characterized by the elemental analysis and other physicochemical

Chapter 3

techniques While attempting to synthesize the ligands with substitution at the 5th

position of phenyl ring by using various -para substituted salicylaldehydes, only the

methyl substituted H3MeSglu ligand has been successfully isolated and its copper(II)

complex III-3 was obtained But, the attempts to isolate corresponding Ni(II)

complex either in the form of bulk or single crystals were also not successful Our attempts to obtain 1:1 complex of Cu(II) and Ni(II) with H2Sapg were not successful Nevertheless, the complexation of the ligand H2Sapg with nickel in 1:1 stiochiometry

has furnished III-7 having 2:1 (ligand: metal) stiochiometry

3-2-1 Description of crystal structures

3-2-1-1 [Cu(HSglu)(H 2 O)].H 2 O, III-2

The asymmetric unit of III-2 contains a copper(II) center with slightly distorted

square pyramidal geometry (τ = 0.033)2as shown in Figure 3-2 Coordination of copper with secondary amine nitrogen atom (Cu(1)-N(1), 1.978(2) Å ), α carboxylate oxygen atom (Cu(I)-O(2), 1.970(2) Å), oxygen atom of aqua ligand (Cu(1)-O(6), 1.964(2) Å) and the carboxylate oxygen atom of neighboring molecule (Cu(1)-O(4), 1.921(2) Å) completes the basal plane of the square pyramid Interestingly, the axial position of square pyramid is occupied by the phenolic oxygen atom (Cu(1)-O(1), 2.345(2) Å) which is protonated Selected bond lengths and bond angles are given in Table 3-1 Very few structural evidences exist for such phenolic group coordinating axially to the Cu(II) ions without deprotonation in different mononuclear,3-4

dinuclear5-7 and ternary copper(II) complexes.8 The Cu-OH(phenol) bond distances observed in these reports lie in the range of 2.432(4)-2.601(4) Å which is still within the range of 2.2-2.9 Å known for the common axial Cu-O bond lengths.9

Figure 3-2 Perspective view of the repeating unit in III-2

The connectivity of each Cu(HSglu) unit by the carboxylate oxygen atom of the

neighboring molecule generated 1D zigzag coordination polymeric structure along c

axis in III-2 as shown in Figure 3-3

Figure 3-3 A segment of 1D polymeric structure propagating along c axis in III-3

(C-H Hydrogen atoms omitted)

O(4)a-Cu(1)-O(6) 89.14(9) O(4)a-Cu(1)-O(2) 92.69(8)

Figure 3-4 View of the portion of 2D hydrogen bonded sheets along c axis in III-2

(The C-H hydrogen bonds are omitted for clarity)

Table 3-2 Hydrogen bond lengths (Å) and bond angles (º) in III-2

3-2-1-2 [Cu(HMeSglu)(H 2 O)].2H 2 O, III-3

The asymmetric unit of the Cu(II) complex III-3 containing HMeSglu ligand

contains the mononuclear square pyramidal copper(II) building blocks (Figure 3-5), with two lattice water molecules (O(7) and O(8)) The geometry around copper ion is distorted square pyramidal (τ = 0.105),2 and similar to that present in III-2 Selected

bond lengths and bond angles are given in Table 3-3

Chapter 3

Figure 3-5 Perspective view of the building block III-3

The connectivity of the copper center by the coordination of neighboring carboxylate oxygen atom (Cu(1)-O(4A)) generated a zigzag 1D coordination polymer

(Figure 3-6) along c axis in III-3 as observed in III-2 But the molecular packing is

slightly different from III-2 due to the presence of methyl group and additional water

molecule

Figure 3-6 Portion of 1D polymeric structure propagating along c axis in III-3 (C-H

Hydrogen atoms omitted)

Table 3-3 Selected bond lengths and bond angles in III-3

(O(6)-H(6A)···O(2)) to form a 2D hydrogen bonded sheet structures along c axis as

shown in Figure 3-7 These 2D sheet structures are further sustained by H(6B)···O(8) hydrogen bonds formed between aqua ligand (O(6)) and lattice water molecule (O(8)) In addition to these interactions, intermolecular hydrogen bonds are also observed between two lattice water molecules (O(8)-H(8A)···O(7)) and the other carboxylate oxygen atoms (O(7)-H(7C)···(O5), O(7)-H(7D)···O(4), O(8)-H(8B)···O(5)) The intermolecular connectivity between 1D strands via aqua ligands, N-H hydrogen atoms and lattice water molecules is clearly shown in Figure 3-8 The hydrogen bond parameters are summarized Table 3-4

Chapter 3

Figure 3-7 Packing of III-3 along c axis showing H-bonding

Figure 3-8 Packing diagram of III-3 along b axis

Table 3-4 Hydrogen bond lengths (Å) and bond angles (º) in III-3

3-2-1-3 [Ni(HSas)(H 2 O)], III-5

The asymmetric unit in III-5 consists of mononuclear Ni(II) center with octahedral

geometry Octahedral geometry around nickel ion is completed by HSas-2 ligand via coordination through phenolic oxygen (Ni(1)-O(1) = 2.092(4) Å), aqua ligand (Ni(1)-O(6) = 2.068(5) Å), amine nitrogen (Ni(1)-N(1) = 2.041(5) Å), α-carboxylate oxygen (Ni(1)-O(2) = 2.054(4) Å), the other carboxylate oxygen atom of the side chain (Ni(1)-O(4) = 2.049(4) Å) and the neighboring carboxylate (Ni(1)-O(5A) = 2.033(4)

Å) as shown in Figure 3.9 The Ni-O and Ni-N bond lengths observed in III-5 are in

agreement with the available octahedral Ni(II) complexes obtained with reduced Schiff base ligands containing N2O2 donors.10a-b The Ni-OH distance of 2.092(4) Å in

III-5 has been found to be in agreement with the previous reports showing the Ni

atom coordinating to the protonated oxygen atoms (either from phenolate or from solvents such as alcohols).10c-g The aqua ligands on the adjacent Ni atoms are bonded

trans to each other Interconnectivity of mononuclear repeating units via coordination

of neighboring carboxylate oxygen (Ni(1)-O(5) = 2.033(4) Å) generated 1D zigzag

coordination polymeric structure in III-5 which propagates along c axis Selected

bond lengths and bond angles are given in Table 3-5

Molecular packing resulted in the connectivity of all 1D zigzag chains along c axis

to form 2D hydrogen bonded network structures through intermolecular hydrogen bonding as shown in Figure 3-10

Figure 3-10 Packing of III-5 viewed from c axis

Figure 3-11 Packing view along b axis in III-5

These intermolecular hydrogen bonds are generated between the α-carboxylate oxygen atoms, aqua ligands and amine hydrogen atoms The α-carboxylate oxygen

atoms are involved in syn-anti mode of hydrogen bonding with amine hydrogen atoms

and aqua ligand Figure 3-11 clearly displays the packing pattern of 1D zigzag chains into 2D hydrogen bonded sheets through aqua ligands and N-H hydrogen atoms The hydrogen bond parameters are displayed in Table 3-6

Chapter 3

Table 3-6 Hydrogen bond lengths (Å) and bond angles (º) in III-5

3-2-1-4 [Ni 2 (Smet) 2 (H 2 O) 2 ], III-6

The molecular structure of III-6 displays dinickel(II) units with octahedral Ni(II)

centers The dinuclear core Ni2O2 is formed by the bridging phenolate oxygen atoms with a Ni···Ni separation of 3.167 Å

Figure 3-12 Perspective view of III-6

Each Ni(II) center completes octahedral geometry through the coordination of two bridging phenolate oxygen atoms (Ni(1)-O(1), 2.033(2) Å), amine nitrogen (Ni(1)-N(1), 2.050(2) Å), α-carboxylate oxygen (Ni(1)-O(2), 2.064(2) Å), sulfur atom from the side arm of the amino acid (Ni(1)-S(1), 2.517(8) Å) and aqua ligand (Ni(1)-O(4), 2.066(2) Å) as shown in Figure 3-12 The bond distances observed are in consistent with the available few phenoxo bridged dinuclear and octahedral Ni(II) complexes

obtained with similar type of ligands.11 Similar Ni-S bond distances were observed in the earlier reports on the related nickel complexes. 10a, 12 The two aqua ligands are

oriented in cis fashion to each other Selected bond lengths and bond angles are given

O(1)a-Ni(1)-N(1) 167.93(8) O(1)a-Ni(1)-O(2) 106.3(8)

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

Figure 3-13 Hydrogen bonded 1D polymeric strand in III-6 along a axis

Chapter 3

Interestingly, the packing of the dimeric molecular units in III-6 has generated the

hydrogen bonded 2D sheet structures in which all these dimeric units are connected to

the adjacent molecules via intermolecular hydrogen bonds generated by the

carboxylate oxygen atoms (O(2) and O(3)) of the neighboring molecule with both

N-H hydrogen atoms (N(1)-N-H(1)···O(3)) and aqua ligands (O(4)-N-H(4A)-O(2)) These hydrogen bonding interactions resulted in the formation of 1D hydrogen bonded polymeric structure as shown in Figure 3-13 Further connectivity of each 1D hydrogen bonded polymeric strand to the neighboring 1D polymeric chains via medium strong intermolecular C-H···S interactions (C(8)-H(8)-S(1)) generated hydrogen bonded 2D sheet structures as shown in Figure 3-14 and Figure 3-15

Figure 3-14 A portion of 2D sheet structure in III-6 sustained by C-H…S

interactions

Figure 3-15 2D hydrogen bonded sheets in III-6 along c axis showing C-H···S

interactions

The C–H···S hydrogen bond, apart from C-H···O and other π···π interactions, has also been found to play very important role in self-assembly process resulting in interesting supramolecular network structures.12 Any H···S contact <3.05 Å may considered to be a significant interaction Thus, the observed H···S contact of 2.82 Å

in III-6 can be medium strong compared to the other H···S contacts14 of ~ 2.91 Å However, the C-H···S interactions to sulfur in a C–S–M linkage were found to be non-directional and do not display directional hydrogen-bond characteristics in metallo-

organic solids, i.e C-H···S interactions may not be considered to be

structure-directing.15 Furthermore, it has also been shown that C-H···S interactions with H···S contacts <3.3 Å do not display any primary and true structure-directing influence due

to the fact that in the presence of other stronger interactions, they will readily be distorted.12b The hydrogen bond parameters in III-6 are given in Table 3-8

Table 3-8 Hydrogen bond lengths (Å) and bond angles (º) in III-6

Chapter 3

3-2-1-5 [Ni(HSapg) 2 ], III-7

X-ray crystal structure of III-7 reveals the presence of mononuclear octahedral

Ni(II) centers Two ligand species are bonded to the nickel ion via coordination through two carboxylate oxygen atoms (Ni(1)-O(2), 2.062(2) Å; Ni(1)-O(6), 2.060(2) Å), two oxygen atoms from the amido groups (Ni(1)-O(4), 2.049(3) Å; Ni(1)-O(8),

2.044(3) Å) in trans fashion, and two amine nitrogen atoms (Ni(1)-N(1), 2.107(3) Å;

Ni(1)-N(3), 2.110(3) Å) completing the octahedral geometry as shown in Figure 3-16 Selected bond lengths and bond angles are given in Table 3-9

Figure 3-16 Perspective view of III-7

When the side arm of ligand is modified by incorporating the -CONH2 donor group

in the place of COO-, the mode of coordination of the ligand is different from the usual behavior observed for reduced Schiff base ligands as described above Usually the phenolate oxygen atom coordinates to the metal centers to give either dinuclear

M2O2 cores via bridging mode or mononuclear metal complexes in 1:1 stoichiometry

with respect to the ligand to metal ratio However, in case of III-7 which is a 2:1

compound phenolic group remained inert towards coordination Nevertheless, compared to the available 2:1 Ni(II) complexes 10a containing similar type of ligands

in which either phenolic oxygen or S atom of thiophenolate group participated in

coordination with Ni(II) centers, the formation of III-7 is probably due to the

experimental conditions employed for synthesis

Table 3-9 Selected bond lengths and bond angles in III-7

Packing of molecules in III-7 generated a hydrogen bonded 3D network structure

as shown in Figure 3-17 The hydrogen atoms of each non-coordinating phenolate groups, the carboxylate oxygen atoms and the hydrogen atoms from the -CONH2

donor group are involved in generating three dimensional intermolecular hydrogen bonds The two coordinated carboxylate oxygen atoms are intermolecularly hydrogen bonded to each phenolate hydrogen atoms (O(1)-H(1)···O(2), O(5)-H(5)···O(6) and O(5)-H(5)-O(7)) from the neighboring molecule The two hydrogen atoms of each –CONH2 group generated hydrogen bonding with both free carboxylate oxygen atom (N(2)-H(2B)···O(3) and N(4)-H(4A)···O(3)) as well as the coordinated carboxylate