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

47
Chapter 2


Dinuclear Copper(II) Complexes as
Functional Models for the Catechol
Oxidase




Chapter 2

48
2-1. Prelude to Parts A and B

Next to iron, copper is the most important bioessential element and its biological
relevance is recognized to the highest degree in the last decades due to the rapid
development of bioinorganic chemistry which offers a successful interaction between
model complexes and metalloenzyme chemistry. Transport, activation and
metabolism of dioxygen are very important processes in living systems.
Metalloenzymes containing one or more copper centers are responsible for these
functions. Many copper containing metalloenzymes such as hemocyanin (dioxygen
carrier), tyrosinase (hydroxylation of monophenols and melanin pigment formation),
catechol oxidase (oxidation of catechols), dopamine β-hydroxylase (production of
catecholamine for nerve and metabolic function), superoxide dismutase (disposal of
potentially damaging radicals formed during normal metabolism), plastocyanin of
plant chloroplasts (electron transport for photosynthesis), celuroplasmin (potential
extra cellular free radical scavenger) and other enzymes such as tryptophan

oxygenase, ascorbate oxidase have been discovered.
1-4


Proteins containing dinuclear copper centers play paramount roles in biology,
including dioxygen transport or activation, electron transfer, reduction of nitrogen
oxides and hydrolytic chemistry.
5
Copper proteins are classified as Type 1, Type 2
and Type 3. Among them many copper enzymes have oxidase or oxygenase activities.
Type 3 copper proteins are characterized by an antiferromagnetically coupled
dicopper core with three histidine ligands on each copper ion and µ-hydroxo bridging
in the met Cu(II)-Cu(II) form which results in the EPR silent active site. Among the
well-known representatives of Type 3 copper proteins, catechol oxidase with active
dicopper(II) sites is a ubiquitous enzyme in living systems for catalyzing the oxidation
Chapter 2

49
of a wide range of ortho-diphenols (catechols) to ortho-diquinones (Figure 2-1). The
subsequent auto polymerization of the highly active quinones into polyphenolic
catechol melanins is considered to be responsible for the defense mechanism observed
in plants against pathogens or pests.
6
While tyrosinase catalyzes the hydroxylation of
tyrosine to dopa (cresolase activity) and the oxidation of dopa to dopaquinone
(catecholase activity) with electron transfer to dioxygen, catechol oxidase exclusively
catalyzes the oxidation of catechols to quinones without acting on tyrosine.
7
This
reaction is of great importance in medical diagnosis for the determination of the

hormonally active catecholamines adrenaline, noradrenaline and dopa.
8
Oxidation of
mono- and diphenol-containing neurotransmitters such as dopamine, epinephrine,
norepinephrine and serotonin have been found associated with the Fe(II) and Cu(II)
centered redox chemistry related to Alzheimer’s disease.
8b-d


Figure 2-1. Catecholase reaction in natural systems.

In fact, the two copper atoms of dicopper(II) bio-active centers present in different
metalloenzymes are found to act cooperatively within the proximity of ~3.5 Ǻ with
each Cu(II) centre coordinated by three histidine donors.
5
As confirmed by the recent
X-ray crystal structure analysis, catechol oxidase in the met oxidized form contains
the dicopper active centers with a Cu···Cu distance of 2.9 Å.
9

Chapter 2

50
The crystal structure of catechol oxidase reported recently by Krebs and co-workers
shown in Figure 2-2 reveals new insight into the functional properties of the Type III
copper protein which includes the closely related and well known tyrosinase as well
as hemocyanin.
9a
These proteins have a dinuclear copper center and have similar
spectroscopic behavior and functional relationships.

Figure 2-2. Overall structure of catechol oxidase from sweet potato (ipomoea
batatas). Copper atoms are shown in orange,
α
helices in blue,
β
sheets in green.
9a


One of the interesting structural aspects encountered in the active site of catechol
oxidase from sweet potatoes (Ipomoea batatas) and in the active sites of some
hemocyanins is an unusual thioether bond between a carbon atom of one of the
histidine ligands and the sulfur atom of a nearby cysteine residue from the protein
backbone. A cysteinyl-histidinyl bond has also been reported for other Type 3 copper
proteins such as tyrosinase from Neurospora crassa,
10a
and hemocyanin from Helix
pomatia
10b
and Octopus dofleini.
10c
A thioether bond between cysteine and tyrosine is
also present in the mononuclear copper enzyme galactose oxidase.
10d
Biomimetic
models mimicking this unique feature were reported by Belle and Reedjik
10e
and
Wieghart and co-workers.

10f
In an attempt to further mimic this quite unusual
Chapter 2

51
structural feature, recently, Krebs et al. demonstrated that adjacent thioether group
enhanced the activity of dicopper(II) complexes by weakening the exogenous acetate
bridging.
11h


Figure 2-3. Oxy and met forms during the activity of tyrosinase and/or catechol
oxides.

Based on the spectroscopic and biochemical evidences as well as the recent X-ray
crystallographic structural findings of catechol oxidase,
9
a plausible mechanism has
been proposed for catecholase activity of tyrosinase and /or catechol oxidase.
9a, 12
The
catalytic cycle begins with the oxy and met states. A diphenol substrate binds to the
met state, followed by the oxidation of the substrate to the first quinone and the
formation of the reduced state of the enzyme. Binding of the dioxygen leads to the oxy
state which is subsequently attacked by the second diphenol molecule. Oxidation to
the second quinone forms the met state again and closes the catalytic cycle. Thus, in
short, the catecholase activity of tyrosinase and catechol oxidase is carried out by the
oxy form (Cu(II)-O
2
2-

-Cu(II)) and by the met form (Cu(II)-Cu(II)) of the enzymes
through a two electron-transfer reaction as shown in Figure 2-3. Coordination sphere
of the dinuclear copper center in the met state is shown in Figure2-4.
9a

Chapter 2

52

Figure 2-4. Coordination sphere of the dinuclear copper center in the met state.
9a


In order to gain deeper insight into the copper-mediated substrate oxidations and to
understand the influence of various parameters that determine bimetallic reactivity
both in natural metalloenzymes and in synthetic analogues, studies of the well defined
and appropriate dicopper(II) complexes are obviously essential. For this reason, quite
a number of mono- and dinuclear copper(II) complexes have been investigated as
biomimetic catalysts for catechol oxidation.
11-14
Krebs and Reim

demonstrated that
that complexes with strained structures show catalytic activity, whereas complexes
present in relaxed and energetically favored conformations are essentially inactive
towards catechol oxidation.
11j


Krebs et al. have shown the dicopper(II) complex [Cu

2
bbpen](ClO
4
)
2
.3MeOH as a
structural and functional model for catechol oxidase.
14a
In all the modeling studies,
11-
14
a common and convenient model substrate, 3,5-DTBC, has been employed as a
model substrate (Figure 2-5) since its low redox potential makes it easy to oxidize,
15

and the bulky substituents prevent further side reactions such as ring opening.
Chapter 2

53

Figure 2-5. Biomimetic oxidation of 3,5-DTBC catalyzed by dicopper(II) complex.

Figure 2-6. Formation of 3,5-DTBQ band from the oxidation of 3,5-DTBC catalyzed
by [Cu
2
bbpen
2
](ClO
4
)

2
·3MeOH. The inset shows the course of the absorption
maximum at 405 nm with time for 10 and 100 equivalents of 3,5-DTBC.
14a


Further, the oxidation product, 3,5-DTBQ is sufficiently stable and displays a strong
absorption at ca. λ
max
= 390 nm the growth of which can be monitored by UV-Vis
spectroscopy as shown in Figure 2-6.
14a


For a dicopper(II) complex to act as an efficient catalyst towards the oxidation of
3,5-DTBC, a steric match between substrate and complex is believed to be the
determining factor: two metal centers have to be located in the proximity of 3 Å to
facilitate proper binding of the two hydroxyl oxygen atoms of catechol prior to the
electron transfer.
16
(Figure 2-7). This view is supported by the observation that
Chapter 2

54
dinuclear copper complexes are generally more active towards the oxidation of
catechol than the corresponding mononuclear complexes.
16
Further, Nishida et al.
have shown that square-planar mononuclear copper(II) complexes are less active than
non-planar mononuclear copper(II) complexes.

17


Figure 2-7. Proposed steric match and binding of substrate with complex.


With dinuclear copper(II) complexes, only a few structurally characterized
complex/substrate adducts are reported. The first, described by Karlin et al.
18
was
prepared by the oxidative addition reaction from a phenoxo-bridged dicopper(II)
complex and tetrachloro-o-benzoquinone (tcbq) which displayed a bridging
tetrachlorocatechol (tcc) between the two copper(II) ions with a Cu···Cu distance of
3.248 Å. Recently, crystal structures of the different adducts of complex/substrate
exemplifying various coordination modes of catecholate have been reported as shown
in Figure 2-8.
19


Figure 2-8. TCBQ complexes, [HL
3
Cu
2
(TCC)(H
2
O)
2
]
2
.ClO

4
(left) and
[HL
4
Cu
2
(TCC)(H
2
O)]
2
.ClO
4
(right) as models for substrate binding.
19b

Chapter 2

55
Comparing the activity of a series of dicopper(II) complexes containing various
endogenous and exogenous bridging moieties, Mukherjee and Mukherjee reported
that nature of the bridging group has profound effects on the ability of the complexes
to perform the oxidation.
20
It has been assumed that for the square pyramidal
dicopper(II) complex to act as an active catalyst, the dissociation of bridging
group/axial donor must occur so that a vacant coordination site will be readily
available for the binding of substrate in a bridging mode. As the binding strength of
exogenous bridge decreases, the ability of the complex to bind to the substrate
increases and the oxidation becomes more efficient. The compounds [Cu
2

(L
5
-
O)
2
(OClO
3
)
2
] (L
5
-OH = 4-methyl-2,6-bis(pyrazol-1-ylmethyl)phenol) (Figure 2-9
(left)) was found inactive, probably the bridging structure changes due to reaction
with catechol.
20


Figure 2-9. Dicopper(II) complexes studied by Mukherjee et al. (left)
20a
and Jager et
al. (right).
11f

The complex, {1,2-O-isopropylidene-6-N-(3-acetyl-2-oxobut-3-enyl)amino-6-
deoxy-glucofuranoso}copper(II) (Figure 2-9 (right) reported by Jager et al. has been
found to be highly active among the model complexes.
11f
These investigations also
confirmed that the copper(II) complexes are reduced to copper(I) complexes during
Chapter 2

56
catalysis and electron transfer from catechol to the copper(II) complex begins after the
formation of a copper-catechol intermediate which could be prevented by the
competitive formation of copper-quinone complex.
16



Reactivity of the dicopper(II) complexes towards catechols have established both
geometry around the copper(II) ions and Cu···Cu distance as two key factors in
determining the catalytic ability of the complexes.
11f, 20-21
The short Cu···Cu distance
allows the bridging catechol coordination compatible with the distance between the
two o-diphenol oxygen atoms.
11f, 20-21
Jager and Klemm
11f
proposed a mechanism
which was adapted from Karlin
22
and Casella
23
as shown in Figure 2-10. In this
mechanism, dinuclear copper(I) species with the o-quinone, µ-peroxo moieties with
copper(II) are the key intermediates.

Figure 2-10. Mechanism of 3,5-DTBC oxidation catalyzed by dicopper(II)
complexes.

11f

Chapter 2

57
Neves et al.
24
postulated a mechanism for the pH dependent oxidation of 3,5-
DTBC by the acetato-bridged complex [Cu
2
(H
2
bbppnol)(µ-OAc)(H
2
O)
2
]Cl
2
.2H
2
O.
According to the proposed mechanism (Figure 2-11), in the first step, a pre-
equilibrium exists between the complex and its deprotonated form since the reaction
depends on pH.

As the incoming catecholate is stronger than acetate, acetate is replaced by the
coordinating diphenol as a bridging ligand prior to the intramolecular electron-transfer
reaction. The electron-transfer reaction, as the rate-determining step, results in the
oxidation of catechol substrate to the corresponding o-quinone and the reduction of
copper centers to copper(I). The oxidation of Cu(I)-Cu(I) complex with 4-coordinated

Cu(I) centers back to the original form in the presence of dioxygen completes the
catalytic cycle as shown in Figure 2-11.


Figure 2-11. Proposed mechanism for the pH dependent oxidation of 3,5-DTBC by
acetate bridged complex, [Cu
2
(H
2
bbppnol)(µ-OAc)(H
2
O)
2
]Cl
2
.2H
2
O.
24

Chapter 2

58
Torelli et al. investigated a series of µ-OH dicopper(II) complexes
25
(for example,
[Cu
2
(L
R

)(μ-OH)](ClO
4
)
2
) (L
R
= (2,6-bis[{bis(2-pyridylmethyl)amino}-methyl]-4-R-
substituted phenol, R = -OCH
3
, -CH
3
, -F) as model systems for the catechol oxidase
regarding the binding of catechol substrate in the first step of the catalytic cycle. It is
shown that ortho-diphenol binds monodentately one copper(II) center with the
concomitant cleavage of the OH bridge. The mechanism shown in Figure 2-12
displays the substrate fixation step as the first step: the first adduct corresponds to the
one proposed by Krebs
12c
and the other adduct corresponds to the one proposed by
Solomon.
12a


Figure 2-12. Proposed mechanism for the interaction between dinuclear copper(II) µ-
OH complexes and the 3,5-DTBC.
25
Insert: (A) intermediate proposed by Krebs;
12c

(B) intermediate proposed by Solomon.

12a


The complete deprotonation of the monodentate bound catechol leads to a bridging
catecholate prior to the electron transfer. The µ-OH appears to be a key factor to
Chapter 2

59
achieve the complete deprotonation of the catechol, leading to a bridging catecholate.
This view is further supported when Belle and Reedjik demonstrated the effect of
substituting the µ-hydroxo bridge with chloride or bromide ion.
10e
Upon substitution
of the µ-hydroxo bridge with the halogen anion, no proton transfer occurred
precluding the binding of catecholate in bidentate bridging fashion, and thus the
subsequent catalytic cycle.

Many attempts to establish a correlation between the structural parameters of the
complexes and their catalytic activity in the oxidation of catechols were described in
the literature.
11-14, 16-25
Some of the crucial factors dictating the catecholase activity
can now be highlighted based on these investigations made on the dicopper(II)
complexes: the distance between the Cu(II) centres,
21
the nature of the bridging group
between the copper ions,
10, 20
electronic properties of the complexes
21a

and the
geometric changes of the dicopper core.
19b
Furthermore, the factors such as the
number of donor sites, nature of the donors and the rigidity of the ligand or the
bridging moiety imposing strain in the complex have also been found to play a role.
11j




60
Chapter 2




Part A

Synthesis, Characterization, Structural
Properties and Catecholase Activity of
Dicopper(II) Complexes of reduced Schiff
base Ligands



Chapter 2 (Part-A)

61
2-A-1. Introduction

In this section, we present a series of dicopper(II) complexes of reduced Schiff base
ligands formed between various substituted salicylaldehydes and
aminocyclopentane/cyclohexanecarboxylic acids and L-alanine (Chaper 1, Figure 1-
20, p-31). Driven by our recent results demonstrating the effect of the chelating side
arms of the dicopper(II) complexes,
27
we are prompted to investigate the role of
electron-donating and withdrawing groups at the benzene ring of the ligand on
catecholase activity. Despite the plethora of the reports on catecholase activity of
various dicopper(II) complexes, studies modulating the activity in terms of electronic
properties
21
via substituted bridging phenolates have not been well documented. Such
investigation on the structure-reactivity patterns is essential to enhance the
mechanistic understanding of the parameters affecting the catecholase activity. This
section describes the structural features and catecholase activity of the dicopper(II)
complexes of various reduced Schiff base ligands containing substituted phenolate
moiety. Further, variable temperature magnetic studies on the representative complex
have also been discussed.

2-A-2. Results and Discussion
2-A-2-1. Synthesis
The dinuclear copper(II) complexes, [Cu
2
(RScp11)
2
(H
2
O)
2

] [R = H (IIA-1), Cl
(IIA-2), CH
3
(IIA-3), OH (IIA-4)], [Cu
2
(RSch11)
2
(H
2
O)
x
] [R = H and x = 1 (IIA-5),
R = Cl (IIA-6), R = CH
3
and x = 2 (IIA-7)], [Cu
2
(RSch12)
2
(H
2
O)
2
] [R = H (IIA-8),
CH
3
(IIA-10), [Cu
2
(ClSch12)
2
].2H

2
O (IIA-9)], [Cu
2
(Diala5)(H
2
O)
2
].H
2
O, IIA-11;
[Cu
2
(Diala4)(H
2
O)
2
].H
2
O, IIA-12 and [Cu
2
(Diala3)(H
2
O)
2
].H
2
O, IIA-13 have been
synthesized in good yields by the complexation of copper(II) acetate monohydrate or
Chapter 2 (Part-A)

62
copper(II) nitrate trihydrate with the corresponding reduced Schiff base ligands
according to the procedures described in the experimental section. Owing to their
poor solubility in common solvents except in DMF and DMSO, our attempts to get
single crystals of the bulk as such were not successful. However, for [Cu
-
2
(Scp11)
2
(MeOH)
2
] (IIA-1a), [Cu
2
(ClScp11)
2
(DMF)(H
2
O)].MeCN (IIA-2a),
[Cu
2
(MeScp11)
2
(MeOH)
2
].2MeOH (IIA-3a), [Cu
2
(ClSch11)
2
(MeOH)
2

].2MeOH
(IIA-6a), [Cu
2
(ClSch12)
2
].2MeOH, IIA-9a and
[Cu
2
(Diala4)
2
(DMSO)
2
]⋅2DMSO⋅2Acetone, IIA-12a the single crystals with the
solvates different from bulk were obtained following the technique of slow diffusion
of solvents (see experimental section). But the complexes IIA-4 and IIA-8 afforded
single crystals from the reaction mixture as described in the experimental section.

2-A-2-2. Description of crystal structures
In all these complexes, the reduced Schiff base ligands display tridentate
coordination mode with binucleating ability through bridging phenolate oxygen
atoms. The ONO donor set arising from amine nitrogen, phenolate oxygen and one of
the carboxylate oxygen atoms completes the tridentate coordination of the ligands to
the Cu(II) ions (Figure 2-13). All these complexes contain phenolato bridged
dinuclear Cu
2
O
2
cores with Cu···Cu distance of ca 3 Å. With respect to the ligands,
IIA-1a, IIA-2a, IIA-3a and IIA-4 contain 1-aminocyclopentanecarboxylate side arm
as a common structural moiety but different substituents on the 5-position of the

benzene ring; H in IIA-1a, Cl in IIA-2a, CH
3
in IIA-3a and OH in IIA-4. All these
complexes, IIA-1a, IIA-2a, IIA-3a, IIA-4 and IIA-12a display Cu(II) centres with
distorted square pyramidal geometry (τ = 0.168, 0.043, 0.038, 0.228 and 0.003
respectively)
28
while square planar Cu(II) centers are present in IIA-9a
Chapter 2 (Part-A)

63
Crystallographic centre of inversion is present in the dimeric structure of IIA-1a, IIA-
3a, IIA-4, IIA-6a and IIA-9a.

Figure 2-13. Schematic representation of dicopper(II) complexes. The axial positions
are mostly occupied by the solvents.

In all these complexes, basal plane of the square pyramid is completed by the
coordination of each Cu(II) to the two bridging phenolate oxygen atoms, amine
nitrogen and carboxylate oxygen. The Cu-O bond distances due to the coordination of
each Cu(II) to the bridging phenolate and carboxylate oxygen atoms in the basal plane
fall in the range of 1.934(2)-2.002(3) Å and 1.876(2)-1.982(7) Å respectively while
the bond distances due to Cu-N (amine nitrogen) are observed in the range of
1.950(2)-2.004(8) Å. The apical position is occupied by aqua ligands as in IIA-4, by
solvent molecules such as methanol as in IIA-1a and by both aqua ligand and DMF
solvent as in IIA-2a giving the Cu-O bond distances in the range 2.213(2)-2.660(3) Å.
The common 1-aminocyclopentanecarboxylate side arm of the ligands in IIA-1a,
IIA-2a, IIA-3a and IIA-4 resulted in the formation of five membered rings where as
the bridging phenolate oxygen generated four membered Cu
2

O
2
rings.
Chapter 2 (Part-A)

64
The crystal packing of dimers in IIA-1a, IIA-2a, IIA-3a and IIA-12a resulted in
the formation of 1D hydrogen bonded polymeric structures while the packing pattern
in IIA-4 resulted in the 3D hydrogen bonded network structures.

2-A-2-2-1. [Cu
2
(Scp11)
2
(MeOH)
2
], IIA-1a
The complex IIA-1a as a methanol adduct crystallized with two methanol
molecules in monoclinic system with space group C2/c with Cu···Cu separation of
3.0245(7) Å. Selected bond lengths and bond angles are given in Table 2-1. The
apical positions of square pyramid of the two Cu(II) atoms are occupied by methanol
molecules in trans fashion (Figure 2-14).


Figure 2-14. A perspective view of the dimer, IIA-1a.




Chapter 2 (Part-A)

65
Table 2-1. Selected bond distances (Å) and bond angles (º) for IIA-1a
Cu(1)-O(2) 1.945(2) Cu(1)-O(1)
a
1.958(2)
Cu(1)-O(4) 2.213(2) Cu(1)-O(1) 1.961(2)
Cu(1)-O(1) 1.983(3) Cu(1)-Cu(1A)
a
3.0245(7)

O(2)-Cu(1)-O(1)
a
102.78(9) O(1)-Cu(1)-Cu(1)
a
39.44(6)
N(1)-Cu(1)-Cu(1)
a
131.92(8) C(1)-O(1)-Cu(1)
a
133.66(19)
C(1)-O(1)-Cu(1) 118.37(18) Cu(1)
a
-O(1)-Cu(1) 101.04(9)
Symmetry transformations used to generate equivalent atoms: a = -x+1,-y+1,-z+1

Complementary MeOH···O=C (O4-H4···O2) and N-H···O=C (N1-H1···O3)
intermolecular hydrogen bonding generates 1D polymeric structure in the solid state
(Figure 2-15). Table 2-2 contains the hydrogen bond parameters.

Figure 2-15. A view showing a portion of the 1D hydrogen-bonded structure in IIA-
1a.






Chapter 2 (Part-A)

66
Table 2-2. Hydrogen bond distances (Å) and angles (º) for IIA-1a
D-H d(D-H) d(H··A)
∠DHA
d(D··A) A Symmetry
N1-H1 0.80(3) 2.35(4) 148(4) 3.053(3) O3 x-1, y, z-½
O4-H4 0.72(4) 1.97(4) 174(5) 2.691(4) O2 x-1, y, z-½

2-A-2-2-2. [Cu
2
(ClScp11)
2
(DMF)(H
2
O)]. MeCN, IIA-2a
Each Cu(II) atom adopts distorted square pyramidal geometry. The two copper
atoms in the dimer are separated by distance of 3.0428(6) Å. Selected bond lengths
and bond angles are given in Table 2-3. The apical site of square pyramid is occupied
by DMF [Cu(1)-O(8), 2.230(3) Å] with Cu(1) and aqua ligand [Cu(2)-O(7), 2.299(3)

Å ] with Cu(2) in trans to each other. Each dimer is associated with a molecule of
acetonitrile solvent in the asymmetric unit (Figure 2-16).

Figure 2-16. A perspective view of the unit cell contents of IIA-2a.




Chapter 2 (Part-A)

67
Table 2-3. Selected bond distances (Å) and bond angles (º) for IIA-2a
Cu(1)-O(4) 1.949(2) Cu(1)-N(1) 1.962(3)
Cu(1)-O(2) 1.964(3) Cu(1)-O(1) 2.002(3)
Cu(1)-O(8) 2.230(3) Cu(1)-Cu(2) 3.0428(6)

O(4)-Cu(1)-N(1 68.25(13) O(4)-Cu(1)-O(2) 102.42(11)
N(1)-Cu(1)-O(2) 82.48(12) O(4)-Cu(1)-O(1) 79.17(10)
N(1)-Cu(1)-O(1) 93.46(12) O(2)-Cu(1)-O(1) 165.66(11)

There are very weak interactions between the carbonyl oxygen atoms and the
copper atoms (Cu(1)⋅⋅⋅O3(1-x, 1-y, 1-z), 2.885 Å and Cu(2) )⋅⋅⋅O6(1-x, y, z), 2.922 Å)
which are around the sum of the van der Waals radii, 2.9 Å. These Cu⋅⋅⋅O interactions
generates 1D polymer along the b-axis (Figure 2-17) sustained by hydrogen bonds
between carboxylate oxygen and aqua ligand, O(7) H(7B)···O(3). Further O(7)-
H(7A)···N(4S) hydrogen bonds have also been observed. Hydrogen bond parameters
are shown in Table 2-4.


Figure 2-17. Hydrogen bonded association in IIA-2a.

Chapter 2 (Part-A)

68
Table 2-4. Hydrogen bond distances (Å) and angles (º) for IIA-2a
D-H D(D-H) d(H··A)
∠DHA
d(D··A) A Symmetry
O7-H7A 0.73(6) 2.26(4) 162(6) 2.961(9) N4S x+1, y, z
O7-H7B 0.81(6) 1.92(4) 177(5) 2.728(4) O3 x-1, y-1, z-1

2-A-2-2-3. [Cu
2
(Mescp11)
2
(MeOH)
2
].2MeOH, IIA-3a
The complex IIA-3a displays square pyramidal geometry at each Cu(II) centre
(Figure 2-18). Basal plane of square pyramid is occupied by two phenolate oxygen
atoms [Cu(1)-O(1) 1.960(2) Å and Cu(1)-O(1A) 1.944(2) Å], amine nitrogen [Cu(1)-
N(1) 1.961(2) Å] and the carboxylate oxygen [Cu(1)-O(2) 1.939(2) Å]. Table 2-5
shows the selected bond distances and bond angles.

Figure 2-18. A perspective view of IIA-3a.

The apical sites the square pyramid at each Cu(II) centre are occupied by methanol
molecules in trans fashion with Cu···O distance of 2.59 Å. The two square planar

copper centres resulted in the Cu···Cu distance of 3.0056(5) Å through phenolate
bridging.
Chapter 2 (Part-A)

69
Table 2-5. Selected bond distances (Å) and bond angles (º) for IIA-3a
Cu(1)-O(2) 1.939(2) Cu(1)-O(1) 1.960(2)
Cu(1)-O(1)
a
1.944(2) Cu(1)-N(1) 1.961(2)
Cu(1)-Cu(1A)
a
3.0056(5)

O(2)-Cu(1)-O(1)
a
102.02(7) O(2)-Cu(1)-O(1) 174.04(7)
O(1)
a
-Cu(1)-O(1) 79.33(7) O(1)
a
-Cu(1)-N(1) 171.71(7)
O(1)-Cu(1)-N(1) 93.79(7) O(2)-Cu(1)-Cu(1)
a
141.57(5)
Symmetry transformations used to generate equivalent atoms: a = -x+1,-y+1,-z+1


Packing of dimers along a axis displays intermolecular hydrogen bonding between
amine hydrogen and lattice methanolic oxygen (N-H···O(5)) and also between

methanolic hydrogen and carboxylate oxygen atoms (O(4)-H(4)···O(3) and O(5)-
H(5)···O(2)). Carboxylate oxygen atoms of the neighboring molecules involved in
weak interactions with each Cu(II) ion in syn-anti fashion giving the Cu···O distance
of 2.885 Å. Hydrogen bond distances and angles are tabulated in Table 2-6. The solid
state structure of this compound is a 1D polymer supported by all these interactions
(Figure 2-19).

Table 2-6. Hydrogen bond distances (Å) and angles (º) for IIA-3a
D-H D(D-H) d(H··A)
∠DHA
d(D··A) A Symmetry
N1-H1 0.87(3) 2.10(3) 159(3) 2.932(3) O5 -x,-y+1,-z+1
O4-H4 0.76(4) 2.05(4) 163(3) 2.785(3) O3 x+1, y, z
O5-H5 0.78(6) 2.14(5) 156(5) 2.864(4) O2

Chapter 2 (Part-A)

70

Figure 2-19. Packing diagram of IIA-3a.

2-A-2-2-4. Crystal structure of [Cu
2
(OHScp11)
2
(H
2
O)
2
], IIA-4

Figure 2-20. A perspective view of the dimer IIA-4.

The crystal structure of IIA-4 consists of the basic dimeric building block
[Cu
2
(OH)Scp11)
2
(H
2
O)
2
] in which both Cu(II) centres have square pyramidal
geometry. The two lattice water molecules are in close interaction with Cu(II) atoms
Chapter 2 (Part-A)

71
with Cu-O distance of 2.66(3) Å occupying the apical positions to complete the
square pyramidal geometry around each Cu(II) centre (Figure 2-20). The two Cu(II)
atoms bridged by the two phenolate oxygen atoms resulted Cu···Cu distance of
2.9924(8) Å. Table 2-7 shows the selected bond lengths and angles.

Table 2-7. Selected bond distances (Å) and bond angles (º) for IIA-4
Cu(1)-O(2) 1.920(2) Cu(1)-O(1)
a
1.934(2)
Cu(1)-O(1) 1.930(2) Cu(1)-N(1) 1.973(2)
Cu(1)-O(4) 2.660(3) Cu(1)-Cu(1A)
a

2.9923(7)

O(2)-Cu(1)-O(1)
a
102.10(8) O(1)-Cu(1)-O(1)
a
78.48(8)
O(2)-Cu(1)-N(1) 84.57(9) O(1)-Cu(1)-N(1) 94.67(9)
O(1)
a
-Cu(1)-N(1) 165.21(9) O(1)
a
-Cu(1)-O(4) 105.32(8)
Symmetry transformations used to generate equivalent atoms: a = -x+1,-y+1,-z+1

Packing of the dimers along b axis showed one of the aqua ligands involving in the
hydrogen bonding with the oxygen atom of the hydroxyl group on the phenolate
moiety (O4-H4B···O5) and the carboxylate oxygen atom (O4-H4A···O3). N-H···O
hydrogen bonds are formed between the N-H hydrogen atom and carboxylate oxygen
atom (N1-H1···O3). In addition, there is also hydrogen bonding between the hydrogen
atom of the hydroxyl group and oxygen atom of aqua ligand (O5-H5···O4). These
hydrogen bonds generate 3D hydrogen bonded network connectivity in IIA-4.
Hydrogen bond distances and angles are shown in Table 2-8. A portion of the
hydrogen bonding connectivity in IIA-4 has been shown in Figure 2-21 and Figure 2-
22.