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Structural Diversity on Copper(I) Schiff Base Complexes

187

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Features of Structure, Geometrical, and Spectral

Characteristics of the (HL)

[CuX

] and

(HL)

[Cu

] (X = Cl, Br) Complexes

Olga V. Kovalchukova

Peoples’ Friendship University of Russia

Russian Federation

1. Introduction

Coordinate compounds of copper(II) are widely spread both as biological objects (metallo-

proteins and metallo-enzymes), and in engineering. Among the functions of the copper

proteins are: the electron transfer involving the Cu(I)/Cu(II) couple; mono- terminal-

oxidases, which form either water or hydrogen peroxide from dioxygen; oxygenases, which

incorporate an oxygen atom into a substrate; superoxide degradation to form dioxygen and

peroxide; and the oxygen transport. From a structural point of view, there are three main

types of biologically active copper centres found in the copper proteins (Cowan, 1993).

These are “blue” copper centres, where copper atoms are normally coordinated to two

nitrogens and two sulphurs, “non-blue” copper centres, where copper atoms are

coordinated to two or three nitrogens as well as oxygens, and copper dimers. The nitrogens

come from histidine groups, the sulfur from methionine and cysteine, the oxygens from the

carboxyllic acid in the protein. So called “non-blue” and dimeric copper-containing proteins

are of a a great similarity with complex halo (chloro-, bromo-) cuprates (Abolmaali et al.,

1998). Thus, studies of structural and spectral characteristics of anionic complex halides of

copper(II) can help to explain electronic structures as well as high reactional abilities and

selectivities of active sites of copper-containing biopolymers in catalytic processes.

It is also evident that anionic halocuprate(II) complexes are catalytically active species

responsible for the increased reactivity in a lot of organic reactions (oxidation and

polymerization of phenols, reactions of tertiary ammines, dimerization of primary alkyl

groups et al.). Various investigations show that catalytic activities of complex copper(II)

halides depend upon structures of their coordination polyhedra (Allen et al., 2009).

And finally, cupric halo-complexes relate to classic magneto-active systems containing 3d-

metals, magnetic properties of which significantly depend on features of the spatial

structure of complex anions (Rakitin & Kalinnikov, 1994).

-Electronic subshell of Cu(II) is responsible for distortions of symmetry of the

coordination polyhedron (Gerloch & Constable, 1994). This deals with the Jahn-Teller effect

(as a result of electron-vibrational interactions), and a large spin-orbital interaction constant.

These two effects are of comparable values, and this fact complicates the prediction of

structures of complexes of such types as well as physico-chemical properties and biological

activity of Cu(II) complexes are in many respects determined by features of their structures.

Current Trends in X-Ray Crystallography

192

The X-Ray analysis on single crystals can unequivocally determine structures of substances

but isolation of single crystals is a complicated process which may not be achieved

successively. Thus, a great role should belong to site-methods of structure determination

(spectral, magnetic et al). Such correlations like “structure – spectral parameters – magnetic

characteristics” help to describe features of the structure of complexes and as a result to

predict their possible physical properties and areas of application.

2. Electronic structure and coordination geometry of Cu(II)-compounds

The ground state for the Cu

cation in its octahedral coordination is

)

, and the

one for the square coordination is

222

222221

gxzyzzxyxy

Ed d d d d



The only excited state might

relate to

)

with the regular and distorted tetrahedral coordination (energy

difference 10Dq) (Cotton & Wilkinson, 1966). The maximal coordination number of

copper(II) is 6 which relates to octahedral complexes of the (t

)

–

222

ddd

222



22 2

222 21

dddd d



configurations. As the



orbital, when compared

to the

d one, contains only one electron, it forms stronger Cu-to-ligand bonds. Thus, four

planar ligands are stronger joint to the central ion than the two axial ligands which are

bounded to Cu(II) along the

z-axis. Sometimes the difference is so great that Cu(II)

complexes may be considered as square. Most often, the coordination numbers of Cu(II) are

4 (square) or 6 (distorted octahedron). Tetrahedral coordination (Т

symmetry) and

distorted tetrahedral coordinations (D

) also exist. The distorted octahedron is

characterized by four short Сu–L bonds at one plane and two longer axial bonds in the

trans-position (4+2 coordination) or 2 short and 4 long bonds (2+4 coordination). The square

environment of Cu(II) appears as a limiting case of a tetragonal distortion of an octahedron.

The copper(II) also forms a lot of complexes with the coordinate number 5. This can be

explained by blocking of one of the tops of the octahedron by an e

-lone electron pair. As a

result, a square pyramid is formed (4+1 configuration). The square-pyramidal configuration

exists for example in copper(II) pycrate diaquaacetylccetonate and K[Cu(NH

)

](PF

)

(Adman, 1991). It usually appears in copper(II) complexes with pyridine and other bases,

and is also typical for a Cu–β-alanyl-L-hystidine compound which is used as a model of M–

protein interactions (Gerloch & Constable, 1994). In the case that ligands are

stereochemically movable, a more symmetric structure of a trigonal bipyramid is formed as

in [CuBr

]

or [CuDipy

(Gillard & Wilkinson, 1963).

The copper(II) forms both cationic and anionic complexes. The anionic ones – cuprates(II)

are most often formed in excess of hydrohalic (HCl, HBr), cyanic (HCN) or thiocyanic

(HSCN) acids. HI usually reduces Cu(II) into Cu(I). The complex anions of the type of

[CuХ

] and М

[СuХ

] (Х = Cl

, Br

, CN

, SCN

) are stabilized by counter-ions the most

simple of which are cations of alkaline metals (Willett & Geiser, 1984). For example, the

structure of a red CsCuCl

(Schluetes et. al., 1966) contains Cu(II) ions octahedrically

surrounded by six chloride anions. The CuCl

in a yellow Cs

CuCl

(Helmholz & Kruh,

1952) is in tetragonal distortion. Brownish-red compounds of LiCuCl

2Н

O (Vossos et al.,

1963) and KCuBr

(Geiser et al., 1986a)

consist of planar Сu

anions (X = Cl, Br) with

symmetrical Cu–X–Cu bridges.

Actually, М

[СuX

], X = Cl, Br compounds with inorganic monovalent cations in the outer

sphere can be mostly considered as double salts of a very low stability. For example, for

Features of Structure, Geometrical, and Spectral Characteristics

of the (HL)

[CuX

] and (HL)

[Cu

] (X = Cl, Br) Complexes

193

[CuCl

]

the formation constants were found to be log 

4.0; log 

4.7; log 

1.96; log 

0.23

(Khan & Schwing-Weill, 1976). The complexes are easily destroyed in polar solvents. Halogen-

ions in the inner sphere are easily replaced by other ligands (ammonia, water at al). Majority of

complex halides are not stable in air and destroyed by absorption of water vapors.

Much more interesting are anionic halocuprates(II) containing protonated organic molecules

as counter-ions. They are stabilized with the help of formation of a system of intra- and

intermolecular hydrogen bonds (H-bonds) or extra coordinate bonds via lone electron pairs

of donating atoms (N, O, S) or vacant molecular orbitals of organic molecules (so-called

dative bonds with the metal-to-ligand charge transfer M→L). The present chapter belongs to

development of features of the structures and properties of such type of compounds.

3. Structure characteristics and properties of CuCl

species

Stereochemistry of copper(II) halides is rather rich (Smith, 1976). The latest results are

summarized in the review (Murphy & Hathaway, 2003). Two types of coordination exists:

the “common” one with ionic radii of about 0.5 Ǻ and semi-coordinated where the bond

lengths are 0.3 – 1.0 Ǻ longer (Hathaway, 1982). The shape of coordination polyhedra

changes from square planar (Harlow et al., 1975) to distorted tetrahedral (Diaz et al., 1999).

The degree of distortion of CuX

coordination polyhedra is determined by the mean value

of the flattering or trans-angle

 (Fig.1). It is evident that the non-distorted tetrahedral

configuration (T

) of CuХ

corresponds to mean -values up to 109 deg. as the planar

distortion increases its value up to 180 deg.

Fig. 1. Determination of the degree of distortion of coordination polyhedra of Cu(II)

complexes

3.1 Structure description

Since the last century, a lot of structures of common formulae (HL)

CuCl

and (HL)

CuBr

where L is an organic base were determined by X-Ray crystallography. It was stated that

the geometry of CuX

depends upon stability of H-bonds and other electrostatic and

steric interactions between counter-ions and halide-ions of tetrahalocuprate(II) fragments

(Murphy & Hathaway, 2003). H-bonds flatten the structure towards D

configuration.

The decrease in the abilities of organic cations to form H-bonds with the inorganic anion

leads to the decrease in distortion of its tetrahedral geometry. Really, in Cs

[CuX

] the

mean values of trans-angle

 is determined to be 124 deg. for X = Cl (Helmholz & Kruh,

1952) and 128.4 deg. for X = Br (Morosin & Lingafelter, 1960) which corresponds to

slightly distorted tetrahedron, as well as tetrahalocuprates containing diprotonated 3-

amminopyridine as counter-ions are characterized by planar structures of inorganic

anions (

 170.60 deg. for tetrachlorocuprate and 170.56 deg. for its bromo-analogue (Willet

et al., 1988). More complicated organic compounds provoke intermediate characters of

inorganic anions (Table 1).

Current Trends in X-Ray Crystallography

194

θ(CuCl

) ,

deg

Ref.

θ(CuBr

deg

Ref.

Cs+ 124

Helmholz & Kruh,

1952

128.4

Morosin &

Lingafelter, 1960

NNH

133 Antolini et al., 1981 125.6

Place & Willett,

1988

134.4

Kovalchukova et al.,

2008a

131.64

Koval’chukova et

al., 2009b

134.69

Bhattacharya et al.,

2004

134.2

Bhattacharya et al.,

2004

NH NH

140

Place & Willett,

1987b

137

Place & Willett,

1987b

170.56 Willet et al, 1988 170.6 Willet et al, 1988

177.4 Long et al., 1997 178 Long et al., 1997

NCH

180 Tichý et al., 1978 180

Rubenacker et al.,

1984

Table 1. Some characteristics of coordination polyhedra of some (HL)

[CuX

] structures.

Plotting of degrees of distortion of the CuХ

coordination polyhedron vs. the type of the

halogenide-anion (Fig 2) gives a straight-line dependence with closely related values of

θ(CuCl

) and θ(CuBr

) for the same organic cations (Koval’chukova et al., 2009b). Thus,

the degree of distortion of coordination polyhedron slightly depends upon the nature of X

(Cl

, Br

), and is mainly determined by the nature of the organic cation.

Fig. 2. The dependence of flattening angle of CuX

coordination polyhedron on the nature

of inorganic anion (Koval’chukova et al., 2009b).

Features of Structure, Geometrical, and Spectral Characteristics

of the (HL)

[CuX

] and (HL)

[Cu

] (X = Cl, Br) Complexes

195

Among the features of crystal structures of tetrabromocuprates(II) in the D

configuration

stabilized by cations of organic aliphatic amines (such as cyclopentylamine (Luque et al., 2001),

phenylethylamine (Arend et al., 1978), methyl(2-phenylethylamine (Willett, 1990)) there are

tight interactions of tetrabromocuprate(II) anions. This leads to the appearance of two semi-

coordinate bonds with Cu–Br distances 3.0 – 3.1 Ǻ as compared with common 2.41 – 2.45 Ǻ. As

a result, the coordination number of copper becomes 6 (4+2 coordination) (Fig. 3).

The same type of (4+2) coordination of Cu

was found in (HL)

[CuCl

], L = 2-methyl-l,3-di-

2-pyridyl-2-propanol (Schlemper et al., 1989). Cu

ions are in distorted square-bipyramidal

coordination. The bridging angles Cu'ClCu are 125.1 and 129.5 deg, and Cu-СІ distances

2.457 and 2.868 Ǻ.

Fig. 3. The formation of a layer of (CuBr

)

in (PhC

)

CuBr

(Arend et al., 1978)

Tetrachlorocuprate(II) ions may also coordinate other d-metals. Paramagnetic metallic ions

form bridging anti-spin magnetic chained systems which are of a great interest for physics

(Landee et al., 1988). The Cu-Cl distances of bridging fragments may reach 2.325 Ǻ, and

those of terminate character shorten till 2.181 Ǻ.

The possibility of bromination of organic cations with using for neutralization of anionic

bromocuprates may also take place. That was described for example by (Place & Willett,

1987a). While the formation of anionic tetrabromocuprates(II) containing protonated 2-

amino-3-methylpyrydine as a counter-ion, partial bromination of an organic specie was

observed. Both 2-amino-3-methylpyrydinium and 2-amino-5-bromo-3-methylpyrydinium

cations neutralize the negative charge of tetrabromocuprate anion in the crystal lattice.

Fig. 4. The crystal structure of (H

(CuBr

)(CuBr

)H

O3CH

OH (Sundberg et al., 1992).

Current Trends in X-Ray Crystallography

196

Sundberg and co-workers (Sundberg et al., 1992) showed that while complex formation of

CuBr

with 6-amino-1,3-dimethyl-5-((2-carboxyphenyl)azo)uracyl (HL) in the presence of

HBr, partial reduction of Cu(II) into Cu(I) is observed. As a result, a complex

(CuBr

)(CuBr

)H

O3CH

OH contains copper atoms in oxidation states +2 and +1.

The coordination number for Cu(I) is 3 (CuBr

as a planar anion), and 4 for Cu(II) (CuBr

as a distorted tetrahedron). In lattice, both (H

(CuBr

) and (H

(CuBr

) exist as

independent fragments. So the above complex may be considered as co-crystallization of

two complexes containing copper in two different oxidation states. Water and methanol

molecules have a lattice character (Fig. 4).

As it was intimated (Place & Willett, 1987a), the H-bonding between organic protonated

molecules and inorganic tetrahalocuprate(II) anions is not released for structures with

 <

132 deg. Such types of structures are described, for example, for bis(4-azafluorene-9-onium)

tetrabromocuprate(II) monohydrate (Kovalchukova et al., 2009a) (

 130.98 deg.) crystal

package of which is present on Fig. 5. It is evident that cations and anions form altermating

layers with no H-bonds between them. The only one intermolecular interaction existing in

the structure belongs to the hydrated water molecule.

On the other hand, H-bonds are found in the (H

L)[CuBr

] structure with the flattening

angle

 120 deg (L - 2,4,6-tris(N,N-dimethylamino)methylphenol) (Kovalchukova ett al.,

2010). The Cu-Br distances in the distorted CuBr

anion range from 2.469(2) to 2.515(1) Ǻ.

The minimal Cu–N distance is 3.65 Ǻ, and its value rejects the possibility of formation of a

ligand-to-metal coordinate bond. Despite a small value of the flattening angle

, a hydrogen

N-H…Br bond exists between an H-atom of a protonated tertiary amino-group of the

organic dication and one of a Br-atom of the inorganic anion (r N-H 1.05; H…Br 2.42; N…Br

3.46 Ǻ;

 NHBr 169.5 deg.).

Flattening of the CuX

structure increases the number and strength of the H-bonds. As an

example, for (HL)

[CuX

] (L = 2-methylimidazolium, X = Cl,  134.4 deg (Kovalchukova et

al., 2008a); X = Br,

 132.64 deg (Koval’chukova et al., 2009b)), multiple H-bonds connect

counter-ions in crystals (Fig. 6). Similar structures were found for N-benzylpiperazinium

tetrachlorocuprate(II) (Antolini et al., 1981) containing an unequally flattened [CuCl

]

2–

tetrahedron (

 166.4 deg.)

(fig. 7) as well as for bis(ethylenediammoniummonobromide)

tetrabromocuprate(II),

 159.2 deg (Anderson & Willett, 1971), and a square-planar limit (

180 deg) for creatinium tetrachlorocuprate(II) (Battaglia et al., 1979). Probably H-bondings

play the crucial role in these limiting planar structures as compared with crystal packing

forces or the cation bulk. Nearly linear strong H-bonds (Fig. 7) produce such a reduction of

the ligand-to-ligand electrostatic repulsion to allow a nearly square-planar geometry,

favored by the crystal field stabilization (Smith, 1976).

H-bonds change not only bond angles but also Cu-Х bond lengths. In (Antolini et al., 1981)

the shortest Cu-Cl distances (~2.260 Å) were found for chlorine atoms which do not form H-

bonds. Abnormally long Cu-Cl bonds (~2.334 Å) were determined for Cl-atoms engaged in

a strong bifurcate H-bounding (Halvorson et al., 1990). Graphical correlations of parameters

of H-bonds, i.e.

r(H-Br) on r(N-Br) (Koval’chukova et al., 2009b),

...NHCl

angle on r(Cu-

Cl) (Kovalchukova et al., 2008a) (Fig. 8, 9) are linear but the dependencies of the NHBr

angles on θ (Fig. 10) for tetrachlorocuprates(II) and tetrabromocuprates(II) have opposite

derivative signs.