**Features of Structure, Geometrical, and Spectral Characteristics of the (HL)2[CuX4] and (HL)2[Cu2X6] (X = Cl, Br) Complexes**

Olga V. Kovalchukova *Peoples' Friendship University of Russia Russian Federation* 

#### **1. Introduction**

190 Current Trends in X-Ray Crystallography

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Chem. 47 (2008) 10729-10739.

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5978-5985.

Coordinate compounds of copper(II) are widely spread both as biological objects (metalloproteins 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- terminaloxidases*,* 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 3dmetals, magnetic properties of which significantly depend on features of the spatial structure of complex anions (Rakitin & Kalinnikov, 1994).

d9-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.

Features of Structure, Geometrical, and Spectral Characteristics

**3. Structure characteristics and properties of CuCl4**

distortion increases its value up to 180 deg.

complexes

**3.1 Structure description** 

inorganic anions (Table 1).

of the (HL)2[CuX4] and (HL)2[Cu2X6] (X = Cl, Br) Complexes 193

[CuCl4]2- the formation constants were found to be log 1 4.0; log 2 4.7; log 3 1.96; log 4 0.23 (Khan & Schwing-Weill, 1976). The complexes are easily destroyed in polar solvents. Halogenions in the inner sphere are easily replaced by other ligands (ammonia, water at al). Majority of

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

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 CuX42- 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 (Td) of CuХ42- corresponds to mean -values up to 109 deg. as the planar

**2- species** 

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

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

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

Since the last century, a lot of structures of common formulae (HL)2CuCl4 and (HL)2CuBr4 where L is an organic base were determined by X-Ray crystallography. It was stated that the geometry of CuX42- 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 D4h 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 Cs2[CuX4] 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

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.
