Spectrochimica Acta Part A 55 (1999) 2559–2564

Local structure analysis of some Cu(II) theophyllinecomplexes

L. David a,*, O. Cozar a, E. Forizs b, C. Craciun a, D. Ristoiu a, C. Balan a

a Faculty of Physics, Babes-Bolyai Uni6ersity, Kogalniceanu 1, 3400 Cluj-Napoca, Romaniab Faculty of Chemistry, Babes-Bolyai Uni6ersity, 3400 Cluj-Napoca, Romania

Received 17 March 1999; accepted 28 April 1999

Abstract

The CuT2L2·2H2O complexes [T=Theophylline (1,3-dimethylxanthine); L=NH3, n-propylamine (npa), 2-aminoethanol (ae)] were prepared and investigated by ESR spectroscopy. Powder ESR spectrum ofCuT2(NH3)2·2H2O is axial (g��=2.255, gÞ=2.059). ESR spectrum of CuT2(npa)2·2H2O with (g��=2.299, gÞ=2.081)is a superposition of one axial (g��=2.299, gÞ=2.073) and one isotropic component (g0:2.089), in the same amount.The axial spectra of the former complexes are due to a static Jahn–Teller effect (EJT:2880 cm−1). ESR spectrumof CuT2(ae)2·2H2O is orthorhombic (g1

c =2.199, g2c =2.095, g3

c =2.037). The local symmetries around the Cu(II) ionsremain unchanged by DMF solvating, by adsorbing these solutions on NaY zeolite or by lowering the temperature.© 1999 Elsevier Science B.V. All rights reserved.

Keywords: Theophylline; Cu(II) complexes; ESR

www.elsevier.nl/locate/saa

1. Introduction

The copper(II) complexes with organic ligandshave been intensely studied in the last years owingto their medical implication [1]. The biologicalactivity of these compounds is influenced by themanner in which the copper(II) ions coordinatethe ligands molecules. As part of our work aboutmetal complexes with molecules of biological in-terest [2–6], we investigate the Cu(II)-theophyllinecompounds. Transition metal complexes contain-

ing theophylline (T=1,3-dimethylxantine) andamine type ligands may serve as a model forcoordination of metal ions to nucleic acids attheirs oxopurine base. Structural and thermal sta-bility studies of these compounds can provideadditional data for a better understanding of theirpossible genetic role and/or antitumoral activityderived by interactions between nucleic acids andcertain metal ions [7,8]. In previous papers,theophylline (T=1,3-dimethylxantine) (Fig. 1)was studied as a model for guanine–metal ioninteractions, of great importance in problems con-cerning the division and replication of DNAmolecules [9,10].

* Corresponding author. Tel.: +40-64-405300; fax: +40-64-191906.

E-mail address: leodavid@phys.ubbcluj.ro (L. David)

1386-1425/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved.

PII: S1386 -1425 (99 )00115 -8

L. Da6id et al. / Spectrochimica Acta Part A 55 (1999) 2559–25642560

Fig. 1. The molecular structure of theophylline.

Fig. 2. Powder ESR spectrum of pseudotetrahedralCuT2(NH3)2·2H2O complex at room temperature.

In order to obtain further information concern-ing the local structure and the influence of theamine type in some Cu(II)-theophylline com-pounds we have prepared and investigated theCuT2L2·2H2O (L: NH3; npa=n-propylamine;ae=2-aminoethanol) complexes by ESR spec-troscopy. The coordination of the Cu(II) ion tothe theophylline molecules is usually realized at itsN(7) atom, by deprotonation [11]. If the prepara-tion of theophylline complexes is attempted fromaqueous solutions of ammonia or primary amines,then coordination compounds consisting of boththeophyllinato and amine ligands can occur, withstructures that in general are not predictable [12].The amine molecules can alter the local structurearound the Cu(II) ion, depending on the mono(NH3, npa) or bidentate (ae) character of thesemolecules and the interplay between steric andelectronic contributions.

The CuT2L2·2H2O compounds were preparedaccording to the procedure described in papers[12,13]. ESR measurements were performed at 9.4GHz (X band) using a standard JEOL-JES-3Bequipment.

2. Results and discussion

The IR spectra indicate a similar behavior ofthe CuT2(NH3)2·2H2O and CuT2(npa)2·2H2Ocomplexes, the Cu(II) ion being coordinate ineach case by corresponding nitrogen atoms oftheophylline and two amine molecules. The corre-sponding chromophore is CuN2N*2 [14]. In thecase of CuT2(ae)2·2H2O, one 2-aminoethanolmolecule coordinate Cu(II) ion also by its oxygenatom (this molecule manifests a bidentate charac-ter), the local unit being CuN2N*2 O [13]. Thesestructures are also confirmed by X-ray data [13].The H2O molecules presents in unit cell are not

involved in the coordination at Cu(II) ion.The distinct mode in which the amine molecules

influence the local structure of the Cu(II)-theophylline complexes is well evidenced by ESRmeasurements. All complexes have been investi-gated in powder samples and in DMF solutions inthe 143–293 K temperature range and also inDMF solutions adsorbed on NaY zeolite.

2.1. The compressed pseudotetrahedralCuT2(NH3)2·2H2O and CuT2(npa)2·2H2Ocomplexes

X-ray measurements have revealed similarstructures for CuT2(NH3)2·2H2O and CuT2(npa)2·2H2O and the presence of the same chromophore(CuN4).

The axial powder ESR spectrum ofCuT2(NH3)2·2H2O compound with g��=2.255 andgÞ=2.059 values suggests a compressed pseu-dotetrahedral symmetry around the metallic ion(Fig. 2). The 4.5 value of the G parameter (G=(g��−2.0023)/(gÞ−2.0023)) [15], inside of theusual interval (4}5), indicates that the experi-mental data suit molecular g values. The shape ofthe spectrum and the g tensor values remainunchanged by lowering the temperature at 143 K.

The powder ESR spectrum of CuT2(npa)2·2H2O complex (Fig. 3, thick solid line) is verylarge (g��=2.299, gÞ=2.081). We have made thesimulation of the powder spectrum of CuT2(npa)2·2H2O, considering a superposition of one axialspectrum (g��=2.299, gÞ=2.073) and oneisotropic spectrum (g0=2.089) [16]. These compo-nents contribute to the whole spectrum in thesame proportion. The g values remain un-

L. Da6id et al. / Spectrochimica Acta Part A 55 (1999) 2559–2564 2561

Fig. 3. Powder ESR spectrum of pseudotetrahedralCuT2(npa)2·2H2O complex at room temperature. The experi-mental spectrum (thick solid line) and simulated spectra (thinsolid line and dashed line, respectively).

Fig. 5. ESR spectrum of DMF CuT2(npa)2·2H2O solutionadsorbed on NaY zeolite at room temperature (a). Extendedperpendicular absorption (b).

changed when the temperature decreases at T=143 K.

The axial spectra of the powder complexes con-taining (NH3) and (npa) are temperature-indepen-dent, and are due to the compression of the CuN4

tetrahedron lengthwise of one S4 axis, owing to astatic Jahn–Teller effect (resulting in a flattenedtetrahedral local configuration). This effect isshown by the vibronical coupling between theelectronic states of T2 symmetry and the active Evibrational mode in the cubic group [17]. Theisotropic spectrum which appears forCuT2(npa)2·2H2O complex corresponds to onetrigonal specie compressed lengthwise on a C3 axisowing to a dynamic Jahn–Teller effect (T2�T2

vibronical coupling) [17,18].The monomeric compressed pseudotetrahedral

species are present in DMF solutions too (Fig. 4).

The isotropic values g0=2.106; A0=62 G forCuT2(npa)2·2H2O and g0=2.125; A0=64 G forCuT2(NH3)2·2H2O and the shape of the spectraare similar to that obtained for the other reportedCu(II)-proteins and Cu(II)-enzymes with themetallic ion surrounded by four nitrogen atoms[19].

Both complexes present axial symmetriesaround the metallic ion in DMF solutions ad-sorbed on NaY zeolite (Fig. 5). The g�� valuescorrespond to a dominant square-planar localsymmetry (or strong compressed pseudotetrahe-dral symmetries) and �A��� values are typical to aCuN2N*2 chromophore (Table 1). The ligandmolecules (theophylline and amine) are not substi-tuted by the solvent molecules.

In gÞ region of CuT2(npa)2·2H2O spectrumthere are nine superhyperfine lines resolved owingto the interaction of the paramagnetic electronwith four magnetic equivalent nitrogen atoms(aN=15 G). For the CuT2(NH3)2·2H2O complex,the superhyperfine structure is not resolved andthe signals of the parallel band are broader thenthose in the previous case. This is a result of thesmall dimension of the (NH3) molecules whichallows the proximity of Cu(II) ions and the exis-tence of dipolar interactions between them.

The degree of the pseudotetrahedral compres-sion lengthwise of the S4 axis of the CuN4

configuration can be evaluated using the interpo-lation of the R(v) ratio for CuN4 chromophore

Fig. 4. ESR spectrum of CuT2(npa)2·2H2O in DMF solutionat room temperature.

L. Da6id et al. / Spectrochimica Acta Part A 55 (1999) 2559–25642562

Table 1ESR parameters of DMF Cu(II)-theophylline solutions adsorbed on NaY zeolite at room temperature

Compound g�� gÞ �A��� (G) R (cm) v (°) a2 b2 d2

2.060 178 121CuT2(NH3)2 302.254 0.83 0.61 0.602.057 174 124 36 0.81 0.61 0.60CuT2(npa)2 2.246

[20], where R (cm)=g��/�A��� and v is the dihedralangle between each of the two coordinationplanes defined by a CuN2 moiety [21]. Using theg�� and A�� experimental values and the dependenceR(v) from the paper [22] we have evaluated the v

value. The value of the angle v varies from 0 to90° with changes in stereochemistry from square-planar to tetrahedral geometry. Also, for square-planar structure, R ratio has typical values105–135 cm−1 and greater ones for dominantpseudotetrahedral symmetry. The obtained R-val-ues (Table 1) show a dominant square-planarlocal structure around the Cu(II) ion. The distor-tion v angle shows only a small deviation (:15°)of the Cu–N bonds out of the Oxy plane. Thisstrong compression of the tetrahedron can beunderstood in terms of first order vibronic cou-pling effects of the Jahn–Teller type [17]. Thelowering of the ground state by the tetragonaldistortion (along an S4 axis) can be estimatedusing the following expression [18]:

2EJT=13

es(5−18 cos2 u+9 cos4 u)

−94

ep(2−9 cos2 u+9 cos4 u)

where the energy values es and ep in the angularoverlap model (hole formalism) are determinedfrom the diffuse reflectance spectra and the angleu between one Cu–N bond and the Oz axis fromthe interpolations of the electronic transitionsD��=D(dxy−dx2−y2) and DÞ=D(dxy−dxz,yz) likea function of this angle [20]. Using the approxi-mate values D��=13 400 cm−1 and DÞ=15 400cm−1 for the compounds containing (NH3) and(npa), u :73° and the relation between es, ep andD��, DÞ [22], a value of :2880 cm−1 results forthe Jahn–Teller energy EJT, in good agreementwith the other reported values for CuN4 species[17,18]. Owing to the small value of the spin-orbitcoupling energy (:820 cm−1) comparative with

the former, the lowering of the ground state isinduced especially by static Jahn–Teller effect andnot the spin-orbit coupling.

In the case of species presenting a static Jahn–Teller effect, the molecular coefficients (a, b, d)have been evaluated with the help of LCAO-MOprocedure typical for square-planar configuration[23]. The obtained values (Table 1) show a domi-nant ionic character of the s bond in the (Oxy)plane and a strong covalent character of the p

bonds in or out of the plane.

2.2. The square-pyramidal CuT2(ae)2·2H2Ocomplex

The X-ray data indicate a square-planar struc-ture for CuT2(ae)2·2H2O, the Cu(II) ion beingpentacoordinated.

The powder CuT2(ae)2·2H2O complex shows atroom temperature an orthorhombic (g1

c =2.199,g2

c =2.095, g3c =2.037) ESR spectrum (Fig. 6).

This spectrum is difficult to interpret because ofthe proximity of gi

c values. This situation can arisefrom the existence of a magnetically undilutedsample, the obtained g values being that crys-talline and not molecular [24,25]. This appears

Fig. 6. ESR spectrum of pentacoordinated CuT2(ae)2·2H2Ocomplex at room temperature.

L. Da6id et al. / Spectrochimica Acta Part A 55 (1999) 2559–2564 2563

Fig. 7. ESR spectrum of DMF CuT2(ae)2·2H2O solution at143 K.

g1c =g sin2 g+gÞ cos2 g

g2c =g cos2 g+gÞ sin2 g

g3c =gÞ

where the indices (1,2,3) are (x,y,z), respectively.These expressions yield gÞ=2.037, g��=2.257,

2g=118°, but also an unreasonable G\5 value.This fact suggests that the local symmetry of onemolecule is not perfectly axial as we have consid-ered above. Choosing gÞ=2.066 as the mean ofthe g2

c and g3c we find a more convenient G=4.0

value.For dilute systems with a C46 geometry (a

square-pyramidal arrangement) and a dz2 groundstate, the orbital reduction factors (k��, kÞ), whichare a measure of covalence, can be estimated fromthe following relations [28]:

gÞ=gx=gy=2.0023+2uÞ−4u 2

g =gz=2.0023+8u −3uÞ2 −4u uÞ

with u��=k ��2l0/D��; uÞ=kz

2l0/Dz ; D��=E(B2)−E(B1), DÞ=E(E)−E(B1), l0= −828 cm−1. Us-ing D��=13400 cm−1 and DÞ=15 400 cm−1, theorbital reduction factors k��=0.72 and kÞ=0.74are derived from the above equations. These typesof values are frequently obtained for copper (II)complexes with nitrogen atoms in a square-pyra-midal configuration [26,27,29]. The kB1 valuessuggest a covalent environment around the metal-lic ions.

3. Conclusions

For the Cu(II)-theophylline complexes withsome amine ligands the local symmetry aroundthe Cu(II) ion is strongly influenced by the natureof the amines.

The studied complexes are monomeric, but ow-ing to the packing structure, the superexchangeand dipolar interactions between the Cu(II) ionsare present too. The CuT2(npa)2·2H2O andCuT2(NH3)2·2H2O complexes are pseudotetrahe-dral compressed, while CuT2(ae)2·2H2O has asquare-pyramidal local symmetry.

Two different structural species appear in theCuT2(npa)2·2H2O due to the vibronical couplingby the static or dynamic Jahn–Teller effect. These

very well after the sample dilution in DMF whenwe obtain g values near that for NH3 and npa.Also, taking into account that gÞ= (g2

c +g3c)/2

and g��=g1c, the G=2.8 value, inferior to 3.5 is

obtained. This shows the presence of exchangecoupled-species [26,27].

The shape of the spectrum is unchanged whenthe temperature is lowered to 143 K.

The room temperature ESR spectrum ofCuT2(ae)2·2H2O in DMF solution exhibits thefour hyperfine lines (g0=2.125; A0=77 G) result-ing from the coupling of the unpaired electron ofcopper (II) with the nuclear spin (I=3/2) of both63Cu (63%) and 65Cu (31%). Lowering the temper-ature at 143 K an axial spectrum (gÞ=2.054,g��=2.252, A��=180 G) is obtained (Fig. 7). Thecorresponding G=4.8 value shows the magneticdilution of the sample in DMF, the g tensor beingthat of one molecule with square-pyramidal localstructure.

The shape of the spectrum of the DMFCuT2(ae)2·2H2O solution adsorbed on NaY zeo-lite is similar to the previous spectra of this type,the obtained values being: gÞ=2.057, g��=2.254,A��=181 G, aN=14 G.

If we return to the powder spectrum we arriveat these g values taking two molecules per unitcell (confirmed by X-ray measurements) withprincipal axes turned ones given the others.

In the case of the unit cell containing twononequivalent exchange-coupled axial moleculeswith the main axes turned by the angle 2g, therelations between the crystal g values (g1

c, g2c, g3

c)and molecular g values (g��, gÞ) are as follows[26,27]:

L. Da6id et al. / Spectrochimica Acta Part A 55 (1999) 2559–25642564

species, one stable and another unstable with thetemperature (and not only), could have differentbiological effects, especially in tumoral medicine.On the other hand, the appearance of two differ-ent species can influence positively or inhibit theprocess of DNA replication.

In the case of CuT2(NH3)2·2H2O complex thegreat difference between the dimension of theamine and theophylline molecules leads to thestabilization of one state with respect to the other,the resulting chromophore being CuN2N*2 . Thedifference between ligand nitrogen atoms is morereduced in the CuT2(npa)2·2H2O complex and thelocal environment is CuN4. For CuT2(ae)2·2H2Ocomplex, two weak coupled molecules appear inunit cell. The rotated angle between these twomolecules is around 120°.

In conclusion, one may state that the geometryof Cu2+ polyhedra is determined by steric andelectronic effects. While the former may be stabi-lized by the square-pyramidal geometry, the lattermay induce the expected bond length anomalies.

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