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Inorganica Chimica Acta 304 (2000) 170–177

Superoxide dismutase activity of ternary copper complexes ofsulfathiazole and imidazole derivatives. Synthesis and properties of

[CuL2(R-Him)2] [HL=4-amino-N-(thiazol-2-yl)benzenesulfonamide, R-Him=4-methylimidazole,

4,4-dimethylimidazoline or 1,2-dimethylimidazole]. Crystalstructure of [CuL2(4,4-dimethylimidazoline)2]

Javier Casanova, Gloria Alzuet, Sacramento Ferrer, Julio Latorre,Jose Antonio Ramırez, Joaquın Borras *

Departamento de Quımica Inorganica, Facultad de Farmacia, Uni6ersidad de Valencia, A6enida Vicent Andres Estelles s/n, 46100-Burjassot,Valencia, Spain

Received 21 October 1999; accepted 26 January 2000

Abstract

New ternary copper(II) complexes of sulfathiazole (4-amino-N-(thiazol-2-yl)benzenesulfonamide)(HL) and methyl imidazolederivatives have been synthesised and characterised. The crystal structure of the complex [CuL2(4,4-dmHim)2] (1) [4,4-dmHim=4,4-dimethylimidazoline] has been determined. The copper centre has a quasi regular square planar environment with Cu-nitrogenbond lengths ranging from 1.952 to 2.010 A, . From the spectroscopic properties of the complexes [CuL2(1,2-dmHim)2] (2)[1,2-dmHim=1,2-dimethylimidazole] and [CuL2(4-mHim)2] (3) [4-mHim=4-methylimidazole] a distorted tetragonal octahedralgeometry is deduced. The compounds showed SOD mimetic activity in fact, a low concentration of the complexes catalyses thedismutation of superoxide at biological pH. This SOD activity is correlated with their structural properties. Using ExtendedHuckel Molecular Orbital Calculations the one-electron energy levels of the CuN4 chromophore in complex 1 are reported andcorrelated with the data of the CuN6 and CuN5 chromophores of the previously reported [CuL2(Him)2]·MeOH and[CuL2(mim)2]·H2O compounds. In addition, the influence of the geometry distortion on the composition and energy of themolecular orbitals is described using idealised models. © 2000 Elsevier Science S.A. All rights reserved.

Keywords: Sulfathiazole complexes; Imidazole complexes; Copper complexes; Crystal structures; Superoxide-dismutase

1. Introduction

A unique feature of metalloproteins is their capabilityto discriminate between various metal ions which arepresent in their natural surroundings. The active site ofa metalloprotein is built up in such a way that onlyspecific metal ions fit in, thus making the metal ions an

integral part of the active site. Copper ions, as centresof the active site of various metalloproteins, play anessential role in biological processes like electron trans-fer, oxidation and dioxygen transport. Except for theCu(I) metallothioneins, all copper proteins studied con-tain one or more imidazole residues of histidine boundto copper ion. Among these metalloproteins the super-oxide-dismutase has the metal ion bound to three imi-dazole from histidines and one imidazolato ion thatacts as a bridging ligand between the copper and zincions [1]. In a previous paper [2], we reported ternary

* Corresponding author. Tel.: +34-96-386 4530; fax: +34-96-3864960.

E-mail address: joaquin.borras@uv.es (J. Borras)

0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.

PII: S0020-1693(00)00080-3

J. Casano6a et al. / Inorganica Chimica Acta 304 (2000) 170–177 171

copper complexes of sulfathiazole with imidazole (Him)and with its N-methyl derivative (mim). From theircrystal structures we deduced the importance of thesubstituents of the imidazole ring in the coordinationpolyhedron of the copper ion likewise in the chro-mophore. While in the [CuL2(Him)2]·MeOH the metalion is in a very distorted octahedron the stereochem-istry of the [CuL2(mim)2]·H2O is intermediate betweenthe square pyramidal and trigonal bipyramidal. In thefirst compound the geometry adopted by the Cu(II) isprobably due to the stacking interactions between thephenyl rings from the sulfathiazole ligands and theimidazole rings. In the latter, the methyl group linkedto the N-imidazole avoids the intramolecular stackinginteractions observed in the [CuL2(Him)2]·MeOH com-plex so that, a five coordinate geometry is adopted bythe copper ion. Both compounds have high superoxidedismutase mimetic activity. The parameters involved inthe higher or lower SOD mimetic activity of the coppercomplexes in vitro, are in study. Among these factors, itseems important a limited steric hindrance to the ap-proach of the superoxide anion, likewise the presence ofcoordination sites belonging to nitrogen heteroatomicrings such as imidazole or pyridine. In the cited paper,we proposed a correlation between the axial bonddistance and the superoxide mimetic activity. In orderto obtain a better understanding about the possiblecorrelation between structural factors and the SODmimetic activity likewise the influence of the sub-stituents of the imidazole ring in the coordination poly-hedron of the sulfathiazole copper complexes, we havesynthesised and characterised several complexes withdifferent methyl imidazole derivatives that are reportedin the present paper.

2. Experimental

2.1. Materials

Copper chloride dihydrate, 4-methylimidazole (4-mHim), 1,2-dimethylimidazole (1,2-dmHim), 4,4-dimethylimidazole (4,4-dmHim) and sulfathiazole (HL)were reagents of grade and used without purification.Xanthine, xanthine oxidase, superoxide dismutase,bovine serum albumine and nitrobluetetrazolium werepurchased from Sigma.

2.2. Physical measurements

Analytical data (C, H, N, S), IR, UV–Vis, EPRspectra and magnetic measurements were carried out asdescribed previously [3].

2.3. Synthesis

2.3.1. [CuL2(4,4-dmHim)2] (1)CuCl2·2H2O (0.17 g, 1 mmol) was added to a solu-

tion of HL (0.5 g, 2 mmol) in hot methanol (75 cm3).Then, to the resulting yellowish mixture 4,4-dmHim(0.98 g, 10 mmol) was added and the solution turneddark green. It was stirred for about 15 min and thenpropan-2-ol (35 cm3) was added. The final solution wasleft to stand at 4°C. After 2 months prismatic browncrystals suitable for X-ray diffraction measurementswere formed. Anal. Calc. for C28H36CuN10O4S4 requiresC, 43.8; H, 4.7; N, 18.2; Cu, 8.3; S, 16.7. Found: C,43.5; H, 4.5; N, 18.0; Cu, 8.3; S, 16.9%. IR: nmax/cm−1

3440 and 3350 (N-Hsulfathiazole); 3120 (N-Himida-zole); 1460 (thiazole ring); 1320, 1150 and 560 (SO2);930 (S�N).

2.3.2. [CuL2(1,2-dmHim)2] (2)The synthesis of the 1,2-dmHim complex was similar

but 1,2-dmHim (0.96 g, 10 mmol) was added instead of4,4-dmHim. After 3 days a green precipitate was ob-tained. Anal. Calc. for C28H32CuN10O4S4 requires C,44.0; H, 4.2; N, 18.3; Cu, 8.3; S, 16.8. Found: C, 43.7;H, 4.3; N, 18.2; Cu, 8.1; S, 17.0%. IR: nmax/cm−1 3440and 3350 (N-Hsulfathiazole); 3120 (N-Himidazole);1450 (thiazole ring); 1320, 1150 and 560 (SO2); 940(S�N).

2.3.3. [CuL2(4-mHim)2] (3)A solution of HL (0.255 g, 1 mmol) in 50 ml of

methanol was added to 25 ml of a methanolic solutioncontaining 4-mHim (0.328 g, 4 mmol) andCu(OAc)2·H2O (0.2 g, 1mmol) in an ice bath. Underthese conditions the resulting mixture was stirred for 8h and then a green solid was obtained. Anal. Calc. forC26H28CuN10O4S4 requires C, 42.4; H, 3.8; N, 19.0; Cu,8.6; S, 17.4. Found: C, 41.8; H, 3.9; N, 18.8; Cu, 8.7; S,17.7%. IR: nmax/cm−1 3440 and 3390 (N-Hsulfathia-zole); 3120 (N-Himidazole); 1480 (thiazole ring); 1320,1150 and 560 (SO2); 940 (S�N).

2.4. X-ray crystallography of complex 1

2.4.1. Crystal dataC28H36CuN10O4S4, M=767.54, space group=P1( ,

a=8.849(2), b=9.2945(9), c=11.1876(8) A, , a=92.491(9)°, b=95.041(12)°, g=109.780(9)°, Z=2,V=859.9(2) A, 3, Dc=1.61 g cm−3.

2.4.2. Data collection and processingAnalysis on single crystals of C28H36CuN10O4S4 (ap-

proximate size 0.15×0.20×0.20 mm) were carried outwith an Enraf–Nonius CAD-4 single crystal diffrac-tometer (l=0.71073 A, ). The unit cell dimensions weremeasured form the angular setting of 25 reflections withu between 15 and 25°.

J. Casano6a et al. / Inorganica Chimica Acta 304 (2000) 170–177172

The reflections were measured in the hkl range (0,−11, −13) to (10, 11, 13) between limits: 1.83°BuB24.97°. The v–2u scan technique and a variable scanrate with a maximum scan time of 60 s per reflectionwere used. The intensity of the primary beam waschecked throughout the data collection by monitoringthree standard reflections every 3600 s. The final drifcorrections factors were in the range of 0.98 and 1.02.Profile analysis was performed on all reflections [4,5] asemiempirical absorption correction, c-scan based, wasperformed [6]. In total, there were 3019 reflections, ofwhich 2295 had Fo\4sFo. Lorentz and polarisationcorrections were applied and the data were reduced toFo values. The structure was solved by the Pattersonmethod using the program SHELXS-86 [7] running on anIBM PENTIUM II 300 computer. Isotropic least-squares refinement was performed by means of theprogram SHELXL-93 [8]. Hydrogen atoms were placedin calculated positions.

During the final stages of the refinement the posi-tional parameters and the anisotropical thermal

parameters of the non-hydrogen atoms were refined.The hydrogen atoms were refined with a common ther-mal parameter. The final conventional agreement fac-tors were R1=0.0633 and wR2=0.1693. The maximumshift of the e.s.d ratio in the last full matrix least-squares cycle was 0.001. The final difference Fouriermap showed no peaks higher than 1.293 e A, −3 and notdeeper than −2.096 e A, −3. Atomic scattering factorswere taken from the International Tables for X-raycrystallography [9]. The molecular plots were producedby program ORTEP [10].

2.5. Superoxide-dismutase assay

The superoxide dismutase activity of the metal com-plexes was determined according to Oberley and Spitz[11] with minor modifications [3].

2.6. EHMO calculations

All calculations were performed by using the Packageof Programs for Molecular Orbital Analysis by Mealliand Proserpio [12] based on CDNT (atom Cartesiancoordinate calculations), ICON (extended Huckelmethod with the weighted Hij formula) and FMO(fragment molecular orbital), including in the drawingprogram CACAO (computer aided composition ofatomic orbitals).

The extended Huckel parameters are as follows: Hij :Cu 4s, −11.40 eV; Cu 4p, −6.06 eV; Cu 3d, −14.00eV; N 2s, −26.00 eV; N 2p, −13.401.950 eV; S 3s,−20.30 eV; S 3p, −14.00; O 2s, −32.30 eV; O 2p,−14.80 eV; C 2s, −21.40 eV; C 2p, −11.40 eV; H 1s,−13.60 eV. Orbital exponents (Contraction Coeffi-cients in Double-j Expansion given in Parentheses): Cu4s, 4p, 2.20; Cu 3d, 5.95(0.5933), 2.30(0.5744); N 2s, 2p,1.950; S 3s, 3p, 1.900; O 2s, 2p, 2.275; C 2s, 2p, 1.625;H 1s, 1.300.

3. Results and discussion

3.1. Crystal structure of [CuL2(4,4-dmHim)2] (1)

Relevant bond distances and angles are given inTable 1. Fig. 1 shows the numbering scheme andthermal ellipsoids drawn at the 30% probability level.

The copper ion is bound centrosymmetrically by fourligands forming a square planar arrangement that con-sists of two imidazole nitrogen atoms and two thiazolenitrogen atoms of the sulfathiazolato anions in a transdisposition. The Cu�Nthiazole distances are 2.010(3) A,which are similar to those determined for related Cu(II)coordination compounds [2,3]. The Cu�Nimidazole dis-tances are 1.954(3) A, , such short Cu�N distances arealso present in previously reported Cu complexes withimidazole ligands [13,14].

Table 1Selected bond lengths (A, ) and bond angles (°) for [CuL2(4,4-dmHim)2] a

1.952(3)Cu�N(2) Cu�N(2)c1 1.952(3)Cu�N(1)c1 2.010(3) Cu�N(1) 2.010(3)

180.0 N(1)c1�Cu�N(1) 180.0N(2)�Cu�N(2)c188.0(1)88.0(1) N(2)c1�Cu�N(1)N(2)�Cu�N(1)c1

91.9(1) 91.9(1)N(2)c1�Cu�N(1)c1 N(2)�Cu�N(1)

a Symmetry transformations used to generate equivalent atoms:c1 −x,−y,−z.

Fig. 1. ORTEP drawing of the complex [CuL2(4,4-dmHim2] (1) (ther-mal ellipsoids drawn at the 30% probability level.

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Fig. 2. Reflectance spectra. (a) [CuL2(4,4-dmHim)2] (1); (b)[CuL2(1,2-dmHim)2] (2); (c) [CuL2(4-mHim)2] (3); (d)[CuL2(Him)2]·MeOH; (e) [CuL2(mim)2]·H2O.

3.2. Infrared spectra

The IR spectra of the compounds have in commonthe bands for the stretching and bending modes of theN�H groups from sulfathiazole and imidazole. As itwas found for other copper–sulfathiazole complexes[2,3] the bands at 3500 and 3380 cm−1 assigned tostretching vibrations of the N�H groups of the sul-fathiazole are shifted to higher frequencies respect tothose of the free ligand. The band corresponding tothe stretching N�H imidazole ring vibration appears atapproximately 3230 cm−1. In all the complexes thebands attributed to the SO2 vibrations remain un-changed. Evidence of the deprotonation of the sul-fathiazole ligand and its coordination through thethiazole N atom arises from the significant changes ofthe bands assigned to the thiazole ring and the S�Nstretching vibrations at 1550 and at 920 cm−1, respec-tively, which appear at 1480–1450 and at 950–940cm−1 in the complexes.

3.3. Electronic absorption spectra

Fig. 2 shows the diffuse reflectance spectra of thecomplexes 1, 2 and 3 and the corresponding ones tothe previously reported [CuL2(Him)2]·MeOH and[CuL2(mim)2]·H2O. Several interesting aspects relatedwith the nature of the chromophore of the complexescan be deduced from comparison of these diffusereflectance spectra. The solid spectrum of [CuL2(4,4-dmHim)2] (1) exhibits three maxima at 638, 455 and340 nm. The first one corresponds to a d–d band in asquare planar environment around Cu(II) which is inagreement with the crystal structure of the compound.The second one could be attributed to a d–d or to acharge transfer ligand-to-metal transition. Square pla-nar copper (II) complexes with a regular CuN4 chro-mophore show a characteristic d–d band at 500 nm[16]. Since the crystal structure of [CuL2(4,4-dmHim)2](1) reveals a quasi regular square planar CuN4 entityit seems reasonable to suggest that the intense band at455 nm includes both transitions. The third band isassigned to a ligand-to-ligand transition of the sul-fathiazolato anion. The [CuL2(1,2-dmHim)2] (2) andthe [CuL2(4-mHim)2] (3) compounds display spectralfeatures different from those of [CuL2(4,4-dmHim)2].Both spectra have a similar pattern to that of the[CuL2(Him)2]·MeOH suggesting the same chro-mophore. On the basis of the crystal of the Himcomplex, that showed a very distorted tetragonal elon-gated octahedral CuN6 entity, we propose a similarcoordination polyhedra for compounds 2 and 3.

3.4. Magnetic measurements and EPR spectra

Magnetic moments values at room temperature (r.t.)of 1.80 MB for all the complexes are consistent with the

The coordination geometry of the metal in thecomplex is virtually square planar with N�Cu�Nbond angle deviations from 90° of less than 2°.The Cu(II) is in the N4 plane. Tetrahedrality forany tetracoordinate copper complex can be deter-mined from the angle subtended by two planes,each encompassing the copper and two adjacent atoms[15]. For strictly square planar complexes with D4h

symmetry, the tetrahedrality is 0°. For tetrahedralcomplexes with D2d symmetry, the tetrahedralityequals 90°. The dihedral angle for [CuL2(4,4-dmHim)2]complex of 0° agrees with its regular square planargeometry.

The sulfathiazolato anion acts as a monodentateligand through the N atom of the thiazole ring. Itshould be noted that this coordination behaviour, thathas been also observed in the [CuL(py)3Cl] complex[3], differs from that exhibited by the sulfathiazoleligand in the [CuL2(Him)2]·MeOH and [CuL2-(mimH)2]·H2O compounds [2]. In contrast to the semi-coordination of the sulfonamidate N atom found inthese complexes (Cu�N bond lengths, 2.68 and 2.57 A, ,respectively), in the [CuL2(4,4-dmHim)2] such interac-tion is not presented, being the Cu�N�sulfonamidatedistance of 3.146 A, . Though there is no obvious expla-nation, the lack of bonding may be due to crystalpacking requirements owing to the steric hindrance ofthe two methyl substituents of the imidazole ring.Moreover, this fact leads to a different copper (II)coordination polyhedra in the three complexes. Whilethe [CuL2(4,4-dmHim)2] presents a square planar envi-ronment, the [CuL2(Him)2]·MeOH is a very distortedoctahedra and the [CuL2(mim)2]·H2O exhibits aslightly distorted square pyramidal geometry.

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presence of one unpaired electron in monomericCu(II) coordination compounds.

The polycrystalline X-band EPR spectra of the com-plexes 1, 2, and 3 at room temperature are shown inFig. 3. These EPR spectra are axial with g��\gÞ andare consistent with a distorted tetragonal geometry.The EPR parameters were extracted by simulation[17]. The EPR of complex 1 has a parallel regionpoorly resolved, being the EPR parameters: g��:2.20and gÞ=2.07. The lack of copper(II) hyperfine cou-pling in this complex is likely due to dipole–dipoleinteractions between copper atoms of neighboringmolecules. The EPR of the complexes 2 and 3 showthe hyperfine structure in the g�� region. The EPRparameters are g��=2.28 and 2.25; gÞ=2.08 for bothcomplexes and A��=165 and 170 G, respectively. Ac-cording to Sakaguchi and Addison [18] the value ofthe g��/A�� quotient of 138 cm for compound 2 and of132 cm for complex 3 suggests a distorted tetragonal

octahedral geometry around the Cu(II) which is ingood agreement with the electronic spectra. The lowerg�� for complex 1 compared to those of the complexes2 and 3 is consistent with the square planar stereo-chemistry of the former.

3.5. Superoxide-dismutase assays

The superoxide dismutase activity of the complexeshas been measured. All the compounds exhibit cata-lytic activity toward the dismutation of the superoxideanions. Fig. 4 shows percentage inhibition of the re-duction of nitrobluetetrazolium plotted against theconcentration of the complex [CuL2(4,4-dmHim)2] (1).Superoxide was supplied enzymatically from hypoxan-thine–xanthine oxidase reaction to the evaluating sys-tem. The mode of generation of superoxide anionsmust be emphasized: it has been shown through theuse of pulse radiolysis techniques for supplying super-oxide anions that most of the copper compounds cata-lyze the dismutation with similar efficiency to that ofthe native SOD [19]. This is due to the fact that pulseradiolysis techniques provided O2

− in high concentra-tion (10−5 M). When the superoxide is provided froma biological source, the concentration is about 10−15

M and only few copper complexes exhibit SOD activ-ity because of the reversibility of the first step of thedismutation reaction.

In the experimental conditions used to evaluate theSOD like activity, the phosphate ion is present in avery large excess with respect to the complexes. It hasbeen demonstrated that the species [Cu(HPO4)] ismuch less active than some dipeptide copper (II) com-plexes and obviously less than the active enzyme itself[20]. So, the SOD mimetic activity of the complexesreported here could not be due to the formation ofsome amount of [Cu(HPO4)] species.

The IC50 value of 0.742 mM for complex 1, 1.03 mMfor complex 2 and 0.586 mM for complex 3 indicatesthat they are strong mimetic superoxide dismutasecompounds.

The percentage of inhibition of NBT reduction wastested in solutions containing 0.75 mM (complex 1), 1.0mM (complex 2) and 0.6 mM (complex 3) and 1.5 mM ofBSA (bovine serum albumin). The inhibition of NBT(%) was 49.6 for complex 1, 49.8 for complex 2 and51.5 for complex 3. The same solutions without BSAgive inhibition values of 51.2, 47.3 and 52.6%, respec-tively. As consequence, the activity of the complexeswas not significantly influenced by the physiologicalchelator albumin. Recently it was proposed that onlyflexible Cu(II)/Cu(I) chelates are genuine active siteanalogues of SOD [21]. In fact, SOD mimicking sys-tems should permit the coordination of both Cu(II) andCu(I) ions. The SOD activity is related to the equatorial

Fig. 3. EPR spectra. (a) [CuL2(4,4-dmHim)2] (1); (b) [CuL2(1,2-dmHim)2] (2); (c) [CuL2(4-mHim)2] (3).

Fig. 4. Percentage inhibition of nitroblue tetrazolium reduction plot-ted against the concentration of the [CuL2(4,4-dmHim)2] complex.Each point represents the mean9standard deviation of triplicatedetermination.

J. Casano6a et al. / Inorganica Chimica Acta 304 (2000) 170–177 175

Fig. 5. Energy levels of d orbitals for the CuN6, CuN5 and CuN4 idealised models.

field experienced by the copper(II) ions [20,22–25]. Astrong equatorial field opposes the interaction of thecomplexes with superoxide radical, disfavouring theprobable formation of the intermediate copper-superox-ide adduct [26]. Moreover, a strong field is a factorwhich disfavours the reduction process from Cu(II) toCu(I). The higher IC50 value and therefore the lowerSOD activity for complex 1 compared with complex 3and the previously reported [CuL2(Him)2]·MeOH(IC50=0.664 mM) and [CuL2(mHim)2]·H2O (IC50=0.429 mM) compounds could be correlated with thestrong field experienced by the Cu(II) in a squareplanar geometry. Considering this factor, it must beexpected that complex 2 would have a superoxide dis-mutase activity similar to that of complex 3 and higherthan the corresponding one to complex 1. However, itsIC50 value indicates the lowest activity. Although thereis not an obvious explanation it could be attributed tothe higher steric hindrance of the two methyl groupsthat could avoid the approach of the superoxide anionin the coordination sphere of the metal ion.

3.6. Molecular orbital calculations

We have obtained crystal structures of copper-sul-fathiazolato complexes with Him, mim and 4,4-dmHimwhose coordination polyhedron ranges from a veryirregular distorted octahedral through a distortedsquare pyramidal to a quasi regular square planargeometries. To investigate the electronic structures ofthe three complexes we have carried out ExtendedHuckel Molecular Orbital (EHMO) calculations bymeans of the CACAO program [12]. This programpermits us to use the crystallographic coordinates

derived from the crystal structures, so it is possible tocalculated the complete molecular correlation diagramsfor the three complexes. From these, it must be notedthat in the SOMO’s of the three complexes, besides thecontribution of the metal orbitals, the only atomicorbitals of the ligands that participate are the Nimida-zole and Nthiadiazole donor atoms, being their relativecontribution around the 25%. As it can expected fromthe similar Cu�N bond distances observed in the crystalstructures there is not significative difference in theparticipation of both Nimidazole and Nthiazole atoms.

With respect to the d copper orbitals involved in theSOMO’s, the main contribution corresponds to the dxy

(27%) and dz 2 (9%) for the [CuL2(4,4-dmHim)2] (1)complex, to the dx 2−y 2 (16%); dyz (10%) and dz 2 (8%)for the [CuL2(mim)2]·H2O complex and to the dyz (26%)and dxz (8%) for the [CuL2(Him)2]·MeOH complex.

The systems are so complicated that it is very difficultto obtain results that can be correlated to the electronicproperties of the complexes. To clarify these calcula-tions we have designed three idealised models substitut-ing the six, five and four donor atoms of the ligands inthe three copper complexes by ammonia moleculesusing the coordinates of the N donor atoms of theligands obtained from the crystallographic data. In thisway, in the idealised models the copper ion and the Natoms retain the sites found in the crystal structuresand the distortions observed in the coordination poly-hedron of the three complexes remain. The influence ofthe distortion in the energy and the nature of themolecular orbitals involving the copper d orbitals canbe established from the diagram deduced from theEHMO calculations for the three idealised modelswhere that for a regular octahedral Cu(NH3)6

2+ is also

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included (Fig. 5). From this we can deduced severalimportant aspects:

The SOMO’s present a participation of the dxy or-bital of 43% in all the models. Furthermore, the energyof the SOMO’s is slightly different, being the corre-sponding to the regular octahedron the most stable(−10.934 eV) and that of the distorted one the mostunstable (−10.745 eV).

In all the models the dz 2 orbital is the main com-ponent of the HOMO. At the same time is the orbitalthat have more energy variation when go from theregular octahedron (−12.106 eV) to the distorted one(−12.890 eV), to the distorted square pyramid(−12.999 eV) and to the quasi regular square planar(−13.793 eV).

The influence of the distortion in the coordinationpolyhedron is clearly observed in the HOMO-1,HOMO-2 and HOMO-3. In the regular geometries theyare constituted by individual d orbitals, being the en-ergy order dx 2−y 2\dxz]dyz while in the two distortedmodels they are a mixture of the dx 2−y 2, dxz and dyz

orbitals because of the distortion leads to a significantinteraction of the dxz and dyz orbitals with the ligandones.

According from these calculations in the electronicspectrum of the complexes it can be expected onlyone band at about 415 nm for the [CuL2(4,4-dmimH)2] (1) complex, two bands at approximately595 and 425 nm for the [CuL2(mim)2]·H2O complex,three bands at about 580, 475 and 405 nm for the[CuL2(Him)2]·MeOH complex and two bands near 500and 422 nm for the idealised hexacoordinateCu(NH3)6

2+.If we compare these theoretical electronic transitions

with those of the reflectance spectra of the complexes(Fig. 2) we observe a good agreement although anaccurate assignment is difficult to made due to thewidth of the bands of the experimental spectra and thetrouble to individualise the d metal atomic orbitalscontribution to the molecular orbital of the complexesas a consequence of the distortion of the coordinationpolyhedron and of the known limitations of this type ofcalculations.

Although several attempts to interpret the visiblespectrum of the transition metal complexes using thedeconvolution curves have been reported, from theresults obtained in this paper it seems very difficult todistinguish the energy levels of the individual d orbitalswhen some distortion takes places.

4. Supplementary material

Atomic coordinates, thermal parameters, bondlengths and angles and structure factors are availablefrom the authors upon request.

Acknowledgements

We greatly appreciate financial support fromComision Interministerial de Ciencia y Tecnologıa(PM97-0105-C02-01) and the Generalitat Valenciana(GV-D-CN-09-145-96).

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