Bogdanović_2001_Polyhedron

Polyhedron 20 (2001) 22312240

Transition metal complexes with thiosemicarbazide-based ligands.Part XLI. Two crystal structures of cobalt(III) complexes with

salicylaldehyde S-methylisothiosemicarbazone and theoretical studyon orientations of coordinated pyridines

Goran A. Bogdanovic a,1, Vesna B. Medakovic b, Ljiljana S. Vojinovic c,Valerija I. C esljevic c, Vukadin M. Leovac c, Anne Spasojevic-de Bire a,

Snezana D. Zaric b,*a Laboratoire de Structures, Proprietes et Modelisation des Solides (SPMS), UMR 8580 du CNRS, Ecole Centrale Paris,

Grande Voie des Vignes, 92295 Chatenay-Malabry Cedex, Franceb Department of Chemistry, Uniersity of Belgrade, Studentski trg 16, P.O. Box 158, 11001 Belgrade, Yugoslaia

c Institute of Chemistry, Faculty of Sciences, Uniersity of Noi Sad, Trg Dositeja Obradoica 3, 21000 Noi Sad, Yugoslaia

Received 31 January 2001; accepted 26 April 2001

Abstract

Two compounds, [CoIII(L)(py)3][CoII(py)Cl3]EtOH and [Co

III(L)(py)3]I3 (H2L=salicylaldehyde S-methylisothiosemicarbazone,py=pyridine), were synthesized and the crystal structures determined by single-crystal X-ray diffraction. In both structures thegeometries of the cation are very similar to a thiosemicarbazide-based ligand coordinated in the mer configuration. The axialpyridines are in mutual perpendicular orientation, the angles between the planes being 85.3(2) and 82.5(2). The plane of theequatorial pyridine is tilted with respect to the equatorial plane by about 40. The orientations of the pyridines were studied inmodel systems by quantum chemistry calculations. It was shown that the interactions between axial and equatorial pyridines areresponsible for the orientation of pyridines in the complex cation; consequently, there are very similar geometries of the complexcation in both crystal structures. The compounds were also characterized by elemental analysis, molar conductivity, magneticsusceptibility and electronic absorption spectra. 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Crystal structures; Thiosemicarbazide based ligands; DFT calculations; Co(III) complexes; Preferential pyridine orientation

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1. Introduction

Because of their biological activity and analyticalapplication, thiosemicarbazides and thiosemicarba-zones, as well as their metal complexes have been thesubject of many studies [1]. Numerous metal complexeshave been synthesized with S-alkylisothiosemicar-bazide-based ligands and one of the most extensivelyused ligands is salicylaldehyde S-alkylisothiosemicarba-zone [2,3]. This ligand can coordinate to the metal in

the monoanionic (resulting from deprotonation of thephenolic hydroxyl) or dianionic form (formed by anadditional deprotonation of the 2NH group of theisothiosemicabazide moiety).

In the cation complexes of CoIII presented in thispaper salicylaldehyde S-methylisothiosemicarbazoneligand is ONN-tridentate coordinated, the three othercoordinated positions are occupied by three pyridines.The orientations of histidines, axially ligated to heme,are considered to have a strong influence on the func-tion of heme cofactors in proteins. Different conforma-tions can shift the redox potential [4] and they cancontrol the coordination of substrates to heme-proteins[5]. Therefore, orientations of the axial ligand havebeen studied in heme proteins [69]. From experimen-

* Corresponding author. Fax: +381-11-638785.E-mail address: [email protected] (S.D. Zaric).1 Present address: Vinca Institute of Nuclear Sciences, Laboratory

of Theoretical Physics and Condensed Matter Physics, P.O. Box 522,11001 Belgrade, Yugoslavia.

0277-5387/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved.PII: S02 7 7 -5387 (01 )00819 -1

G.A. Bogdanoic et al. / Polyhedron 20 (2001) 223122402232

tal data on heme model systems and from molecularmechanics studies it was shown that the orientations ofaxial ligands have an influence on the porphyrin ringconformations. In complexes with parallel orientationof two planar axial ligands, the porphyrin ring remainsplanar. For porphyrin complexes, which have two pla-nar axial ligands in perpendicular orientation, the por-phyrin ring is almost invariably distorted fromplanarity [712]. The DFT calculations on [Fe-(por)(py)2] and [Fe(por)(py)2]+ showed that for the FeII

complex there is no difference in the preferential orien-tations (parallel or orthogonal) of axially coordinatedpyridines, but for the FeIII complex the orthogonalorientation is more stable [11]. The calculations on[Fe(por)(im)2] and [Fe(por)(im)2]+ showed that for bothcomplexes there is no difference in the preferentialorientations (parallel or orthogonal) of axially coordi-nated imidazoles [12].

Recently, [Fe(TMP)(5-MeHim)2]ClO4 has been ob-tained in two distinct crystalline forms with differentrelative axial ligand orientations: one is almost paralleland the other is almost perpendicular [13]. Hence, onecan conclude that the two conformational isomers areenergetically nearly equivalent.

In this work the synthesis and crystal structure deter-mination of [Co(L)(py)3][Co(py)Cl3]EtOH (1) and[Co(L)(py)3]I3 (2) (H2L=salicylaldehyde S-methyliso-thiosemicarbazone, py=pyridine) have been per-formed. Due to some ambiguities in the previous pub-lished structure of 1, a re-investigation of the structurehas been done [2]. Very similar geometries of the com-plex cation in both crystal structures, with perpendicu-lar orientations of axial pyridines, indicated that thereare intramolecular interactions influencing the orienta-tion of coordinated pyridines. Quantum chemistry cal-culations have been done on model systems in order tofind out factors influencing the orientation of the ax-ially coordinated pyridines.

2. Experiments and results

2.1. Synthesis and physicochemical properties

2.1.1. [Co(L)(py)3][Co(py)Cl3] EtOH (1)EtOH (5.0 cm3) and pyridine (0.4 cm3) were added to

a mixture of 0.60 g (2.5 mmol) of CoCl26H2O and 0.26g (1.25 mmol) of salicylaldehyde S-methylisothiosemi-carbazone (H2L). The reactants were dissolved by stir-ring and mild heating. After 24 h at room temperature(r.t.) crystals were formed. The crystals were filteredfrom the green solution and washed out with EtOH andether. The yield was 0.70 g (75%).

The molar conductivity of a 1103 mol dm3

compound solution in dimethyl formamide (DMF) is41.3 S cm2 mol1. The compound is paramagnetic and

its effective magnetic moment is 4.28 B. The substanceloses 3.42% in mass at 80C (calculated 2.99%). At105C there is no further loss in mass.

Anal. Found: C, 46.74; H, 4.37; Co, 14.56; N, 12.10.Calc. for Co2C31H35N7O2SCl3: C, 46.90; H, 4.44; Co,14.85; N, 12.35%.

2.1.2. [Co(L)(py)3]I3 (2)0.60 g (2.5 mmol) of CoCl6H2O was added to a

warm solution of 0.75 g (5.0 mmol) NaI in 5.0 cm3 ofEtOH. The reactants were dissolved by mild heating.After 15 min of cooling at r.t., crystals of NaCl wereseparated. 0.26 g (1.25 mmol) of H2L and 0.4 cm3 ofpyridine were added to the solution of CoI2. The reac-tants were dissolved again by mild heating. After 2 daysat r.t. crystals were filtered from a dark solution, andwashed out with EtOH and ether. The yield was 0.69 g(55%).

The molar conductivity of a 1103 mol dm3

compound solution in DMF is 70.2 S cm2 mol1.Anal. Found: C, 7.25; H, 2.97; Co, 7.25; N, 9.71.

Calc. for CoC24H24N6OSI3: C, 32.60; H, 2.74; Co, 6.66;N, 9.51%.

2.2. Physicochemical measurements

Elemental analysis was carried out by standardmicromethods.

Magnetic susceptibility measurements were made atr.t. using a MSB-MKI magnetic susceptibility balance(Sherwood Scientific Ltd, Cambridge, UK). The datawere corrected for diamagnetic susceptibilities. Molarconductivities of freshly prepared 1103 mol dm3

solutions were measured on a Jenway 4010 conductivitymeter.

Electronic absorption spectra were taken on a Cary219 spectrophotometer.

2.3. Crystal structure determinations

Data were collected on an EnrafNonius CAD4 dif-fractometer [14] using Mo K radiation (=0.71069 A )and 2 scans in the 1.2325.98 range for complex1 and 1.6427.97 range for complex 2. Cell con-stants and an orientation matrix for data collection,obtained from 24 centered reflections in the range14.9717.98 for 1 and 13.0417.14 for 2, corre-sponded to a triclinic and monoclinic cell, respectively.The dimensions are given in Table 1. The data werecorrected for Lorentz and polarization effects [15]. AGaussian-type absorption correction [16b] based on thecrystal morphology was applied (Tmin=0.535, Tmax=0.713 for 1; Tmin=0.254, Tmax=0.531 for 2).

The structures were solved by the heavy atommethod [17a] and difference Fourier methods andrefined on F2 by a full-matrix least-squares method

G.A. Bogdanoic et al. / Polyhedron 20 (2001) 22312240 2233

[17b]. All H atoms were placed at the calculated posi-tions except the H atoms from the isothiosemicarbazidemoiety, which were determined from difference Fouriersynthesis and freely refined. Idealized H atoms wererefined with isotropic displacement parameters set to1.2 (1.5 for methyl groups) times the equivalentisotropic U value of the parent atom.

Special attention has to be given to the refinement ofcomplex 1. In a previous paper [2] the reported struc-ture did not contain any solvent molecule. Nevertheless,a precise analysis of the structure with the PLATONprogram [16,18] has shown that solvent accessible voidsexist in the crystal lattice with a volume of 208.8 A 3.Theoretical volumes for EtOH dimer as solventmolecules fit very well with the calculated space. Etha-nol molecules can be expected in the crystal lattice sincesynthesis is done in that solvent. Moreover, the experi-mental density of 1 is 1.48 Mg m3, and calculateddensities are 1.493 and 1.406 Mg m3 with and withoutEtOH, respectively. The elemental analysis is also inagreement with the presence of solvent molecules in thecrystal lattice.

In order to take into account the presence of EtOHmolecules in the crystal, two different refinements havebeen performed on the new dataset:

1. The positions of non-hydrogen atoms of EtOH weredetermined from the difference Fourier map. How-ever, because of the very high disorder of themolecule, these atoms were refined with isotropicthermal parameters (quoted as refinement 1a).

2. Using the SQUEEZE procedure [16b] a new reflectionfile was formed, where the disordered solvent contri-bution (as determined in the previous step) is sub-tracted from the observed data. Further refinementsof the whole complex were done using this new file(quoted as refinement 1b).

Crystallographic data and refinements parameters forcomplexes 1 and 2 are summarized in Table 1. Themaximum and minimum peaks on the final differenceFourier map are located within 0.5 A of the Cl and Coatoms. The structure of complex 1 obtained from thetwo refinement strategies (1a and 1b) are very similar;therefore, Tables 24 present the results deduced fromprocedure 1b compared to the structure of complex 2.Figs. 1 and 2 present the structures of complexes 1 and2, respectively.

2.4. Computational methods

The calculations were performed on three differentmodel systems of the complex cation [CoIII(L)(py)3]+.

Table 1Crystal data, data collection and structural refinement details

Complex 1aa Complex 1bb Complex 2c

C31H35Cl3Co2N7O2SEmpirical formula C29H29Cl3Co2N7OS C24H24CoI3N6OSgreen; prismColor; habit green; prism brown; prism0.290.500.58Size (mm) 0.290.500.58 0.180.360.50

Temperature (K) 293(2) 293(2) 293(2)triclinicCrystal system monoclinictriclinic

P21/nP1P1Space groupUnit cell dimensions

a (A ) 9.024(5) 9.024(5) 14.646(3)11.844(3) 12.513(2)11.844(3)b (A )16.923(4) 16.622(2)16.923(4)c (A )

90.00100.91(2) () 100.91(2) () 92.29(6) 92.29(6) 106.100(10)

95.11(6) () 95.11(6) 90.00V (A 3) 2926.8(8)1766(2)1766(2)

2 42ZDcalc (Mg m

3) 1.493 1.406 2.0053.8501.265 1.265Absorption coefficient (mm1)

h, k, lData collection limits h, k, l h, k, l7370Reflections collected 7370 7827C31H35Cl3Co2N7O2SEmpirical formula C29H29Cl3Co2N7OS C24H24CoI3N6OS

7046 [Rint=0.0267]6910 [Rint=0.0231] 6910 [Rint=0.0223]Independent reflections5537/0/392 5114/0/3235531/0/404Data for [I2(I)]/restraints/parameters

Goodness-of-fit on F2 1.073 1.078 1.0370.0667, 0.1990R1 , wR2 [I2(I)] 0.0654, 0.2011 0.0366, 0.0941

R=FoFc/Fo2, Rw= [(w(Fo 2Fc2)]/[w(Fo2)2]0.5.a For 1a: w=1/[2(Fo2)+(0.1346P)2+3.1230P ] where P= (Fo2+2Fc2)/3.b For 1b: w=1/[2(Fo2)+(0.1480P)2+1.4970P ] where P= (Fo2+2Fc2)/3.c For 2: w=1/[2(Fo2)+(0.0465P)2+4.4300P ] where P= (Fo2+2Fc2)/3.


Table 2Selected geometric parameters (A , ) for [CoIII(L)(py)3]+

Complex 1b Complex 2 MS1Complex 1 [2] MS2a MS2b MS3

Bond lengths1.893(7)Co1O1 1.884(4) 1.880(2) 1.870 1.865 1.854 1.873

1.884(3) 1.879(3) 1.885 1.867 1.869 1.879Co1N1 1.911(8)1.902(5) 1.891(4) 1.9151.889(9) 1.913Co1N3 1.908 1.911

1.963(7)Co1N6 1.976(4) 1.991(3) 1.971 2.025 2.000 1.9651.970(3) 1.963(3) 1.974Co1N5 1.9601.953(7) 1.977 1.9921.980(4) 1.974(3) 2.0221.960(8) 2.031Co1N4 2.040 2.012

1.76(1)SC1 1.766(5) 1.763(4)1.802(5) 1.779(3)SC1S 1.80(1)1.319(6) 1.313(6) 1.3551.33(1) 1.361C1N2 1.365 1.359

1.34(1)C1N3 1.321(6) 1.314(6) 1.397 1.391 1.381 1.3911.285(6) 1.279(4) 1.359 1.355N1C2 1.3551.27(1) 1.3551.387(5) 1.411(5) 1.4751.40(1) 1.476N1N2 1.477 1.474

1.35(1)O1C4 1.315(5) 1.316(5) 1.347 1.347 1.347 1.343

Bond angles95.1(2) 95.8(1) 96.195.4(5) 97.1O1Co1N1 97.9 96.4

176.8(5)O1Co1N3 176.1(2) 177.4(1) 177.7 178.2 178.6 178.681.4(2) 81.6(2) 82.7N1Co1N3 83.581.6(6) 82.9 82.689.2(2) 90.0(1) 90.789.8(5) 89.5O1Co1N6 88.7 90.6

90.2(6)N1Co1N6 89.8(2) 87.8(2) 89.1 88.9 89.8 90.1N3Co1N6 89.1(6) 88.9(2) 90.3(2) 91.3 88.8 91.2 90.5

88.4(2) 87.1(1) 88.488.9(5) 88.8O1Co1N5 88.4 87.690.1(6)N1Co1N5 89.9(2) 90.6(1) 89.1 91.2 90.6 89.2

93.4(2) 92.5(2) 89.5N3Co1N5 92.992.2(6) 92.8 91.3177.6(2) 176.5(1) 177.9178.6(6) 178.3N6Co1N5 176.0 178.0

88.6(5)O1Co1N4 88.5(2) 88.2(1) 87.7 86.9 89.1 87.6175.9(6)N1Co1N4 176.3(2) 175.0(2) 176.2 174.7 173.0 175.8

95.0(2) 94.3(2) 93.494.4(6) 92.4N3Co1N4 90.2 93.390.4(6)N6Co1N4 91.0(2) 89.4(1) 91.3 87.7 89.4 91.089.4(6)N5Co1N4 89.5(2) 92.4(1) 90.6 92.3 90.7 89.8

34.0(4) 37.4(3) 35.234(1)O1Co1N4C1a 37.983.8(4) 86.3(3) 70.5 4.8C1bN5N6C1c 87.384(1)

Table 3Total puckering amplitudes and torsion angles for six-membered chelate rings

Complex 2 MS1 MS2a MS2b MS3Complex 1

0.1627(1) 0.05QT (A ) 0.120.1545(5) 0.02 0.10O1CoN1C2 () 8.0(3)11.0(4) 4.1 5.1 0.6 8.5

0.9(5) 3.0 1.43.7(6) 1.3CoN1C2C3 () 5.34.3(7)N1C2C3C4 () 5.5(6) 1.5 3.7 0.1 2.4

C2C3C4O1 () 4.0(6)0.1(6) 4.2 3.9 2.1 4.516.7(5) 1.9 12.711.5(6) 2.7CoO1C4C3 () 1.4

16.6(3)N1CoO1C4 () 1.714.8(3) 11.6 1.3 6.5

The geometries for different conformers of the threemodel systems were optimized and energies were cal-culated using the B3LYP method. These B3LYP cal-culations have been carried out with the GAUSSIAN-98program [20]. In the B3LYP geometry optimizations,STO-3G basis sets were chosen for the carbon, nitro-gen, oxygen and hydrogen atoms and LANL2DZ forthe cobalt atom. The DFT method is used as it givesgood results for all transition metal complexes includ-

ing transition metal complexes of the first row[11,12,21]. In cases where conformers were not station-ary points some torsion angles were fixed in order tokeep the desired orientations of pyridines. Startingwith these optimized structures, the B3LYP singlepoint energy calculations were made using the 6-31Gbasis sets. The partial charges, which are presented inthe article, are charges calculated with 6-31G basissets.


Table 4Interligand contacts (A ) in crystal structures and model systems of complex cations

Complex 2 MS1 MS2a aComplex 1 MS2b b MS3

A HB system2.38(1)A=O1, B=C1a (or N4) 2.422.44(1) 2.47 c 2.60 c 2.47

A=O1, B=C5b (or N5) 2.55(1)2.44(1) 2.81 d 2.53 2.50 2.412.48(1) 2.55 d 2.34 2.432.60(1) 2.67A=O1, B=C5c (or N6)

AH HB system3.30(1)A=C1a (or N4), B=C5b (or N5) 2.55 d3.35(1) 2.45 c 2.41 c 3.433.41(1) 2.65 d 2.49 c3.32(1) 2.42 cA=C1a (or N4), B=C5c (or N6) 3.33

a Model system 2 with perpendicular orientations of axial pyridines.b Model system 2 with parallel orientations of axial pyridines.c The distances are with H of NH3 in equatorial position.d The distances are with H of NH3 in axial position.

3. Discussion

3.1. Synthesis and physicochemical properties

The reaction of warm EtOH solutions of CoX26H2O(X=Cl, I) with salicylaldehyde S-methylthiosemicarba-zone (H2L), synthesized according to Ref. [22], andpyridine in a mole ratio 2:1:5 in the presence of the airoxygen resulted in complexes 1 and 2. A commoncharacteristic of both compounds is that the sameoctahedral complex cation of cobalt(III) is coordinatedwith the dianion of the tridentate ONN isothiosemicar-bazone and with three pyridine molecules. In the caseof complex 1, the cation is stabilized by the tetrahedral[CoIICl3Py] anion. In contrast, the reaction with CoI2yielded no [CoIII3Py], obviously less stable, and com-plex 2 presents I3 as the anion. Therefore, complex 2is diamagnetic and complex 1 paramagnetic, with avalue eff=4.28 B, which is in the range characteristicof the tetrahedral CoII complexes [23].

The complexes are highly soluble in DMF, less solu-ble in MeOH, EtOH, and Me2CO, and insoluble inH2O and Et2O.

The molar conductivity (M) of the freshly preparedDMF solution of complex 2 corresponds to the 1:1 typeof electrolyte [24], whereas M of complex 1, probablybecause of the presence of the voluminous anion, islower. In any case, DMF solutions of these complexesare not stable. In both cases, after several days, thebrown complex of the non-electrolyte type[CoIII(HL)L], described earlier [25], is formed. In con-trast to the DMF solutions, the Me2CO solutions aremore stable, which in the case of complex 1 is alsoobserved as the preservation of its green color.

The electronic absorption spectra of the DMF andMe2CO solutions of both complexes are characterizedby the presence of the absorption maxima at 400 nm(=900022 000 dm3 mol1 cm1) which can be as-cribed to the octahedral CoIII ion [26]. The spectrum ofcomplex 1 in both solvents contains additional weaker

absorption bands (=640900 dm3 mol1 cm1) witha maxima in the range of 600675 nm, which undoubt-edly belong to the tetrahedral anion.

3.2. Re-inestigation of the structure of 1

The re-investigation of the structure of complex 1leads us to the following results: (i) The position of thesolvent molecule has been determined; the observed

Fig. 1. ORTEP [19] drawing of [CoIII(L)(py)3][CoII(py)Cl3] with the

atom numbering scheme. The thermal ellipsoids correspond to 50%probability.

Fig. 2. ORTEP [19] drawing of [CoIII(L)(py)3]I3 with the atom number-ing scheme. The thermal ellipsoids correspond to 50% probability.


geometry is a dimer exhibiting mutual H-bonds be-tween oxygen lone pairs and hydrogen atoms from CH2groups (I). (ii) The cation complex structure remainsglobally the same [2], but as expected the values aremore accurate (Table 2). (iii) The main discrepancy isobserved in the salicylaldehyde S-methylisothiosemicar-bazone ligand. After final full-matrix least-squaresrefinement with anisotropic temperature factors fornon-H atoms, there was no suitable peak indicating thepresence of a hydrogen atom near the N2 atom.

3.3. Crystal structure of complexes 1 and 2. Generalbehaior

Globally, the two compounds differs from theircounter anions. In complex 1, the anion is [CoIICl3Py].The Co2 atom is in a tetrahedral environment formedby three Cl atoms [2.282(2)2.313(3) A ] and one pyri-dine N atom [2.053(5) A ]. The crystal structure of thecomplex 2 anion consists of the tri-iodide anion, whichis almost linear with the I1I2I3 angle of 175.86(2).The I1I2 and I2I3 bond lengths are 2.9330(6),2.8867(6) A , respectively.

The Co atom in complex cations lies in an octahedralenvironment with salicylaldehyde S-methylisothiosemi-carbazone coordinated in the mer configuration. In allpreviously published compounds [3] the salicylaldehydeS-methylisothiosemicarbazone (H2L) is coordinated totransition metals as a planar tridentate NNO ligandforming fused five- and six-membered chelate rings.The same trend is observed in the structure of com-plexes 1 and 2 (Figs. 1 and 2). However, this ligand canbe coordinated as a monoanionic (HL) or dianonic(L) ligand. As observed for complex 1 (vide supra)there is no hydrogen atom near the N2 atom. Thestructure quality of complex 2 is good enough to allowthe detection of all hydrogen atoms and therefore onecan conclude that the salicylaldehyde S-methylisoth-iosemicarbazone is coordinated in both complexes as adianion. This means that both octahedral coordinatedatoms have an oxidation state of +3, instead of +2,as previously published [2] for complex 1. This highoxidative state is in agreement with the electronic spec-tra of complexes. It can be confirmed by re-investigat-ing the structure [2] in which the distance between thedummy hydrogen atom bonded to N2 nitrogen andthe hydrogen atom from the methyl group bonded to Sis 1.8 A . Such a short distance is physically meaningless

due to the steric hindrance. This steric hindrance couldbe avoided by a cis position of the methyl group withrespect to N3 nitrogen, which is the case in somecomplexes with coordinated salicylaldehyde S-methylisothiosemicarbazone as a monoanion [3ac,f,g].

In the crystal packing, there are a few hydrogenbonds of N3H group: N3H(N3)=1.030 A ,H(N3)Cl2=2.370 A , N(3)H(N3)Cl2=166.1and N3H(N3)=1.030 A , H(N3)I3=2.916 A ,N3H(N3)I3=174.1 for structures 1 and 2, respec-tively [27]. These H-bonds do not have an importantinfluence on the structure of the cation.

The complex cations in both structures are verysimilar, which is clear when comparing the bond dis-tances and angles (Table 2). It is interesting to noticethat both complex cations are stereochemically almostidentical (conformations of chelate rings, mutual orien-tations of axial ligands, orientation of equatorial lig-and). It means that different anions, and differentpacking do not have any influence on the conformationof the [CoIII(L)(py)3]+ complex cation; the conforma-tion of the cation is determined by intramolecularinteractions in the cation. In order to investigate thesemutual ligands influences, quantum chemical calcula-tions on different conformers of model systems of thecomplex cation were performed.

3.4. Geometries of complex cations in the crystalstructures and in the calculated model systems

The quantum chemical calculations have been doneon the model systems with the equatorial chelate ligandsmaller than in the real molecule (Fig. 3). In order toinvestigate mutual influence of coordinated pyridinesthree different model systems were used. In modelsystem 1, two NH3 groups are coordinated at axialpositions and one pyridine is coordinated in equatorialposition. In model system 2, one NH3 is coordinated inequatorial position and two pyridines are coordinatedin axial positions. In model system 3, three pyridinesare coodinated, one in equatorial and two in axialpositions. The geometries for a few different conform-ers of each of the three model systems were obtained.However, Tables 24 contain only geometric data ofstationary point conformers, obtained with full opti-mization. In model system 1, the conformer with theequatorial pyridine (MS1) is tilted by about 40 withrespect to the equatorial plane; in model system 2, thereare two conformers, with perpendicular (MS2a) andwith parallel orientations of axial ligands (MS2b); andin model system 3, the conformer has the axial ligandsin perpendicular orientation (MS3).

Table 2 gives geometric parameters for the crystalstructures of the complex cation and for the optimizedgeometries of the model systems. There are slight differ-ences between experimental and calculated bond


Fig. 3. Perspective view of the model systems: (a) model system 1 (MS1); (b) model system 2 (MS2a); and (c) model system 3 (MS3).

lengths. The bond angles around Co1 show that theupper pyridine is slightly bent towards O1. In thecrystal structures and in the calculated model systemswith axial pyridines (MS2 and MS3) the O1Co1N5angle is smaller than 90, the N3Co1N5 angle islarger than 90, and the N6Co1N5 angle is smallerthat 180. The values of these angles indicate that thereis an attraction between hydrogen atoms on C1b andO1 due to some weak CHO hydrogen bonds (Table4) , caused by substantial positive charge on the hydro-gen atom (calculated to be around 0.2) and negativecharge on the oxygen atom (calculated to be around0.6).

In the crystal structures and in the calculated modelsystems (MS1 and MS3) the equatorial pyridine is tiltedby about 40 to the equatorial plane (Table 2). Thisorientation of the equatorial pyridine is a compromisebetween two effects: one is the attraction between hy-drogen atom on C1a of the pyridine and O1, and theother is the repulsion of the hydrogen atom on N3(with calculated positive charge of 0.28) and the hydro-gen atom on C5a (with calculated positive charge of0.22). Because of these interactions the O1Co1N4angle is smaller than 90 and the N3Co1N4 angle islarger than 90. Relevant interligand interactions aregiven in Table 4.

The six-membered chelate rings in both crystal struc-tures are very similar (Table 3) and they are almostplanar. The conformation of these rings can be de-scribed as half-chair. Non-planar conformation of thechelate ring is a probable consequence of an attractionbetween the hydrogen of the C1a atom and the O1atom.

In the model systems six-membered chelate rings areeven more planar than in the crystal structures, theyhave smaller total puckering amplitudes (Table 3).Since there is no condensed aromatic ring, there isbetter delocalization of electrons in the chelate ring.The ring is the most planar when there are no axialpyridines (MS1) or for parallel orientation of axial

pyridines (MS2b). The torsion angles in six-memberedchelate rings show that non-planarity behavior is verysimilar in different structures (Table 3). For the con-former of MS3 when axial pyridines are parallel, theequatorial pyridine falls in the equatorial plane, and theequatorial chelate ligand is planar. The orientation ofequatorial pyridine and planarity of the equatorialchelate ligand is caused by the steric interactions, whichare equivalent above and below the equatorial plane.The planarity of the equatorial ligand when the axialligands are in mutual parallel orientation has alreadybeen observed in cases of porphyrinato complexes [712], and shows analogy of our complexes with por-phyrinato complexes.

Similarity of the bonding angles around Co1 and ofthe conformations of six-membered rings in the crystalstructures and in the calculated structures indicates thatour model systems are suitable for investigation offactors that influence the orientations of axial ligands incomplexes 1 and 2.

3.5. Preferential orientations of the pyridines in thecalculated cation complexes

The calculations on different conformers, even withfixed torsion angles, of the model systems describedabove were performed in order to investigate the fac-tors that have influence on the orientations of coordi-nated pyridines.

3.5.1. Orientation of the equatorial pyridine in theabsence of axial pyridine

Calculations on model system 1 (Fig. 3(a)), with twoNH3 in axial positions and pyridine in equatorial posi-tion, were performed in order to show what the pre-ferred orientation of the equatorial pyridine isindependent of interactions with axial pyridines. Thegeometries and the energies were calculated for threeconformers. In the first conformer the structure wasfully optimized and the pyridine was in a similar posi-


tion to the crystal structure, the angle between theplane of the ring and the equatorial ligand being35.2. In the other two conformers, which are notminima, the pyridine was fixed in the equatorial plane(torsion angle O1Co1N4C1a=0.0) and fixed to beperpendicular to the equatorial plane (torsion angleO1Co1N4C1a=90.0) (Table 5). The first con-former, fully optimized, is the most stable. Hence, theorientation of the equatorial pyridine with an angle ofabout 40 to the equatorial plane is the preferableorientation of the equatorial pyridine. The structurewith the pyridine in the equatorial plane and the struc-ture with the pyridine perpendicular to the equatorialplane are less stable by 0.79 and 8.38 kcal mol1,respectively.

3.5.2. Orientation of the axial pyridines in the absenceof equatorial pyridine

In order to see what the preferable orientations ofaxial pyridines in the absence of the equatorial pyridineare, calculations were performed on model system 2(Fig. 3(b)), with two axial pyridines and the NH3 in theequatorial position. The geometries and energies werecalculated for three conformers: with two pyridinesperpendicular to each other, with parallel pyridinesabove the six-membered ring, and with parallel pyridi-nes above the five-membered ring. The structures withperpendicular pyridines and with parallel pyridinesabove the five-membered ring have very similar ener-gies, the second one being only 0.67 kcal mol1 morestable. The structure with the parallel pyridines abovethe six-membered ring is less stable by 4.02 kcal mol1.Since that structure is not a stationary point, it wasnecessary to fix torsion angles to keep axial pyridines inparallel position. This structure has higher energy since

there is steric hindrance of hydrogens on pyridines,H(C1a) and H(C1c), and the H(N3) from the five-mem-bered equatorial ring, since there is substantial positivecharge on these hydrogen atoms, around 0.18 onH(C1b) and H(C1c) and around 0.29 on H(N3). Thesecalculations show that the axial pyridines do not preferperpendicular orientation if the equatorial pyridine isnot present. The results are very similar to the results ofcalculations on FeII complexes with porphyrine andaxial pyridines or imidazoles, where it was found thatthe structures with perpendicular and parallel orienta-tions of axial ligands had almost the same stability[11,12]. These show that for the same d6 metal systemsthere is no important influence of chelate equatorialligands on the orientation of the axially coordinatedpyridines.

3.5.3. Mutual orientations of the three pyridinesCalculations on model system 3, with three pyridines,

were done for the perpendicular mutual orientation ofaxial pyridines and for two parallel orientations of axialpyridines, above the five-membered ring and above thesix-membered ring. The conformation with a perpendic-ular orientation of axial pyridines is the most stable(Table 5). This is a fully optimized structure where thetorsion angle between two pyridines is 87.3, and theangle of the plane of the equatorial pyridine to theequatorial plane is 38.0. The structures with parallelaxial pyridines are not stationary points and the torsionangle was fixed. These structures are less stable by 4.45and 7.00 kcal mol1 than the structure with a perpen-dicular orientation of axial pyridines. Again, the struc-tures with parallel axial pyridines above thefive-membered equatorial ring is more stable than thestructure with parallel axial pyridines above the six-membered ring.

Table 5Relative energies of calculated conformers of model systems a

Label in Section 3.4 Torsion angle () E (kcal mol1) b

O1Co1N4C1a C1bN5N6C1c

MS1Model system 1 35.2 0.00.0 c 0.79

8.3890.0 c

MS2aModel system 2 4.8 d 0.0MS2b 70.5 0.67

0.0e,f 4.02MS3 38.0Model system 3 87.3 0.0

4.4511.5 0.9d,f

7.006.4 0.0e,f

a Model systems are shown in Fig. 3.b Relative energies with respect to the most stable conformer of the particular model system.c In these structures orientations of equatorial pyridine with respect to equatorial plane is fixed.d Parallel pyridines are situated above the five-membered ring.e Parallel pyridines are situated above the six-membered ring.f In these structures orientations of axial pyridines with respect to equatorial ligand are fixed.


These studies on model systems show that axialpyridines, in the absence of an equatorial pyridine, canbe in perpendicular or parallel orientation. The equato-rial pyridine, in the absence of axial pyridines, prefersto be tilted with respect to the equatorial plane byabout 40. In model systems with three pyridines themost stable structure has a tilted equatorial pyridine,and axial pyridines in normal mutual orientation. Theseorientations of three pyridines are the most stable be-cause there is no steric crowding between axial pyridi-nes and the equatorial one (Table 4). That is, there issubstantial positive charge on the hydrogen atoms on-C of coordinated pyridine molecules, of around 0.2.

In the structures where axial pyridines are parallelthere is much more steric crowding. These steric inter-actions are responsible for a large instability of parallelstructures. That is, in the absence of axial pyridines, thetilted orientation of the equatorial pyridine in onlymore stable by 0.79 kcal mol1 than the orientation inthe equatorial plane. However, in presence of axialpyridines, the tilted orientation is more stable by 4.45kcal mol1; hence, the difference of 3.66 kcal mol1 isa consequence of interactions of axial pyridines withthe equatorial one.

4. Conclusions

The crystal structures of two compounds,[CoIII(L)(py)3][CoII(py)Cl3]EtOH and [CoIII(L)(py)3]I3,show that the geometries of the complex cations arealmost the same, with perpendicular orientation of axialligands, regardless of different anions and lattice pack-ing. This indicates that intramolecular interactions inthe complex cation are responsible for the orientationsof coordinated ligands.

The DFT calculations on the model systems showthat the equatorial pyridine, because of interactionswith the chelate equatorial ligand, prefers to be tiltedby about 40 with respect to the equatorial plane. Axialpyridines, in the absence of an equatorial pyridine, donot prefer to be perpendicular, the calculations showingthat parallel and perpendicular orientations are almostequally stable. This result is similar to the calculationson the porphyrinato Fe complexes, indicating that in d6

systems chelate equatorial ligands do not have an im-portant influence on the orientation of axially coordi-nated pyridines. However, in presence of a tiltedequatorial pyridine the perpendicular orientation ofaxial pyridines is more stable, because in this orienta-tion there are no steric constraints among coordinatedpyridines.

In the crystal structures and in the calculated struc-tures of complex cations there are attractive electro-static interactions of hydrogen atoms on -C ofpyridine and the coordinated oxygen atom of the equa-torial chelate ligand.

5. Supplementary material

Crystallographic data for the structural analysis havebeen deposited with the Cambridge CrystallographicData Centre, CCDC Nos. 154792 and 154793 for com-plexes 1 and 2, respectively. Copies of this informationmay be obtained free of charge from The Director,CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK(fax: +44-1223-336033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk).

Acknowledgements

We would like to thank Professor E.W. Knapp foruseful discussions. This work was supported by theMinistry for Science and Technology of the Republic ofSerbia (Grant No. 02E09). The calculations were car-ried out at the computer center of the Free Universityof Berlin.

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