Solid State Sciences 9 (2007) 295e302www.elsevier.com/locate/ssscie

The characterization of cobalt(II) derivatives of selectedsubstituted meso-tetraphenyl and tetrapyridyl

porphyrins by EPR spectroscopic study

D. Skrzypek a,*, I. Madejska a, J. Habdas b

a A. Chelkowski Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Polandb Institute of Chemistry, University of Silesia, Szkolna 7, 40-007 Katowice, Poland

Received 20 September 2006; received in revised form 23 November 2006; accepted 29 December 2006

Available online 22 February 2007

Abstract

The cobalt complexes of: (a) meso-tetratolyl-, (b) 5-(4-carboxyphenyl)-10,15,20-tritolyl, (c) 5,10,15,20-tetra-(4-N-methylpyridyl)tetraiodide,and (d) 5,10,15,20-tetra-(4-pyridyl) porphyrins have been synthesized and characterized by MS, electronic spectra, X-ray diffraction and EPRspectroscopy studies. The complex EPR spectra for examined compounds in solid state were obtained in temperature range between 3 K and300 K. This result makes the connection between the crystal and/or an amorphous form of cobalt porphyrins with different substituents recordedby XRD and analyzed EPR spectrum possible.� 2007 Elsevier Masson SAS. All rights reserved.

Keywords: Cobalt(II) complexes; Porphyrins; Electronic spectra; Electron Paramagnetic Resonance spectra; X-ray diffraction

1. Introduction

The porphyrin compounds form an important class ofmaterials considering their application in the photodynamictherapy (PDT) in the diagnosis and treatment of cancer. Forexample meso-tetra-4-hydroxyphenylporphyrin has for severalyears been used as a photosensitizer, approved for clinical use[1]. Apart from that, the complexes of the 3d metals (above allcobalt and iron) with porphyrins, when fixed on various car-riers, can be used as efficient catalysts [2]. Usually, heteroge-nization is obtained either by forming covalent bond betweenthe peripheral ligand atoms of a complex and a carrier or bycoordinating its central metal ion with a donor group on thecarrier surface. It is known that electron-donating or elec-tron-withdrawing properties of porphyrins are critically depen-dent on the nature of the substituents located on the periphery

* Corresponding author. Tel./fax: þ48 322588431.

E-mail address: dskrzyp@us.edu.pl (D. Skrzypek).

1293-2558/$ - see front matter � 2007 Elsevier Masson SAS. All rights reserved

doi:10.1016/j.solidstatesciences.2006.12.005

of the ring [3,4]. The subject of our investigations was metal-loporphyrins with methyl, carboxylic substituents in para-position of meso-linked phenyl or pyridyl rings. We decidedto investigate these porphyrins as a result of their chemicalproperties. Carboxylic group allows forming an amide bond,e.g. with amino acids, what is characteristic for proteins.The presence of such substituent opens the possibility todesign compounds with desired biochemical properties [5].meso-Tetrapyridylporphyrins are compounds, which may berelatively easy converted in soluble in water. This transforma-tion makes them more useful for biological or medical appli-cations [6,7].

A number of results of the investigations of cobalt porphy-rins have been summarized in papers by Subramanian [8] andWalker [9]. In cited papers the EPR studies of paramagneticcobalt porphyrins have been conducted on solutions at roomtemperature or frozen solutions at low temperature. The ob-tained results exhibit significant influence of different solventson coordination and local symmetry around the central ion.We report here the EPR studies of cobalt porphyrins with

.

296 D. Skrzypek et al. / Solid State Sciences 9 (2007) 295e302

different substituents in its solid state and at temperature rangebetween 300 K and 3 K.

2. Experimental

2.1. Synthesis of compounds

meso-Tetratolylporphyrin (TTP) and 5-(4-carboxyphenyl)-10,15,20-tritolylporphyrin (MCP) were prepared using theprocedure of Adler et al. [10]. The above-mentioned com-pounds were synthesized by heating 4-methylbenzaldehydeand 4-carboxybenzaldehyde with pyrrole in boiling propionicacid. Overnight cooling gave purple crystals, which were sep-arated from the reaction mixture on Buchner funnel, washedtwice with water and finally with hot water in order to removetraces of propionic acid and partially formed tars. The ob-tained porphyrins were purified on chromatography column(4� 50 cm) filled with silica gel (60e230 mesh) where mix-ture of chloroform/methanol, v/v 9:1 was applied as an eluent.

meso-Tetrapyridylporphyrin was obtained in a similar wayby using 4-(pyridylaldehyde) and pyrrole in molar ratio 1:1.The product was only 5,10,15,20-tetra-(4-pyridyl)-porphyrin(TPyP). In order to convert this product into porphyrin solublein water, methyl iodide in nitromethane was added. Refluxingof this mixture for 1 h gave after evaporation quaternary am-monium salt of 5,10,15,20-tetra-(4-pyridyl)-porphyrin, thatis, 5,10,15,20-tetra-(4-N-methylpyridyl)-porphyrin tetraiodide(TMePyP). The structural formulas of these compounds arepresented in Fig. 1.

The cobalt porphyrins were obtained by standard method[11] of refluxing cobalt acetate in DMF for 1 h. The metallo-porphyrins were precipitated by adding water and then

separated from solvent on Buchner funnel. The successfulmetal insertion was checked by infrared spectroscopy, MSspectroscopy, visible absorption spectroscopy and thin layerchromatography (TLC).

2.2. Methods

MS spectra were measured on TSQ 700 Finigan-Mat usingESI technique. Molecular peaks proved molecular masses ofinvestigated metalloporphyrins: (a) cobalteTTP: m/e¼ 729;(b) cobalteMCP: m/e¼ 759; (c) cobalteTMePyP: m/e¼ 1245;(d) cobalteTPyP: m/e¼ 677.

The IR spectra were measured on IR 560 Magma Nicoletspectrometer as KBr pellets in the spectral range of 450e4000 cm�1 The absorbance spectra of compounds weremeasured at normal incidence at room temperature in the rangeof 350e700 nm by a Jasco UV-spectrometer in propionic acidsolutions (10�6 M and 10�4 M).

The X-ray powder diffraction (XRD) analysis was carriedout with the use of Siemens D5000 diffraction equipment.The filtered Cu Ka radiation was selected and the scanswere recorded in a (5�e100�) 2q range.

Electron Paramagnetic Resonance spectra were recorded onRadiopan SE/X spectrometer with TE102 rectangular cavityand 100 kHz field modulation, equipped with an OxfordInstruments ESR 910 helium flow cryostat. The microwavefrequency was measured using Hewlett Packard 534 micro-wave frequency counter and the magnetic field strength wasmonitored by an NMR teslameter. The temperature depen-dence measurements were performed in the temperature rangefrom 3 K to 300 K. All EPR parameters were confirmed bysimulation using Bruker-Symphonia software package.

Fig. 1. The molecular structure of base porphyrins.

297D. Skrzypek et al. / Solid State Sciences 9 (2007) 295e302

3. Results and discussion

3.1. IR, electronic spectra and X-ray diffractionmeasurements

The principal infrared absorption bands for the presentedmetalloporphyrins in this work are listed in Tables 1 and 2.Our assignments are, in general, in agreement with thosepreviously reported in literature [12,13]. The absence of repre-sentative NeH stretching vibrations of free base porphyrins inthe region of 3300e3360 cm�1 or sharp peaks at 740 cm�1

and 680 cm�1 associated with NeH out-of-plane deformationsindicates a successful cobalt insertion into porphyrin ring. Thepresence of substituents of phenyl ring in p-position is ob-served. CeO stretching vibration of carboxylic group absorbsat 1701 cm�1 for cobalteMCP. For cobalteTTP and cobalteMCP is observed typical stretching mode for aliphatic hydro-carbons around 2925 cm�1 and 2923 cm�1, respectively.

The absorption spectral data of the cobalt complexes aregiven in Table 3 and are shown in Fig. 2(aed). In spite of‘‘hypso’’ type of cobalt derivatives of porphyrins (blue shiftedspectra, what is observed here in Q region), the Soret bandsshow dramatic red shifts. It is well known that Soret bandfor porphyrins in solution depends on the type of solvents.Red shifts are always observed in polar solvents (likepropionic acid) relative to non-polar solvents [14,15]. Fromthe absorption spectra of studied metalloporphyrins (exceptcobalteTTP) we observed that the second Q bands at about600 nm (for CoeMCP at 598 nm, for CoeTPyP at 598 nm,for CoeTMePyP at 591 nm) have been almost disappeared.The explanation suggested by Kaizu et al. [16] was that metalion position is out of plane of the porphyrin ring and the at-tachment the solvent molecule in axial position disturbs thesymmetry of the system.

The crystalline structure was studied by powder X-ray dif-fraction. The X-ray diffraction patterns of obtained complexesare shown in Fig. 3(aed). It can be seen that cobalteTTP iswell crystallized, cobalteMCP is moderately crystallized,cobalteTMePyP is weakly crystallized and cobalteTPyP isamorphous. The synthesis of various cobalt(II) derivatives of

Table 1

Infrared absorption bands of formed cobalt derivatives of tetraphenylporphyrins

CobalteTTP

[cm�1]

CobalteMCP

[cm�1]

Types of vibrations

3025 3022 CeH stretch in phenyls

2925 2923 CeH stretch in methyl groups

1701 CeO stretch in carboxylic group

1591 1588 C]C stretch in phenyl

1562 1559 C]C stretch in porphyrin

1471 1463 C]C stretch in porphyrin

1350 1350 C]H bend in phenyl

1247 1256 CeH bend, CeN stretch in porphyrin

1182 1182 CeH bend in phenyl

1001 1000 In-plane bonding in porphyrin

967 964 In-plane bonding in porphyrin (ring vibration)

800 801 CeH out-of-plane p-substituted phenyl

616 Out-of-plane deformation porphyrin

tetraphenyl and tetrapyridyl porphyrins was performed in thesame manner, but the products are crystallized in variousdegrees.

3.2. EPR studies

The powder EPR spectra of: (a) cobalteTTP, (b) cobalteMCP, (c) cobalteTMePyP, and (d) cobalteTPyP and theirtemperature evolution are shown in Figs. 4e7. For all com-pounds the spectra are complex and contain the groups of linesdenoted as A, B, C, D and R.

3.2.1. CobalteTTP (see Fig. 4)The signal B of the spectrum is characterized by single

asymmetrical line, which was simulated with gt¼ 2.3 andgk ¼ 2.07. The signal B was detectable at whole temperaturerange (from 300 K to 3 K) and its intensity gradually increasedwith decreasing temperature. At T< 80 K this line is coveredby EPR signal labeled A in Fig. 4. The signal A consists ofa series of eight nuclear hyperfine lines. Its best simulation(T¼ 4 K) with parameters listed in Table 4 is shown in the in-set of Fig. 4. This feature with g around 3.2 is weak, broad andunresolved at ambient temperature. The prominent feature ofthe spectrum at room temperature is the intense signal atg¼ 2 (denoted as R), which appears to be identical to freeradical observed in the base porphyrin. When the temperaturedecreased, this signal was completely replaced by a new asym-metrical signal C with gt¼ 2.00 and gk ¼ 2.04.

3.2.2. CobalteMCP (see Fig. 5)As can be seen from Fig. 5, the signal recorded as type-B is

distorted by an overlying narrow resonance at g¼ 2 and the

Table 2

Infrared absorption bands of formed cobalt derivatives of tetrapyridylporphyrin

CobalteTPyP

[cm�1]

CobalteTMePyP

[cm�1]

Types of vibrations

3120 3118 CeH stretch in pyridine

2885 CeH stretch in methyl groups

1649 1641 In-plane bonding in pyridine

1559 1563 C]C stretch in porphyrin

1490 C]C stretch in porphyrin

1417 1416 In-plane bonding in pyridine

1211 1216 CeC stretch in porphyrin

1009 1006 In-plane bonding in porphyrin

970 969 In-plane bonding in porphyrin

(ring vibration)

801 800 CeH out-of-plane in porphyrin

725 725 Out-of-plane deformation pyridine

Table 3

The optical parameters of studied cobalt porphyrins

Compound Soret

band [nm]

Q bands

[nm]

3(n)� 105 [mol�1 cm�1]

CobalteTTP 441 547,612 9.05 2.55; 2,55

CobalteMCP 430 545 8.41 1.7

CobalteTMePyP 437 550 14.0 15.7

CobalteTPyP 436 551 7.0 6.12

298 D. Skrzypek et al. / Solid State Sciences 9 (2007) 295e302

Fig. 2. The absorption spectra of: (a) cobalteTTP; (b) cobalteMCP; (c) cobalteTMePyP; (d) cobalteTPyP.

shoulder on low-field side. With decreasing temperature theperpendicular component of signal A-type splits. Both perpen-dicular and parallel hyperfine components are visible at liquidhelium temperature (simulation parameters e see Table 4).The C-type signal partially splits in the temperature rangefrom T¼ 3 K to 80 K. Apart from that, below T< 50 K,a new signal, denoted as D was detected. It shows a sharpfeature in the low-field region of the spectrum (see Fig. 5);the g-values are g¼ 6.04 in maximum and g¼ 5.75 in cross-ing point.

3.2.3. CobalteTMePyP (see Fig. 6)The B-type EPR signal of cobalteTMePyP complex can

be simulated with identical parameters as those observed forcobalteTTP. Signal A is undetected at room temperature; thehyperfine component around gt¼ 3.15 occurs while coolingthe sample. The best splitting was observed at liquid nitrogentemperature and hyperfine parameters of the same order ofmagnitude as for CoeTTP were calculated. This signal isvery weak. In comparison, the strong, broad and unresolvedsignal of D-type was detected in the range of the lowest tem-peratures. The signal of C-type is characterized by asymmet-rical, resolved line centered at g¼ 2.0. In the inset of Fig. 6,the extension of this signal and its simulation (T¼ TLN) areshown. The signal C is observed at whole temperature range,however, it is unresolved at the liquid helium temperature.

3.2.4. CobalteTPyP (see Fig. 7)The type-B signal of cobalteTPyP (see Fig. 7) is character-

ized by three transitions with g-values at 2.32, 2.21, and 2.14,

Fig. 3. X-ray diffraction pattern of: (a) cobalteTTP; (b) cobalteMCP; (c)

cobalteTMePyP; (d) cobalteTPyP.

299D. Skrzypek et al. / Solid State Sciences 9 (2007) 295e302

respectively (the simulated spectrum with these parameters isshown as inset in Fig. 7). The line C shows unresolved featurewith gav¼ 2.013. The dominant feature of EPR spectrumfor CoeTPyP is signal of type D, which occurs belowT¼ 65 K. It is an unresolved signal, whose linewidth stronglydepends on temperature.

Fig. 4. The temperature evolution of the EPR spectra of cobalteTTP. The inset

shows the comparison of the experimental and simulated signals A-type at

T¼ 3 K (simulated spectrum for the parameters, which are listed in Table 4).

Fig. 5. The temperature evolution of the EPR spectra of cobalteMCP.

3.3. Discussion

The characteristic feature of Co(II) EPR spectrum is itsspectroscopic complexity with respect to the orbital degener-acy of the ground state and coupling of excited state andalso with respect to influence of changes in the coordinationenvironment [17,19,26,32,33].

Fig. 6. The temperature evolution of the EPR spectra of cobalteTMePyP. The

inset shows the comparison of the experimental and simulated signals C-type

at T¼ 3 K (simulated spectrum for the parameters which are listed in Table 4).

Fig. 7. The temperature evolution of the EPR spectra of cobalteTPyP. The

inset shows the comparison of the experimental and simulated signals B-type

at T¼ 3 K (simulated spectrum for the parameters which are listed in Table 4).

300 D. Skrzypek et al. / Solid State Sciences 9 (2007) 295e302

Table 4

EPR parameters of obtained metalloporphyrins

Compound Type of spectrum Diffraction pattern

A B C D R

CobalteTTP T¼ 3 K T¼ 300 K T¼ 99 K e Present Well crystallized

gt¼ 3.20 gt¼ 2.30 gt¼ 2.00

gk ¼ 1.81 gk ¼ 2.07 gk ¼ 2.04

At¼ 350 At¼ 20 At¼ 9

Ak ¼ 147 Ak ¼ 60 Ak ¼ 15

VS S W e S

CobalteMCP T¼ 3 K T¼ 300 K T¼ 3 K T¼ 3 K Present Moderately crystallized

gt¼ 3.22 gt¼ 2.20 gav¼ 2.00 gmax¼ 6.04

gk ¼ 1.79 gk ¼ 2.00 gapp¼ 5.75

At¼ 370

Ak ¼ 147

W S W S W

CobalteTMePyP T¼ 113 K T¼ 300 K T¼ 91 K T¼ 3 K Present Weakly crystallized

gt¼ 3.15 gt¼ 2.30 gx¼ 1.990 gx¼ 2.95

gk ¼ 1.81 gk ¼ 2.07 gy¼ 2.028 gy¼ 4.30

At¼ 350 gz¼ 2.079 gz¼ 5.11

Ax¼ 13

Ay¼ 10

Az¼ 15

VW S W VS VW

CobalteTPyP e T¼ 3 K T¼ 185 K T¼ 3 K e Amorphous

gx¼ 2.32 gav¼ 2.013 gx¼ 2.80

gy¼ 2.21 gy¼ 3.90

gz¼ 2.14 gz¼ 4.90

e S VW VS e

A in 10�4 cm�1; VS e very strong; S e strong; VW e very weak; W e weak indicate the intensity of individual features of EPR spectra.

The parameters calculated from the EPR spectra of thecompounds studied are listed in Table 4.

The reviews of the typical results for frozen solutions of themetalloporphyrins with symmetric D4h or C4 square-planarsymmetry were done by Subramanian [8] and Walker [9].These data imply that A and B the EPR features are consistentwith the d7 e configuration in the low-spin state, leading to theS0 ¼ 1/2 system in which the single unpaired d electron islocalized on the dz

2 orbital. The axially symmetrical spin-Hamiltonian [17]

H ¼ mB

hgkBzSz þ gt

�BxSx þBySy

�iþAkSzIz

þAt

�SxIx þ SyIy

�ð1Þ

[in which S¼ 1/2 and I¼ 7/2 (for 59Co e 100% abundance)]generates the computer spectrum almost identical with theexperimental pattern of signal A-type with g- and A-parametersimilar to those reported by several authors [8,7,17e19]. It isworth noting that in CoeTPyP complex the signal A does notoccur.

The signals denoted as B, detectable even at ambient tem-perature and present in all examined cobalt porphyrin deriva-tives are consistent with the formation of a five-coordinateCo(II) species, which are typical low-spin cobalt(II) com-plexes [20e24]. The obtained EPR parameters are listed inTable 4. The form of the EPR spectra for low-spin Co(II)

complexes with gt> gk> 2 suggests that the dz2 orbital is

favoured for the unpaired electron. However, the positive shiftfrom the free-spin value for gk(zþ0.1) is unusual for puredz

2 complexes [25]. This can be explained formally, in termsof dz

2 and dx2�y2 mixing, which has the effect of moving thegk features to low field and the gt features to high field. Theaddition of a fifth ligand to a square-planar complex is knownto have a strong destabilizing effect on its dz

2 orbital [26].Mc Garvey’s theory [27] has shown that to adequately

explain the EPR data on low-spin cobalt complexes, it is nec-essary to include the spineorbit mixing of excited quartetstates into the doublet ground state. The set of equations forg- and A-parameter reproduced in the simplified versions byBarzaghi et al. [28] has been obtained on the basis of two pos-sible ground states:

2A1 ¼�x2� y2

�2ðxz or yzÞ2ðyz or xzÞ2�z2�

2B1 ¼�x2� y2

�2�z2�2ðyz or xzÞ2ðxz or yzÞ ð2Þ

It was shown that for low-symmetry complexes the large an-isotropy in g- and A-value may result from the extensive mixingof dz

2 and dx2�y2 orbitals which is allowed in lower symmetries.The separation between the excited and ground states is a verysensitive function of axial ligands. For example, the results ofEPR investigations of tetraarylporphyrin complexes of Co(II)in the presence of a number of solutes are as follows [19]: for

301D. Skrzypek et al. / Solid State Sciences 9 (2007) 295e302

the Lewis base donor, piperidine, gt is smallest (2.214); for pdonors e gt ranges from 2.4 to 2.7; for strong p acceptors egt� 3.0.

Among the complexes studied, the signal type-B forCoeTPyP was simulated with rhombic spin-Hamiltonian.This means that the Co atom is slightly out of the plane of thefour nitrogen atoms of the porphyrin ring. The same resultwas observed for Lewis base complexes of Co(II) porphyrins,when the cobalt is five-coordinate [19]. Therefore, in our opin-ion, the presence of the five-coordinated cobalt(II) species in allcompounds obtained suggests the possibility of coordination byeluent molecules during preparation.

The appearance of feature type C in the EPR spectra of ex-amined compounds means that these porphyrins are partiallyoxygenated. This signal is associated with Co(III)eO2

� com-plexes [29e31]. The value of gt> gk means the transfer ofan electron from the Co(II) ions to oxygen. If the superoxidehas several different conformations, each will have slightlydifferent coupling to cobalt, resulting in smearing out of thehyperfine structure, what is observed in our experiment.

The signal of type D is undetected from 300 K up to 60 K.This fact and EPR lineshape indicate that high-spin Co(II) spe-cies are present in CoeMCP, CoeTMePyP and CoeTPyPcomplexes.

The splitting of spectroscopic states of HS Co(II) in coordi-nation complexes is due to the combined effects of the symme-try of the crystal field and spineorbit coupling [17]. Theycorrespond to the HS d7 configuration either in an orbitallynondegenerate ground state (4A2) or in an orbitally degenerateground state (4T1) in which the orbital levels are separated byspineorbit coupling. As a result, the combined effects ofspineorbit coupling and distortion of the crystal fields fromhigh symmetry lead to a series of Kramers doublets. Thesurvey of the theoretical expectations regarding the EPR spec-tra of high-spin cobalt(II) was made in Bencini et al.’s [32] andBanci et al.’s [33] papers. The EPR spectra for four-, five- andsix-coordinate HS cobalt(II) complexes were catalogued astype I (4T1-ground state) which includes octahedral andsquare-pyramidal complexes, and type II (4A1-ground state),which includes tetrahedral and trigonal bipyramidal com-plexes. It is known [34] that the EPR spectra of high-spincobalt(II) are examples of extreme sensitivity to low-symmetrycomponents. As a matter of fact, the g-values are completelyanisotropic.

The D-feature of EPR spectrum can be interpreted accord-ing to the Hamiltonian [17]:

H ¼X

i¼x;y;z

½mBgiBiSiþAiSiIi� ð3Þ

with an effective spin of S¼ 1/2.The hyperfine splitting for the lowest field feature does not

occur. The resulting large linewidth corresponds to the sum ofeight overlapping components with the hyperfine splittingaround 200� 10�4 cm�1. This magnitude of A-parameterwas reported for octahedral and square-pyramidal cobaltcomplexes by a number of authors [26,32e34]. The values

of g-parameters, which were estimated for studied compoundsare listed in Table 4.

4. Conclusions

Four cobalt porphyrins with different substituents wereprepared and characterized in polycrystalline form on the basisof the EPR data. The data in Table 4 show that EPR spectra ofexamined compounds are consistent with the formation ofsquare-planar LS Co(II) species (A), the five-coordinate LSCo(II) (B) and HS Co(II) (D) species. Besides, EPR spectraof all porphyrins indicate that these compounds are partiallyoxygenated and exhibit the typical signal Co(III)eO2

� aroundg¼ 2 with the same features (axial symmetry and hyperfinestructure) already shown for cobalt(II)eformylbiliverdin com-plexes [30].

The change in relative intensities of individual features ofEPR spectra is worth noting. As can be seen from Table 4,there is a connection between the character of diffractionpattern of porphyrins studied and amount of individual species(the intensity of individual features of EPR spectra is labeledas S, VS, W, VW). The cobalteTTP complex was the bestcrystallized and the prevailing feature of its spectrum is patternof lines type-A, which is associated with square-planar cobaltcomplexes. At the same time, for cobalteTPyP complex,which is an amorphous compound, the signal from high-spincobalt(II) is a dominant feature of the EPR spectrum. Thisobservation makes the connection between a crystal formand/or an amorphous form of the cobalt porphyrins with dif-ferent substituents recorded by XRD and the analyzed EPRspectrum possible.

The appearance of HS Co(II) species in weakly crystallizedporphyrins can be explained by use of the calculation for themetaledonor atom bond lengths corresponding to high-spinand low-spin cobalt(II) which was carried out by Tiwary andVasudevan [35]. This bond length change, Dr¼ rHS� rLS z0.1 A, is relatively small (with comparison with iron(II),where Dr z 0.2 A), because only one electron is transferredbetween the eg and t2g orbitals in the DS¼ 1 transitions:(eg)2(t2g)5 4 (eg)1(t2g)6. As a consequence of the reduced vol-ume changes associated with change in spin state for Co(II),together with the possibility of spin-state mixing throughspineorbit coupling, spin interconversion is expected to bemore rapid for Co(II) than for Fe(II). Recently, the comprehen-sive review of spin crossover phenomena was published [36].The spin crossover in solid transition metal compounds isalways accompanied by changes of crystal lattice parameters,which also involve modifications of bond angles betweenthe metal and ligand atoms, changes of intraligand bondlengths and angles and possibly changes of orientations of non-coordinating anions and solvent molecules. All these structu-ral changes are predominantly displacive but not bond breakingin nature. The EPR method is very sensitive, to quote someresults of studies of [Co(MeC5H3NCOO)2(H2O)2] and [CoCl2(C5H4NCOOPri)2] crystals, which were obtained by using XRDand EPR techniques by March et al. [37]. In these investigationsthe signals belonging to two different Co(II) chromophores

302 D. Skrzypek et al. / Solid State Sciences 9 (2007) 295e302

were observed, what were undetected in the X-ray diffractionanalysis.

It is proposed in this paper that the complexity of the EPRspectra for examined cobalt porphyrins results from extremesensitivity to geometry of cobalt(II) chromophore.

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