resonance raman characterization of cationic co(ii) and co(iii)...

JOURNAL OF RAMAN SPECTROSCOPYJ. Raman Spectrosc. 2003; 34: 868–881Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jrs.1068

Resonance Raman characterization of cationic Co(II)and Co(III) tetrakis(N-methyl-4-pyridinyl)porphyrinsin aqueous and non-aqueous media

Sergei N. Terekhov,1∗ Sergei G. Kruglik,2 Vladimir L. Malinovskii,3 Victor A. Galievsky,1

Vladimir S. Chirvony1 and Pierre-Yves Turpin4

1 Institute of Molecular and Atomic Physics, National Academy of Sciences of Belarus, F. Skaryna Ave. 70, Minsk 220072, Belarus2 B. I. Stepanov Institute of Physics, National Academy of Sciences of Belarus, F. Skaryna Ave. 70, Minsk 220072, Belarus3 A. V. Bogatsky Physico-Chemical Institute, National Academy of Sciences of Ukraine, Odessa, Ukraine4 Universite Pierre et Marie Curie, Case Courrier 138, 4 Place Jussieu, LPBC, 75252 Paris Cedex 05, France

Received 26 February 2003; Accepted 10 July 2003

Resonance Raman (RR) spectra of Co(II) and Co(III) complexes of the water-soluble cationic tetrakis(N-methyl-4-pyridinyl)porphyrin [CoIITMpyP(4), CoIIITMpyP(4)] were obtained in aqueous and non-aqueousmedia, using both B- and Q-band excitation wavelengths. RR spectra of CoIITMpyP(4) are observed forthe first time, and they are compared with those of CoIIITMpyP(4). Wavenumbers of the n2, n4, n8 andn6 oxidation state-sensitive bands depend on pH in aqueous solution and on the nature of the solvent(methanol, ethanol, DMF, DMSO): they lie in distinct spectral ranges for Co(II) vs Co(III) porphyrins. Inaddition, wavenumber shifts induced by axial ligation cover regions twofold broader for CoIIITMpyP(4)than for CoIITMpyP(4). This shows that the electronic influence of axial ligands on the p-system of themacrocycle is more effective when cobalt is in a higher oxidation state. This can be due to (i) smaller axialbond lengths of Co(III) porphyrin and, hence, more efficient cobalt–axial ligand interaction as comparedwith Co(II) complexes, and (ii) CoIIITMpyP(4) being involved in bis-adducts whereas CoIITMpyP(4) ismainly bound with only one axial ligand. The response of the porphyrin ring to the binding of axialligands having various electron-donating–withdrawing properties was monitored by the behaviour ofthese oxidation-state sensitive RR bands: their shifts observed in a series of solvents were comparedwith Gutmann donor and acceptor numbers (DN, AN) of the solvents. A linear positive correlation wasobserved between Gutmann AN and n2, n4, n8 and n6 band wavenumbers for CoIIITMpyP(4), whereas alinear negative correlation was observed for the same bands of CoIITMpyP(4) when DN increases. Thismeans that enhanced electron-accepting properties of axial ligand result in a scarcity of cobalt dp-orbitals,which in turn leads to a decrease in p back-donation from cobalt to the porphyrin macrocycle. Copyright 2003 John Wiley & Sons, Ltd.

KEYWORDS: resonance Raman; cobalt porphyrins; axial ligands

INTRODUCTION

Cationic water-soluble (metallo)porphyrins have attractedconsiderable interest in view of their potential biomedi-cal applications.1 – 5 The crucial role played by porphyrin

ŁCorrespondence to: Sergei N. Terekhov, Institute of Molecular andAtomic Physics, National Academy of Sciences of Belarus,F. Skaryna Ave. 70, Minsk 220072, Belarus.E-mail: [email protected]/grant sponsor: Belarus State Program of Basic Research;Contract/grant number: Spektr-07.Contract/grant sponsor: Foundation of Basic Research of theRepublic of Belarus; Contract/grant number: F02R-105.

derivatives in biological systems is due to their very highbinding affinity to nucleic acids6 – 11 and their ability tocleave DNA.12 – 15 Moreover, site-selective binding of tran-sition metal derivatives of cationic porphyrins with nucleicacids has recently been proposed for use as a specific probeof DNA structure.16,17 Porphyrin–nucleic acid complexesare stabilized mainly by coulombic interactions between thepositive porphyrin periphery and negative phosphates of thenucleic acids chain. However, the binding modes depend onvarious factors, such as the structure of the porphyrin macro-cycle, the relative porphyrin–nucleic acid concentration, theenvironmental properties, the presence of a central metal

Copyright 2003 John Wiley & Sons, Ltd.

Characterization of cobalt porphyrins 869

and its affinity for axial ligation, etc.6 – 11,18 – 22 Hence studiesof the various aspects of the interaction of water-soluble por-phyrins with nucleic acids are of fundamental importancefor understanding DNA binding interactions in general.

One of the main factors affecting metalloporphyrin–nu-cleic acid binding modes is porphyrin axial ligation. It isknown that cationic free base porphyrins and their Ni2C,Cu2C and Au3C derivatives, which have no axial or weaklybound axial ligands, are able to intercalate preferably in GC-rich DNA regions.6,9 On the other hand, axially coordinatedgroups hinder intercalative insertion of the porphyrin planebetween adjacent base pairs. Thus, porphyrins havingstrongly bound axial ligands, such as Mn3C, Fe3C and Co3C

derivatives, are usually considered to be ‘outside bound’in the DNA groove, preferentially at AT-rich regions.1,9

In the case of weak coordination, the presence of nucleicacids can promote processes of axial ligand release. Forexample, the six-coordinate Ni complex of 5,10,15,20-meso-tetrakis(N-methyl-4-pyridinyl)porphyrin [NiTMpyP(4)] insolution with an excess of poly(dG–dC)2 loses its axial waterligands and subsequently intercalates at GC sites.7,10

Processes of axial ligand release/coordination in por-phyrin–biopolymer mixtures can occur via changes in solu-tion properties23 or upon photoexcitation.24,25 This can lead tochanges in the binding mode or to the formation of complexesbetween the porphyrin in its excited state and the biopoly-mer. Recently, we have shown23 that upon a decrease in thebuffer ionic strength, ZnTMpyP(4) loses its axially boundwater molecule, which induces the porphyrin intercalationin the DNA helix. On the other hand, CuTMpyP(4), whichhas no axial ligand in the ground electronic state, is knownto form exciplexes upon photoexcitation in the presence of T-or U-containing polynucleotides, probably due to the bind-ing of T (or U) C O groups to the central Cu2C ion.24,25

However, studies which are still in progress show that theyield of formation of exciplex species depends drastically onthe structure of the (oligo-) polynucleotide counterpart, andthat an exciplex is sometimes also observable in the presenceof nucleic acids having no carbonyl groups. Hence such aprocesses is of interest from the point of view of local DNAstructure and dynamic probing.

The above examples show that binding of cationic por-phyrins to nucleic acids is significantly mediated by pro-cesses of axial ligation. At the same time, the porphyrinlability is frequently a complicating factor in understand-ing the mechanisms and binding modes of metallopor-phyrins to biopolymers. Therefore, prior to investigatingporphyrin–nucleic acid binding interactions, in a first step itis necessary to monitor the effects of axial ligation, and thiscan be performed on axially ligated porphyrin species in freesolutions.

In a recent attempt to investigate the complexation ofCoIITMpyP(4) with DNA, we observed26 that the porphyrin,which spontaneously oxidizes in water,27 – 29 becomes sta-ble in the presence of DNA or polynucleotides such as

N

N

N

N

N

N

N

N

Co

L1

L2

++

+

+

L1, L2 = H2O, OH−, methanol, ethanol, DMF, DMSO

Co = Co(II), Co(III)

Figure 1. Structure of CoTMpyP(4).

poly(dA–dT)2. It was suggested that this is due to axialligation of the porphyrin by a C O group of thymine oranother group from the surroundings. In order to under-stand the processes of interaction of CoIITMpyP(4) withnucleic acids, we decided to undertake a comparative inves-tigation of cationic Co(II) and Co(III) porphyrins (Fig. 1) ina series of free solutions. Under these conditions, solventmolecules can serve as axial ligands, and it is therefore possi-ble to obtain spectroscopic characterization of various axiallyligated species.

Cobalt derivatives are involved in a number of biolog-ical and technological processes.30 – 34 Water-soluble Co(II)porphyrins are growth inhibitors of human malignantmelanoma cells.30 Other complexes are also known as themost important components in sensors used for the electro-catalytic determination of oxygen dissolved in solution,31 andas very effective catalysts for dioxygen electroreduction.32,33

Moreover, non-water-soluble Co(II) porphyrin derivativesare attractive synthetic models of hemoglobin and myo-globin natural systems, owing to their unusual ability to formreversible dioxygen complexes.34 Cationic CoIIITMpyP(4)can produce breaks in DNA in the presence of a varietyof oxidizing agents.8 Most of these properties of Co por-phyrins are related to the enhanced ability of axial ligandsubstitution at the metal center. A systematic study of theinteractions of Co porphyrins with axial ligands and variousaspects of their chemical features are indeed of great generalinterest.

Here we present a study of Co(II) and Co(III) deriva-tives of the water-soluble cationic porphyrin H2TMpyP(4)by a resonance Raman (RR) technique. RR spectroscopycan address questions concerning axial ligation and elec-tronic effects in metalloporphyrins via well-known Ramancore-size- and oxidation-state (OS)-sensitive bands.35 – 43 Sur-prisingly, RR data for water-soluble Co(II) porphyrins havenot yet been reported in the literature; their aqueous solu-tions are very unstable owing to their sensitivity to molecular

Copyright 2003 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2003; 34: 868–881

870 S. N. Terekhov et al.

oxygen dissolved in water. For Co(III) derivatives of water-soluble porphyrins, RR spectra have been published,10,44,45

but spectroscopic characterization of their ligated species hasnot been carried out.

Comparative investigation of Co(II) and Co(III) com-plexes in various media was performed with special empha-sis on the effect of axial ligation on the porphyrin macrocycle.The relative extent of transmission of electronic influencealong the axial ligand ! cobalt ! porphyrin ring pathwaywas followed by RR OS-sensitive band wavenumbers. Theirdisplacements were compared with empirical parameters forelectron donor and acceptor properties of the solvents.

EXPERIMENTAL

MaterialsCobalt(II) tetrakis(N-methyl-4-pyridinyl)porphyrin tetraio-dide and cobalt(III) tetrakis(N-methyl-4-pyridinyl)porphyrinpentaiodide, and those with ClO4

� and PF6� counterions (we

will use the designation CoIIP and CoIIIP when the natureof the counterion is not specified), were obtained by usingmethods described in the literature.46,47

pH was adjusted in aqueous samples by convenientaddition of HNO3 or NaOH to doubly distilled water.Phosphate buffer solution (PBS) at ionic strengths � D 0.03and 0.2 M (25 mM, pH 6.8) was used in some cases. Dimethylsulfoxide (DMSO), methanol (MeOH) and ethanol (EtOH)were of spectroscopic grade (99.9% purity), purchased fromSigma Chemical. Dimethylformamide (DMF) (99.9%), HPLCgrade, was purchased from Sigma Aldrich Chemical. Allsolvents were used without further purification.

Sample preparationSamples of appropriate concentrations (1.10�3 –1.10�5 M)were prepared just before measurements. To obtain CoIIPsamples in aqueous solutions with the lowest possiblecontribution of Co(III) species, we performed an extensiveN2 bubbling procedure (for 30 min) with the solvent,in specific laboratory-made double-compartment air-tightcuvettes, before dissolving the porphyrin in the degassedsolvent. Immediate measurements of absorption and Ramanspectra after preparation allowed the spectral characteristicsof the CoIIP species to be determined (see the first section inResults).

Since CoIIP oxidation in organic solvents is much lessefficient than in water, it was not necessary to degasthese solutions from O2. Selection of convenient excitationconditions was sufficient to remove the Co(III) speciescontribution in the RR spectra of CoIIP.

Absorption spectraAbsorption spectra were obtained on Beckman UV5270 andUvikon XL UV–visible spectrophotometers. All measure-ments were carried out in 10 ð 10, 10 ð 4 or 10 ð 2 mmquartz cells at room temperature.

Resonance Raman spectroscopyRR spectra were recorded with a 90° scattering geometry ona Jobin-Yvon T64000 Raman spectrometer equipped with acharge-coupled device (CCD) detector (Spectrum 1) cooled to140 K with liquid nitrogen. Continuous-wave (cw) excitationat 441.6 nm was provided by a Liconics He–Cd laser. Thewhole set of lines (454.5, 457.9, 465.8, 472.7, 476.5, 488.0and 514.5 nm) of a Spectra-Physics Stabilite 2017 argon ionlaser was also used for cw excitation. Pulsed excitation wassupplied by a Datachrom 5000 dye laser which was pumpedby a Q-switched Nd3C:YAG laser (repetition rate 10 Hz, pulseduration 20 ns). Excitation at 422–440 nm was obtained byfrequency mixing the laser emission of LD 700 dye withthe 1064 nm fundamental wavelength of the Nd3C:YAG ina non-linear crystal. Samples were contained in standard10 mm spectrofluorimetric quartz cells. All measurementswere made at room temperature. Before each measurementthe wavenumber scale was calibrated by using toluene bands.Stationary absorption spectra were recorded before and afterRaman measurements to check the sample integrity.

The spectra shown in the figures are the unsmoothedsums of several accumulations, after appropriate back-ground subtraction. Typical experimental conditions werea 3–5 cm�1 spectral slit width, 0.8 cm�1 increment and 20–30accumulations each of integration time 20 s. Peak positions ofthe Raman lines in fast Fourier transform smoothed spectrawere obtained with a special laboratory-produced program.Highly overlapping lines were decomposed into Lorentziancomponents (however, exactly the same wavenumber val-ues were obtained by fitting the spectra with Gaussianbands). Wavenumber peak positions were accurate to withinš1.0 cm�1. The reproducibility of band maxima determina-tion was evaluated as š0.2 cm�1. This should be taken intoaccount when considering the plots shown in Figs 10 and 11(correlations between RR wavenumbers of Co porphyrinsand electronic properties of the solvents).

RESULTS

Characterization of CoIITMpyP(4) versusCoIIITMpyP(4) in waterCo(II) porphyrins are easily oxidized by molecular oxygendissolved in solution, making their spectroscopic investi-gation difficult. Figure 2 shows the electronic absorptionspectra of CoIIP in PBS (at � D 0.20 M) measured eitherimmediately after preparation or over some periods of time.The fresh solution of CoIIP is characterized by a Soret max-imum at 429 nm [trace (a)], i.e. in agreement with literaturedata.47 Changes in the Soret band position can be detected10 min after preparation. One hour later the Soret band isat 430 nm [trace (b)]. The absorption spectrum of CoIIP oxi-dized species is similar to that of CoIIIP [traces (c) and (d),respectively]. Finally, the CoIIP spectrum measured 48 h afterpreparation (not shown) completely coincides with that ofCoIIIP.



Figure 2. Absorption spectra of Co porphyrins in PBS at� D 0.2 M. CoIITMpyP(4): (a) fresh solution; (b) after 1 hstorage in air; (c) after 24 h. CoIIITMpyP(4): (d) fresh solution.

Preliminary Raman investigations showed that even afresh solution of CoIIP always contains some percentage ofCoIIIP, this proportion being sufficient (and the Raman cross-section being high enough) that bands of the Co(III) speciesare predominant in the RR spectrum obtained at a 441.6 nmexcitation wavelength. To find adequate conditions to recordthe RR spectra of CoIIP we studied the effect of the solventproperties on the porphyrin oxidation rate. The process ofCoIIP ! CoIIIP transformation in PBS at two different ionicstrengths (� D 0.2 and 0.03 M), and in aqueous solutions

at various pHs, was monitored by measuring the electronicabsorption at 10 min intervals, after 1 h and after 1 dayfollowing the solution preparation. The general trend is thatthe higher the PBS ionic strength and the lower the pH ofwater solution (WS), the faster is the oxidation. The relativeefficiency of CoIIP to CoIIIP transformation follows the orderWS < WS (pH 11.5) < PBS (� D 0.03) < PBS (� D 0.2) ³WS (pH 8) < WS (pH 6.8) < WS (pH 2).

For RR experiments we used doubly distilled water inwhich the oxidation rate of CoIIP was the lowest. In orderto limit the percentage of Co(III) species, CoIIP powderwas dissolved in N2-bubbled water. The CoIIP Soret bandmaximum was found at 428 nm in water bubbled withN2 for 30 min, indicating a drastic decrease in the CoIIIPcontribution in comparison with that observed in solutionwithout bubbling.

Preliminary N2 bubbling and excitation at the ‘blue’side of the Soret band (where resonance conditions forCoIIIP modes are less favorable) allowed the resonanceRaman spectrum of CoIIP to be obtained. Figure 3 showsthe RR spectrum of CoIIP in water measured with 422 nmexcitation and the spectra of CoIIIP aqueous solution withB- and Q-band excitations. The observed wavenumbersand polarization properties of the RR bands and theirassignments are summarized in Table 1; assignments forCoIIIP are based on results reported previously44 and on dataobtained for metal tetraphenylporphyrins (M-TPP).42,48 – 50

Assignments for CoIIP are analogous to those of CoIIIP.Comparison of the CoIIP and CoIIIP spectra reveals appre-

ciable differences in the positions and relative intensities

Figure 3. Resonance Raman spectra of Co porphyrins in water: (a) CoIITMpyP(4) bubbled for 30 min with N2, pulsed excitation at422 nm; (b) CoIIITMpyP(4), pulsed excitation at 432 nm. Spectra are normalized on the υ(pyr) vibrational band at 1220 cm�1.(c) CoIIITMpyP(4), cw excitation at 514.5 nm, in parallel (jj) and perpendicular (?) polarization.



Table 1. RR wavenumbers (cm�1) and band assignments for CoIITMpyP(4) and CoIIITMpyP(4) in aqueous mediaa

CoIITMpyP(4) CoIIITMpyP(4)

Band No.b In water pH 11.5 pH 2 pH 8 pH 11.5 Assignmentc

277, vw 277, vw 290, vw, p 293, w, p 298, w, p υ(por) C �(M–N), A1g

308, w 309, w 306, vw, p 306, vw 306, sh υ(por) C �(M–N), A1g

�8 377, s 376, s 387, s, p 384, s, p 381, s, p υ(por) C υ(pyr), A1g

665, vw 666, vw, p 665, vw, p 664, vw, p υ(pyr) C υ(C–NC –CH3�A1g

794, w 794, w 794, m, p 794, m, p 794, m, p pyr �(CC) C � (NC –CH3)906, w 906, w 907, w, p 907, w, p 907, w, p υs(por), A1g

�6 998, w 999, w 1010, w, p 1008, m, p 1006, s, p �s (C˛Cm), A1g

1057, vw 1059, vw 1057, w, p 1057, w, p 1057, w, p pyr υ(CH), A1g

1098, w 1099, w 1103, w, p 1101, w, p 1100, w, p υs(CˇH), A1g

1192, m 1193, m 1192, m, p 1192, m, p 1192, m, p υ(pyr), �(NC –CH3)1220, s 1220, s 1221, m, p 1221, s, p 1221, s, p υ(pyr), A1g

1253, m 1254, m 1257, m, p 1257, m, p 1257, m, p �(Cm –pyr) A1g

�4 1367, s 1368, s 1374, s, p 1372, m, p 1371, m, p �s(C˛N), A1g

1467, m 1468, m 1475, vw, p 1475, vw, p 1476, vw, p �s(C˛Cˇ�, A1g

1516, w, dp 1516, w, dp 1516, w, dp �(CˇCˇ), B1g

1521, vw 1522, vw 1522, vw, p 1521, vw, p 1521, vw, p υ(pyr), A1g

1550, w, ap 1550, w, ap 1548, w, ap �as(C˛Cm), A2g

�2 1573, s 1574, s 1580, m, p 1579, m, p 1578, m, p �(CˇCˇ C C˛Cm), A1g

1597, w, dp 1597, w, dp 1597, w, ap B1g

1643, m 1643, m 1644, m, p 1644, m, p 1644, m, p υ(pyr), A1g

a Abbreviations: p, polarized; dp, depolarized; ap, anomalously polarized; s, strong; m, medium; w, weak; vw, very weak; por,porphyrin core; pyr, N-methylpyridinium group; �, stretching; υ, bending; subscripts as and s, antisymmetric and symmetric modes,respectively.b Band numbering according to Ref. 51.c Band assignments according to Refs 42, 44 and 48–50.

for a majority of the bands. Most noticeable changes arefound for �2 [�(CˇCˇ� C �(C˛Cm), ¾1580 cm�1], �4 [�s(C˛N),¾1370 cm�1] and �8 [υ(por) C υ(pyr), ¾390 cm�1)] (bandnumbering according to Ref. 51), which are known to beOS markers in the RR spectra of Fe porphyrins.52 – 55 Ongoing from Co(III) to Co(II), significant shifts (up to 12 cm�1)also affect the bands at ca 1470 [�s(C˛Cˇ�] and 1000 cm�1

[�s(C˛Cm�]. Moderate shifts (4 cm�1) are observed for thebands at ca 1100 [υ(CˇH)] and 1250 cm�1 [�(Cm-pyr)]. Itis also noteworthy that a change in the cobalt OS drasti-cally affects the relative intensities. Indeed, as can be seenfrom Fig. 3, the intense CoIIP �2 mode decreases appreciablyon going to CoIIIP. In contrast, the CoIIP �6 and �4 bandsand a band at 1250 cm�1 have a higher intensity comparedwith CoIIIP. Other bands of medium intensity (at ca 1190,1220 and 1640 cm�1), mainly due to the N-methylpyridylgroup vibrations, retain their positions and intensities almostunchanged.

We also tried to obtain RR spectra of intermediateproducts of CoIIP oxidation. It is known46,56 – 58 that whenCo(II) porphyrins interact with dioxygen in solution, they canform adducts with O2 and �-peroxy dimer. We monitored

the 422 nm RR spectrum of CoIIP in water during theCoIIP ! CoIIIP transition. No additional feature that couldbe related to intermediate oxidized species was detected(spectra not shown), probably because these oxidized speciesare short-lived at room temperature58 (or excitation is notadequate). The RR spectrum of the final products of CoIIPoxidation completely coincides with that of CoIIIP.

It is remarkable that for both CoIIP oxidized and genuineCoIIIP species two spectral forms have been observed, oneof them increasing with time for CoIIIP. Figure 4 shows the�8 band region of both Co porphyrins in water obtainedat two excitation wavelengths. Under excitation into the‘blue’ region of the Soret band (422 nm) of CoIIIP, an intenseand fairly narrow band at 386 cm�1 occurs for the freshlyprepared sample. This feature widens on excitation intothe ‘red’ region of the Soret band (441.6 nm) [Fig. 4(b)].After storage of the solution in the open air for 24 h, theposition of this maximum shifts to 392 cm�1. It can be seenthat the contour and peak position of this band completelycoincide with those of the corresponding band detected forthe CoIIP oxidized species with 441.6 nm excitation. Twosimilar spectral forms were also observed for CoIIIP in



Figure 4. Low-wavenumber frequency region of theresonance Raman spectra of Co porphyrins in water excited at422 and 441.6 nm: (a), (b) CoIIITMpyP(4) fresh solution;(c) CoIIITMpyP(4) after 24 h storage in air; (d) oxidized speciesof CoIITMpyP(4).

water at pH 2. These data will be presented in the nextsection.

Co porphyrins in aqueous solutions at various pHCoIIP in aqueous solutions at low pH was found to beoxidized very efficiently. Therefore, we were not able toobtain its RR spectrum in acidic media even with 422 nmexcitation. At pH 11.5, the CoIIP spectrum obtained withthe same excitation contains no contribution from CoIIIP(spectrum not shown). The observed wavenumbers anda comparison with those obtained for CoIIP in water aregiven in Table 1. The Raman wavenumber shifts with pHincrease are negligible (or lie within a š1 cm�1 range) in thiscase.

CoIIIP in solution at various pH exhibits three kindsof ligated species,47,59 – 62 i.e. at pH 2, 8 and 11.5 the axialpositions of CoIIIP are occupied predominantly by (H2O)2,(H2O, OH�) and (OH�)2, respectively [we will use thefollowing designation: CoIIIP(H2O)2, CoIIIP(H2O)(OH) andCoIIIP(OH)2]. Pasternack and Cobb59 found the followingrelative concentrations of these ligated species: at pH 2,[CoIIIP(H2O)2] is 104 times as large as [CoIIIP(H2O)(OH)];at pH 8, [CoIIIP(H2O)(OH)] is 100 times as large as both[CoIIIP(H2O)2] and [CoIIIP(OH)2]; and at pH 11.5, CoIIIP(OH)2

strongly predominates in solution. It should also be notedthat the absorption maxima of these three ligated species inthe Soret region are well separated (inset in Fig. 5). All theabove creates favorable conditions for investigating the RRspectra of each of the forms separately, provided that eachof them is excited near its absorption maximum.

Figure 5. Resonance Raman spectra of CoIIITMpyP(4)aqueous solutions in the 150–950 cm�1 region: (a) at pH 2, cwexcitation at 454.5 nm; (b) at pH 2, pulsed excitation at422 nm; (c) at pH 8, cw excitation at 441.6 nm; (d) at pH 11.5,cw excitation at 441.6 nm. All spectra are normalized on theintensity of the υ(pyr) 1640 cm�1 band. Inset: UV–visibleabsorption spectra of corresponding aqueous solutions ofCoIIITMpyP(4) at pH 2 (dot-dashed line), pH 8 (solid line); andpH 11.5 (dashed line).

Figures 5 and 6 show low-and high-wavenumber regions,respectively, of the RR spectra of CoIIIP aqueous solutionsat pH 2, 8 and 11.5. First let us consider the RR spectrumobtained at pH 2. There are two kinds of CoIIIP species, hav-ing distinct RR spectra displayed in Figs. 5 and 6, traces (a)and (b): 422 nm spectra (b) correspond to CoIIIP(H2O)2, sinceit corresponds to the spectrum of the main product in freshsolution. To clarify the nature of the additional spectral form,we monitored the dependence of the RR spectrum on theexcitation wavelength over the range 422–514.5 nm. Figure 7shows the �8 band region (i.e. where the most striking spectraldifferences between the two CoIIIP species can be seen)obtained with a set of pulsed and cw excitation wavelengths.With 422–426 nm excitations a fairly narrow and symmetri-cal contour, assignable to CoIIIP(H2O)2, peaks at 387 cm�1. Atlonger excitation wavelengths a high-wavenumber shoulderappears, which peaks at a maximum of 397 cm�1 for 454.5



Figure 6. Same as Fig. 5, but high-wavenumber region.

and 457.9 nm excitations. Then its intensity again decreaseson going to 514.5 nm excitation.

Thus, in addition to CoIIIP(H2O)2, another species [weshall call it CoIIIP(L1�(L2�] that absorbs at ¾456 nm exists inaqueous solution at pH 2. In principle, the large red shift(ca 23 nm) of the Soret band can be due to the axial ligationby a strong electron donor, i.e. by iodide ions. However,counterions such as I� (or ClO4

� and PF6�), which are

present in solution owing to the complete dissociation of theporphyrin in aqueous media46,56 and which have sufficientlybasic properties, could hardly serve as axial ligand(s) sincethey are very weak as ligands to replace water moleculesaxially bound to cobalt. Hence the origin of CoIIIP(L1�(L2)species in aqueous solution at pH 2 (and also in pure waterafter prolonged storage in air) remains to be understood.This aspect requires special investigation.

Let us now compare the RR spectra of CoIIIP(H2O)2

[Figs 5(b) and 6(b)] with those of CoIIIP(H2O)(OH) andCoIIIP(OH)2 [traces (c) and (d), respectively]. First, we areinterested in whether there is evidence for cobalt–ligandvibrations in the low-wavenumber region. It can be seenfrom the comparison of the spectra in Fig. 5 that nearly thesame set of features is observed for pH 2, 8 and 11.5 solu-tions. One exclusion is the band at 453 cm�1, the intensity ofwhich is lost in the spectrum obtained for the pH 2 solution.Previously, the bands at 443 and 541 cm�1 in the RR spec-tra of FeIIITMpyP(2) solution at high pH were assigned to�sym[Fe–(OH��2] and Fe–OH� stretchings, respectively.63 Itcould be suggested that, in our case, the 453 cm�1 band isconnected with cobalt–hydroxide stretching vibrations, e.g.the 453 cm�1 band can be assigned to Co–OH� vibrations(pH 8 and 11.5 solutions) and a shoulder at 400 cm�1 is due to�sym[Co–(OH��2] (pH 11.5 solution), or in both solutions the

453 cm�1 band can be due to Co–OH� stretching. In any case,it is surprising that the intensity of the 453 cm�1 band for asolution of pH 2 is approximately equal to that for pH 11.5,in spite of the considerable difference in the relative concen-trations of CoIIIP(OH)2 [or CoIIIP(H2O)(OH)] for these twosolutions. Therefore, we believe that Co–hydroxide bandsare hardly enhanced in the RR spectra of Co(III)P at pH 8 and11.5. Moreover, we measured the RR spectra of CoIIIP in waterand deuterated water (spectra not shown). In pure water allthree CoIIIP ligated species can be found, with a predomi-nance of the CoIIIP(H2O)(OH) form. No differences betweenthe RR spectra of Co(III)P in H2O and in D2O were observed(with the same excitation wavelengths). Hence no any evi-dence of Co–ligand vibrations was found in RR spectra.

Generally, all weak bands in the low-wavenumber regionare barely sensitive to pH, with the exception of the 290 cm�1

band, which shifts up to 298 cm�1 when the pH increases.In contrast, the �8, �6, �4 and �2 modes and the 1100 cm�1

band gradually shift downwards on going from pH 2 to 8and 11.5. In addition, there also are intensity variations: the�2, �4 and the 1257 cm�1 band decrease and �6 increases ongoing to basic solutions. It is remarkable that all of thesespectral changes follow the same tendency in the seriesCoIIIP(H2O)2 ! CoIIIP(H2O)(OH) ! CoIIIP(OH)2. For laterdiscussion, all the spectral changes induced by pH variationsare listed in Table 1.

Co porphyrins in non-aqueous mediaTo date, RR studies of water-soluble cationic metallopor-phyrins in non-aqueous media have not been reported. Wehave monitored effects of axial ligation on the RR spec-tra of CoIIP and CoIIIP in organic solvents having variouselectron-accepting–donating properties, where the Co por-phyrin solubility is high enough and the type of coordination



Figure 7. Excitation wavelength dependence of the resonanceRaman spectra of CoIIITMpyP(4) (pH 2 aqueous solution) in the�8 band region.

to cobalt is well known. Although oxidation of CoIIP in non-aqueous media is less effective than in aqueous solutions,there is still some contribution of oxidized species, andtherefore the excitation conditions are also important forRR measurements of CoIIP in non-aqueous media. Our andliterature56 data show that the Soret band always shifts upwhen the cobalt OS changes from II to III. Consequently, exci-tation to the ‘blue’ side of the Soret band will be preferredfor obtaining RR spectra of CoIIP with a minimum CoIIIPcontribution.

Figure 8 shows the RR spectra of CoIIP solutions inmethanol, DMSO and DMF obtained with 425 nm excita-tion. Solvent bands, indicated by dotted lines, have beensubtracted; however, they do not overlap OS markers and,consequently, the subtraction has essentially no influence onthe position of the porphyrin bands.

Comparison of the RR spectra of CoIIP measured inorganic solvents (Fig. 8) with those obtained in water (Fig. 3)again reveals that noticeable downshifts occur for the �8,�6, �4 and �2 modes, as for changes in cobalt OS. Thedownshifts remain weak for methanol solution but increaseon going from water to DMSO and DMF solutions. It is worthnoting that the N-methylpyridyl wavenumber vibrations arealso sensitive to solvent: the CoIIP bands seen at 1220 and1643 cm�1 in water move to 1225 and 1223 cm�1 and to1641 cm�1 in DMSO and DMF solutions, respectively.

Figure 9 compares the high-wavenumber regions ofCoIIIP RR spectra obtained in DMSO, DMF, ethanol,methanol and aqueous solution at pH 8, excited at the redside of the Soret band (441.6 nm) to minimize the possiblecontribution of CoIIP. It is seen that most of the porphyrinmodes (and in particular �8, �6, �4 and �2 OS markers) aresensitive to the solvent; they gradually shift downwards ongoing from pH 8 aqueous solution to DMF/DMSO solutions.Again, as for CoIIP, the wavenumbers of N-methylpyridylvibrations at ca 1220 and 1640 cm�1 in DMSO and DMFare noticeably shifted in comparison with aqueous solution.This is due to solvation effects: in aqueous solution, CoIIPcounterions are fully dissociated,46 whereas in DMSO andDMF these counterions are mainly located at the pyridylrings close to the positively charged nitrogens.56 In such anarrangement, N-methylpyridyl groups are partly protectedfrom interaction with solvent molecules. This situation isprobably also valid for CoIIIP. Indirect argument in favorof such a mechanism is the fact that the N-methylpyridylmodes of CoIIIP are not sensitive to pH (Fig. 6).

DISCUSSION

Background: axial ligation of CoIITMpyP(4) andCoIIITMpyP(4)CoIIP dissolved in water is in a monomeric form with oneor two axially coordinated water molecules.46,47,61 AlthoughCoIIP in acidic media was thought to be a diaqua adduct,62

there is no detailed information in the literature on thepH-dependent structure of CoIIP ligated species. At lowpH in aerobic conditions, efficient CoIIP oxidation occurs,which makes the investigation of ligated species difficult.At high pH we observed a red shift (7 nm) of the Soretband maximum of CoIIP relative to its position in neutralaqueous solution. This may be due to coordination of cobaltby OH� groups, since upon axial ligation of the porphyrincenter metal by electronegative groups a pronounced redshift of the Soret band would be expected,64,65 i.e. a 10 nmshift was observed for the cobalt derivative of TMpyP(4)in aqueous solution with an excess of ethoxide,31 and alsoan 8 nm red shift occurred upon formation of CoIIIP(OH)2

(see inset in Fig. 5). Taking into account that hydroxide is astronger ligand than water, CoIIP in aqueous solution at pH11.5 probably exists as CoIIP(OH)2.



Figure 8. Resonance Raman spectra of CoIITMpyP(4) obtained with 425 nm pulsed excitation: (a) in methanol; (b) in DMSO; (c) inDMF. Dotted lines: RR spectra of the corresponding solvents.

In non-aqueous media such as DMF and DMSO, CoIIP isknown to be axially coordinated with one solvent molecule.56

We assume a similar coordination in methanol by analogywith the case of Co(II) protoporphyrin in this solvent.66,67

For CoIIIP, which is a diamagnetic low-spin d6 com-plex,47,59 – 61 its chemistry in aqueous46,47,59 – 62 and non-aqueous56,68 solvents has been studied in detail. CoIIIPis monomeric in solution61 because axial ligands preventassociation. As mentioned above, it binds two axial watermolecules at low pH, water and hydroxide at neutral pH,and two hydroxide groups at high pH.

In DMF or DMSO, it has been shown that two solventmolecules are bound to cobalt through oxygen atoms.56,68

Additionally, it was suggested that a counterion in solutioncan be ligated in a trans-position to DMSO. One couldtherefore expect that the corresponding RR spectra woulddepend on the counterion species. We obtained RR spectraof CoIIIP in DMSO with three different counterions (I�,ClO4

� and PF6�, in the 420–457 nm excitation range for I�):

no difference was observed between any of these spectra.Therefore, we assume that only CoIIIP(DMSO)2 featurescontribute to the RR spectra, i.e. counterion ligated speciesdo not contribute or do not exist.

We found no published data concerning ligated speciesof CoIIIP in alcoholic solution. However, it is known thatthe protoporphyrin Co(III) derivative is a bis-adduct inmethanol.67 Since cationic porphyrins are relatively poorin electrons in comparison with protoporphyrin, owing tothe electron-withdrawing capability of pyridyl substituents,and in addition since alcohol possesses strong coordinating

properties, we assume that CoIIIP in methanol and ethanolexists as CoIIIP(Me)2 and CoIIIP(Et)2. Hence in both aqueousand non-aqueous solutions, CoIIIP has two axial ligands,considered to be bound to cobalt through oxygen atoms.

RR characterization of Co porphyrins: Co(II)versus Co(III) complexesThe RR spectra of a series of Fe porphyrin derivatives withvarious iron OS/spin states have been extensively studied.In particular, for Fe complexes of tetraphenylporphyrin(FeTPP), �4 and �2 porphyrin core bands were found tobe sensitive to the central metal OS and porphyrin coresize.48,52 – 55 For the water-soluble cationic Co porphyrinsinvestigated here, the same modes, the intensities of whichare resonance enhanced under Soret excitation, also shiftdownwards strongly with a cobalt OS change from III to II(Fig. 3). Therefore, by analogy with FeTPP, it can be assumedthat �4 and �2 can serve as OS and structure marker bandsfor water-soluble Co porphyrins.

The intense and polarized �8 mode at 390 cm�1 alsocan serve as an excellent OS marker,52 although it alsopartly depends on other factors. This mode is due toporphyrin and pyridinium deformation vibrations.44 Arecent normal coordinate treatment has shown that asimilar band for M-TPP derivatives primarily consists ofM–Npyrrole and C˛ –Cm bond stretching and methine bridgebending motions.49 Our present data show that the �8

mode wavenumber is very sensitive to Co(II) ! Co(III)conversion and its behavior matches that of Fe(II) and Fe(III)porphyrins.54



Figure 9. High-wavenumber region of resonance Raman spectra of CoIIITMpyP(4) solutions in: (a) DMSO; (b) DMF; (c) ethanol;(d) methanol; (e) aqueous solution at pH 8. Dotted lines: RR spectra of the corresponding solvents. All spectra are taken with441.6 nm cw excitation. Asterisks: laser plasma lines.

In addition, the �2 and �8 shifts are nearly equal forboth Co and Fe complexes (under an Fe low-spin state).Since the sensitivity to OS is connected with alternationsin � back-donation from the metal to the porphyrin ring,it is suggested that the change of d� –eŁ

g interactions withOS change occurs to nearly the same extent for both Coand Fe complexes. On the other hand, the shift of theremaining �4 mode is noticeably lower for Co than forFe derivatives. It is known from studies of Fe porphyrinsthat the �4 mode (related to C–N breathing vibrations)is sensitive not only to electronic � back-donation effects,but also to the porphyrin ring core size.52 – 54 Therefore, weassume that the core size barely changes upon CoIIIP ! CoIIPconversion as compared with FeTPP complexes. Indeed, itis known from x-ray data69,70 that the Co—N distancesin non-water-soluble CoIIITPP and CoIITPP derivatives areessentially the same.

The �6 mode, essentially �s(C˛Cm) in character, is notknown to be sensitive to OS. However, we did observe thatit follows the same trends as the �2, �4 and �8 modes withchange of cobalt OS; hence it can also serve as an OS marker,and in the following discussion we will focus on the behaviorof these four skeletal modes.

Solvent-dependent RR spectra of Co porphyrinsSolutions at various pHIn contrast to CoIIP, whose RR spectra in pure water andbuffer solution at pH 11.5 are similar, CoIIIP is appreciablypH dependent in both Raman and electronic absorption.On going from pH 2 to 8 and 11.5, the gradual redshift of the Soret band can be explained by the increasein electronegativity of the axial ligands in the seriesCoIIIP(H2O)2 ! CoIIIP(H2O)(OH) ! CoIIIP(OH)2. In pH-dependent RR spectra the most pronounced changes occur



for OS-sensitive modes, which shift downwards when theacidity decreases (Figs 5 and 6). Evidently, when the pHincreases the RR features change in the same direction ason going from CoIIIP to CoIIP. It is therefore reasonable tolink pH-dependent changes in the RR spectra with electroniceffects of axial ligands on the porphyrin ring, which aretransferred through d-orbitals of cobalt.

The binding between the cobalt center and axial ligandmolecules involves �- and �-types of interaction. Co3C ionwith a d6 low-spin structure has an unoccupied dz2-orbital.When water, i.e. a moderate �-donor, binds to cobalt, the dz2-orbital is slightly destabilized by antibonding interactionwith the lone pair of water. Since the electronegativityof hydroxides is higher than that of water, its enhancedinteraction with the dz2-orbital on going from CoIIIP(H2O)2

to CoIIIP(H2O)(OH) would result in a shortening of theCo—O distance. For symmetry reasons a change of thedz2-orbital occupation does not lead to any perturbationof the �-electron system of the porphyrin ring. On theother hand, since hydroxide is a strong �-donor, electronicdensity can be transmitted through filled d�-orbitals ofCo to the antibonding eŁ

g-porphyrin orbital via � back-donation. Population of the antibonding eŁ

g orbital is knownto lead to wavenumber shifts of modes associated withpyrrole breathing modes, known as OS RR markers. Thus,the change in � interaction between cobalt and axialligand on going from water to hydroxide is reflected inthe extent of d� –eŁ

g overlap; this effect increases in theseries CoIIIP(H2O)2 ! CoIIIP(H2O)(OH) ! CoIIIP(OH)2.Therefore, the wavenumbers of �2, �4, �6 and �8 OS-sensitivebands of CoIIIP in pH-increasing aqueous solutions becomelower. The relatively small wavenumber shifts observedsuggest relatively weak (but still significant) changes incobalt d� ! eŁ

g back-donation in comparison with thatobserved for Co(II) ! Co(III) conversion.

Non-aqueous solutionsFor both Co(II) and Co(III) porphyrins in non-aqueous media,OS marker modes appeared also to be more sensitive thanother RR modes to the kind of solvent, whose molecules areinvolved in axial ligation. The trends of the wavenumbershifts for each of these OS sensitive modes as a function ofsolvent are also similar. It can be suggested that the electroniceffects of axial ligands on the porphyrin ring are responsiblefor these spectral changes. However, the efficiency of theelectronic density transmission through cobalt–porphyrin �back-donation has been found to be different for Co(II) andCo(III) complexes. Analysis of the wavenumber shifts forboth Co porphyrins shows that the extent of wavenumbervariations for various solvents is at least twofold narrowerfor CoIIP than for CoIIIP, indicating that the response of theporphyrin macrocycle of the Co(II) complex to axial uptakeis less sensitive than that of CoIIIP.

Only a weak sensitivity of RR spectra of non-water-soluble Co(II) porphyrins to the environment has been

observed for oxy- and deoxycobalt(II)–myoglobins.71,72 Also,minor wavenumber changes have been detected in vibra-tional studies of CoIITPP adducts with CO, NO and O2 in gasmatrices.73 The difference in magnitudes of the wavenumbershifts of OS marker bands observed here between CoIIP andCoIIIP as a function of the solvent may be accounted for bytwo main reasons, as follows.

(i) CoIIP is coordinated by only one solvent molecule (1 : 1complex), whereas CoIIIP form, bis-adducts (2 : 1 complex).The addition of a second ligand probably induces an increasein � back-bonding efficiency. Calculations of d-orbitalenergies and excited states of Co porphyrins have shownthat the effect of axial coordination of solvent molecules ismore pronounced in 2 : 1 than 1 : 1 complexes.74

(ii) Co(II) and Co(III) complexes have different equatorialand axial bond lengths.69,70 Whereas the equatorial distancesare only slightly longer for Co(II) complexes, the axialbond lengths are substantially longer in Co(II) than inCo(III) derivatives, owing to the occupancy of the 3dz2-orbital in Co(II) compounds. Actually, in CoIITPP(pip)2 theCo—N axial distance is 0.37 A longer than in the Co(III)counterpart.70 It is reasonable to assume that a similar trendcan be expected for cationic Co porphyrins. Consequently,the interaction of cobalt with axial ligands is more efficientin CoIIIP than in CoIIP. Hence axial ligation in CoIIIP willproduce a stronger influence on the porphyrin eŁ

g orbital.

Correlation between spectral parameters and axialligand electronic propertiesObviously, axial ligands having different electronic prop-erties will induce different perturbations of the porphyrin�-electron system. Depending on the donor/acceptor abili-ties of axially bound groups, the total charge in the cobaltd-orbitals is increased or decreased, this in turn leadingto variations in the extent of d� ! eŁ

g back-donation. Ithas been pointed out earlier56 that the absorption bands ofCoIITMpyP(4) shift to higher wavelengths as the Gutmanndonor number (DN) of the solvent increases (donor numberscharacterize solvent nucleophilic properties; they are definedas the molar enthalpy of interaction of the electron-donatingsolvent with SbCl5, which serves as a reference acceptor).75

Hence it is not completely unexpected that, in the RR spectraof both Co porphyrins investigated here, there is an obvioustendency for the vibrational wavenumbers to decrease whenthe donicity of axially bound molecules increase. Figure 10displays this tendency (compared with the accuracy limits ofthe peak positions) for the �2, �4, �6 and �8 modes of CoIIP asa function of the solvent Gutmann DN taken from Ref. 76.

The tendency is even more pronounced in the caseof CoIIIP. Figure 11 displays a nearly linear dependencebetween the same four OS marker band wavenumbers andsolvent Gutmann acceptor numbers (AN) [acceptor numbersrepresent the measure of the solvent electrophilicity; theyhave been determined77 from chemical shifts in 31P NMRspectra of (C2H5�3PO in various solvents]. Noticeably there



Figure 10. Correlation between wavenumbers of theCoIITMpyP(4) OS-sensitive RR bands and Gutmann solventdonor numbers (DN).76

exists some deviation for DMF solutions from linear plotsfor the �2 and �4 modes: their wavenumbers are higherthan those observed in DMSO. This may reflect competitionbetween �-donation and �–d� back-donation between theaxial ligand and cobalt, since the changes in DN and AN forDMF and DMSO are in opposite directions.

In our case, when the solvent molecules are axiallyligated, both electron-withdrawing and -donating propertiesresult in changes in the effective charge on the metal ion.Since the shifts of RR OS-sensitive bands are connectedwith changes in d� –eŁ

g interactions, the influence of the axialligands on d�-orbitals is expected to be important in termsof spectral consequences. The mechanism of the electronicinfluence of axial ligands on the d�-orbital depends on a seriesof factors, such as the balance between �- and �-donatingand �-accepting properties of the ligands, competing effectsfrom trans-ligands in the case of bis-adducts, steric factors,etc. The difference in correlations for CoIIP and CoIIIP(RR wavenumbers vs DN and AN, respectively) may bedue to the last two factors. However, it is expected thatcompetition of the porphyrin macrocycle in �-acceptancewith axial ligands is a decisive factor in the variation ind� –eŁ

g back-donation. The correlation of RR wavenumberswith AN for CoIIIP probably reflects such a competition.

Figure 11. Correlation between wavenumbers of theCoIIITMpyP(4) OS-sensitive RR bands and Gutmann solventacceptor numbers (AN).76

For a detailed consideration of the effects discussed above,quantum-chemical calculations and geometry optimizationare necessary. We plan to perform these studies in the nearfuture.

CONCLUSION

The cationic cobalt porphyrins investigated here provideinteresting systems in which the effects of axial ligation canbe studied under the same spin and the same oxidationstates of the central metal. Therefore, it is possible totune the electronic properties/effects of axial ligands andto follow the response of the �-electron system of theporphyrin ring without major reorganization of the metald-orbital population. Porphyrin ring changes, induced bysolvent ligation, were monitored by following the shifts ofRaman bands sensitive to the oxidation state of the metal;these shifts follow empirical parameters characterizingthe solvent donor–acceptor properties. There is a lineartendency of wavenumber downshift when the solventdonicity (Gutmann DN) increases for CoIIP, whereas a linearpositive correlation of OS marker band wavenumbers withGutmann AN is observed for CoIIIP. Thus, axial ligand-induced changes in cobalt porphyrins can be characterized by



RR data and estimated on the basis of well-known empiricalparameters of solvent properties. Conversely, environmentalproperties can be estimated on the basis of RR data. Moregenerally, the results obtained here are of interest for theinvestigation of metalloporphyrin–biopolymer complexes,where the effects of axial exchanges can play a key role.

AcknowledgmentsThis study was partly supported by the Belarus State Program ofBasic Research (Project Spektr-07) and by the Foundation of BasicResearch of the Republic of Belarus (F02R-105). S.N.T. acknowledgesthe Service des Bourses de Recherche Scientifique et Technique duTraite de l’Atlantique Nord (NATO) and personal visiting grantsfrom the French Embassy in Belarus.

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