The Study discusses the possible binding of molecular oxygen to some new Co11 complexes with the Schiff base ligands of the Co(Salen) type. The investigated complexes of Co11 with the Schiff bases are: (bis(5-nitro-salicylaldehyde)ethylenediaminato)cobalt(II), Co-(NSalen); (bis(a-ethylene-salicylaldehyde)ethylenediaminato)cobalt(II), Co(EtSalen); (bis(a-ethylene-3,5-diiode-salicylaldehyde)-ethylenediaminato)cobalt(II), Co(DIEtSalen); (bis(a,5-dimethyl-3-iode-sahcylaldehyde)ethylenediaminato)cobalt(II), Co(DMISalen) and (bis(salicylaldehyde)methylene-p,p\u27-diphenylenediaminato)cobalt(II), Co(Salmbfn). The studies were based on cyclic voltammetry and UV-Vis spectrometry, in DMF solution. Electron-withdrawing substituents on the Schiff bases were found to decrease the affinity of the cobalt Schiff bases for dioxygen. Equilibrium dioxygen uptake measurements over a large range of temperatures provide the ΔH° and ΔS° values of oxygenation that fali in the range -37 to -50 kJ mol-1 for ΔH° and -117 to -157 J mol-1 K-1 for ΔS0, which is in line with the values reported for analogous dioxygen complexes described in literature

Pui, Aurel

Aurel Pui

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ISSN-0011-1643CCA-2790 Original Scientific PaperBinding of Molecular Dioxygen to the Cobalt(II)Complexes in Nonaqueous SolutionAurel PuiFaculty of Chemistry, »Al. I. Cuza« University, 6600-Iasi, RomaniaRecieved July 24, 2000; revised November 7, 2001; accepted November 14, 2001The study discusses the possible binding of molecular oxygen to somenew CoII complexes with the Schiff base ligands of the Co(Salen)type. The investigated complexes of CoII with the Schiff basesare: (bis(5-nitro-salicylaldehyde)ethylenediaminato)cobalt(II), Co-(NSalen); (bis(-ethylene-salicylaldehyde)ethylenediaminato)co-balt(II), Co(EtSalen); (bis(-ethylene-3,5-diiode-salicylaldehyde)-ethylenediaminato)cobalt(II), Co(DIEtSalen); (bis(,5-dimethyl-3-iode-salicylaldehyde)ethylenediaminato)cobalt(II), Co(DMISalen) and(bis(salicylaldehyde)methylene-p,p’-diphenylenediaminato)cobalt(II),Co(Salmbfn). The studies were based on cyclic voltammetry andUV-Vis spectrometry, in DMF solution. Electron-withdrawing sub-stituents on the Schiff bases were found to decrease the affinity ofthe cobalt Schiff bases for dioxygen. Equilibrium dioxygen uptakemeasurements over a large range of temperatures provide the H°and S values of oxygenation that fall in the range –37 to –50 kJmol–1 for H and –117 to –157 J mol–1 K–1 for S, which is in linewith the values reported for analogous dioxygen complexes descri-bed in literature.Key words: cobalt(II) complexes, dioxygen binding, Schiff bases.INTRODUCTIONThe ability of the CoII complexes to bind molecular dioxygen has beenknown for quite a long time.1 In the last decades, the specialists’ attentionhas been focused on this aspect with the aim to some synthetic carriers ofoxygen that should mimic the natural biological systems in fixation, release,* Author to whom correspondence should be addressed.(E-mail: aurel@uaic.ro)CROATICA CHEMICA ACTA CCACAA 75 (1) 165¿173 (2002)stocking and activation of dioxygen.2,3 The complexes’ behaviour depends ontheir physical state. Thus, in the solid state, they may evidence spontaneousactivity towards dioxygen, but only after a preliminary activation phase.4 Insolution, in donor organic solvents, several complexes manifest activity to-ward O2 in a spontaneous manner. Interesting studies on this issue havebeen elaborated by Basolo, Martell, Pâris, etc.5–7 Most of them were perfor-med on octahedral chelates of the bis(salicylaldehyde)ethylenediimine)-co-balt(II) type, Co(Salen), known to possess two potentially vacant axial posi-tions. For O2 fixation, the presence of a base or a sufficient donor solvent isnecessary, in order to occupy one of the axial positions, the other permittingreversible binding dioxygen.8 Studies performed in an aqueous solution –with a higher degree of difficulty – depend equally on the nature of the lig-ands and complexes, on pH, etc.9The present paper analyzes dioxygen binding, as performed by CoII com-plexes with the Schiff bases, by cyclic voltammetry and UV-Vis spectroscopy,in dimetylformamide (DMF) solutions. The structures of the used complexesare presented in Figure 1.The CoII complexes with the Schiff bases are: (bis(5-nitro-salicylaldehyde)-ethylenediaminato)cobalt(II), Co(NSalen); (bis(-ethylene-salicylaldehyde)-ethylenediaminato)cobalt(II), Co(EtSalen); (bis(-ethylene-3,5-diiode-salicyl-aldehyde)ethylenediaminato)cobalt(II), Co(DIEtSalen); (bis(,5-dimethyl-3-iode-salicylaldehyde)ethylenediaminato)cobalt(II), Co(DMISalen) and (bis-(salicylaldehyde)methylene-p,p’-diphenylenediaminato)cobalt(II), Co(Salmbfn).166 A. PUIONO2NOO2NCoN NO OCoCH2CH3CH2CH3NNO OCoCH2CH3CH2CH3IIIINCoCH3CH3CH3CH3IINONONONOCH2Co CoCH2Co(NSalen)Co(DMISalen)Co(EtSalen)Co(Salmbfn)Co(DIEtSalen)Figure 1. Structures of the cobalt(II) complexes.EXPERIMENTALThe synthesis of coordinative compounds was done by the Bailles and Calvinmethod.10 They were characterized by elemental analysis, UV-Vis and IR spectros-copy, X-ray diffraction and cyclic voltammetry.11Cyclic voltammetry was developed on a PAR 263 A voltammeter, operated by acomputer, which employed the PAR 270 M software. The recordings were made on asolution with a concentration of 2 mmol dm–3, in DMF, using 0.1 mol dm–3 LiClO4, asa supporting electrode. The working electrode was vitreous carbon, with the electro-active area A = 3.8 mm2. Saturated Ag/AgCl was used as a reference and a platinumwire as an auxiliary electrode. Cyclic voltamograms of the complexes’ solution, pre-pared under an argon atmosphere in anhydrous and degassed DMF, were recorded.Then, the oxygen was bubbled for 3–5 min in solution. The excess of O2 from the so-lution was removed through bubbling of Ar (for 5–10 min), and the cyclic voltamo-grams were recorded. Then an efficient bubbling of Ar followed for 20–30 min, ac-companied by stirring of the solution to remove O2 from the complex, and the cyclicvoltamograms were recorded once more.The qualitative and quantitative determinations of O2 binding were performedby UV-Vis spectroscopy. The comparative UV-Vis spectra of the complexes and ad-ducts were obtained in DMF solution on a double ray DES device, equipped with acomputer, using the Safas for Windows 95 software. Determinations were made inDMF solution, at an optimum concentration of pyridine (previously determined) andat temperatures ranging between 1–45 C.RESULTS AND DISCUSSIONIn cyclic voltammetry, the complexes – with the exception of Co(Salmbfn)– exhibit a quasi-reversible behaviour. The presence of pyridine in the com-plex solution in DMF (cpy ranging between 55–95%), leads to the formationof the CoL(DMF)(py) compound, characterized by lower values of the redoxpotentials compared to the initial complex.12 Cyclic voltamograms of the so-lution of complexes indicate significant modifications, both in the absenceand in the presence of O2, Figure 2.The reaction that most probably occurs in the presence of O2 is:CoIIL(DMF)(py) + O2 M CoIIIL(py)(O2) + DMF (1)In the absence of O2, the complex is stable, evidencing a quasi-reversiblebehaviour (Figure 2, curve 1). The adduct formed in the presence of O2 isvery unstable, while the CoIII complex subjected to reduction through cyclicvoltammetry (Figure 2 curve 2) is very quickly reoxidated to the CoIII com-plex, which is much more stable.13 If O2 is reversibly bound to the Co com-plex, the former should be eliminated, and the complex should exhibit thesame behaviour as the initial one (Figure 2 curve 3). In the case of theBINDING OF MOLECULAR DIOXYGEN TO THE COBALT(II) COMPLEXES 167Co(DMISalen) complex, one may observe that the cyclic voltamograms, forboth the initial form and the one after O2 elimination, are almost identical,which indicates binding of O2 by this complex. Similar behaviour has beenrecorded for all CoII complexes, evidencing a reversible or quasi-reversiblebehaviour in cyclic voltammetry, while the Co(Salmbfn) complex shows anirreversible behaviour in cyclic voltammetry.UV-Vis spectroscopy shows oxygen’s qualitative and quantitative fixation.Bubbling of oxygen, at various partial pressures and temperatures, throughthe complex solution, enables to observe an increase of the values of absor-bants (Figure 3).168 A. PUI–45.00–35.00–25.00–15.00–5.005.0015.0025.00I/A300.0 100.0 –100.0 –300.0 –500.0 –700.0 –900.0E vs/ V . Ag / AgClFigure 2. Cyclic voltamograms of the Co(DMISalen) complex; 1 – solution preparedinitially in inert atmosphere; 2 – complex in the presence of O2; 3 – complex after O2elimination, cCoL = 0.2 mmol dm–3, cpy = 0.2 mol dm–3, v = 0.1 V s–1.0.00.20.40.60.81.01.24321Absorbancen / m600500400300Figure 3. Evolution of absorbance in O2 bubbling in the Co(DMISalen) complex;cCoL = 10–4 mol dm–3, T = 286 K, 1 – initial, 2,3 – in the presence of oxygen (air), 4 –in the presence of pure oxygen.With the increase of the partial pressure of O2, the value of absorbancealso increases, up to a value that is maintained constant. Bubbling of an in-ert gas, accompanied eventually by heating of the solution, leads to a reductionof absorbance, as compared with the initial one. New oxygen bubbling leadsagain to an increase of absorbance, a second oxygenation cycle being thusrealized. Figure 4 represents the two oxygenation cycles for the Co(DIEtSalen)complex. The DMF concentration is considered approximately constant, hav-ing no influence on the complex concentration.The oxygenation constant, corresponding to reaction (1), is defined ac-cording to relation (2), where B is an axial ligand, e.g., py or DMF:14,15KPO2OLCoBOLCoB22[ ][ ](2)If, at the wave length considered in the study, the only absorbing speciesare LCoB and LCoBO2, one may calculate the oxygenation constant, KO2 ,as a function of absorbance modifications of the present species, by applyingEq. (3).6KA AA A PtOO220 1		 (3)where: A0 – initial absorbance,At – total absorbance,A – absorbance at various oxygen pressure values,PO2 – oxygen pressure during semi-oxygenation.BINDING OF MOLECULAR DIOXYGEN TO THE COBALT(II) COMPLEXES 169300 350 400 450 500 550 6000.20.40.60.8 a b321300 350 400 450 500 550 6000.00.20.40.60.8321AbsorbanceAbsorbancen / m n / mFigure 4. The first two cycles (a and b) of oxygenation for the Co(DIEtSalen) complex,cCoL = 10–4 mol dm–3, T = 274 K, 1 – initial, 2 – in the presence of oxygen (air), 3 – inthe presence of pure oxygen.The oxygenation constants determined using relation (3), at differentworking temperatures, are listed in Table I. If the experimental values ofKO2 , at different temperatures, obey the van’t Hoff equation, then the ther-modynamic parameters may be determined from the graph of function (4),Figure 5,ln KO2 = f (1/T) (4)with relation (5)ln KO2 = – (H/RT) + (S/R) (5)The data given in Table I show the decrease of the oxygenation con-stants with the increase of temperature. They are correlated with the valuesof the halfwave potentials corresponding to the complexes. The higher is thehalfwave potential E1/2, corresponding to the lower oxygenation constants,KO2 , (Table II) the corresponding complex resist a larger number of oxyge-nation-deoxygenation cycles.This correlation may be related to the nature of the substituents graftedon ligands. The presence of some ligands possessing (–NO2) attracting ef-fects induces a decrease in the density of the electrical charge on the centralion, which will result in its more difficult oxidation to more reduced poten-tials. The presence of some relatively bulky groups correlated with the pres-ence of some electron-attracting substituents on ligands (–CH3, –CH2–CH3)protects the central ion, so that it may participate in a higher number ofoxygenation-deoxygenation cycles. The Co(NSalen) complex resists at least170 A. PUI3.35 3.40 3.45 3.50 3.55 3.60 3.651.01.52.02.53.04321lnKO21000 / TFigure 5. Van’t Hoff diagrams for the complexes: 1 – Co(NSalen), 2 – Co(EtSalen),3 – Co(DIEtSalen), 4 – Co(DMISalen).10 cycles of this type, Co(DMISalen) at least 7, and the others at least 5such cycles. The Co(Salmbfn) complex cannot bind O2 in a reversible man-ner.CONCLUSIONSThe complexes of CoII with tetradentate Schiff bases bind the molecularoxygen reversibly, with the highest efficiency in the presence of aminic baseBINDING OF MOLECULAR DIOXYGEN TO THE COBALT(II) COMPLEXES 171TABLE IValues of the oxygenation constant and of the thermodynamic parametersfor the CoL complexesComplexcmmoldm–3cpymoldm–3TKKO2atm–1H°kJmol–1S°Jmol–1 K–1Co(NSalen) 0.1 0.01 274 10.52 –37.56 –117.5Co(NSalen) 0.1 0.01 286 5.20Co(NSalen) 0.1 0.01 298 2.43Co(NSalen) 0.1 0.01 318 1.08Co(EtSalen) 0.2 0.02 274 16.5 –49.66 –156.7Co(EtSalen) 0.2 0.02 286 8.75Co(EtSalen) 0.2 0.02 298 2.96Co(DIEtSalen) 0.1 0.01 274 18.5 –46.50 –131.7Co(DIEtSalen) 0.1 0.01 286 7.40Co(DIEtSalen) 0.1 0.01 298 3.05Co(DMISalen) 0.2 0.02 274 17.36 –45.2 –129.8Co(DMISalen) 0.2 0.02 286 12.2Co(DMISalen) 0.2 0.02 298 6.75TABLE IICorrelation between the halfwave potentials, E1/2, and theoxygenation constant, KO2 (T = 286 K)Compound E1/2 / V KO2 / atm–1Co(NSalen) –0.52 5.20Co(DIEtSalen) –0.82 7.40Co(EtSalen) –0.88 8.75Co(DMIsalen) –0.93 12.20which, in solution, occupies one of the axial positions of the CoII complexand favours substitution of the other solvent molecule with O2. Removal ofO2 may be effected by efficient bubbling of inert gas, which may be accompa-nied by heating of the solution. The cyclic voltammetry of the initial com-plex (non-oxygenated, oxygenated and deoxygenated), reveals such modifi-cations. Spectrophotometric studies confirm reversible binding of oxygen.From the KO2 variation with temperature one may determine the values ofthe thermodynamic parameters of oxygenation. They are correlated withthe oxygenation constants and with the values provided by the oxygenationcycles, depending on the values of the semiwave potentials, on the nature ofthe substituents on ligands and on the steric hindrance effects.Acknowledgment. – I thank Professor Irène Morgenstern-Badarau and AuroraGref of the Paris-Sud University for their assistance with the electrochemical equip-ment and for helpful discussions.REFERENCES1. G. McLendon and A. E. Martell, Coord. Chem. Rev. 76 (1976) 1–39.2. R. D. Jones, D. A. Summerville, and F. Basolo, Chem. Rev. 79 (1979) 139–179.3. L. I. Simàndi, Dioxygen Activation and Homogeneous Catalytic Oxydation, Elsevier,Amsterdam, 1991.4. D. J. Aymes and M. R. Pâris, J. Chem. Educ. (1989) 854–856.5. E. C. Niederhoffer, J. H. Timmons, and A. E. Martell., Chem. Rev. 84 (1984) 137–203.6. D. J. Aymes and M. J. Pâris, Bull. Soc. Chim. Fr. (1976) 1717–1721.7. D. J. Aymes, A. Suifil, and M. J. Pâris, Bull. Soc. Chim. Fr. 127 (1990) 137–144.8. D. Chen and A. E. Martell, Inorg. Chem. 26 (1987) 1026–2652.9. S. Cabani, React. Funct. Polym. 28 (1996) 167–182.10. R. H. Bailles and M. Calvin, J. Am. Chem. Soc. 69 (1947) 1886–1893.11. A. Pui, Ph.D. Thesis, »Al. I. Cuza« University, Iasi, 1999.12. E. Eichhorn, A. Rieker, B. Speiser, and H. Stahl, Inorg. Chem. 36 (1997) 3307–3317.13. S. K. Dhar, Inorg. Chim. Acta 240 (1995) 609–61414. M. J. Carter, D. P. Rillema, and F. Basolo, J. Am. Chem. Soc. 96 (1974) 392–340.15. D. Chen, A. E. Martell, and Y. Sun, Inorg. Chem. 28 (1989) 2647–2652.SA@ETAKVezivanje molekula kisika na komplekse kobalta(II)u nevodenim otopinamaAurel PuiIstra`ivani su slijede}i kompleksi kobalta(II) sa Schiffovim bazama: (bis(5-nitro-salicilaldehid)etilendiaminato)kobalt(II), Co(NSalen); (bis(-etilensalicilaldehid)eti-172 A. PUIlendiaminato)kobalt(II), Co(EtSalen); (bis(-etilen-3,5-dijodo-salicilaldehid)etilen-diaminato)kobalt(II), Co(DIEtSalen); (bis(,5-dimetil-3-jodo-salicilaldehid)etilendia-minato)kobalt(II), Co(DMIsalen) i (bis(salicilaldehid)metilen-p,p-difenilendiamina-to)kobalt(II), Co(Salmbfn). Istra`ivanja su provedena metodama cikli~ke voltamme-trije i UV-Vis spektroskopije, za {to su ispitivani uzorci bili otopljeni u dimetilforma-midu. Na|eno je da afinitet kompleksa Schiffovih baza s kobaltom prema moleku-lama kisika opada s porastom sposobnosti supstituenata u Schiffovim bazama daodvla~e elektrone. Ravnote`na mjerenja vezivanja kisika na prou~avanim kom-pleksima preko {irokog temperaturnog podru~ja utvrdila su da H° ima rasponvrijednosti od 37 do 50 kJ mol–1, a S° od 117 do 157 J mol–1 K–1, {to je u skladu sliteraturnim vrijednostima vezivanja kisika na srodnim kompleksima.BINDING OF MOLECULAR DIOXYGEN TO THE COBALT(II) COMPLEXES 173

Binding of Molecular Dioxygen to the Cobalt(II) Complexes in Nonaqueous Solution

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