Study on Oxygenation Process in Mixed Ligand

        System  Cobalt(II) - 2,2’-Bipyridil - Serine               

                      Tatyana S. Spolitak, Dmitriy M. Palade

                              Donetsk National Technical University

                      58 Artema Str., 83000 Donetsk, Ukraine        

                           E-mail:   tspolitak@yahoo.com 

Key words: Complexation and oxygenation  processes, mechanisms,  equilibrium constants, cobalt complexes, mixed ligand systems, oxygenated complexes 

            On the basis of detailed  pH–metric, manometric and spectrophotometric studies there was  determined mechanism of  oxygenation process in mixed ligand system Co²⁺ – dipy – Ser that  confirms proposed in our previous research mexanism of oxygenation process for mixed ligand systems with oxyaminoacids. There were calculated equilibrium constants for all steps taking into consideration formation of binary and ternary cobalt(II) complexes including those binding dioxygen. Some kinetic characteristics of the oxygenation process were determined which allowed to conclude  that oxygenation process proceeds by a pathway including formation of mixed ligand hydroxodimer (in which serine behave in a bidentate manner) and its further oxygenation resulting in formation of binuclear oxygenated monobridged cobalt complexes (in which serine is tridentate ligand).                                     

 Introduction.  In our previous work [1] complexation and oxygenation processes were studied on the basis of pH-metric and manometric data obtained for mixed ligand system involving cobalt(II), diamine (2,2’-bipyridil) and a-amino-b-oxyaminocarbonic acid (threonine). There was proposed oxygenation mechanism for systems involving oxyaminoacids taking into consideration the peculiarities  of oxyaminoacids to behave as bidentate and tridentate ligands. It should be mentioned that interest to such complexes which have been studied thoroughly is determined by the fact that these systems may be considered as active centers for breathing pigments and as intermediary stages in oxidation processes [2 - 4].

       The detailed studies on complexation and oxygenation processes based both on pH-metric, manometric and spectrophotometric studies on some quantitative kinetic characteristics of oxygenation processes have been extended to the other  mixed  ligand  system  involving   cobalt(II), diamine (2,2’-bipyridil)  and serine  that confirms proposed previously mechanism of oxygenation process [1].

Experimental Section. The chemicals used were of reagent grade (D,L-Serine, Co(NO₃)₂·6H₂O, 2,2’-bipyridile (dipy), 0,1 N KOH free from carbonates). All pH-metric equilibrium determinations were performed in a jacketed titration cell at 25 ± 0.1 ºC. The ionic strength of each solution was adjusted to 0.10  M by the addition of KNO₃. The temperature of the cell was maintained by thermostat having accuracy of  ±  0.1 ºС. Kinetic data were obtained on the basis of spectrophotometric stopped-flow studies and  a recording spectrophotometer “Specord M 40” having thermostat equipped with a solution mixing device for  kinetic studies. All the calculations were done with the aid of the computer program described in [5] on a computer of type IBM PCAT on the basis of   Neilder and Mid’ s method [6] with using fixed the acid dissociation constants of the ligands and the stability constants for binary and ternary species estimated under an inert atmosphere under identical conditions  as non-refinable parameters [7]. Investigation on complexation and oxygenation procrsses were conducted on the basis of pH-metric  studies (Ionometer EV-74 ) at 25 ºC and ionic strength 0.1 (KNO₃).

Results and Discussion.  In earlier  studies [1], for mixed ligand system Co(II)-dipy-Thr on the analysis of experimental manometric studies (mol O₂ /mol Co) it was inferred that oxygenation process in mixed ligand systems with oxyaminoacids proceeds by a way including formation and oxygenation of mixed ligand hydroxodimer in which coordinated threonine anion behaves in a bidentate manner (the experimental  manometric curve becomes independent on pH in the range corresponding adding approximately more than 1.6 equivalents of  KOH /mol Co – 1 equivalent of base is used for titration of –NH₃⁺-group, and  half an equivalent - for formation of hydroxodimer). It has been shown in [8, 9] that a such half – integer number of base per mol of Co is characteristic for hydroxodimer formation prior to dioxygen binding.  At the same time, the inflection point observed at a value of 2.0 equivalents of base/mol Co for our systems allows to conclude that titration of OH-group of coordinated  anion of threonine occurs and binuclear oxygenated dibridged cobalt complexes become monobridged oxygenated species (as a result of titration of OH-group) in which oxyaminoacid anions behave in a tridentate manner.

       As mentioned in [1], the entire process can be described by a multiple-equilibria scheme  involving deprotonation of  NH₃⁺-group of oxyaminoacid, formation of binary oxyaminoacid,   bisdiamine     and   mixed  ligand  complexes of cobalt(II), formation of hydroxodimers and oxygenation of both  diamine and mixed ligand diaminoxyaminoaid complexes, and as a result of deprotonation of OH-group in binuclear oxygenated dibridged complexes formation of binuclear monobridged oxygenated  cobalt(II) complexes. Thus the process of complexation and oxygenation process in mixed ligand systems with oxyaminoacids in aqueous solutions is described by the following equilibrium scheme (1-20):     

in which LH^- - bidentate ligand of oxyaminoacid (serine, threonine). Mathematical model of complexation and oxygenation  processes is represented in [1] as well.

       In order to understand more closely above-mentioned   mechanism of oxygenation process and to obtain kinetic evidence for the process   spectrophotometric (including stopped-flow) studies were presented for Co(II)-dipy-Ser system which are in accord with the proposed mechanism of formation of hydroxodimer and its further oxygenation followed by deprotonation  of OH-group that leads to formation of binuclear oxygenated monobridged cobalt complex.

       To simplify the procedure for studies kinetic characteristics of oxygenation process were observed in the presence of excess of serine that allowed not to take into account oxygenation of bisdiamine complex  in the system due to insignificant amount of the latter under  conditions employed.  Specrtrophotometric data of oxygenation process studies  in mixed ligand Co⁺² –dipy - Ser system are represented in Table 1.

       When an excess of initial complex was added deoxygenation rate

 (k_-1 [(CodiamLH)₂)OHO₂⁺] (diam – 2,2’-bipyridil)  might be ignored so that oxygenation rate  of binuclear oxygenated dibridged complex (CodiamLH)₂)OHO₂⁺ formation may be described as follows:

   W _x =  k₁ [(CodiamLH)₂)OH⁺]   [O₂] = k_ef  [O₂],                                                                             (21) 

and the rate  constant  should  be  the  pseudo-first-order  under  conditions employed in study.

     Taking into consideration that

                                                    K₁₄ K₁₅

[(CodiamLH)₂)OH⁺]    =         ^{--------------------     [(CodiamLH+] 2                                                          (22)}    

                                                        [H⁺]

k_ef  may be described as

                                    [(CodiamLH⁺] ²

k₁ K₁₄ K₁₅    =     --------------------------------     =      k_ef                                                                        (23)

                                           [H⁺]

                                                                                                                                        Table 1

           Spectrophotometric  data of oxygenation process studies  in the system Сo²⁺- dipy – Ser   at component ratio   C_Co : C_diam : C_Ser =1:1:2,  t = 25  ºC, l = 385 nm (using “Specord M40”   and stopped-flow studies)

Thus if the oxygenation process includes formation and oxygenation of hydroxodimer which is the predominant in the observed pH range (and this is  the rate-determining step for the process since deprotonation of OH-group and formation of binuclear monobridged form is assumed to be rapid reaction) the constant  value of  k_ef  [H⁺] / [(CodiamLH⁺] ²  (Table 2)  that is equal constants product  k₁ K₁₄ K₁₅ should be expected.

       First order rates calculated for the spectrophotometric data (Table 1)  by plotting log (D_∞ - D₀)/D_∞ - D) vs. time, where D represents optical density (at a given wave length, and D_∞ is the observed absorbance at equilibrium after the system came to an apparent equilibrium. Good first-order rate plots were obtained over 65 –75% reaction.

       The data recorded in Table 2 show that values of   k₁ K₁₄ K₁₅  (equal k_ef  [H⁺] / [(CodiamLH⁺]²) obtained on the basis of k_ef  (as first order) and value of [(CodiamLH)₂)OH⁺]           (described      according      to    eq. (22)     in     values    of [(CodiamLH⁺] ²  and [H⁺] concentrations are in agreement with the proposed above oxygenation mechanism for these systems.

    The value of k₁ estimated on condition of all these accepted for studies assumptions is  22.36 ± 0.3 l/mol ^-1 c^-1 that is in accord with those obtained for formation of binuclear dibridged oxygenated diaminaminoacid cobalt(II) complexes involving two bidentate ligands  [12] .

       Thus all this may be considered as additional evidence based on spectrophotometric studies  (performed at different pH values and different cobalt concentrations) that confirms above-mentioned  mechanism of oxygenation process which includes formation and oxygenation of hydroxodimer.

Fig. 2. Cobalt distribution curves as function of pH in presence of oxygen for mixed liganf system Co(II)-dipy-Ser at component ratio С_Сo : C_dipy  : C_Ser = 1:1:1; C_O2 = 2,5· 10^-4 mol/l;   t = 25 ˚C;  μ = 0,1 (KNO₃).   1 - Co²⁺;  2 - CoLH⁺;   3 - Co(LH)₂;  4 - Codipy²⁺;  5 - Codipy₂²⁺;  6 - Codipy₃²⁺; 7 - (Codipy)₂OHO₂³⁺;  8 - CodipyLH⁺;  9 - CodipyLHOH; 10 - (CodipyLH)₂OH⁺;  11 - (CodipyLH)₂OHO₂⁺;

12 - (CodipyL)₂O₂;  13 - LH₂; Codipy₂LH⁺, (Codipy₂)₂OH³⁺ (<0,02); Codipy(LH)₂,  Codipy₂OH⁺  (~0,002). 

The experimental data of manometric and pH-metric studies on oxygenation process in mixed ligand system Co(II) – dipy- Ser in aqueous solution are represented in Fig. 1 and Table 3, and all equilibrium constants are recorded in Table 4.

       In comparison with [1], formation constants  for oxygenation process in Сo²⁺- dipy – Ser system are somewhat less than those obtained for   Сo²⁺- dipy –Thr system. That is due to the fact that donor characteristics of threonine are somewhat greater  that leads  to enhanced stability of formation constants of threonine complexes for   Сo²⁺- dipy –Thr system.

       Based on the values of K₁ - K₁₀, K₁₄ - K₂₀ predicted curves of oxygen absorbed in mixed ligand  Сo²⁺- dipy – Ser system agreed with the  manometric studies data within the uncertainty of the experiments (Fig. 1) that confirms adequacy of the proposed oxygenation mechanism to the real process.

              Cobalt distribution curves as a function of pH for Сo²⁺- dipy – Ser system are represented in Fig. 2. It should be mentioned the same qualitative features in the amounts of different species in comparison with those described in  [1]. The content of binuclear oxygenated dibridged cobalt complex  (CodipyLH)₂OHO₂⁺   was found  to have maximal value at pH ~ 9 (a ~ 0.25). As a result of deprotonation of OH-group these dibridged oxygenated complexes become binuclear monobridged species with content increasing at pH > 9 that accompanied  in decreasing amount  of   (CodipyLH)₂OHO₂⁺   form respectively. At pH >11 there was found one oxygenated form    ( CodipyL)₂O₂  in which anion of serine is coordinated in tridentate manner. The other oxygenated binuclear dibridged bisdipyridil cobalt(II) complex is characterized by insignificant outcome with maximal value (a~0.02) at pH = 8.25.

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