Mixed ligand complex formation of Fe III with boric acid and typical N-donor multidentate ligands

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1 Proc. Indian Acad. Sci. (Chem. Sci.), Vol., No., June 00, pp 6 7 Indian Academy of Sciences Mied ligand comple formation of III with boric acid and typical N-donor multidentate ligands. Introduction G N MUKERJEE* and ANSUMAN DAS Department of Chemistry, Calcutta University, University College of Science, 9, Acharya Prafulla Chandra Road, Kolkata , India gurunath@cucc.ernet.in MS received October 00; revised 5 April 00 Abstract. Equilibrium study of the mied ligand comple formation of III with boric acid in the absence and in the presence of, -bipyridine,,0-phenanthroline, diethylenetriamine and triethylenetetramine (L) in different molar ratios provides evidence of formation of (O), (O), (L), ( BO ), (O)( BO ), (O) ( BO ), (L)( BO ) and (L) (BO ) complees. (L), (L) ( BO ) and (L) (BO ) complees are also indicated with, -bipyridine and,0-phenanthroline. Comple formation equilibria and stability constants of the complees at 5 ± 0 C in aqueous solution at a fied ionic strength, I = 0 mol dm (NaNO ) have been determined by potentiometric method. Keywords. Mied ligand- III -borate complees; comple equilibria; N-donor multidentate ligands. Enhancement of acidity of boric acid ( BO ) in presence of cis-,-diols, cis--hydroy acids and cis-,-dicarboylic acids due to the formation of chelated structures with tetracovalent electron deficient B III is well-known. Acidity of aqueous solution of BO is found to be enhanced in the presence of cis-diaqua Co III complees: cis- [Co III (N )( O) ], where, (N ) = (, -bipyridine) or (,-diaminopropane) or triethylenetetramine, due to the formation of mononuclear () and binuclear () mied ligand borate complees through pseudo substitution reactions, in which Co O bonds remain intact and substitution of Co O moiety takes place on the electron-deficient B III centre with release of the proton. O O () (N )Co B () O O (N )Co O O B O O Co(N ) Acidity of aqueous solution of BO is also enhanced in the presence of coordinately unsaturated labile complees, [M II (L)( O) ], M = Co, Ni, Cu and Zn, L = (N ), (N ) amine formed in situ in ternary M II -L- BO mitures. With the view to studying the *For correspondence 6

2 6 G N Mukherjee and Ansuman Das effect of coordinately unsaturated [ III (amine)( O) ] complees on the acidity of BO, comple formation equilibria of III with BO in the absence and in the presence of, -bipyridine (bipy),,0-phenanthroline (phen) (hereafter, L ) and diethylenetriamine (den), triethylenetetramine (trien) (hereafter, L ) have been investigated by potentiometric methods using different molar ratios of the reactants III, (L or L ) and BO in aqueous solution at a fied ionic strength I = 0 mol dm (NaNO ) at 5 ± 0 C. Stoichiometry and the formation constants of the complees and comple formation equilibria have been evaluated by a computerised method.. Eperimental Boric acid, bipy, phen, sodium nitrate, nitric acid were of AR grade. Den and trien (AR) were converted into their corresponding nitrate salts, den.no and trien.no respectively, air-dried and analysed 5. rric nitrate solution was prepared by dissolving freshly precipitated, alkali-free (O) in standard NO and standardised by combined acid base, ion echange and compleometric titrations 5. The equilibrium study involved p-metric titrations in aqueous medium of a series of solutions (0 05 dm ) containing known amounts ( mol dm ) of BO and/or L (L = bipy, phen, den and trien in their protonated forms) and known amount (0 0 mol dm ) of free NO, in the absence and in the presence of known amounts ( mol dm ) of III -nitrate, keeping the III : L: BO ratios ::, ::, :: and :: as required, with carbonate-free 6 standard 0 mol dm NaO solution, maintaining a fied ionic strength, I = 0 mol dm (NaNO ). Ionic product of water at the eperimental temperature and activity coefficient of ion at the eperimental ionic strength were obtained from literature 7,8. Analytical concentrations of ion corresponding to the p-meter readings were obtained by the usual procedure 9. Formation constants (table ) were calculated with the aid of the SCOGS computer program. Comple formation equilibria were elucidated with the help of speciation curves.. Results and discussion. Proton-ligand equilibria In the p-range of investigation bipy and phen up to two protons, den bind up to three and trien binds up to four protons (table ). BO titrates as a weak mono- basic acid ( pk BO = 9 00) in aqueous solution due to its ionisation, not as a proton donor, but as a Lewis acid, accepting an O according to equilibrium () 0 : BO O B(O) O, () BO = ( K [ O] [B(O) ])/[ BO ]. (a) Charges are not shown in the epressions for clarity.. ydrolytic equilibria of III III is so etensively hydrolysed in aqueous solution that (aq) hardly eceeds 5% even at p <, in as low as 5 0 mol dm concentration.

3 III L BO mied ligand comple equilibria 65 Table. Formation constants* of mied ligand III L borate complees with L = bipy, phen, den and trien in aqueous solution. I = 0 M (NaNO ); Temperature = 5 C (a) ydrolysis constants of (aq) ions (b) Formation constants of III BO complees (i) Overall formation constants ( BO ) logβ 67 (O)(BO ) logβ 6 (O) (BO ) logβ 7 (ii) Effect of coordination with (aq), (O) and (O) BO on ionisation of [ BO O] log K [(O) BO O] log K [(O) BO O] log K ( ) ( ) 0 89 Ligands (L) Constants bipy phen den trien (c) Proton ligand constants L L L L 56 (d) III L binary constants (i) Overall formation constants L logβ (L)(O) ( ) logβ (L)(O) ( ) L logβ (L) (O) 7 6 ( ) (L) (O) logβ ( ) 07 (Continued )

4 66 G N Mukherjee and Ansuman Das Table. (Continued) Ligands (L) Constants bipy phen den trien (ii) ydrolysis constants of (L) (L) (L) (L) 7 9 (L) (e) III L BO ternary constants (i) Overall formation constants logβ logβ logβ 8 8 logβ (ii) Effect of coordination with (L) and (L) on ionisation of BO [(L)(O) BO ] [(L)(O) BO ] 8 6 [(L) BO O] [(L) BO O] [(L) (O) BO] [(L) BO O] [(L) BO O] [(L) (O) BO] *Limits of error in the constants: ± ( ) in log scale ( ) O (O), () K = ([(O) ][] )/[], ( =,, ). (a) The predominant III species in the range p ~, are (O) and (O). The concentration of (O), is negligibly small. Although III has a profound tendency to form polynuclear µ-dioo species, concentrations of such species in the eperimental concentration range are negligible.

5 III L BO mied ligand comple equilibria 67. III BO binary equilibria In the presence of III, the buffer region corresponding to the ionisation () of BO is shifted to lower p regions due to the formation of ( BO ), ( BO )(O) and ( BO )(O) complees according to ( ) (O) BO O (O) ( BO ), ( = 0,, ). () p measurements, however, could not be etended above p due to commencement of precipitation, probably of electroneutral (O) and/or ( BO ), which respectively constitute ~ 0% and 5% of total III. Formation constants of [(O) ( BO )] complees may be defined according to B(O) O (O) ( BO ) ( ), () (O) (BO ) β = ([(O) ( BO )][] () )/([][B(O) ]). (a) (O) (BO ) β values may be obtained as computer output, from which the equilibrium constants of the reactions () may be calculated using [(O) BO BO ] = logβ (O) ( BO ). (a) Speciation curves of III - BO system also indicate the formation of the [(O) ( BO )] complees according to the reproportionation equilibria, ( ) (O) ( BO ) (O) ( BO ), (5) for which the ( ) values may be calculated using (O) ( BO ) (BO ) ( ) = log β logβ. (5a) Predominant III species in this system are (O), (O) and (O) ( BO ). Percentages of ( BO ) and (O)( BO ) are very small in the entire p range studied. Negative values of ( ) (table ) indicate much less relevance of the reproportionation equilibria (5) as compared to that of equilibria ().. III (L /L ) binary equilibria (L = bipy, phen; L = den, trien) Predominant III species at lower p (~ ) values in III L BO systems are (L )(aq), which with rise of p undergo hydrolysis to produce (L ( ) )(O) according to ( ) (L ) O (L )(O), (6) K = ([(L )(O) ][] )/[(L )], ( =,, ). (6a) (L )

6 68 G N Mukherjee and Ansuman Das The overall formation constants ( β ) of these ternary hydroo complees, (L )(O) (L ( ) )(O), defined according to ( ) L O (L )(O), (7) β = ([(L )(O) ][] )/([][L ]), (7a) (L )(O) may be obtained as computer output, from which the hydrolysis constants, K, may (L ) be calculated using the relations K (L ) (L )(O) (L ) = log β log, (7b) where to K is the stability constant of the binary (L ) comple, defined according (L ) L (L ), (8) K = [L ]/([][L ]). (8a) (L ) In the range p, the ligands L (den and trien) eist in their protonated (L y y ) forms (y =,,, ). : binary (L )(aq) complees are formed according to (aq) L y y (L )(aq) y, (9) K = [L ]/([][L ]). (9a) (L ) Speciation curves of these systems also indicate reproportionation equilibria of the type (5), for which the ( ), ( =, ) values corresponding to (O) (L) ( ) complees may be calculated using relations of the type (5a) substituting K (L = L, (L) L ) for β ( BO ). While the ( ) values for (O) ( BO ) are more negative than the statistical values, those of (O) (L ) ( ) are found to be less negative than the statistical values or even positive (table ). Since both O and BO are σ-basic only ligands, whereas, L are σ-basic and π-acidic ligands, formation of ternary (O) ( BO ) complees are disfavoured due to electronic repulsion arising from ( BO ) O σ-bonding, whereas, the formation of (O) (L ) ( ) complees are favoured due to (L ) π-back bonding, which lowers electron electron repulsion arising from L O σ-bonding and strengthens the L - bonds by synergistic effect. ( ) values (table ) indicate slightly higher π-acidity of phen over bipy in these complees. The difference between the hydrolysis constants of the uncompleed (aq) ion and that of the compleed (L)(aq) ions ( K and K (L) respectively) on the log scale (table ) gives a measure of the difference in the acidity of coordinated O in free and compleed III ions and also the relative strengths of the L III (σ)- and L III (π)- ( ) bonds in these (L)(O) complees. It is observed that coordination of σ-basic only ligands (L ), viz., den (N -σ) and trien (N -σ) to III lowers the acidity of the coordinated O. On the other hand, in the coordination of σ-basic and π-acidic ligands

7 III L BO mied ligand comple equilibria 69 (L ), viz., bipy, phen (N -σ, π) and (bipy) and (phen) (N -σ, π), although the acidity of the coordinated O tend to increase, their π-acidic effects cannot outweight their σbasic effects. As a result, acidity of the O ligands coordinated to III -L complees are in the order: (aq) > (N -σ, π)(aq) > (N -σ, π)(aq) (N -σ)(aq) > (N - σ)(aq)..5 III L BO general equilibria Comple formation equilibria of III with BO in the presence of another ligand, L, in aqueous solution may be described according to the general equilibria (0): p ql rb(o) [ p(l) q ( s BO ) r ] (prs 5r) r( s), (0) ( p rs 5r) β pqsr = ([ p (L) q ( s BO ) r ][ ] r( s) )/([ ] p [L] q [B(O) ]r ), (0a) where p, q and r are stoichiometric numbers of, ligand L and BO 5 group and s is the number of ions bound to the BO 5 groups in the formula of the comple. The borate ion, B(O) is derived from the ionisation () of BO, in aqueous solution. β pqsr may be obtained as computer output along with the species distribution data for elucidation of the comple formation equiibria..6 III L BO ternary equilibria (L = bipy, phen) In both :: and :: III : L : BO systems, the concentrations of the (L ) species are small from the very beginning of the reactions (p ~ ), in which (L )(O) and (L )(O) are the predominant III species. The mied ligand III - borate complees, (L )(( BO ) and (L )(BO )(L ) are formed according to the following equilibria at p values above 5. (L )(O) BO [(L )( BO )] O, () (L )(O) BO (L ) [(L )(BO )(L )] O, () (L )(O) BO [(L )(BO )(L )] O, () (L )(O) BO (L ) [(L )(BO )(L )]. () Binuclear [(L )(BO )(L )] complees are formed from the very beginning of the reactions and represent the most predominant III species in the :: III : L : BO systems. The overall formation constants, β and β 0, of [(L )( BO )] and [(L )(BO )(L )] complees, defined as below, β = ([(L )( BO )][] )/([][L ][B(O) ]), (5) β 0 = ([ (L ) (BO )][] )/([] [L ] [B(O) ]), (6) may be obtained as computer output along with the speciation data of these complees.

8 70 G N Mukherjee and Ansuman Das 00 (a) concentration(%) (b) p Figure. Speciation curves for III bipy BO systems. III : bipy: BO = (a) : : and (b) ::; () BO, () (bipy), () (bipy) (O), () (bipy) (O), (5) (bipy) ( BO ), (6) (bipy) (BO ). In the :: and :: III : L : BO systems (figure ) the predominant III species at the beginning (p ~ ) are (L ), (~ 95%) and (L ) (O), ( 5%). The dihydroo complees, (L ) (O), appear at p ~ and their concentrations hardly eceed 0% at the end of the reactions (p ~.5). The major mied ligand III -borate complees that constitute about 90% of III and p in the :: III : L : BO systems are (L ) ( BO ), formed mostly according to (7) and at least to some etent according to (8): (L ) BO O [(L ) ( BO )], (7) (L ) (O) BO [(L ) ( BO )]. (8) Concentrations of the binuclear [(L ) (BO )(L ) ] complees, however, hardly eceed ~ 5% in the :: III : L : BO systems. owever, in the :: III :L : BO systems, the mononuclear [(L ) ( BO )] complees appear to be minor species where the binuclear [(L ) (BO )[(L ) ] complees are formed mainly according to (9) and only slightly according to (0) and () at p : (L ) BO O [(L ) (BO )(L ) ] 5, (9)

9 III L BO mied ligand comple equilibria 7 (L ) BO (L ) (O) [(L ) (BO )(L ) ], (0) (L ) (O) BO [(L ) (BO )(L ) ] O. () Overall formation constants of [(L ) ( BO )] and [(L ) (BO )(L ) ] complees, defined according to () and () respectively, β = ([(L ) ( BO )][] )/([][L ] [B(O) ]), () β 0 = ([ (L ) (BO )][] )/([] [L ] [B(O) ]), () are obtained as computer output along with the speciation data of the complees. Using computer-refined values of β pqsr, the equilibrium constants of the comple formation reactions () () and (7) () may be calculated with the aid of the relations (a) (a) and (7a) (a): = log β log β, (a) [(L )(O) BO ] BO (L )(O) [(L ) (L )(O) BO] = logβ 0 log β (L )(O) BO, (L ) (a) = log β log, (a) 0 BO β [(L )(O) BO ] (L )(O) [(L ) (L )(O) BO ] = log β 0 log β (L )(O) BO, (L ) (a) = logβ logβ, (7a) [(L ) BO O] BO (L ) = logβ log, (8a) BO β [(L ) (O) BO] (L ) (O) 5 = logβ log, (9a) 0 BO β [(L ) BO O] (L ) [ (L ) (L ) (O) BO] = logβ 0 log β (L ) BO logβ (L ) (O), (0a) = logβ log. (a) 0 BO β [(L ) (O) BO ] (L ) (O)

10 7 G N Mukherjee and Ansuman Das.7 III L BO ternary equilibria (L = den, trien) Binary (:) III : L complees are formed in both in the :: and :: III : L : BO systems from the very start of the reactions (p ) according to (9). Concentrations of ( BO ), (O)( BO ) and (O) ( BO ), (O)(L ) and (O) (L ) are found to be negligible. The dominant III species at the early stages of the reactions in these systems are (L )(aq), (O)(aq) and (O) (aq). Therefore, ionisation of BO at lower p ( 5) values in the presence of III and L ligands is obviously due to the formation of mied ligand III L BO complees according to the following equilibria as the speciation curves (figure ) imply. (O) L y y BO [(L ) ( BO )] (y ) O, () (L ) BO O [(L ) ( BO )], (5) (L ) BO O [(L )(BO )(L )] 5, (6) (a) 60 0 concentration(%) FM 5 8,9 0,, 7 6 (b) FM 5 8,9 0,, p Figure. Speciation curves for III trien BO system: III : trien: BO = (a) :: and (b) ::; () BO, () trien, () trien, () (O), (5) (O), (6) (O), (7) (trien), (8) (trien)(o), (9) (trien)(o), (0) ( BO ), () ( BO )(O), () ( BO )(O), () (trien)( BO ), () (trien) (BO ).

11 III L BO mied ligand comple equilibria 7 (L ) (L ) ( BO ) [(L )(BO )(L )]. (7) Equilibria () (6) are common to both :: and :: III : L : BO systems, whereas the equilibrium (7) is identifiable only in the :: systems. The overall formation constants, β and β 0 of the complees [(L )( BO )] and [(L )(BO )(L )] respectively, may be obtained as computer output, from which the equilibrium constants of () (7) may be calculated using the relations (a) (7a) below respectively. ( y ) [(O) L y BO ] BO y L y = logβ, (a) BO K [(L ) BO O] (L ) = log β log, (5a) 5 0 BO K [(L ) BO O] (L ) = log β log, (6a) [(L ) (L )( BO )] 0 K (L ) = log β log β log. (7a) Equilibrium (7) is a unique one of its kind as the equilibrium constant K [(L ) (L )(BO )] gives a measure of the coordination tendency of the coordinated ( BO ) ligand species to (L )(aq) ion having two O ligands in cis-positions. The effect of coordination of borate species with (aq), (L-σ) and (L-σ, π) on the enhancement of acidity of BO may be compared on the basis of the formation constants (table ) of III -borate and III -L-borate complees from the reactions of BO with the corresponding III -species. As a matter of fact, the enhancement of acidity of BO due to the formation of mied ligand borate complees: (L ) z ( BO ) and (L ) z (BO )(L ) z, where z =,, is much higher than that due to the formation of simple binary ( BO ) comple, because of (L ) π-bonding. owever, this effect is not observed due to the formation of ternary (L )( BO ) and (L )(BO )(L ) complees because of the absence of such π-bonding with these ligands. The difference between the two constants, (a) and [(L ) (O) BO ] [(O) BO ], (a) shows a tremendous enhancement of acidity of BO (by a factor of ~ 0 ) due to its reaction with (L ) (O) relative to (O). In this regard the present III -amine complees may be arranged in the order: III (bipy) z > III (phen) z III (den) > III (trien); z =, and complees with (z = ) > those with (z = ). Although the binary III (phen) z, z =, complees are relatively more stable than their bipy analogues, (bipy) z ( BO ) and (bipy) z (BO )(bipy) z are more stable than their phen analogues. Bipy because of free rotation about the C C bond, can adjust much better than phen to the tetrahedral bond angles imposed on III by the borate ligands BO 5 and BO. Due to the structural rigidity of phen, the chelate rings of (phen) z ( BO ) and (phen) z (BO )(phen) z are strained at the bond angles around III, consequently, the phen complees are less stable than their bipy analogues. The etent of metal to ligand π-bonding and hence the stability is obviously higher for the (L ) and (L ) (O) complees than those with (L ) and (L )(O)

12 7 G N Mukherjee and Ansuman Das complees (table ). Moreover, III in (L ) and (L ) (O) may eist in low spin 5 (t g ) states, and consequently act as a better π-electron donor than the high spin ( t g e g ) in (L ) and (L )(O) complees. This is also consistent with the order: III (N - σ, π) > III (N -σ, π) III (N -σ) > III (N -σ) with regard to the effectiveness of the present III -amine complees in enhancing the ionisation of BO through mied ligand borate comple formation.. Conclusions Labile (L)(aq) complees, L = bipy, phen (N ), den (N ); (bipy), (phen), trien (N ), formed in situ, are structurally compatible with,-cis-diols, like the substitution inert cis-[(n )M III ( O) ] complees (M = Co, Cr), with regard to the enhancement of acidity of boric acid in aqueous solution due to the formation of mononuclear and binuclear mied ligand III -borate complees of the types [(L)( BO )] and [(L)(BO )(L)] respectively, in which the electron deficient B III attains tetracovalency through chelation. The binuclear [(L)(BO )(L)] complees are found to be more stable as compared to the mononuclear [(L)( BO )] complees. The π-acidic nature of the ligands (L) increases the acidity of the coordinated O molecules in III (L)(aq) complees, which in turn, enhances the acidity of BO to a greater etent than those without having metal to π-bonding. Acknowledgement AD is grateful to the Council of Scientific and Industrial Research, New Delhi for a fellowship. References. Boeseken J and Vermaas N 9 J. Phys. Chem Mukherjee G N and Das A 00 J. Indian Chem. Soc (a) Mukherjee G N and Das A 000 J. Indian Chem. Soc. 77 6; (b) Mukherjee G N and Das A 00 J. Indian Chem. Soc (a) Sayce I G 968 Talanta 5 97; (b) Sayce I G 970 Talanta 8 65; (c) Sayce I G and Sharma V S 97 Talanta Kolthoff I M, Sandell E B, Meehan E J and Brukenstein S 967 Quantitative chemical analysis (London: Macmillan) ch. 6. Schwarzenbach G and Biderman W 98 elv. Chim. Acta 7. Wolleym E M, urkot D G and epler L G 970 J. Phys. Chem arned S and Owen B B 958 Physical chemistry of electrolytic solutions (New York: Reinhold) 9. Irving M, Milles M G and Pettit L D 967 Anal. Chim. Acta Cotton F A and Wilkinson G 988 Advanced inorganic chemistry 5th edn (New York: Wiley Interscience). Flynn C M (Jr) 98 Chem. Rev. 8. Sigel 975 Angew. Chem., Int. Ed. Engl. 9