Banerjea, D.

Basak, A. K.

English

NOPR

Indian Journal of ChemistryVoL 14A, March 197', pp. 184-188Studies on Complex Formation by Di/z-pyridyljamine &2-Aminopyridine with Metal Ions in Aqueous Solution: Evaluation ofEnthalpies & Entropies of Complex Formation with Mn(II), Co(II),Ni(II), Cu(II), Zn(II) & Cd(II) & Ligand Field StabilizationEnergies of the Metal ComplexesA. K. BASAK & D. BANERJEA *Department of Chemistry, Calcutta University, Calcutta 700009Received 20 August 1975; accepted 6 October 1975The formation constants of the complexes forrned by di(2-pyridyl)amine and 2-aminopyridinewith Mn(II), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) in aqueous solution have been evaluatedpH-Illetrically at different temperatures in the range of 25-40°, following the rnethod of Rossottiand Rossotti and from these data the corresponding values of enthalpies and entropies of COIll-plex forrnation have been evaluated, as well as the ligand field stabilization energies of theCo(II), Ni(II) and Cu(II) complexes following the Illethod of George and McClure. The resultshave been compared with literature data on the complexes of 2,2'-dipyridyl. Di (2-pyridyl)-amine has been found to forrn rnuch more stable complexes than 2-aIllinopyridine and thusresembles 2,2'-dipyridyl. 2-Atninopyridine also forrns complexes which are rnuch more stablethan those of pyridine. Ligand field stabilization energy values indicate that the b.s-comptexformed by Cu(II) with di(2-pyridyl)aIlline is of low tetragonality while the corresponding bis-complex of 2-aIllinopyridine has a high degree of tetragonal distortion.FROM structural considerations the complexingability of di(2-pyridyl)amine and a comparisonof its behaviour in this regard with thoseof 2,2' -dipyridyl and 2-aminopyridine appearedworthy of investigation. The formation constantsof the complexes formed by 2,2'-dipyridyl withMn2+, C02+, Ni2+, Cu2+, Zn2+ and Cd2+ ions arereported in the literature". The results of studieson the complexes formed by these metal ions withdi(2-pyridyl)amine and 2-aminopyridine are reportedin this paper.Materials and Methods •Di(2-pyridyl) amine, ClOH9N3 (Reilly Tar &Chemical Corp., USA) was purified before use byrecrystallization from hot ethanol and its puritychecked by its melting point (95°).2-Aminopyridine, C5H6N2' a pure product obtainedfrom E. Merck was used. Its aqueous solution(,.....,0·03M) was prepared and standardized by PH-metric titration with acid.Stock solutions containing the metal ions wereprepared by dissolving the corresponding nitrates(GR, E. Merck or AR, BDH) in doubly distilledwater containing a small amount of reagent gradeHN03 (freed from nitrous fumes by aeration afterdilution with water as usual). The solutions werestandardized by usual methods for the estimationof the metals. From a knowledge of the metalconcentration the amount of the free acid in thesolution was ascertained by eluting a known volume*Author to whom all correspondence to be made.184of the solution through a strong cation exchanger(Dowex 50WX-8 in the H+ form) and titratingthe HN03 in the effluent with standard alkali asusual.All other chemicals used were of reagent quality;distilled water redistilled with a little KMn04 andKOH in an all-glass still was used in making allthe solutions which were stored in Jena glass vessels.A Philips pH-meter, PR 9405M, was used forPH measurements (accuracy better than ± 0·01)using glass and calomel electrodes. The set wasfirst calibrated before use with solutions of known[H+], so that from the measured PH values thecorresponding [H+J could be ascertained accurately.Ionic strength of the experimental solution wasmaintained constant at O·lM with requisite quantityof KN03 ([KN03J+[HN03J= O·lM).Evaluation of equilibrium constant for the proto-nation of the ligands and the formation constants ofthe complexes - The equilibrium constants for theprotonation of the ligands were evaluated graphi-cally (d. Figs. 1 and 2, for data at 25°) frommeasurements of pH values of solutions containingthe ligand and HN03 in different proportions in themanner described earlier>. The results indicatethat both the ligands add only one proton withpractically no tendency to add a second protonin weakly acidic solutions.From the measured pH values of the solutionscontaining known concentrations of a metal ion,a ligand and HN03 the value of the formationfunction ii, i.e. the average number of moles ofthe ligand bound per mole of the metal ion, could~ ,Bf------."l:\~Zl~1I21~6---6-6~:8A~ j<l!~:t·= 0 " 0 j/"0 1-5(2-;;.)[I-I'J 6>---....:..:...:--. 10(/-iiL)Fig _ 2 - Graphical evaluation of protonation constant ofligand (25°; (.I., O·lM) [(A) Di(2-pyridyl)amine, 0·005 M and(B) 2-aminopyridine, 0'01M]BASAK & BANERJEA: Di(2-PYRIDYL)AMINE & 2-AMINOPYRIDINE COMPLEXES OF BIVALENT METALS1·00810'610-/10-2 A..JIt::],00'80-6"'l 80'2, I ,30 4'0 5'0 6'0\\\ j\17'0 8'00-5TABLE 1 - LIGAND PROTONATION CONSTANTS ATDIFFERENT TE11PERATURES([J.= O'lMjLigand log kt at25° 30° 35° 40°6'87 6·79 6·76 6-736'89 6·66 6·48 6·285'33(ref. 1,TableNo. 275)4·44(ref. 1,)Di (2-pyrid yl)amine2·AminopyridinePyridine-to'][;,']J iff. 1 - Protonation of the ligands (25°; (.I., O'lM) [(A) 2,2'-DipyridylDi(2-pyridyl)amine, 0'005M and (B) 2-aminopyridine, O'OlM]be evaluated". The formation curves for 25° areshown in Fig. 3 for the different metal complexesof both the ligands. From the ii values thus ob-tained for solutions having a particular concen-tration of metal ion and different concentrationsof a ligand the values of the overall formationconstants (~1 and ~2) were evaluated graphically(d. Figs. 4 and 5 for results at 25°) following themethod of Rossotti and Rossotti- as describedearlier",The nature of plots in Fig. 5 indicates that nodetectable amount of the tris or higher complexesare formed under the experimental conditions.Evaluation of thermodynamic parameters and ligandfield parameters energies - From the values of theformation constants (~1and ~2) obtained at differenttemperatures the enthalpy (!:iHx) and entropy(!:iSx) of complex formation were evaluated graphi-cally from the plot of log ~x (x = 1, 2) vs liT asusual. Using the !:iHx values, the values of theligand field stabilization energies, ~Hl and 8H2,for the mono- and the bis-complexes respectivelyof the transition metal ions were evaluated followingGeorge and McClure4• The !:iH~ (x = 1, 2) valueswhich represent the enthalpy changes in the for-mation of the complexes in solution from the gaseousmetal ion and the ligand in solution have beenevaluated from Eq. (1)!:iH~=!:iHh+!:iHxwhere !:iH h is thegaseous metal ion.... (1)enthalpy of hydration of the*Values better than ± 0·02.Results and DiscussionValues of the protonation constant, Kv of eachligand at four different temperatures are given inTable 1. The values of the formation constantsat four different temperatures are given in Table 2.It appears that although di(2-pyridyl) amineresembles 2-aminopyridine very closely in its basicity,it forms much more stable complexes than thelatter and in this respects it resembles 2,2' -dipyridyl.This seems to suggest that in the formation of metalion complexes of di(2-pyridyl)amine bonding fromthe nitrogen atoms of each of the two pyridinerings takes place as in the case of 2,2' -dipyridyl.Data in Table 2 further indicate that 2-aminopyri-dine forms more stable complexes than pyridine.The observed sequence of stabilities of both themono- and the bis-complexes of di(2-pyridyl)amineand the mono-complexes of 2-aminopyridine withdifferent metal ions of the first transition seriesstudied is Cu2+'>Ni2+>C02+>Zn2+'>Mn2+, whichis in agreement with the Irving-Williams naturalorder of stabilities", Since the value of the for-mation constant depends on the !:iH and !:is values(Table 3), the order of stabilities may show a devia-tion from the Irving-Williams sequence in somesystems. Indeed for the bis-complexes of 2-amino-pyridine the observed order of stabilities is Cu2+>Mn2+>Ni2+>C02+>Zn2+ and the following orderhas been observed earlier- in the case of the185IKDIAN J. CHEM., VOL. HA, MARCH 1976TABLE 2 - METAL-LIGANDFORMATIONCONSTANTSAT DIFFERENT TEMPERATURES({L = O·lM)r:ation Temp., Di(2-pyridyl)amine(a) 2-Aminopyridine(b) Pyridinetc) 2,2'-Dipyridyl(c)°C --------- --------log [31 log [3. log [31 log [32 log [31 log [32 log [31 log [32Mn2+ 25 1'78 (5'94) 2·19 (5-43) l'84(b) 3'45(d) 2-62 4-6230 1-64 (5'90) 2·07 (5,33)35 1·50 (5'86) 1·97 (5'22)40 1·40 (5'82) l·SS (5'06)Co'-\' 25 4-98 8·90 (8'91) 3·06 (4'87) l·14(e) l'54(e) 5·65 11·25-30 4·95 8·81 (8'79) 3·05 (4-81)35 4·92 8·71 (8'76) 3·03 (4'75)40 4·89 8·61 (8·63) 3·02 (4'70)Ni2+ 25 6'76 12·84 (13'05) 3'20 (5'36) 2-13(d) 3'79(d) 7-07 13·93-30 6'70 12·72 (12·93) 3·17 (5,19)35 6·64 12·62 (12'82) 3·14 (5'02)40 6·60 12·52 (12-64) 3·11 (4'84)Cu2-\' 25 7·60 14·40 (14·38) 5·43 (9-61) 2'46(d) 4·41 (d) 6·33 16·28-30 7·57 14·34 (14'30) 5·42 (9-44)35 7'53 14·29 (14'27) 5·40 (9'27)40 7·50 14'24 (14'20) 5·38 (9·11)In·+ 25 3·70 7·16 (7-27) 2·72 (4064) 2'08ld) 3'77(d) 5·16 8·930 3-62 7·07 (7·16) 2·69 (4'56)35 3'56 6'98 (7-06) 2·66 (4-45)40 3·49 6·90 (6'95) 2·63 (4'38)Cd2+ 25 2·89 (4-98) 2·54 (5'24) l'27(e) 2·07(e) 4·5 10·030 2·86 (4'86) 2·49 (5,19)35 2·83 (4'74) 2·44 (5'14)40 2·80 (4064) 2·38 (5'09)(a) This work: [31values from intercepts of Y1 vs Xl plot and [32values from intercepts of Y2 vs X. plots. [32values.given within parentheses were obtained from the slopes of the Y1 vs Xl plots (see ref. 2 and 3). In those cases wherefor practical reasons f! values much higher than 1 could not be reached within the permissible range of pH even withexcess of the ligand, the [32values obtained from the slopes of the Y1 vs Xl plots are only available. Accuracy of log [3x-values is better than ± 0·02. (b) At 25°; d. ref. 1, Table 275. (c) At 25°; cf. ref. 1. (d) At {L = l·OM. (e) At {L = 0·5M.TABLE 3 - VALUES OF ENTHALPY (t:.H, kcal mole'<) AND ENTROPY (t:.5, e.u) CHANGESAND LIGAND FIELDSTABILIZATIONENERGIES (3H, kcal mole-1)(a)Thermo- H+ Mn2+ C02+ NP+ Cu"-\' ln2+ Cd'+dynamicparametersDI-(2pYRIDYL)AMINEt:.H\~) -654 -697 -716 -716·9 -701-1t:..H1 -2·8 -10·9 -2,7 -4·4 -2,5 -5·6 -2,1t:.H~ -664·9 -699'7 -720,4 -719'4 -706'7t:.51 +22·1 -28·3 +13'9 +16'3 +26·6 -1·8 +6'23H1 0 18·1 30·5 21·1 0 0t:..H2-3-3 -8-5 -9·0 -4,8 -7-3 -10·0sn; -657·3 -705·5 -725 -721-7 -708-4ss, +16'2 +12·4 +28-9 +50-1 +8-4 -10-73Hz 0 27-7 37·0 23-4 0 02-AMINOPYRIDINEsu, -17-3 -9-1 -1-2 -2-3 -1-5 -2-6 -406sn; -663·1 -698-2 -718·3 -718·4 -703-7t:.51 -26-4 -20·4 +10·0 +7·0 +19'9 +3-8 -3-73H1 0 19·2 31-0 22·9 0 0t:.H2 -10·7 -4-9 -14'8 -14·3 -7-7 -4-2su; -664-7 -701·9 -730-8 -731·2 -708·8su, -10'9 +6-0 -25,0 -3-8 -4-5 +10·03Hz 0 20·7 40'4 31-7 0 0(a) t:.H" and t:..5" values reported here account for log [3" (and log K1) values at the different temperatures (25-40° ~to within ± 0·02 of the experimental values. _ __(b) The values of the enthalpy of hydration (t:..Hh) of the metal ions which were used in calculating the 3HJII"(x = 1, 2) values were obtained from ref. 4.186BASAK & BANERJEA: Di(2-PYRJDYL)AMINE & 2-AMINOPYRIDINE COMPLEXES OF BIVALENT METALS2'.."" 2-0 Mn'"~..."'t;1-(;N:,-::,t>~. 1-2..~....0S-It; 0"6ol),....."'c:; 0"4~ AIt;~2-D 3-0 8-0so- 10g[zJ6-0Fig. 3 - Complex formation curves (25°; [J.,0-IM) [(A) M2+ -di(2-pyridyl)amine systems:ligand, 0-005M; M2+, O-OOIM in all the casesexcept MnH, 0-00085M and (B) M2+-2-arninopyridine systems: ligand, O-OIM; M2+,0-OOlM in all the cases except Mn2+,0-00085MJ1'Z100-80'2.mon ocomplexes of these metal ions with hydrazine:Cu2+>Mn2+>Zn2+>Ni2+>C02+.From the !1H; values (Table 3) it is seen thatfor a particular series of complexes of the differenttransition metal ions the sequence of !1H; isMn2+<C02+ <Ni2+ ..•....Cu2+<Zn2+ which is in keepingwith the expected trend. It is also seen from thedata in Table 3 that of the mono- and the bis-complexes of di(2-pyridyl)amine and of 2-amino-pyridine, Ni2+ complexes have the highest 3Hxvalues (Table 3), with the values for the C02+ andCu2+ complexes being much less; the order beingNi2+~Cu2+>C02+ for the mono-complexes of boththe ligands and Ni2+~C02+>Cu2+ for the bis-complexes of di (2-pyridyl) amine, while for thebis-complexes of 2-aminopyridine the observedsequence is Ni2+~Cu2+~C02+.For perfectly octahedral weak field complexesof Ni(II), Co(II) and Cu(II) it is known that theligand field stabilization energy values are 12, 8and 6Dq respectively. However, due to Jahn-Teller distortion the complexes of Cu(II) are alwaystetragonal which further stabilizes this d9 system.In most cases because of this additional stabili-zation due to Jahn-Teller effect the overall stabili-j2,0 3'0 4'0 5-0 6-0 7'0 8-0-IOfj{l.] (Mnl,+CO~+Ni ~+cuz~ Znz+)& 2f/ofj [l.]] (Cd 2+)zation energies of complexes of Cu(II) are higherthan those of the corresponding complexes of Co(II).However, in the case of Cu(II) complexes of lowtetragonality, ligand field stabilization energy ofthe copper(II) complex may be nearly the sameas or even lower than that of the correspondingcobalt (II) complex. According to jorgensen" therelatively low value of Vcu/VNi = 1-2, where vCuand VNi are the wavenumbers of the principalligand field bands of the tris-dipyridyl complexesof Cu(II) and Ni(II) respectively, is indicative oflow tetragonality of Cu(dipy)~+ and in agreementwith this the order of 3H3 for the tris-dipyridylcomplexes is Ni2+~Cu2+ ..•....C02+, with the valuefor the Cu(II) complex being only 1 kcal mole!higher than that of the Co(II) complex+, Hence,the sequence which has been observed now forthe bis-complexesof di(2-pyridyl)amine indicateslow tetragonality of the bis-complex of Cu(II)with di(2-pyridyl) amine. However, for the bis-complexes of 2-aminopyridine the observed sequence,particularly the much higher 3H 2 value of theCu(II) complex compared to that of the Co(II)complex, indicates a tetragonal distortion of highdegree in this Cu(II) complex.187INDIAN J. CHEM., VOL. 14A, MARCH 1976J A,II1'6~<It'l:l1'2...x-:;I;1:: jBa·o 12-0 16'0Xl x/olfJ20'0 24.0r:::=' : Ie c~· I80 .- 0 1~4 6 .. A .2+6 • N, I""6-o~.~ I" I>: 401-'oF· 0 ~cu H• ~ • z,,2+I , , , I I0'5 10 1'5 2'0 2'5 3'0 3'5Xz)( 10'(;Fig. 5 - Graphical evaluation of 132 values of the metalcomplexes of di(2pyridyl)amine (25°; (1., O'IM) [M2+ andligand concen tratior.s as in Fig. 4X = (3-ii) [Ll. Y = n-(1-il)~1[Ll. p = 4 (Cc12+)2 (2-n) , 2 (2-n) [Ll2 ' ,7 (Zr;2+), 8 (C02+), 12 (Ni2+), 14 (Cu2+); q = 2 (Cd2+), 3(CoH, Zn2+), 5 (Ni2+), 6 (Cu'+)J188Fig. 4 - Graphical evaluation of formation constants of metal complexes(25°; (1., O'IM)X _ (2-n) [L]. y _ n1 - (1-n) , 1 - (1-n) [L][(A) M2+-di(2-pyridyl)amine systems:p = 3 (Mn2+, Cd H) , 4(Zn2+), 5 (C02+),7 (Ni2+), 8 (Cu2+); q = 4 (MnH, Zn2+),5 (C02+), 7 (Ni2-l'), 8 (Cu2+) 3 (Cd2+).(B) M-2-aminopyridine systems: p=3(Mn2+, C02+, Ni2+, Zn2+, Cd2+), 5 (Cu2+);q = 4 (Mn2-f), 3 (Co2+, Ni2+, Zn2+,Cd2+), 6 (Cu2+)]AcknowledgementThis investigation has been carried out withfinancial assistance from the CSIR, New Delh.The award of a research fellowship to one of us(A.K.B.) by the said council is thankfully ack-nowledged.References1. SILLEN, L. G., Stability constants of metal-ion complexes(The Chemical Society, London), spl. publication No.17, 1964 and No. 25, 1971, Table No. 783.2. BANERJEA, D. & SINGH, 1. P., Z. anorg . allgem. Chem.,349 (1967), 213.3. ROSSOTT!, F. J. C. & ROSSOTT!, H. S., Acta. chem, Scand.,9 (1955), 1166.4. GEORGE, P. & MCCLURE, D. S., Progress in inorganicchemistry Vol. 1, edited by F. A. Cotton (Iriterscience,New York), 1959, 428.5. IRVING, H. & WILLIAMS, R. J. P., Nature, Lond., 162(1948), 746.6. JORGENSEN, C. K., Absorption spectra and chemical bondingin complexes (Pergamon Press, Oxford), 1962, 124.

Studies on Complex Formation by Di(2-pyridyl)amine & 2-Aminopyridine with Metal Ions in Aqueous Solution: Evaluation of Enthalpies & Entropies of Complex Formation with Mn(II), Co(II), Ni(II), Cu(II), Zn(II) & Cd(II) & Ligand Field Stabilization Energies of the Metal Complexes

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Studies on Complex Formation by Di(2-pyridyl)amine & 2-Aminopyridine with Metal Ions in Aqueous Solution: Evaluation of Enthalpies & Entropies of Complex Formation with Mn(II), Co(II), Ni(II), Cu(II), Zn(II) & Cd(II) & Ligand Field Stabilization Energies of the Metal Complexes

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