Indian Journal of ChemistryVo\. 27 A, December 1988, pp. 1111-1113

Evaluation of Formation Constants ofPyridine Adducts of (Resacetophenone

oximato )nickel(II)

ARVIND T RANE* & DEEPAWATl R NEPALI

Tata Institute of Fundamental Research, Bombay 400 005

Received 22 December 1987; revised 7 March 1988; accepted24 March 1988

Changes in the absorbance of nickel chelate of resacetophe-none oxime are observed. on the addition of heterocyclicN-bases to a solution of the nickel chelate in cyclohexanone.This behaviour is attributed to the adduct formation of nickelchelate with pyridine or substituted pyridines. All the N-basesform monoadducts with the nickel chelate, except I, lO-phen-anthroline which forms a diadduct: The stabilities of the nick-el adduct increase in the following order of the bases: 3,5-luti-dine < 2,4,6-collidine < 3-picoline < 4-picoline < pyridine < 2,2'-bipyridyl < 2,9-neocuproin < I, lO-phenanthroline.

Resacetophenone oxime (2,4-dihydroxyacetophe-none oxime; abbreviated as RAPOX) has beenused as an analytical reagent in spectrophotometryfor several metal ions I.

Recently, cobalt-RAPOXimate complex was is-olated in solid state and a spectrophotometricstudy of the pyridine and substituted pyridine ad-ducts of the said complex was undertaken in mon-ophase system of cyclohexanone and formationconstants of the pyridine adducts were evaluated",The present investigation is an extension of ourearlier work".

2-Picoline, 3-picoline, 4-picoline, 2,4-lutidine,3,5-lutidine (all Fluka AG), pyridine and 2,4,6-col-lidine (both BDH, AR) were dried over KOH andpurified by distillation. o-Phenanthroline (FlukaAG), 2,2'-bipyridyl (Eastman) and neocuproin(2,9-dimethyl-1,1O-phenanthroline, BDH, AR)were used as such. Nickel ammonium sulphateand methanol (BDH, AR) were used for the pre-paration of nickel chelate complex.

The reagent RAPOX (CSH903N) was synthes-ized in the laboratory and recrystallised from etha-nol. Nickel-RAPOXimate was prepared as report-ed earlier".

ProcedureStock solution of nickel RAPOXimate was pre-

pared by dissolving requisite quantity of the nickelcomplex in freshly distilled cyclohexanone. Speci-fic volumes of cyclohexanone solution of the nick-el complex were pipetted into. standard volumetric

flasks contauung solutions of varying amounts ofthe adducting base in cyclohexanone and the vo-lume was adjusted to the mark with cyclohexa-none.

The absorption spectra were recorded on aCary 17D spectrophotometer in the range 500-650 nm employing 10 mm cuvettes.

The spectrum of Ni-RAPOXimate in cyclohex-anone exhibits one distinct peak at 595 nm(E max = 132 g mol- 1 em-I). No shift in the wave-length of maximum absorbance was observed onthe addition of base but the absorbance decreasedgradually and the peak subsequently disappearedwhen the adduct formation was complete, i.e.[B1~ [NiRzl. Below 530 nm, all the curves mergeand go beyond the scale. The absorbance at 595nm was taken for the calculation of adduct forma-tion constant.

The formation of the nickel adduct with base Bcan be represented by Eq. (1)[NiR21+ nB .=[NiR2.nBl ... (I)The adduct formation constants were evaluatedusing Eq. (2) which was originally derived byMath and Freiser for the pyridine adducts of nick-el dithiozonate and 8-mercaptoquinolinate4

•

Ao-Alog K AD =npB + log -;;:-- ... (2)

In Eq. (2) Ao and A are. the absorbances in theabsence and presence of the pyridine base (B), theconcentration of which is denoted by its negativelogarithm, pB.

Plots of log (Ao- A)I A versus [B] for variousbases are shown in Fig. 1. Intercepts of these plotsfor various adduct forming bases, on y-axis givedirectly the values of adduct formation constants.The values of adduct formation constants at dif-ferent temperatures are given in Table 1. FromFig. 1, it could be seen that the slopes of all theplots excepting that of 1,lO-phenanthroline areone indicating that only one molecule is involvedin the formation of the adduct, NiR2.B. In case of1,1O-phenanthroline, however, the slope of theplot happens to be two, giving rise to the forma-tion of a diadduct, NiRz.2B. Surprisingly, 4-pico-line and 3,5-lutidine, though contain reactive me-thyl groups and no other groups at 2 and o-posi-tions capable of offering steric hindrance fail to

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INDIAN J. CHEM., VOL. 27 A, DECEMBER 1988

Table 1- Adduct Formation Constants of Nickel RAPOXimate with Pyridine Bases and Thermodynamic Parametersof Adduct Formation at 26°C

Base pK log K AD in cyclohexanone at -Ea -6H' -65'(ref. 7) (kJ mol : '] (kJ mol- I) (JK -I mol- I)

26· 35· 44·C

Pyridine 5.2 0.93 0.85 0.72 20.84 23.33 2974-Picoline 6.08 0.72 0.55 0.27 66.4 68.9 3693-Picoline 5.68 0.573,5-Lutidine 6.14 0.33 0.24 0.15 16.4 18.88 3022,2'-Bipyridyl 4.37 2.53 1.97 1.5 87.36 89.85 4971,1O-Phenanthroline 4.95 6.23 6.14 6.18 16.9 19.4 5642,9-Neocuproin 5.85 2.82 2.832,4,6-CoUidine 7.48 0.43

3·°1---------------.

.1·0

~I~:0>.£ 0

-1·0

-2,0

-3<l -2·0log[BJ

-1-0 o

Fig. I-Plots showing adduct formation between [Ni(RA-POXhl and various heterocyclic nitrogen bases in cyclohexa-none [(I) 1,1O-phenanthroline; (2) 2,9-neocuproin; (3) 2,2'-bi-pyridyl; (4) pyridine; (5) 4-picoline; (6) 3-picoline; (7) 2,4,6;

collidine; (8) 3,5-lutidinel

form diadducts. Thus. amongst all the bases used,1,10-phenanthroline forms a stable adduct, as seenfrom the log K AD value (Table 1) and favours hex-acoordinate structure for the nickel adduct whichis the most stable configuration. In the remainingcases, the adducts contain only one mol of base,accounting for five coordination sites around thenickel atom. The presence of any water moleculeoccupying the sixth position is ruled out as the ex-periments were carried out in the monophase sys-tem of anhydrous cyclohexanone and using oven-dried nickel complex. It is not uncommon to havea pentacoordinate structure around nickel ion.Pentacoordination in nickel has been observed in

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1:1 adducts of the N-bases with nickel chelates ofother S-containing ligandsv' and oxines", Only li-gands such as 1,10-phenanthroline behave like bi-dentate ligands as indicated by the substantial in-crease in log K AD' Similar monoadducts were ob-served in the case of cobalt(U) RAPOXimatewhere all the pyridine derivatives, except the par-ent pyridine, gave rise to monoadducts. Pyridine,however, showed diad duct formation". No shift aswell as change in the absorbance in the spectrumof Ni-RAPOXimate was observed on the additionof 2,4-lutidine, 2,6-lutidine and 2-picoline. Thesebases were unable to produce any adduct withnickel RAPOXimate. The adduct formation con-stants with various pyridine bases decreased in thefollowing order of the bases: 1,lO-phenanthro-line> 2,9-neocuproin > 2,2'-bipyridyl > pyridine> 4-picoline > 3-picoline > 2,4,6-collidine> 3,5-lutidine. Surprisingly 2,4,6-collidineformed a stronger adduct than 3,5-lutidine. Thismay be due to the fact that 2,4,6-collidine is morebasic than 3,5-lutidine because of the presence ofone additional methyl group. The methyl group atposition-4 is stronger and more reactive. Its reac-tivity and basicity neutralise the steric hindrancecaused by methyl groups at 2 and 6 positions, giv-ing rise to a stronger adduct.

Absorption spectra of Ni-RAPOXimate withmost of the pyridine bases were also recorded at3SO and 44°C besides at ambient temperature(26°) and the adduct formation constants were cal-culated as usual. These values are given in Table1. The values were found to decrease with in-crease in temperature. In the case of 2,9-neocu-proin, however, the values at different tempera-tures were almost identical. The energy of activa-tion, Ea, was calculated- from the slope of the Ar-rhenius plot of log K AI) versus liT, by the formu-la In K AI) = - E /RT. Preexponential factor A was

obtained from the vertical intercept of this plot.The activation enthalpy !l.Hf and the activationentropy !l.Sf were obtained from Eqs (3) and (4)!l.HI=E -lIT ... (3)a

!l.Sf = R[ln(AhlkT)-1] ... (4)

In Eq. (4) h is the Plank constant; k, the Boltz-mann constant; and R, the gas constant. Thesethermodynamic adduct formation constants of

.nickel RAPOXimate at 26°C, are also given inTable 1.

NOTES

References1 Rane A T, J Scient ind Res, 43 (1984) 88.2 Rane A T & Nepali D R, Trans Met Chern 12 (1987) 330.3 Bhatki K S, Rane A T & Kabadi M B, Bull chem Soc Ja-

pan, 36 (1963) 1689.

4 Math K S & Freiser H, Analyt Chern, 41 (1969) 1682.5 Sgamelloti A, Furlani C & Magrini F, J inorg nucl Chern,

30 (1968) 2655.

6 Bhatki K S, Rane A T & Freiser H, lnorg chim Acta, 26(1978) 183.

7 Clarke K & Well R, J Arn chem Soc, (1960) 1885.

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