Indian Journal of ChemistryVol. 23A, February 1984, pp. 120-121

Azo-Hydrazone Tautomerism in Arylazophenoxy Acetic Acids & TheirOrganotin Complexes

T K CHAlTOPADHYAY, A SENGUPTA & B MAIEP

Department of Chemistry, North Bengal University, Darjeeling 734430

Received 8 March 1983; revised and accepted 23 September 1983

Azo-hydrazone tautomeric equilibria in arylazophenoxyacetic acids and their organotin derivatives have beenquantitatively studied. The absence of any marked effect of stannylation of the carboxyl group on the tautomeric equilibriumshows that the N-atom adjacent to the phenoxy ring is involved in N....•Sn coordination in the organotin derivatives,supporting the previously proposed structures for the organotin arylazophenoxy acetates.

Azo-hydrazone tautomerism is well known in arylazocompounds with an ortho- or para-hydroxyl group">",Although azophenols exist almost completely in theazo form, 2-(arylazo)-5-hydroxyphenoxyacetic acids(I) and their organotin derivatives't-? exist as solvent-dependent equilibrium mixtures of the two forms, Theformation of N -> Sn coordinate bond in the organotincomplexes is likely to compete with the azo-hydrazoneequilibrium (Eq. I). Since both involve the partici-pation of the azo group, a quantitative study of thisequilibrium is expected to provide a clue to the natureof bonding between the tin atom and the azo group inits complexes. This is particularly important since theparticipation or otherwise of the azo group inorganotin complexes of functionally substituted azocompounds has so far been inferred only from thebathochromic shifts in their electronic absorptionbands10,11,

LigandL1HH'L2HH'UHH'L4HH'L'HH'L6HH'L3HMeL3H'MeL'H'Me

X

5-0H5-0H5-0H5-0H5-0H5-0H5-0CH3

5-OH5-0H

Y

H2'-OCH~2'-CI2'-F4'-OCH3

3'-OH2'-CI2'-CI2'-OCH3

R

HHHHHHHCH3

CH3

H' and H refer to 5-0H and the carboxylic protons respectively.

120

Materials and MethodsThe methods of preparation of the ligands (I) and

their organotin derivatives, and the analytical andphysico-chemical data are given elsewhere'':".

Results and DiscussionThe electronic absorption spectra of I are

characterised by two solvent-dependent, overlappingbands in the regions 360-390 and 420-460 nm insteadof a single absorption band in the region 350-390 nmobserved in similar azo dyes, e.g., 4-hydroxyazoben-zenes 1 ~, suggesting the presence of a solvent-dependent equilibrium. Since both the bands arepresent even in non-polar aprotic solvents like benzeneand Beer's law is obeyed in the concentration rangestudied, presence of inter- or intra-molecular hydrogenbonding or association-dissociation processes may beruled out. That the two absorption bands arise fromazo-hydrazone equilibrium (Eq. I) is shown by themethylation of the 5-0H group which eliminates thepossibility of azo-hydrazone tautomerism,

The methoxy derivatives have a single absorption?in the region 360-390 nm, similar to those in thesubstituted azobenzenes, e.g., 4-hydroxyazobenzenes,which are known to exist only in the azo form 12.

Incidentally, the stannoxy derivative, Ph3SnL3Me,which was prepared to test whether the stannylation ofthe 5-0H group has the same effect as methylation,shows a single absorption at 375 nrn". Obviously, the420-460 nm band arises from the hydrazone form(II B). In fact, hydrazone form always absorbs at alonger wavelength as compared with the position in thecorresponding azo form 13. The integrated absor-

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CHATTOPADHYAY et a/.: TAUTOMERISM IN ARYLAZOPHENOXY ACETIC ACIDS

Table I-Integrated Molar Absorbance of the 1<-11*

Transitions Due To Azo and Hydrazone Forms and KT ofSome Typical Arylazophenoxyacetic Acids and Their

Organotin DerivativesCompound Solvent Integrated molar Kr

absorbs nee x 108

(litre mol I em I see ')--.-------.--------

A, Ah A,+Ah

L'HH' Methanol 1.18 0.10 l.28 0.08Benzene 0.99 0.37 1.36 017

Bu,SnL1H' Methanol 1.15 0.15 1.30 o. !3Benzene 0.73 0.59 1.32 0.81

Ph~SnL'H' Methanol 1.15 0.13 l.28 0.11Benzene 0.77 0.53 1.30 0.68

L2HH' Methanol 1.14 0.16 1.30 0.14Ph.lSnL2H' Methanol 1.15 0.15 1.30 0.13BU.lSnL2H' Methanol 1.14 0.18 1.32 0.16L3HH' Benzene 1.15 0.05 l.20 0.04Ph3SnL3H' Benzene 1.13 0.05 1.18 0.04L'HH' Methanol 1.05 0.07 1.12 0.06Ph3SnL'H' Methanol 1.08 0.08 1.16 0.07

Benzene 1.04 0.08 1.12 0.08L6HH' Methanol 1.31 0.17 1.48 O. I3Bu3SnUH' Methanol 1.41 0.09 1.50 0.06

Benzene 0.73 0.74 1.47 1.01

bance 12.14.15 due to the azo and the hydrazone formspresent in the solution was then determined using acomputerised band resolution technique whicheffectively carries a least squares fit of the experimentalabsorption curves in terms of the sum of two gaussiancurves corresponding to the azo and hydrazoneforms!". The integrated absorbance data for somerepresentative arylazophenoxyacetic acid derivatives,given in Table 2, show that the total integratedabsorbance due to both the forms in similarcompounds remains nearly constant although therelative intensities of the component bands vary widelyindicating that the integrated absorbances of thecomponent bands are proportional to the con-centrations of each of the tautorners in the solution sothat KT is given by

K1 = [Hydrazone forrn]. [Azo form]= All/A"

where Ah and A. are rcspec.rvely the integratedabsorbances of the hydrazone and the azo forms.

Tautomeric equilibrium constants, K,. calculatedusing Eq. (2) are also given ill Table I.

In methanol. azo form predominates over thehydrazone form both in the ligands and the org.n.otinderivatives (Table I), presuma hly due to greaterstabilization of the azo form by solvent-soluteinteraction through the - OH group. The equilibriumshifts to some extent towards the hydrazone form innon-polar solvent benzene

Stannylation of the carboxyl group does not haveany significant effect on the azo-hydrazoneequilibrium except for L IHH' and L6HH' in benzene,This is consistent with the proposed structures of theorganotin derivatives+? where the a-nitrogen atom isused to coordinate the tin atom without affecting the f3-nitrogen atom which can participate in the azo-hydrazone equilibrium both in the ligand and theorganotin derivatives(III). In the 6-coordinate

R3SnL IH' and R3SnL 6H', the steric crowding aboutthe tin atom is considerably reduced in the hydrazoneform because of the non-planarity of the two rings ofthe ligand, The other ligands with a donor group at 2'-position (y = F, Cl, OMe) form 7-coordinatederivatives (see structures V and VII in ref. 9) withthree annelated rings thereby making the structurerigid, Very little extra stabilization is, therefore,achieved in the hydrazone form in these compounds.This explains the nearly similar KT values in bothbenzene and methanol for those compounds, Again, inL sHH' and its organotin derivative the resonanceinvolving the 4'-OCH3 group stabilizes the azo-formresulting in low KT value in both the solvents.

On the other hand, involvement of the f3 - Natom orthe formation of a n-complex through the azo group asa whole would have shown significant effect on theazo-hydrazone equilibrium in going from the ligand toits organotin derivatives, This supports the previouslyproposed structures for organotin arylazophenoxyacetates". showing the formation of a 5-memberedchelate ring,

. .(2)

ReferencesI Price R. The chemistry 0.1synthetic dv e,1 , '1013 (Academic Press,

New York), 1970 .2 Bur a way A, Salem A G & Thompson A R, J chem Soc. (1952)

4973.3 Buraway A & Thompson A R. J chem Soc. (1953) 1443.

4 Fischer E & Frei Y . .i chem S,)(. (1959) 3159.5 MorganKJ,Jchem SO(',(1961) 2151.6 Juwik P & Sundby B. Acta chem scand, 27 (1973) 1645.

., Fierz-David H E.. Blangey L & Merian E, He/v chim Acta, 34(1951) 846.

8 Sengupta A, Studies on some organotin aryloxy acetates.synthesis of potential hept a-coordinated triorganotincarboxylates. PhO thesis. North Bengal University, 1981.

9 Sengupta A, Chattopadhyay T K & Majee B. Indian J Chern. 21/\(]982) 1090