Indian Journal of ChemistryVol. 19A, June 1980, pp, 567-569

Physicochemical Study of the Structural Rearranagement of SchiffBases on Complex Formation: Part II-Lewis Acid Base Adducts of

Sn(IV) with Tetradentate Ligands Derived from SalicylaldehydeS. N. PODDAR*, N. s. DASt & A. K. DAS

Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta 700032

Received 17 April 1978; revised 14 November 1979; accepted 3 December 1979

A series of 1 : 1 adducts of tin(IV) iodide with schiff bases (derived from salicylaldehyde and diamines) havebeen synthesized and studied in the solid state and in solution. The structures of the complexes have been assigned onthe basis of their reaction with tertiary bases, conductance, IR and UV spectral data. The solid adducts having theketo-amine form of the coordinated Iigands are readily dissociated in solution and the rearrangement of this tauto-mer in solution is affected by solvent polarity as well as by the influence of external bases.

THE adducts of transition and non-transitionmetals with schiff bases, so far studied, areshown to have the neutral ligands coordi-

nating to the metal ions either in their phenol-imineform1,2 or in the tautomeric keto-amine form+",Previous studies of such compounds of Sn(IV) halides,SnX4 (LH2) and R2SnX2 (LH2), furnished ambiguousresults due mainly to the interpretation of activeconfiguration of ligand molecules. Recently,Ruddick and Sams", on the basis of Mossbauerspectroscopic studies of SnX4 (LH2) [LH2 = BSEDH2,

Acacenl-lj], suggested a non-ionic structure in con-trast to the ionic structure previously suggested byMurray et al.1o•

The present paper describes the behaviour of Snl ,towards a series of potential tetradentate ligands.The ligands used were derived from salicylaldehydeand diamines like ethylenediamine (I, BSEDH2),

propylenediamine (II, BSDPH2), 2-hydroxypropy-lenediamine (III, BSDPOH3), phenylenediamines(IV-VI, 0-, m-, p-BSDPH2) and benzidine (VII,BSBZH2)·

Materials and MethodsThe adducts were prepared by adding the ligand

(1 mmole) dissolved in dry benzene/Cf'I, dropwiseto a solution of SnI4 (1 mmole) in the same solvent.In the case of BSBZH~, nitrobenzene was used asthe solvent. The mixture was stirred under an at-mosphere of N2. An orange to red precipitateappeared. It was allowed to settle, filtered, washedwith solvent and dried in vacuo. The yield isalmost quantitative.

Sn, I and N in the compounds were estimated bythe methods discussed earlier+, Conductivitymeasurements were made using a Philips PR9 500conductivity bridge. The electronic spectra in theUV region were made on a Hilger-Uvispec instru-

+Present address : Central Fuel Research Institute, Dhanbad:828 108.

ment and the solid state IR spectra were obtained inKBr and nujol on Perkin-Elmer and BeckmannIR 20 instruments. Rigorously dried solvents wereemployed in measuring the spectra in solution toavoid any hydrolytic cleavage of the )C = N bondof the ligands.

Results and DiscussionThe isolated complexes are orange, red or brown

solids, quite stable at room temperature in the drystate, but they liberate iodine when heated above100°. The analytical data (Table 1) correspond tothe compositions, Snl , (LH2), for the complexes,where LH2 is a molecule of the ligand.

The probable structures of the complexes havebeen deduced mainly from their IR spectral data, ascompared with those of the free ligands which areknown to exist in enol-imine form in the solid state.

TABLE1- ANALYTICALAND CONDUCTIVITYDATA FOR Sn(IV)ADDUCTS

Adduct Decomp, Found (Calc.), % AmTemp. (ohm-1

(0C) Sn N I cm'fmole)

SnI.(BSEDH.) 62 12.95 3.21 55.80(13.30) (3.13) (56.76) 258.5*

SnI.(BSDPH2) 82 12.98 3.12 55.65(13.03) (3.08) (55.88)

SnIiBSDPOH3) 110 11.97 3.02 54.70 216.8*(12.86) (3.03) (54.92)

SnI.(o-BSPDH,) 210 12.51 3.12 50.90 10.42t(12.62) (2.97) (53.87) 178.4*

SnI.(m-BSPDH2) 145 12.72 3.01 54.00 26.0t(12.62) (2.97) (53.87) 269.3*

SnI.(p-BSPDH2) 160 12.60 2.89 53.81 197.2*(12.62) (2.97) (53.87)

SnI.(BSBZH2) 180 11.40 2.98 49.25 222.5*(11.69) (2.75) (49.85)

*in DMF at 30°, t in dioxan at 30°,tin nitrobenzene at 29.so.

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INDIAN 1. CHEM., VOL. 19A, JUNE 1980

But the IR spectra of some of the complexes suggestthe existence of the keto tautomers of the ligands inthe coordinated state as discussed below:

(i) The disappearance of the ligand bands,[vO-H (H-bonded to azomethine nitrogen)12-14centered at 2700 cm", ~O-H (in plane) around1280 crrr ! and ~O-H (out of plane) at 860 cm-I(both the latter bands appearing in the N - alkylschiff bases only)15-16] clearly indicates the absenceof the phenolic -OH in the adducts. Although thisobservation would be expected in the case with thedeprotonated chelates of the metal ions-', suchdeprotonation in the present cases is strictly excludedand any ionic structure involving NH+r species iscompletely ruled out, as in such events the complexesshould have exhibited characteristic absorption bandin the 2600-2400 cm-I region".

(ii) Unlike the ligands [except BSDPOH3, whichgives a medium band around 3450 em'? due to secon-dary alcoholic vO-HI9], the adducts show a medium,broad band around 3300 cm-I. The extent of theshift from the original position of vO-H (H-bonded)at 2700 cm ? and the change in its intensity seemto suggest a considerable change in the structuralarrangement of the ligand molecule in the adducts.Absorption for (unbonded) -OH generally occurs"near 3450 cm". The observed frequency at 3300 cm-1is obviously too high to be assigned to the absorptiondue to any H-bonded -OH, even if the nitrogen iscoordinated to the metal ion in the formal pentavalentstate 20. The observed bands in the present casesare assigned to vN-H vibration arising from theketo-amine form of the schiff bases>",

(iii) Coordination through the phenolic oxygenusually results in an increase in the frequency of theband around 1300 em'? (1280 em'? for N-alkyl schiffbases) due to the increasing covalent character of themetal-oxygen linkage15,16,20,21. But, no such bandis observed in the spectra of the complexes whichcould be assigned to this mode of vibration.

(iv) The highest intensity band around 1635-1625cm", assigned to the vC=N mode of the ligand>,is replaced in the spectra of the complexes by astronger band in the higher frequency side ([:; v=15-25 cm"). In the spectra of the ligands, the vC=Nvalue, which depends in part on the nature of thecharge on sp2-hybridised nitrogenI5,18'2I, decreaseswith the increasing length of the conjugate chain inthe ligand molecule [in the order N-alkyls, N-(m-phenylene), N-aryls]. The highest frequency bandsin the spectra of the complexes in this region do notfollow this trend. The band may thus be assignedto the vC=O vibration which appears in the lowerfrequency side due to either H-bonding with enaminenitrogen or through coordination with the metal ion.

(v) The band at 1265 em"? is assigned as the stret-ching vibration of =C-N-C-group, according tothe assignment made earlier-".

The vSn-I bands, which are known to be sensitiveto the nature of the donor ligands in Sn(IV)-adducts,are expected to occur in the lower energy region24,21>.The stronger and broader bands around 600-250em'? and less intense bands around 400-350 em"!are taken as vSn-O and "Sn-N modes respectively.Sn-N bonding seems to be preferred in most of the

adducts as reported in the earlier assignments20,26,27_The PMR spectra of most of the adducts could not

be studied due to their insolubility in the availablesolvents. The spectra of the only soluble adduct,SnI4 (m-BSPDH2) in CDCl3 (recorded using 60 MHzVarian instrument) shows the absence of any tin-ligand proton coupling, suggesting a considerabledegree of dissociation in solution. The high con-ductivity values in DMF and DMSO (Table 1) corres-pond to those of SnI4 in solution. This result is inaccord with the earlier observations. However,the adduct with o-BSPDH2 in dioxane shows 110n-electrolytic nature, whereas the conductance value ofSnI4 (m-BSPDH2) in nitrobenzene roughly corres-ponds to 1 : 2 ionisation in the solvent'",

The nature of the individual bands in the UVspectra of the ligands and their sodium salts has beendiscussed earlier!'. The enol-imine form of the ligandis characterised by strong enol bands around 250-270 (7t-lt*)and 320-350 nm (It-lti). Weak bandsof the keto-amine form appear around 270-300(It-7tt ) and 400 nm (7t-rr;). The spectra of thepresent adducts show either much less intense enolbands or their complete absence. Since the intensityof the band of any form is expected to be dependenton the contribution of this form in solution29,30,.the above spectra suggest greater contribution of theketo form of the ligands in solution in the presenceof lewis acid, SnI4. The higher wave-length It-7tiketo band found as a weak band around 390 nmin the spectra of other complexes is probably due tohigh dilution of the solutions used.

The rearrangement of the ligand molecule to its,keto-tautomer in the complexes is further evidencedby the spectrum of SnI4 (BSEDH2) in benzene (cone,,....,10-6 M). The spectrum shows much stronger ketoband (at 290 , 380 nm) and weaker 7t-lti -enol bandat 320 nm, instead of the stronger enol-band at 318nm and a weak 7t-lti[ -keto band (at 275 nm) ofthe ligand's spectra, indicating greater contributionof the keto form of the ligand in the adduct's solution.. Effect of external bases-Reaction of the complexes.

with triethylamine gives deprotonated chelates of thetype SnI4(L)11, where the ligand molecules functionin the dianionic enolic form. The base-catalysedrearrangement is rapid for the complexes with N-alkyl schiff bases and with o-BSPDH2, but is slow forother N-aryl schiff bases. The ad ducts with N-alkylschiff bases can easily bedeprotonated with pyridine,picolines or even without the addition of base inethanolic solution. The keto-enol tautomeric re-arrangement of such ligands is known to depend onthe basicity of the azomethine nitrogen and thestrength of Fl-bonding-". In the present cases thebasicity differences in the two classes of schiff bases"cannot explain the behaviour of the adducts, since aseries of adducts with ligands of weaker basicity havebeen found to be deprotonated with greater ease32.So the cause of such phenomenon may be associated.with the strength of H-bonding in the keto-ligands.Obviously, the stronger the H-bonded structure of theketo-ligands in the complexes, the easier is the tauto-meric proton interchange and greater will be theattack by bases. From the experimental results,structure I is preferred for the complexes with N-

PODDAR et al. : SCHIFF BASE ADDUcrS OF Sn(IV)

cx,,..--,, H....N/A'\.N."))'\1/1::". ~..Ji ,H.. .. o ~

Sn/I'\.

I I I<1)

[~r/:'rc~ r'~o1.~!-d~JI1

(lI)

alkyl schiff bases and o-BSPDH2, since such a struc-ture would explain the case of deprotonation of thecomplexes. Complexes with other schiff bases maybe supposed to have the structure II. The conduc-tivity data for SnI4 (o-BSPDH2) in dixoane and SnI4{m-BSPDH2) in nitrobenzene, as mentioned earlier,also support, such structures.

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