Synthesis , Characterization and Mössbauer Studies of Fe ( II ) and Fe ( III ) Complexes of 2-Acetylpyridine Thiosemicarbazone

Foram obtidos e caracterizados três complexos de ferro da 2-acetilpiridina tiossemicarbazona (2APT): [Fe(HAPT)(APT)]Cl2 (I), [Fe(HAPT)2]Cl2 (II), e [Fe(APT)2] (III), nos quais HAPT representa o ligante protonado no nitrogênio N(2’) da cadeia lat eral e APT o ligante desprotonado em N(2’). A coordenação ao íon metálico ocorre através do sistema tridentado N py ∩ N(1’) ∩ (S), com a formação de complexos do tipo L:M 2:1. Os parâmetros Mössbauer δ (desvio isomérico) e ∆ (desdobramento quadrupolar) são característicos de Fe(III) para o complexo I e de Fe(II) para os complexos II e III, todos de spin baixo e simetria octaédrica. O valor de δ diminui quando se passa do complexo [Fe(HAPT) 2]Cl 2 (II, δ = 0,264 ± 0,001 mm/s) ao complexo [Fe(APT) 2] (III, δ = 0,227 ± 0,001 mm/s), em razão da maior densidade eletrônica de eletrons s no metal no segundo complexo, resultante do efeito indutivo eletrodoador do grupo metila e da carga negativa no enxofre de tiolato que o tornam um melhor doador sigma do que o enxofre de tiona do primeiro complexo. O desdobramento quadrupolar ∆ aumenta quando se passa do complexo protonado, II (∆ = 0,537 ± 0,002 mm/s) ao desprotonado, III ( ∆ = 0,755 ± 0,002 mm/s), provavelmente em razão da diferença na natureza dos dois ligantes, principalmente na ordem das ligações metal-enxofre.

thiosemicarbazones de rived from 2-acetylpyridine have been shown to pres ent a broad range of chemotherapeutic ac tiv i ties and have been eval u ated for their antimalarial, antitumor, an ti bac te rial and antifungal ac tion 2 .The pharma co log i cal ac tiv ity of these com pounds has of ten been related to their che lat ing abil ity with metal ions 3 .
In pre vi ous works we have un der taken stud ies on the metal com plexes of 2-formylpyridine thiosemicarbazone [4][5][6] .The pres ent re port deals with the prep a ra tion, char ac ter iza tion and Mössbauer data of Fe(II) and Fe(III) com plexes of 2-acetylpyridine thiosemicarbazone.
The ligand is a tridentate che lat ing N py ∩ N(1') ∩ S system (see Fig. 1) that can co or di nate metal ions ei ther un der the protonated (HAPT) or the deprotonated (APT ) form at N(2').The for mer con tains thione sul fur and the lat ter thiolate sul fur.
The com plex [Fe(APT) 2 ] (III) was ob tained by ad di tion of 1.8 mmole (0.3492 g) of HAPT to a hot so lu tion of FeCl 2 (0.9 mmole, 0.1141 g) in 35 mL of eth a nol.Af ter the ad dition of the ligand, an equal vol ume of deionized wa ter and a few drops of con cen trated am mo nia were added giv ing a green mix ture.Af ter 2 h at 70 °C the re ac tion mix ture was cooled and the brown ish green pre cip i tate was fil tered, washed with eth a nol, fol lowed by ether and dried un der vac uum over drierite (yield 23%).The C,H,N anal y sis was com pat i ble with the for mula C 16 H 18 N 8 S 2 Fe, M = 442.12g/mole Found: C%:43.01;H%:3.94;N%:24.95.Calcd.: C%:43.01;H%:3.88;N%:25.34.
All three com plexes de com pose upon heat ing: I at 262 °C, II at 197 °C and III at 303 °C.

Ap pa ra tus
The atomic ab sorp tion spec tra were ob tained by means of a Varian Techtron AAG spec trom e ter and the neutronic ac ti va tion anal y sis of chlo ride was ob tained by means of a Cambera equip ment pro vided with a 7229 Ge/Li de tec tor.The mo lar con duc tance data were ob tained by means of a YSI Con duc tiv ity Bridge, model 31, equipped with a condutimetric cell of cell con stant equal to 0.088 cm -1 , using 10 -4 mol/L so lu tions in meth a nol.KCl, KBr, BaCl2 and Mg(NO3)2.6H20were used as stan dards.
IR spec tra were run in a Perkin Elmer 283 B in stru ment, us ing Nujol mulls.
Mössbauer spec tra were ob tained us ing a con ven tional con stant ac cel er a tion spec trom e ter, mov ing a Co-Rh source at room tem per a ture.All spec tra were com puter fitted as sum ing lorentzian line shapes.

Re sults and Dis cus sion
For all com pounds the C,H,N anal y sis in di cated the pres ence of 1:2 metal to ligand com plexes.In the case of com plex I this anal y sis sug gested the pres ence of one protonated and one deprotonated ligand at tached to the metal ion (Fe(III)), whereas two protonated and two deprotonated lig ands at tached to Fe(II) are sug gested in the case of com plexes II and III re spec tively.In or der to confirm the ex is tence of one protonated and one deprotonated ligand in the same com plex, the per cent ages of the metal and chlo ride were also de ter mined for com plex I, cor rob orat ing our as sump tion.Fur ther more, the mo lar con ductance of this com plex (148.5 S cm 2 mol -1 ) in di cated that the com plex is a 1:2 elec tro lyte, again con firm ing our sup po sition.In fact, some works re port the syn the sis of 2:1 ligand-to-metal com plexes of other thiosemicarbazones hav ing the two ligand mol e cules in dif fer ent protonation states 8 .
The in fra red spec tra of the com plexes pres ent char acter is tic fea tures com pared to that of the free ligand (Table 1).The co or di na tion of the pyridine ni tro gen can be ev idenced by the shift ing of the ligand ab sorp tions at 985 and 970 cm -1 , at trib uted to the ring breath de for ma tion 9 to lower fre quen cies in the spec tra of the com plexes.Fur thermore, the ligand ab sorp tions at 630 and 375 cm -1 , as cribed to the in plane and out of plane ring de for ma tions re spectively 10 shift to higher fre quen cies in the spec tra of the com plexes.In ad di tion, the band at 1605 cm -1 , at trib uted to the ν CC ring stretch ing vi bra tion also is shifted to higher frequen cies upon complexation 11 .How ever, the lat ter normally oc curs in the same re gion as the NH 2 de for ma tion (1610-1615 cm -1 ) or as the ν CN stretch ing around 1580 cm -1 not al low ing an unumbiguous in ter pre ta tion of the spec tra in this re gion 12 .
The co or di na tion of the azomethine ni tro gen N(1 ' ) is in di cated by the shift ing of the ligand ab sorp tions at 1550, 1490 and 1425 cm -1 , as cribed to the stretch ing νCN or to the νCN + δNH2 mode (thio amide II), to higher fre quen cies in the spec tra of the com plexes [13][14][15] .
A set of three in tense bands at 1135, 1095 and 1075 cm -1 is be lieved to have a ν CS con tri bu tion 16 .This set of bands is shifted to lower fre quen cies upon co or di na tion, ap pear ing with lower in ten sity in the spec tra of the complexes, in di cat ing the at tach ment of the sul fur to the metal in all cases.More over, the spec trum of the free ligand exhib its two strong bands at 835 and 815 cm -1 , which are usually as signed mainly to the ν CS vi bra tion.These ab sorp tions are shifted 35 to 45 cm -1 to lower fre quen cies with dim i nu tion of in ten sity in the spec tra of the complexes, sug gest ing the co or di na tion of the sul fur [17][18] .Other au thors ob served a sim i lar shift ing of the ν CS vi bra tion upon complexation 10,15,19 .In fact, when the protonated HAPT is co or di nated, the thione sul fur is able to ac cept elec tron den sity from the metal through back π bond ing, lead ing to an in crease of the M-S bond or der and the concom i tant de crease in the C-S bond or der.On the other hand, when co or di nated as the deprotonated (APT -) form, the tiolate sul fur is not as good a π ac cep tor 20 .Thus, the CS bond pres ents a dou ble bond char ac ter due to the high delocalization of the neg a tive charge in the ligand's chain, lead ing again to a strength en ing of the M-S bond with the cor re spond ing de crease in the C-S bond or der.There fore, the in fra red data do not al low a clear dis tinc tion be tween thione and thiolate sul fur.In con clu sion, the IR spec tra clearly in di cate the co or di na tion of the ligand as a tridentate che lat ing N py ∩ N(1') ∩ S sys tem.
The Fe-S vi bra tion is as signed to the ab sorp tions in the 340-380 cm -1 re gion, based on re ports of other au thors 10,21 , al though the lit er a ture also con tains re ports atributing these bands to the Fe-N vi bra tion and that at 400 cm -1 to the Fe-S vi bra tion 22 .
In a re cent work 23 we re ported the vis i ble spec tral assign ments for com plexes I, II and III.In aque ous so lu tion the protonated Fe(II) com plex (II), shows a band at 575 nm with a shoul der at 518 nm.Upon deprotonation re sult ing in the for ma tion of com plex III, the max i mum at 575 nm shifts to 600 nm and the shoul der to 545 nm.From res onance Raman data ob tained pre vi ously for the anal o gous com plexes of 2-formypyridine thiosemicarbazone (HFPT), we at trib uted the spec tral max ima to ligand-to-metal charge trans fer tran si tions (LMCT) from a pπ or bital of the thione/thiolate sul fur to a σ * or bital of the low spin Fe(II).The shoul der was in di cated to be due to a vibronic struc ture cor re spond ing to a tran si tion to an excited vi bra tional level 11 .The same as sign ment is sug gested for the bands in the vis i ble spec tra of com plexes II and III (both low spin, as shown by Mossbauer spec tros copy, vide in fra).In fact, on deprotonation the en ergy of this tran si tion de creases due to the the lower en ergy of the charge trans fer from the thiolate sul fur or bital com pared to the thione sulfur or bital.This en ergy dif fer ence re sults be cause the the thiolate sul fur is more polarizable.That is, the greater p char ac ter of the sp 3 hy brid iza tion of the thiolate sul fur compared to the sp 2 hy brid iza tion of the thione sul fur makes the thiolate a better pπ do nor.As we dem on strated be fore, reso nance Raman data as well as the low en ergy of the ab sorption max ima in the vis i ble pre cluded the as sign ment of the peak to a σ(S)-σ*(M) charge trans fer tran si tion 11,24 .The lat ter is prob a bly su per im posed to the intraligand tran sitions in the ul tra vi o let range (Fig. 2).The vis i ble spec trum of the Fe(III) com plex (I) shows a shoul der at 440 nm, which we as signed to a pπ(S)-σ*(M) LMCT, sim i lar to the pre vi ously stud ied Fe(III) com plex of HFPT 11 .Be cause of the larger 10 Dq as so ci ated with low spin Fe(III) (Mossbauer data, vide in fra) com pared to that of low spin Fe(II) re sult ing from the higher charge 25 , the charge trans fer tran si tion to σ*(Fe(III)) is higher in en ergy than to σ*(Fe(II)).
The Mössbauer spec tra of com plexes I, II and III are shown in Fig. 3. Ta ble 2 gives the Mössbauer ex per i men tal data, as well as those of the Fe(III) com plex of the re lated ligand diacetyl monoxime thiosemicarbazone (IV), stud ied by other au thors 13 .Com plex IV is rep re sented by [Fe(DMT-H)(DMT-2H)] in which DMT-H and DMT-2H stand for the ligand diacetyl monoxime thiosemicarbazone con tain ing a thione and a thiolate sul fur re spec tively (i.e., the other hy dro gen is lost from the monoxime moi ety).
The Mössbauer spec trum of com plex I is an asym metric dou blet, the sec ond line be ing only 65% as in tense as the first one.The nar row band width of the dou blet (Γ = 0.242 mm/s) in di cates that the most in tense line at around 1.25 mmm/s is not due to any im pu rity but prob a bly to a pref eren tial ori en ta tion of the crys tals or to vi bra tional ani so tropy of the Mossbauer iso tope in the crys tal lat tice.
The Mossbauer spec trum of com plex I was also recorded at room tem per a ture, in which the plane of the sample was ro tated by 50° with re spect to the γ-ray di rec tion.The pa ram e ters ob tained were δ = 0.063 ± 0.002 mm/s, ∆ = 2.426 ± 0.004 mm/s and Γ = 0.248 ± 0.003 mm/s, that are essen tially the same as the ones re ported in Ta ble 2, ex cept that now the sec ond line is 98% as in tense as the first.This re sult in di cates that the asym me try of the dou blet of complex I de pends on the ori en ta tion of the crys tals in the sample holder, which is char ac ter is tic of the tex ture ef fect 26 .There fore, the asym met ric Mossbauer dou blet shown by  com plex I is mainly due to pref er en tial ori en ta tion of the crys tals.The spec tra of com plexes II and III show sym metric dou blets.
The iso mer shift (δ) rel a tive to me tal lic iron and the quadrupole split ting (∆) pa ram e ters are char ac ter is tic of low spin Fe(III) for com plexes I and of low spin Fe(II) for com plexes II and III, and all com pounds hav ing ap prox imately oc ta he dral ge om e try.The iso mer shift of [Fe(HAPT) 2 ]Cl 2 (II, ∆ = 0.264 ± 0.001 mm/s) is higher than that of [Fe(APT) 2] (III, δ = 0.227 ± 0.001 mm/s, Ta ble 2).The lower δ value of the deprotonated form can be in terpreted in terms of an in crease in the s elec tron den sity at the metal re sult ing from the neg a tively charged thiolate sulfur 20 .This greater elec tron den sity is also fa cil i tated by the acetyl methyl group's do na tion via the con ju gated sys tem of the an ionic thiosemicarbazone ligand.In fact, thiolate sul fur is better a σ as well as a pπ do nor to Fe(II) than thione sul fur.Both sul furs pos sess va cant dπ orbitals that can be used for dπ-dπ metal-sulfur bond ing.Al though thione sulfur is less polarizable than thiolate sul fur, it ap pears to be a better dπ elec tron ac cep tor than the neg a tively charged thiolate sul fur 26 .There fore, the elec tronic den sity at the iron(II) cen ter is lower for the protonated (thione) com plex (II) than for the deprotonated (thiolate) com plex (III), in accor dance with the higher δ value for com plex II.More over, as we stated be fore, the en ergy of the LMCT tran si tion in the elec tronic spec trum is lower for the deprotonated complex (III), again in ac cor dance with the higher do nor abil ity of the thiolate sul fur.
For the Fe(II) com plexes the quadrupole split ting increases ap pre cia bly in go ing from the protonated (II, ∆ = 0,537 ± 0.002 mm/s) to the deprotonated (III, ∆ = 0.755± 0.002 mm/s) form.In or der to ex plain this vari a tion ligand inequivalencies must be taken into con sid er ation, a sig nif icant con tri bu tion be ing at trib uted to the dif fer ence in the Fe-S bond or der.Thiolate sul fur is able to form one σ and two π bonds with the metal whereas thione sul fur can only form one π and one σ bonds.An other sig nif i cant con tri bution may be as cribed to the neg a tive charge at the thiolate sul fur, the thione sul fur be ing neu tral 20 .Again these re sults are in per fect agree ment with the lower en ergy of the thiolate-to-metal charge trans fer tran si tion , due to its higher do nor abil ity.
At this stage it is worth not ing the sim i lar i ties of the Mössbauer pa ram e ters of the Fe(III) com plexes [Fe(HAPT)(APT)]Cl (I, δ = 0.065 ± 0.001 mm/s , ∆ = 2.430 ± 0.002 mm/s) and its an a log [Fe(DMT-H)(DMT-2H)] (IV, δ = 0.07 mm/s, ∆ = 2.2 mm/s).Both the iso mer shift and the quadupole split ting of I and IV are com pa ra ble, in di cat ing that our for mu la tion of com pound I as con tain ing one protonated and one deprotonated lig ands is cor rect.

Ac knowl edg ments
This work was sup ported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).
We are in debted to pro fes sor Douglas X. West for helpful dis cus sion and sug ges tions.

Fig ure 1 .
Fig ure1.Struc ture of the ligand 2-acetylpyridine thiosemicarbazone un der the thione and thiol tau to meric forms.

Fig ure 2 .
Fig ure2.A sim pli fied mo lec u lar or bital di a gram il lus trat ing the poten tial ligand-to-metal charge trans fer tran si tions.Tran si tions to t2g are ab sent in the low spin Fe(II) com plexes.