Structure of Cobalt ( II ) Perchlorate Adsorbed on Silica Gel Surface Chemically Modified with Benzimidazole Molecule

Mo lé cu las de ben zi mi da zol co va len te men te li ga das à su per fí cie da si li ca gel, ≡SiL (onde L = N-propilbenzimidazol), ad sor vem Co(ClO4)2 de sol ven te não aquo so for man do com ple xos na su per fí cie se gun do a re a ção: m ≡SiL + Co(ClO4)2 → (≡SiL)mCo(ClO4)2. As cons tan tes de equi lí brio e ca pa ci da de de ad sor ção, fo ram de ter mi na das apli can do-se a equa ção de Lang mu ir for ne cen do res pec ti va men te, b = 3.0 x 10 L mol e Ns = 0.098 x 10 mol g. O me tal está li ga do atra vés do áto mo de ni tro gê nio e o íon per clo ra to não está co or de na do. O es tu do de ESR mos trou que o com ple xo pos sui es sen ci al men te uma ge o me tria com dis tor ção te tra go nal, com os elé trons dos qua tro ni tro gê ni os in te ra gin do com o ion me tá li co cen tral co bal to no pla no equa to ri al. So men te uma es pé cie com ple xa foi de tec ta da na su per fí cie.


Intro duc ti on
Orga nic mo le cu les con ta i ning ni tro gen atoms have been che mi cally bon ded to a si li ca gel sur fa ce in or der to ad sorb me tal ions from aque ous or non aque ous sol vents [1][2][3][4][5] .As in so lu ti on pha se, many com plex spe ci es may be for med at the so lid-solution in ter fa ce by the re ac ti on of the me tal ion with the im mo bi li zed func ti o nal or ga nic mo le cu le [6][7][8] .Tech ni ques such as 13 C CPMAS NMR 9,10 and elec tron spin re so nan ce (ESR) 8,11 have been used to de ter mi ne the structu res of the se com plex spe ci es.The se cond tech ni que has been par ti cu larly use ful for cop per com ple xes 10,12 .
Co balt (II) is ad sor bed from ace to ne so lu ti on by the atta ched ben zi mi da zo le yi el ding a pale blue com plex 13 .The ad sorp ti on of the me tal ion oc curs ac cor ding to the re ac tion: m ≡SiL + CoX 2 → ( ≡SiL) m CoX 2 , whe re ≡SiL is the atta ched ne u tral func ti o nal or ga nic group on the si li ca sur fa ce.The step wi se com plex for ma ti on, which con sists of a chan ge in the ave ra ge num ber of the li gand, m, co or dina ting one me tal ion is a func ti on of the me tal lo a ding in the pre sent case be ca u se the sur fa ce den sity of at ta ched li gands is cons tant.The cor re la ti on bet we en the struc tu re of the com ple xes in the so lu ti on pha se and that on the so lid sur face is not stra igh tfor ward be ca u se from a ste ric po int of view Ar ti cle the at ta ched li gands have li mi ted mo bi lity whi le in the solu ti on pha se this res tric ti on does not exist.
In this work the struc tu re of co balt per chlo ra te ad sor bed on the sur fa ce of si li ca mo di fi ed with ben zi mi da zo le is repor ted.The per chlo ra te coun te ri on was cho sen be ca u se it has low co or di na ti on abi lity and thus, the re sul ting struc ture of the re ac ti on bet we en Co(ClO 4)2 and ≡SiL may be exclu si vely de ter mi ned by the in te rac ti on of the im mo bi li zed li gand and the me tal ion.On the ot her hand, this study is of gre at in te rest be ca u se the si li ca gel sur fa ce is morp ho lo gically very he te ro ge ne ous, the li gands are not uni formly distri bu ted on the sur fa ce and to our know led ge, lit tle in for ma ti on on com plex for ma ti on in po rous ma te ri als is ava i la ble in the li te ra tu re.

Pre pa ra ti ons
Si li ca gel 60 (Merck) ha ving a par ti cle size bet we en 0.2 and 0.05 mm was ac ti va ted at 420 K un der va cu um (0.13 Pa).This si li ca ( ca.50 g) was sus pen ded in dry xyle ne (200 cm 3 ) and 3-chloropropyltrimethoxysilane (15 cm 3 ) was ad ded; the mix tu re was stir red for 24 h at 380 K un der a ni tro gen at mosp he re.The re sul ting mo di fi ed si li ca was immer sed in pure di methylfor ma mi de (150 cm 3 ) and ben zimi da zo le (17.5 g, 0.15 mol) was ad ded.The mix tu re was stir red for 24 h at 380 K un der a ni tro gen at mosp he re.The re sul ting mo di fi ed si li ca was fil te red off and was hed with di methylfor ma mi de and et ha nol.The pro duct was he a ted for 8 h at 353 K un der va cu um (0.13 Pa).The amount of ben zi mi da zo le at ta ched to the si li ca sur fa ce was No= 0.48 x 10 -3 mol g -1 .
The re ac ti ons which des cri be the pre pa ra ti on can be repre sen ted by the equa ti ons in Sche me 1, whe re for sim pli fica ti on (A) will he re af ter be de sig na ted as SiL.

Isot herm of Adsorp ti on
The isot herm ad sorp ti on of Co(ClO 4)2 from an ace to ne so lu ti on by ≡SiL was de ter mi ned at 298 K.About 0.1 g of ≡ SiL was im mer sed in 50 mL so lu ti ons of Co(ClO 4 ) 2 of va ria ble con cen tra ti ons.The sus pen si on was sha ken for 60 min and then al lo wed to stand for 20 min, time ne ces sary for all the so lid ma te ri al to de po sit at the bot tom of the flask.About 10 mL of the su per na tant so lu ti on was eva po ra ted, the re si due dis sol ved in wa ter and the me tal was analy zed by con ven ti o nal EDTA com ple xo me tric ti tra ti on.The quan tity of the ad sor bed me tal was de ter mi ned by ap plying the equa ti on:

N
Na Ne m f = whe re Na is the ini ti al me tal mole num ber, Ne the me tal mole num ber in equi li bri um with the so lid pha se and m the mass of the ad sor bent.

Infra red spec tra
Infra red spec tra of the sam ples were ob ta i ned in the regi on 2000 to 1000 cm -1 , using the tech ni que of self sup ported disk des cri bed el sew he re 3 .The equip ment used was a Ni co let FT-IR spec trop ho to me ter.

Elec tro nic spec tra
The elec tro nic spec tra of the sam ples were ob ta i ned by im mer sing the so lid with Co(ClO 4 ) 2 ad sor bed on the sur face, ( ≡SiL) mCo(ClO 4)2, in spec tral gra de car bon te trach lo ride in a quartz cell ha ving 1mm path length.A Cary 2300 spec trop ho to me ter was used.Good qua lity spec tra were ob ta i ned sin ce the re frac ti ve in dex of both, si li ca and the liquid, are ne arly the same 14 .

ESR spec tra
Spec tra of the sam ples were ob ta i ned in a quartz tube at 77 K, at band X (9 Ghz), 40 mW mi cro wa ve po wer, mo dula ti on fre quency of 100 kHz and a mag ne tic fi eld with swee ping of 1000 Ga uss cen te red at 2850 or 2900 Ga uss.The equip ment used was a Bru ker ECS 106 elec tron spin re sonan ce spec tro me ter.

Isot herm of ad sorp ti on
Fi gu re 1 shows the isot herm of ad sorp ti on of Co(ClO 4 ) 2 by the so lid pha se ≡SiL, from ace to ne so lu ti on.The ad sorpti on ca pa city, Ns, and the equi li bri um cons tant, b, were deter mi ned by ap plying the Lang mu ir equa ti on: whe re C is the me tal con cen tra ti on in the so lu ti on pha se in equi li bri um with the so lid pha se.Plot ting C/Nf vs. C gave a stra ight line with cor re la ti on co ef fi ci ent r = 0.997 was ob tai ned.The cal cu la ted cons tants were: b = 3.0 x 10 3 L mol -1 and Ns = 0.098 x 10 -3 mol g -1 .The mag ni tu de of the constant b is the same as that de ter mi ned for re ac ti on of the pendant ben zi mi da zo le li gand and co balt (II) in non aque ous sol vents 3 .The che mi cal spe ci es for med on the sur fa ce is ther mody na mi cally sta ble and ad sorp ti on oc curs by ni trogen-metal bond for ma ti on.The re ac ti on at the sur fa ce can be re pre sen ted by the equa ti on: -counte ri on and then, the ad sorp ti on oc curs as an ad sorp ti on of a ne u tral spe ci es.The equa ti on also shows that as the con centra ti on of the me tal ion in cre a ses at the so lid-solution in ter face, the ave ra ge num ber of the at ta ched li gands co or di na ted to the me tal ion de cre a ses be ca u se the sur fa ce den sity of the li gands is cons tant.The re fo re, it is very im por tant to study the stru cu tu re of the spe ci es ta king into ac count the quan tity of the me tal ad sor bed on the sur fa ce 8 .

Infra red spec tra
The in fra red spec tra of ( ≡ SiL) m Co(ClO 4 ) 2 as a func tion of me tal lo a ding is pre sen ted in Fig. 2. The bands ob served bet we en 1570 and 1450 cm -1 are due to the vi bra ti o nal mo des in vol ving the ben zi mi da zo le ring.The band at 1500 cm -1 ob ser ved in Fig. 2a (sam ple wit hout ad sor bed me tal, Nf/No = 0) is slightly shif ted to hig her fre quency, 1504 cm -1 (Fig. 2b, Nf/No = 0.065), and for the sa tu ra ti on con diti on is shif ted to 1518 cm -1 (Fig. 2c, Nf/No = 0.21).This par ti cu lar band is as sig ned to the ske le ton vi bra ti o nal mode of the ring which in vol ves cou pled vi bra ti ons, νCN + νCC 15,16 .This band nor mally is shif ted to ward hig her frequency upon me tal-nitrogen bond for ma ti on 3,8 .Other bands ob ser ved in the spec tra are not sen si ti ve to ni trogen-metal bond for ma ti on.
If the ad sorp ti on oc curs ex clu si vely at the ben zi mi dazo le ba sic site i.e. , due to the for ma ti on of a ni tro gen-metal bond, the sa tu ra ti on va lue should give an idea of the ave rage num ber of li gands co or di na ted to the cen tral me tal ion.Ho we ver, it must be re mem be red that the sur fa ce is he te roge ne ous and the li gands are not dis tri bu ted uni formly on this sur fa ce and thus, many of them may re ma in un co or dina ted even at sur fa ce sa tu ra ti on con di ti ons.

Elec tro nic spec tra
The elec tro nic spec tra for the com plex with Nf/No 0.065, 0.16 and 0.21 were re cor ded.Ho we ver, no spec tral chan ges (Fig. 3) were ob ser ved as the de gree of lo a ding was va ri ed, which may sug gest that in every case only one che mi cal spe ci es was for med on the sur fa ce.A dif fe rent beha vi or was ob ser ved for Cu(II) com ple xes ad sor bed on ben zi mi da zo le-modified si li ca, whe re the spec tra chan ged for dif fe rent de gre es of me tal lo a dings 8 .In the last case, more than one spe ci es was for med on the sur fa ce.

ESR spec tra
Fi gu re 4 shows the esr spec tra ob ta i ned for (≡SiL) mCo(ClO 4)2, un der the ex pe ri men tal con di ti ons: Nf/No = 0.065 and 0.21.For Nf/No = 0.21 (Fi gu re 4a) one can dis tin guish very well nine su perh yper fi ne line struc tures, cha rac te ris tic of the in te rac ti on of ben zi mi da zo le ni trogen atoms with the co balt un pa i red elec tron, cor res pon ding to the tran si ti on M s = -1/2 to 1/2, and Co nu cle ar spin I = 7/2.The ot her ex pec ted tran si ti ons are ba rely seen due to the bro a de ning ef fects but the re are re si du al bro a de ned superh yper fi ne li nes that may be iden ti fi ed by com pa ring the se cond har mo nic ex pe ri men tal spec trum with the si mu lated one.The less con cen tra ted spe ci es (Fig. 4b) shows broa de ned su perh yper fi ne li nes in all hyper fi ne tran si ti ons in clu ding tho se of M = -1/2 to 1/2.The spec tra (Fig. 4b) as a who le re sem bles that cor res pon ding to the more con cen trated per chlo ra te spe ci es (Fig. 4a).This be ha vi or is in di ca tive that the ove rall symmetry of the co balt-benzimidazole com plex is not chan ged when co balt is di lu ted i.e. , the in terac ti on with four ben zi mi da zo le mo le cu les per sists.The per sis tent in ten se line (star red pe aks) with g = 2.00 vi si ble in all spec tra for ≡SiL or (SiL) m Co(ClO 4 ) 2 , is due to the Fe 3+ im pu rity.
The spec tra shown in Fig. 4 may be analy zed con side ring a squa re pla nar struc tu re with the me tal ion at the ori gin and four li gands at the equa to ri al po si ti ons.A pow der si mu la ti on spec trum was ge ne ra ted using the hamil to ni an pa ra me ters in Ta ble 1 and the line sha pe consis ting of a ra tio of Ga us si an to Lo rent zi an equal to 50 per cent and line widths of 35 ga uss in the pa ral lel and 30 ga uss in the per pen di cu lar po si ti ons, res pec ti vely.The cal cu la ti ons were car ri ed out using the ac cep ted va lu es for P = 254 cm -1 17 and the spin or bit cou pling cons tant λ = 178 cm -1 18 .The cal cu la ted per pen di cu lar and pa ral lel di rec tions of the su perh yper fi ne cou pling cons tant of the li gand, K N , by si mu la ti on up to se cond or der in the per tur ba ti on tre at ment for the su perh yper fi ne spec tra 19 , gave: K N ⊥ = 16 and K N || = 8 ga uss.The re sul ting si mu la ted spec trum, which agre es fa irly well with the ex pe ri men tal spec trum, is shown in Fig. 5.

Con clu si ons
Only one ad sor bed com plex spe ci es is de tec ted for diffe rent sur fa ce lo a dings with the me tal ion.The ESR study in di ca ted that we have es sen ti ally an oc ta he dral com plex with te tra go nal dis tor ti on in a low spin con fi gu ra ti on.Under the D 4h crystal li ne elec tric fi eld, Co(II) in te racts with four ni tro gen nu clei of ben zi mi da zo le mo le cu les in the equa to ri al pla ne.Accor ding to the fo re go ing analy sis  we have es sen ti ally a 2 A 1 ground sta te with the elec tron in a dz 2 or bi tal.About 58 per cent of the nor ma li zed an gu lar vari a ti on of this or bi tal is in dx 2 -y 2 and this jus ti fi es the ob serva ti on of su perh yper fi ne li nes with the four ni tro gen nu clei in the equa to ri al pla ne of symmetry of the com plex.
Fi gu re 5. Se cond har mo nic si mu la ted spec trum using data from Table 1.The agre e ment is fa irly good for the cen tral tran si ti on (M = -1/2 to 1/2) whe re su perh yper fi ne li nes 1 to 9 are in di ca ted.In the real spec trum the ot her tran si ti ons are bro a de ned but some struc tu re is visi ble by com pa ring with the si mu la ted spec trum.

m 4 -
≡SiL (s) + Co 2+ (sln) + 2ClO (sln) =(≡SiL) m Co(ClO 4 ) 2(s)The equa ti on shows that when the me tal ion dif fu ses into the so lid-solution in ter fa ce it is fol lo wed by the ClO 4