spectroscopic and theoretical study of cu(ii), zn(ii), ni(ii), co(ii) and cd(ii) complexes of...

Spectrochimica Acta Part A 63 (2006) 403–415

Spectroscopic and theoretical study of Cu(II), Zn(II), Ni(II),Co(II) and Cd(II) complexes of glyoxilic acid oxime

Ivelina Georgievaa, Natasha Trendafilovaa,∗, Gunther Bauerb

a Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgariab Institute of Chemical Technologies and Analytics, Technical University of Vienna, Vienna A-1060, Austria

Received 20 April 2005; accepted 19 May 2005

Abstract

The paper presents a detailed experimental and theoretical study of five metal complexes of glyoxilic acid oxime (gaoH2), Cu(gaoH)2(H2O)2(1), Zn(gaoH)2(H2O)2 (2), Co(gaoH)2(H2O)2 (3), Ni(gaoH)2(H2O)2 (4) and [Cd(gaoH)2(H2O)2]·H2O (5). The electronic and vibrational spectrawere measured and discussed as to the most sensitive to the M–L binding bands. Two different types of coordination were considered forgaoH− ligand: bidentate through the carboxylic oxygen and oxime nitrogen in1–4 and mixed bidentate and bridging through the COO group in5 (d,p))c ries,e xes studied.T©

K

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afi(aweRsTselaao

itorsnitro-

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tral,effi-

rs ofp-ntatecar-ng, (2)itro-ndgs,ur-,

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. It is shown that the spectral behavior of theν(COO) modes can be used to predict bridging ligand coordination. DFT(B3LYP/6-31++Galculations on model compounds: neutral, anionic and radical forms of gao and Cu(gaoH)2, have been carried out to correlate geometlectronic and vibrational structures. The results obtained were used to assist the electronic and vibrational analysis of the complehe effect of the metal–ligand interactions (electrostatic and covalent) on the geometry structure of the ligand was investigated.2005 Elsevier B.V. All rights reserved.

eywords: Glyoxilic acid oxime; Cu(II), Zn(II), Co(II), Ni(II) and Cd(II) complexes; DFT calculations; Electronic and vibrational structure

. Introduction

There is a growing interest in the molecular designnd in the coordination chemistry of structurally modi-ed bio-ligands containing different donor groups. The2-hydroxyimino)carboxylic acids (2-hica) are structuralnalogues of amino acids, where the amino group (NH2)as substituted with hydroxyimino group (NOH). Thextensive studies of 2-(hydroxyimino)carboxylic acids,

C( NOH)COOH, both in solution[1–6] and in the solidtate[7–13]are due to their original coordination properties.he coordination ability of 2-hica has applications ineveral fields: in analytical chemistry and metallurgy as veryffective complexing agents[14] in metal oxide ceramics as

ow temperature precursors[15] in organometallic reactionss suitable matrices[16] in molecular magnetism for designnd synthesis of polynuclear assemblies[17]. Differentximes and their metal complexes have shown versatile

∗ Corresponding author. Tel.: +359 2 9792592; fax: +359 2 8705024.E-mail address: [email protected] (N. Trendafilova).

bioactivity as chelating therapy agents, as drugs, as inhibof enzymes and as intermediates in the biosynthesis ofgen oxide. The 2-(hydroxyimino)carboxylic acids formnew class of compounds used as models of metal–printeractions[18].

By versatile conditions (pH and temperature) the neuanionic and dianionic species of 2-hica are specific andcient coordinating ligands. The alternative donor centeoxime group (ON, N) and carboxylic group (O, O) presupose an abundance of M-(2-hica) bindings. The monodebinding could occur either through the N or through thebonyl O atoms[13]. A variety of bidentate modes of bindiare possible: (1) through both carboxylic oxygens (O, O)through one of the carboxylic oxygen and the oxime ngen (O, N)[1–10]and (3) through one of the carboxylic aoxime oxygens (O, ON). In the cases of bidentate bindin(1), (2) and (3), the ligand forms with the metal stable fofive- and six-membered rings, respectively.

Glyoxilic acid oxime, gaoH2 (IUPAC name: 2(hydroxyimino)acetic acid) is the simplest 2-hica a(HC( NOH)COOH) and hence it is a suitable model

386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2005.05.027

404 I. Georgieva et al. / Spectrochimica Acta Part A 63 (2006) 403–415

investigation of the M–L interactions in this class of com-pounds. Our previous structural (X-ray) and vibrational studyhas shown that in the solid state, gaoH2 forms H-bondedtetramers stabilized with two type H-bonds, O· · ·H O andN· · ·H O[19,20]. In aqueous solution, however, the ligand isdeprotonated and acts as anion, gaoH− [7]. Recently, on thebasis of accurate post-HF and DFT(B3LYP/6-31G++(d,p))calculations, the most probable conformation for coordina-tion of gaoH− anion to metal ions was predicted[21]. Tocheck the reliability of the theoretical prediction, the coordi-nation ability of glyoxilic acid oxime was studied in coordi-nation reaction with series of transition metals.

In the present paper we present synthesis, physico-chemical and spectroscopic characterization of transitionmetal complexes of gao. The coordination properties of gaoligand as well as the type of the M–L interactions in Cu(II)–,Zn(II)–, Co(II)–, Ni(II)– and Cd(II)–gao complexes werestudied on the basis of spectroscopic (electronic, IR andRaman) and theoretical (DFT and MO) data. The geometric,electronic and vibrational changes of the ligand, producedby metal binding were estimated with the help of gaoH− andCu(gaoH)2 model calculations. The results were used (1) tospecify the ligand bond lengths, sensitive to the Cu(II)Nand Cu(II) O binding and to assist the vibrational assign-ments and (2) on the basis of the vibrational analysis of thecomplexes studied to suggest the metal(II)–ligand bondings thel esti-m andb etali ox-i r toe talsi

2

2

hodd s-t .1 da(i on-s top( redw x-i a,H tica on-ds

oxime oxygen pKa(2) = 11.61. At the same time, dueto the electron-withdrawing effect of 2-cyano group,HONC(CN)COOH has very acid carboxylic oxygen(pKa(1) = 1.25) and oxime oxygen (pKa(2) = 6.61)[1,2].

2.2. Preparation of Cu(II), Zn(II), Co(II), Ni(II) andCd(II) complexes with glyoxilic acid oxime

The metal salts used, Cu(CH3COO)2·H2O,Zn(CH3COO)2·2H2O, CoSO4·7H2O, NiSO4·6H2O,and Cd(CH3COO)2, were analytical grade purity products.Complexes of the form M(gaoH)2(H2O)2 for M = Cu(II),Zn(II), Co(II) and Ni(II) and [M(gaoH)2(H2O)2]·H2Ofor M = Cd(II) were obtained by treating gaoH2 with thecorresponding metal salts in a M:L = 1:2.2 molar ratio inaqueous solution.

Cu(gaoH)2(H2O)2. [trans-bis(2-Hydroxyiminoacetato-N,O)diaquacopper(II)] (1). A concentrated aqueoussolution of the gao ligand (0.1469 g, 0.165 mmol in 1.5 mlH2O) was added dropwise to an aqueous solution ofCu(CH3COO)2·H2O (0.1497 g, 0.075 mmol in 1 ml H2O),pH of the mixture is∼4. The reaction was carried out atroom temperature, in a closed system and without stirring.After 3 days, the blue-green crystals formed were filteredoff and washed with cold water. Yield: 0.1137 g, 55%.The monocrystals for X-ray diffraction determination wereo tiono omt ,2 0%.MD

-N art oli mw rayd1 7%.MD

-N ds oli erefi %.A 4.F ity,Λ

-N ds in1 erefi g,5 ;N lar

trength. The effect of the intermolecular H-bonds onigand bond lengths and their frequency modes was also

ated. Vibrational criteria was obtained for unidentateridging binding of COO group of gao to the transition m

on. The coordination ability and gas phase basicity of glylic acid oxime was compared with that of glycine in ordexplain the higher selectivity of gao to the transition me

n solution with low pH.

. Experimental section

.1. Characterization of the ligand

Glyoxilic acid oxime was prepared using the metescribed in the literature[22]. The product was recry

allized from an ethylacetate–nC7H16 mixture (m.p45–150◦C). The pKa values of gaoH2 could be uses a measure for the carboxylic (O−) and oximeON

−) oxygen basicities and reveal their�-donor abil-ties. GaoH2 exhibits two measurable protonation ctants, pKa(1) = 3.05 and pKa(2) = 7.56, correspondingrotonation of the carboxylic (COO−) and hydroxyiminoC NO−) group, respectively. These values compaell with literature data about two derivatives of glyo

lic acid oxime, (2-hydroxyimino)propionic acid (2-hipONC(CH3)COOH) and 2-cyano-(hydroxyimino)acecid (2-chiaa, HONC(CN)COOH). Due to the electronating effect of the methyl group, HONC(CH3)COOHhowed more basic carboxylic oxygen pKa(1) = 3.25 and

btained by recrystallization from water and evaporaf the solvent from the filtered reaction mixture at ro

emperature. Anal. Calc. for C4H8N2O8Cu: C, 17.43; H.93; N, 10.16%. Found: C, 18.20; H, 3.23; N, 10.2olar conductivity, ΛM: 40.4 cm2 �−1 mol−1 in MeOH.ecomp.:∼270◦C.Zn(gaoH)2(H2O)2. [trans-bis(2-Hydroxyiminoacetato

,O)diaquazinc(II)] (2). Complex2 was synthesized similo 1, using Zn(CH3COO)2·2H2O (0.1591 g, 0.0725 mmn 1 ml H2O). Yield: 0.066 g, 33%. Recrystallization froater yields colorless monocrystals suitable for X-iffraction determination. Anal. Calc. for C4H8N2O8Zn: C,7.32; H, 2.91; N, 10.1. Found: C, 17.93; H, 3.3; N, 9.6olar conductivity, ΛM: 5.5 cm2 �−1 mol−1 in MeOH.ecomp.:∼340◦C.Co(gaoH)2(H2O)2. [trans-bis(2-Hydroxyiminoacetato

,O)diaquacobalt(II)] (3). Complex 3 was synthesizeimilar to 1, using CoSO4·7H2O (0.2512 g, 0.0893 mm

n 1 ml H2O). After 7 days, the rose crystals formed wltered off and washed with cold water. Yield: 0.072 g, 30nal. Calc. for C4H8N2O8Co: C, 17.73; H, 2.98; N, 10.3ound: C, 18.05; H, 2.92; N, 10.47%. Molar conductivM: 4.2 cm2 �−1 mol−1 in MeOH. Decomp.:∼300◦C.Ni(gaoH)2(H2O)2. [trans-bis(2-Hydroxyiminoacetato

,O)diaquanickel(II)] (4). Complex 4 was synthesizeimilar to 1, using NiSO4·6H2O (0.1978 g, 0.7525 mmolml H2O). After 6 days, the light blue crystals formed wltered off and washed with cold water. Yield: 0.10140%. Anal. Calc. for C4H8N2O8Ni: C, 17.74; H, 2.98, 10.35%. Found: C, 17.9; H, 3.22; N, 13.73%. Mo

I. Georgieva et al. / Spectrochimica Acta Part A 63 (2006) 403–415 405

conductivity, ΛM: 0.8 cm2 �−1 mol−1 in DMF. Decomp.:∼255◦C.

[Cd(gaoH)2(H2O)2]·H2O. [m-(2-Hydroxyiminoacetato-O,O)-trans-bis(2-hydroxyimino-acetato-N,O)diaquacadmium(II)] (5). Complex5 was synthesizedsimilar to 1, using Cd(CH3COO)2 (0.1761 g, 0.0764 mmolin 1 ml H2O). After 7 days, the colorless crystals, suitablefor X-ray analysis, were obtained. Yield: 0.061 g, 23%.Anal. Calc. for C4H10N2O9Cd: C, 14.03; H, 2.94; N, 8.18%.Found: C, 19.62; H, 3.08; N, 8.22%. Molar conductivity,ΛM: 5.3 cm2 �−1 mol−1 in DMF. Decomp.:∼300◦C.

2.3. Measurements and analysis

The melting points of the complexes1–5 were measuredwith Boetius apparatus and were not corrected. The elemen-tal analysis for C, H and N were performed according tostandard microanalytical procedures. The electrical conduc-tivity measurements were performed using a Conductivitymeter type OK-102/1, Radelkis, for 10−3 mol l−1 solutionsat 25◦C in dimethylformamide (DMF) for4 and 5 and inmethanol for1–3. The solid-state infrared spectra of thecomplexes studied were recorded in the 4000–150 cm−1 fre-quency range (in KBr 4000–400 cm−1 and in CsI in the400–150 cm−1) by FTIR 113V Bruker spectrometer. TheR werem trom-e -c .A ser( eso-l inH ) int il-i field1

3

98A 6-3 ramp ss-f cu-l s ofg -t

4

tenta f thec

Zn(II), Co(II) and Ni(II) and (2) [M(gaoH)2(H2O)2]·H2Ofor M = Cd(II). The molar conductivity values of0.8–5.3 cm2 �−1 mol−1 in DMF (for 4 and 5) and inMeOH (for 2 and 3) indicated non-electrolytic behaviorof the complexes. In the studied series of complexes, ahigher conductivity value for1 in methanol was obtained,40.4 cm2 �−1 mol−1; it is however lower than those for1:1 electrolytes (80–115 in CH3OH) [27]. This valuecould be explained in terms of partial dissociation of1,which takes place in solution. All the complexes studieddecomposed above 250◦C. They were found to be solublein water, methanol, ethanol and DMF and insoluble incommon organic solvents (CH3COCH3, CCl4, CHCl3). Thecomplexes2 and5 are slightly hygroscopic.

4.1. Molecular structures of Cu(II), Zn(II) and Cd(II)complexes of gaoH−

Selected bond lengths and valence angles for Cu(II),Zn(II) and Cd(II) complexes as obtained from X-ray diffrac-tion analysis, are given inTable 1 [28]. According to thestructural data, Cu(gaoH)2(H2O)2 and Zn(gaoH)2(H2O)2complexes are isostructural and crystallize in the triclinicspace groupP-1 (Ci) with one formula unit in the unit cell(Fig. 1). Two gaoH− molecules are located centrosymmet-rically around the metal atom. Both gaoH− molecules acta h then orm-i HlN hec osi-t -t

ledsoC rM icityo edt ,s urs,w atedOw hipa[ )m r( II)cflc ,p)c db e Ni hea

aman spectra of the compounds mentioned aboveeasured in the solid state with a SPEX-Ramalog specter (double monochromator with 1152× 298 pixels CCDamera detector) in the 4000–200 cm−1 frequency rangen argon ion laser (514.5 nm) as well as krypton ion la

647.1 nm) were used for excitation. In all cases the rution was 1 cm−1. The UV–vis spectra were recorded

2O solution on a Specord UV–vis (Carl-Zeiss, Jenahe 50 000–10 000 cm−1 region. The magnetic susceptibty was determined at room temperature and magnetic0 000 Oe, using the Faraday method.

. Computational procedure

All computations were performed using GAUSSIAN.9 program package using B3LYP method and1++G(d,p) basis set as implemented within the progackage[23,24]. The B3LYP method has proven succe

ul in determining the conformational behavior, molear geometries and the harmonic vibrational frequencielycine [25] and its Cu(II) complex[26]. TDDFT calcula

ions were done with GAUSSIAN03 program package.

. Results and discussion

The values for carbon, hydrogen and nitrogen conre in agreement with the suggested general formula oomplexes studied: (1) M(gaoH)2(H2O)2 for M = Cu(II),

s bidentate ligands and coordinate to the metal througitrogen and the deprotonated hydroxyl oxygen atoms, f

ng two planar five-membered chelating rings. The gao−ies in a plane intrans position (O1 Cu(Zn) O1 = 180.0◦,

Cu(Zn) N = 180.0◦). Two water molecules complete toordination sphere of the metal atom, taking the axial pions of a distorted octahedron with equal MO4 bond disances.

Both Cu(II) and Zn(II) structures have reveahorter M O bond lengths in comparison with MNnes (Cu O = 1.9445(13)A, Zn O = 2.0421(11)A,u N = 2.0630(15)A, Zn N = 2.1780(13)A). The shorte

O bond lengths were explained with the higher basf the carboxylic oxygen. Our DFT calculations show

hat by optimization of the N-protonated gaoH− moleculepontaneous H-proton transfer from N to O atom occhich indicates higher proton basicity of the deprotonthan that of the N atom. The same trend (MO < M N)

as observed for other bis-chelate complexes: with 2-8,9] and with glycine[29]. However, for the bis(glycinoetal complexes the difference (MN) − (M O) is smalle

∆ ∼ 0.04A) in comparison with that for the studied M(omplexes with gaoH− (∆ ∼ 0.13A). The larger∆ obtainedor our complexes is obviously due to the longer MN bondength (the M O bond lengths in bis(glycino) and gaoH−omplexes are nearly equal). Our B3LYP/D95++(dalculations showed that the longer MN bond length coule explained with the lower gas-phase basicities of oxim

n gaoH2 (183.87 kcal/mol) in comparison with that of tmino N in glycine (204.4 kcal/mol)[25].


Table 1Experimental bond distances (inA) and valence angles (in degrees) of Cu(II)–, Zn(II)– and Cd(II)–gao complexes[28]

Name definition Cu(gaoH)2(H2O)2 (1) Zn(gaoH)2(H2O)2 (2) [Cd(gaoH)2(H2O)2]·H2O (5)

Bond distancesC1 O2 1.227(2) 1.2341(19) 1.248(2)C1 O1 1.283(2) 1.2769(18) 1.263(2)C1 C2 1.503(2) 1.5064(19) 1.497(2)C2 N 1.271(2) 1.270(2) 1.274(2)N O3 1.3710(19) 1.3808(15) 1.3762(19)O3 H1 0.69(3) 0.72(2) 0.85(3)O4 H41a 0.70(3) 0.73(2)O4 H42a 0.73(3) 0.79(3)O8 H81b 0.79(3)O8 H82b 0.81(3)M O1 1.9445(13) 2.0421(11) 2.3198(19)M O5b 2.3630(15)M N(1) 2.063(15) 2.1780(13) 2.4600(15)M O4 2.395(14) 2.1665(14) 2.3783(14)M O7b 2.3413(14)M O8b 2.2406(17)

Bond anglesO1 M N(1)c 80.90 (5) 77.85(5) 67.88(5)O1 M N(2)d 99.10(5) 102.15(5) 70.14(5)O1 M O4a 90.47(5) 90.04(5)O4 M O4a 180.0 180.0O5 M N1b 86.12(5)O5 M O4b 70.38(5)O8 M O7b 175.77(6)C1 C2 N 115.31(16) 116.80(13) 117.56(16)O1 C1 C2 115.61(15) 116.74(12) 117.48(15)N O3 H1 109.0(2) 105.80(17) 104.1(17)

a The atom numbering for complexes1 and2 are given inFig. 1.b The atom numbering for complex5 is given inFig. 2.c Bite angle.d Interligand angle.

As compared to Cu(II) complex, in Zn(II) the ZnO1and Zn N bond lengths increase, whereas ZnO4 decreases.The axial Cu O4 bond lengths (2.395A) are longerthan Zn O4 (2.1665A) and Cd O4 (2.3413, 2.2406A)ones and correspond to the so-called semi-coordination.Due to the Jahn–Teller effect the Cu(II) complex is astrongly elongated (along the O4Cu O4′ axis) octahedron(Cu O1 = 1.9445A, Cu N = 2.063A, Cu O4 = 2.395A).Our B3LYP/6-31G++(d,p) calculations showed that the dz2orbital in Cu(II) complex is filled and to reduce the repul-sion along thez axis, the water coordinates at much longerdistances than those in Zn(II) and Cd(II) complexes. Theunpaired electron occupies the 3dxy orbital and thus the repul-sion in the plane of the two ligands is minimized. As it isexpected for closed-shell metal ions, the axial ZnO4 bondlengths approach ZnO1 and Zn N in the plane and thus,almost a regular octahedron is realized,Table 1.

Cd(II) complex has shown different crystal structure ascompared to that of Cu(II) and Zn(II) complexes,Fig. 2andTable 1. The Cd atoms are seven-coordinate and the com-plex crystallized in the monoclinicP21/c (C2h) space group,with four molecules in the unit cell. Two gaoH− moleculescoordinate to Cd atom, which is not in a center of sym-metry. As in the case of Cu(II) and Zn(II) complexes both

gaoH− ligands coordinate through the nitrogen and deproto-nated hydroxyl oxygen atoms intrans position. However, thegaoH− ligands in the Cd(II) complex are of different crys-tallographic type. The complex keeps a planar arrangementof the two ligands, which close five-membered chelate ringsincluding the metal atom. X-ray data showed that CdN bondlengths are longer (CdN = 2.461, 2.460A) than Cd O ones(Cd O = 2.378, 2.32A) and both Cd N and Cd O are longeras compared to the corresponding in1 and2. In the planeof the two gaoH− ligands, a third CdO coordination wasdetected with CdO5 = 2.363A. However, the third gaoH−molecule, which acts simultaneously as bridging (throughCOO− group) and chelating ligand, lies in a plane, differentfrom that of the other two gaoH− ligands. Two free coordi-nation sites of the Cd(II) complex are occupied with watermolecules which are located above and below the plane ofthe other two ligands with CdO7 and Cd O8 bond dis-tances being 2.341 and 2.240A. Both Cd O7 and Cd O8bond lengths are shorter in comparison with CdO4 (Cd O1)and Cd N1 (Cd N2) ones and hence a compressed along theaxial direction polyhedron is realized[28].

The O M N bite angle in Cu(II) complex is 80.9◦ and itdecreases in Zn(II) (77.85◦) and Cd(II) (67.88◦ and 65.77◦)complexes,Table 1. This finding correlates with the increas-


Fig. 1. Molecular structure of M(gaoH)2(H2O)2 complexes (M = Cu(II), Zn(II) and Co(II))[28].

ing M O and M N bond lengths in Cu(II), Zn(II) and Cd(II)complex order and it is in agreement with the metal ionicradii: Cu(II) (87 pm) < Zn(II) (88 pm) < Cd(II) (109 pm)[30].The smallest OCd N bite angle and the longest CdO andCd N distances in the plane are due to the largest ionic radiusof Cd(II), which allows the fifth CdO5 contact in the plane.

4.2. Intermolecular H-bondings

Three types intermolecular H-bond were detected inCu(II) and Zn(II) complexes,Fig. 1, Table 2: (1) between theoxygen of the coordinated water, O4′, and the oxime H1 ofadjacent gaoH− molecule, O4′· · ·H1, (2) between H41 of thecoordinated water and the deprotonated hydroxyl O1′ coordi-nated to another metal, H41· · ·O1′ and (3) between H42 of thecoordinated water and the carbonyl O2′′ of adjacent gaoH−,H42· · ·O2′′. In the first H-bond, the water molecule acts asa proton acceptor and in the other two—as a proton donor.X-ray results showed shorter O4′· · ·H1 distance and stronger

Table 2Intermolecular hydrogen-bonding distances (inA) in Cu(gaoH)2(H2O)2,Zn(gaoH)2(H2O)2 and [Cd(gaoH)2(H2O)2]·H2O [28]

H-bonding Cu(II)a Zn(II)a Cd(II)b

O4· · ·H1′ 1.968 2.022 –O4· · ·O3′ 2.65 2.736 –H41· · ·O1’ 2.009 1.943 –O4· · ·O1’ 2.639 2.632 –H42· · ·O2′′ 1.973 1.845 –O4· · ·O2′′ 2.678 2.629 –O4′· · ·H1 – – 1.776O4′· · ·O3 – – 2.624O9′· · ·H81 – – 1.945O9′· · ·O8 – – 2.724O9′· · ·H72′ – – 2.058O9′· · ·O7′ – – 2.829O1· · ·H3 – – 2.083O1· · ·O6 – 2.710

a The atom numbering is in agreement withFig. 1.b The atom numbering is in agreement withFig. 2.


H-bond in Cu(II) as compared to Zn(II) complex. The CuNis shorter than ZnN and hence higher proton donor abilityof H1 is suggested for Cu(II) complex. On the other hand,the longer CuO4 distance supposes higher proton acceptorability of the O4′ atom in1. As a result, both factors pro-duce strengthening of O4′· · ·H1 H-bond in1. The other twointermolecular H-bond distances, H41· · ·O1′ and H42· · ·O2′′in 1 are found longer and the H-bonds are weaker than thecorresponding in2. The stronger H42· · ·O2′′ and H41· · ·O1′H-bonds in2 could be explained with the shorter ZnOH2bond distances, which lead to strengthen of the water pro-ton donor ability as compared to that of1. As a result, onthe basis of the experimental geometry parameters a relationbetween M N and M O binding strength and intermolec-ular H-bond strength was established for Cu(II) and Zn(II)complexes. The M–L binding affects the proton donor andproton acceptor ability of the atoms included in H-bondingsand thus, influences the intermolecular H-binding strength.

The hydrogen bonds in Cd(II) complex are of differenttype in comparison with those in Cu(II) and Zn(II) complexes,Table 2, Fig. 2: (1) two intermolecular H-bonds betweena crystallized water (H91′ O9′ H92′) on one side and thecoordinated water molecules on the other, O9′· · ·H81 andO9′· · ·H72′; (2) one intermolecular H-bond between car-boxylic O4′ and oxime H1, O4′· · ·H1 and (3) one intramolec-ular H-bond between the carboxylic O1 and oxime H3,O

becomes possible due to decrease of the interligand angle(O1 Cd N2 = 70.14◦) upon the fifth Cd O5 coordination inthe same plane. On the basis of the experimental O· · ·O dis-tances one may conclude that the H-bonds of Cd(II) complexare weaker than those in1 and2, Table 2.

4.3. Effect of metal–ligand interactions on the geometrystructure of gao estimated on the basis of DFT modelcalculations

In order to specify the ligand bonds, which are sensitive toCu(II) N and Cu(II) O binding the changes in the gao dueto its bidentate coordination to Cu(II) were followed with thehelp of model calculations of the isolated ligand (in neutral,anion and radical form) and Cu(gaoH)2, Table 3. As it wasalready mentioned, the active form of gaoH2 in solution is theanion form, gaoH−. It is, however, impossible to obtain exper-imentally geometrical parameters of gaoH− and thereforethey were obtained from DFT(B3LYP/6-31++G(d,p)) calcu-lations. The calculated Cu(gaoH)2 structure was found as aglobal minimum inCs symmetry and the obtained structuralparameters are in a very good agreement with experiment.Since the water molecules in axial positions are at quite longdistances for Cu(II) (2.395A) we do not expect that theyhave significant effect on the CuO and Cu N parametersand hence, they were not considered. On the other hand,i the
1· · ·H3. The intramolecular H-bond (O1· · ·H3 = 2.083A)
Fig. 2. Molecular structure of [Cd(ga

n the frame of the model used it was possible to trace

oH)2(H2O)2]·H2O complex[28].


Table 3Selected calculated parameters of different gao species: neutral (ectt), anion (e1−), radical (e1•) and Cu(gaoH)2 and experimental parameters ofCu(gaoH)2(H2O)2

Name definition Calculated (B3LYP/6-31++G(d,p)),Cs symmetry Experimental[28]

gaoH2 (ectt) gaoH− (e1−) gaoH• (e1) Cu(gaoH)2 Cu(gaoH)2(H2O)2

C O 1.216 1.263 1.258 1.218 1.227(2)C O 1.349 1.251 1.27 1.314 1.283(2)C C 1.483 1.551 1.475 1.527 1.503(2)O HCOOH 0.972 – – – –C N 1.28 1.275 1.280 1.278 1.271(2)C H 1.089 1.094 1.090 1.088 0.91(3)N O 1.385 1.451 1.375 1.366 1.3710(19)O HNOH 0.969 0.966 0.969 0.977 0.69(3)M O – – – 1.917 1.9445(13)M N – – – 1.972 2.063(15)O Cu Na 83.7 80.9O Cu Nb 96.3 99.1

a Bite angle.b Interligand angle.

geometrical changes of gaoH−, produced only by the M–Linteractions, but not by the interligand interactions (H-bonds)and the crystal-packing effects.

GaoH2 and gaoH• are global minima inCs symmetry.GaoH−, however, is a global minimum inC1 and a localminimum in Cs symmetry (with 0.35 kcal mol−1 above theglobal minimum). Our calculations showed that gaoH− bondlengths inC1 and Cs symmetry do not differ significantlyand gaoH− parameters inCs could be used for compari-son with Cu(gaoH)2. The calculations showed that the lowestelectronic state of Cu(gaoH)2 structure is2A′ and thus, theopen-shell orbital lies in a symmetry plane.

According to the calculations made, significant changeswere found going from the neutral gaoH2 to the deproto-nated gaoH− form, Table 3. Due to delocalization of thenegative charge on the carboxyl group in gaoH− the C Obonds are equalized. At the same time, CC, C H, N Oare elongated and CN and O H are shortened. The struc-tural parameters obtained for gaoH− are further comparedwith those in the Cu(gaoH)2 complex. The binding of twogaoH− to Cu(II) in trans position through O and N atomsproduced an elongation of C1O1, C2 N and O3 H1 bondlengths and a shorten of C1O2, C1 C2, C2 H2 and N O3ones. The trends obtained from the calculations are in excel-lent agreement with the experimental data for Cu(II) complex(Table 3) and this finding allowed us to use the informationf c-t

r-a lentM s oft u-b� Hs italsuN ereas

C1 O1, C2 N, C2 H2 and O3 H1 became more polar-ized and elongate in Q(2+)-gaoH−. In order to estimate thegeometry changes of gao which occur upon gaoH− → Cu(II)�-donation, we simulate the electron density depletion con-sidering the radical, gaoH•. The comparison of the geometryparameters of gaoH• and gaoH− showed the same geometrychanges as those produced from Q(2+) (Table 3). Obviously,the bond length changes in gaoH− produced by the doublepoint charge and by the gaoH− → Cu(II) �-donation are inthe same direction and they follow the changes obtained ingaoH− upon the Cu–gaoH interaction.

A survey of the calculated bond lengths of gaoH− andthe experimental ones of gaoH− in 1, 2 and 5 allow usto trace the ligand bond length changes sensitive to thestrength of the M–L interactions. The experiment showed thatC1 O2 and C1 O1 bond lengths are sensitive to the MO1binding: when the MO1 bond length becomes shorter,the C1 O2 becomes shorter whereas the C1O1—longer(Tables 1 and 3). Further, the NO3 bond length was foundalso to be sensitive to MN binding. In the studied M(II) com-plexes, CuN bond is the shortest one and NO3 showed thelargest shortening. The analysis of the ligand bond lengths inthe metal complexes studied will assist further the vibrationalanalysis as well as it will be used for suggestion of M(II)–gaobinding strength in Ni(II)– and Co(II)–gao complexes.

A comparison of the geometrical parameters of gaoH−a omX wst etali ionsiba genbT terss erf d is

rom Cu(gaoH)2 calculations for interpretation of the eleronic and vibrational spectra of Cu(II) complex.

The geometrical changes of gaoH− upon Cu–gaoH intection could be attributed to the electrostatic and cova–L contributions and they are estimated: (1) by change

he MO polarization of gaoH− upon the influence of the dole point charge of the metal (+2) and (2) by gaoH− → Cu(II)-donation. The NBO analysis of gaoH and Q(2+)-gao−howed significant polarization of the gaoH bonding orbpon the double point charge,Fig. 1: the C1 O2, C1 C2 and

O3 bonds became less polarized and shorten, wh

nd Cu(gaoH)2 models from one side and that obtained fr-ray analysis of Cu(II) complex from the other one allo

o distinguish the ligand changes that occur upon the mnteractions and that—upon the intermolecular interactn the crystal. In Cu(II) complex the experimental O3H1ond length is quite shorter than the calculated one (Table 3)nd could be explained with the intermolecular hydroond, O4′· · ·H1 O3 and O4′ → H1 electron transfer,Fig. 1.he comparison of Zn(II) and Cu(II) structural paramehowed that the O4′· · ·O3 and O4′· · ·H1 distances are shortor Cu(II) complex and the intermolecular hydrogen bon


stronger. The stronger hydrogen bond in Cu(II) complex sug-gests larger O4′ → H1 electron transfer, which produces theobserved shorter O3H1 bond length. In the three complexesstudied, C C, C N and C H bonds vary only slightly andthey are not indicative to the M–L binding strength.

4.4. Electronic spectra and structure. DFT and MOcalculations

The electronic spectrum of glyoxilic acid oxime in watersolution showed one absorption band at 48 480 cm−1. Ver-tical excitation energies for the low-lying singlet and tripletelectronic states have been calculated for the gaoH− form(e1−) in water solution using TDDFT/B3LYP/6-31++G(d,p)method. The excitation energy of the most intensive tran-sition was calculated at 47 057 cm−1 and this value wasin good agreement with the observed strong absorptionpeak at 48 480 cm−1. According to the calculations themost intensive transition in the gaoH− electronic spectrumrevealed mixed� → �* character (with carboxylic O pxand py orbital contributions) and� → �* character (withoxime O pz and N pz orbital contributions). In the elec-tronic spectrum of 2-hipa (where H(C) atom in gaoH− issubstituted with CH3 group) this transition was observedat lower wavenumbers (38 000 cm−1) [5]. Obviously, theCH group, which posses a positive polarization effect, low-e isono wedt tiond (II),Z herf nd4 lexe ciesw sa atest nicc theCa ics tionw con-fi ition.T ddi-t mA forC mra ng oft /6-3 )T ed:dmw es-t pen-

shell orbital is dxy (x and y coordinates bisect the biden-tate ligands) whereas in monodentate square-planar com-plexes the open shell orbital is dx2–y2 (x and y axes aredirected along the M–L bonds)[31]. Thus, four spin-allowed d–d transitions are possible in the visible region:dz2(A′) → dxy(A′), dyz(A′′) → dxy(A′), dxz(A′′) → dxy(A′)and dx2–y2(A′) → dxy(A′). The broad and symmetrical bandat 16 160 cm−1 in Cu(gaoH)2 complex spectrum couldbe assigned as dz2(A′) → dxy(A′) or dx2–y2(A′) → dxy(A′).The bands at 37 760 and 30 320 cm−1 were associ-ated with L–Cu(II) charge transfer:�(O)→ dxy(Cu(II)) or�(N) → dxy(Cu(II)) transitions. For [Cu(gly)2], the bandaround 40 000 cm−1 has been assigned to O(�) → dx2–y2transition[32]. At the same time, the band at 27 778 cm−1

for Cu-chiaa has been associated with Nox → Cu(II) tran-sition [3]. Our MO calculations suggested that HOMO ofgaoH− as well as the high-energy MOs of Cu(gaoH)2 containmainly contributions of both carboxylic oxygens, whereasthe lower energy MOs have oxime N and O orbital con-tributions. Hence, the band at 30 320 cm−1 were associ-ated with O(�) → dxy(Cu(II)) transition and the band at37 760 cm−1 to N(�) → dxy(Cu(II)) transition. The calcu-lated Mulliken atomic charges in gaoH− and in Cu(gaoH)2showed that O1 and N charges become less negative (O1:−0.626→ −0.568; N:−0.313→ −0.197) and Cu(II) sig-nificantly lowers its positive charge, 2.0→ 0.032 in the com-pN ento t3 -l ast eva rs.

ex( e ins I)c and2 theca[ -ti nd1 oO

isi e ins II)co d thes a-h edd( llyi two-

3rs the energy of the intraligand transition. A comparf the electronic spectra of the complexes studied sho

hat the vertical excitation energy of intraligand transiepends on the nature of the metal ion. In Ni(II), Con(II) and Cd(II) complexes this band is shifted to hig

requencies with∼400 cm−1 (at 48 960, 48 800, 48 816 a8 960 cm−1, respectively), whereas in the Cu(II) complectronic spectrum it appears at much lower frequenith 1120 cm−1 (at 47 360 cm−1). The highest shift of thibsorption observed in the Cu(II) complex spectrum indic

hat gaoH− undergoes significant geometric and electrohanges upon Cu(II) coordination. It is expected thatu(II) coordination mainly affects the HOMO(O px, O py)nd LUMO (N py, ON px) of the ligand. In the electronpectra of Zn(II) and Cd(II) complexes only this absorpas observed (closed-shell metal ions) and this findingrmed the assignment of the band as an intraligand transhe electronic spectrum of Cu–gao complex shows in a

ion three absorptions at 16 160, 30 320 and 37 760 c−1.s it was stated in the literature, the d–d transitionsuN2O2 chromophore, appeared in the 13 000–20 000 c−1

egion. Thus, the broad band at 16 160 cm−1 could bessigned as a d–d transition. For better understandi

his absorption, molecular orbital calculations at B3LYP1G++(d,p) level of theory were performed for Cu(gaoH2.he following energy order of the d-orbitals was obtainxy(A′) > dz2(A′) > dyz(A′′) > dxz(A′′) > dx2–y2(A′). As alreadyentioned, the unpaired electron occupies the dxy orbitalith A′ symmetry. This is in agreement with the sugg

ion that in bidentate square-planar complexes the o

lex. The values obtained suggested OgaoH→ Cu(II) andgaoH→ Cu(II) �-donations and confirmed the assignmf the band at 30 320 cm−1 to O(�) → dxy(Cu(II)) and a7 760 cm−1 to N(�) → dxy(Cu(II)) transitions. The calcu

ated effective magnetic moment of Cu(II) complex wemperature independent, withµeff = 2.34 BM. The negativalue of the Wiess constant (θ =−186 K) indicated mediumntiferromagnetic interaction between the copper-cente

The effective magnetic moment for Ni(II) compl3.20 BM) suggested high-spin state of Ni(II) and middltrength crystal field[33]. The electronic spectrum of Ni(Iomplex showed four bands at 13 522, 15 608, 18 8006 560 cm−1. Assuming an octahedral coordination ofomplex, the bands at 13 522, 15 608 and 18 800 cm−1 werettributed to spin allowed d–d transitions [3A2g→ 3T2g] (ν1),

3A2g→ 3T1g(F)] (ν2) and [3A2g→ 3T1g(P)] (ν3), respecively. For comparison, the d–d transitions for Ni(hipa)2 (hipas a CH3-substituted gaoH2) were observed at 10 352 a6 892 cm−1 [2]. The band at 26 560 cm−1 was assigned t(�) → Ni(II) transition.The µeff value of Co(II) complex is 4.76 BM which

n agreement with a high-spin state of Co(II) and middltrength crystal field[34]. In the electronic spectrum of Co(omplex, two bands at 20 448 cm−1 and at 35 120 cm−1 werebserved. The first one is attributed to d–d transition anecond one to O(�) → Co(II) transition. Assuming an octedral coordination for Co(II) complex, three spin allow–d transitions are expected:4T1g→ 4T2g (ν1), 4T1g→ 4A2gν2) and4T1g→ 4T1g(P) (ν3). The first band appeared usuan the near IR region, whereas the second transition is


Fig. 3. IR spectra of M(gaoH)2(H2O)2 complexes (M = Cu(II), Zn(II), Co(II), Ni(II) and Cd(II)).

electronic process and therefore its band is with low-intensity.The broad band with maximum at 20 448 cm−1 correspondsto the highest-energy4T1g→ 4T1g(P) (ν3) transition.

4.5. Vibrational analysis

As it was obtained from X-ray diffraction analysis, inthe solid state, the glyoxilic acid oxime, gaoH2, forms H-bonded tetramer and frequency modes concerning NOH andCOOH groups are affected from the H-bondings. Therefore,the solid-state IR and Raman spectra of gaoH2 are not suit-able base for tracking the ligand frequency changes in thecomplexes and hence for understanding the metal–ligandbinding mode. Moreover, in solution gaoH2 is deprotonatedand the active form in metal–ligand reaction is gaoH−. How-ever, experimental IR data about gaoH− are lacking. The IRspectrum of Na complex with gaoH− revealed features ofbidentate and bridging coordination and hence it was alsonot suitable for comparison with the IR spectra of the com-plexes studied. For that reason the vibrational analysis of the

complexes studied was supported with calculations of thevibrational spectra and frequency assignments for gaoH− andCu(gaoH)2 models at B3LYP/6-31++G(d,p) level of theory.The experimental IR and Raman spectra of Cu(II), Co(II),Ni(II), Zn(II) and Cd(II) complexes and the assignments ofthe vibrational frequencies are given inTable 4. The experi-mental IR spectra of the complexes studied are presented inFigs. 3 and 4.

The spectral behaviors of Cu(II), Zn(II), Co(II) and Ni(II)complexes are similar in the 2000–400 cm−1 and quite dif-ferent in the 4000–2000 and 400–200 cm−1 regions.

4000–2000 cm−1. Broad and low-intensity bands dueto ν(O H) modes of NOH groups and coordinated H2Owere observed in the 4000–3000 cm−1 frequency region. InCu(II) complex spectrum the bands at 3292 and 3143 cm−1

were assigned toν(OH)NOH andν(OH)w, respectively. Theν(OH)NOH modes in the complexes studied were observedin the following order: 3292 cm−1 for Cu(II), 3226 cm−1

for Zn(II), 3226 cm−1 for Co(II) and 3216 cm−1 for Ni(II)complex. The calculated structural parameters of gaoH−


Table 4Experimental (IR and Raman) frequencies of Cu(II)–, Zn(II)–, Ni(II)–, Co(II)– and Cd(II)–gao complexes and their approximate description

Cu(gaoH)2x(H2O)2,IR and R

Zn(gaoH)2x(H2O)2,IR and R

Ni(gaoH)2x(H2O)2,IR and R

Co(gaoH)2x(H2O)2,IR and R

[Cd(gaoH)2(H2O)2]·H2O, IR

Approximate descriptiona

– – – – 3592w,br ν(OH)w (crys.)3529w

3292w (IR) 3226w (IR) 3216w (IR) 3216w (IR) 3177w ν(OH)NOH

3143w (IR) 3106w (IR) 3084w (IR) 3084w (IR) 3077vw,br ν(OH)w (coor.)3053br (R) 3035m (R) 3038m (R) 3038m (R) 3038m ν(CH)3038m (IR) 3035m (IR) 3037m (IR) 3016m (IR)1659m (R) 1650m (R) 1650s (R) 1645s (R) 1592s νas(CO)1670s (IR) 1668s (IR) 1664s (IR) 1662s (IR)1648sh (IR) 1649sh (IR) 1650sh (IR) 1654sh (IR) 1661m, 1653m δ(HOH)w1642m,d (IR) 1635m (IR) 1636m (IR) 1634m (IR) 1641m,br. ν(C N)1634sh (R) 1610w (R) 1610w (R) 1610w (R)1471m (R) 1478m (R) 1471w (R) 1471m (R) 1501w δ(NOH)i.p.

1466m,d (IR) 1475m,d (IR) 1469m,d (IR) 1470m,d (IR) 1487m1377m (IR) 1386m (IR) 1386m (IR) 1380m (IR) 1391m νs(CO)1368m (R) 1377m (R) 1378m (R) 1372m (R)1285s (R) 1294s (R) 1291s (R) 1290s (R) 1281m, δ(NCH)i.p. + δ(CCH)i.p.

1275s (IR) 1283s (IR) 1282s (IR) 1280s (IR) 1260m,d1077m (IR) 1050s (IR) 1056s (IR) 1047s (IR) 1051s ν(NO)1072w (R) 1050m (R) 1052w (R) 1044w (R)

1019w991m (R) 990m (R) 994m (R) 992m (R) 990w δ(CCH)o.p.+ δ(NCH)o.p.

986w (IR) 985w (IR) 988w (IR) 987w (IR) 976w952m (R) 959m (R) 956w (R) 954m (R) 946w ν(CC)953w (IR) 960w (IR) 958w (IR) 956w (IR)837w,br (IR) 873m,br (IR) 899m,br (IR) 883m,br (IR) 933w, 923w,br δ(OCO)i.p.

780m (IR) 790m (IR) 800m (IR) 796m (IR) 763m δ(OCO)o.p.

767w (R) 770w (R) 769w (R)669w (IR) 745w (IR) 744w (IR) 747w (IR) 502 vw,br H2O libration645br (IR) 721w (IR) 732w (IR) 732w (IR) (w,r,tw)

706w,br (IR) 693w,br (IR) 692w,br (IR)747w (R) 745w (R) 746w (R)

572vw (IR) 563vw (IR) 571vw (IR) 565vw (IR) 537vw δ(CCO)i.p. + δ(CCN)i.p.

563w (R) 553w (R)512w (IR) 499w (IR) 499w (IR) 499w (IR) 485w δ(NOH)o.p.

510s (R) 498w (R) 502m (R) 498m (R)395vw (IR) 390vw (IR) 387vw δ(CCN)i.p. + δ(CCO)i.p.

385w (R) 384w (R)380w (IR) 380w (IR) 378w (IR) 378w (IR) 380w δ(CNO)i.p.

375w (R) 368w (R) 380w (R) 365w (R)361w (IR) 342w Overlap. 356w (IR) 350vw δ(CNO)o.p.

344w (R)340m (IR) 305m (IR) 329m (IR) 317m (IR) 223w ν(MO)342w (R) 328w (R) 329w (R) 330w (R)270m (IR) 257m (IR) 292m (IR) 284m (IR) 209m ν(MN)

291w (R)231m (IR) 246m (IR) 266m (IR) 254m (IR) 239sh ν(MO)w

212m (IR) 225m (IR) 237m (IR) 232m (IR) Overlap. δ(OMN)i.p.b

185w, br (IR) 180w (IR) 192vw (IR) 194w (IR) 183w δ(OMO)o.p.

163w (IR) Overlap. 177w (IR) 176vw (IR) 157w δ(CCN)o.p.

w, weak: s, strong; m, medium, sh, shoulder; br, broad; b.a., bite angle; o.p., out of plane; i.p., in plane.a The vibrational assignment was done on the basis of calculated vibrational frequencies and their assignments for gaoH− and Cu(gaoH)2 models and of

literature data[29].b Bite angle.

and Cu(gaoH)2 suggested that the oxime OH bond lengthincreases upon Cu–gaoH interaction. Thus, we expect that theν(OH)NOH mode will shift to lower frequencies with increas-ing of M N bond strength. Since the CuN bond length isshorter than the ZnN one, in the Cu(II) complex spectrum,the ν(OH)NOH band ought to appear at lower frequencies

than in the Zn(II) complex. However, the O4′· · ·H1 hydro-gen bonds in the Cu(II) and Zn(II) structures affect the oximeO3 H1 bond lengths. The comparison of the Cu(II) andZn(II) data showed that shorter O4′· · ·H1 bond in Cu(II) com-plex (Table 2) correlates with the shorter (OH)NOH distance(discussed in Section4.3) and higherν(OH)NOH frequency


Fig. 4. FIR spectra of M(gaoH)2(H2O)2 complexes (M = Cu(II), Zn(II),Co(II), Ni(II) and Cd(II)).

(3292 cm−1) than that in Zn(II) one (3226 cm−1). Obviously,theν(O H)NOH mode in the complexes studied is more sen-sitive to the hydrogen O4′· · ·H1 bonding strength than to theM N bond strength.

The bands due toν(OH)w modes of the coordinatedwater were observed in the following order: 3143 cm−1

for Cu(II), 3106 cm−1 for Zn(II), 3084 cm−1 for Ni(II)and 3084 cm−1 for Co(II) complex. For Cu(II) and Zn(II)structures, theν(OH)w modes correlate with O4H41 andO4 H42 bond lengths. The longer (OH)w distance (lower-frequencyν(OH)w mode) is in agreement with strongerO2′′· · ·H42 bonding in Zn(II) complex in comparison withthat in Cu(II) one (Tables 1 and 2).

For Cd(II) complex there are two types crystallographicwater (coordinated) and two crystallographically differentgaoH ligand. Hence, we expect four bands for the waterν(OH)w modes and two bands for the oximeν(OH) modes. Inaddition, two O H oscillators are expected due to crystalliza-tion water. In the IR spectrum of Cd(II) complex, the peaksat 3592 and 3529 cm−1 were assigned toν(OH)w modes ofcrystallization water, one peak at 3177 cm−1 to ν(OH)NOHmodes of oxime groups and one broad peak at 3077 cm−1 toν(O H)w modes of the coordinated water. The broadν(OH)wband consists of four components of theν(OH)w mode.

2000–400 cm−1. The COO stretching vibrations usuallys pec-t en-tsN ndZ eci theν etala ions.I -

metricν(COO) frequencies vary in the 1670–1662 cm−1 fre-quency region (Fig. 3) and the symmetricν(CO) ones—inthe 1377–1386 cm−1 frequency region. Theν(COO) fre-quency separation increases in the following order of met-als: Ni(II) (∆ = 278 cm−1) < Zn(II) (∆ = 282 cm−1) ∼= Co(II)(∆ = 282 cm−1) < Cu(II) (∆ = 293 cm−1). The same order hasbeen obtained for transition metal complexes of glycine[29].The only exception was the Co–glycine complex, whichhas shown larger separation than that of Cu–glycine. Thelower ν(OH)w frequency in Co(II) complex (3084 cm−1)compared to that in Cu(II) one (3143 cm−1) suggestedstronger O2′′· · ·H42 bonding in the first one. This assumptioncould explain the smaller∆ value for Co(II) complex com-pared to Cu(II) one. Disregarding the O2′′· · ·H42 H-bondingeffects, the∆ order indicates that with an increase of theCOO group asymmetry, metal–oxygen interaction becomesstronger. Thus,ν(COO) stretching modes of the complexesstudied indicated strongest Cu(II)O bonding and weakestNi(II) O one.

In the Cd(II) complex both CO are coordinate andhence the asymmetricν(COO) mode shifts significantly tolower wavenumbers (1592 cm−1) and it was found below theν(C N) (1647 cm−1), Fig. 3. The same trends were observedfor an analogue complex compound of gao, Na-hipa wherethe ligand acts also simultaneously as a chelate and bridg-ing, ν(C N)—1661 cm−1 and ν(C1O2)—1599 cm−1 [13].I sa ses(c I)–a( ra-ti seb ies(

-p vior,T edt Og hestf ni i-ac

qc -r ess om-pb

st

how two strong and characteristic absorptions in the IR srum. They are sensitive to the M–L binding mode (unidate, chelating bidentate or bridging) and to the MO bondtrength. According to the X-ray analysis both gaoH− areoxime,Ocarboxylic-bidentate coordinated to the Cu(II) an(II). From the other side the COO− group is unidentatoordinate and the non-coordinated carboxylic O2′′ atom

s involved in intermolecular hydrogen bonding. Thus,(COO) should be affected by the coordination of the ms well as by the intermolecular hydrogen bond interact

n the Cu(II), Zn(II), Ni(II) and Co(II) complexes the asym

n COO bridging complexes, where the CO bond lengthre equalized, theν(CO) frequency separation decrea∆ (1592–1391 cm−1) = 201 cm−1 for Cd(II) complex) inomparison with the separation in Cu(II)–, Zn(II)–, Ni(Ind Co(II)–gao complexes with unidentate COO− binding∆ = 278–293 cm−1). Thus, on the basis of the detail vibional analysis for complexes with unidentate (1,2) and bridg-ng (5) COO− binding, differentν(COO−)as behavior wastablished: strong absorption at∼1665 cm−1 for unidentateinding (1, 2) and strong absorption at lower frequenc1592 cm−1) for the bridging binding (5).

In the region 940–800 cm−1 the IR spectra of the comlexes studied showed very different vibrational behaable 4andFig. 3. The bright band in this region is assigno δ(OCO)i.p. mode. Due to the bridging binding of the COroup in the Cd(II) complex, this band is observed at hig

requencies (933, 923 cm−1) in comparison with its position the other complexes (899–837 cm−1). The frequency vartion ofδ(OCO)i.p. mode in Cu(II), Co(II), Zn(II) and Ni(II)omplexes is related to the O2′′· · ·H42 bonding strength.

The medium bands observed in the 1642–1634 cm−1 fre-uency range were assigned toν(C N) mode,Fig. 3. Thealculations showed that the higherν(C N) frequency coresponds to shorter M(II)N bond length, thus indicattronger binding. This frequency varies slightly in the clexes studied and it is not sufficiently indicative for the MNinding strength.

Our calculations showed that theν(NO) modehould appear at 864 cm−1 for gaoH−. It was foundhat upon CuNgaoH interaction, the NO bond length


decreased andν(N O) appeared at higher frequencies(1077 cm−1). The positive ν(NO) shift should indicatestrengthening of the MN bond. In the complexes stud-ied the ν(NO) modes were observed in the followingorder: Co(II) (1047 cm−1) < Zn(II) (1050 cm−1) ∼= Cd(II)(1051 cm−1) < Ni(II) (1056 cm−1) < Cu(II) (1077 cm−1).Therefore, on the basis of theν(N O) andν(C N) frequencybands of the complexes studied the strongest Cu(II)N andthe weakest Co(II)N bindings are expected.

The libration modes of coordinated H2O in Cu(II) com-plex were observed at lower frequencies (669, 645 cm−1)in comparison with those of Zn(II) (745, 721, 706 cm−1),Co(II) and Ni(II) complexes,Table 4. This finding is in agree-ment with longer CuO4 bond length (2.395A) (i.e. weakerCu O4 bond) than ZnO4 bond length (2.166A).

400–200 cm−1. As it is expected from a symmetryconsideration thetrans-isomer (Ci point group) shouldhave only one IR active ligand–M stretching vibration(M = Cu(II), Zn(II), Ni(II) and Co(II)). Thus, in the lowfrequency region the spectra of thetrans-complexes studiedexhibit oneν(MO), one ν(MN) and oneν(MO4) modes.Unlike ν(COO) andν(NO) modes, which are affected byH-bonding strength,ν(MO) and ν(MN) modes could beused for estimation of the MO and M N bond strength.As it was already discussed, the experimental MO bondlengths are shorter than MN ones (M = Cu(II), Zn(II),C ss erν lf xper-ia n-c(f der:C( int(

ndNc t ofC ther

5

beenp cido wnt tionsa nif-i to thesb ption

shift of Cu(II) complex correlated with the strongest CuOand Cu N interactions, indicating that gaoH− undergoes thelargest geometric and electronic changes in Cu(II) complex.The calculations for Cu(gaoH)2 indicated that the unpairedelectron occupies the dxy orbital and dz2(A′) → dxy(A′) tran-sition corresponds to 16 160 cm−1 band. The calculated Mul-liken atomic charges of O and N charges are less negative inthe Cu(II) complex, whereas Cu2+ charge lowers and thusOgaoH→ Cu(II) and NgaoH→ Cu(II) �-donations were sug-gested. In Cd(II) complex theν(COO)as frequency lowerssignificantly andδ(COO)i.p. one increases as compared to thatof Cu(II) and Zn(II) complexes and thus they could be usedto predict bridging COO− coordination The vibrational spec-tra showed that theν(COO) frequency separation increasedin the order: Cd(II) < Ni(II) < Zn(II)∼= Co(II) < Cu(II), sug-gesting the highest covalent character for CuO interaction.Theν(M O) spectral behavior supported the trend obtained.The higher basicity of O atom than that of N atom corre-lated with the shorter MO distance and determined strongerM O interaction as compared to MN one. The higher calcu-lated and experimentalν(MO) frequency in comparison withν(MN) one is also in agreement with stronger MO interac-tions. The vibrational behavior ofν(NO) mode was selectedto predict M N binding strength. The vibrational behav-ior in the 2000–500 cm−1 region of Co(II)– and Ni(II)–gaocomplexes with that of Cu(II)- and Zn(II)-complexes is verys

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d(II) (∆ = (M N) − (M O)∼= 0.13A)). Our calculationhowed that the shorter MO bond length produced high(M O) in comparison withν(M N). The experimentarequencies were assigned in agreement with the emental bond lengths, MO, M N and M O4, Fig. 4nd Table 4. The ν(MO) modes shift to higher frequeies in the order: Cd (223 cm−1) < Zn (305 cm−1) < Co317 cm−1) < Ni (329 cm−1) < Cu (340 cm−1). Theν(M N)requencies shift to higher frequencies in the ord (209 cm−1) < Zn (257 cm−1) < Cu (270 cm−1) < Co

284 cm−1) < Ni (292 cm−1). Theν(MOw) modes appearhe following order: Cu (231 cm−1) < Cd (239 cm−1) < Zn246 cm−1) < Co (254 cm−1) < Ni (266 cm−1).

There is a lack of X-ray diffraction data for Co(II) ai(II) complexes of gao. As it is seen fromFig. 3, theseomplexes reveal very similar vibrational behavior to thau(II) and Zn(II) complexes. Hence, this finding gives us

eason to conclude that they are isostructural.

. Conclusions

Spectroscopic and theoretical investigations haveerformed for a series M(II) complexes with glyoxilic axime. DFT(B3LYP/6-31++G(d,p)) calculations have sho

hat the metal–ligand (electrostatic and covalent) interacnd the interligand (H-bonding) interactions produced sig

cant changes in the ligand geometry. The most sensitivetrength of the MO and M N binding are the CO and N Oond lengths, respectively. The largest intraligand absor

imilar and suggests that they are isostructural.

cknowledgments

The authors thank the Computer Centre of Vienna Tical University for the computational facilities and servirovided. The authors are grateful to Prof. D. Mehand

or the magnetochemical measurements and to Doz. Droskova for the conductivity measurements.

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spectroscopic and theoretical study of cu(ii), zn(ii), ni(ii), co(ii) and cd(ii) complexes of...

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