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Inorganica Chimica Acta 317 (2001) 324–330

Note

The cis/trans-isomerism on cobalt(III) complexes with1,4,8,11-tetraazacyclotetradecane-1,8-bis(methylphosphonic acid)

Jan Kotek, Petr Hermann, Ivana Cısarova, Jan Rohovec, Ivan Lukes *Department of Inorganic Chemistry, Faculty of Science, Uni�ersita Karlo�a (Charles Uni�ersity), Hla�o�a 2030,

128 40 Prague 2, Czech Republic

Received 31 October 2000; accepted 22 January 2001

Abstract

The X-ray structure determinations show that 1,4,8,11-tetraazacyclotetradecane-1,8-bis(methylphosphonic) acid H4L formsboth cis- and trans-O,O isomeric complexes with Co(III). Orientation of the other donor atoms indicates the formation ofcis-O1,O2-trans-N1,N8-cis-O1,N4-[Co(HL)]. The kinetically preferred cis-isomer is easily formed due to the strong hydrogenbond between the neighboring phosphonic acid groups. The presence of the hydrogen bond in aqueous solution is confirmed bythe value of pKa which increases by four orders of magnitude in comparison with the trans-isomer. Formation of thetrans-[Co(HL)] occurs only after long heating of a Co(II) solution with the ligand under inert atmosphere before oxidation, whichis explained by insertion of the Co(II) ion in the plane of nitrogen atoms of 1,4,8,11-tetraazacyclotetradecane. © 2001 ElsevierScience B.V. All rights reserved.

Keywords: Cyclam complexes; Phosphonic acid complexes; Cobalt complexes; Crystal structures

1. Introduction

Polyaza rings with coordinating arms are superiorligands for transition metal ions as well as lanthanides[1,2]. They form thermodynamically very stable com-plexes and show a high selectivity to a particular metalion [1–3]. Polydentate ligands, such as 1,4,7,10-te-traazacyclododecane-1,4,7,10-tetraacetic acid (H4dota)and 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetra-acetic acid (H4teta) (Scheme 1), form thermodynami-cally and kinetically very stable complexes even withlabile metal ions such as the first row transition metaldivalent ions or trivalent lanthanides [2]. Properties ofsuch ligands have successfully been explored in design-ing the magnetic resonance imaging (MRI) contrastagents [4] based on Gd3+ and diagnostic/therapeuticradiopharmaceuticals utilizing metal radionuclides [5].

In addition, azacyclic sequestering agents for toxicheavy metals such as lead, cadmium and mercury weresynthesized [6]. In search for other ligands with similaror better properties than common acetate derivatives,research has also been focused on synthesis and investi-gation of azamacrocycles with phosphonic [7] or phos-phinic [8] acid arms. Complexes with the phosphorusligands exhibit higher selectivity in complexation andsufficient thermodynamic stability [9].

Phosphorus derivatives of cyclam were investigatedmuch less than those of cyclen. Only one derivativecontaining four methylenephosphonic acid arms(H8tetp, Scheme 1) was studied [10,11]. It was foundthat the ligand contains two very basic ring nitrogenatoms [10,11] and shows some selectivity for large metalions [9b]. It was suggested that not all pendant armscan be coordinated to the same transition metal ion dueto bulkiness of the phosphonate moiety and, therefore,the full coordination ability of the ligand cannot beinvestigated. We suppose that the limitation can beovercome by synthesis of cyclam-based ligands with

* Corresponding author. Tel.: +420-2-21952357; fax: +420-2-21952378.

E-mail address: lukes@prfdec.natur.cuni.cz (I. Lukes).

0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 0 20 -1693 (01 )00348 -6

J. Kotek et al. / Inorganica Chimica Acta 317 (2001) 324–330 325

Scheme 1.

only two phosphonic acid arms containing four nitro-gen and two oxygen donor atoms, which should beparticularly suitable for complexation of octahedralions of the first transition metal row. Investigation ofthe first cis/trans-isomerism and coordination ability ofthe 1,4,8,11-tetraazacyclotetradecane-1,8-bis(methyl-phosphonic) acid H4L [12] (Scheme 1) to a typicaltransition metal ion, Co(III), is the subject of thepresent paper.

Even though the cis/trans-isomerism of Co(III) com-plexes has been widely investigated and documented,only a few examples were found in the literature ofcyclam and its derivatives. In addition to trans- andcis-[Co(cyclam)Cl2]+ [13], cis-[Co(cyclam)(oxalato)]-(NO3)2 [14] and cis-[Co(cyclam)(en)]Cl3 [15] were syn-thesized and structurally characterized. Ware et al. [16]used complex [Co(edda)(bcpe)]+ (H2edda=ethylenedi-aminediacetatic acid and bcpe=N,N �-bis(3-chloro-propyl)ethylenediamine) in the template synthesis of[Co(1,4-bcc)]+ where H2bcc=1,4-bis(carboxymethyl)-cyclam. However, we have not found any example ofthe cis/trans-isomerism of H4teta complexes orother cyclam derivatives with coordinating arms inliterature.

Phosphonate complexes of Co(III) have been studiedonly marginally as well. Two complexes,[Co(NH3)4(H2mdp)]Cl and [Co(en)2(H2mdp)]ClO4·H2O,of methylenediphosphonic acid (H4mdp) were charac-terized structurally [17]. The phosphonate ligand makesa six-membered O,O-chelate ring protonated on eachphosphonate group. Investigation of complexation ofthe phosphonate complexons with (Co(en)2)2+ and(Co(en)2(NH3))2+ species in aqueous solution showedthat the complexons are bound only through phospho-nate group(s) [18]. Isomerism in complexation ofCo(III) with glyphosate ((HO)2PCH2NHCH2COOH,N-(phosphonomethyl)glycine, H3pmg) was studied insolution and the structure of sodium salt of one iso-meric anion, all-trans-[Co(pmg)2]3−, was determined byX-ray diffraction [19].

2. Experimental

2.1. General

H4L [12] and Na3[Co(CO3)3]·H2O [20] were synthe-sized by literature methods. The ion-exchange resin wasobtained from Fluka. NMR spectra were recorded on aVarian Unity Plus at 400 MHz for 1H (internal refer-ence t-BuOH), 161 MHz for 31P (external reference 85%H3PO4) and 100 MHz for 13C (internal reference t-BuOH) at room temperature. Carbon and hydrogenatom assignments were based on HMQC and RELAY-H experiments. The concentration was 0.01 M (in D2Ofor 1H and 13C, in H2O for 31P) and pH was adjustedusing dilute KOH or aqueous HCl. Elemental analyseswere performed in the Institute of MacromolecularChemistry of the Academy of Sciences. Ther-mogravimetry (TG) experiments were done on a Stan-ton–Redcroft in the laboratory of the Institute ofChemical Technology (Prague) in air (25–300°C,10°C min−1). Potentiometric titrations were run on anautomatic titrator consisting of a PHM 240 pH-meter(Radiometer), an ABU 900 burette (Radiometer) and aGK 2401B electrode (Radiometer, Denmark). The solu-tions of the complexes prepared were titrated withstandard KOH solution at I=0.1 M KNO3 at 25°C.Protonation constants were calculated by the OPIUM

program [21]. Details concerning pH-metric titrationsand calculations are described elsewhere [12]. UV–Visspectra (200–1100 nm) were recorded on a Unicam UV300. Cyclic voltammetry (CV) was run in three-elec-trode arrangement (hanging mercury drop electrodeagainst saturated calomel electrode (SCE), Pt auxiliaryelectrode) in aqueous 0.1 M KNO3, scan rate 200mV s−1.

2.2. X-ray structure analysis

The data were collected using Mo K� (�=0.71069A� ) radiation using a CAD4 diffractometer (Enraf–No-nius). The lattice parameters of both the compounds

J. Kotek et al. / Inorganica Chimica Acta 317 (2001) 324–330326

Table 1Experimental data for X-ray diffraction studies

Parameter cis-[Co(HL)]· trans-[Co(HL)]·6H2O3.5H2O

C12H34CoN4-Formula C12H39CoN4O12P2

O9.50P2

507.30M 552.34293(1)150(2)T (K)triclinicCrystal system monoclinicP1� (no. 2)Pn (no. 7)Space group

Unit cell dimensions7.363(5)8.852(5)a (A� )

16.592(5)b (A� ) 8.835(5)13.450(5)c (A� ) 9.477(5)

107.250(5)90� (°)95.090(5)� (°) 103.210(5)102.950(5)90� (°)

1923.2(14)U (A� 3) 565.8(6)14Z

1.752dcalc (g cm−3) 1.6211.118� (mm−1) 0.965

2921068F(000)0.57×0.85×0.32Crystal dimensions (mm) 0.61×0.50×0.32

2.28–28.97� range (°) 1.23–29.09−10�h�9,0�h�11,Index ranges−12�k�11,0�k�21,0�l�12−18�l�17

Reflections 5052/5015 3005/2980[Rint=0.0096][Rint=0.0174]collected/unique

5015/2/594Data/restraints/parameters 2980/6/219Goodness-of-fit on F2 1.0831.079

0.0308; 0.0824 0.0272; 0.0785Final R ; R � indices[I�2�(I)] a

R ; R � indices (all data) a 0.0281; 0.07910.0323; 0.08340.651; −0.844 0.416; −0.850Largest difference peak

and hole (e A� −3)

a R=� �Fo−Fc�/��Fc�; R �= [� w(Fo2−Fc

2)2/� w(Fo2)2]1/2; w=1/

[�2(Fo2)+(AP)2+BP ] (SHELXL-97 [24]).

elution with water) and then isolated after diffusion ofacetone vapor into its concentrated aqueous solution.Purple leaflets of the product were separated by filtra-tion and dried in air. The yield was 0.047 g (86%).Single crystals suitable for X-ray analysis were pickedup from the bulk. Elemental analysis: Anal. Found: C,27.98; H, 6.48; N, 10.20. Calc. for C12H27CoN4O9.5P2

(MW=507.11): C, 28.40; H, 6.75; N, 11.04%. TGA:one broad step between 40 and 140°C (12.43%) (calcu-lated for 3.5H2O, 12.63%). CV (Co(II)/Co(III)): −0.28V (reversible).

2.4. trans-[Co(HL)] ·6H2O

H4L (0.050 g, 0.11 mmol) was refluxed with 0.030 g(0.13 mmol) of CoCl2·6H2O in 10 ml of deoxygenatedwater in argon atmosphere for 15 h. Sodium hydroxide(2 ml of 2% water solution) was added dropwise duringthe first 3 h of refluxing. Subsequently, the solution wasrefluxed for 3 h in air. After chromatography on 15 mlof Amberlite 50 (H+ form, elution with water), theproduct was isolated by diffusion of acetone vapor intoits concentrated aqueous solution. Brown-purple cubiccrystals were separated by filtration and dried in air.Single crystals for X-ray study were prepared in thesame way. The yield was 0.037 g (62%). Elementalanalysis: Anal. Found: C 25.48; H 6.92; N 9.53. Calc.for C12H39CoN4O12P2 (MW 552.14): C 26.08; H 7.11; N10.14%. TGA: 19.10% in two steps between 50–90 and90–130°C in an approximate ratio of 2:1 (calculated for6H2O, 19.57%). CV (Co(II)/Co(III)): −0.58 V(reversible).

3. Results and discussion

H4L reacts with Co(II) in the neutral aqueous solu-tion forming [CoL]2− species as was confirmed byUV–Vis spectroscopy. Pink fibrous solids, inconvenientfor X-ray analysis, were also obtained from the solu-tion. When the solution was left standing in air forseveral weeks, oxidation of Co(II) to Co(III) and theformation of predominantly cis-[Co(HL)] in a wideregion of pH was observed using UV–Vis spectroscopy.Nevertheless, a common route starting fromNa3[Co(CO3)3] was used for the preparation of cis-[Co(HL)]. Oxidation of Co(II) with hydrogen peroxidewas not used to avoid partial cleavage of the C�P bondas was found for lanthanide complexes with analogousligands [25]. The procedure for the preparation oftrans-[Co(HL)] consists of two steps: long reflux of aCo(II) solution with H4L in argon atmosphere, givingprobably trans-[CoL]2−, and oxidation in air. On thebasis of synthetic experience and the X-ray structureresults mentioned later, the long reflux is essential toplace the central atom in the N4-plane of the ring.

were determined from 25 reflections. Lorenzian-polar-ization corrections were made using program JANA-98

[22]. Both structures were solved by direct methods andrefined by full-matrix least-squares techniques (pro-grams SIR92 [23] and SHELXL-97 [24]. All the hydrogenatoms were found in the structure of the trans-[Co(HL)]·6H2O; for refinement of the structure of cis-[Co(HL)]·3.5H2O, hydrogen atoms on the ring carbonswere fixed in theoretical sites. Experimental details aregiven in Table 1.

2.3. Synthesis of cis-[Co(HL)] ·3.5H2O

H4L (0.050 g, 0.11 mmol) was dissolved together with0.043 g (0.12 mmol) of Na3[Co(CO3)3]·3H2O in 10 ml ofcold water. The mixture was allowed to stand for oneweek in a refrigerator. The product was purified bychromatography on 50 ml of Amberlite 50 (H+ form,

J. Kotek et al. / Inorganica Chimica Acta 317 (2001) 324–330 327

For both the isomers, crystals convenient for X-rayanalysis were isolated and their structures were deter-mined. The structures are shown in Figs. 1 and 2; andTables 2 and 3 list the selected bond lengths and angles.In both the isomers, the HL3− is coordinated toCo(III) by four nitrogen atoms of the macrocycle andtwo oxygen atoms of two phosphonate groups.

The trans-[Co(HL)] molecules are centrosymmetricwith the Co(III) atom placed in the N4-plane of thetetraazacyclo-tetradecane ring. Oxygen atoms are virtu-ally normal to the N4-plane. The Co�O and Co�Ndistances lie in the range expected for this type ofcompounds and the N�Co�N, N�Co�O and O�Co�Oangles are close to 90 and 180° (see Table 2); thus thecoordination sphere has a shape of the slightly distortedoctahedron. Coordination around the phosphorusatoms does not correspond to a regular tetrahedron.The P�O bonds for non-coordinated oxygen atomsdiffer, the longer one, P�O2, is influenced by the strong

Table 2Selected bond lengths (A� ) and angles (°) for trans-[Co(HL)]·6H2O

Bond lengthsCo(1)�O(1) 1.9098(12)Co(1)�N(4) 1.9878(15)Co(1)�N(1) 2.0150(14)P(1)�O(3) 1.4976(12)

1.5339(13)P(1)�O(1)P(1)�O(2) 1.5409(13)P(1)�C(8) 1.8215(19)

AnglesO(1)�Co(1)�N(4) 88.28(5)O(1)�Co(1)�N(1) 90.03(7)O(1)�Co(1)�O(1)c1 180.0N(1)�Co(1)�N(4) 87.13(6)N(1)�Co(1)�N(1)c1 180.0N(4)1�Co(1)�N(4)c1 180.0O(3)�P(1)�O(1) 114.28(7)O(1)�P(1)�O(2) 108.17(7)O(1)�P(1)�C(8) 102.68(6)

108.34(7)O(2)�P(1)�C(8)O(2)�P(1)�O(3) 113.59(8)

109.09(7)O(3)�P(1)�C(8)115.64(7)P(1)�O(1)�Co(1)

Fig. 1. View of trans-[Co(HL)] with the atom numbering scheme.

centrosymmetric hydrogen bond O2···H···O2� (2.422 A� )and the length of the shorter one P�O3, indicates itsdouble character. The short hydrogen bonds link theoctahedrons into polymeric chains as shown in Fig. 3,similar to the structure of [Co(NH3)4(H2mdp)]Cl [17a].Conformation of the tetraazatetradecane ring is virtu-ally the most common observed conformation of transcomplexes with cyclam ligands [26], for example thesame as was observed for trans-dichloro complex ofCo(III) with cyclam [13a]. The hydrogen atom of N4and the arm of N1 are oriented to the opposite direc-tions and the structure is centrosymmetric; therefore,the isomer should be considered as the trans-III.

In the unit cell of cis-[Co(HL)] two independentmolecules were found; however, the bond lengths forboth the units are virtually the same within the 3�

interval. The Co�O and Co�N distances also lie in therange expected for this type of compounds; however,the N�Co�N, N�Co�O and O�Co�O angles differ from90 and 180° (see Table 3), and the structure is moredistorted octahedron than that found for the trans-iso-mer. Conformation of the azacycle, cis-V, correspondsto the conformation observed for cis-dichloro [13b] oroxalato [14] complexes of Co(III) with cyclam. Aninteresting feature observed in the structure of cis-[Co(HL)] is the strong hydrogen bond O12−H···O22(2.459 A� ) connecting the phosphonic groups inside thecoordination polyhedron. This hydrogen bond stabi-lizes the structure and promotes the formation of thecis-isomer.

Both the structures are stabilized by a network ofhydrogen bonds between phosphonic acid groups andFig. 2. View of cis-[Co(HL)] with the atom numbering scheme.

J. Kotek et al. / Inorganica Chimica Acta 317 (2001) 324–330328

Table 3Selected bond lengths (A� ) and angles (°) for cis-[Co(HL)]·3.5H2O

Molecule bMolecule a

Bond lengthsCo(1)�O(21) 1.883(3)1.871(3)

1.906(3)1.908(3)Co(1)�O(11)Co(1)�N(4) 1.939(4) 1.944(3)

1.968(3)1.975(3)Co(1)�N(11)2.006(3)Co(1)�N(1) 2.008(3)2.017(3)2.009(3)Co(1)�N(8)

1.494(3)P(1)�O(11) 1.496(3)1.536(3)1.514(3)P(1)�O(12)

1.502(4)P(1)�O(13) 1.513(4)P(1)�C(15) 1.812(4)1.863(5)

1.485(4)1.496(4)P(2)�O(23)1.487(3)P(2)�O(22) 1.508(3)1.533(3)1.534(3)P(2)�O(21)1.869(4)P(2)�C(16) 1.799(5)

AnglesO(21)�Co(1)�O(11) 96.42(12)96.11(12)

174.51(14)173.69(15)O(21)�Co(1)�N(4)O(11)�Co(1)�N(4) 83.29(14) 83.51(13)

83.49(13)84.96(14)O(21)�Co(1)�N(11)173.50(12)O(11)�Co(1)�N(11) 174.32(13)97.20(14)96.25(15)N(4)�Co(1)�N(11)

86.92(14)O(21)�Co(1)�N(1) 87.19(13)92.99(13)92.38(13)O(11)�Co(1)�N(1)

86.82(15)N(4)�Co(1)�N(1) 87.33(14)93.25(14)N(11)�Co(1)�N(1) 93.50(14)

89.72(13)90.32(15)O(21)�Co(1)�N(8)84.42(12)O(11)�Co(1)�N(8) 84.96(12)95.74(13)95.88(15)N(4)�Co(1)�N(8)89.08(13)N(11)�Co(1)�N(8) 89.47(14)

175.72(13)175.95(13)N(1)�Co(1)�N(8)112.2(2)O(11)�P(1)�O(13) 115.37(18)110.97(19)106.27(19)O(11)�P(1)�O(12)

111.60(19)O(13)�P(1)�O(12) 112.42(17)105.33(18)103.70(17)O(11)�P(1)�C(15)111.2(2)O(13)�P(1)�C(15) 113.1(2)104.16(17)106.0(2)O(12)�P(1)�C(15)

116.54(16)P(1)�O(11)�Co(1) 116.15(14)111.12(19)110.6(2)O(23)�P(2)�O(22)

112.74(19)O(23)�P(2)�O(21) 115.77(18)106.40(18)O(22)�P(2)�O(21) 111.0(2)112.9(2)111.9(3)O(23)�P(2)�C(16)105.68(19)O(22)�P(2)�C(16) 105.3(2)104.16(16)104.95(18)O(21)�P(2)�C(16)

115.47(15)P(2)�O(21)�Co(1) 115.56(15)

Scheme 2.

water molecules and water molecules alone. The P�Obonds as well as Co�O bonds in both the complexes aredistinctly shorter than the corresponding bonds in theother Co(III) phosphonate complexes [17,19].

If we consider all possible isomers with cis orienta-tion of oxygen atoms, we can deduce the formation ofthree different isomers: (i) with different cis/trans orien-tation of N1 and N8 atoms bearing pendant arms and(ii) two isomers of the trans-N1,N8, with cis or transorientation of O1,N4. All the possibilities are shown inScheme 2. Comparing the possibilities mentioned andthe structure found for cis-[Co(HL)], it is evident thatthe isomer should be described as cis-O1,O2-trans-N1,N8-cis-O1,N4-[Co(HL)].

UV–Vis spectra of both the isomers in solutioncorrespond to those observed for other Co(III) com-plexes with the N4O2 coordination sphere [27]. Theparameters 10Dq and B, 21 000 and 475 cm−1 (cis-iso-mer) and 20 900 and 475 cm−1 (trans-isomer) are alsoin the range expected for this type of complexes. Forthe cis-isomer, the 1T2g transition is unchanged, 376 nm(�=121 dm3 mol−1 cm−1) and the maximum of 1T1g

transition is shifted from 535 (620sh) nm (�=111dm3 mol−1 cm−1) at pH 10.07 to 523 nm (�=107dm3 mol−1 cm−1) at pH 0.00. For the trans-isomer, thesplitting of 1T2g transition to 1B2g and 1Eg was not

Fig. 3. Linkage of trans-[Co(HL)] octahedrons through the phosphonic acid groups into the infinite chain.

J. Kotek et al. / Inorganica Chimica Acta 317 (2001) 324–330 329

Table 4Protonation constants of both isomer compounds

trans-[Co(HL)]cis-[Co(HL)]

log �1=pKa(1)a 6.98(1) 2.97(1)

log �2 4.84(1)8.72(1)1.871.74pKa(2)= log �2−log �1

a �i are protonation constants defined �i= [HiCoL(i−1)+]/[H+]i-[CoL−]. Ka(i) are corresponding dissociation constants: Ka(1) corre-sponds to the dissociation of the neutral complex HCoL, Ka(1)= [H+]-[CoL−]/[HCoL]; and Ka(2) corresponds to the dissociation of proto-nated species H2CoL+, Ka(2)= [H+][HCoL]/[H2CoL+].

bonds.Potentiometric titration starting from the acid region

(pH�1.7) gave two protonation (dissociation) con-stants for each isomer as shown in Table 4. Coordina-tion of the oxygen atom of a phosphonic group isusually associated with a pKa decrease of about 2–3 logunits from a value of about 6 observed for the freeacids (5.36 and 6.78 for H4L [12,28]). This decrease wasobserved for trans-[Co(HL)]. However, the changesfound for cis-[Co(HL)] indicate the reverse shift. Thispoints to the stability of the hydrogen bond in a wideregion of pH, and consequently to the stability of thecoordination sphere for the cis-isomer. The values ofpKa(2), which are associated with the formation of the[Co(H2L)]+ species for both the isomers are very close.This protonation does not influence the coordinationsphere of the species. The protonation takes place onan oxygen atom of the second phosphonic acid group.

Trivalent cobalt is highly stabilized in both the com-plexes. It is illustrated by Co(II)/Co(III) redox potential(−0.28 V for cis- and −0.58 V for trans-[Co(HL)] vs.SCE). The value for trans-[Co(HL)] is even lower thanthat found for [Co(1,4-bcc)]+ (−0.53 V vs. SCE) [16a]and is the lowest value known for cobalt(III) with N4O2

environment. The value is comparable to those ofcobalt(III) N6 complexes [29].

The 1H and 13C NMR spectra for both the isomerstogether with assignments are listed in Table 5. Thenumbering scheme for NMR experiments is shown inScheme 3. The spectra are dependent on the pH of thesolution and the parameters in Table 5 correspond toalmost 100% abundance of monoprotonated form[Co(HL)] in both cases. From the results it is evidentthat in solution both the isomers could be described byone half of their molecules. The trans-isomer is cen-trosymmetric in the solid state and thus, it is an ex-pected property in solution. The X-ray analysis showed,as mentioned above, that the structure of the cis-isomerconsists of two independent molecules in the unit cell.

observed and the maximum is also unchanged, 374 nm(�=101 dm3 mol−1 cm−1) depending on pH. On theother hand, transition 1T1g spans 1A2g+

1Eg and theirmaxima shift, depending on pH, from 480 (�=75dm3 mol−1 cm−1) and 573 nm (�=69 dm3 mol−1

cm−1) at pH 9.90 to 477 (�=76 dm3 mol−1 cm−1) and576 nm (�=72 dm3 mol−1 cm−1) at pH 0.00. Thus, pHinfluence is negligible and no changes in the coordina-tion sphere are expected. Parameter Dt estimated fortrans-isomer is positive (390 cm−1) and it correspondsto the weaker axial field than the equatorial field andconfirms coordination of oxygens in axial positions insolution. More pronounced differences are seen at theregion of the CT bands. For the cis-isomer, the CTband shifts from 233 nm (�=16 600 dm3 mol−1 cm−1)at pH 10.07 to 247 nm (�=17400 dm3 mol−1 cm−1) atpH 0.00. For the trans-isomer, the intensity of theoxygen-to-cobalt CT band increases with increasing pH(�=9100 dm3 mol−1 cm−1 at pH 0.00, �=14 900dm3 mol−1 cm−1 at pH 9.90) and the maximum is notshifted (�=230 nm). The nitrogen-to-cobalt CT bandmoves to higher wavelength with increasing pH (259nm, �=10 500 dm3 mol−1 cm−1 at pH 0.00 and 267nm, �=11 400 dm3 mol−1 cm−1 at pH 9.90). Suchchanges are consistent with protonation of the phos-phonate moieties and strengthening of the Co�N

Table 5Comparison of NMR spectra of both isomer compounds

1H NMR 13C NMR

trans-[Co(HL)]cis-[Co(HL)]trans-[Co(HL)]cis-[Co(HL)]62.21, sC22.80, m 69.47, d, 3JPC=4.5 Hz2.58, mH2a

H2b 53.87, s3.42, dt, 2JHH=13.9 Hz, 3JHH=4.4 Hz 3.14, m C3 56.06, sH3a 49.91, s2.73, dd, 2JHH=12.8 Hz, 3JHH=4.4 Hz 2.91, m C5 51.30, s

27.75, s26.94, d, 4JPC=1.2 HzC6H3b 3.06, m2.86, dt, 2JHH=13.0 Hz, 3JHH=4.4 HzC7 64.31, d, 3JPC=12.3 Hz 63.75, d, 3JPC=4.5 HzH5a 2.44, m2.12, m

H5b 57.38, d, 1JPC=132.2 Hz67.57, d, 1JPC=137.3 HzC82.63, m2.39, m2.12, m1.97, mH6a

1.97, mH6b 2.42, m2.58, m 2.59, mH7a

3.01, m3.07, mH7bH8a 3.03, m 2.97, m

3.70, t, 2JPH=16.0 Hz, 2JHH=16.0 Hz 2.98, mH8b

J. Kotek et al. / Inorganica Chimica Acta 317 (2001) 324–330330

Scheme 3.

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In the solid, possible two-fold symmetry is broken dueto the presence of the unsymmetrical hydrogen bondand consequently, the C�P bond lengths also differ (seeTable 3). However, the bonds lying farther from theC�P bonds show virtually the same lengths for bothparts of the molecule. Thus the NMR results indicatethat in solution at slightly acidic pH, the proton of thehydrogen bond is rapidly exchanged and, consequently,the molecule behaves as symmetrical. The 31P NMRspectra were recorded at different pH giving P 35.8ppm (pH 10.3, 100% [CoL]−), 39.2 ppm (pH 4.22,100% [Co(HL)] and 40.7 ppm (pH 0.0, 100%[Co(H2L)]+ for the cis-isomer and 38.0 ppm (pH 7.8,100% [CoL]−), 39.7 (pH 2.8, 65% [Co(HL)]) and 43.3(pH 0.0, 100% [Co(H2L)]+) for the trans-isomer. Themeasurement confirms protonation of the phosphonategroup as the P increases with the acidification of thesolution [12].

Acknowledgements

We thank the Grant Agency of the Czech Republic(grant no. 203/97/0242 to I.C.), Ministry of Educationof the Czech Republic (grant No. VS96140) and pro-gramme EU COST (COST D18) for financial support.Our thanks are also due to Dr. J. Ederova (Institute ofChemical Technology, Prague) for TG measurements.

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