Journal of Inorganic Biochemistry 94 (2003) 291–299www.elsevier.com/ locate/ jinorgbio

T hermodynamic, kinetic and structural studies on the mixed ligandcomplexes of palladium(II) with tridentate and monodentate ligands

a a b a ,*´ ´ ´ ´ ´ ´ ´ ´Zoltan Nagy , Istvan Fabian , Attila Benyei , Imre SovagoaDepartment of Inorganic and Analytical Chemistry, University of Debrecen, H-4010 Debrecen, Hungary

bDepartment of Physical Chemistry, University of Debrecen, H-4010 Debrecen, Hungary

Received 19 July 2002; received in revised form 13 December 2002; accepted 19 December 2002

Abstract

21Stability constants of the complexes formed in the reaction of [Pd(bpma)] [bpma5bis(pyridin-2-ylmethyl)amine] with monodentatenitrogen and thioether ligands including uridine, MeUH, cytidine, MeC, EtGH, AcHis, AcHm, AcLys and AcMet were determined by

21 21potentiometric method. The coordination chemistry of [Pd(bpma)] shows a significant similarity to that of [Pd(terpy)] , but it is21 21different from [Pd(dien)] . The formation of hydroxo and dinuclear complexes is especially enhanced in the case of [Pd(bpma)] and

21[Pd(terpy)] , but the affinity of palladium(II) ions for the coordination of thioether residues is reduced in the presence of pyridinenitrogen atoms. Stopped-flow kinetic measurements reveal that the substitution reactions of the thioether ligand AcMet are much faster

21than those of the N-donor cytidine. The presence of the two pyridyl residues significantly enhances the kinetic reactivity of [Pd(bpma)]21as compared to that of [Pd(dien)] . The Pd–S(thioether) bonded species can be important intermediates in multicomponent systems, but

the equilibrium state is characterised by the formation of Pd–N bonded species. The complex [Pd(bpma)NO ]NO has been prepared in3 3

solid state and its structure was elucidated by single crystal X-ray diffraction method. 2003 Elsevier Science Inc. All rights reserved.

Keywords: Palladium(II); Bis(pyridin-2-ylmethyl)amine; Thioether donors; Potentiometry; Stopped-flow; X-ray diffraction

1 . Introduction other equilibrium studies for platinum(II) complexes.Coordination geometry and complex formation processes

The antitumor activity of various platinum containing of palladium(II) are very similar to those of platinum(II),drugs is generally explained by the platination of DNA via thus palladium(II) ions are frequently used to mimic thebinding to guanine [1,2]. Platinum(II) compounds are, binding properties of various platinum(II) species.however, known to exhibit a strong preference for soft In the previous papers [8,9] we reported both thermo-donor atom ligands, such as glutathione, cysteine or dynamic and kinetic parameters for the mixed ligandmethionine. These amino acids and peptides are present in complexes of palladium(II) formed in the reaction of

21 21significant concentrations in the intracellular environment, [Pd(dien)] and [Pd(terpy)] with various nitrogen andthus Pt–S bonded intermediates can play an important rolethioether sulfur donor ligands. In the case of the

21in both the transport and toxicity of platinum containing [Pd(dien)] –AcMet–cytidine ternary system these param-drugs. The intra- and intermolecular competitions between eters provided a clear evidence for the existence of a

1the sulfur and nitrogen donor ligands have been studied bymetastable intermediate [Pd(dien)(AcMet)] containingseveral authors in the past few years [3–7]. Most of these Pd–S bond, but the equilibrium state was described by thestudies revealed the kinetic preference of thioether coordi- predominant formation of the Pd–N bonded species [9].nation over the substitution reactions of various nitrogen Comparison of the stability constants of various mixeddonor ligands. The slow formation kinetics, however, ligand palladium(II) complexes [8–12] led to some other

21generally rules out the stability constant determination or important observations. The complex [Pd(terpy)] con-taining three pyridine-type, trigonal nitrogen donor atomshad much less affinity for thioether binding than that of*Corresponding author. Tel.:136-52-512-900; fax:136-52-489-667.

21´ ´ ´E-mail address: sovago@delfin.klte.hu(I. Sovago). [Pd(dien)] containing only tetrahedral, amino-type nitro-

0162-0134/03/$ – see front matter 2003 Elsevier Science Inc. All rights reserved.doi:10.1016/S0162-0134(03)00009-6

292 Z. Nagy et al. / Journal of Inorganic Biochemistry 94 (2003) 291–299

gen donors. In a more recent study, on the interaction of equivalents) and this solution was added dropwise to the1[Pd(terpy)Cl] with various sulfur containing molecules, warm solution of K [PdCl ]. The pH of the solution was2 4

the thioether ligands were found to be unreactive [13] and adjusted to 5 by the addition of KOH. The pale yellowsimilar observation have been reported for the corre- solution was filtered, evaporated and left to stand at lowsponding platinum(II) species [14]. The evaluation of the temperature (08C) until yellow crystals of [Pd(bpma)Cl]Clstability constants obtained for nitrogen donor ligands were precipitated. In aqueous solution and in the presencerevealed that the palladium(II) species containing aromatic of chloride ion the species [Pd(bpma)Cl]Cl dominates over

21 21 21nitrogen donors (e.g. [Pd(terpy)] and [Pd(pic)] ) have [Pd(bpma)(H O)] , therefore it was necessary to trans-2

reduced affinity for binding of tetrahedral, amino-type form the chloride salt to the corresponding nitrates beforenitrogen donors and an increased tendency for hydrolysis the equilibrium studies. For this purpose the solid[8,12]. These studies strongly support that complex forma- [Pd(bpma)Cl]Cl was redissolved in water containing twotion reactions of the coordinatively unsaturated pal- equivalents of HNO to avoid hydrolytic reactions and the3

ladium(II) complexes are significantly influenced by the chloride content of the sample was precipitated by theother donor functions present in the coordination sphere of addition of two equivalents of silver(I) nitrate. The whitethe metal ion. precipitate (AgCl) was filtered and the filtrate was partly

Most of the previous studies were performed on the evaporated and left to stand at low temperature until21 21mixed ligand complexes of [Pd(dien)] and [Pd(terpy)] crystals of [Pd(bpma)NO ]NO were formed. The bright3 3

containing only amino- and pyridine-type nitrogen donor yellow crystals were filtered, washed with cold water andatoms, respectively. In a most recent publication the results dried in air. Major part of the solid was used for the

21 21 21obtained for the interaction of [Pd(dien)] , [Pd(terpy)] preparation of stock solutions of [Pd(bpma)(H O)] for221and [Pd(bpma)] [bpma5bis(pyridin-2-ylmethyl)amine] kinetic and equilibrium studies and some crystals were

with thiol compounds were reported and the highest used for single crystal X-ray diffraction measurements.21reactivity of [Pd(terpy)] has been demonstrated [15].21The complex [Pd(bpma)] contains two trigonal nitrogen 2 .3. Potentiometric measurements

donor atoms in thecis and one tetrahedral nitrogen in thetrans position to the free coordination site. Now in this The stability constants of the ternary complexes werepaper we report the results of combined equilibrium, determined by potentiometric measurements. In the case ofkinetic and structural studies on the mixed ligand complex- nitrogen donor ligands complex formation is accompanied

21es of [Pd(bpma)] with nitrogen and thioether donor by the change of the protonation states of the ligands, thusligands including AcLys, AcHis, AcHm, uridine, cytidine, a direct potentiometric procedure can be applied as de-MeC, MeUH, EtGH and AcMet. The major point of this scribed previously [12]. Complex formation reactions ofwork was the parallel determination of kinetic and equilib- thioether ligands are, however, not affected by pH and anrium parameters, which can be easily performed only in indirect potentiometric method, using uridine as a competi-the case of the relatively labile palladium(II) ions. tive ligand, should be applied. The experimental details of

this procedure have been reported elsewhere [9].All pH-metric titrations were performed in 10 ml

23 232 . Experimental samples in the concentration range 2310 to 4310 M21at the ratios [Pd(bpma)(H O)] :L52:1, 1:1 and 1:2.2

2 .1. Materials Argon was bubbled through the samples to ensure theabsence of oxygen and carbon dioxide and to stir the

The ligands bis(pyridin-2-ylmethyl)amine (bpma),N- solutions. All pH-metric measurements were carried out atacetyl-L-histidine (AcHis), N-acetylhistamine (AcHm),N- 298 K and at a constant ionic strength (0.2 M KNO ).3

acetyl-L-lysine (AcLys), uridine, cytidine, N-acetyl-L- Measurements were made with a Radiometer PHM 93 pHmethionine (AcMet), 1-methylcytosine (MeC), 1- meter equipped with Metrohm 6.0219.100 double junctionmethyluracil (MeUH) and 9-ethylguanine (EtGH) were electrode to avoid formation of chloro complexes even inpurchased from Sigma and used without further purifica- low concentrations. Carbonate-free potassium hydroxide oftion. Stock solutions of palladium(II) ions were prepared known concentration was used for titration with the help offrom K [PdCl ] (Fluka) and two equivalents of acid Metrohm 715 Dosimat automatic burette. pH readings2 4

(HNO or HCl) was added to suppress hydrolytic re- were converted to hydrogen ion concentration and the3

actions. stability constants were calculated by means of a generalcomputational program (PSEQUAD) as described previously

2 .2. Crystallisation of [Pd(bpma)NO ]NO [9,12].3 3

(C H N O Pd)12 13 5 6

2 .4. Kinetic measurementsK [PdCl ] (1.4794 g, 4.532 mmol) was dissolved in 1002 4

ml 0.1 M HCl (|2 equivalents). A 1 ml bpma ligand Kinetic measurements were made in aqueous solution(4.533 mmol) was dissolved in 70 ml 0.2 M HCl (|3 under pseudo-first-order conditions with an Applied Photo-

Z. Nagy et al. / Journal of Inorganic Biochemistry 94 (2003) 291–299 293

physics DX-17MV sequential stopped-flow instrument bpma with a square planar geometry for the central metalusing 10 mm optical path at 340 and 355 nm for the ion. Tridentate coordination of the ligand has been proved

21 21[Pd(bpma)] –cytidine binary and [Pd(bpma)] –cytid- in the copper(II) complexes of bpma, too [19]. In thisine–AcMet ternary systems, respectively. The concentra- study the mixed ligand complex of copper(II) with bpma

21tion of the metal species [Pd(bpma)] varied in the range and pycolinate has been prepared and the equatorial25 245310 to 5310 M, while the metal ion to ligand ratio coordination of the tridentate bpma ligand was demon-

was between 1:5 and 1:40. The kinetic traces were strated. These data suggest that the tridentate coordinationevaluated with the software package provided with the is a common feature of bpma ligand with a series ofinstrument. Each rate constant was obtained from the transition elements and this binding mode can exist also inaverage of at least five replicate runs, which were re- ternary systems.producible within 5%. The crystal structure of [Pd(bpma)NO ]NO is depicted3 3

in Fig. 1 and in agreement with the earlier findings itshows a slightly distorted square planar coordination2 .5. NMR measurementsgeometry of palladium(II) ion. Selected examples of bond

1 lengths and angles are collected in Table 1, while someH NMR studies were used for the detection of the1parameters of the complexes [Pd(dien)NO ] [20] and2various isomeric species formed in the reaction of pal-

1[Pd(terpy)OH] [21] are compared to those ofladium(II) complexes with the imidazole containing ligand1[Pd(bpma)NO ] in Table 2.3AcHis. The spectra were recorded in D O on a Bruker2 It is clear from Fig. 1 and Table 1 that the coordinationAM360 FT-NMR spectrometer using TSP (sodium 3-tri-

geometries of the palladium(II) complexes of bpma aremethylsilyl-propionate) as an internal reference. The pDvery similar in both the chloride [15] and the nitrate salts.values were determined by the use of a Radiometer pHHowever, the complex prepared in our laboratory containsmeter equipped with a Metrohm 6.0222.100 combinedone of the oxygen atoms of the nitrate ions in transglass–calomel electrode and by addition of 0.4 to the pHposition to the amino-type nitrogen atom instead of themeter readings.chloride ion [15]. As a consequence, a slight difference canbe observed in the Pd–N bond lengths of the two com-

12 .6. Structure analysis and refinement pounds. In the case of [Pd(bpma)Cl] all Pd–N bonddistances were almost the same, while in the case of

1Bright yellow crystals of [Pd(bpma)NO ]NO ,3 3 [Pd(bpma)NO ] the Pd–N bondtrans to the oxygen atom3C H N O Pd, size 0.5430.430.2 mm), M5429.67,12 13 5 6 is slightly shorter than the others.˚ ˚orthorhombic,a512.8487(10) A,b513.4676(10) A,c5 The comparison of the structural parameters of the three3˚ ˚17.1239(10) A,V52963.1(4) A ,Z58, space group:Pbca, tridentate, nitrogen donor ligands in Table 2 reveals some23

r 51.926 g cm . Data were collected at 293(1) K,calc small differences in their coordination geometries. TheEnraf Nonius MACH3 diffractometer, MoKa radiation three Pd–N bond lengths are almost the same in the

˚l50.71073 A,v 2 2u motion, u 525.338, 2691 mea-max palladium(II) complex of dien, but slightly shorter dis-sured reflections of which 1778 were unique withI .2s(I), decay: none. The structure was solved using the

2SIR-92 software [16] and refined onF usingSHELX-97 [17]program, publication material was prepared with the

2WINGX-97 suite [18],R(F )5 0.0557 andwR(F )5 0.1588for 2691 reflections, 221 parameters. Hydrogen atoms werefixed in geometry position except H N which could be2

found at the difference electron density map. Residual3˚electron density: 1.969/20.893 e/A close to the Pd atom,

3˚other peaks are smaller than 0.5 e/A .

3 . Results and discussion

3 .1. Crystal structure of [Pd(bpma)NO ]NO3 3

(C H N O Pd)12 13 5 6

The crystal structure of the palladium(II) complex of theligand bis(pyridin-2-ylmethyl)amine (bpma) has alreadybeen reported in the form of chloride salt([Pd(bpma)Cl]Cl?H O) [15]. In accordance with the ex- Fig. 1. ORTEP view (50% probability level) and numbering scheme of2

pectations the data indicated tridentate coordination of [Pd(bpma)NO ]NO .3 3

294 Z. Nagy et al. / Journal of Inorganic Biochemistry 94 (2003) 291–299

Table 1Selected bond lengths and angles for the compound [Pd(bpma)NO ]NO3 3

˚Bond lengths (A) Bond angles (8)

Pd–N(1) 1.995(6) N(1)–Pd–N(2) 84.0(2)Pd–N(2) 1.977(5) N(2)–Pd–N(3) 82.9(2)Pd–N(3) 2.015(6) N(3)–Pd–O(4) 96.5(2)Pd–O(4) 2.034(4) N(1)–Pd–O(4) 96.5(2)N(1)–C(1) 1.380(9) N(1)–C(1)–C(2) 120.5(7)N(1)–C(5) 1.353(9) N(3)–C(19)–C(29) 120.5(7)N(3)–C(19) 1.328(9) C(1)–N(1)–C(5) 119.3(6)N(3)–C(59) 1.357(9) C(19)–N(3)–C(59) 120.8(6)N(2)–C(6) 1.489(9) C(6)–N(2)–C(69) 115.4(5)N(2)–C(69) 1.461(9) C(1)–C(6)–N(2) 110.4(6)C(1)–C(6) 1.501(10) C(19)–C(69)–N(2) 108.8(5)C(19)–C(69) 1.520(10)N(5)–O(4) 1.291(8)N(5)–O(5) 1.215(8)N(5)–O(6) 1.208(8)

tances were obtained for the complexes of bpma and terpy containing the tridentately coordinated ligand and onecontaining trigonal nitrogen atoms. The decrease of bond coordination site available for substitution reactions. Thelengths is especially pronounced for thetrans Pd–N(2) stability constants reported for the interaction of

21 21bonds and they are shortening in the order dien, bpma and [Pd(dien)] and [Pd(terpy)] with the same ligands areterpy. The distortion of bond angles is the highest for terpy also included in Table 3. In the previous experiments [8]and decreasing in the order bpma and dien. On the other the equilibrium parameters were determined in the pres-hand, the distortion from planarity is the lowest for terpy ence of chloride ions, but now these values are transformedand increasing in the order bpma and dien. The changes of to chloride free conditions taking into account the equa-

–these parameters are in a good agreement with the high tion:K5K9(11K ?[Cl ]), where K and K9 denote theCl21flexibility of the complex [Pd(dien)] containing only stability constants of [ML] obtained in the absence and

tetrahedral nitrogen donors and with the rigidity of presence of chloride ions, respectively [22,23].K standsCl21[Pd(terpy)] built up exclusively from trigonal carbon for the stability constants of corresponding chloro com-

and nitrogen atoms. These data also suggest that thetrans plexes and the values logK 53.11 and 2.74 were usedCl1 1influence of the various nitrogen donor ligands should for the formation of [Pd(dien)Cl] and [Pd(terpy)Cl] ,

increase in the order dien,bpma,terpy. At the same respectively.time, the steric hindrance at the free coordination site It is obvious from Table 3 that the complex formationshould be the highest for terpy and decreasing for bpma reactions of the three tridentate N-donor ligands are similarand dien. to each other and only small differences can be observed in

the equilibrium parameters of the various mononuclear3 .2. Equilibrium studies complexes. These differences, however, reveal some sys-

tematic tendencies upon replacing the three amino-type NStability constants of the mixed ligand complexes were donors of dien to pyridine nitrogens of terpy. The forma-

determined by potentiometric measurements and the data tion of hydroxo complexes is favoured in the order:are collected in Table 3. terpy.bpma.dien, while the monodentate coordination

21In Table 3 M is used for the species [Pd(bpma)] of the N(3) donor atoms of pyrimidine bases (uridine,

Table 2Selected bond lengths and angles of palladium(II) complexes of three tridentate, nitrogen donor ligands

1 1 1Complex [Pd(dien)NO ] [20] [Pd(bpma)NO ] [Pd(terpy)OH] [21]2 3

˚Bond lengths (A)Pd–N(1) 2.041 2.001 2.022Pd–N(2) 2.047 1.975 1.934Pd–N(3) 2.048 2.016 2.035

Bond angles (8)N(1)–Pd–N(2) 85.3 84.0 81.2N(2)–Pd–N(3) 83.6 83.0 80.8N(1)–Pd–N(3) 167.2 166.4 162.0

Deviation from planarity (8) 1.7 0.6 0.0

Z. Nagy et al. / Journal of Inorganic Biochemistry 94 (2003) 291–299 295

Table 3Stability constants (logb ) of mixed ligand complexes of palladium(II) (T5298 K, I50.2 M, standard deviations are in parentheses)pqr

21 21 21Ligand (L) Species* [Pd(dien)] [Pd(bpma)] [Pd(terpy)][8] [8]

2OH MH 27.50 27.08(4) 26.8421

Uridine ML 8.47 8.90(2) 8.29

Cytidine ML 5.66 5.83 –

MeUH ML 8.56 9.26(2) 8.42

MeC ML 5.62 5.84(2) 4.60M LH 2 1.76(20) 3.382 21

EtGH MLH 15.74 15.06(7) 15.37ML 8.20 8.11(8) 8.44M L 14.68 14.95(6) 14.562

AcHis MLH 10.38 11.58(5) –ML 9.65 8.47(4) 7.06M LH 6.86 6.99(10) 5.032 21

AcHm ML 7.50 7.72(3) 6.68M LH 4.80 6.13(10) 3.692 21

AcLys ML 7.88 – –

AcMet ML 5.61 3.41(4) 3.6621 21 21*M stands for the species [Pd(dien)] , [Pd(bpma)] and [Pd(terpy)] .

21MeUH and MeC) is slightly preferred with [Pd(bpma)] . dinuclear complexes requires the coordination of both N(1)Similar observations have already been reported for the and N(3) atoms of imidazole rings. This reaction takes

21 21mixed ligand complexes of [Pd(en)] and [Pd(pic)] place in a rather slow process and the dinuclear complex[12] and were explained by both electronic and steric formation overlaps with hydrolytic reactions thus the erroreffects. On the other hand, the preference of hydrolytic of thermodynamic parameters is significantly increased. Inreactions suppresses the coordination of amino group of spite of the relatively high inaccuracy of these equilibrium

21 21AcLys in the ternary systems with [Pd(terpy)] and parameters, it can be stated that [Pd(terpy)] has the21[Pd(bpma)] . lowest affinity for imidazole binding either in the mono- or

In the case of MeC the deprotonation and coordination dinuclear species. The close similarity in the valuesof the exocyclic amino groups result in the formation of obtained for dien and bpma rules out the significantdinuclear complexes with [M LH ] stoichiometry. The influence of steric factors, thus the reduced affinity of2 21

21formation of the dimeric species takes place above pH 4 imidazole binding of [Pd(terpy)] may be explained byfor terpy and pH 5 for bpma ligand, while this process the electronic effect caused by the replacement of amino-Noccurs only under alkaline conditions with dien. In the by pyridine-N donor atom in thetrans position.

21[Pd(terpy)] –MeC system the structure of [M LH ] has The stability constants obtained for the coordination of2 21

been determined by X-ray crystallography and the en- the thioether donor function of AcMet reveal the similarity21 21hanced stability of the dinuclear complex was explained by of [Pd(terpy)] and [Pd(bpma)] . In another study—in

the strong stacking interactions between the N(3) and parallel with our work—it was found that the thioetherN(4)-coordinated terpy residues [24]. The formation of a residue ofL-methionine or S-methyl-L-cysteine is not asimilar species was observed with bpma, but the lower metal binding site in the reaction with [Pd(terpy)Cl]Clthermodynamic stability of the corresponding complex [13]. However, the presence of chloride ions suppressessuggests some weakening in the stacking interaction of thioether binding and the use of the mixed solvent in thebpma residues as compared to those of terpy. Dinuclear other study can be another reason for the absence ofcomplexes were also formed with the purine base EtGH thioether coordination under these experimental conditions.and the imidazole ligands AcHis and AcHm. In the case of Our data in Table 3 and the values reported earlier for theEtGH the binding sites of the dinuclear species can be thioether binding of AcMet in dipeptide complexes (logeasily explained by the metal ion coordination of both K54.89 for [Pd(GlyGlyH )], 4.91 for [Pd(GlyAlaH )]21 21

N(1) and N(7) donor atoms of guanine. This type of and 3.24 for [Pd(GlyMetH )] [9]) clearly support that all21

coordination rules out stacking of the pyridyl residues and tridentately coordinated palladium(II) species can bind thethe three metal species possess very similar equilibrium thioether donor functions. Thermodynamic stability of theparameters. thioether adducts is, however, rather low and it is especial-

21In the case of AcHis and AcHm the formation of the ly true for the interaction with [Pd(terpy)] ,

296 Z. Nagy et al. / Journal of Inorganic Biochemistry 94 (2003) 291–299

21[Pd(bpma)] and [Pd(GlyMetH )]. In the case of2121 21[Pd(terpy)] and [Pd(bpma)] the reduced stability of

the thioether complexes can be explained by the sterichindrance caused by theortho-hydrogens of pyridineresidues and the methyl group of the thioether function.The low thermodynamic stability of the thioether adduct of[Pd(GlyMetH )] as compared to those of the pal-21

ladium(II) complexes of the other dipeptides, however,suggests that electronic effects, e.g. the presence of softdonor atoms (sulfur or pyridine-N), also play a role in thereduced affinity for thioether binding.

3 .3. NMR studies

1H NMR studies were performed for the detection of21linkage isomers formed in the reaction of [Pd(bpma)]

with AcHis containing imidazole-N donor atoms. Mono-protonated imidazole derivatives are always characterisedby the existence of tautomeric forms in which one of theN(1) and N(3) donor atoms can be protonated. In the metalcomplexes of histidine or histamine the formation of the(NH ,N(3)) 6-membered chelates are favoured, but N(1)2

and N(3) linkage isomers can be present with N-protectedderivatives as is demonstrated by Scheme 1. The factorsinfluencing the formation of these linkage isomers havealready been studied by Appleton et al. in the case of

21[Pd(dien)] and some other palladium(II) or platinum(II)complexes [25,26]. It was found that the isomers havedifferent chemical shifts for both C(2)H and C(5)Hprotons of imidazole and the ratio of isomers depends onboth steric effects and pH values if other protonation sitesare also available in the molecules. Fig. 2 shows the NMR

21spectra obtained in the solutions of [PdL] –AcHis sys-tems at pH 4.4. Relatively low pH values and high ligandexcess were used in these experiments to avoid theformation of dinuclear complexes, which make the assign-ment of C(2) and C(5) proton resonances more difficult. Itcan be seen from Fig. 2 that the intensities of the C(2)Hand C(5)H proton resonances are different for the N(1)and N(3) linkage isomers. The approximate integral valuesN(3) /N(1)51.25, 0.9 and 2.0 were obtained for the

Scheme 1.palladium(II) complexes of dien, bpma and terpy, respec-tively. The data indicate that the ratio of linkage isomersdepends on the other donor functions present in the 21[Pd(dien)] [9]. Metal ion coordination of the thioethercoordination sphere of the metal ion.

residue results in some increase of light absorption in thewavelength range below 270 nm and the absorbance values

3 .4. Kinetic studies measured at 255 nm were used for the evaluation. Thecomplexation with cytidine is accompanied with some

Stopped-flow kinetic measurements were performed in decrease of molar absorptivity and the absorbance values21 21the [Pd(bpma)] –cytidine, [Pd(bpma)] –AcMet and obtained at 340 and 355 nm were used for calculation of

21[Pd(bpma)] –cytidine–AcMet ternary systems. The pos- kinetic parameters. The rate constants obtained from thesesible pathways for these reactions are shown in Scheme 2. measurements are included in Table 4 and the data

21The experiments and the evaluation of the kinetic measure- reported for the mixed ligand complexes of [Pd(dien)]ments were carried out under the same conditions as are also involved for comparison.reported earlier for the corresponding complexes of One of the major differences in the kinetic parameters of

Z. Nagy et al. / Journal of Inorganic Biochemistry 94 (2003) 291–299 297

Table 421Kinetic parameters for the reaction of [Pd(bpma)] with AcMet and

cytidine (T5298 K, I50.2 M, Standard deviations of the kineticparameters are in 2%)

Reaction Parameter Rate constants

L5dien [9] L5bpma21 21 21 4 6[PdL] 1AcMet k (M s ) 3.8310 $10S

21 23k (s ) 0.1 .33102S

21 21 21 3 3[PdL] 1cytidine k (M s ) 3.4310 4.8 x10N21 23 23k (s ) 1.7310 7.13102N

1 21 21[PdL(AcMet)] 1cytidine k (M s ) 3.1 556SN21 21k (M s ) 13.1 2.1NS

interference of these species in the kinetic studies is morepronounced than in the other systems. As a consequence,the kinetic parameters were obtained from a relativelynarrow pH range (3.5,pH,4.5) and their uncertainty wassomewhat bigger than usual.

It can be seen from Table 4 that the increase of rate21constants is much higher in the [Pd(bpma)] –AcMet

system than was obtained for cytidine. The reaction is toofast on the time scale of the stopped-flow technique, thuswe can only give a lower estimate for the rate constants of

21the reactions. This means that [Pd(bpma)] ions react30–100 times faster with the thioether donor AcMet than

21 1[Pd(dien)] . In accordance with this expectation the H21NMR spectra of [Pd(bpma)] –AcMet system gives a

single and relatively broad band for the methyl resonances1 21Fig. 2. H NMR spectra of the [PdL] –AcHis (1:5) systems (L5dien, of the thioether residue indicating the fast ligand exchange

bpma and terpy) at pH 4.4 (O) free ligand, (^) N(1) and (�) N(3)reactions. The NMR signals of the free and coordinatedlinkage isomers.ligands were observed separately in all other systemsincluded in Table 4.

21 21 21the complexes of [Pd(dien)] and [Pd(bpma)] is that In the reaction of [Pd(bpma)] with an equimolarthe substitution reactions of the latter metal species are mixture of the two entering ligands the formation of

21significantly faster than those of [Pd(dien)] . In the case cytidine adduct is favoured thermodynamically (log21of the [Pd(bpma)] –cytidine system the increase of the K 55.83), but according to the kinetic data thecytidine

rate constant is not very significant, it is only about 40%. ternary complex should form in a much faster reactionBecause of the enhanced stability of the hydroxo and with AcMet than with cytidine. As a consequence, the

21 1especially the dinuclear complexes of [Pd(bpma)] the thioether adduct [Pd(bpma)(AcMet)] is expected to ac-

Scheme 2.

298 Z. Nagy et al. / Journal of Inorganic Biochemistry 94 (2003) 291–299

cumulate quickly in the initial phase of the reaction and the monodentate ligands, while the other coordination sitesremain intact.cytidine complex is probably formed in a subsequent slow

The stability constants of the complexes formed in theprocess. It is essential to determine how the two binary21reaction of [Pd(bpma)] with N or S donor ligands werecomplexes are converted into each other in the ternary

determined by potentiometric titrations. It was found thatsystems. One possibility is a solvolytic path, namely the21coordination chemistry of [Pd(bpma)] is very similar todissociation of the binary complex and the coordination of

21that of [Pd(terpy)] , but in some respects it is differentthe other ligand in the subsequent reaction step. As an21from [Pd(dien)] . The differences are reflected in a seriesalternative, a direct ligand exchange reaction should also

of thermodynamic parameters including: (i) increasedbe considered between the binary complexes as shown on21tendency for hydrolytic reactions with [Pd(bpma)] andthe right hand side of Scheme 2. In the case of

2121 [Pd(terpy)] , (ii) deprotonation and coordination of the[Pd(dien)] –cytidine–AcMet system the ligand exchangeexocyclic amino group of MeC under slightly acidicreactions could be followed either by monitoring the

1 21 conditions resulting in the formation of dinuclear complex-[Pd(dien)(AcMet)] –cytidine or [Pd(dien) (cytidine)] –21 21es with [Pd(bpma)] and [Pd(terpy)] , (iii) stable adductAcMet substitution reactions. However, in the case of

1 21bpma only the reaction [Pd(bpma)(AcMet)] –cytidine formation between AcLys and [Pd(dien)] , while this21could be followed, because of the very fast formation reaction was not observed with [Pd(bpma)] and

21kinetics of the thioether complexes. On the other hand, it [Pd(terpy)] , (iv) low thermodynamic stability of the21 21also should be considered that the metal ion coordination thioether complexes of [Pd(bpma)] and [Pd(terpy)] .

of the thioether residues results in very small spectral Monodentate coordination of the various pyrimidine-,changes in the presence of pyridine nitrogen atoms and it purine- or imidazole-type nitrogen donor ligands (includ-significantly decreases the accuracy of kinetic parameters ing uridine, MeUH, cytidine, MeC, EtGH, AcHis andin the systems containing bpma ligand. AcHm) is, however, not much affected by the change of

The kinetic parameters obtained for the ligand exchange the triamine ligands and relatively stable Pd–N bondedreactions are also involved in Table 4 and these data adducts are formed in all cases. The thermodynamicsupport again the significant increase in the rate of stabilities of these mixed ligand complexes of N donorsubstitution reactions of bpma. In the case of the ligands always exceed those of the thioether coordinated

1[Pd(dien)(AcMet)] –cytidine system the contribution of species. As a consequence, the metal ion speciation of a21the exchange path tok was around 80% in the presence multicomponent system containing [Pd(bpma)] , AcMetobs

of high ligand excess, while it is almost 95% for bpma and the above mentioned nitrogen donor ligands inunder the same conditions. Kinetic parameters for the equimolar concentrations is characterised by the predomi-substitution reactions in palladium(II)–triamine–thioether nant formation of the Pd–N bonded complexes at physio-systems have not been reported in the literature to date. logical pH.However, the corresponding platinum(II) complexes of The kinetic parameters obtained from the stopped flowtriamines and sulfur containing ligands have already been measurements indicate that the substitution reactions of theinvestigated. The kinetic data for the reactions of thioether ligand AcMet are much faster than those of the

21 21 21[Pt(dien)] , [Pt(bpma)] and [Pt(terpy)] with several N-donor cytidine. Similar observations have already been21thiourea ligands have been published recently [27] and the reported for the ligand exchange reactions of [Pd(dien)] ,

values reveal the enhancement of the kinetic reactivity by but the presence of the two pyridyl residues in21the increase of the number of pyridine nitrogen donors. [Pd(bpma)] significantly enhances the kinetic reactivity

of the palladium(II) complexes. The increase of the rateconstants is only 40% for the N-donor cytidine, but there is

4 . Conclusions about 30- to 100-fold increase in the kinetic parameters of21thioether coordination with [Pd(bpma)] . Thus, the for-

1The ligand bis(pyridin-2-ylmethyl)amine (bpma) con- mation of the thioether adduct [Pd(bpma)(AcMet)] takestaining two pyridine- and one amino-type nitrogen donor place in a very fast reaction and the equilibration requiresatoms readily forms a stable complex with palladium(II) only 3 ms, while it is about 1 s for the formation of

1ion even under strongly acidic conditions. The nitrate salt [Pd(dien)(AcMet)] . On the other hand, the species1of the complex [Pd(bpma)NO ]NO has been prepared in [Pd(bpma)(AcMet)] can be an important intermediate in a3 3

solid state and structurally characterised. The ligand is multicomponent system containing both N- and S-donorcoordinated tridentately via the three nitrogen atoms and ligands. It is best represented by the time-dependent metal

21the fourth coordination site of the slightly distorted square ion speciation of the [Pd(bpma)] –AcMet–cytidine sys-planar palladium(II) ion is occupied by the oxygen atom of tem shown in Fig. 3.a nitrate ion in the solid state or by a water molecule in It is clear from Fig. 3 that the Pd–S (thioether) bondeddiluted aqueous solutions. As a consequence, the fourth species can be readily obtained even in the presence of

21coordination site of the species [Pd(bpma)(H O)] is excess of N-donor ligands, but the lifetime of the sulfur2

easily available for substitution reactions with various coordinated complexes is very low. The equilibrium state

Z. Nagy et al. / Journal of Inorganic Biochemistry 94 (2003) 291–299 299

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A cknowledgements

This work was supported by the Hungarian ScientificResearch Fund (OTKA T034361, M028244 and TS040685).