Studies on Pt±S bonds. Complexes of methylsulfanyl- andmethylsulfynil-acetate, benzoate and phenolate with

the diethylenetriamineplatinum(II) moiety

Alessandro Pasini*, Cristiano FioreDipartimento di Chimica Inorganica Metallorganica e Analitica, the University and CNR Centre, Via Venezian 21, 20133 Milan, Italy

Received 12 May 1998; accepted 27 July 1998

Abstract

This paper reports on some complexes of formula [Pt(dien)(L-S]�, where dien is diethylenetriamine and L are methylsulfanyl- and

methylsul®nyl-acetate, benzoate and phenolate. These ligands are monodentate and bound to Pt via the sulfur atoms. Reactions with

guanosinemonophosphate (GMP) give, in all cases, [Pt(dien)(GMP)]2ÿ, with GMP coordinated through the N(7) atom. The reactivities of

the [Pt(dien)(L-S)]� complexes are compared with those of [Pt(en)(L-S,O)]� (en, ethylenediamine), in which the ligands L are chelated to

Pt via the sulfur and the carboxylato, or phenolato oxygen atoms. # 1999 Elsevier Science S.A. All rights reserved.

Keywords: Platinum complexes; Diethylenetriamine complexes; Sulfur ligands complexes

1. Introduction

The interest in the properties of the platinum±sulfur bonds

arises because the interaction of platinum with the sulfur

nucleophilic centres of certain bio-molecules is believed to

play important roles in the metabolism of cisplatin or of its

analogues [1±4]. Examples are: Pt binding to the sulfhydryl

groups of kidney proteins, the likely origin of the renal

toxicity of cisplatin [5±7]; interaction with intracellular

glutathione, which leads to inactivation of the drug and cell

resistance [8,9]; the proposed use of extracellular gluta-

thione, as protection of cisplatin toxicity [10], and of sulfur

nucleophiles (such as dithiocarbamates) as rescue agents in

the case of acute toxicity [11]; Pt binding to DNA poly-

merase-a, probably connected with the mechanism of cyto-

toxicity [12±14]; it has also been proposed that Pt-

methionine complexes, formed in vivo, may provide a route

for DNA platination [15,16] and/or a way to dispose of

platinum, since these kinds of complexes have been found in

the urine of patients treated with cisplatin [17], and in those

of animals treated with carboplatin [18]. Finally a number of

platinum complexes with sulfur ligands (sulfoxides,

thioethers and even thiourea) have been proposed as cispla-

tin analogues [19±24].

In our laboratory we have been exploring the properties of

a series of complexes [Pt(en)(L-S,O)]� in which the

Pt(II)(en) moiety is chelated by the ligands L1±L6 (see

Scheme 1). In particular we have studied the cytotoxicity

and the reactivity towards GMP of such complexes [25,26],

as well as the competition of the ligands L1±L6 for the Pt(en)

group [27]. Certain results of these studies could be ratio-

nalised considering the different stabilities of the various

chelate rings formed by L1±L6, which could mask the

reactivities and/or stabilities of the different Pt±S bonds,

consequently we decided to study the series of complexes

1±6, [Pt(dien)L]�, in which the tridentate dien ligand leaves

only one coordination position occupied by the sulfur atoms

of the ligands L1±L6 (see below). This paper reports such

studies.

2. Experimental

Analyses (Table 1) are from the Microanalytical Labora-

tory of our department. Infrared and multinuclear NMR

spectra were recorded on Jasco FT/IR 5000 and Bruker AC

200 respectively. FAB MS spectra were obtained on a VCA

Analytical 7070 EQ from 3-nitrobenzyl alcohol with xenon

as the FAB source, isotope cluster abundance was checked

by computer simulations.

Inorganica Chimica Acta 285 (1999) 249±253

*Corresponding author. Tel.: +39-02-26680676; fax: +39-02-2362748.

0020-1693/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.

PII: S0020-1693(98)00347-8

The ligands L1±L6 were obtained according to Ref. [26].

Chlorotriethylenediamineplatinum(II) chloride was pre-

pared following Ref. [28].

2.1. Nitrato(triethylenediamine)platinum(II) nitrate

The 51.3 ml of 0.1 mol lÿ1 water solution of AgNO3 were

added dropwise to a solution of 0.948 g of [PtCl(dien)]Cl

(2.57 mmol) in 30 ml of water. The slurry was stirred for

24 h in the dark, ®ltered twice over celite and the ®ltrate was

evaporated to drops under reduced pressure. The compound

was obtained by addition of cold ethanol. Yield: 96%,

1.034 g.

2.2. Diethylenetriamine(dimethylsulfoxide)platinum(II)

nitrate

This was obtained by adding 0.220 g of [Pt(dien)-

(NO3)]NO3 to an aqueous solution of (Me)2SO (0.041 g

in 10 ml). After 15 h at room temperature the solution was

concentrated to 2 ml at the rotary evaporator. Addition of a

large excess of acetone gave an oily product which became

crystalline on stirring. Yield: 94%, 0.244 g.

2.3. Preparation of complexes 1±6

Diethylenetriamine(methylsulfanylacetate)platinum(II)

nitrate, [Pt(dien)(sa)]NO3 (1). A solution of 0.131 g

(1.23 mmol) of methylsulfanylacetic acid in 5 ml of water

was treated with 0.029 g of LiOH and 0.520 g of [Pt(dien)-

(NO3)]NO3. After 6 h at 408C the solution was evaporated

to dryness under vacuum, the residue treated with 20 ml of

methanol and ®ltered, obtaining 0.323 g of a white product

(56.5%). The other compounds were prepared by the same

procedure. Compound [Pt(dien)(soph)]ClO4 (6), was crys-

tallised by the addition of LiClO4.

2.4. Diethylenetriamine(guanosinemonophosphato)-

platinum(II), Pt(dien)(GMP)

A water solution of equimolar amounts of GMP (di-

sodium salt) and Pt(dien)(NO3)NO3 (0.47 mmol in 30 ml)

was heated at 408C for 5 h in the dark, ®ltered, concentrated

to 5 ml in vacuo and treated with methanol obtaining

Scheme 1.

Table 1

Analytical and other relevant characterisation data

Compound Analyses (%) 195Pt NMRa Relevant IR data FAB�, m/zb

(calculated values in parentheses) � (ppm) (KBr pellets, cmÿ1)

C H N

[Pt(dien)(sa)]NO3 17.9 (18.1) 3.7 (3.9) 11.9 (12.0) ÿ3376 �COO � 1608 403 (80/100)

[Pt(dien)(soa)]NO3�H2O 16.5 (16.8) 4.1 (4.0) 11.2 (11.3) ÿ3429 �COO � 1604; 419 (60/100)

�SO � 1113

[Pt(dien)(sb)]NO2 27.9 (28.2) 4.1 (3.9) 10.9 (11.0) ÿ3340 �COO � 1582 465 (50/100)

[Pt(dien)(sob)]NO3�2H2O 25.7 (25.6) 4.4 (4.3) 9.6 (9.9) ÿ3445, �COO � 1615; 481 (80/100)

ÿ2499c �SO � 1120, 1005d

[Pt(dien)(sph)]NO3�2H2O 25.5 (24.3) 4.5 (4.5) 10.5 (10.4) ÿ3330 437 (100/50)

[Pt(dien)(soph)]ClO4�2H2O 23.0 (24.2) 4.4 (4.1) 10.2 (10.0) ÿ3410 e 453 (100/75)

[Pt(dien)(NO3)](NO3) 11.5 (11.4) 2.8 (2.6) 16.5 (16.7) ÿ2482 1377f

1506, 1283, 990g

[Pt(dien)(Me2SO)](NO3)2 14.3 (14.4) 3.5 (3.8) 14.3 (14.0) ÿ3450 �SO � 1132

a D2O solutions; � values from [PtCl6]2ÿ.b In parentheses the ratio of the abundances of the peak due to the cation and the peak at m/z 297, corresponding to [Pt(dien)ÿH]�, see text.c This peak is probably due to a dien aquo complex derived by dissociation of sob.d This band is probably due to free sobÿ.e �SO buried under the ClO4

ÿ band.f Ionic nitrate.g Coordinated nitrate, this spectrum was taken from Nujol mull.

250 A. Pasini, C. Fiore / Inorganica Chimica Acta 285 (1999) 249±253

0.298 g of the white product, impure of NaNO3. This was

used as such for the spectroscopic characterisation. 1H

NMR (D2O solution, 408C): 6.24, JH±H 4.33 Hz, H(10);9.00, JPt±H 25 Hz, H(8). 13C NMR (D2O solution): 50.3,

54.2 (dien CH2); 62.8, 70.4, 75.7, 85.1, 88.2 (ribose moiety);

114.1, C(5); 140.3, C(8); 150.9, C(4); 154.9, C(2); 157.6,

C(6). 195Pt NMR (D2O solution): � ÿ2868, cfr. [Pt(dien)-

(histidine-N(1))]2�, ÿ2861 [28]. FAB� MS: m/z � 660:

[Pt(dien)(GMP)�H]�.

2.5. Reactivity studies

Reactions of complexes 1±6 with GMP. Weighed

amounts of a platinum complex and of three-fold excess

of GMP (disodium salt) were mixed in an NMR tube. D2O

was then added in order to obtain a 10ÿ2 mol lÿ1 concen-

tration of the Pt complex. Under these conditions, pH* was

about 7. The tube was thermostated at 408C. The course of

the reaction was followed by measuring the ratio of the

integrals of the resonances of H(8) of free and complexed

GMP. Due to the broadening of the Pt satellites of the latter,

the uncertainty of its integral is about 15%.

3. Results and discussion

3.1. Synthesis and characterisation

Complexes 1±6 were obtained by reaction of [Pt(dien)-

(NO3)]NO3 with the lithium salt of the ligands L1±L6. The

cationic complexes 1±5 were isolated as nitrate salts, while

6 was crystallised as the perchlorate (caution!). They are 1/1

electrolytes in water solutions. The IR spectra of 1±4 show

bands attributable to �asym. of ionic carboxylate groups

(Table 1). This suggests S-coordination of the ligands,

con®rmed also by multinuclear NMR spectroscopy (D2O

solutions). 195Pt NMR: (Table 1) chemical shift values in

the range reported for PtN3S chromophores, in particular the

slightly different � values observed for the sulfanyl (around

ÿ3360 ppm) and sul®nyl derivatives (about ÿ3430 ppm)

compare well with those of similar complexes [29±35], see

also the value of [Pt(dien)(Me2SO)]�. 1H NMR (Table 2):

down ®eld shift of the CH3S resonances (with respect to

those of the free anionic ligands [25,26,30±33]) and Pt±H

coupling of about 40 Hz for the sulfanyl and 20 Hz for the

sul®nyl ligands. 13C NMR (Table 3): again low ®eld shift of

the CH3 resonances and Pt±C coupling (about 15 Hz for

CH3S and 48 Hz for CH3SO) [25,26,30±33].

These spectra are stable for days with the exception of the

sob derivative 4, as this ligand dissociates in water solution.

The conductivity of a freshly prepared solution of this

complex is that of a 1/1 electrolyte, but it slowly increases

with time, approaching that of a 1/2 electrolyte after one

day. NMR data are also in accordance with such a dissocia-

tion (Tables 1±3). It is likely that this compound is partly

dissociated also in the solid state, since its IR spectrum

shows bands attributable to both S-coordinate and free

sulfoxide (1120 and 1005 cmÿ1 [25]) plus a medium inten-

sity band at 1050 cmÿ1 which is not present in the sb

complex and must be due to the sul®nyl group (h2-S=O?).

The FAB MS spectra of all complexes gave cluster of

peaks corresponding to the cation [Pt(dien)L]�. These

spectra showed also a rather intense peak at m/z � 297,

corresponding to [Pt(dien)±H]�, which, with the exception

of the sph and soph case, is the most abundant (see Table 1).

Such a low stability of the Pt±S bonds in complexes 1±6under FAB conditions is in sharp contrast with what was

observed for the previously described [Pt(en)(L-S,O)]�

derivatives [26], for which the peak at m/z � 254, which

corresponds to [Pt(en)±H]�, arising from the detachment of

L, has a very low abundance (from 0 to 25% for the sa and

sob derivatives), a clear consequence of the stability of the

chelate rings of L's.

3.2. Reactivity with guanosinemonophosphate

Complexes 1±6 react with GMP yielding Pt(dien)(GMP-

N(7)), identi®ed by comparison with an authentic sample.

The course of these reactions was followed by 1H NMR

spectroscopy at 408C in D2O solution by monitoring the

decrease of the resonances of H(8) of free GMP and the

growth of a resonance at 9.00 ppm (at 408C) due to H(8) of

N(7) coordinated GMP. Since some reactions are rather slow

we used a three-fold excess of GMP, which turned out to be

the highest excess which allowed reliable measurements of

the ratio of the integrals of the two H(8) peaks. Under these

conditions, pH* (the pH reading uncorrected for D2O) was

around 7. The half lives of these reactions are presented in

Table 4.

All the Pt±S bonds of the complexes studied in this work

are substituted by GMP; the order of ligands substitution is:

soph > soa � sob >> sb > sph > sa.

Table 21H NMR data for ligands and complexesa

Compound CH3S CH2 (dien) Others

saÿ 2.24 3.39 (CH2S)

[Pt(dien)(sa)]� 2.73 (42) 3.00±3.60 3.87 (37, CH2S)

soaÿ 2.94 3.75 (CH2S)

[Pt(dien)(soa)]� 3.75 (20) 3.10±3.70 b

sbÿ 2.59 7.40±7.65

[Pt(dien)(sb)]� 3.07c 2.80±3.60 7.60±8.10

sobÿ 3.08 7.75±8.18

[Pt(dien)(sob)]� 3.73 (23); 3.08 d 2.85±3.65 7.75±8.25

sphÿ 2.59 7.00±7.50

[Pt(dien)(sph)]� 2.72 (41) 2.80±3.50 6.80±7.60

sophÿ 2.97 6.80±7.70

[Pt(dien)(sopph)]� 3.79 (21) 2.90±3.60 6.85±7.80

[Pt(dien)(Me2SO)]2� 3.55 (23) 2.80±3.20

a D2O solutions; 408C; � values in ppm vs. Me4Si; JPt±H in parentheses.b The CH2 protons are buried under the HOD resonance [25].c Pt satellites could not be observed because buried under the dien

resonances.d Due to ligand dissociation, see text.

A. Pasini, C. Fiore / Inorganica Chimica Acta 285 (1999) 249±253 251

The reactivity of the sob complex may seem anomalous,

in view of the fact that this ligand is partly dissociated in

water solution and that the reaction of the aqua complex

[Pt(dien)(H2O)]2� is rather fast (t1/2 < 0.3 h), however dis-

sociation of sob from 4 is a slow process.

Table 4 reports also the t1/2 values, evaluated in a pre-

ceeding work [26], of the reactions of an excess GMP with

[Pt(en)(L-S,O)]�, in which the ligands are chelated through

the sulfur and the oxygen (carboxylato or phenolato) atoms.

Substitution of chelated L's, to give [Pt(en)(GMP)2]2ÿ

occurs in two steps: displacement of the carboxylato, or

phenolato oxygen atoms gives, initially, [Pt(en)(GMP)-

(L-S))]ÿwhich then yields the bis GMP derivatives by slower

substitution of the sulfur donor atoms [26]. Comparison of

the reactivities of the two series is interesting. (i) Mono-

dentate soph is the most reactive ligand, but in the chelated

series it is more inert than soa and sob. Such a difference

may be attributed to the inertness of the ®ve-membered

chelate ring which involves the phenolato moiety. (ii)

Monodentate S-coordinated sph is displaced, although

slowly, by GMP, whereas chelated sph in [Pt(en)(sph-

S,O)]� is unreactive, a difference which must again be

attributed also to the rather high inertness of the chelate

ring of sph, rather than to thermodynamic stability, in fact

[Pt(en)(GMP)2]2ÿ is inde®nitely stable in the presence of an

excess of sph [26]. (iii) The behaviour of monodentate sa is

noteworthy: reaction of [Pt(en)(sa-S,O)]� with GMP

quickly gives [Pt(en)(GMP)(sa-S)] which does not react

further in the presence of an excess of GMP [26], on the

contrary [Pt(dien)(sa-S)] reacts, albeit slowly, with GMP.

4. Conclusions

4.1. Pt±sulfinyl bonds

These are easily reversible. Interestingly Pt sulfoxide

complexes possess also a fair practical stability, they are

easily prepared and handled, and dimethylsulfoxyde com-

plexes have been used as synthons for the preparation of

various Pt complexes [28,36]. Pt±sul®nyl complexes have

also been proposed as cisplatin analogues, but their activity

is usually low [19,25,26].

4.2. Pt±sulfanyl bonds

We have con®rmed that these bonds are reversible in the

case of Pt-dien complexes [15,16], however if such bonds

are part of a chelate ring, as in the en derivatives of sa, they

may be rather reluctant to substitution (Table 4 and Ref.

[26], see also the FAB data). Moreover, if we compare the

slow, but signi®cant reactivity of S-coordinate sa in the dien

complex (and of other dien thioether complexes [15,16])

with the inertness of this ligand in [Pt(en)(GMP)(sa-S)]ÿ as

well as that of a similar Pt-en-methionine-S complex [37],

one is led to invoke some role of the ancillary ligands, such

as, for instance, a trans labilizing effect of the secondary

amine group of dien. Since ammonia does not possess such a

trans effect, mixed Pt(NH3)(thioether) complexes, which

Table 313C NMR data a

Compound CH3S Other C resonances CO2ÿ CH2 (dien)

saÿ 18.1 41.8 (CH2S) 180.7

[Pt(dien)(sa)]� 22.3 (18) 45.6 (13) 174.8 53.0 (10), 55.4 (24)

soaÿ 39.2 62.8 (CH3S) 173.0

[Pt(dien)(soa)]� 45.6 (48) 59.8 170.1 52.4 (no), 55.5 (24)

sbÿ 17.80 127.6, 127.9, 142.8, 151.2, 139.3, 138.4 178.5

[Pt(dien)(sb)]� 24.3 (12) 131.3, 131.4, 133.1, 131.2, 128.3 (27, CS), 141.7 (CCO2) 176.4 52.9, 55.6 (18)

sobÿ 45.3 124.9, 132.7, 133.2, 134.5, 135.7 (CS), 146.5 (CCO2) 174.3

[Pt(dien)(sob)]� 47.2 (48)b 125.2, 133.2, 133.4, 135.4, 132.7 (CS), 147.2 (CCO2)b 175.9b 52.5, 55.4b

sphÿ 16.8 165.3, 128.8, 129.7, 120.0, 129.2, 118.4

[Pt(dien)(sph)]� 22.9 (no) 135.1, 132.6, 123.0, 117.5, 168.4 (CO), 117.9 (CS), 115.7 53.2 (18), 55.7 (22)

sophÿc 40.3 167.4, 131.6, 126.6, 123.3, 136.2, 117.6

[Pt(dien)(soph)]� 45.6 (49) 116.4, 124.7, 127.3, 138.0, 167.9 (CO), 112.8 (CS) 52.4, 53.4 (26)

a D2O solutions, room temperature; � values in ppm vs. Me4Si; JPt±C (Hz) in parentheses when observed; the spectra of the anionic ligands are from Ref. [26].b In addition to these resonances we also observed the peaks of sobÿ and [Pt(dien)(H2O)]2� (52.4, 57.5) due to dissociation, see text.

Table 4

Half lives (h) for the reactions of Pt(dien)(L-S)]� and [Pt(en)(L-S,O)]�

with GMP

t1/2 (h)

L [Pt(dien)(L-S)]�a [Pt(en)(L-S,O)]�b

sa 22c (0.45)d

soa 1.6 0.67

sb 7.2 2.25

sob 1.8 0.38

sph 12.3 unreactive

soph 0.9 1.55

a Reaction conditions: 10ÿ2 mol lÿ1 D2O solutions, three-fold excess of

GMP, 408C, uncertainty is around 15%.b From Ref. [26], same reaction conditions, but with a ten-fold excess of

GMP.c The different values reported in Ref. [27] are due to the different

conditions employed.d This value is referred to the formation of [Pt(en)(GMP)(sa-S)]ÿ, which

does not react further, see text.

252 A. Pasini, C. Fiore / Inorganica Chimica Acta 285 (1999) 249±253

could form in vivo through the interaction of cisplatin with,

e.g., methionine, could be rather inert. The consequence of

such an inertness can be either the inactivation of cisplatin

itself, or the formation of stable DNA-protein cross-links.

Moreover, the relatively high trans effect of thioethers could

also lead to ligand scrambling. Such a scrambling has been

observed in some instances for the en and dien complexes

[38±40], but it can become an important process in the case

of cisplatin itself [38], indeed Pt-methionine complexes

have been found as metabolites of platinum anticancer

drugs [17,18].

Acknowledgements

We thank the Italian MURST (Ministero dell'Universita'

e della Ricerca Scienti®ca e Tecnologica) for ®nancial help.

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