Role of the phosphine ligands on the stabilization of monoadducts of the model nucleobases 1-methylcytosine and 9-methylguanine in platinum(II) complexes

Article abstract of DOI:10.1021/ic7020319

The addition of 1-methylcytosine (1-MeCy) or 9-methylguanine (9-MeGu) to solutions of cis-(PPh₃)₂Pt(ONO₂)₂ (1a), in a molar ratio of 1:1, affords the monoadducts cis-[(PPh ₃)₂Pt(1-MeCy)(ONO

Full text of DOI:10.1021/ic7020319

Inorg. Chem. 2008, 47, 2688-2695
Role of the Phosphine Ligands on the Stabilization of Monoadducts of
the Model Nucleobases 1-Methylcytosine and 9-Methylguanine in
Platinum(II) Complexes
Diego Montagner,† Ennio Zangrando,‡ and Bruno Longato*,†
Dipartimento di Scienze Chimiche, UniVersita´ di PadoVa, Via Marzolo 1, 35131 PadoVa, Italy,
and Dipartimento di Scienze Chimiche, UniVersita´ di Trieste, Via Giorgieri 1, 34127 Trieste, Italy
Received October 13, 2007
The addition of 1-methylcytosine (1-MeCy) or 9-methylguanine (9-MeGu) to solutions of cis-(PPh3)2Pt(ONO2)2 (1a),
in a molar ratio of 1:1, affords the monoadducts cis-[(PPh3)2Pt(1-MeCy)(ONO2)]NO3 (2a) and cis-[(PPh3)2Pt(9-
MeGu)(ONO2)]NO3 (3a) and only trace amounts of the bisadducts cis-[(PPh3)2Pt(1-MeCy)2](NO3)2 (4a) and cis-
[(PPh3)2Pt(9-MeGu)2](NO3)2 (5a), respectively. The X-ray structural determination of 2a and 3a indicates a strong
π-π stacking interaction between one of the PPh3 phenyl groups and the pyrimydinic N3-platinated cytosine or
the imidazole part of the N7-coordinated guanine base. The addition of a further equiv of nucleobase to the
monoadducts forms quantitatively the bisadducts that have been isolated as pure compounds 4a and 5a. Under
the same experimental conditions, the dinitrato analogue cis-[(PMePh2)2Pt(ONO2)2] (1b) forms the monoadducts
2b and 3b in equilibrium with a relatively high concentration (20–30%) of the bisadducts cis-[(PMePh2)2Pt(1-
MeCy)2](NO3)2 (4b) and cis-[(PMePh2)2Pt(9-MeGu)2](NO3)2 (5b), which have been structurally characterized by
single-crystal X-ray analysis. The characterization of the isolated complexes by multinuclear NMR spectroscopy is
also described.
Introduction
L is PMe2Ph and PMePh2,3 whereas the mononuclear
species cis-[L2Pt{9-MeAd(-H)-N6N7}]NO3 is formed when
L is PPh3.4 Similarly, deprotonation of 1-methylcytosine (1-
MeCy) leads to the polynuclear adducts cis-[L2Pt{1-MeCy-
In the last 40 years, the interactions of model nucleobases
with metal ions have been studied in great detail, in particular
toward PtII centers having amine or diamine as ancillary
ligands.1 Recently, we have demonstrated that the use of
monodentate phosphines (PR3) as ligands led to the formation
of platinum(II) complexes in which the nuclearity of the
adducts and/or the binding mode of the nucleobase appear
to depend on the nature of the substituents R on the
phosphorus atoms. For instance, deprotonation of 9-substi-
tuted adenine (9-MeAd) promoted by the hydroxo complexes
cis-[L2Pt(µ-OH)]2(NO3)2 affords the dinuclear species cis-
[L2Pt{9-MeAd(-H)-N1N6}]2(NO3)2 when L is PMe32 and the
trinuclear adducts cis-[L2Pt{9-MeAd(-H)-N1N6}]3(NO3)3 when
5
(-H)-N3N4}]3(NO3)3 with the less hindered phosphines PMe3
and PMe2Ph,6 while the mononuclear species cis-[L2Pt{1-
MeCy(-H)-N4}(1-MeCy-N3)](NO3) is quantitatively obtained
when L is PPh3.6
In this paper, we report the synthesis and characterization
of the complexes cis-[(PPh3)2Pt(nucleobase)(ONO2)]NO3 and
cis-[(PPh3)2Pt(nucleobase)2](NO3)2, containing the neutral
nucleobases 1-MeCy and 9-methylguanine (9-MeGu) acting
as monodentate ligands. Early studies have shown that the
nitrato complexes cis-L2Pt(ONO2)2, obtained from the cis-
* To whom correspondence should be addressed. E-mail: bruno.longato@
unipd.it.
(3) (a) Longato, B.; Pasquato, L.; Mucci, A.; Schenetti, L. Eur. J. Inorg.
Chem. 2003, 128. (b) Longato, B.; Pasquato, L.; Mucci, A.; Schenetti,
L.; Zangrando, E. Inorg. Chem. 2003, 42, 7861.
(4) Montagner, D.; Longato, B. Inorg. Chim. Acta, 2007, doi: 10.1016/
j.ica.2007.02.025.
†
Universita´ di Padova.
Universita´ di Trieste.
‡
(1) Lippert, B.; Müller, J. Concepts and Models in Bioinorganic Chemistry;
Kraatz, H.-B., Metzler-Nolte, N., Eds.; Wileys-VCH: Weinheim,
Germany, 2006; p 137.
(5) Trovò, G.; Bandoli, G.; Casellato, U.; Corain, B.; Nicolini, M.;
Longato, B. Inorg. Chem. 1990, 29, 4616.
(2) Schenetti, L.; Mucci, A.; Longato, B. J. Chem. Soc., Dalton Trans.
1996, 299.
(6) Longato, B.; Montagner, D.; Zangrando, E. Inorg. Chem. 2006, 45,
8179.
2688 Inorganic Chemistry, Vol. 47, No. 7, 2008
10.1021/ic7020319 CCC: $40.75  2008 American Chemical Society
Published on Web 03/08/2008

Model Nucleobases 1-MeCy and 9-MeGu
5.36 (d, 2JPP ) 22.6 Hz, 1JPPt ) 4080 Hz), 5.12 (d, 2JPP ) 22.6 Hz,
platin analogues cis-L2PtCl2 (L ) tertiary phosphines), in
reactions with 1 equiv of these nucleobases, form mixtures
of cis-[L2Pt(nucleobase)]2+ and cis-[L2Pt(nucleobase)2]2+, in
equilibrium with the unreacted starting nitrate. The relatively
high stability of the bisadducts precludes the isolation of the
intermediate species cis-[L2Pt(nucleobase)]2+.3 However, the
presence of PPh3 ligands stabilizes enough such intermediates
to allow their isolation as pure compounds. The single-crystal
X-ray analyses of cis-[(PPh3)2Pt(1-MeCy)(ONO2)]+ and cis-
[(PPh3)2Pt(9-MeGu)(ONO2)]+ are reported here along with
a study of their behavior in solution by NMR spectroscopy.
With the less hindered phosphine PMePh2, we were able to
isolate only the bisadducts cis-[(PMePh2)2Pt(1-MeCy)2]2+ and
cis-[(PMePh2)2Pt(1-MeCy)2]2+, whose X-ray structures are
also described. Finally, the relative stabilities of the monoad-
ducts cis-[L2Pt(nucleobase)]2+ (L ) PMePh2, PPh3) have
been evaluated.
2
2
1JPPt ) 3536 Hz), 4.60 (d, JPP ) 21.4 Hz), 4.02 (d, JPP ) 21.4
Hz) with relative intensities 9:1, respectively. 31P{1H} NMR in
2
1
DMF (δ): two AB multiplets at 5.07 (d, JPP ) 22.3 Hz, JPPt
)
4015 Hz), 4.51 (d, 2JPP ) 22.3 Hz, 1JPPt ) 3488 Hz), 5.02 (d, 2JPP
) 21.4 , JPPt ) 4015 Hz), 4.48 (d, JPP ) 21.4 Hz, JPPt ) 3488
Hz) with relative intensities 1:1, respectively. The same synthesis,
carried out in CH2Cl2, followed by precipitation with Et2O affords
1
2
1
1
2a, with a yield of 70%. H NMR in CD2Cl2 (δ): 9.47 (br s, 1H,
NH2), 7.71–7.28 (c m, 30H, PPh3), 7.12 (br s, 1H, NH2), 6.81 (d,
3JHH ) 7.0 Hz, 1H, H6], 5.85 [d, 3JHH ) 7.0 Hz, 1H, H5], 3.20 (s,
3H, NCH3). 31P{1H} NMR in CD2Cl2 (δ): AB multiplet at 5.11 (d,
2JPP ) 21.3 Hz, 1JPPt ) 3939 Hz), 3.58 (d, 2JPP ) 21.3 Hz, 1JPPt
)
3455 Hz). 195Pt NMR (δ) in CH2Cl2: -4163 (dd, 1JPPt ) 3430 and
3960 Hz). Crystals for X-ray analysis, having the composition
2a·3DMF, were obtained from a DMF solution of 2a by slow
diffusion of diethyl ether vapors, at room temperature.
Synthesis of cis-[(PPh3)2Pt(9-MeGu)(ONO2)](NO3)·2DMF
(3a). By using the procedure of 2a, complex 3a was isolated with
a yield of 55%. Diffusion of Et2O into a DMF solution afforded
colorless crystals having the composition cis-(PPh3)2Pt(9-MeGu)-
(NO3)2 ·2DMF. Elem anal. Calcd for C42H37N8O8P2Pt: C, 48.56;
H, 3.60; N, 10.78. Found: C, 48.91; H, 3.72; N, 10.31. 1H NMR in
DMSO-d6 (δ): 11.5 (br s, 1H, N1H), 8.66 (s, 1H, H8), 7.6–7.3 (c
m, 30H, PPh3), 6.95 (br s, 2H, NH2), 3.38 (s, 3H, NCH3). DMF
resonances: 7.94 (s, 1H, CHO), 2.88 (s, 3H, CH3), 2.72 (s, 3H,
CH3). 31P{1H} NMR in DMSO-d6 (δ): AB multiplets at 7.94 (d,
Experimental Section
Instrumentation and Materials. NMR spectra of various
solvents at 300 K were obtained in solution in 5-mm sample tubes
1
for H, 31P, and 195Pt (operating at 300.13, 121.5, and 64.2 MHz,
respectively) with a Bruker AVANCE 300 MHz spectrometer
equipped with a variable-temperature apparatus and for 15N
(operating at 40.6 MHz) with a Bruker 400 AMX-WB spectrometer.
The 1H NMR chemical shifts were referenced to the residual
impurity of the solvent. The external references were H3PO4 (85%
w/w in D2O) for 31P, Na2PtCl4 in D2O (adjusted to δ ) -1628
ppm from Na2PtCl6) for 195Pt, and CH3NO2 (in CDCl3 at 50% w/w)
for 15N. Inverse-detected spectra were obtained through hetero-
nuclear multiple-bond (HMBC) correlation experiments, using
parameters similar to those previously reported.2
2JPP ) 22.6 Hz, 1JPPt ) 3482 Hz), 5.12 (d, 2JPP ) 22.6 Hz, 1JPPt
)
4048 Hz). 31P{1H} NMR in DMF (δ): two AB multiplets at 7.12
(d, 2JPP ) 22.6 Hz, 1JPPt ) 3465 Hz), 4.99 (d, 2JPP ) 22.6 Hz, 1JPPt
2
1
) 4032 Hz), 5.71 (d, JPP ) 22.6 Hz, JPPt ) 3465 Hz), 4.92 (d,
2JPP ) 22.6 Hz, 1JPPt ) 4032 Hz). There were minor resonances at
5.73 and 5.46 ppm (apparent singlets). 3a is insoluble in chlorinated
solvents. Crystals for X-ray analysis, having the composition
3a·2DMF, were obtained from a DMF solution of 3a by slow
diffusion of diethyl ether vapors, at room temperature.
The solvents CDCl3, DMSO-d6, DMF-d7, and CD2Cl2 and the
reagent 9-MeGu (Aldrich) were used as received unless otherwise
stated. cis-(PPh3)2PtCl2,7 cis-(PMePh2)2Pt(ONO2)2 (1b),3 cis-
(PMe3)2Pt(NO3)2,5 and 1-MeCy8 were synthetized as previously
reported. cis-(PPh3)2Pt(ONO2)2 (1a), recently described by us,4
crystallized from dimethylformamide (DMF) in the presence of
diethyl ether vapors, gave crystals having the composition 1a ·DMF,
Synthesis of cis-[(PPh3)2Pt(1-MeCy)2](NO3)2 (4a). To a solution
of cis-[(PPh3)2Pt(NO3)2]·DMF (35.3 mg, 0.04 mmol) in CH2Cl2
(2.5 mL) was added 9.8 mg of 1-MeCy (0.08 mmol), and the
suspension was stirred at room temperature for 17 h. The resulting
solid was recovered by filtration, washed with pentane, and dried
under vacuum. The product was purified by dissolution in DMF
and precipitated with Et2O. The yield of 4a was 28.9 mg (68%).
Elem anal. Calcd for C46H44N8O8P2Pt: C, 50.50; H, 4.05; N, 10.24.
1
as shown by H NMR and elemental analysis. Elem anal. Calcd
for C39H37N3O7P2Pt: C, 51.09; H, 4.07; N, 4.58. Found: C, 51.21;
H, 3.93; N, 4.27. A CHCl3 solution of 1a, allowed to concentrate
at room temperature, released crystals appropriate for X-ray
analysis, having the composition cis-(PPh3)2Pt(ONO2)2 ·CHCl3.
Preparation of the Complexes. Synthesis of cis-[(PPh3)2Pt-
(1-MeCy)(ONO2)](NO3) (2a). To a solution of 1a·DMF (73.4 mg,
0.08 mmol) in DMF (3 mL) was added 10.2 mg of 1-MeCy (0.08
mmol), under stirring at room temperature, obtaining a solution in
ca. 10 min. The addition of Et2O (10 mL) afforded a white
precipitate, which was recovered by filtration and dried under
vacuum. The yield of a solid, having the composition cis-
(PPh3)2Pt(1-MeCy)(NO3)2 · DMF, was 55 mg (66%). Elem anal.
Calcd for C44H44N6O8P2Pt: C, 50.72; H, 4.26; N, 8.07. Found: C,
49.76; H, 4.14; N: 8.08. 1H NMR in DMSO-d6 (δ): 8.72 (br s, 1H,
NH2), 8.08 (br s, 1H, NH2), 7.60–7.35 (c m, 31H, PPh3 and H6),
5.60 and 5.45 [d, 3JHH ) 7.1 Hz H5], 2.91 and 3.08 (s, 3H, NCH3).
DMF resonances: 7.94 (s, 1H, CHO), 2.88 (s, 3H, CH3), 2.72 (s,
3H, CH3). 31P{1H} NMR in DMSO-d6 (δ): two AB multiplets at
1
Found: C, 50.56; H, 4.48; N, 10.49. H NMR in DMSO-d6 (δ):
9.04 (br s, 1H, NH2), 7.93 (br s, 1H, NH2), 7.6–7.3 (c m, 16H,
PPh3 and H6), 5.78 (d, 3JHH ) 7.3 Hz, 1H, H5), 2.91 (s, 3H, NCH3).
31P{1H} NMR in DMSO-d6 (δ): -0.81 (s, 1JPPt ) 3525 Hz). 195Pt
1
NMR in CH2Cl2 (δ): -4198 (s, JPPt ) 3540 Hz).
Synthesis of cis-[(PMePh2)2Pt(1-MeCy)2](NO3)2 (4b). A solu-
tion of cis-(PMePh2)2Pt(ONO2)2 (80.8 mg, 0.11 mmol) and 28 mg
of 1-MeCy (0.22 mmol) in DMF (4 mL) was stirred at room
temperature for 1 h. The addition of Et2O (25 mL) afforded a
powdered solid, which was recovered by filtration, washed with
ether, and dried under vacuum. The product was purified by
dissolution in DMF and precipitated with Et2O. The yield of cis-
[(PMePh2)2Pt(1-MeCy)2](NO3)2 was 69 mg (65%). Elem anal. Calcd
for C36H40N8O8P2Pt: C, 44.59; H, 4.17; N, 11.55. Found: C, 44.34;
H, 4.48; N, 11.80. 1H NMR in DMSO-d6 (δ): 8.88 (br s, 1 H, NH2),
3
7.97 (br s, 1 H, NH2), 7.6–7.3 (c m, 10H, PMePh2), 7.35 (d, JHH
(7) Chock, P. B.; Halpern, J.; Paulick, F. E. Inorg. Synth. 1973, 14, 90.
(8) Kistenmacher, T. J.; Rossi, M.; Caradonna, J. P.; Marzilli, L. G. AdV.
Mol. Relax. Interact. Processes 1979, 15, 119.
) 7.2 Hz, 1H, H6), 5.55 (d, 3JHH ) 7.1 Hz, 1H, H5), 3.06 (s, 3H,
2
NCH3), 1.89 (d, JHP ) 9.2 Hz, 3H, PMePh2). 31P{1H} NMR in
Inorganic Chemistry, Vol. 47, No. 7, 2008 2689

Montagner et al.
Table 1. Crystallographic Data and Details of Structure Refinements for Compounds 1a-3a, 4b, and 5b
1a·CHCl3
2a·3DMF
3a·2DMF
4b·2DMF
5b·4DMF
formula
Mr
C37H31Cl3N2O6P2Pt
963.02
C50H58N8O10P2Pt
1188.07
C48H51N9O9P2Pt
1155.01
C42H54N10O10P2Pt
1115.98
C50H68N16O12P2Pt
1342.23
cryst syst
space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/n
12.043(3)
20.007(3)
15.846(3)
triclinic
P1
monoclinic
P 21/n
15.046(3)
22.337(3)
17.389(3)
monoclinic
P21/n
10.501(2)
25.022(3)
17.850(3)
triclinic
P1
j
j
10.809(3)
13.033(3)
19.209(4)
87.99(2)
74.77(2)
88.41(2)
2608.9(11)
2
11.754(3)
15.387(3)
18.652(4)
68.05(3)
76.46(3)
76.54(3)
3002.2(12)
2
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
95.35(2)
112.17(2)
97.81(2)
3801.4(13)
4
1.683
4.034
1896
5412.1(16)
4
1.418
2.710
2328
4646.7(13)
4
1.595
4.743
2256
Dcalcd (g cm-3
)
1.512
2.814
1198
1.485
2.461
1368
Mo KR (mm-1
F(000)
)
θ range (deg)
reflns collected
unique reflns
Rint
1.98–31.08
53 841
10 800
0.0449
7449
460
0.898
0.0313
0.0713
0.674, –0.613
2.48–30.61
37 778
14 664
0.0337
12 450
660
2.34–25.68
64 637
10 062
0.0883
4519
613
0.819
0.0443
0.0966
0.739, –0.832
5.52–31.75
26 946
5260
0.0430
4993
563
1.157
0.0495
0.1114
1.153,b –0.878
2.00–26.37
34 058
9970
0.0771
4942
734
0.834
0.0539
0.1087
1.798,c –1.074
obsd I > 2σ(I)
refined param
GOF (F2)
0.963
R1 [I > 2σ(I)]a
wR2a
0.0315
0.0798
0.602, –0.842
2
residuals (e Å-3
)
a R1 ) ∑||Fo| - |Fc||/∑|Fo|, wR2 ) [∑w (Fo2 - Fc )2/∑w (Fo )2]1/2
.
b Residual located near a lattice DMF molecule. c Residual located at 1.1 Å from the
2
metal ion.
1
DMSO-d6 (δ): -11.13 (s, JPPt ) 3322 Hz). Crystals for X-ray
analysis, analyzed as 4b·2DMF (by NMR), were obtained by slow
diffusion of Et2O vapors into a DMF solution of 4b.
λ ) 1.000 00 Å, T ) 100(2) K]. Cell refinement, indexing, and
scaling of all of the data sets were carried out using Denzo9 and
Scalepack.9 All of the structures were solved by direct methods
and subsequent difference Fourier analyses10 and refined by the
full-matrix least-squares method based on F2 with all observed
reflections.10 Besides the nitrate anions, the difference Fourier maps
revealed that some solvent residuals successfully interpreted as
chloroform in 1a and dimethylformamide in 2a, 3a, 4b, and 5b,
accounting for three, two, two, and four molecules for metal units,
respectively; some of these were found to be disordered over two
positions. All hydrogen atoms were located at geometrical positions,
except those of disordered solvent molecules; those of cytosine
exocyclic amino groups in 2a and 5a were derived from the
difference Fourier map. The crystal packing of both compounds
evidence about 30% of the unit cell accessible to the solvent, and
three lattice DMF molecules per complex unit in 2a (two in 3a)
were detected on the difference Fourier map and successfully
refined. The volume left out in the crystal by the complexes and
nitrate anions accounts for about 22 and 38% of the unit cell in 4b
and 5b, and two and four solvent DMF molecules (per complex
unit), respectively, occupy this area. All of the calculations were
performed using the WinGX system, version 1.70.05.11
Synthesis of cis-[(PPh3)2Pt(9-MeGu)2](NO3)2 (5a). To a solu-
tion of cis-[(PPh3)2Pt(ONO2)2]·DMF (50.9 mg, 0.06 mmol) in 3
mL of CH2Cl2was added 18.6 mg of 9-MeGu (0.12 mmol), and
the suspension was stirred at room temperature for 36 h. The
resulting white solid was recovered by filtration, washed with
CH2Cl2, and dried under vacuum. The sample was purified from
DMF/Et2O. The yield of 5a was 55 mg (84%). Elem anal. Calcd
for C48H44N10O8P2Pt: C, 49.10; H, 3.78; N, 14.32. Found: C, 48.73;
H, 3.71; N, 14.29. 1H NMR in DMSO-d6 (δ): 11.3 (br s, 1H, N1H),
7.74 (s, 1H, H8), 7.6–7.3 (c m, 15H, PPh3), 6.81 (br s, 2H, NH2),
3.27 (s, 3H, NCH3). 31P{1H} NMR in DMSO-d6 (δ): 1.54 (s, 1JPPt
) 3507 Hz). 15N NMR (inverse detected) in DMSO-d6 (δ): -203
(N7), -216 (N9), -226 (N1H), –297 (N2H2).
Synthesis of cis-[(PMePh2)2Pt(9-MeGu)2](NO3)2 (5b). With the
procedure reported for 5a, 5b was prepared with a yield of 67%
from 1b (102 mg, 0.14 mmol) and 9-MeGu (46 mg, 0.28 mmol) in
CH2Cl2 (10 mL). Elem anal. Calcd for C38H40N12O8P2Pt: C, 43.48;
H, 3.85; N, 16.00. Found: C, 43.52; H, 4.46; N, 15.97. 1H NMR in
DMSO-d6 (δ): 11.16 (br s, 1H, N1H), 7.87 (s, 1H, H8), 7.7–7.1 (c
m, 10H, PMePh2), 6.84 (br s, 2H, NH2), 3.28 (s, 3H, NCH3), 1.93
(d, JHP ) 8.7 Hz, 3H, PMePh2). 31P{1H} NMR in DMSO-d6 (δ):
2
Results and Discussion
-9.21 (s, 1JPPt ) 3341 Hz). 15N NMR (inverse detected) in DMSO-
d6 (δ): -198 (N7), -216 (N9), -227 (N1H), –297 (N2H2). Crystals
for X-ray analysis were obtained by slow diffusion of Et2O vapors
into a DMF solution of 5b.
1. Structural Characterization of 1a. The removal of
the chloride ligands in cis-[(PPh3)2PtCl2], using AgNO3, leads
to the dinitrato complex 1a and its crystallization in DMF,
in the presence of diethyl ether vapors, or in CHCl3 affords
crystals having the composition 1a · DMF or 1a · CHCl3,
X-ray Structure Determinations. Crystal data and details of
structural refinements are reported in Table 1.
Diffraction data for compounds 1a–3a and 5b were collected
on a Nonius DIP-1030H system equipped with Mo KR radiation
(λ ) 0.710 73 Å) at ambient temperature, except the data collection
of 2a, which was performed at T ) 200(2) K. Because of the small
crystal dimensions (approximately 0.15 × 0.14 × 0.10 mm), the
intensity data of 4b were measured at the RX diffraction beamline
of Elettra Synchrotron, Trieste, Italy [CCD MarResearch detector,
(9) Otwinowski, Z.; Minor, W. Processing of X-ray Diffraction Data
Collected in Oscillation Mode. In Methods in Enzymology; Carter,
C. W., Jr., Sweet, R. M., Eds.; Academic Press: New York, 1997;
Vol. 276, p 307.
(10) Sheldrick, G. M. SHELX97, Programs for Crystal Structure Analysis,
release 97-2; University of Göttingen: Göttingen, Germany, 1998.
(11) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
2690 Inorganic Chemistry, Vol. 47, No. 7, 2008

Model Nucleobases 1-MeCy and 9-MeGu
Figure 1. ORTEP drawing of complex 1a (only ipso-carbon atoms are
labeled for clarity). Selected bond lengths (Å) and angles (deg): Pt-O1
2.093(2), Pt-O4 2.105(3), Pt-P1 2.239(1), Pt-P2 2.248(1); O1-Pt-O4
84.70(10), O1-Pt-P1 174.61(7), O4-Pt-P1 90.89(8), O1-Pt-P2 87.44(8),
O4-Pt-P2 172.14(7), P1-Pt-P2 96.97(4), N1-O1-Pt1 113.8(6),
N2-O4-Pt1 115.6(7).
respectively. The X-ray structure of 1a · CHCl3 reveals one
independent complex cocrystallized with a molecule of
chloroform.
Figure 2. 31P{1H} NMR spectra (0.1 M) of 1a in (a) CDCl3, (b) DMSO-
d6, (c) DMF-d7 (25 °C), and (d) DMF-d7 (-60 °C) and (e) an expansion of
part d.
Figure 1 reports the ORTEP drawing of the metal complex.
The platinum displays a distorted square coordination, bound
to the P donors and O nitrate atoms, in a cis configuration.
The nitrato groups are directed on opposite sides with respect
to the coordination plane, and their mean planes form
dihedral angles of 69.1(1) and 84.9(1)° with it. No significant
displacement of the metal from the P2O2 plane is detected,
and the donor atoms are coplanar within (0.03 Å. Phenyl
rings C13 and C19 are involved in π-π stacking interaction
[centroid-to-centroid distance of 3.644(2) Å], but no inter-
molecular phenyl interaction is detected in the crystal. The
coordination bond lengths and angles, as well as the
orientation of nitrate anions, are very similar to those detected
nances (at δ 6.68 and 4.28 ppm, with 1JPPt values in the range
4050–4260 Hz), which does not change after some hours at
room temperature. When the solution is cooled at -60 °C
(Figure 2d), the spectrum shows two AB multiplets, one
2
sharp (at δ 7.15 and 4.7 ppm, JPP ) 22.9 Hz, relative
intensity 39%) and the second (at δ 5.7 and 3.5 ppm, 2JPP
)
23.3 Hz) less intense (27%) and broader. Both of them are
consistent with the presence of monosubstituted species of
the type cis-[(PPh3)2Pt(ONO2)(DMF)]+, in which the NO3
–
group behaves as a monodentate ligand, and/or as a bridging
ligand, in polynuclear species. The remaining resonances
(Figure 2e) at δ 4.93 (slightly broad singlet, relative intensity
26%) and δ 3.99 ppm (singlet, relative intensity 8%) are
tentatively attributed to the species cis-[(PPh3)2Pt(DMF)2]2+
and unsolvated species cis-(PPh3)2Pt(ONO2)2. The absence
of resolved 195Pt satellites (at -60 °C) precludes a deeper
insight into the nature of species in equilibrium.
13
in the PMePh2,3 PMe3,12 and PEt3 derivatives. This last
feature, however, contrasts with that found in the analogue
cis-(NH3)2Pt(ONO2)2, in which the nitrate groups lie on the
same side of the ligand plane.14
In a solution of coordinating solvents, 1a undergoes an
extensive solvolysis of the Pt-O bonds, with the formation
of complexes in which one or both of the NO3- ligands are
replaced by solvent molecules. The process is clearly
evidenced by 31P NMR spectroscopy, as is illustrated in
Figure 2.
2. X-ray Structures of cis-[(PPh3)2Pt(nucleobase)-
(ONO2)]+ (nucleobase ) 1-MeCy, 2a; 9-MeGu, 3a) and
cis-[(PMePh2)2Pt(nucleobase)2]2+ (nucleobase ) 1-MeCy,
4b; 9-MeGu, 5b). The addition of 1 equiv of 1-MeCy to a
DMF solution of 1a, followed by condensation of diethyl
ether vapors, afforded crystals having the composition
In CDCl3 at 27 °C (Figure 2a), the 31P{1H} NMR spectrum
of 1a exhibits a sharp singlet at δ 3.35 ppm flanked by 195Pt
NMR satellites (1JPPt ) 4018 Hz), while in DMSO-d6 (Figure
2b), the signal is broad and shifted at lower field (δ 4.6 ppm,
1
2a · 3DMF, as established by H NMR and X-ray structure
determination. Similarly, with 9-MeGu the analogue complex
3a · 2DMF has been isolated in good yield. In the presence
of 2 equiv of nucleobase, the bisadducts 4a and 5a are formed
in quantitative yield (by NMR).
The monoadduct complexes 2a and 3a, reported in Figures
3 and 4, present close similarities not only in the conforma-
tion but also in the crystal packing and will be discussed
together.
1
with JPPt ) ca. 4070 Hz), indicative of chemical exchange
between the NO3- ligands and the solvent molecules. Figure
2c shows the spectrum of a freshly prepared solution of 1a
in DMF-d7 (ca. 0.1 M), characterized by two broad reso-
(12) Suzuki, Y.; Miyamoto, T. K.; Ichida, H. Acta Crystallogr., Sect. C
1993, 49, 1318.
(13) Kuehl, C. J.; Tabellion, F. M.; Arif, A. M.; Stang, P. J. Organometallics
2001, 20, 1956.
The Pt ion exhibits a square-planar coordination sphere
achieved through the phosphorus atoms, the nitrogen donor
from the nucleobase, and the nitrate oxygen atom. The
(14) Lippert, B.; Lock, C. J. L.; Rosemberg, B.; Zvagulis, M. Inorg. Chem.
1977, 16, 1525.
Inorganic Chemistry, Vol. 47, No. 7, 2008 2691

Montagner et al.
Table 2. Coordination Bond Lengths (Å) and Angles (deg) and
Geometrical Parameters for 2a and 3a
2a
3a
Pt-N3/7a
Pt-O1
Pt-P1
2.100(2)
2.117(2)
2.2508(9)
2.2846(9)
2.075(5)
2.140(5)
2.247(2)
2.2780(18)
Pt-P2
N3/7-Pt-O1
N3/7-Pt-P1
N3/7-Pt-P2
O1-Pt-P1
86.37(10)
90.81(7)
169.08(7)
173.61(7)
83.53(7)
86.9(2)
90.38(17)
171.40(17)
173.88(15)
84.51(13)
98.08(7)
O1-Pt-P2
P1-Pt-P2
98.82(3)
N2-O1-Pt
116.37(18)
112.92(19)
124.9(2)
C2-N3-Pt
C4-N3-Pt
N4-O1-Pt
117.9(5)
130.0(5)
123.0(5)
76.91(9)
61.7(2)
C5-N7-Pt
Figure 3. ORTEP drawing (35% probability ellipsoid) of the complex
cation of 2a. Only ipso-carbon atoms are labeled for the sake of clarity.
C8-N7-Pt
base/coord plane NOP2 (deg)
NO3/coord plane NOP2 (deg)
76.38(8)
68.3(1)
a N3 refers to 2a and N7 to 3a.
On the other hand, the nitrate group is more bent, and the
mean plane figures out a dihedral angle of 65° (mean value
for the two structures), with nitrate oxygen O3 pointing
toward the cytosine exocyclic amino group N4 in 2a (at a
hydrogen-bond distance from the latter, O3-N4 ) 2.99 Å)
and toward the guanine carboxyl oxygen in 3a (O3-O6 )
3.09 Å). In both complexes, the phosphine phenyl group C31
is oriented in such a way as to favor an intramolecular π-π
interaction with the model nucleobase (centroid-to-centroid
distance of ca. 3.5 Å). Moreover, an intramolecular stacking
interaction is detected in 3a between phenyl rings C11 with
C41 [distance between centroids ) 3.587(7) Å]. The cor-
responding distance in 2a is slightly longer [4.007(2) Å],
indicating a different, although small, difference in the
conformations of the phosphine ligands.
Whereas we were unable to grow crystals of 4a and 5a
for X-ray analyses, this was possible for the PMePh2
analogues, 4b and 5b, easily prepared from 1b in the
presence of 2 equiv of 1-MeCy and 9-MeGu, respectively.
The X-ray structural analysis of 4b and 5b confirms the
formation of the bisadducts species cis-[(PMePh2)2Pt(1-
MeCy)2]2+ and cis-[(PMePh2)2Pt(9-MeGu)2]2+, and an ORTEP
view of these complexes, showing similar geometrical
features, are illustrated in Figures 5 and 6, while the structural
data are collected in Table 3.
Figure 4. ORTEP drawing (35% probability ellipsoid) of the complex
cation of 3a. Only ipso-carbon atoms are labeled for the sake of clarity.
nucleobase is bound to the metal through N3 of 1-MeCy
and N7 of 9-MeGu, which represent the preferential binding
sites for pyrimidine and purine bases, respectively. The bond
distances and angles are reported in Table 2 and do not show
any anomaly. The Pt-N3 bond distance in 2a [2.101(2) Å]
is slightly longer than the corresponding distance Pt-N7 in
3a [2.075(5) Å] and can be ascribed to the endocyclic
C-Nd-C angle (Nd being the donor site), narrower in the
five-membered ring than in the six-membered ring, which
allows a closer approach of the ligand to the metal center.
Both of the bases have an unsymmetrical coordination; in
fact, the C2-N3-Pt angle of the cytosine (on the side of
the exocyclic amino group) and the C5-N7-Pt one of the
guanine (carbonyl group side) are larger by ca. 12° and ca.
7° than the C4-N3-Pt and C8-N7-Pt angles, respectively.
As far as Pt-P bond lengths are concerned, it is worth noting
the shorter value measured for the phosphine trans to the
nitrate anion, and the narrower N-Pt-O1 angle [86.39(10)
and 86.9(2)° in 2a and 3a, respectively] in comparison to
the P1-Pt-P2 one [98.83(3) and 98.08(7)°], likely induced
by steric requirements.
The bases, bound through N3 and N7 in 4b and 5b,
respectively, assume a head-to-tail arrangement that is the
most frequent orientation of two bases in a square-planar
geometry.15 The Pt-N and Pt-P coordination bond lengths
follow a trend as observed in the monoadduct complexes.
The C4-N3-Pt and C8-N7-Pt angles in 4b and 5b are
significantly larger than the C2-N3-Pt and C5-N7-Pt
ones, respectively, confirming an unsymmetrical coordination
of the bases (Table 3). Coordination Pt-N distances compare
well with those found in the correspondent PMe3 deriva-
The 1-MeCy and 9-MeGu bases are oriented almost
normal to the coordination mean plane, forming a close
comparable angle with the latter of ca. 76° in both complexes.
(15) Zangrando, E.; Pichierri, F.; Randaccio, L.; Lippert, B. Coord. Chem.
ReV. 1996, 156, 275.
2692 Inorganic Chemistry, Vol. 47, No. 7, 2008

Model Nucleobases 1-MeCy and 9-MeGu
Table 3. Coordination Bond Lengths (Å) and Angles (deg) and
Geometrical Parameters for 4b and 5b
4b
5b
Pt-N3a/7aa
Pt-N3b/7b
Pt-P1
2.106(7)
2.095(8)
2.265(2)
2.275(2)
2.087(7)
2.082(7)
2.268(3)
2.256(3)
Pt-P2
N3a-Pt-N3b
86.3(3)
N7a-Pt-N7b
84.7(3)
89.2(2)
172.9(2)
174.7(2)
90.2(2)
N3a/7a-Pt-P1
N3b/7b-Pt-P1
N3a/7a-Pt-P2
N3b/7b-Pt-P2
P1-Pt-P2
89.5(2)
174.3(2)
176.1(2)
91.5(2)
92.88(9)
113.4(6)
125.2(6)
114.4(7)
123.1(6)
95.91(9)
C2a-N3a-Pt
C4a-N3a-Pt
C2b-N3b-Pt
C4b-N3b-Pt
C5a-N7a-Pt
121.5(6)
131.1(6)
123.7(6)
130.6(6)
79.5(1)
C8a-N7a-Pt
Figure 5. ORTEP drawing (35% probability ellipsoid) of the complex
cation of 4b. Phosphine carbon atoms as spheres of the fixed radius are
labeled for the sake of clarity.
C5b-N7b-Pt
C8b-N7b-Pt
base a/coord plane N2P2 (deg)
base b/coord plane N2P2 (deg)
base a/base b (deg)
80.6(2)
79.8(2)
77.7(2)
86.25(1)
76.99(1)
a N3a,b refers to 4b and N7a,b to 5b.
each base ring and a phenyl group [base a-phenyl C3,
centroid-to-centroid distance 3.480(6) Å; base b-phenyl C21,
3.448(7) Å]. In 5b, the intramolecular π-π interaction
between the imidazole ring of base b and phenyl C21 is even
present [centroid-to-centroid distance 3.488(7) Å]. On the
other hand, the different conformation assumed by the
phosphine P1, leading the methyl group C1 to lay ap-
proximately in the coordination plane (Figure 6), induces a
weaker π-π stacking linking the phenyl groups C3 and C15
[distance 3.784(7) Å]. In the solid-state structure, a hydrogen-
bonding interaction of type C-H · · · O between a phenyl
group and the guanine oxygen O6 in 3a (O6b in 5b) is also
present. The C · · · O distances are 3.361(11) and 3.246(13)
Å, respectively. All of these interactions are likely to stabilize
the crystal packing and the solid-state conformations of these
complexes. It is worth noting the crystal packing of 5b, where
each guanine is connected through hydrogen bonds via amino
group N2 and nitrogen N3 of the centrosymmetric moiety,
forming a mismatched homo base pairing [NH2 · · ·N ) 3.03
Å (base a) and 2.98 Å (base b)].18 The atoms of the two
nucleobases are coplanar within (0.03 and (0.02 Å in 4b
and 5b), respectively. Moreover, N1 and N2 nitrogen atoms
act as hydrogen-bond donors toward a nitrate. Figure 7 shows
the hydrogen-bonding pattern, which gives rise to a zigzag
complex polymer with pendant nitrate anions.
Figure 6. ORTEP drawing (35% probability ellipsoid) of the complex
cation of 5b.
tives,16,17 although the structural characterization of the cis-
[(PMe3)2Pt(9-MeGu)2]2+ species17 is less accurate. The Pt-P
bond distances appear only slightly longer in the present
complexes containing the bulkier PMePh2 (2.270 and 2.262
Å mean values in 4b and 5b, respectively) when compared
with the average values of 2.255 and 2.250 Å of the PMe3
complexes.
The complex 4b presents a pseudo-2-fold symmetry in
the solid state for the conformation assumed by the PMePh2
ligands (methyl groups on different sides of the coordination
plane) and a head-to-tail arrangement of the bases. This
allows the formation of intermolecular hydrogen bonds
between each exocyclic NH2 group and the carbonyl of the
adjacent base (N4 · · · O2 distances of 2.87 and 2.94 Å). The
other NH2 hydrogen atom of each cytosine forms a hydrogen
bond with a nitrate anion (N4-H · · · O of 2.85 Å, mean
distance). Intramolecular π-π interactions occur between
3. NMR Characterization of cis-[L2Pt(nucleobase)-
(ONO2)]+ and cis-[L2Pt(nucleobase)2]2+. In solution, the
isolated bisadducts cis-[L2Pt(nucleobase)2]2+ do not manifest
particular features. The 1H and 31P NMR spectra of the isolated
complexes, obtained in different solvents, show a single set of
resonances indicative of the chemical equivalence of the two
(18) (a) Roitzsch, M.; Lippert, B. Chem. Commun. 2005, 5991. (b)
Freisinger, E.; Rother, I. B.; Lu¨th, M. S.; Lippert, B. Proc. Natl. Acad.
Sci. U.S.A. 2003, 100, 3748. (c) Erxleben, A.; Metzger, S.; Britten,
J. F.; Lock, C. J. L.; Albinati, A.; Lippert, B. Inorg. Chim. Acta 2002,
339, 461.
(16) Trovò, G.; Valle, G.; Longato, B. J. Chem. Soc., Dalton Trans. 1993,
669.
(17) Longato, B.; Bandoli, G.; Trovò, G.; Marasciulo, E.; Valle, G. Inorg.
Chem. 1995, 34, 1745.
Inorganic Chemistry, Vol. 47, No. 7, 2008 2693

Montagner et al.
Figure 7. Crystal packing of 5b showing the one-dimensional polymers
built by hydrogen-bond interactions (dotted lines).
Figure 9. 31P{1H} NMR spectra (central part) of 2a in (a) DMSO-d6, (b)
DMF-d7, and (c) CD2Cl2 and (d) an expansion of part c.
1
field in Figure 8, having JPPt ) 3934 Hz, can be assigned
to the phosphine trans to the nitrate ligand or, more likely,
to a solvent molecule. The upfield doublet, with 1JPPt ) 3453
Hz, consequently is attributable to the phosphine trans to
the nucleobase. Similar results were obtained with 9-MeGu
(in DMSO-d6: δ -11.46 (1JPPt ) 3870 Hz), -2.72 (1JPPt
3430 Hz, with JPP ) 23.4 Hz).
)
2
Figure 8. 31P{1H} NMR spectra in DMSO-d6 (central part) of a mixture
(0.1 M) of 1b and 4b (1:1).
For both of the nucleobases investigated, the monoadducts
cis-[L2Pt(nucleobase)]2+ (L ) PMePh2) are in equilibrium
with relatively high concentrations of the bisadducts cis-
[L2Pt(nucleobase)2]2+ (and dinitrate). Thus, from the relative
intensities of the signals in Figure 8, cis-[(PMePh2)2Pt(1-
MeCy)]2+ accounts for 63% of all of the phosphine-
containing species. However, when L is PPh3, the relative
stability of the monoadducts 2a and 3a appears largely
increased. In Figure 9a, the 31P NMR spectrum of a solution
of ca. 0.1 M of 2a in the DMSO-d6 is shown. The bisadduct
4a, in equilibrium with the monoadduct, is observed as a
singlet at δ -0.81 ppm and its relative intensity is about
4%. The resonance due to cis-(PPh3)2Pt(NO3)2 is undetect-
able, being overlapped with those of the monoadduct,
characterized by two AB multiplets.
nucleobases. As previously noticed, the platination of the
cytosine at the N3 site shifts at lower field the resonance of the
exocyclic NH2 protons, observed as broad singlets with the same
relative intensities, at δ 9.04 and 7.93 ppm for L ) PPh3, in
DMSO-d6. Similarly, the platination of 9-MeGu at the N7 site,
as confirmed through 1H-15N heterocorrelated spectra (see the
Supporting Information), causes deshielding of the H1 proton,
up to 0.8 ppm for 5a.
The coordination of two nucleobases to cis-L2Pt(ONO2)2
(in DMSO-d6) determines an upfield shift of the 31P NMR
1
resonances and a decrease of the JPPt values. For 1-MeCy,
the chemical shift differences (∆δ) are 4.3 and 5.4 ppm for
PMePh2 and PPh3, respectively, while for 9-MeGu ∆δ are
2.4 and 3.1 ppm. As an example of the spectroscopic
changes observed upon coordination of 1-MeCy to cis-
(PMePh2)2Pt(ONO2)2, the 31P NMR spectrum obtained im-
mediately after dissolution of equimolar amounts of
dinitrate and 1-MeCy, in DMSO-d6 [or, alternatively, cis-
(PMePh2)2Pt(ONO2)2 and cis-[(PMePh2)2Pt(1-MeCy)2](NO3)2
in a 1:1 molar ratio] is shown in Figure 8.
The spectrum is characterized by two AX doublets (2JPP
) 24.3 Hz) with the same relative intensity, at δ -12.36
ppm (1JPPt ) 3934 Hz) and δ -3.54 ppm (1JPPt ) 3453 Hz),
attributed to the monoadduct cis-[(PMePh2)2Pt(1-MeCy)]2+,
a broad singlet at δ -6.90 ppm of unreacted 1b, and a sharp
singlet at δ -11.13 ppm (1JPPt ) 3322 Hz) due to the
bisadduct 4b. This last signal is the only detectable one when
an additional equiv of nucleobase is added, while the initial
AX multiplet disappears immediately. The doublet at lower
The main resonances, at δ 5.36 (1JPPt ) 4080 Hz) and
5.12 (1JPPt ) 3536 Hz), with 2JPP ) 22.3 Hz, can be assigned
to the solvent complex cis-[(PPh3)2Pt(1-MeCy)(DMSO)]2+,
while the weaker multiplets δ 4.60 and 4.02 (2JPP ) 21.4
Hz) are attributable to the species cis-[(PPh3)2Pt(1-MeCy-
)(ONO2)]+ in which the nitrate ion is in the coordination
sphere of the metal, as found in the solid-state structure. The
intensities of these latter signals, in fact, increase when the
-
NO3 ions are added (such as the Ph4AsNO3 salt). The
unusually small chemical shift differences of the two
phosphines (∆δ ) 0.24 and 0.58 ppm), compared with that
found for the PMePh2 analogue (∆δ ) 8.82 ppm; Figure 8),
can be accounted for by the presence of strong intraligand
π-π interactions, as are found in the solid state, which
prevent the free rotation around the Pt-P and/or Pt-N3
bonds. The imposed rigidity causes a strong chemical shift
2694 Inorganic Chemistry, Vol. 47, No. 7, 2008

Model Nucleobases 1-MeCy and 9-MeGu
anisotropy and a consequent shift at lower field of one of
the phosphine resonances.19
for the 9-substituted guanines unprecedented for platinum(II).
The spectroscopic parameters of this species, which is the
main component of the equilibrium mixture (ca. 60%), are
similar to those of the species found in DMSO. The addition
of 1 equiv of 9-MeGu to the DMF or DMSO solution of 3a
affords the immediate formation of the bisadduct 5a in
quantitative yield (by NMR).
Similar considerations apply to the 31P NMR spectrum of
2a in DMF. As shown in Figure 9b, two AB multiplets have
almost the same relative intensity and very similar chemical
shifts (see the Experimental Section). These data suggest the
presence of the solvent complex cis-[(PPh3)2Pt(1-MeCy)-
(DMF)]2+ in which the DMF and the nucleobase are arranged
in different conformations on the coordination plane of the
metal. The resonances at lower field, having higher values
of 1JPPt (4015 Hz), are assigned to the PPh3 trans to the DMF
ligand.
Conclusions
The coordination of two molecules of 1-MeCy and
9-MeGu to the metal center of cis-L2Pt(ONO2)2 (L )
PPh2Me, PPh3) appears kinetically facile, as expected for
-
A different pattern of the 31P NMR spectrum is observed
in chlorinated solvents. Figure 9c refers to a solution of 1a
and 1-MeCy (ca. a 1:1.1 molar ratio) in CD2Cl2. In addition
to the singlets at δ -0.61 and +3.49 ppm, due to species 4a
and 1a, respectively, the spectrum is characterized by two
AX doublets (2JPP ) 21.3 Hz) at δ 5.11 (1JPPt ) 3939 Hz)
and 3.58 ppm (1JPPt ) 3455 Hz) having slightly different
line widths. The doublet at higher field, on the basis of its
the presence of good leaving groups, such as the NO3
ligands. The stereochemistry of the bisadducts cis-[L2Pt-
(nucleobase)2]2+ (L ) PMePh2) is that of a head-to-tail
arrangement of the two cytosines and guanines, N3- and N7-
coordinated, respectively. The X-ray structures confirm that
the two nucleobases do not determine steric crowding in the
metal coordination sphere (the sums of bond angles P-Pt-P
and N-Pt-N are 179.2° and 180.6° in 4b and 5b, respec-
tively). Despite the apparent thermodynamic stability of the
corresponding PPh3 analogues, we were unable to crystallize
the complexes cis-[(PPh3)2Pt(nucleobase)2]2+ (4a and 5a), a
failure likely related to the higher steric requirements of the
PPh3 (cone angles of 145° and 136° for L ) PPh3 and
PMePh2, respectively).20 It is worth noting that in the strictly
related bisadduct cis-[(PPh3)2Pt(1-MeCy-N3)(1-MeCy(-H)-
N4)]NO3, containing a neutral and a NH2-deprotonated
cytosine, the P-Pt-P and N3-Pt-N4 angles are 98.22(4)°
and 87.51(13)°, respectively.6 The platination of both cy-
tosines at the N3 atom (or the guanine at N7) was expected
to further increase such deviations from the ideal value of
90°.
1
lower value of JPPt, is assigned to the phosphine trans to
the N3-coordinated cytosine. This signal is overlapped with
the singlet due to unreacted 1a. The resonance centered at δ
1
5.11 ppm exhibits a JPPt value (3939 Hz) typical for a
phosphine trans to an oxygen donor ligand. Moreover, this
doublet is characterized by very weak satellites (separation
of ca. 11 Hz; Figure 9d) likely due to long-range 31P-195Pt
coupling. Such satellites, although less resolved, are detect-
able also for the doublet at higher field and suggest the
presence of a polynuclear species in which the neutral
nucleobase acts as a bidentate bridging ligand likely through
the N3 and O2 atoms. Attempts to grow crystals of 2a from
chlorinated solvents in order to prove this unusual binding
mode were unsuccessful.
Although in the solid state complexes 2a and 3a exhibit
strong analogies, in solution they disclose important differ-
ences. The 31P NMR spectrum of 3a in DMSO-d6 shows a
single set of resonances (AB multiplet at δ 7.94 and 5.12
ppm) indicative of a complete solvolysis of the nitrate Pt-O
bond. Moreover, the multiplicity of signals observed in a
DMF solution (see the Experimental Section) points to the
presence of other species in equilibrium, in addition to that
found in the solid state and the solvent complex, cis-
[(PPh3)2Pt(9-MeGu-N7)(DMF)]2+. The relatively high dif-
ference between the two chemical shift values (∆δ ) 2.88
ppm) could be assigned to the chelated complex cis-
[(PPh3)2Pt(9-MeGu-N7O6)]2+ in which the nucleobase binds
the metal through the N7 and O6 atoms, a coordination mode
Finally, the relatively high stability of the monoadducts
cis-[L2Pt(nucleobase)]2+ when L ) PPh3 ligands can be
accounted for by the presence in both complexes of a strong
intramolecular π-π interaction occurring between a phenyl
group and the pyrimydinic (or purinic) ring, which leads the
P-Pt-N3/7 angle to assume a value near to ideality.
Acknowledgment. This work was financially supported
by the Ministero dell’Universita´ e della Ricerca Scientifica
e Tecnologica, PRIN 2004.
Supporting Information Available: Crystallographic data in
CIF format for the structures reported in this paper and 15N-1H
HMBC spectra of 5a in DMSO-d6. This material is available free
of charge via the Internet at http://pubs.acs.org.
IC7020319
(19) Garrou, P. E. Chem. ReV. 1977, 77, 313.
(20) Tolman, C. A. Chem. ReV. 1981, 81, 229.
Inorganic Chemistry, Vol. 47, No. 7, 2008 2695