Complexes of Platinum(II) Containing Neutral and Deprotonated 9-Methyladenine

Article abstract of DOI:10.1021/ic034406z

The dinuclear hydroxo complex cis-[L₂Pt(μ-OH)] ₂(NO₃)₂ (L = PMePh₂, 1), in CH ₂Cl₂, CH₃CN, or DMF solution, deprotonates the NH₂ group of 9-methyladenine (9

Full text of DOI:10.1021/ic034406z

Inorg. Chem. 2003, 42, 7861−7871
Complexes of Platinum(II) Containing Neutral and Deprotonated
9-Methyladenine. Synthesis, X-ray Structures, and NMR Studies on the
Cyclic Trimer cis-[L2Pt{9-MeAd(−H)}]3(NO ) and the Dinuclear
3 3
cis-[L2Pt(ONO ){9-MeAd(−H)}PtL2](NO ) (L ) PMePh2)
2
3 2
Bruno Longato,*,†,‡ Lucia Pasquato,‡ Adele Mucci,§ Luisa Schenetti,§ and Ennio Zangrando|
Istituto di Scienze e Tecnologie MolecolarisCNR, c/o Dipartimento di Chimica Inorganica,
Metallorganica ed Analitica, UniVersita` di PadoVa, Via Marzolo 1, 35131 PadoVa, Italy,
Dipartimento di Chimica Inorganica, Metallorganica ed Analitica, UniVersita` di PadoVa,
Via Marzolo 1, 35131 PadoVa, Italy, Dipartimento di Chimica, UniVersita` di Modena e Reggio
Emilia, Via Campi 183, 41100 Modena, Italy, and Dipartimento di Scienze Chimiche, UniVersita`
di Trieste, Via Giorgieri 1, 34127 Trieste, Italy
Received April 17, 2003
The dinuclear hydroxo complex cis-[L2Pt(µ-OH)]2(NO ) (L ) PMePh2, 1), in CH Cl2, CH CN, or DMF solution,
3 2
2
3
deprotonates the NH group of 9-methyladenine (9-MeAd) to give the complex cis-[L2Pt{9-MeAd(−H)}]3(NO ) , 2,
2
3 3
which was isolated in good yield. The X-ray structure shows that the nucleobase binds symmetrically the metal
centers through the N(1),N(6) atoms forming a cyclic trimer with Pt‚‚‚Pt distances in the range 5.202(1)−5.382(1)
Å. Dissolution of 2 in DMSO or DMF determines the partial (or total) dissociation of the cyclic structure to form
several fragments. A multinuclear NMR analysis of the resulting mixture supports the presence of the mononuclear
species cis-[L2Pt{9-MeAd(−H)}]+, 3, in which the deprotonated nucleobase chelates the metal center with the
N(6),N(7) atoms. Addition of a stoichiometric amount of the nitrato complex cis-[L2Pt(ONO ) ] (L ) PMePh2, 4) to
2 2
a DMSO or DMF solution of 2 affords quantitatively the diplatinated compound cis-[L2Pt(ONO ){9-MeAd(−H)}-
2
PtL2](NO ) , 5. The single-crystal X-ray analysis shows that the adenine behaves as a tridentate ligand bridging
3 2
two cis-L2Pt units at the N(1) and N(6),N(7) sites, respectively [Pt(1)−N(1) ) 2.109(5) Å, Pt(2)−N(6) ) 2.095(7)
Å, Pt(2)−N(7) ) 2.126(7) Å]. The N(1)-bonded metal center completes the coordination sphere through an oxygen
atom of a nitrate group, and its coordination plane is arranged orthogonally with respect the second one. The Pt−O
distance [2.109(5) Å] is similar to those found in the nitrato complex 4 [2.110 Å, average]. The related complex
cis-[{L2Pt(ONO )}2(9-MeAd)](NO ) , 6, containing the neutral adenine platinated at the N(1),N(7) atoms, was isolated
2
3 2
and its stability in solution investigated by NMR spectroscopy. In DMSO, 6 undergoes decomposition forming a
mixture of the species 4, 5, and the adenine mono- and bis-adducts cis-[L2Pt(9-MeAd)(DMSO)]2+, 7, and cis-[L2-
Pt(9-MeAd)2]2+, 8, respectively. This last complex, quantitatively formed upon addition of 9-MeAd (Pt/adenine )
1:2) to the mixture, was also isolated and characterized.
Introduction
nents toward the platinum has been carried out on complexes
of PtII (and PtIV) having NH3 or amines as ancillary ligands.1,2
In the frame of our interest in the reactivity of the cisplatin
analogues cis-[L2PtCl2] (L ) phosphorus donor atom of a
tertiary phosphine) with model nucleobases, we described
the binding modes of 9-substituted methyladenine (9-MeAd)
in the neutral and cationic complexes cis-[(PMe3)2Pt(ONO2)2]
Since the discovery of the biological activity of cisplatin,
a large part of the coordination chemistry of DNA compo-
* To whom correspondence should be addressed. E-mail: longato@chin.
unipd.it.
† Istituto di Scienze e Tecnologie MolecolarisCNR, c/o Dipartimento
di Chimica Inorganica, Metallorganica ed Analitica, Universita` di Padova.
‡ Dipartimento di Chimica Inorganica, Metallorganica ed Analitica,
Universita` di Padova.
§ Universita` di Modena e Reggio Emilia.
(1) Lippert, B. Prog. Inorg. Chem. 1989, 37, 1-97 and references therein.
(2) Lippert, B. Coord. Chem. ReV. 2000, 200-202, 487-516.
| Universita` di Trieste.
10.1021/ic034406z CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/31/2003
Inorganic Chemistry, Vol. 42, No. 24, 2003 7861

Longato et al.
Scheme 1
Scheme 2
and cis-[(PMe3)2Pt(µ-OH)]22+, respectively.3 In the first case
the substitution of the nitrate ligands afforded the bis-adduct
cis-[(PMe3)2Pt(9-MeAd)2]2+, in which the neutral adenines
are selectively platinated at the N(1) site, while the cationic
hydroxo complex deprotonated the exocyclic amino group
to form cis-[(PMe3)2Pt{9-MeAd(-H)}]22+. The X-ray char-
acterization of this dinuclear compound, carried out on the
9-ethyl-substituted adenine, disclosed a cyclic structure in
which the nucleobase acts as bridging ligand through the
N(1),N(6) atoms.
More recently, a detailed NMR study on the reaction of
cis-[(PMe2Ph)2Pt(ONO2)2] with variable amounts of nucleo-
base in DMSO-d6 allowed us to characterize a diplatinated
complex (species B in Scheme 1) resulting on the simulta-
neous binding of two cis-{(PMe2Ph)2Pt} units at the N(1)
and N(7) atoms of the adenine with concomitant reVersible
deprotonation of the exocyclic NH2 group. By increasing the
Pt/adenine ratio to the value 1:2, in fact, the complete
disappearance of the chelated complex B was observed with
the almost quantitative formation of the bis-adduct cis-
[(PMe2Ph)2Pt(9-MeAd)2]2+ in which the selectivity of the
metal center toward the N(1) site of the adenine was
maintained.5
Ph)2Pt{9-MeAd(-H)}]33+, obtained by reacting cis-[(PMe2-
Ph)2Pt(µ-OH)]2(ΝÃ3)2 with 9-MeAd, was treated with cis-
[(PMe2Ph)2Pt(ONO2)2], according to the reaction scheme
depicted in Scheme 2.
Now, the extension of this study to the complexes
stabilized by the less basic but more sterically demanding
PMePh2 (L) ligand, i.e., cis-[L2Pt(µ-OH)]2(ΝÃ3)2, 1, and cis-
[L2Pt(ONO2)2], 4, has permitted the determination of the
crystal structures of cis-[L2Pt{9-MeAd(-H)}]3(NO3)3 (2) and
cis-[L2Pt(ONO2){9-MeAd(-H)}PtL2](NO3)2 (5), both de-
scribed in this paper. Complex 2 and its PMe2Ph analogue
represent the first examples of cyclic trimers of the N(1),N-
(6)-platinated adenine ion, whereas the binding mode of the
adenine found in 5 (species B in Scheme 1) has been only
recently proved for a platinum(IV) organometallic com-
pound.6 The new compounds have been characterized by
multinuclear NMR spectroscopy showing that 2 in coordinat-
ing solvents is in equilibrium with the mononuclear species
cis-[L2Pt{9-MeAd(-H)}]+ (3) in which the nucleobase
chelates the metal center with the N(6),N(7) atoms. In
addition, in this paper we describe the synthesis of cis-[{L2-
Pt(ONO2)}2(9-MeAd)](NO3)2, (6), containing the neutral
adenine bridging two metal centers through the N(1) and
N(7) atoms. The spectroscopic characterization (31P NMR)
reveals that 6, in DMSO, forms a mixture of the species 4,
5, and cis-[L2Pt(9-MeAd)(DMSO)]2+, 7, which are quanti-
tatively converted into the bis-adduct cis-[L2Pt(9-MeAd)2]-
(NO3)2 (8) upon addition of a stoichiometric amount of
The complete spectroscopic characterization of B was
possible by taking advantage of the observation that this
complex was formed when the trinuclear species cis-[(PMe2-
(3) Schenetti, L.; Mucci, A.; Longato, B. J. Chem. Soc., Dalton Trans.
1996, 299-303.
(4) Trovo`, G.; Bandoli, G.; Nicolini, M.; Longato, B. Inorg. Chim. Acta
1993, 211, 95-99.
(5) Longato, B.; Pasquato, L.; Mucci, A.; Schenetti, L. Eur. J. Inorg.
Chem. 2003, 128-137.
(6) Zhu, X.; Rusanov, E.; Kluge, R.; Schmidt, H.; Steinborn, D. Inorg.
Chem. 2002, 41, 2667-2671.
7862 Inorganic Chemistry, Vol. 42, No. 24, 2003

Pt(II) Complexes Containing 9-Methyladenine
Chart 1
OH)]2(NO3)2 (L ) PMePh2, 1)9 was synthesized with a procedure
similar to that described for the PMe2Ph analogue.3
9-MeAd. A graphical scheme of all the characterized adenine
complexes is presented in Chart 1.
Synthetic Work. cis-[(PMePh2)2Pt{9-MeAd(-H)}]3(NO3)3 (2).
A suspension of 1, cis-[(PMePh2)2Pt(µ-OH)]2(NO3)2 (150 mg, 0.11
mmol), and 9-MeAd (33 mg, 0.22 mmol) in CH3CN (2.5 mL) was
stirred at room temperature for ca. 20 min. The addition of Et2O to
the resulting solution afforded a white microcrystalline solid, which
was filtered and dried under vacuum. The isolated product weighed
140 mg (yield 78%, calculated as anhydrous product). The crystals
used for the X-ray analysis, obtained by slow evaporation of a
dichloromethane/toluene (1:1) solution of 2 at ambient conditions,
Experimental Section
Instrumentation and Materials. H, 13C, 15N, 31P, and 195Pt
1
NMR spectra were obtained with a Bruker 400 AMX WB and a
Bruker AVANCE 300. Reagent grade chemicals were used as
received unless otherwise stated. cis-[(PMePh2)2PtCl2]7 and 9-MeAd8
were prepared according to the literature methods. cis-[L2Pt(µ-
(7) Jenkins, J. M.; Shaw, L. J.Chem. Soc. A 1966, 773-775.
(8) Talman, E. G.; Bru¨ning, W: Reedijk, J.; Spek, A. L.; Veldman, N.
Inorg. Chem. 1997, 36, 854-861.
(9) Longato, B.; Dolmella, A.; Bandoli, G. Eur. J. Inorg. Chem., submitted.
Inorganic Chemistry, Vol. 42, No. 24, 2003 7863

Longato et al.
Table 1. Crystallographic Data and Details of Structural Refinement
Parameters for 2, 4, and 5
had the composition 2‚0.75H2O whereas the elemental analysis was
more consistent with the formulation 2‚3H2O. Anal. Calcd for
C32H32N6O3P2Pt‚H2O: C, 46.66; H, 4.16; N, 10.20. Found: C,
46.84; H, 4.02; N, 9.95. 31P NMR (121.5 MHz, in CD3CN): δ
2‚0.75H2O
4‚0.5CH2Cl2
5‚2DMF
formula
C96H97.5N18-
C26.50H27ClN2-
O6P2Pt
761.98
monoclinic
P21/c
10.947(3)
13.230(4)
21.344(5)
C64H72N10-
O11P4Pt2
1671.38
triclinic
P1h
13.805(4)
14.515(4)
20.672(5)
76.72(2)
75.59(3)
65.96(3)
3625.1(17)
2
2
-8.50 (1JPPt 3192 Hz), -9.34 (1JPPt 3372 Hz) with JPP 23.3 Hz.
O9.75P6Pt3
1H NMR (400.13 MHz, in CDCl3): δ major component (ca. 90%)
8.27 (s, 1H, H(8)), 7.65 (s, 1H, H(2)), 8.2-6.4 (m, 20 H, Ph), 7.11
(br s, 1H, NH), 3.45 (s, 3H, NMe), 2.75 (d, 2JHP 11.4 Hz, 3H, PMe),
fw
2430.51
monoclinic
P21/n
30.144(6)
18.216(4)
36.300(6)
crystal system
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
2
1.20 (d, JHP 10.3 Hz, 3H, PMe); minor component 8.14 (s, 1H,
H(2)), 6.82 (s, 1H, H(8)), 8.2-6.4 (m, 20 H, Ph), 5.8 (br s, 1H,
NH), 3.73 (s, 3 H, NMe), 2.33 (d, 2JHP 10 Hz, 3H, PMe), 1.83 (d,
2JHP 10 Hz, 3H, PMe). 13C NMR (100.61 MHz, in CDCl3): δ 166.2
(C-6), 155.8 (C-2), 154.5 (C-4), 141.8 (C-8), 128.3 (C-5), 30.2
(NCH3), 16.2 and 13.3 (PCH3).
1H NMR (400.13 MHz, in DMSO-d6): δ major component (ca.
60%) 8.04 (s, 1H, H(8)), 6.52 (s, 1H, H(2)), 5.25 (br s, 1H, NH),
3.56 (s, 3H, NMe), 2.23 (d, 2JHP 10 Hz, 3H, PMe), 2.12 (d, 2JHP 10
Hz, 3H, PMe). 13C NMR (100.61 MHz, in DMSO-d6): δ 164.9
(C-6), 157.8 (C-2), 147.5 (C-4), 140.1 (C-8), 128.3 (C-5), 30.6
(NCH3), 13.2 and 12.1 (PCH3).
93.58(4)
90.11(2)
19893(7)
8
1.623
7.003
9612
3091.2(15)
4
1.637
4.769
1492
Z
D
calcd, g cm-3
1.531
4.004
1660
µ, mm-1
F(000)
θ range, deg
reflns collected
reflns unique
reflns I > 2σ(I)
no. of params
R(int)
2.50-33.13
47103
25026
20185
1285
0.0549
1.021
0.0746
0.2081
2.154, -1.191
2.45-27.10
11190
6149
4654
346
0.0507
1.053
0.0492
0.1424
1.085, -0.888
1.79-27.88
28765
15805
10820
800
0.0519
1.054
0.0553
0.1578
1.300, -2.576
With the same procedure, complex 2 was obtained operating in
CH2Cl2 (isolated product 276 mg, yield 77%) and DMF (160 mg,
yield 88%).
GOF on F2
R1a
wR2a
cis-[(PMePh2)2Pt(ONO2)2] (4). A solution of AgNO3 (0.52 g,
3.06 mmol) in EtOH (8 mL) and H2O (1.5 mL) was slowly added
to a CH2Cl2 (13 mL) solution of cis-[(PMePh2)2PtCl2] (1.02 g, 1.53
mmol). The resulting precipitate was eliminated by filtration and
the solvent evaporated to dryness under vacuum, yielding 1.05 g
of product (yield 95%). The isolated solid contained small amounts
residuals, e‚Å-3
a R1 ) ∑||Fo| - |Fc||/∑|Fo|, wR2 ) [∑w(Fo - Fc )2/∑w(Fo )2]1/2
.
2
2
2
13C NMR (100.61 MHz, in CDCl3): δ 153.3 (C-2), 152.2 (C-6),
147.8 (C-8), 146.6 (C-4), 115.5 (C-5), 30.9 (NCH3), 13.1, 12.4,
and 9.5 (PCH3).
1
of CH2Cl2, seen by H NMR. Anal. Calcd for C26H26N2O6P2Pt‚
0.5CH2Cl2: C, 41.77; H, 3.57; N, 3.68. Found: C, 41.72; H, 3.42;
cis-[(PMePh2)2Pt(9-MeAd)2](NO3)2 (8). 9-MeAd (57 mg, 0.4
mmol) was added to a solution of 4 (137 mg, 0.2 mmol) in CH3-
CN (7 mL) and stirred for 1 h. The resulting solution, left at ambient
temperature for 5 days, separated a white solid, which was recovered
by filtration, washed with Et2O, and dried under vacuum. The yield
of 8 was 170 mg (88%). Anal. Calcd for C38H40N12O6P2Pt: C,
44.84; H, 3.96; N, 16.51. Found: C, 44.71; H, 3.92; N, 16.46. The
solid, insoluble in chlorinated solvents, was characterized in DMSO.
1H NMR (400.13, in DMSO-d6): δ major conformer (ca. 55%)
8.95 (s, 2H, H(2)), 8.6 (br s, 4H, NH2), 8.12 (s, 2H, H(8)), 7.7-
N, 3.44. 1H NMR (300 MHz, in DMSO-d6): δ 7.64-7.43 (m, 20H,
2
Ph), 2.02 (d, JHP 10.6 Hz, 6H, PMe). 31P NMR (121.5 MHz, in
1
DMSO-d6): δ -6.90 (s, JPPt 3903 Hz).
cis-[(PMePh2)2Pt(ONO2){9-MeAd(-H)}Pt(PMePh2)2]-
(NO3)2 (5). To a solution of 2 (135 mg, 0.056 mmol) in DMF (5
mL) was added 4 (121 mg, 0.168 mmol), and the resulting solution
was stirred at room temperature for 2 h. Addition of Et2O gave a
white solid, which was filtered, washed with Et2O, and dried under
vacuum. Crystallization of the crude product, by dissolution in DMF
and vapor diffusion of Et2O, afforded X-ray quality crystals of 5
(192 mg, yield 68%) having the formula cis-[(PMePh2)2Pt(ONO2)-
{9-MeAd(-H)}Pt(PMePh2)2](NO3)2‚2DMF (5‚2DMF). Anal. Calcd
for C64H72N10O11P4Pt2 (1671.4): C, 45.99; H, 4.34; N, 8.38.
2
6.8 (m, 20 H, Ph), 3.60 (s, 6H, NMe), 2.32 (d, JHP 11 Hz, 6H,
PMe); minor conformer (45%) 8.71 (s, 2H, H(2)), 8.6 (br s, 4H,
NH2), 8.11 (s, 2H, H(8)), 7.7-6.8 (m, 20 H, Ph), 3.57 (s, 6H, NMe),
2.32 (d, 2JHP 11 Hz, 6H, PMe). 13C NMR (100.61 MHz, in DMSO-
d6): δ more abundant conformer 155.0 (C-6), 152.1 (C-2), 148.8
(C-4), 144.8 (C-8), 120.5 (C-5), 29.6 (NCH3), 10.8 (PCH3); less
abundant conformer 154.4 (C-6), 153.1 (C-2), 148.7 (C-4), 144.8
(C-8), 120.7 (C-5), 29.6 (NCH3), 10.8 (PCH3).
1
Found: C, 45.69; H, 4.35; N, 8.64. H NMR (400.13, in DMSO-
d6): δ 8.64 (s, 1H, H(2)), 7.8-7.2 (m, 40 H, Ph), 6.52 (s, 1H,
H(8)), 5.30 (br s, 1H, NH), 3.56 (s, 3H, NMe), 2.50 (s, 6H, PMe),
2.26 (d, 2JHP 11.1 Hz, 3H, PMe), 1.97 (d, 2JHP 11.1 Hz, 3H, PMe);
DMF resonances at 7.9(s), 2.89 (s), and 2.73 (s). 13C NMR (100.6
MHz, in DMSO-d6): δ 164.9 (C-6), 157.8 (C-2), 147.5 (C-4), 140.1
(C-8), 124.9 (C-5), 30.6 (NCH3), 13.2 and 12.1 (PCH3).
X-ray Structure Determinations. Crystal data, data collection,
and refinement parameters for compounds 2, 4, and 5 are sum-
marized in Table 1. Due to the small crystal dimensions (ap-
proximately 0.15 × 0.14 × 0.10 mm), data collection of 2 was
carried out at the X-ray diffraction beamline of the Synchrotron
Elettra, Trieste, Italy, using a monochromatized wavelength of 1.000
Å. A total of 60 frames were collected on a 165 mm CCD
MarResearch (rotation of 3° about æ, fixed dose of radiation,
detector at 35 mm from the crystal, T ) 100(2) K). Data collections
of 4 and 5 were performed on a Nonius DIP-1030H system
equipped with Mo-KR radiation (λ ) 0.71073 Å) graphite-
monochromatized (30 frames, exposure time of 20 min, rotation
of 6° about æ, detector at 80 mm from the crystal, T ) 293(2) K).
Cell refinement, indexing, and scaling of all the data sets were
cis-[{(PMePh2)2Pt(ONO2)}2(9-MeAd)](NO3)2 (6). 9-MeAd (18.8
mg, 0.13 mmol) was added to a suspension of 4 (182 mg, 0.25
mmol) in CHCl3 (8 mL) and stirred at ambient temperature for 1
h. Addition of Et2O to the resulting solution afforded a white solid,
which was recovered by filtration, washed several times with Et2O,
and dried under vacuum. The yield of 6 was 160 mg (80%). Anal.
Calcd for C58H59N9O12P4Pt2 (1588.2): C, 43.86; H, 3.74; N, 7.94.
1
Found: C, 43.35; H, 3.44; N, 7.87. H NMR (400.13, in CDCl3)
δ: 11.08 (br s, 1H, NH2), 9.94 (s, 1H, H(8), 8.53 (br s, 1H, NH2),
8.01 (s, 1H, H(2)), 8.0-6.9 (m, 40 H, Ph), 3.52 (s, 3H, NMe),
2.39 (d, 2JHP 11.6 Hz, 3H, PMe), 2.26 (d, 2JHP 11.3 Hz, 3H, PMe),
2.13 (d, 2JHP 11.3 Hz, 3H, PMe), 1.96 (d, 2JHP 11.4 Hz, 3H, PMe).
7864 Inorganic Chemistry, Vol. 42, No. 24, 2003

Pt(II) Complexes Containing 9-Methyladenine
performed using programs Mosflm and Scala.10 The structures were
solved by Patterson and subsequent Fourier analyses and refined
by the full-matrix least-squares method based on F2 with all
observed reflections.11 In 2 all the phenyl rings were fitted to regular
hexagons and isotropically refined; four disordered nitrate anions
with restrained geometry were treated isotropically (fixed U ) 0.25);
three residuals were interpreted as water molecules with half-
occupancy. A difference Fourier map revealed the presence of a
disordered CH2Cl2 molecule (0.5 occupancy) in 4 and of two DMF
molecules in 5. The contributions of hydrogen atoms at calculated
positions were included in final cycles of refinements. All the
calculations were performed using the WinGX System, version
1.64.12
Results and Discussion
Synthesis and Structure of the Trinuclear Complex cis-
[(PMePh2)2Pt{9-MeAd(-H)}]3(NO3)3, 2. We have already
shown that the dinuclear hydroxo complexes cis-[L2Pt(µ-
2+
OH)]2 in DMSO deprotonate the exocyclic amino group
of the 9-substituted methyladenine to form the dinuclear
2+
3
complex cis-[L2Pt{9-MeAd(-H)}]2 when L is PMe3 or
3+
the trinuclear species cis-[L2Pt{9-MeAd(-H)}]3 when L
is PMe2Ph.5 Both are cyclic compounds in which the
nucleobases act as bridging ligands binding two cis-L2Pt units
through the N(1),N(6) atoms.
The hydroxo complex, 1, stabilized by the bulkier phos-
phine PMePh2, is also very reactive toward this model
nucleobase. 31P NMR experiments carried out in DMSO-d6
show a rapid and complete disappearance of the signals
typical of 1 when the stoichiometric amount of 9-MeAd is
added. In this case, however, the multiplicity of resonances
observed indicates the formation of a complex mixture of
products. On the contrary, in acetonitrile or chlorinated
solvents, the spectra of the resulting solutions show signals
due to the predominant species cis-[(PMePh2)2Pt{9-MeAd-
(-H)}]33+, 2, which has been isolated in fairly good yields.
Crystallization of the product from a mixture of CH2Cl2/
toluene afforded small but well-shaped crystals, suitable for
an X-ray diffraction study, having the composition cis-
[(PMePh2)2Pt{9-MeAd(-H)}]3(NO3)3‚0.75H2O (2‚0.75H2O).
The X-ray analysis confirms for 2 the trinuclear arrangement
with a pseudo-3-fold symmetry. Two crystallographically
independent complexes, but with a close comparable con-
formation, were found in the unit cell. As depicted in Figure
1, the deprotonated 9-methyladenine acts as a bridging ligand
through N(1) and N(6) donors toward the platinum centers.
The Pt-N(6) mean bond lengths are fairly shorter when
compared with those involving N(1) (mean values of 2.075-
(10) and 2.117(10) Å, respectively), although the low
accuracy on bond distances does not allow a detailed
analysis. The Pt-P bond distances average to 2.280(4) Å.
The intermetallic distances vary from 5.202(1) to 5.382(1)
Å, and one of these (ca. 5.20 Å) is significantly shorter with
respect to the other two (mean 5.36 Å) in both the complexes
Figure 1. Molecular structure of one of the two crystallographically
independent cations cis-[(PMePh2)2Pt{(9-MeAd(-H)}]33+ in 2 (phenyl rings
omitted for clarity): (a) along the pseudo-3-fold axis; (b) side view.
Table 2. Selected Coordination Bond Lengths (Å) for 2
1st unit
2nd unit
Pt(1)-Pt(2)
5.204(1)
5.382(1)
5.320(1)
2.124(10)
2.107(9)
2.118(10)
2.069(11)
2.055(10)
2.081(10)
Pt(4)-Pt(5)
5.360(1)
5.202(1)
5.362(1)
2.114(9)
2.117(11)
2.122(9)
2.066(9)
2.087(10)
2.094(10)
Pt(2)-Pt(3)
Pt(1)-Pt(3)
Pt(1)-N(1a)
Pt(2)-N(1b)
Pt(3)-N(1c)
Pt(1)-N(6c)
Pt(2)-N(6a)
Pt(3)-N(6b)
Pt(4)-Pt(6)
Pt(5)-Pt(6)
Pt(4)-N(1d)
Pt(5)-N(1e)
Pt(6)-N(1f)
Pt(4)-N(6f)
Pt(5)-N(6d)
Pt(6)-N(6e)
(Table 2). The nucleobases are disposed with dihedral angle
of 55-76° with respect to the metal Pt3 plane to form a
pinched cone arrangement. The crystal structure evidences
the two independent molecules packed head-to-head (Figure
2), in such a way that in the dimer the bases give origin to
a cavity, similar to that, filled with water molecules, detected
in the crystal packing of [PtIVMe3{9-MeAd(-H)}]3‚OEt2.6
Of the disordered water oxygen atoms detected in the crystals
of 2, none of these are inside the described voids. On the
other hand, a nitrate anion is located on the phosphine side
of each complex molecule, with a close approach of nitrate
oxygens to Pt apical position and a short Pt-O distance of
3.26 Å being observed for Pt(2).
(10) Collaborative Computational Project, Number 4. Acta Crystallogr.,
Sect. D 1994, 50, 760-763.
(11) Sheldrick, G. M. SHELX97 Programs for Crystal Structure Analysis,
release 97-2; University of Go¨ttingen: Go¨ttingen, Germany, 1998.
(12) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837-838.
Inorganic Chemistry, Vol. 42, No. 24, 2003 7865

Longato et al.
Table 5. 15N NMR Data (δ in ppm; J in Hz) for the Adenine
Complexes 2, 3, 5, 6, and 8 in DMSO-d6 or CDCl3 at 27 °C
compound (solvent)
N(1)
N(3)
N(6)
N(7)
N(9)
2 (CDCl3)a
-205.0
-162.0 -283.6
2JNP ) 60
-132.0
-227.0
2JNP ) 50 Hz
-132.0
3 (DMSO-d6)b
5 (DMSO-d6)
-155.3 -244.7
-190.4
-217.8
-214.9
2JNP ) 50 JNP ) 60
2
-188.7
-147.2 -240.0
-188.3
2JNP ) 60
2JNP ) 60 JNP ) 60
2
6 (CDCl3)
-193.8
-148.9
c
-198.6
-218.3
2JNP ) 66
2JNP ) 66
8 (DMSO-d6)d
-200.2
-142.6 -284.0
-144.4 -284.0
-132.8
-132.8
-218.6
-218.6
2JNP ) 60
-202.8
2JNP ) 60
a Data refers to the 90% component. b The data refer to the major
component in the mixture. c The linewidth (lw1/2 70 Hz) of NH2 protons,
at δ 11.08 and 8.53 ppm, prevented the detection of the N(6) in inverse-
detection mode. d First row refers to the 55% conformer.
Figure 2. The dimer in the crystal packing of 2 showing the nitrate anions
inside the area delimited by the coordination metal planes (phosphine phenyl
rings and methyls not shown).
Table 3. 1H NMR Data (δ in ppm) for the Adenine Complexes 2, 3, 5,
6, and 8 in DMSO-d6 and/or CDCl3 at 27 °C
2, obtained in chlorinated solvents, show two sets of
resonances. The most intense one (ca. 90% of relative
intensity) is attributable to the trinuclear species cis-[(PMePh2)2-
Pt{9-MeAd(-H)}]33+. The determination of the adenine
coordination sites in this complex was achieved through
appropriate two-dimensional NMR experiments. As already
shown in our previous reports,3,5 the first step of this process
is the correct assignment of the carbons and protons of the
adenine units, through 1H,13C heteronuclear multiple-quantum
(HMQC)13 and multiple-bond (HMBC)14 correlation experi-
ments in inverse-detection mode. The signals at δ 8.27 and
7.65 ppm were assigned to H-8 and H-2, respectively, on
the basis of their different one-bond coupling constants
[1J(C,H) ) 216 and 208 Hz for H-8 and H-2, respectively]
and of their long-range correlations with carbons at 128.3
(C-5), 154.5 (C-4), and 166.2 (C-6) ppm. Once H-2 and H-8
proton resonances were properly attributed, the sites of
platination were determined from the 1H,15N inverse-detec-
tion experiments (Figure 3), which show that platination
occurs at N(6) and N(1) atoms of adenine moiety. The
nitrogen atoms involved in the platination process are
identified by the presence of a 2J(N,P) coupling constant of
about 60 Hz, in the second dimension. This coupling is found
compound (solvent) H(2) H(8)
NH2/NH
NCH3
PMe
2 (CDCl3)a
b
7.65 8.27
8.14 6.82
8.04 6.52
8.64 6.52
7.11
5.80
5.25
5.30
3.45
3.70
3.56
3.51
3.52
1.20, 2.75
1.83, 2.35
2.12, 2.23
1.96, 2.25
2.39, 2.26
2.13, 1.96
2.32
3 (DMSO-d6)c
5 (DMSO-d6)
6 (CDCl3)
8.01 9.94 11.08; 8.53
8 (DMSO-d6)d
8.95 8.12
8.71 8.11
8.6
8.6
3.60
3.57
2.32
a First row refers to the 90% component. b Second row refers to the 10%
component. c The data refer to the major component in the mixture. d First
row refers to the 55% conformer.
Table 4. 31P and 195Pt NMR Data (δ, in ppm; J in Hz) for the Adenine
Complexes 2, 3, 5, 6, and 8 in DMSO-d6 and/or CDCl3 at 27 °Ca
complex (solvent)
δ
31P
1JPPt
δ
195Pt
2 (CDCl3)b
-8.29 (1.20, 7.11) 3150 -4217 (1.20, 2.75, 7.11)
-9.85 (2.75, 7.65) 3360
c
-7.93 (2.35)
-8.14 (1.83)
3750 -4351
3050
3 (DMSO-d6)d
5 (DMSO-d6)
-5.50 (2.23, 6.52) 3717 -4348 (2.12, 2.23, 5.25)
-6.28 (2.12, 5.25) 3086
-4.81 (1.96, 8.64) 3305 -4231 (1.96)
-10.35 (1.96)
3780
2
for the nitrogen at -283.6 ppm [1J(N,H) ) 80 Hz, J(N,P)
-5.82 (2.25, 6.52) 3660 -4377 (2.25, 5.30)
-7.15 (2.25, 5.30) 3222
) 60 Hz], detected through the directly bonded NH proton
at 7.11 ppm, and for the nitrogen at -205.0 ppm [2J(N,P)
) 50 Hz], detected through H-2 at 7.65 ppm.
6 (CDCl3)
-4.13(2.39, 8.01) 3431 -4235 (1.96, 2.39, 8.01)
-13.74 (1.96)
3790
The cyclic trimer 2 is substantially stable in chlorinated
solvents whereas it undergoes decomposition in DMSO or
DMF. As an example, in Figure 4 is shown the 31P NMR
spectrum of 2 obtained immediately after the sample dis-
solution in CD2Cl2 (trace a) and in DMSO-d6 (trace b),
respectively. Thus, the AB multiplet observed at δ -8.30
(1JPPt 3174 Hz) and -9.68 ppm (1JPPt 3394 Hz) in CD2Cl2,
attributed to the trinuclear species 2, is completely replaced
by several signals in the range δ -2.9 to -10.6 ppm in
DMSO-d6. Among these, the AB multiplet at δ -5.50 and
- 6.28 ppm, with 2JPP ) 22.1 Hz, exhibits the highest relative
-4.76 (2.26, 9.94) 3450 -4218 (2.13, 2.26, 9.94)
-14.78 (2.13) 3700
8 (DMSO-d6)e
-13.94 (2.32, 8.95) 3261 -4289 (2.32, 8.95)
-13.26 (2.32, 8.71) 3249 -4281 (2.32, 8.71)
a In parentheses are reported the chemical shift values of the protons
correlating with the phosphorus and platinum. b 90% component. c 10%
component. d The data refer to the major component in the mixture. e First
row refers to the 55% conformer.
2
Stability of the Cyclic Complex cis-[(PMePh )2Pt{9-
MeAd(-H)}]33+ (2) in Solution. The 1H, 31P, 195Pt, and 15
N
NMR data of complex 2 are collected in Tables 3, 4, and 5,
respectively, whereas 13C NMR data are reported in the
Experimental Section. As anticipated, the NMR spectra of
(13) Bax, A.; Griffey, R. H.; Hawkins, B. L. J. Magn. Reson. 1983, 55,
301-315.
(14) Bax, A.; Summers, M. F. J. Am. Chem. Soc. 1986, 108, 2093-2094.
7866 Inorganic Chemistry, Vol. 42, No. 24, 2003

Pt(II) Complexes Containing 9-Methyladenine
Figure 5. 1H,15N inverse-detected spectra of complex 3 in DMSO-d6
solution: (a) HMBC spectrum (evolution time 50 ms); (b) HMQC spectrum
(5.56 ms).
Figure 3. 1H,15N inverse-detected spectra of complex 2 in CDCl3
solution: (a) HMBC spectrum (evolution time 50 ms); (b) HMQC spectrum
(5.56 ms).
Scheme 3
and the directly bonded nitrogen at -244.7 ppm [1J(N,H)
2
) 80 Hz and J(N,P) ) 50 Hz] and between H-8 at 6.52
ppm and the platinated nitrogen at -190.4 ppm, attributable
to N(7) [2J(N,P) ) 60 Hz], clearly indicate that the platination
occurs at N(6),N(7) atoms.
Figure 4. 31P{1H} NMR spectrum (central part) of cis-[(PMePh2)2Pt{(9-
MeAd(-H)}]3(NO3)3, 2, dissolved in (a) CD2Cl2 and (b) DMSO-d6. The
label b denotes the trinuclear species 2 whereas 9 denotes the mononuclear
species 3.
We conclude that the cyclic complex 2 in DMSO is
unstable and its fragmentation leads, in addition to several
unknown species, to the chelated complex 3. The formation
of a chelated species is supported by 195Pt and 31P NMR
data which are very similar to those obtained for the N(6),N-
(7) coordinated PtL2 moiety in complex 5 (see below), and
this binding mode of the adenine finds precedents on Mo-
(IV) and Ru(II) organometallic complexes.15
intensity. On the basis of multinuclear NMR heterocorrelated
experiments, analogous to those performed on 2 in CDCl3,
we attributed this multiplet to the mononuclear species 3
containing the deprotonated N(6),N(7)-chelated nucleobase,
depicted in Scheme 3. The pertinent 1H, 195Pt, and 15N NMR
data are collected in Tables 3-5.
The change in the coordination sites of the adenine moiety,
induced by the variation of the solvent, is evidenced by
1H,15N inverse-detection experiments on 3 (Figure 5). The
detected correlations, between the NH proton at 5.25 ppm
The tendency of the trinuclear species 2 to rearrange,
although to a minor extent, is clear also in chlorinated
(15) (a) Kuo, L. Y.; Kanatzidis, M. G.; Marks, T. J. J. Am. Chem. Soc.
1987, 109, 7207-7209. (b) Korn, S.; Sheldrick, W. S. Inorg. Chim.
Acta 1997, 254, 85-91.
Inorganic Chemistry, Vol. 42, No. 24, 2003 7867

Longato et al.
solvents. In fact, a very weak resonance at ca. δ -8.0 ppm
is present in the 31P NMR of 2 in CD2Cl2 (see Figure 4a),
better characterized in CDCl3 in which is observed an AB
multiplet with ca. 10% of relative intensity (see Table 4).
The intensity of these signals increases upon addition of
DMSO, and the chemical shift values undergo a downfield
shift (in dependence on the DMSO fraction) approaching
the values found for complex 3 in pure DMSO-d6.
The disruption of 2, which is apparently complete in
DMSO (see Figure 4b), is only partial in DMF (about 20%)
as estimated on the relative intensities of the signals in the
31P NMR spectrum. One of the products appears to be the
chelated species 3 as inferred from the 31P parameters (δ
1
1
-5.58, JPPt 3719 Hz, and -6.28 ppm, JPPt 3045 Hz, with
2JPP ) 22.3 Hz), very similar to those obtained in DMSO. It
is important to note that the formation of 3 upon dissolution
of 2 in DMF is a reversible process. In fact, complex 2
prepared in this solvent (yield 88%) exhibits NMR spectra
identical to those of the product isolated from CH2Cl2.
Synthesis and X-ray Structure of the Diplatinated
Complex cis-[(PMePh2)2Pt(ONO2){9-MeAd(-H)}Pt(PMe-
Ph2)2](NO3)2 (5). We have recently reported that the analogue
of complex 2 stabilized by the phosphine PMe2Ph undergoes
platination at the N(7) site of the adenine forming mainly
the dinuclear species analogue of 5 depicted in Scheme 2.
A similar reaction occurs when a stoichiometric amount of
2 and cis-[(PMePh2)2Pt(NO3)2], 4, are mixed in DMSO or
DMF, leading to the quantitative formation of the diplatinated
complex 5. The coordination of two cis-L2Pt units and the
simultaneous involvement of N(1), N(6), and N(7) atoms of
the same adenine in the platination process are indicated by
NMR data obtained through heterocorrelated experiments.
In particular, the 1H,15N spectrum shows that the 15N signal
at δ -188.7 (Table 5), detected through the H-2 signal at δ
8.64 (Table 3), is attributed to the N(1) atom, whereas the
resonances at -240.0 [1J(N,H) ) 80 Hz] and -188.3 ppm,
detected through the NH signal at 5.30 and H-8 signal at
6.52 ppm, are attributable to the N(6) and N(7) atoms,
respectively. The 31P{1H} NMR spectrum, displayed along
the f1 axis in Figure 6, shows the presence of two pairs of
AB multiplets with different bandwidth, those at δ -4.81
and -10.35 ppm and broader than those at δ -5.82 and
-7.15 ppm. These two last signals are attributed to the
phosphine trans to the N(7) and N(6) atoms, respectively,
on the basis of the correlations of the H-8 signal with the
31P signal at -5.82 ppm, and of the NH resonance at 5.30
ppm with the 31P signal at -7.15 ppm. The signal at δ -4.81
of the broader AB system shows correlation with the H-2
signal at 8.64 ppm, and is thus attributed to the phosphine
trans to the N(1) adenine atom, whereas that at -10.35 ppm
is attributed to the phosphine trans to a solvent molecule, or
to the oxygen atom of a coordinated NO3- group (as found
in the solid state structure). The competition between the
solvent and the nitrate group is probably at the origin of the
high line width of these two 31P resonances.
Figure 6. 1H,31P inverse-detected spectra of complex 5 in DMSO-d6
solution: (a) aromatic and (b) aliphatic regions.
enable the assignment of all the relevant NMR data (i.e.,
the methyl, phosphorus, and platinum resonances) relative
to the two L2Pt moieties. Nevertheless, the unambiguous
attribution of each metal center to the proper nucleobase
coordination sites can be derived only through the analysis
1
1
of the H,31P (Figure 6) and H,195Pt correlations (when
observed) involving adenine protons.
The product obtained in DMF was isolated in good yield,
and the elemental analysis of the crystals obtained from Et2O
indicates the composition [L4Pt2{9-MeAd(-H)}](NO3)3‚
2DMF (5‚2DMF). The single-crystal X-ray analysis shows
that one of the nitrate groups is involved in the coordination
at the metal center so that the correct formula of the cationic
complex is [L2Pt(ONO2){9-MeAd(-H)}PtL2]2+. The struc-
ture of the dinuclear complex 5 is shown in Figure 7, and a
selection of bond lengths and angles is reported in Table 6.
The deprotonated adenine bridges the L2Pt moieties acting
as monodentate to Pt(1) via N(1) and as chelating ligand to
Pt(2) through N(6) and N(7) atoms. The restricted bite of
the N(6), N(7) chelate induces a remarkable deviation from
the ideal 90° angle (N(6)-Pt(2)-N(7) ) 81.3(2)°), but both
the coordination geometries, likely for steric reasons, result
considerably distorted (Table 6).
For complex 5 two different 195Pt signals are observed in
the 1H,195Pt spectrum (Table 4). It is worthwhile noting that
The nucleobase, which is almost orthogonal to the
coordination plane of Pt(1) (dihedral angle of 83.4(1)°),
results coplanar with that of Pt(2). Thus, the dihedral angle
1
1
the methyl correlations in H,195Pt and the H,31P spectra
7868 Inorganic Chemistry, Vol. 42, No. 24, 2003

Pt(II) Complexes Containing 9-Methyladenine
Figure 7. ORTEP drawing of 5 (ellipsoids at 40% probability level) with
labeling scheme of relevant atoms. For clarity, carbon atoms of phenyls
rings are sketched with a fixed radius.
Table 6. Selected Bond Lengths (Å) and Angles (deg) for 5
Figure 8. ORTEP drawing of 4 (ellipsoids at 40% probability level) with
atom-labeling scheme.
Pt(1)-N(1)
Pt(1)-O(1)
Pt(1)-P(1)
Pt(1)-P(2)
2.091(6)
2.109(5)
2.255(2)
2.278(2)
Pt(2)-N(6)
Pt(2)-N(7)
Pt(2)-P(3)
Pt(2)-P(4)
2.095(7)
2.126(7)
2.238(3)
2.267(3)
Table 7. Selected Bond Lengths (Å) and Angles (deg) for 4
Pt-O(1)
Pt-O(4)
2.125(6)
2.095(6)
Pt-P(1)
Pt-P(2)
2.239(2)
2.230(2)
N(1)-Pt(1)-O(1)
N(1)-Pt(1)-P(1)
O(1)-Pt(1)-P(1)
N(1)-Pt(1)-P(2)
O(1)-Pt(1)-P(2)
P(1)-Pt(1)-P(2)
C(2)-N(1)-Pt(1)
C(6)-N(1)-Pt(1)
N(11)-O(1)-Pt(1)
85.8(2)
90.72(19)
173.92(15)
171.25(18)
85.42(16)
97.95(8)
117.7(5)
121.8(5)
117.0(5)
N(6)-Pt(2)-N(7)
N(6)-Pt(2)-P(3)
N(7)-Pt(2)-P(3)
N(6)-Pt(2)-P(4)
N(7)-Pt(2)-P(4)
P(3)-Pt(2)-P(4)
C(6)-N(6)-Pt(2)
C(8)-N(7)-Pt(2)
C(5)-N(7)-Pt(2)
81.3(2)
92.43(19)
173.14(19)
170.69(19)
89.46(19)
96.87(10)
112.7(5)
O(4)-Pt-O(1)
P(2)-Pt-P(1)
O(1)-Pt-P(1)
O(1)-Pt-P(2)
84.5(3)
97.07(8)
86.83(18)
175.44(16)
O(4)-Pt-P(1)
O(4)-Pt-P(2)
N(1)-O(1)-Pt
N(2)-O(4)-Pt
171.20(18)
91.62(19)
114.7(4)
113.5(5)
of nitrate ions are very similar to those detected in the PMe316
151.5(6)
105.1(5)
17
and PEt3 derivatives.
Synthesis and NMR Studies on the N(1),N(7)-Diplati-
nated Complex cis-[{(PMePh2)2Pt(ONO2)}2(9-MeAd)]-
(NO3)2 (6). When 9-MeAd is added in a 1:2 molar ratio to
a suspension of the nitrato complex 4 in chlorinated solvents,
the complex cis-[{L2Pt(ONO2)}2(9-MeAd)](NO3)2, 6, is
formed in quantitative yield. The reaction, followed by 31P
NMR in CDCl3, is complete in ca. 12 h, at ambient
temperature. The multinuclear NMR characterization shows
that the neutral adenine acts as a bifunctional ligand through
the N(1) and N(7) atoms. The sites of platination were
is also close to the angle formed by the two metal coordina-
tion planes (82.2(1)°). Beside the phosphine groups, Pt(1)
completes the square planar coordination through a nitrate
oxygen at 2.109(5) Å, comparable with the values found in
the dinitrate complex 4 (see below). It is worth noting in
the complex cation the stacking interaction between the base
and one of the phenyl rings of the P(1) phosphine which
leads the ipso carbon atom to be at 3.00 Å from adenine
nitrogen N(1). A similar coordination mode for the nucleo-
base has been reported only in two crystallographic forms
of the neutral trinuclear complex [PtIVMe3{9-MeAd(-H)}]3.6
However, in this adduct, the Pt-N distances average to 2.20
Å for the presence of strong trans influencing methyl groups
and for the geometrical requisites in the formation of the
cycle, and a comparison with present data results useless.
1
identified through long-range H,15N experiments, showing
that H-2 correlates with N(1) and H-8 with N(7) (see Table
5).18
The 31P NMR spectrum of 6 exhibits two AX multiplets
due to two pairs of coupled resonances (Table 4), as derived
from a 31P{1H}-homonuclear correlation experiment. The two
signals at -4.13 and -4.76 ppm are attributed to the
phosphorus atoms trans to N(1) and N(7) of adenine, on the
The same coordination mode for the nitrate group found
in 5 was observed in the complex 4. The X-ray structure of
4 shows (Figure 8) the platinum bound to the P and O nitrate
atoms in a distorted square planar geometry and, as expected,
with a cis configuration. A selection of bond lengths and
angles is reported in Table 7. The ONO2 groups are directed
on opposite sides of the coordination plane, which exhibits
significant tetrahedral distortions with deviations of (0.45
and (0.57 Å for nitrogen and phosphorus donors, respec-
tively. The orientation of two phenyl rings is such to ensure
a weak intramolecular π-interaction, the ipso carbon atoms
C(9) and C(15) being at a distance of 3.50 Å (see Figure 8).
The coordination bond lengths and angles and the orientation
1
basis of their long-range H,31P correlations with H-2 and
H-8, respectively. The resonances at δ -13.74 and -14.78
ppm are assigned to the phosphines trans to the NO3- ligands.
1
The H,195Pt correlation spectrum (Figure 9) allows us,
on the basis of the correlation between H-2, H-8, and methyl
(16) Suzuki, Y.; Miyamoto, T. K.; Ichida, H. Acta Crystallogr., Sect. C
1993, 49, 1318-1320.
(17) Kuehl, C. J.; Tabellion, F. M.; Arif, A. M.; Stang, P. J. Organometallics
2001, 20, 1956-1959.
(18) The linewidth (lw1/2 70 Hz) of NH2 protons, at δ 11.08 and 8.53 ppm,
prevented the detection of the N(6) in inverse-detection mode. The
large difference between these two resonances suggests the involve-
ment of the one shifted to low-field in an intramolecular hydrogen
bond, probably with a nitrate ion.
Inorganic Chemistry, Vol. 42, No. 24, 2003 7869

Longato et al.
Figure 10. 31P{1H} NMR spectrum (central part) of 6 in (a) CDCl3 and
(b) DMSO-d6; (c) sample b, immediately after the addition of 9-MeAd (Pt/
9-MeAd ) 1:2). The labels denote the following complexes: b (4), [ (5),
O (7), and 0 (8).
MeAd)2]2+, 8. The last species is characterized by sharp
singlets at δ -13.24 and -13.92 ppm, having similar relative
intensities, in agreement with the presence of two conformers,
as previously shown for the PMe2Ph and PMe3 analogues.3,5
The relatively broad doublets at δ -11.64 and -3.13 ppm
are attributed to the monoadduct 7 (Chart 1), in which one
of the ligands is a solvent molecule, whereas the doublet at
-3.75 ppm is unassigned. All these species were quantita-
tively converted into the bis-adduct cis-[L2Pt(9-MeAd)2]2+
when 9-MeAd was added (9-MeAd/Pt ) 2), as shown in
Figure 10 (trace c).
Figure 9. 1H,195Pt inverse-detected spectra of complex 6 in CDCl3
solution: (a) aromatic and (b) aliphatic regions. The correlation between
H-2 and platinum at -4235 ppm is very low and partially overlapped by
correlations and t1 noise of aromatic phosphine protons.
Complex 8 was prepared by addition of 2 equiv of
nucleobase to 4 in CH3CN, characterized by elemental
analysis and through multinuclear NMR spectroscopy. All
NMR spectra display two major sets of resonances (Tables
3-5) attributed to the presence of two conformers at
equilibrium (head-to-head and head-to-tail) in a 55:45 ratio.
Also in the present case the coordination of the adenine
occurs mainly (g83%, in DMSO-d6) at the N(1) atom, as
evidenced by 1H,15N heteronuclear correlation experiments.
The minor resonances in the 31P NMR spectrum of Figure
10c can be likely attributed to the coordination isomer cis-
[L2Pt(9-MeAd-N1)(9-MeAd-N7)]2+, in which one of the two
adenine molecules is N(7)-coordinated.
protons with 195Pt signals, to establish that the signal at δ
-4235 is due to the platinum bonded to N(1), whereas that
at -4218 ppm is due to platinum bonded to N(7). Again,
this correlation experiment is fundamental for the attribution
of the methyl resonances (and the related phosphines) to the
proper platinum atom and for the assignment of the metal
centers to the platinated nitrogens.
Pt(II) complexes containing N(1),N(7)-coordinated 9-MeAd
have been structurally characterized in several cases.19 We
were unable to obtain crystals of 6 for the X-ray structure.
As a matter of fact, a solution of 6 in CH2Cl2/EtOH separated
crystals of the starting complex 4 (used for its X-ray
analysis). The lability of 6 observed in ethanol is confirmed
in DMSO. In this solvent it undergoes complete decomposi-
tion as shown in Figure 10 in which the 31P NMR spectra of
6, in CDCl3 (trace a) and DMSO-d6 (trace b), are reported.
In this latter solvent, the following species are identified:
(a) the dinitrato complex 4, as broad singlet at δ -6.88
(relative intensity 37%); (b) the diplatinated species 5, as
sharp AB multiplet, associated with the relatively broad AX
pattern (29% of the total intensities); (c) the mononuclear
adducts cis-[L2Pt(9-MeAd)(DMSO)]2+, 7, and cis-[L2Pt(9-
Conclusions
The results reported in this paper can be summarized as
follows: (i) The deprotonation of the adenine at the NH2
2+
group induced by the hydroxo complex cis-[L2Pt(µ-OH)]2
affords the species cis-[L2Pt{9-MeAd(-H)}]33+ (L ) PMe-
Ph2, 2), which represents the first example of a cyclic trimer
of the nucleobase N(1),N(6)-platinated structurally character-
ized. (ii) Unlike its PMe2Ph analogue,5 complex 2 in
coordinating solvents undergoes solvolysis, and one of the
reaction products is the mononuclear species 3, in which the
adenine is N(6),N(7)-chelated. (iii) The trinuclear complex
2 can be quantitatively converted into the dinuclear species
(19) (a) Lock, C. J. L.; Speranzini, R. A.; Turner, G.; Powell, J. J. Am.
Chem. Soc. 1976, 98, 7865-7866. (b) Jaworski, S.; Menzer, S.;
Lippert, B.; Sabat, M. Inorg. Chim. Acta 1993, 205, 31-34.
7870 Inorganic Chemistry, Vol. 42, No. 24, 2003

Pt(II) Complexes Containing 9-Methyladenine
5 in which the adenine exhibits a new binding mode,
previously characterized in solution for the PMe2Ph deriva-
tive. (iv) The NMR study of complex 6 in DMSO gives a
clear evidence that the simultaneous presence of two cis-
L2Pt units at the N(1),N(7) sites of the adenine promotes
the reversible deprotonation of the exocyclic amino group.
As expected, the addition of the strong base 1,8-bis-
(dimethylamino)naphthalene (“proton sponge”) to the solu-
tion of 6 leads to the quantitative formation of the depro-
tonated species 5. (v) The information derived from the
multinuclear NMR analysis is essential for the structural
characterization in solution of these platinum-adenine
complexes stabilized by phosphine with phenyl groups. It is
apparent that the chemical shift of the adenine protons H-2,
H-8, and NH cannot be directly used for the assignment of
the platination sites. The position of these signals, in fact, is
largely variable (especially for H-8), likely as the result of
anisotropy effects due to the presence of aromatic rings on
the phosphine ligands.
Acknowledgment. Thanks are due to the Centro Inter-
dipartimentale Grandi Strumenti of Modena University for
the use of the Bruker AMX 400 WB spectrometer. This work
was financially supported by the Ministero dell’Universita`
e della Ricerca Scientifica e Tecnologica, PRIN 2001.
Supporting Information Available: Crystallographic data for
the structures reported in this paper. This material is available free
of charge via the Internet at http://pubs.acs.org.
IC034406Z
Inorganic Chemistry, Vol. 42, No. 24, 2003 7871