Platinum(II)-mediated coupling reactions of acetonitrile with the exocyclic nitrogen of 9-methyladenine and 1-methylcytosine. Synthesis, NMR characterization, and X-ray structures of new azametallacycle complexes

Article abstract of DOI:10.1021/ic051755f

The hydroxo complex cis-[L₂Pt(μ-OH)]₂(NO ₃)₂, (L = PMePh₂,1a), in CH₃CN solution, deprotonates the NH₂ group of 9-methyladenine (9-MeAd) to give the cyclic trinuclear species cis-[L₂Pt{9-MeAd(-H)}] ₃(NO₃)₃, (L = PMePh₂, 2a), in which the nucleobase binds the metal centers through the N(1), N(6) atoms. In solution at room temperature, 2a slowly reacts with the solvent to form quantitatively the mononuclear azametallacycle cis-[L₂PtNH=C(Me){9-MeAd(-2H)}] NO₃ (L = PMePh₂, 3a), containing as anionic ligand the deprotonated form of molecule N-(9-methyl-1,9-dihydro-purin-6-ylidene)- acetamidine. In the same experimental conditions, the hydroxo complex with PPh₃ (1b) forms immediately the insertion product 3b. Single-crystal X-ray analyses of 3a and 3b show the coordination of the platinum cation at the N(1) site of the purine moiety and to the N atom of the inserted acetonitrile, whereas the exocyclic amino nitrogen binds the carbon atom of the same CN group. The resulting six-membered ring is slightly distorted from planarity, with carbon-nitrogen bond distances for the inserted nitrile typical of a double bond [C(3)-N(2) = 1.292(7) A in 3a and 1.279(11) A in 3b], while the remaining CN bonds of the metallocycle are in the range of 1.335(8)-1.397(10) A. A detailed multinuclear ¹H, ³¹P, ¹³C, and ¹⁵N NMR study shows that the nitrogen atom of the inserted acetonitrile molecule binds a proton suggesting for 3a,b an imino structure in solution. In DMSO and chlorinated solvents, 3a slowly releases the nitrile reforming the trinuclear species 2a, whereas 3b forms the mononuclear derivative cis-[L₂Pt{9-MeAd(-H)}]NO₃ (L = PPh₃, 4b), in which the adeninate ion chelates the metal center through the N(6) and N(7) atoms. Complex 4b is quantitatively obtained when 1b reacts with 9-MeAd in DMSO and can be easily isolated if the reaction is carried out in CH ₂Cl₂. In CH₃CN solution, at room temperature, 4b slowly converts into 3b indicating that the insertion of acetonitrile is a reversible process. A similar metal-mediated coupling reaction occurs when 1a,b react with 1-methylcytosine (1-MeCy) in CH₃CN. The resulting complexes, cis-[L₂PtNH=C(Me){1-MeCy(-2H)}]NO₃, (L = PMePh₂, 5a and PPh₃, 5b), contain the deprotonated form of the ligand N-(1-methyl-2-oxo-2,3-dihydro-1H-pyrimidin-4-ylidene)-acetamidine. The X-ray analysis of 5a shows the coordination of the metal at the N(3) site of the pyrimidine cycle and to the nitrogen atom of the acetonitrile, with features of the six-membered metallocycle only slightly different from those found in 3a and 3b. In CD₃CN/ CH₃¹³CN solution complexes 5a,b undergo exchange of the inserted nitrile, while in DMSO or chlorinated solvents they irreversibly release CH₃CN to form species not yet fully characterized. No insertion of CH₃CN occurs when the hydroxo complexes are stabilized by PMe₃ and PMe₂Ph.

Full text of DOI:10.1021/ic051755f

Inorg. Chem. 2006, 45, 1805−1814
Platinum(II)-Mediated Coupling Reactions of Acetonitrile with the Exocyclic
Nitrogen of 9-Methyladenine and 1-Methylcytosine. Synthesis, NMR Characterization,
and X-ray Structures of New Azametallacycle Complexes
Bruno Longato,*,†,‡ Diego Montagner,‡ Giuliano Bandoli,§ and Ennio Zangrando
Istituto di Scienze e Tecnologie Molecolari-C.N.R., c/o Dipartimento di Scienze Chimiche,
UniVersita’ di PadoVa, Via Marzolo 1, 35131-PadoVa, Italy, Dipartimento di Scienze
Farmaceutiche, UniVersita’ di PadoVa, Via Marzolo 5, 35131-PadoVa, Italy, and
Dipartimento di Scienze Chimiche, UniVersita’ di Trieste, Via Giorgieri 1, 34127 Trieste, Italy
Received October 10, 2005
The hydroxo complex cis-[L2Pt(
of 9-methyladenine (9-MeAd) to give the cyclic trinuclear species cis-[L2Pt
2a), in which the nucleobase binds the metal centers through the N(1), N(6) atoms. In solution at room temperature,
2a slowly reacts with the solvent to form quantitatively the mononuclear azametallacycle cis-[L2PtNH C(Me) 9-
MeAd( 2H) ]NO3 (L PMePh2, 3a), containing as anionic ligand the deprotonated form of molecule N-(9-methyl-
µ
-OH)]2(NO3)2, (L
)
PMePh2, 1a), in CH3CN solution, deprotonates the NH2 group
{
9-MeAd( H) ]3(NO3)3, (L PMePh2,
−
}
)
d
{
−
}
)
1,9-dihydro-purin-6-ylidene)-acetamidine. In the same experimental conditions, the hydroxo complex with PPh3 (1b)
forms immediately the insertion product 3b. Single-crystal X-ray analyses of 3a and 3b show the coordination of
the platinum cation at the N(1) site of the purine moiety and to the N atom of the inserted acetonitrile, whereas the
exocyclic amino nitrogen binds the carbon atom of the same CN group. The resulting six-membered ring is slightly
distorted from planarity, with carbon
N(2) 1.292(7) Å in 3a and 1.279(11) Å in 3b], while the remaining CN bonds of the metallocycle are in the range
of 1.335(8)
−nitrogen bond distances for the inserted nitrile typical of a double bond [C(3)−
)
1
−
1.397(10) Å. A detailed multinuclear H, 31P, 13C, and 15N NMR study shows that the nitrogen atom of
the inserted acetonitrile molecule binds a proton suggesting for 3a,b an imino structure in solution. In DMSO and
chlorinated solvents, 3a slowly releases the nitrile reforming the trinuclear species 2a, whereas 3b forms the
mononuclear derivative cis-[L2Pt{9-MeAd(−H)}]NO3 (L ) PPh3, 4b), in which the adeninate ion chelates the metal
center through the N(6) and N(7) atoms. Complex 4b is quantitatively obtained when 1b reacts with 9-MeAd in
DMSO and can be easily isolated if the reaction is carried out in CH2Cl2. In CH3CN solution, at room temperature,
4b slowly converts into 3b indicating that the insertion of acetonitrile is a reversible process. A similar metal-
mediated coupling reaction occurs when 1a,b react with 1-methylcytosine (1-MeCy) in CH3CN. The resulting
complexes, cis-[L2PtNHdC(Me){1-MeCy(−2H)}]NO3, (L ) PMePh2, 5a and PPh3, 5b), contain the deprotonated
form of the ligand N-(1-methyl-2-oxo-2,3-dihydro-1H-pyrimidin-4-ylidene)-acetamidine. The X-ray analysis of 5a shows
the coordination of the metal at the N(3) site of the pyrimidine cycle and to the nitrogen atom of the acetonitrile,
with features of the six-membered metallocycle only slightly different from those found in 3a and 3b. In CD3CN/
CH313CN solution complexes 5a,b undergo exchange of the inserted nitrile, while in DMSO or chlorinated solvents
they irreversibly release CH3CN to form species not yet fully characterized. No insertion of CH3CN occurs when
the hydroxo complexes are stabilized by PMe3 and PMe2Ph.
Introduction
functionality.1 It is now clear that the coordination of RCt
N to a metal center [M] increases the rate of the nucleophilic
attack to the carbon atom of the CN group. Depending on
the nature of the nucleophile (Nu)sprotic or aproticsthe
product is the imino derivative, [M]-NHdC(Nu)R, or the
The chemistry of metal-activated organonitriles (RCN) is
still intensively investigated owing to the wide variety of
products obtainable by addition reactions to the CtN
* To whom correspondence should be addressed. E-mail:
longato@chin.unipd.it.
(1) (a) Kukushkin, V. Yu.; Pombeiro, A. J. L. Chem. ReV. 2002, 102,
1771-1802. (b) Michelin, R. A.; Mozzon, M.; Bertani, R. Coord.
Chem. ReV. 1996, 147, 299-338. (c) Reisner, E.; Arion, V. B.;
Chioresku, J.; Schmid, W. F. J. Chem. Soc., Dalton Trans. 2005,
2355-2364.
† Istituto di Scienze e Tecnologie Molecolari, Universita’ di Padova.
‡ Dipartimento di Scienze Chimiche, Universita’ di Padova.
§ Dipartimento di Scienze Farmaceutiche, Universita’ di Padova.
Universita’ di Trieste.
10.1021/ic051755f CCC: $33.50
Published on Web 01/26/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 4, 2006 1805

Longato et al.
Chart 1
Chart 2
azavinyledene species [M]-NdC(Nu)R, respectively. As an
example of the first type of reaction, we have recently
reported the characterization of the amidine complex cis-
[(PMe3)2Pt{NHdC(Me)NH2}{1-MeTy(-H)}]+ in which
1-MeTy(-H) is the anion of the model nucleobase 1-me-
thylthymine, metal coordinated at the N(3) site. The com-
pound was obtained in low yield from the cationic species
cis-[(PMe3)2Pt(NtCMe){1-MeTy(-H)}]+, left in acetonitrile
containing stoichiometric amounts of water.2
Azavinyledene complexes can be obtained by formal
addition of a metal-ligand fragment to a nitrile triple bond.
Insertion of acetonitrile into metal-nitrogen bonds of early
transition metal amides, M-NR2, to form the adducts
[M]-NdC(Me)NR2 is well documented (M ) Ta3), and a
similar reaction occurs with M-PR2 bonds (M ) Zr),
generating phosphorus analogues of N,N-dialkylamidinates.4
In this Article we report four examples of insertion of
acetonitrile into platinum-nitrogen bonds of the NH2-
deprotonated nucleobases 9-methyadenine (9-MeAd) and
1-methylcytosine (1-MeCy), depicted in Chart 1, observed
in the reactions of the hydroxo complexes cis-[L2Pt(µ-OH)]2-
(NO3)2 (L ) PMePh2, PPh3) with the model nucleobases in
CH3CN solution.
The structures of the new complexes have been elucidated
by single-crystal X-ray analysis and multinuclear NMR
spectroscopy showing that in solution they can be formulated
as mononuclear azametallacycle species, cis-[L2PtNHd
C(Me){9-MeAd(-2H)}]+ and cis-[L2PtNHdC(Me){1-MeCy-
(-2H)}]+ (L ) PMePh2, PPh3), in which the anionic ligands
are the deprotonated forms of the amidines N-(9-methyl-
1,9-dihydro-purin-6-ylidene)-acetamidine and N-(1-methyl-
2-oxo-2,3-dihydro-1H-pyrimidin-4-ylidene)-acetamidine, re-
spectively, shown in Chart 2.
The stability in solution of this new class of metallocycles
has been also investigated showing that cis-[L2PtNHd
C(Me){9-MeAd(-2H)}]+ in chlorinated solvents release
reVersibly the inserted CH3CN molecule to form the cyclic
species cis-[L2Pt{9-MeAd(-H)}]nn+ in which binding modes
of the deprotonated nucleobase and nuclearity of resulting
cations depend on the nature of L.5 To the best of our
knowledge, the reversible insertion of CH3CN into a metal-
nitrogen bond has been reported only in some tetranuclear
Ir2Ag2 pyrazolyl amidine complexes.6
Moreover, in this Article we show that the insertion of
CH3CN does not occur in platinum nucleobase compounds
stabilized by PMe3 and PMe2Ph ligands. It turns out that
2+
the adenine complexes cis-[(PMe3)2Pt{9-MeAd(-H)}]2
and cis-[(PMe2Ph)2Pt{9-MeAd(-H)}]33+, previously char-
acterized,7,8 can be prepared in acetonitrile and are indefi-
nitely stable in this solvent, even at high temperature.
Similarly, in the reaction of cis-[(PMe2Ph)2Pt(µ-OH)]2(NO3)2,
with 1-MeCy in CH3CN, the deprotonation of the nucleobase
affords the trinuclear species cis-[(PMe2Ph)2Pt{1-MeCy(-
H)}]3(NO3)3 which does not react further with the solvent.
Insertion reactions of the acetonitrile CtN group into metal-
nucleobase bonds have been previously reported for a
rhenium (IV)-adenine complex.9
Experimental Section
Instrumentation and Materials. 1H, 13C, and 31P NMR spectra
were recorded on a Bruker AVANCE 300 spectrometer (at 300.13,
75.47, and 121.49 MHz, respectively), equipped with a variable
temperature apparatus and were calibrated against the residual
signals of the solvent (for 1H and 13C) and external H3PO4 for 31P.
1H,15N heterocorrelation experiments were performed on a Bruker
400 MHz spectrometer, and 15N resonances were calibrated with
nitromethane. The solvents CD2Cl2, CD3CN, and CH313CN (Ald-
rich) were distilled from CaH2.
Reagent grade chemicals were used as received unless other-
wise stated. cis-[(PMe3)2Pt(µ-OH)]2(ClO4)2,2 (Caution: Perchlorates
are potential explosiVes!) cis-[(PMe2Ph)2Pt(µ-OH)]2(NO3)2,7 cis-
[(PMePh2)2Pt(µ-OH)]2(NO3)2,10 and 9-MeAd11 were synthesized as
previously reported.
Synthetic Work. 1. cis-[(PPh3)2Pt(µ-OH)]2(NO3)2 (1b). The
complex was prepared following the procedure described for the
synthesis of the PMePh2 analogue, with a yield of 69%.10 Elemental
Anal. Calcd for C36H31NO4P2Pt: C, 54.14; H, 3.91; N, 1.75.
(6) Carmona, D.; Ferrer, J.; Lahoz, F. J.; Oro, L. A.; Lamata, M. P.
Organometallics 1996, 15 (5), 5175-5178.
(2) Longato, B.; Bandoli, G.; Mucci, A.; Schenetti, L. Eur. J. Inorg. Chem.
2001, 3021-3029.
(7) Schenetti, L.; Mucci, A.; Longato, B. J. Chem. Soc., Dalton Trans.
1996, 299-303.
(3) Broder, C. K.; Goeta, A. E.; Howard, A. K.; Hughes, A. K.; Johnson,
A. L.; Malget, J. M.; Wade, K. J. Chem. Soc., Dalton Trans. 2000,
3526-3533.
(8) Longato, B.; Pasquato, L.; Mucci, A.; Schenetti, L. Eur. J. Inorg.
Chem. 2003, 128-137.
(9) Pearson, C.; Beauchamp, A. L. Inorg. Chem. 1998, 37, 1242-1248.
(10) Longato, B.; Bandoli, G.; Dolmella, A. Eur. J. Inorg. Chem. 2004,
1092-1099.
(4) Segerer, U.; Blaurock, S.; Sieler, J.; Hey-Hawkins, E. Organometallics
1999, 18, 2838-2842.
(5) Longato, B.; Pasquato, L.; Mucci, A.; Schenetti, L.; Zangrando, E.
Inorg. Chem. 2003, 42, 7861-7871.
(11) Talman, E. G.; Bru¨ning, W; Reedijk, J.; Spek, A. L.; Veldman, N.
Inorg. Chem. 1997, 36, 854-861.
1806 Inorganic Chemistry, Vol. 45, No. 4, 2006

New Azametallacycle Complexes
Table 1. Crystal Data and Details of Refinements for Compounds 3a,
3b, and 5a
Table 2. Selected Bond Lengths (Å) and Angles (Deg) in the Cation of
3a and 3b
3a
3b
5a
3a
3b
formula
fw
crystal syst
space group
a (Å)
C34H35N7O3P2Pt C44H39N7O3P2Pt C33H35N5O4P2Pt
Pt-N(1)
2.115(5)
2.017(5)
2.290(2)
2.267(2)
1.292(7)
1.523(9)
1.335(8)
1.355(8)
1.365(8)
1.357(7)
2.124(7)
2.011(7)
2.311(3)
2.281(3)
1.279(11)
1.509(11)
1.358(11)
1.319(9)
1.349(9)
1.397(10)
846.72
triclinic
P1h
970.85
triclinic
P1h
822.69
orthorhombic
Pna21
14.933(4)
18.888(4)
11.860(3)
90.0
90.0
90.0
3345.2(14)
4
Pt-N(2)
Pt-P(1)
Pt-P(2)
10.236(2)
11.563(2)
16.510(3)
107.79(3)
93.34(3)
110.22(3)
1716.40(6)
2
9.741(2)
12.303(3)
17.180(3)
95.95(3)
96.78(3)
105.30(3)
1952.4(7)
2
N(2)-C(3)
C(3)-C(7)
N(6)-C(3)
N(6)-C(6)
N(1)-C(2)
N(1)-C(6)
b (Å)
c (Å)
R (°)
â (°)
γ (°)
vol (Å3)
Z
N(1)-Pt-N(2)
N(1)-Pt-P(1)
N(1)-Pt-P(2)
N(2)-Pt-P(1)
N(2)-Pt-P(2)
P(1)-Pt-P(2)
85.1(2)
84.4(3)
95.08(15)
172.31(15)
172.43(18)
88.17(16)
92.05(7)
93.04(19)
173.5(2)
176.3(3)
89.4(2)
D
calcd (g cm-3
)
1.638
1.651
1.634
λ Mo KR (mm-1
F(000)
)
4.226
3.727
4.335
840
968
1632
unique reflns
reflns I > 2σ(I)
refined params
GOF
6014
8422
9157
93.08(10)
4370
3614
6823
C(6)-N(1)-C(2)
C(6)-N(1)-Pt
C(2)-N(1)-Pt
C(3)-N(2)-Pt
N(2)-C(3)-N(6)
N(2)-C(3)-C(7)
N(6)-C(3)-C(7)
C(3)-N(6)-C(6)
N(1)-C(6)-N(6)
N(1)-C(6)-C(5)
N(6)-C(6)-C(5)
118.6(6)
122.5(4)
118.9(4)
127.1(5)
128.0(6)
117.3(6)
114.7(6)
122.4(6)
127.3(6)
116.1(6)
116.6(6)
119.8(8)
119.1(5)
120.1(6)
128.6(7)
126.5(8)
119.6(9)
113.9(8)
123.1(8)
127.7(8)
113.5(8)
118.7(8)
424
512
406
0.903
0.965
0.906
Flack param
R1 (I > 2σ(I))a
wR2b
0.016(6)
0.0322
0.0667
0.476
0.0398
0.0667
0.0455
0.0652
0.673
max residuals (e/Å3) 1.017
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑w(Fo - Fc )2/∑w(Fo )2]1/2
.
2
2
2
1
Found: C, 53.69; H, 3.84; N, 1.82. H NMR in CDCl3: 2.12 (br
s, 1 H, OH); 7.55-7.11 (cm, 30 H, Ph).{1H}31P NMR in CDCl3:
singlet at δ 8.45 (1JPPt ) 3713 Hz). These spectroscopic data
compare well with those of the BF4 analogue.12
DMSO-d6: AB multiplet at δ -3.05 (1JPPt ) 3182 Hz) and -3.62
(1JPPt ) 3264 Hz) with JPP ) 26.8 Hz.
2
2. cis-[(PMePh2)2PtNHdC(Me){9-MeAd(-2H)}]NO3 (3a). A
suspension of cis-[(PMePh2)2Pt(µ-OH)]2(NO3)2, 1a, (716 mg, 0.5
mmol) and 9-MeAd (158 mg, 1.1 mmol) in CH3CN (26 mL) was
stirred for ca. 1 h, and the resulting pale yellow solution was heated
at 50 °C for 12 h. A trace amount of metallic platinum was removed
by filtration, and the solution was left to evaporate at room
temperature. In 2-3 days pale yellow crystals, suitable for the X-ray
analysis, were formed, which were separated from the solution and
dried under vacuum (ca. 20 mg). Addition of Et2O (25 mL) to the
remaining solution caused the precipitation of a very pale yellow
solid which was collected by filtration, washed with Et2O, and dried
under vacuum. The recovered solid (600 mg) was purified by
dissolution in CH3CN and precipitated by addition of diethyl ether.
The yield of pure 3a (pale yellow microcrystals) was 69%.
Elemental Anal. Calcd for C34H35N7O3P2Pt: C, 48.23; H, 4.17; N,
11.58. Found: C, 48.20; H, 4.10; N, 11.48. 1H and 15N NMR data
are collected in Tables3 and 4, respectively. {1H}31P NMR in CD3-
CN: AB multiplet at δ -3.36 (1JPPt ) 3172 Hz) and -4.01 (1JPPt
) 3265 Hz) with 2JPP ) 27.4 Hz. {1H}13C NMR (in CD3CN): (9-
MeAd resonances) 156.18 (d, 3JCP ) 8.5 Hz, C-2), 150.42 (s, C-6),
149.56 (s, C-4), 144.16 (s, C-8), 30.05 (s, NCH3). PMePh2
3a, dissolved in a mixture of CD3CN and CH313CN (2:1 v/v),
exchanges the inserted CH3CN molecule in a few hours at room
temperature, as shown by the appearance of a 13C resonance at
164.3 ppm, flanked by poorly resolved 195Pt satellites (2JCPt ca. 12
Hz).22
3. cis-[(PPh3)2PtNHdC(Me){9-MeAd(-2H)}]NO3 (3b). A
suspension of cis-[(PPh3)2Pt(µ-OH)]2(NO3)2, 1b, (211 mg, 0.132
mmol) and 9-MeAd (40 mg, 0.27 mmol) in CH3CN (5 mL) was
stirred for 2 h at ca. 25 °C. Addition of Et2O (20 mL) to the resulting
solution afforded a pale yellow solid which was purified by
dissolution in hot CH3CN, filtered to eliminate trace amounts of
undissolved material, and precipitated with Et2O. The pale yellow
solid was collected by filtration, washed with Et2O, and dried under
vacuum. The yield of pure 3b was 169 mg, yield 65%. Small
crystals, suitable for single-crystal X-ray diffraction, were obtained
by vapor diffusion of Et2O into a CH3CN solution of 3b. Elemental
Anal. Calcd for C44H39N7O3P2Pt: C, 54.43; H, 4.05; N, 10.10.
Found: C, 53.42; H, 3.77; N, 10.08. {1H}31P NMR in CD3CN:
AB multiplet at δ 10.27 (1JPPt ) 3398 Hz) and 11.47 (1JPPt ) 3269
2
Hz) with JPP ) 24.5 Hz. Inverse-detected 13C NMR (in DMSO-
d6): (9-MeAd resonances) 155.5 (C-2), 148.3 (C-4), 125.2 (C-5),
149.0 (C-6), 144.1 (C-8), 29.4 (NCH3). PPh3 resonances: 133.2-
126.8. CH3CN resonances: δ 163.0 and 27.0. {1H}31P NMR in
CDCl3: AB multiplet at δ 11.33 (1JPPt ) 3385 Hz) and 10.86 (1JPPt
3
resonances: 133.24 (d, JCP ) 10.3 Hz, C-2 and C-6), 133.05 (d,
2JCP ) 10.7 Hz, C-2′, C-6′), 132.89 (d, 4JCP ) 2.5 Hz, C-4), 132.11
(d, 4JCP ) 2.3 Hz, C-4), 129.84 (d, 3JCP ) 11.1 Hz, C-3 and C-5),
129.54 (d, JCP, ) 10.8 Hz, C-3 and C-5), 128.47 (dd, JCP ) 60.0
and 3.5 Hz, C-1), 126.80 (dd, JCP ) 63.3 and 3.5 Hz, C-1), 14.41
(dd, JCP ) 44.4 and 3.5 Hz, PCH3), 12.82 (dd, 1JCP ) 45.8 and 3.7
Hz, PCH3). CH3CN resonances: 164.96 (s, 2JCPt ) 12 Hz). {1H}31P
NMR in CDCl3: AB multiplet at δ -3.21 (1JPPt ) 3173 Hz) and
) 3369 Hz) with JPP ) 24.7 Hz. {1H}31P NMR in DMSO-d6:
2
AB multiplet at δ 10.91 (1JPPt ) 3260 Hz) and 9.47 (1JPPt ) 3428
2
Hz) with JPP ) 24.6 Hz.
4. cis-[(PMePh2)2PtNHdC(Me){1-MeCy(-2H)}]NO3 (5a). A
suspension of 1a (233 mg, 0.17 mmol) and 1-MeCy (44 mg, 0.3
mmol) in CH3CN (7 mL) was stirred at room temperature for 30
min. Addition of Et2O to the resulting pale yellow solution afforded
a white solid which was separated by filtration and recrystallized
from CH3CN/Et2O. The yield of pure 5a was 230 mg (yield 81%).
Elemental Anal. Calcd for C33H35N5O4P2Pt: C, 48.18; H, 4.29; N,
-3.50 (1JPPt ) 3215 Hz) with JPP ) 27.4 Hz. {1H}31P NMR in
2
(12) (a) Bushnell, G. W.; Dixon, K. R.; Hunter, R. G.; McFarland, J. J.
Can. J. Chem. 1972, 50, 3694-3699. (b) Li, J. J.; Li, W.; James, A.
J.; Holbert, T.; Sharp, T.; Sharp, P. R. Inorg. Chem. 1999, 38, 1563-
1572.
Inorganic Chemistry, Vol. 45, No. 4, 2006 1807

Longato et al.
Table 3. 1H NMR Data (δ in ppm, J in Hz) for Complexes 3a and 3b in Various Solvents and Temperatures
solvent
compound
(°C)
H(2)
H(8)
NH
NCH3
3.52 s
NdCCH3
PMe
Ph
3a
DMSO-d6
(25)
8.16 app t
8.03 s
5.84 br s
2.01 s
2.23 d
7.64-7.28
(4JHP ) 1.8)
(2JHP ) 10.0)
1.91 d
(2JHP ) 9.5)
2.01 d
3a
3a
3a
CD3CN
(25)
8.05 app t
7.87 s
7.89 s
8.12 s
5.39 br s
3.51 s
3.45 s
3.62 s
1.98 s
1.92 s
2.07 s
7.66-7.30
7.64-7.22
7.64-7.28
(4JHP ) 1.7;
3JHPt ) 17.1)
(2JHP ) 10.2)
1.81 d
(2JHP ) 9.6)
1.97 d
CD3CN
(-40)
7.94 app t
5.36 app dd
(4JHP ) 1.8)
(3JHP ) 4-6)
(2JHP ) 8.7)
1.76 d
(2JHP ) 9.3)
2.17 d
CDCl3
(25)
7.85 s
5.38 br s
(2JHP ) 9.2)
1.83
(2JHP ) 8.6)
3b
3b
DMSO-d6
(25)
CD3CN
(25)
8.08 app t
8.17 s
7.86 s
6.11 dd
3.54 s
3.46 s
1.87 s
1.89 s
7.64-7.14
7.61-7.17
(4JHP ) 1-2)
8.08 app t
(3JHP ) 3-7)
5.69 br s
(4JHP ) 1.8;
3JHPt ) 18)
7.94 app t
3b
3b
CD3CN
(-40)
CDCl3
(25)
7.62 s
7.81 s
5.93 app dd
(3JHP ) 5.3)
5.44 br s
3.42 s
3.50 s
1.86 s
1.87 s
7.50-7.16
7.61-7.17
(4JHP ) 1.8)
8.11 app t
(4JHP ) 1.5)
intensity 58%) at δ -4.34 (1JPPt ) 3290 Hz), -5.33 (1JPPt ) 3280
Hz) with 2JPP ) 28.4 Hz and AX multiplet (relative intensity 42%)
at δ -6.31 (1JPPt ) 3360 Hz), -11.16 (1JPPt ) 3360 Hz) with 2JPP
) 26.4 Hz. {1H}31P NMR in DMSO-d6 at 27 °C: AB multiplet at
δ -5.38 (d, JPP ) 26.7 Hz, JPPt ) 3334 Hz) and -8.38 (s, JPPt
) 3310 Hz). In a second experiment, carried out in CD3CN (0.5
mL), a suspension of 44 mg of 1a and 1-MeCy (8.1 mg) was stirred
for 1 h, at room temperature. The resulting pale yellow solution,
after 24 h, separated pale yellow crystals of 5a (ca. 30 mg) which
were used for the X-ray analysis.
Table 4. 15N NMR Data (δ in ppm, J in Hz) for Complexes 3a,b, 2a,
and 4b in DMSO-d6
complex
N(1)
N(3)
N(6)
N(7)
N(9)
CH3CN
3a
-198.0
-138.6 -172.1
-138.8 -172.0
-155.3 -244.7
-127.1
-220.8 -233.5
2
1
1
(2JNP ) 39)
-200.0
(2JNP ) 53)
-221.0 -234.5
(2JNP ) 58)
3b
2a
4b
-126.7
-190.4
(2JNP ) 60)
-132.0
-217.8
(2JNP ) 50) (2JNP ) 60)
-131.5
-153.2 -243.1
-191.5
-216.5
(2JNP ) 56)
5. cis-[(PPh3)2PtNHdC(Me){1-MeCy(-2H)}]NO3 (5b). A
suspension of 1b (81 mg, 0.05 mmol) and 1-MeCy (12.7 mg, 0.10
mmol) in CH3CN (4 mL) was stirred at room temperature for 12
h. Addition of Et2O to the resulting pale yellow solution afforded
a pale yellow solid which was separated by filtration and recrystal-
lized from CH3CN/Et2O. The yield of pure 5b was 58 mg (yield
61%). Elemental Anal. Calcd for C43H39N5O4P2Pt: C, 54.54; H,
4.16; N, 7.39. Found: C, 54.40; H, 4.10; N, 7.29. {1H}31P NMR
in CD3CN at 27 °C: AB multiplet at δ 8.81 (1JPPt ) 3476 Hz) and
8.01 (1JPPt ) 3432 Hz) with 2JPP ) 25.0 Hz; in CDCl3, AB multiplet
Table 5. Selected Bond Lengths (Å) and Angles (Deg) in the Cation of
5a
Pt-N(3)
Pt-N(2)
Pt-P(1)
Pt-P(2)
N(2)-C(3)
2.097(4)
2.036(4)
2.279(1)
2.262(1)
1.295(7)
N(3)-C(4)
N(3)-C(2)
N(4)-C(3)
N(4)-C(4)
C(3)-C(7)
1.358(7)
1.388(6)
1.337(7)
1.356(7)
1.503(8)
N(2)-Pt-N(3)
N(2)-Pt-P(1)
N(2)-Pt-P(2)
N(3)-Pt-P(1)
N(3)-Pt-P(2)
P(1)-Pt-P(2)
C(3)-N(2)-Pt
C(3)-N(4)-C(4)
83.12(19)
169.24(14)
88.93(15)
95.60(12)
172.00(12)
92.37(5)
C(2)-N(3)-C(4)
C(2)-N(3)-Pt
120.9(4)
117.8(3)
121.3(4)
126.7(5)
118.7(5)
114.6(5)
125.6(5)
117.8(5)
116.6(5)
C(4)-N(3)-Pt
N(2)-C(3)-N(4)
N(2)-C(3)-C(7)
N(4)-C(3)-C(7)
N(3)-C(4)-N(4)
N(3)-C(4)-C(5)
N(4)-C(4)-C(5)
2
at δ 8.99 (1JPPt ) 3477 Hz) and 7.77 (1JPPt ) 3419 Hz) with JPP
) 24.8 Hz; DMSO-d6, AB multiplet at δ 9.48 (1JPPt ) 3444 Hz)
124.9(4)
122.6(5)
2
and 8.45 (1JPPt ) 3442 Hz) with JPP ) 25.0 Hz.
6. Decomposition of 5a in Chlorinated Solvents. A solution
of 5a (139 mg) in CH2Cl2 (5 mL) was left at room temperature for
2 weeks. Addition of Et2O afforded a white precipitate which was
recovered by filtration, washed with Et2O, and dried under vacuum.
The elemental analysis of the solid (71 mg), after recrystallization
from CH2Cl2/Et2O, shows a composition significantly different from
the values calculated for the expected cytosine complex [(PMePh2)2-
Pt{1-MeCy(-H)}]n(NO3)n. Calcd for C31H32N4O4P2Pt: C, 47.63;
8.51. Found: C, 48.01; H, 4.08; N, 8.47. 1H NMR data are collected
in Table 6. {1H}31P NMR in CD2Cl2 at 27 °C: AB multiplet at δ
-6.06 (1JPPt ) 3325 Hz) and -9.43 (1JPPt ) 3311 Hz) with 2JPP
)
27.2 Hz; at -40 °C, AB multiplet at δ -4.65 (1JPPt ) 3280 Hz)
and -6.00 (1JPPt ) 3270 Hz) with 2JPP ) 28.1 Hz and AX multiplet
at δ -7.15 (1JPPt ) 3330 Hz) and -13.05 (1JPPt ) 3330 Hz) with
2JPP ) 26.4 Hz, with relative intensities 1.2:1, respectively. {1H}13C
NMR (in CD2Cl2 at 27 °C): 165.75 (s, CH3CN), 161.45 (s, C-4),
1
H, 4.13; N, 7.17. Found: C, 45.99; H, 4.13; N, 6.84. H NMR in
CDCl3 at 27 °C (δ, ppm): (1-MeCy(-H)) 6.73 (d, 3JHH ) 5.1 Hz,
1 H, H(6)), 6.58 (s, 1 H, NH), 6.34 (d, 3JHH ) 6.0 Hz, 1 H, H(5)),
2.89 (s, 3 H, NCH3); PMePh2, 8.06-7.04 (cm 20 H, PPh), 2.75 (d,
2JHP ) 10 Hz, 3 H, PMe), 2.28 (br s, 3 H, PMe). {1H}31P NMR in
CDCl3 at 27 °C: apparent doublet at δ -11.05 (2JPP ) 21 Hz,
1JPPt ) 3356 Hz) and an extremely broad resonance in the range
4
155.21 (s, C-2), 144.37 (s, C-6), 104.22 (d, JCP ) 3.5 Hz, C-5),
37.81 (s, NCH3); 133.0-125.5 (complex multiplets, PMePh2), 12.39
1
(d, JCP ) 42.7 Hz, PMePh2). {1H}31P NMR in CD3CN at 27 °C:
AB multiplet at δ -5.86 (d, 2JPP ) 27.2 Hz, 1JPPt ) 3350 Hz) and
1
-8.80 (s, JPPt ) 3345 Hz); at -40 °C, AB multiplet (relative
1808 Inorganic Chemistry, Vol. 45, No. 4, 2006

New Azametallacycle Complexes
Table 6. 1H NMR Data (δ in ppm, J in Hz) for Complexes 5a and 5b in Various Solvents and Temperatures
solvent
compound
T (°C)
H(5)
H(6)
NH
NCH3
3.62 s
NdCCH3
PMe
Ph
5a
DMSO-d6
(25)
5.84 dd
7.27 d
6.29 br s
1.95 s
2.13 d
7.64-7.28
(3JHH ) 7.1;
5JHP ) 1.3)
(3JHH ) 7.1)
(2JHP ) 10.8)
ca. 2 br s
1.89 d
5a
5a
CD3CN
(25)
5.81 dd
6.93 d
5.50 br s
5.53 br s
2.56 s
2.63 s
1.89 s
2.07 s
7.60-7.29
7.62-7.34
(3JHH ) 7.1;
5JHP ) 1.4)
(3JHH ) 7.1)
(2JHP ) 10.5)
1.93 d
(2JHP ) 10.0)
1.84 d
CD2Cl2
(25)
5.91 dd
7.92 d
(3JHH ) 7.2;
5JHP ) 1.5)
(3JHH ) 7.2)
(2JHP ) 10.0)
1.93 d
(2JHP ) 11.0)
5b
5b
5b
DMSO-d6
(25)
5.86 dd
7.26 d
7.03 app t
3.50 s
2.49 s
2.71 s
1.85 s
1.95 s
1.92 s
7.67-7.28
7.80-7.23
7.69-7.27
(3JHH ) 7;
5JHP ) 0.9)
5.83 dd
(3JHH ) 7.3)
(3JHP ) 4-6)
CD3CN
(25)
6.91 d
5.84 br s
(3JHH ) 7.2;
5JHP ) 1.2)
6.02 dd
(3JHH ) 7.2)
CDCl3
(25)
7.20 d
5.72 br s
(3JHH ) 7.0;
5JHP ) 1.5)
(3JHH ) 7.1)
of -5 to -10 ppm. Similar data were obtained in CD3CN, even
after several days at 45 °C, indicating that the isolated solid does
not react with the solvent.
7. Reaction of cis-[L2Pt(µ-OH)]22+ (L ) PMe3, PMe2Ph) with
9-MeAd and 1-MeCy in Acetonitrile. A suspension of cis-
[(PMe3)2Pt(µ-OH)]2(ClO4)2, (46 mg, 0.05 mmol) and 9-MeAd (15
mg, 0.1 mmol) in 1 mL of CD3CN was stirred at room temperature
for 4 h obtaining a colorless solution. The 31P NMR of the reaction
were refined by the full-matrix least-squares method based on F2
with all observed reflections.16 In 3b the nitrate oxygen atoms were
found disordered over two positions with occupancy factors refined
at 0.43(2) and 0.57(2). The final cycles with fixed contribution of
hydrogen atoms at calculated positions converged to final R1 and
wR2 factors reported in Table 1.
All the calculations were performed using the WinGX System,
version 1.64.05.17
mixture showed the presence of two AB multiplets, flanked by 195
-
Pt satellites, at δ -27.34 (1JPPt ) 3260 Hz) and -28.56 (1JPPt
)
Results and Discussion
2
3117 Hz, JPP ) 25.1 Hz) ppm and δ -29.66 (1JPPt ) 3021 Hz)
Synthesis and Characterization of cis-[L2PtNHdC-
(Me){9-MeAd(-2H)}]NO3 (L ) PMePh2, 3a; PPh3, 3b).
We have recently shown that the hydroxo complex cis-
[(PMePh2)2Pt(µ-OH))]2(NO3)2, 1a, reacts with the 9-substi-
tuted methyladenine (9-MeAd) to give the cyclic trimer cis-
[(PMePh2)2Pt{9-MeAd(-H)}]3(NO3)3, 2a, containing the
NH2-deprotonated adenine bridging the metal centers through
the N(1) and N(6) atoms.5 When the reaction is carried out
in CH3CN, the initially formed complex 2a, slowly converts
into the mononuclear species cis-[(PMePh2)2PtNHdC(Me)-
{9-MeAd(-2H)}]NO3, 3a, according to the reaction shown
in Scheme 1.
2
and -30.86 (1JPPt ) 3218 Hz, JPP ) 26.1 Hz) ppm, with relative
intensities ca. 5:1, respectively. In 2 weeks at room temperature
the first multiplet quantitatively converted in the second one,
attributable to the dinuclear species cis-[(PMe3)2Pt{9-MeAd(-H)}]2-
(ClO4)2. The solution was then heated at 50 °C for 5 days. Further
changes on the 31P NMR spectrum were not observed. Addition of
Et2O afforded a white precipitate which was recovered by filtration
and dried under vacuum to give 25 mg of solid which was further
characterized by elemental analysis, mass spectrometry, and NMR
spectroscopy. Anal. Calcd for C12H24N5ClO4P2Pt: C, 24.23; H, 4.07;
N, 11.77. Found: C, 24.17; H, 4.01; N, 11.69. ESI mass spectrum
in CH3CN: m/z 1089 due to the monovalent cation [(PMe3)2Pt{9-
1
MeAd(-H)}]2(ClO4)+. H and {1H}31P NMR data (in DMSO-d6)
In the same experimental conditions, complex 1b forms
immediately the insertion product cis-[(PPh3)2PtNHdC(Me)-
{9-MeAd(-2H)}]NO3, 3b. Both the complexes have been
isolated in fairly good yield. X-ray analyses of 3a,b show
that the insertion of a CH3CN molecule into the adenine Pt-
N(6) bond had occurred, with formation of a six atoms
metallocycle, as depicted in Figures 1 and 2, respectively.
A selection of bond distances and angles for these
structures is collected in Table 2, indicating a close similarity
of the isolated complex were in agreement those reported for cis-
[(PMe3)2Pt{9-MeAd(-H)}]2(NO3)2.7 With similar procedures, com-
plexes cis-[(PMe2Ph)2Pt{9-MeAd(-H)}]3(NO3)38 and cis-[(PMe2-
13
Ph)2Pt{1-MeCy(-H)}]3(NO3)3 were prepared reacting cis-
[(PMe2Ph)Pt(µ-OH)]2(NO3)2 with 9-MeAd and 1-MeCy, respectively,
in CH3CN. No reaction with the solvent was observed, even at 50
°C within 1 week.
X-ray Structure Determinations. Diffraction data were col-
lected on a Stoe & Cie diffractometer equipped with a STADI4
CCD detector (compounds 3a,b) and on a Nonius DIP-1030H
system (5a) graphite-monochromatized Mo KR radiation. Cell
refinement, indexing, and scaling of the data sets were carried out
using the program X-RED14 (compounds 3a,b) and by programs
Denzo and Scalepack for 5a.15 The structures were solved by direct
methods (SHELX86 and SHELXTL NT) and Fourier analyses and
(15) 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; Macromolecular Crystallography, Part A;
Academic Press: New York, 1997; Vol. 276, pp 307-326.
(16) (a) Sheldrick, G. M. SHELXTL, NT, version 5.10; Brucker Analytical
X-ray System: Madison WI, 1999. (b) Sheldrick, G. M. SHELX97
Programs for Crystal Structure Analysis, release 97-2; University of
Go¨ttingen: Go¨ttingen, Germany, 1998.
(13) Longato, B.; Montagner, D.; Zangrando, E. Manuscript in preparation.
(14) X-RED, version 1.22; Stoe & Cie: Darmstadt, Germany, 2001.
(17) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837-838.
Inorganic Chemistry, Vol. 45, No. 4, 2006 1809

Longato et al.
Scheme 1
of the metal coordination sphere in the two adducts. The
platinum is bound to the nucleobase at the N(1) site, to the
inserted acetonitrile nitrogen N(2), and completes the square
planar coordination through phosphorus donors. The Pt-
N(1) bond lengths are 2.115(5), 2.124(7) Å, in 3a and 3b,
respectively, while Pt-N(2), of 2.017(5) and 2.011(7) Å,
are slightly shorter. The Pt-P bond distances average to
2.287(2) Å, a value comparable to that found in the parent
complex 2a (2.280(4) Å).5 The coordination N2P2 donors
show a slight tetrahedral distortion in 3a, with deviations of
ca. (0.10 Å from its mean plane, while they are coplanar in
3b, with the Pt ion slightly displaced by 0.04 Å in both
complexes. Figure 3 displays a side view along the P(1)-
P(2) vector in 3a (a similar conformation is also exhibited
by 3b), showing the bending orientation assumed by the
adenine-acetonitrile moiety with respect to the coordination
plane. The dihedral angle formed by the mean planes through
these atoms is of 29.2(2)° and 34.7(2)°, in 3a and 3b,
respectively. The metal displacement from the plane defined
by N(1)/C(6)/C(3)/N(2) is of 0.52 and 0.65 Å in the two
complexes.
Figure 1. ORTEP drawing (ellipsoid 40% probability level) of the cation
of 3a.
In all the structures reported the H atom at acetonitrile
nitrogen N(2) was included considering the evidences
achieved from 1H,15N NMR experiments, since the electron
density maps did not allow us to definitely locate this
hydrogen atom. The values of bond distance inside the
chelating unit (Table 2) indicate a significant π electron
delocalization, similar to that found in a related platinum-
(II) metallacycle.18 In particular, the C(3)-N(2) bond
distances for the inserted CH3CN molecule present a value
typical for a double bond,19 being of 1.292(7) and 1.279-
(11) Å in 3a and 3b, respectively. In the C(3)-N(6)-C(6)-
N(1)-C(2) fragment the distances are slightly longer varying
in a range from 1.335(8) to 1.397(10) Å. All other bond
lengths and angles inside the nucleobase are in the usual
range.20 Compound 3b shows an intramolecular phenyl
stacking occurring between rings of ipso carbon C(1c) and
C(1d).
Figure 2. ORTEP drawing (ellipsoid 40% probability level) of the cation
of 3b.
NMR Studies in Solution. The reaction with acetonitrile
has been followed by NMR spectroscopy. Figure 4 shows
the changes of the 31P NMR spectra of a CD3CN solution of
2a in the course of the transformation to 3a.
A freshly prepared solution of 2a (trace a) exhibits a sharp
AB multiplet, flanked by the 195Pt satellites, whose param-
eters are similar to those obtained in chlorinated solvents in
which 2a maintains the trinuclear structure found in the solid
state.5 In several days at room temperature, the multiplet is
replaced by a new AB multiplet at lower field, attributable
(18) Guedes da Silva, M.; Ferreira, C. M. P.; Branco, E. M. P. R. P; Frau´sto
da Silva, J. J. R.; Pombeiro, A. J. L.; Michelin, R.; Belluco, U.; Bertani,
R.; Mozzon, M.; Bombieri, G.; Benetollo, F.; Kukushkin, V. Y. Inorg.
Chim. Acta 1997, 265, 267-270.
(19) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A.
G. J. Chem. Soc., Perkin Trans. 2 1987, S1.
(20) Velders, A. V.; van der Geest, B.; KooiJman, H.; Spek, A.; Haasnoot,
J. G.; Reedijk, J. Eur. J. Inorg. Chem. 2001, 369-372.
1810 Inorganic Chemistry, Vol. 45, No. 4, 2006

New Azametallacycle Complexes
Scheme 2
length found in the two complexes (Pt-N(1) ) 2.117(10)
Å (average)5 in 2a, and 2.115(5) Å in 3a).
In the spectrum of 3a, the methyl protons of the phosphines
exhibit distinct and well-resolved resonances, in agreement
with the chemical inequivalence of the two ligands. The 1H
NMR data, in various solvents and temperatures, are col-
lected in Table 3.
For thorough 1H and 13C resonance assignments the routine
1- and 2-D NMR techniques have been employed. Discrimi-
nation between the adenine H(2) and H(8) protons was
achieved by heteronuclear multiple bond correlations (HMBC)
C/H experiments. Coordination of the adenine at the N(1)
position is confirmed by the splitting of the resonances C(2)
3
4
(δ 156.3, JCP ) 8.5 Hz) and H(2) (δ 8.16, JHP ) 1.8 Hz),
due to the coupling with 31P nuclei, in the {1H}13C and H
1
spectra of 3a. Moreover, HMBC 15N,1H experiments show
that the resonances at δ 5.84 in 3a and 6.11 ppm in 3b
correlate with the 15N resonances at δ -233.5 (3a) and
-234.5 ppm (3b), indicating that the nitrogen atom of the
inserted CH3CN is protonated. The pertinent 15N NMR data
are collected in Tables 4, while the spectra are available as
Supporting Information.
Figure 3. Perspective view of the complexes 3a and 5a along the P1-P2
direction showing the orientation of the nucleobase moiety with respect to
the coordination plane.
These attributions are in line with the observation that the
NH resonance at δ 6.11 in 3b occurs as a doublet of doublets,
due to coupling with 31P nuclei, separated by 3-7 Hz, typical
values for three-bonds 1H-31P interactions. Similarly, in 3a
the NH resonance is a broad singlet at ambient temperature
but exhibits fine structure at -40 °C (See Table 3). The
process responsible of these changes with the temperature
was not investigated in detail. However, the possible
exchanges of the NH proton between the N(2) and the N(6)
atoms for 3a in DMSO-d6 was ruled out through a ROESY
experiment. The whole of these data supports the conclusion
that the anionic ligand in 3a and 3b, abbreviated as NHd
C(Me){9-MeAd(-2H)}, is the deprotonated form of the
amidine N-(9-methyl-1,9-dihydro-purin-6-ylidene)-acetami-
dine (see Chart 2).
Figure 4. 31P{1H} NMR spectra of 2a in CD3CN at 27 °C: (a) fresh
solution; (b) after 24 h; (c) after 2 weeks at room temperature.
to complex 3a. The reaction appears complete in 3 weeks
(trace c), and no intermediates are detectable during the
transformation (trace b).
The formation of 3a causes a downfield shift of both the
31P nuclei (ca. 5.2 ppm), whereas only one of the 1JPPt values
changes significantly (from 3372 Hz in 2a to 3265 Hz in
3a). The resonance at -3.36 ppm, whose value of 1JPPt (3172
Hz) is very similar to one of those found in the parent
complex 2a (3192 Hz), can be attributed to the phosphine
in trans to the adenine N(1) atom. The small change of 1JPPt
for this phosphine reflects the invariance of the Pt-N(1) bond
In solution complexes 3a,b are stable only in acetonitrile,
in which the inserted CH3CN molecule exchanges with the
solvent (see experimental). In chlorinated solvents at room
temperature, 3a slowly releases the inserted CH3CN molecule
reforming the trinuclear species 2a (Scheme 1). The reaction
leads to an equilibrium mixture in ca. 4 weeks and, in a
solution ca. 3.0 × 10-2 M (in CDCl3) the relative intensities
of the 31P NMR signals of 2a and 3a are ca. 3:1. On the
Inorganic Chemistry, Vol. 45, No. 4, 2006 1811

Longato et al.
Scheme 3
contrary, 3b in CH2Cl2 (and DMSO) undergoes a complete
decomposition with formation of the mononuclear species
4b (Scheme 2) in which the NH2-deproptonated nucleobase
chelates the metal center at the N(6), N(7) sites.21
The PMePh2 analogue of this complex was previously
characterized as the main component the mixture of products
obtained when 2a is dissolved in DMSO.5 Complex 4b can
be isolated as pure product by reacting 1b with 9-MeAd in
dichloromethane.21 In CH3CN solution 4b slowly regenerates
3b, indicating a complete reversibility in the insertion
reaction of acetonitrile.
Synthesis and Characterization of cis-[L2PtNHdC-
(Me){1-MeCy(-2H)}]NO3 (L ) PMePh2, 5a; L ) PPh3,
5b). A similar metal-promoted coupling of CH3CN with the
exocyclic nitrogen of 1-methylcytosine (1-MeCy) is observed
when complexes 1a,b react with the nucleobase in acetoni-
trile (Scheme 3).
Mixtures of 1a or 1b and 1-MeCy (molar ratio 1:2) in
CH3CN, in a few hours at room temperature, form pale
yellow solutions from which the compounds cis-[L2PtNHd
C(Me){1-MeCy(-2H)}]NO3 (L ) PMePh2, 5a; L ) PPh3,
5b) can be separated by crystallization (5a) or precipitation
with Et2O (5b). The NMR analysis of the reaction mixture,
performed before the separation of the solid, showed the
quantitative formation of 5b, whereas for 5a the yield was
ca. 90%. The remaining product is a species, not yet
completely characterized, that can be isolated when 5a is
dissolved in chlorinated solvents (see the Experimental
Section).
The X-ray structure analysis of complex 5a (Figure 5)
shows the platinum bound to the phosphines and to the
nitrogen donors of the adduct obtained from cytosine with a
MeCN solvent molecule, the cytosine coordination site being
N(3).
Figure 5. ORTEP drawing (ellipsoid 40% probability level) of the cation
of 5a.
the cytosine ring. The overall complex conformation, as well
as the bond distances, are close to those detected in the
adenine derivatives, indicating a similar geometry in the
fragment resulting from the binding of the acetonitrile
molecule to the exocyclic amino nitrogen N(4). The perspec-
tive view of 5a (Figure 3) shows the orientation assumed
by the chelating ligand with respect to the coordination plane
(dihedral angle of 40.8(1)°).
The characterization of 5a,b in solution was performed
in CD3CN, DMSO-d6 and chlorinated solvents, and the
1
pertinent H NMR data are collected in Table 6.
The pyrimidinic protons occur as sharp doublets (3JHH
)
7.8 Hz), and that attributed to H(5) shows a further splitting
due to the coupling with a 31P nucleus (5JHP ) 1.3-1.5 Hz).
The NH proton in 5a exhibits a broad singlet (in DMSO-
d6), while in 5b this signal has a fine structure (apparent
3
triplet, JHP ) 4-6) Hz) due to coupling with 31P nuclei.
This finding supports the protonation on the inserted aceto-
nitrile nitrogen, as found in 3a and 3b. Thus, the anionic
ligand in 5b can be described as the deprotonated form of
the amidine N-(1-methyl-2-oxo-2,3-dihydro-1H-pyrimidin-
4-ylidene)-acetamidine (Chart 2).
The location of the NH proton in 5a remains unresolved.
Its resonance, observed at room temperature as broad singlet
at δ 6.29 in DMSO-d6, is shifted at δ 5.50 and 5.53 ppm in
CD3CN and CD2Cl2, respectively, and attempts to observe
a fine structure by lowering the temperature were unsuc-
cessful. Moreover, whereas 31P NMR spectra of 5b are
characterized by a sharp AB multiplet, those of the PMePh2
analogue show that one part of the expected AB multiplet
Both the Pt-P and Pt-N bond lengths are comparable to
those measured in 3a and 3b within 2σ (Table 5). The P2N2
square planar geometry exhibits a slight tetrahedral distortion
(atom displacement of about (0.10 Å) with the P2Pt and
N2Pt planes forming a dihedral angle of 10.7°. But more
severe deformations are detected in the six-membered ring
with Pt and N(4) located above the mean plane passing
through N(2), C(3), C(4), and N(3) atoms by 0.78 and 0.13
Å, respectively. The latter plane forms an angle of 15.7° with
1
(at δ -8.0, JPPt ) 3300 Hz) is broad, in contrast with the
sharp resonance at δ -6.5 (1JPPt ) 3300 Hz) as depicted in
Figure 6 (trace a), in which the spectrum of a fresh solution
of 5a in CD2Cl2 is reported.
As the temperature decreases, the spectrum becomes more
complex, a series of changes leading to the appearance of
two sets of resonances at -40 °C (Figure 6, trace b). Similar
temperature dependence and spectral patterns were observed
in CD3CN. In addition, in the {1H}13C spectrum of 5a the
(21) Montagner, D.; Longato, B. Unpublished results.
1812 Inorganic Chemistry, Vol. 45, No. 4, 2006

New Azametallacycle Complexes
Chart 3
OH)]2(NO3)2, in water or DMSO, deprotonates the exocyclic
NH2 group of 1-MeCy affording the cyclic species cis-
[(PMe3)2Pt{1-MeCy(-H)}]2(NO3)2 with the cytosinate ion
bridging two metal centers through the N(3) and N(4) atoms
(Chart 3).23 At 80 °C, it converts quantitatively in the
corresponding trinuclear derivative, cis-[(PMe3)2Pt{1-MeCy-
(-H)}]3(NO3)3, in which the nucleobase maintains the same
coordination mode.24
Some of the polynuclear cyclic complexes, previously
characterized, have been now prepared in acetonitrile and
their stability in this solvent verified. We find that the
perchlorate salt cis-[(PMe3)2Pt{9-MeAd(-H)}]2(ClO4)2 (soluble
in CH3CN, unlike its nitrato derivative),7 can be prepared in
good yield from cis-[(PMe3)2Pt(µ-OH)]2(ClO4)2 and 9-MeAd
in acetonitrile, and it is indefinitely stable in this solvent,
even at 50 °C for a week. A similar stability is exhibited by
the trinuclear species cis-[(PMe2Ph)2Pt{9-MeAd(-H)}]3-
(NO3)3 and cis-[(PMe2Ph)2Pt{1-MeCy(-H)}]3(NO3)3 which
are quantitatively formed when cis-[(PMe2Ph)2Pt(µ-OH)]2-
(NO3)2 reacts with 9-MeAd and 1-MeCy, respectively, in
CD3CN.10,13
Figure 6. 31P{1H} NMR spectra of 5a in CD2Cl2: (a) fresh solution at
27 °C; (b) at -40 °C.
resonances of one phosphine appear broad (phenyl region)
or undetectable (methyl region), while in the proton spectrum
the two phosphines exhibit distinct signals for methyl groups,
2
3
one sharp (at δ 1.87 with JHP ) 10.5 Hz and JHPt ) 36
2
Hz) and the other one broad (at 1.93 ppm JHP ca. 10 Hz
with unresolved 195Pt satellites).
These spectral features, not observed in the spectra of 3a,
are consistent with the presence of different conformations,
related to restricted rotation of one phosphine around the
Pt-P bond in complex 5a and/or a different flexibility of
the metallocycle. It is to be noted that in 3a the {1H}13C
spectrum of the phenyl groups on the same phosphorus ex-
hibits chemical equivalence (see experimental). This requires
free rotation of phosphine ligands around the Pt-P bonds
and/or the presence of a symmetry plane which can be
obtained if the six atom metallacycle and the adjacent purine
ring are coplanar. From a detailed analysis of the crystal
structures of the two complexes it is apparent that there is a
stacking interaction between the cytosine and the C(10)
phenyl ring in 5a (see Figure 5): the distance between their
centroids is 3.86 Å, with torsion angle N(3)-Pt-P(1)-C(10)
of -18.3°. The corresponding values in 3a are 4.20 Å and
32.0° (N(1)-Pt-P(1)-C(1b), respectively, with a tilting of
the phenyl which does not provide a suitable stacking. It
seems likely that the more bulkier carboxyl oxygen O(2) and
methyl C(1) in the cytosine of 5a might hamper or reduce
the free rotation in solution of the adjacent phosphine group,
when compared with that of the C(2)H environment in 3a.
As shown for 3a,b, complexes 5a and 5b in solution are
stable only in acetonitrile. 5a dissolved in a mixture of
CD3CN/CH313CN slowly (several hours at room temperature)
exchanges the inserted acetonitrile molecule, indicating a
kinetic lability of the chelated ligand.22 Moreover, 5a and
5b in chlorinated solvents lose acetonitrile to give species
not yet completely characterized.
The formation of the insertion products 3a,b and 5a,b
seems therefore related to the presence of the PMePh2 and
PPh3 ligands in the starting hydroxo complex. However, the
relative importance of steric and/or electronic factors of the
phosphines on the metal coordination of CH3CN and the
following nucleophilic attack of the deprotonated nucleobase
have to be elucidated. We observe that the trinuclear cation
2a can be isolated from acetonitrile while the formation of
its PPh3 analogue is prevented, also in chlorinated solvents.
As previously noticed, 2a in DMSO-d6 undergoes a complete
and rapid rearrangement with formation of the mononuclear
cation 4a as the major component of the resulting mixture.5
On the contrary, in the same solvent, cis-[(PMe2Ph)2Pt{9-
3+
MeAd(-H)}]3 maintains almost completely its trinuclear
structure.8 Similarly, the cytosine complex cis-[(PMe2Ph)2Pt-
3+
{1-MeCy(-H)}]3 appears stable in DMSO, whereas we
were unable to characterize similar species stabilized by
PMePh2 and PPh3.21
Conclusions
The formation of compounds 3 and 5 here described
represents a rare example of a reaction in which a platinum-
(22) Wagner, G.; Pakhomova, T. B.; Bokack, N. A.; Frau`sto da Silva, J. J.
R.; Vicente, J.; Pombeiro, A. J. L.; Kukushkin, V. Yu. Inorg. Chem.
2001, 40, 1683-1689.
(23) Trovo`, G.; Bandoli, G.; Casellato, U.; Corain, B.; Nicolini, M.;
Longato, B. Inorg. Chem. 1990, 29, 4616-4621.
Role of the Phosphine Ligands in the Activation of
CH3CN. Previous studies showed that cis-[(PMe3)2Pt(µ-
(24) Schenetti, L.; Bandoli, G.; Dolmella, A.; Trovo`, G.; Longato, B. Inorg.
Chem. 1994, 33, 3169-3176.
Inorganic Chemistry, Vol. 45, No. 4, 2006 1813

Longato et al.
nitrogen bond is formally added to a nitrile triple bond. The
quality of the structural data obtained in the solid phase for
these metallacycles does not allow us to discriminate between
the azavinyledene complexes cis-[L2PtNdC(Me){nucleobase-
(-H)}]+ and the imino structures cis-[L2PtNHdC(Me)-
{nucleobase(-2H)}]+, in which the hydrogen on the N(6)
(or N(4)) atom of the NH2-deprotonated nucleobase is located
on the nitrogen of the inserted CH3CN molecule. This latter
structure appears to be only detectable on the solutions of
3a,b and 5b. The existence of a possible tautomeric equi-
librium between the two forms seems also ruled out on the
basis of a detailed analysis of the NMR spectra obtained in
various solvents. This possibility, however, cannot be
excluded for complex 5a.
the N(6), N(7) sites. For 2a and 4b, the insertion of CH3CN
into the platinum-nucleobase bond is a reVersible reaction.
On the contrary, for the cytosine derivatives 5a and 5b, the
relatively low stability of the trinuclear species cis-[L2Pt{1-
MeCy(-H)}]33+, when L is a PMePh2 and PPh3, make the
loss of acetonitrile an irreVersible process.
Acknowledgment. This work was financially supported
by the Ministero dell’Universita’ e della Ricerca Scientifica
e Tecnologica, PRIN 2004. Thanks are due to Consorzio
Interuniversitario di Ricerca in Chimica dei Metalli nei
Sistemi Biologici for a Grant to D.M. We are in debt with
Professor L. Schenetti for the assistance on the 15N,1H NMR
experiments.
All these metallacycles are stable only in acetonitrile
solution. In chlorinated solvents, 3a loses the inserted
molecule of CH3CN to give the trinuclear species 2a, whereas
3b, having the bulkier PPh3 ligands, forms the mononuclear
complex 4b, stabilized by the chelation of the adenine at
Supporting Information Available: Crystallographic data for
the structures reported in this Article. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC051755F
1814 Inorganic Chemistry, Vol. 45, No. 4, 2006