Irreversible insertion of benzonitrile into platinum(II)-nitrogen bonds of nucleobase complexes. Synthesis and structural characterization of stable azametallacycle compounds

Article abstract of DOI:10.1021/ic901508w

Deprotonation of 1 -methylcytosine (1 -MeCy) and 9-methyladenine (9-MeAd) promoted by c/s-[L₂Pt(μ-OH)]₂(N0₃)2 (L = PPh₃, PMePh₂, 1/2dppe) in PhCN causes the irreversible insertion of a nitrile molecul

Full text of DOI:10.1021/ic901508w

Inorg. Chem. 2010, 49, 2103–2110 2103
DOI: 10.1021/ic901508w
Irreversible Insertion of Benzonitrile into Platinum(II)-Nitrogen Bonds of
Nucleobase Complexes. Synthesis and Structural Characterization of Stable
Azametallacycle Compounds
Diego Montagner,† Alfonso Venzo,‡ Ennio Zangrando,§ and Bruno Longato*,†
†
‡
ꢀ
Dipartimento di Scienze Chimiche, Universita di Padova, Via Marzolo 1, 35131 Padova, Italy, CNR, Istituto di
§
Scienze e Tecnologie Molecolari, Via Marzolo 1, 35131 Padova, Italy, and Dipartimento di Scienze Chimiche,
ꢀ
Universita di Trieste, Via Giorgieri 1, 34127 Trieste, Italy
Received July 29, 2009
Deprotonationof1-methylcytosine(1-MeCy) and 9-methyladenine(9-MeAd) promotedbycis-[L2Pt(μ-OH)]2(NO3)2 (L =
PPh3, PMePh2, 1/2dppe) in PhCN causes the irreversible insertion of a nitrile molecule into the Pt-N4 and Pt-N6 bonds
of the cytosinate and adeninate ligands, respectively, to form the stable azametallacycle complexes cis-[L2PtNHd
C(Ph){1-MeCy(-2H)}]NO3 (L = PPh3, 1; PMePh2, 2; 1/2dppe, 3) and cis-[L2PtNHdC(Ph){9-MeAd(-2H)}]NO3 (L =
PPh3, 4; PMePh2, 5) containing the deprotonated form of the molecules (Z)-9-N-(1-methyl-2-oxo-2,3-dihydropyrimidin-
4(1H)-ylidene)benzimidamide and (Z)-N-(9-methyl-1H-purin-6(9H)-ylidene)benzimidamide. Single-crystal X-ray ana-
˚
˚
lyses of 2 and 4 show the metal coordinated to the N3 cytosine site [Pt-N3 = 2.112(7) A] and to the N1 site of adenine
˚
˚
˚
˚
[Pt-N1 = 2.116(6) A] and to the nitrogen atom of the inserted benzonitrile[Pt-N2 = 2.043(6) and 2.010(6) A in 2 and 4,
respectively], with the exocyclic nucleobase amino nitrogen bound to the carbon atom of the CN group. Complex 2, in
solution, undergoes a dynamic process related to a partially restricted rotation around Pt-P bonds, arising from a steric
interaction of the oxygen atom of the cytosine with one ring of the phosphine ligands. The reaction of 4 with acetylacetone
(Hacac) causes the quantitative protonation of the anionic ligand, affording the acetylacetonate complex cis-[(PPh3)2Pt-
(acac)]NO3 and the free benzimidamide NHdC(Ph){9-MeAd(-H)}. In the same experimental conditions, complex 3
reacts with Hacac only partially.
Chart 1. Neutral Forms of the Anionic Ligands Found in Azametalla-
Introduction
cycle Complexes cis-[L2PtNHdC(Me){1-MeCy(-2H)}]þ (Right) and
cis-[L2PtNHdC(Me){9-MeAd(-2H)}]þ (Left) (L=PPh3, PMePh2)
The coordination of a nitrile (RCtN) to a metal center [M]
increases the rate of the nucleophilic attack to the carbon
atomofthe CN group.1 Whenthenucleophile (Nu) is a protic
species, the imino derivative [M]-NHdC(Nu)R is formed.2
In this context, we have recently described the azametalla-
cycles cis-[L2PtNHdC(Me){1-MeCy(-2H)}]þ and cis-[L2-
PtNHdC(Me){9-MeAd(-2H)}]þ (L=PPh3, PMePh2), where
the anionic ligands are the deprotonated forms of the
amidines shown in Chart 1.
These complexes are formed as the result of a formal
insertion of an acetonitrile molecule into a Pt-N bond of the
NH2-deprotonated nucleobases 1-methylcytosine (1-MeCy)
and 9-methyladenine (9-MeAd).3 As an example, the tri-
nuclear complex cis-[(PMePh2)2Pt{9-MeAd(-H),N1N6}]3
,
3þ
*To whom correspondence should be addressed. E-mail: bruno.longato@
unipd.it. Tel.: þ39 049 8275197. Fax: þ39 049 8275161.
containing bridging 9-methyladeninate ligands,4 dissolved in
CH3CN, gives the species cis-[(PMePh2)2PtNHdC(Me){9-
MeAd(-H)}]þ, as shown in Chart 2.
(1) (a) Kukushkin, V. Y.; 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. 2008, 2355–2364.
The reaction requires (i) the formation of a mononuclear
(2) (a) Carmona, D.; Ferrer, J.; Lahoz, F. J.; Oro, L. A.; Lamata, M. P.
Organometallics 1996, 15, 5175–5178. (b) Lopez, J.; Santos, A.; Romero, A.;
Echavarren, A. M. J. Organomet. Chem. 1993, 443, 221–228.
(3) Longato, B.; Montagner, D.; Bandoli, G.; Zangrando, E. Inorg.
Chem. 2006, 45, 1805–1814.
species cis-[L2Pt(NtC-Me){9-MeAd(-H)}]þ containing a
(4) Longato, B.; Pasquato, L.; Mucci, A.; Schenetti, L.; Zangrando, E.
Inorg. Chem. 2003, 42, 7861–7871.
r
2010 American Chemical Society
Published on Web 02/05/2010
pubs.acs.org/IC

2104 Inorganic Chemistry, Vol. 49, No. 5, 2010
Montagner et al.
Chart 2
metal-coordinated solvent nitrile and the monodentate anio-
nic 9-MeAd(-H), likely N1-platinated, and (ii) the intramo-
lecular nucleophilic attack at the carbon atom of the CH3CN
ligand by the N6 imino atom of the adeninate ion with
concomitant migration of the N6H proton at the acetonitrile
nitrogen atom to give the cyclic amidine complex. The tri-
nuclear complex, in turn, was isolated by reacting the hydr-
oxo complex cis-[(PMePh2)2Pt(μ-OH)]22þ with 9-MeAd, in a
CH3CN solution, in which the condensation reaction occurs
quickly.4 In the same experimental conditions, the hydroxo
complex cis-[(PPh3)2Pt(μ-OH)]22þ leads directly to the inser-
tion product cis-[(PPh3)2PtNHdC(Me){9-MeAd(-2H)}]þ.
The adenine derivatives cis-[L2PtNHdC(Me){9-MeAd-
(-2H)}]þ, in chlorinated solvents, release reversibly the in-
serted CH3CN molecule to form the trinuclear cyclic species
cis-[L2Pt{9-MeAd(-H)}]33þ when L is PMePh2 or the mono-
nuclear chelate adeninate complex cis-[(PPh3)2Pt{9-MeAd-
(-H),N6N7}]þ when L is PPh3.5
these compounds are indefinitely stable in a solution of
dimethyl sulfoxide (DMSO) or chlorinated solvents; i.e.,
the formal insertion of PhCN into the Pt-N bond of the
nucleobase is irreversible. Moreover, the protonation of the
anionic ligands in 3 and 4 has been examined. The reaction,
carried out by dissolving 4 in acetylacetone (Hacac), causes
the quantitative formation of the complex cis-[(PPh3)2Pt-
(acac)]NO3, while the replacement of the benzimidamide
ligand occurs only partially in the case of complex 3.
Experimental Section
Instrumentation and Materials. The NMR spectra were ob-
tained in a solution of various solvents at 300 K on a Bruker
AVANCE 300 MHz for 1H and 31P NMR (νo 300.13 and 121.5
MHz, respectively) and on a Bruker 400 AMX-WB spectro-
meter for 15N NMR (νo 40.6 MHz). δ are in parts per million and
J in hertz. The 1H, 31P, and 15N NMR chemical shift values were
referenced to internal Me4Si, external 85% aqueous H3PO4, and
CH3NO2, respectively. The 15N NMR parameters were ob-
tained through HMBC experiments. The heteronuclear Over-
hauser enhancement spectroscopy (HOESY) experiments were
obtained by using a Bruker QNP probe operating in the direct
acquisition mode at 161.98 MHz for 31P NMR and 400.13 MHz
for 1H NMR on a 9.4 T field. The 31P NMR experiments carried
out in nondeuterated solvents, Hacac and hexafluoroacetylace-
tonate (hfHacac), were recorded with an insert of D2O for the
instrumental lock. The labeling of the proton resonances follows
the X-ray numbering scheme used for 2 and 4.
In spite of the large number of studies dealing with metal
nucleobase complexes,6 coupling reactions of nitriles with the
exocyclic nitrogen atoms of 9-MeAd and 1-MeCy have only a
precedent in the literature.7
In this paper, we report the coupling reactions of benzoni-
trile with 1-MeCy and 9-MeAd, occurring when the hydroxo
complexes cis-[L2Pt(μ-OH)]2(NO3)2 {L = PPh3, PMePh2,
1/2dppe [dppe = 1,2-bis(diphenylphosphino)ethane]} react
in a PhCN solution. The insertion products cis-[L2PtNHd
C(Ph){1-MeCy(-2H)}]NO3 (L = PPh3, 1; PMePh2, 2;
1/2dppe, 3) andcis-[L2PtNHdC(Ph){9-MeAd(-2H)}]NO3 (L=
PPh3, 4; PMePh2, 5) have been isolated as pure compounds
and fully characterized by multinuclear NMR studies, and
the molecular structures of 2 and 4 have been confirmed by
single-crystal X-ray analyses. Unlike the CH3CN analogues,
cis-[(PMePh2)2Pt(μ-OH)]2(NO3)2,4 cis-[(dppe)Pt(μ-OH)]2(NO3)2,8
and 1-MeCy9 were prepared as previously reported. 9-MeAd
and all of the solvents were Aldrich products.
Synthetic Work. 1. cis-[(PPh3)2PtNHdC(Ph){1-MeCy-
(-2H)}]NO3 (1). A mixture of cis-[(PPh3)2Pt(μ-OH)]2(NO3)2
(410 mg, 2.57 ꢀ 10-1 mmol) and 1-MeCy (64.2 mg, 5.13 ꢀ 10-1
mmol) in PhCN (20 mL) was stirred at room temperature for ca.
4 h. The resulting yellow solution was filtered to remove trace
amounts of a solid. The addition of Et2O (100 mL) to the filtrate
(5) Montagner, D.; Longato, B. Inorg. Chim. Acta 2008, 361, 1676–1680.
(6) (a) Zangrando, E.; Pichierri, F.; Randaccio, L.; Lippert, B. Coord.
Chem. Rev. 1996, 156, 275–332. (b) Lippert, B. Coord. Chem. Rev. 2000,
200-202, 487–516.
(8) Li, J. J.; Li, W.; Sharp, P. R. Inorg. Chem. 1996, 35, 604–613.
(9) Kistenmacher, T. J.; Rossi, M.; Caradonna, J. P.; Marzilli, L. G. Adv.
Mol. Relax. Interact. Processes 1979, 15, 119–133.
(7) Pearson, C.; Beauchamp, A. L. Inorg. Chem. 1998, 37, 1242–1248.

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2105
afforded 1 Et2O as a pale-yellow solid, which was isolated
and dried under vacuum (yield 96%). Elem anal. Calcd for
C52H51N5O5P2Pt: C, 57.67; H, 4.76; N, 6.46. Found: C, 57.80;
solution used for single-crystal X-ray analyses, appear to be
an anhydrous product. 1H NMR in CDCl3: δ 7.57-7.20 (c m,
35H, Ph-CN and PPh3), 8.28 (s, 1H, H2), 7.95 (s, 1H, H8), 6.32
(br dd, 1H, N2H), 3.60 (s, 3H, N1CH3). 1H NMR in DMSO-d6:
δ 7.71-7.19 (c m, 35H, Ph-CN and PPh3), 8.26 (s, 1H, H2), 8.15
(s, 1H, H8), 6.29 (br s, 1H, N2H), 3.50 (s, 3H, N1CH3).
3
1
H, 4.85; N, 6.55. H NMR in CDCl3: δ 7.70-7.24 (c m, viz.
3
complex multiplet, 35H, Ph-CN and PPh3), 7.14 (d, JHH
=
7.08, 1H, H6), 6.22 (dd, 3JHH = 7.08, 5JHP = 1.52, 1H, H5), 6.05
(br s, 1H, N2H), 2.76 (s, 3H, N1CH3). 1H NMR in DMSO-d6: δ
7.84-7.22 (c m, 35H, Ph-CN and PPh3), 7.21 (d, 3JHH = 7.75,
1H, H6), 6.12 (d, 3JHH = 7.75, 1H, H5), 6.60 (br s, 1H, N2H),
2.58 (s, 3H, N1CH3). The presence of one molecule of Et2O was
confirmed by 1H NMR.
With a similar procedure, complex 5 was prepared by reacting
cis-[(PMePh2)2Pt(μ-OH)]2(NO3)2 (20 mg, 1.4 ꢀ 10-2 mmol) and
9-MeAd (4.3 mg, 2.8 ꢀ 10-2 mmol) in PhCN (2 mL). The 31P
NMR of the reacting mixture indicated an almost quantita-
tive formation of 5. The 31P NMR in PhCN is an AB multiplet
at δPA = -2.80 (1JPPt = 3242) and δPB = -3.30 (1JPPt = 3222).
The addition of Et2O formed a yellow solid, which was isolated
and further analyzed by 1H and 31P NMR in CDCl3. 1H NMR in
CDCl3: δ 7.68-7.24 (c m, 25H, Ph-CN and PPh2), 8.22 (s, 1H,
H2), 7.94 (s, 1H, H8), 6.12 (br s, 1H, N2H), 3.66 (s, 3H, N1CH3),
2.16 (d, 2JHP = 9.00, 3H, PMe), 1.85 (d, 2JHP = 9.00, 3H, PMe).
5. cis-[(dppe)Pt(acac)]NO3. cis-[(dppe)Pt(μ-OH)]2(NO3)2 (50
mg, 3.7 ꢀ 10-2 mmol) was dissolved in 1 mL of Hacac. The
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 for 24 h (yield 88%). Elem anal.
Calcd for C31H31NO5P2Pt: C, 49.34; H, 4.15; N, 1.86. Found: C,
49.28; H, 4.03; N, 1.90. 1H NMR in CDCl3: δ 7.83-7.50 (c m,
20H, PPh2), 5.67 (s, 1H, CH(acac)), 2.86 (d, 2JHP = 11.30, 4H,
P(CH2)2), 1.98 (s, 6H, CH3 (acac)). 31P{1H} NMR in CDCl3:
singlet at δ 30.26 (1JPPt = 3711). 31P{1H} NMR in Hacac:
singlet at δ 31.41 (1JPPt = 3879).
The 31P{1H} NMR data of complexes 1-5 in different
solvents are reported in Table 2.
2. cis-[(PMePh2)2PtNHdC(Ph){1-MeCy(-2H)}]NO3 (2). A
mixture of cis-[(PMePh2)2Pt(μ-OH)]2(NO3)2 (210 mg, 1.55 ꢀ
10-1 mmol) and 1-MeCy (38.9 mg, 3.1 ꢀ 10-1 mmol) in PhCN
(15 mL) was stirred at room temperature for ca. 2 h. The
addition of Et2O (90 mL) to the resulting solution afforded a
pale-yellow solid, which was isolated by filtration and dried
under vacuum. Dissolution of the crude product in PhCN and
vapor diffusion of Et2O at room temperature afforded crystals
of 2 0.5Et2O suitable for X-ray analyses (yield 79%). Elem anal.
3
Calcd for C40H42N5O4.5P2Pt: C, 52.12; H, 4.60; N, 7.59. Found:
1
C, 52.35; H, 4.52; N, 7.66. H NMR in CD2Cl2: δ 7.65-7.23
(c m, 25H, Ph-CN and PPh2), 6.96 (d, 3JHH = 7.17, 1H, H6),
6.15 (d, 3JHH = 7.17, 1H, H5), 6.07 (br s, 1H, N2H), 2.68 (s, 3H,
N1CH3), 2.01 (br d, 2JHP 8.51, 3H, PMe), 1.86 (d, 2JHP 8.51, 3H,
1
PMe). H NMR in CD2Cl2 (-75 °C): δ 7.86-6.91 (c m, 52H,
=
Ph-CN, H6, and PPh2); the main conformer, δ 6.10 (d, 3JHH
6.43, 1H, H5), 5.88 (br s, 1H, N2H), 2.63 (s, 3H, N1CH3), 1.77
6. cis-[(PPh3)2Pt(acac)]NO3. cis-[(PPh3)2Pt(μ-OH)]2(NO3)2
(42.4 mg, 2.65 ꢀ 10-2 mmol) was suspended in 3 mL of Hacac.
In few minutes, a pale-yellow solution was obtained, which was
further stirred for 2 h at room temperature. The addition of
Et2O (30 mL) afforded a white solid, which was recovered by
filtration, washed several times with Et2O, and dried under
vacuum (yield 82%). Elem anal. Calcd for C41H37NO5P2Pt: C,
(d, 2JHP = 8.91, 3H, PMe), 1.30 (d, 2JHP = 7.65, 3H, PMe); the
2
minor conformer, δ 6.17 (d, JHP = 6.58, 1H, H5), 5.97 (br s,
1H, N2H), 2.58 (s, 3H, N1CH3), 2.70 (d, 2JHP = 9.70, 3H, PMe),
2
2.03 (d, JHP = 8.91, 3H, PMe). 1H NMR in DMSO-d6: δ
7.68-7.30 (c m, 25H, Ph-CN and PPh2), 7.18 (d, 3JHH = 7.62,
1H, H6), 6.11 (d, 3JHH = 7.62, 1H, H5), 6.42 (br s, 1H, N2H),
2.61 (s, 3H, N1CH3), 2.08 (br d, 2JHP = 10.92, 6H, PMe). The
presence of Et2O was confirmed by 1H NMR.
1
55.91; H, 4.24; N, 1.59. Found: C, 55.93; H, 4.30; N, 1.56. H
NMR in CDCl3: δ 7.50-7.26 (c m, 30H, PPh3), 5.59 (s, 1H,
CH(acac)), 1.51 (s, 6H, CH3 (acac)). 31P{1H} NMR in CDCl3:
singlet at δ 8.65 (1JPPt = 3850). 31P{1H} NMR in Hacac: singlet
at δ 9.09 (1JPPt = 3868).
3. cis-[(dppe)PtNHdC(Ph){1-MeCy(-2H)}]NO3 (3). A mix-
ture of cis-[(dppe)Pt(μ-OH)]2(NO3)2 (100 mg, 7.43 ꢀ 10-2
mmol) and 1-MeCy (18.6 mg, 1.49 ꢀ 10-1 mmol) in PhCN
(8 mL) was stirred at room temperature for 1 day. The resulting
yellow solution was filtered to eliminate trace amounts of a dark
solid. The addition of Et2O (70 mL) to the filtrate afforded a
pale-yellow precipitate, which was isolated and dried under
vacuum. The isolated solid corresponds to the formula
cis-[(dppe)PtNHdC(Ph){1-MeCy(-2H)}]NO3 (yield 65%).
Elem anal. Calcd for C38H35N5O4P2Pt: C, 51.70; H, 4.00; N,
7.93. Found: C, 51.92; H, 3.96; N, 7.98. 1H NMR in CD2Cl2: δ
7.78-7.31 (c m, 25H, Ph-CN and PPh2), 7.19 (d, 3JHH = 7.08,
1H, H6), 6.21 (d, 3JHH = 7.08, 1H, H5), 7.01 (br s, 1H, N2H), 2.78
X-ray Structure Determinations. Diffraction data for com-
pounds 2 and 4 were collected at room temperature on a Nonius
˚
DIP-1030H system with Mo KR radiation (λ = 0.710 73 A). Cell
refinement, indexing, and scaling of the data set were carried out
using programs Denzo10 and Scalepack.10 The structures were
solved by direct methods and subsequent Fourier analyses11 and
refined by the full-matrix least-squares method based on F2 with
all observed reflections.11 The ΔF map of 2 revealed a molecule
of diethyl ether located on a 2-fold axis (isotropically refined
atoms, 0.5 occupancy, hydrogen atoms not assigned), while the
nitrate anion in 4 was found to be disordered over two positions
(0.5 occupancy each, isotropically refined atoms with restraints
on N-O bond distances and angles). All the calculations were
performed using WinGX, version 1.80.05.12 Crystal data and
details of refinement are collected in Table 1.
1
(s, 3H, N1CH3), 2.53-2.23 (c m, 4H, P(CH2)2). H NMR in
DMSO-d6: δ 7.71-7.25 (c m, 25H, Ph-CN and PPh2), 7.01 (d,
3JHH = 7.48, 1H, H6), 6.42 (d, 3JHH = 7.48, 1H, H5), 6.53 (br s,
1H, N2H), 2.98 (s, 3H, N1CH3), 2.65-2.31 (c m, 4H, P(CH2)2).
4. cis-[L2PtNHdC(Ph){9-MeAd(-2H)}]NO3 (L = PPh3, 4;
Results and Discussion
PMePh2, 5).
A mixture of cis-[(PPh3)2Pt(μ-OH)]2(NO3)2
(92.1 mg, 5.7 ꢀ 10-2 mmol) and 9-MeAd (17 mg, 1.2 ꢀ 10-1
mmol) in PhCN (5 mL) was stirred at room temperature for ca.
48 h. The addition of Et2O (20 mL) to the resulting solution
afforded a pale-yellow solid, which was isolated by filtration and
dried under vacuum. Dissolution of the crude product in PhCN,
followed by precipitation with Et2O at room temperature,
afforded microcrystals of 4 having the composition cis-[(PPh3)2-
Synthesis of the Complexes cis-[L2PtNHdC(Ph){1-MeCy-
1
(-2H)}]NO3 (L = PPh3, 1; PMePh2, 2; /2dppe, 3) and
cis-[L2PtNHdC(Ph){9-MeAd(-2H)}]NO3 (L = PPh3, 4;
PMePh2, 5). We have recently shown that the hyd-
roxo complex cis-[(PPh3)2Pt(μ-OH)]2(NO3)2 reacts with
PtNHdC(Ph){9-MeAd(-2H)}]NO3 H2O (yield 83%). Elem
3
(10) Otwinowski, Z.; Minor, W. Processing of X-ray Diffraction Data
Collected in Oscillation Mode. In Methods in Enzymology; Carter, C. W., Jr.,
Sweet, R. M., Eds.; Academic Press: New York, 1997; Vol. 276, pp 307-326.
(11) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122.
anal. Calcd for C49H43N7O4P2Pt: C, 56.00; H, 4.13; N, 9.32.
Found: C, 55.88; H, 4.20; N, 9.38. The presence of water in
the isolated solid was confirmed by 1H NMR, although the
crystals, obtained by vapor diffusion of Et2O into a PhCN
(12) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837–838.

2106 Inorganic Chemistry, Vol. 49, No. 5, 2010
Montagner et al.
Table 1. Crystallographic Data and Details of Structural Refinement Parameters
for 2 and 4
mixture obtained immediately after dissolution of the
reagents (Figure 1a) and after 48 h (Figure 1b).
The AB multiplets at δ 9.2 (1JPPt = 3870) and 6.0
(1JPPt = 3104) are attributable to the adeninato complex
cis-[(PPh3)2Pt{9-MeAd(-H),N6N7}]þ formed by deproto-
nation of the exocyclic NH2 group of the nucleobase. This
complex, in fact, is the only product formed when the
condensation reaction is carried out in chlorinated sol-
vents.5 Its dissolution in PhCN causes the appearance of
2 0.5OEt2
3
4
formula
fw
C40H42N5O4.50P2Pt
921.82
C49H41N7O3P2Pt
1032.92
triclinic
P1
10.332(3)
13.470(3)
16.910(4)
92.32(2)
99.71(2)
109.18(3)
2179.2(9)
2
cryst syst
space group
monoclinic
C2/c
29.932(5)
9.882(2)
28.250(4)
˚
a, A
˚
b, A
˚
c, A
new AB multiplets at δ 10.9 (1JPPt = 3308) and 9.9 (1JPPt
=
R, deg
β, deg
γ, deg
3425), due to the product 4, and the reaction appears
complete in several hours at 27 °C.
107.45(3)
3
˚
V, A
Z
7971(2)
8
1.536
3.648
3688
2.18-25.68
17 652
6571
3089
432
0.0867
0.787
0.0421
0.0833
0.916, -0.601
Complex 4 has been isolated as a pure compound and
further characterized by multinuclear NMR in a CDCl3
solution. Attribution of the adenine H2 and H8 reso-
Dcalcd, g cm-3
1.574
3.345
1032
μ, mm-1
F(000)
1
nances is supported by H-15N heteronuclear multiple-
θ range, deg
reflns collected
reflns unique
reflns [I > 2σ(I)]
no. of param
R(int)
1.61-24.71
30 566
6647
bond correlation (HMBC) experiments (see Figure S1
in the Supporting Information). The H2 resonance, at
δ 8.28, correlates with the 15N resonances at δ -202
and -143, whereas the H8 proton (at δ 7.95) correlates
with the 15N resonances at δ -136 and -224. The latter
resonance, in turn, correlates with the N9 methyl protons.
The proton resonance at δ 6.32 appears as a poorly
resolved triplet (or a doublet of doublets) due to coupling
with the 31P nuclei and is attributed to the N2H proton of
the inserted PhCN molecule. In fact, it correlates with the
15N resonance at δ -243, with a 1JNH value of ca. 80 Hz.
Moreover, in the 1H-31P HMBC experiment, the adenine
H2 signal exhibits correlations with the 31P resonances, in
line with the metalation of the nucleobase at the N1
position. A broad resonance for the N2H proton, in the
3559
556
0.0560
0.765
0.0389
0.0724
GOF on F2
R1a
wR2a
residuals
0.803, -0.625
P
P
P
P
a R1 = ||Fo| - |Fc||/ |Fo|, wR2 = [ w(Fo2 - Fc2)2/ w(Fo2)2]1/2
.
1-MeCy, in chlorinated solvents, to give the complex
cis-[(PPh3)2Pt{1-MeCy(-H),N4}(1-MeCy,N3)]NO3 con-
taining a NH2-deprotonated and a neutral cytosine acting
as monodentate ligands through the N4 and N3 atoms,
respectively.13 When the same reaction is carried out
in CH3CN, however, the complex cis-[(PPh3)2PtNHd
C(Me){1-MeCy(-2H)}]NO3 is obtained in quantitative
yield.3 The anionic ligand is the deprotonated form of the
N-(9-methyl-2-oxo-2,3-dihydro-1H-pyrimidin-4-ylidene)-
acetamidine molecule shown in Chart 1 and can be des-
cribed as the result of the coupling of the metal-activated
CH3CN molecule with the N4 exocyclic atom of the N3
coordinated cytosine. Analogous reactions, depicted in
Chart 3, occur in benzonitrile, and the new metallacycles
1-5 have been isolated as pure compounds.
The reaction of cis-[(PPh3)2Pt(μ-OH)]2(NO3)2 with 1-
MeCy is fast at room temperature, being complete upon
dissolution of the reagents in PhCN, as shown by mon-
itoring of the reaction mixture by 31P NMR. The yield of 1
was quantitative, and no intermediates were detectable.
A similar behavior was observed with the hydroxo com-
plexes stabilized by PMePh2 and dppe. Compounds 2 and
3 have been isolated in good yield, and no intermediates
were identified when the condensation reaction in benzo-
nitrile was monitored by 31P NMR.
1
range of δ 6.05-7.01, is observed also in the H NMR
spectra of the 1-MeCy derivatives 1-3. In these com-
plexes, coordination of cytosine at the N3 atom causes a
weak interaction of the pyrimidinic H5 proton with the
phosphine in the trans position, leading to the appearance
of this resonance as a well-resolved doublet of doublets
in 1 (3JH5H6 = 7.08 and 5JHP = 1.52).
It is interesting to note that the reaction with PhCN
(Chart 3) does not occur when the phosphine is PMe2Ph.
Thus, the trinuclear complex cis-[(PMe2Ph)2Pt{9-MeAd-
(-H)}]3(NO3)3 can be quantitatively prepared by react-
ing cis-[(PMe2Ph)2Pt(μ-OH)]2(NO3)2 and 9-MeAd in
CH3CN,14 and it remains unchanged upon dissolution
in PhCN even after 12 h at 50 °C. This lack of reactivity
can be interpreted by assuming a high thermodynamic
stability of the trinuclear species cis-[(PMe2Ph)2Pt-
{nucleobase(-H)}]33þ, which prevents the formation of
the key intermediate cis-[(PMe2Ph)2Pt(NtCR){nucleo-
base(-H)}]þ, susceptible to nucleophilic attack of the N4
cytosine or N6 adenine to the metal-activated nitrile
molecule.
Unlike the MeCN analogues (Chart 2), complexes 1-5
are indefinitely stable in a solution of chlorinated solvents
or DMSO (at least 24 h at 50 °C).
NMR Behavior of Complex 2. As anticipated for 4
(Figure 1b), the 31P NMR spectra of these insertion
products exhibit sharp AB multiplets with well-resolved
195Pt satellites (Table 2 and Figure S2 in the Supporting
Information). Only in the case of the PMePh2 complex 2 is
In contrast, the suspension of 9-MeAd and cis-[(P-
Ph3)2Pt(μ-OH)]2(NO3)2 in PhCN, in ca. 1 h, forms a
pale-yellow solution containing, as the main product,
the intermediate species cis-[(PPh3)2Pt{9-MeAd(-H),
N6N7}]þ, previously characterized.5 This compound, in
several hours at room temperature, reacts quantitatively
with the solvent, affording 4. These transformations are
clearly evidenced in the 31P NMR spectra of the reaction
(13) Longato, B.; Montagner, D.; Zangrando, E. Inorg. Chem. 2006, 45,
8179–8187.
(14) Longato., B.; Pasquato, L.; Mucci, A.; Schenetti, L. Eur. J. Inorg.
Chem. 2003, 128–137.

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2107
Chart 3
Figure 2. 31P{1H} NMR spectra of 2 in CD2Cl2: (a) at room tempera-
ture; (b) at -75 °C.
Table 2. 31P NMR Value of Complexes 1-5 in Different Solvents and at
Different Temperatures (δ in ppm and J in Hz)
Figure 1. 31P{1H} NMR spectra (central part) of the reaction between
cis-[(PPh3)2Pt(μ-OH)]2(NO3)2 (*) and 9-MeAd in benzonitrile (D2O as
the inset) at 27 °C: (a) after 2 h; (b) after 48 h at room temperature. (9)
Intermediate complex cis-[(PPh3)2Pt{9-MeAd(-H),N6N7}]NO3. (b) In-
sertion product 4.
compound
solvent (K)
PA (1JPPt
)
PB (1JPPt
)
2JPP
1
1
2
2
CDCl3 (298) 9.03 (3486)
DMSO-d6 (298) 8.22 (3509)
7.77 (3417) 24.9
7.85 (3443) 25.2
CD2Cl2 (298)
CD2Cl2 (223)
-5.14 (3306) -9.27 (3340) 27.8
-3.27 (3283) -5.32 (3277) 28.8 [56%]
-5.95 (3364) -13.41 (3291) 25.9 [44%]
the high-field component observed as a broad signal at
ambient temperature, whereas the low-field resonances
are sharp, as shown in Figure 2a.
2
3
3
4
4
5
DMSO-d6 (298) -4.38 (3354) -8.66 (3301) 26.1
CD2Cl2 (298)
DMSO-d6 (298) 38.45 (3384)
CDCl3 (298) 10.95 (3263)
DMSO-d6 (298) 9.96 (3347)
CDCl3 (298)
39.89 (3358)
35.57 (3408) 11.5
37.26 (3308) 7.17
10.70 (3370) 24.5
9.28 (3436) 25.5
At -75 °C, two sharp AB spin systems are detected
(Figure 2b) whose spectroscopic parameters are collected
in Table 2. Similarly, the 1H NMR spectrum of 2 at room
temperature exhibits a sharp doublet for the methyl
protons of one phosphine (at δ 1.86, 2JHP = 8.51) flanked
by 195Pt satellites (3JHPt = ca. 30) and a broader one (at δ
2.01, 2JHP = 8.51). At low temperature (-75 °C), two sets
of resonances of comparable relative intensities for each
phosphine ligand are detected, as reported in the Experi-
mental Section. A very similar behavior was also ob-
served for the pyrimidinic resonances.
-2.97 (3240) -3.45 (3149) 27.3
These spectral changes are consistent with the occur-
rence of an equilibrium between preferred conforma-
tional arrangements related to a partially restricted
rotation around the Pt-P bonds. These two conforma-
tions arise from the position of the phosphine substituents
with respect to the two sides of the coordination plane of

2108 Inorganic Chemistry, Vol. 49, No. 5, 2010
Montagner et al.
Figure 5. ORTEP drawing (40% ellipsoid probability) of the complex
cation of 4.
Figure 3. 1H-31P NMR HOESY spectrum of 2 at -70 °C in CD2Cl2.
The circles indicate pure dipolar correlations, with the other cross-peaks
being heteronuclear COSY scalar correlations.
˚
Table 3. Selected Bond Lengths (A) and Angles (deg) for 2 and 4
2
4
Pt-N3
Pt-N2
Pt-P1
2.112(7)
2.043(6)
2.281(2)
2.271(2)
1.339(10)
1.342(10)
1.302(10)
1.327(10)
Pt-N1
Pt-N2
Pt-P1
2.116(6)
2.010(6)
2.313(2)
2.280(2)
1.388(9)
1.341(9)
1.308(9)
1.295(8)
Pt-P2
Pt-P2
N3-C4
C4-N4
N4-C3
C3-N2
N1-C6
C6-N6
N6-C3
C3-N2
N3-Pt-N2
N3-Pt-P1
N3-Pt-P2
N2-Pt-P1
N2-Pt-P2
P1-Pt-P2
N3-C4-N4
C4-N4-C3
N4-C3-N2
83.2(3)
N1-Pt-N2
N1-Pt-P1
N1-Pt-P2
N2-Pt-P1
N2-Pt-P2
P1-Pt-P2
N1-C6-N6
C6-N6-C3
N6-C3-N2
84.5(3)
94.73(19)
172.38(18)
169.2(2)
90.0(2)
92.58(9)
127.5(9)
122.8(8)
126.7(8)
93.80(19)
172.7(2)
172.55(19)
88.23(18)
93.33(8)
127.2(8)
124.0(7)
126.5(8)
Moreover, the interconversion between the conformers is
confirmed by the presence of pure dipolar correlations in
the 31P-1H NMR HOESY spectrum shown in Figure 3,
together with the expected heteronuclear 31P-1H NMR
scalar correlations (Figure S3 in the Supporting In-
formation).
Figure 4. ORTEP drawing (40% ellipsoid probability) of the complex
cation of 2.
the complex: i.e., one presents two phenyls on one side
and the methyl on the other, and the second one would
have a phenyl and a methyl on the same side, leaving a
phenyl on the other. At -75 °C, the molar ratio between
the two conformers is measured to be 56/44. The differ-
ence likely arises from a steric interaction of the oxygen
atom of 1-MeCy with the ipso carbon of one phenyl ring
of the phosphine located in trans to the inserted PhCN
molecule and with the phosphorus atom of the same
phosphine (see the structure discussion below).
It is interesting to note that the previously reported
acetonitrile analogue showed identical NMR behavior.3
Crystal and Molecular Structures of 2 and 4. The X-ray
structural determinations of 2 and 4 show that the inser-
tion of a PhCN molecule into the cytosine Pt-N4 and
adenine Pt-N6 bonds had occurred with the formation
of a six-membered ring, as depicted in Figures 4 and 5,
respectively. Selected bond distances and angles are col-
lected in Table 3.
In the two adducts, the Pt is bound to the nucleobase N
donor site, the inserted benzonitrile nitrogen N2, com-
pleting the square-planar coordination through P donors.
The Pt-N(nucleobase) bond lengths are 2.112(7) and
The different line-width values observed at room tem-
perature (Figure 2a) are related to the different Δδ
values exhibited by the two AB multiplets (Figure 2b).

Article
Inorganic Chemistry, Vol. 49, No. 5, 2010 2109
Figure 6. Side views of 2 (left) and 4 (right) showing the conformation of the nucleobase benzimidamide with respect to the coordination plane.
˚
2.116(6) A in 2 and 4, respectively, while Pt-N2 bond
lengths, of 2.043(6) and 2.010(6) A, are slightly shorter.
this interaction (although not observed in the solid state
likely for the packing requirement) is also feasible
in complex 2 (distance between C8-H and centroid C33
˚
The Pt-P bond distances fall in the range from 2.271(2) to
2.313(2) A. These values are comparable to those mea-
˚
˚
ring = 3.06 A).
sured in the acetonitrile derivatives.3 The N2P2 donors
show a slight tetrahedral distortion in 2, with deviations
The difference Fourier maps allow one, in both struc-
tural determinations, to prudently fix the proton on the
benzonitrile nitrogen N2 as determined through NMR.
Reaction of cis-[(PPh3)2PtNHdC(R){9-MeAd(-2H)}]þ
(R = Ph, Me) and cis-[(dppe)PtNHdC(Ph){1-MeCy-
(-2H)}]þ with Hacac. In order to characterize the benzi-
midamide ligands, the reactivity of the new complexes
toward Hacac has been investigated. This weakly acidic
molecule (pKa = 8.95)16 has well-established chelating
properties in its deprotonated form (acac-),17 and it is
expected to replace the anionic ligand in 1-5. In fact,
complex 4 dissolvesin pure Hacac in a few minutesat room
temperature to give a pale-yellow solution. Its 31P NMR
spectrum exhibits two signals at δ 11.58 and 9.09, flanked
by 195Pt satellites. The lower field signal, observed as an
apparent singlet having a relative intensity of 60% with a
rather large line width, is attributable to the unreacted
complex 4. The sharp singlet at δ 9.09 is due to the reaction
product, namely, the complex cis-[(PPh3)2Pt(acac)]NO3,
which contains the chelating acetylacetonate ligand. After
2 days at room temperature, this is the only detectable
signal, indicating that the anionic ligand in 4 has
been quantitatively replaced by acac-. The compound
cis-[(PPh3)2Pt(acac)]NO3, previously characterized as a
BPh4 salt,18 can be quantitatively prepared by dissolving
the hydroxo complex cis-[(PPh3)2Pt(μ-OH)]2(NO3)2 in
Hacac, where it reacts immediately (see the Experimental
Section). The complete characterization of the organic
moiety NHdC(Ph){9-MeAd(-H)} resulting from the
ligand exchange will be reported elsewhere.19 It can be
anticipated that 1H NMR analysis of the solid-state
residue and of the liquid components of the reaction
mixture, separated under vacuum and dissolved in
DMSO-d6 and CDCl3, respectively, rules out the presence
of free 9-MeAd and benzonitrile. Similarly, the acetoni-
trile analogue of 4, cis-[(PPh3)2PtNHdC(Me){9-MeAd-
(-2H)}]NO3,3 reacts easily in pure Hacac to give
quantitatively cis-[(PPh3)2Pt(acac)]NO3, as shown by the
˚
of (0.125 A from their mean plane, while milder defor-
mations ((0.056 A) are measured in 4. Figure 6 displays a
˚
side view of the complexes, showing the conformation
assumed by the nucleobase benzimidamide moiety with
respect to the coordination plane. The dihedral angle
formed by the nucleobase ring with the N2P2 coordina-
tion plane is of 36.5(2)° and 27.3(2)° in 2 and 4, respec-
tively. The phenyl from benzonitrile is almost coplanar
with the cytosine ring in 2, while it is tilted with respect to
the adenine in 4. Inside the six-membered-ring fragment,
the geometrical parameters are comparable in the two
cases and indicate an electron delocalization. The more
apparent difference is observed for the C3-N2 bond
˚
distance, which is 1.327(10) and 1.295(8) A in 2 and 4,
respectively. Making allowance for the estimated stan-
dard deviations, the value measured in 2 is also longer
than the distances measured in the crystal structures with
3
˚
acetonitrile, viz., 1.279(11)-1.295(7) A.
In complex 2, the short distance measured between the
1-MeCy oxygen atom with phosphorus P1 and with the
˚
ipso carbon C14 of the phosphine (2.88 and 2.92 A,
respectively) is an indication of a weak intramolecular
bonding interaction, and it gives support to the observed
behavior in solution (see the NMR results) for this species.
The corresponding values in the acetonitrile derivative
were found to be 3.08 and 3.00 A, respectively. On the
other hand, complex 4 shows an intramolecular stacking
˚
between the phenyl rings C28 and C40 of PPh3 [distance
˚
between centroids of 3.731(6) A; dihedral angle 6.4(5)°],
while the stacking between the six-membered adenine ring
˚
and phenyl C16 appears marginal [3.924(6) A; 37.5(5)°].
Finally, an edge-to-face CH π interaction is observed
between the hydrogen atom C11-H and ring C46
(H-phenyl centroid distance = 2.87 A). This kind of
stabilizing interaction, the so-called π-hydrogen bond,
was sometimes observed in organometallic complexes15
and could support the observed stability of the benzoni-
trilewithrespect to the acetonitrile derivatives. In principle,
3 3 3
˚
(16) Leipoldt, J. G.; Grobler, E. C. Transition Met. Chem. 1986, 11, 110–
112.
(17) De Pascali, S. A.; Papadia, P.; Ciccarese, A.; Pacifico, C.; Fanizzi,
F. P. Eur. J. Inorg. Chem. 2005, 788–796.
(18) Ito, T.; Kiriyama, T.; Yamamoto, A. Bull. Chem. Soc. Jpn. 1976, 49,
3250–3256.
(19) Montagner, D.; Zangrando, E.; Longato, B. Manuscript in prepara-
tion.
(15) (a) Ceccon, A.; Bisello, A.; Crociani, L.; Gambaro, A.; Ganis, P.;
Manoli, F.; Santi, S.; Venzo, A. J. Organomet. Chem. 2000, 600, 94–111.
(b) Bisello, A.; Ceccon, A.; Gambaro, A.; Ganis, P.; Manoli, F.; Santi, S.; Venzo,
A. J. Organomet. Chem. 2000, 593-594, 315–324. (c) Ganis, P.; Ceccon, A.;
€
Kohler; Manoli, F.; Santi, S.; Venzo, A. Inorg. Chem. Commun. 1998, 1, 15–18.

2110 Inorganic Chemistry, Vol. 49, No. 5, 2010
Montagner et al.
31P NMR spectrum recorded immediately after dissolu-
tion of the complex.
nucleobase complexes are described. The compounds cis-[L2-
PtNHdC(Ph){9-MeAd(-2H)}]þ and cis-[L2PtNHdC(Ph)-
{1-MeCy(-2H)}]þ have been obtained when the neutral
ligands L are PPh3, PMePh2, and dppe but not with the
more basic phosphine PMe2Ph.
Unlike the acetonitrile analogues, which release CH3CN in
DMSO or chlorinated solvents,3 the benzonitrile derivatives
here reported are indefinitely stable in solution. The higher
thermodynamic stability of these substituted benzamidine
complexes could be ascribed (i) to a more extended π-electron
delocalization in the organic ligand due to the presence of the
A different behavior was observed for complex 3.
Even though it dissolves easily in Hacac, protonation
of the coordinated ligand NHdC(Ph){1-MeCy(-2H)}
occurs only to a minor extent. In fact, the 31P NMR
spectrum of the resulting solution shows AB multi-
plets of 3 [at δ 40.7 (1JPPt = 3347) and 36.35 (1JPPt
=
3386)] and a singlet at δ 31.41 flanked by 195Pt satellites
(1JPPt = 3879 Hz), having a relative intensity ratio of 4:1.
This latter signal is clearly attributable to cis-[(dppe)Pt-
(acac)]NO3 (see the Experimental Section). The spec-
trum does not change after several days, indicating
that the reaction mixture is at equilibrium. If Hacac
is replaced by the more acidic hexafluoroacetylacet
one (pKa = 4.35),16 the equilibrium appears strongly
shifted toward protonation of 3. In this case, the relative
abundance of 3 and cis-[(dppe)Pt(hfacac)]NO3 was
1:1.5. The latter species, observed in the 31P NMR
phenyl group and (ii) to an edge-to-face CH π interaction
3 3 3
betweena PhCNphenyl hydrogenwith onePPh3 phenylring,
clearly evidenced in the X-ray structure of 4.
The anionic ligands in cis-[(PPh3)2PtNHdC(R){9-MeAd-
(-2H)}]NO3 (R = Me, Ph) can be quantitatively protonated
by Hacac, leading to formation of the free amidines NHdC-
(R){9-MeAd(-H)} and the acetylacetonate complex cis-[-
(PPh3)2Pt(acac)]NO3. The incomplete protonation of the
anionic ligand in 3 by Hacac or hfHacac is likely due to
electronic effects related to the presence of dppe, which is
more basic in comparison with the PPh3 ligand.
1
spectrum as a singlet at δ 37.12 with JPPt = 3860 Hz,
is quantitatively formed when cis-[(dppe)Pt(μ-OH)]2-
(NO3)2 is dissolved in hfHacac.
Conclusions
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.
The first examples of azametallacycles resulting on
the formal insertion of benzonitrile into PtII-N bonds of