Role of the ancillary ligands on the stabilization of the imino-oxo tautomer of 1-methylcytosine in PtII complexes

Article abstract of DOI:10.1039/b818514j

The mixed nucleobases complexes cis-[L₂Pt{1-MeTy(-H)}(1-MeCy, N³)]NO₃ (L = PPh₃, 1a; PMePh₂, 1b), containing the N(3)-deprotonated 1-methylthymine (1-MeTy(-H)) and the neutral 1-methylcytosine (1-MeCy) have been prepared and characterised. The compounds were obtained by reacting the hydroxo complexes cis-[L₂Pt(μ-OH)] ₂(NO₃)₂ with 1-methylthymine (1-MeTy), followed by the addition of 1 equivalent of 1-MeCy. In solution of DMSO, DMF or chlorinated solvents, 1a converts quantitatively into the isomer cis-[L ₂Pt{1-MeTy(-H)}(1-MeCy,N⁴)]NO₃ (2a) containing the tautomeric form of the cytosine stabilized through the coordination at the N(4) atom, as shown by single-crystal X-ray analysis. The structural determination of 2a shows the presence in the unit cell of two crystallographic independent complexes having similar conformation, with a different orientation of the two nucleobases (head-head and head-tail) according to the presence of both isomers in solution. Complex 1b, having the less hindered PMePh₂ ligands, in DMSO solution, contains the tautomeric forms of the cytosine in equilibrium and the migration of the metal from the N(3) to N(4) site occurs only to a minor extent. The Royal Society of Chemistry 2009.

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Role of the ancillary ligands on the stabilization of the imino-oxo tautomer
of 1-methylcytosine in PtII complexes†
Bruno Longato,*a Diego Montagnera and Ennio Zangrandob
Received 20th October 2008, Accepted 24th December 2008
First published as an Advance Article on the web 12th February 2009
DOI: 10.1039/b818514j
The mixed nucleobases complexes cis-[L2Pt{1-MeTy(-H)}(1-MeCy,N3)]NO3 (L = PPh3, 1a; PMePh2,
1b), containing the N(3)-deprotonated 1-methylthymine (1-MeTy(-H)) and the neutral
1-methylcytosine (1-MeCy) have been prepared and characterised. The compounds were obtained by
reacting the hydroxo complexes cis-[L2Pt(m-OH)]2(NO3)2 with 1-methylthymine (1-MeTy), followed by
the addition of 1 equivalent of 1-MeCy. In solution of DMSO, DMF or chlorinated solvents, 1a
converts quantitatively into the isomer cis-[L2Pt{1-MeTy(-H)}(1-MeCy,N4)]NO3 (2a) containing the
tautomeric form of the cytosine stabilized through the coordination at the N(4) atom, as shown by
single-crystal X-ray analysis. The structural determination of 2a shows the presence in the unit cell of
two crystallographic independent complexes having similar conformation, with a different orientation
of the two nucleobases (head–head and head–tail) according to the presence of both isomers in
solution. Complex 1b, having the less hindered PMePh2 ligands, in DMSO solution, contains the
tautomeric forms of the cytosine in equilibrium and the migration of the metal from the N(3) to N(4)
site occurs only to a minor extent.
We have recently shown that the N(3)-bonded cytosine molecule
Introduction
in the mixed nucleobases complex cis-[(PPh3)2Pt{1-MeTy(-H)}(1-
MeCy,N3)]+ (1a) slowly rearranges into its more stable tautomeric
The usual metal binding site of the model nucleobase 1-
methylcytosine (1-MeCy) is the N(3) atom.1 However, the sta-
bilization of the tautomeric form of this molecule through the
coordination at the exocyclic N(4), in particular at PtII centers, have
derivative cis-[(PPh3)2Pt{1-MeTy(-H)}(1-MeCy,N4)]+ (2a).4
In this paper we report the structural characterisation of
this compound by single-crystal X-ray analysis showing that
been well documented.2 In all these cases, the initial coordination
the binding mode of the cytosine, previously established in
of the metal occurs at the N(3) site, followed by the metal migration
solution by multinuclear NMR techniques, is maintained in
at the N(4) position. Such isomerisation implies the shift of one
the solid state. Moreover, the role of the PPh3 ligands in the
of the exocyclic NH2 protons to the endocyclic N(3) atom of the
cytosine ligand, as shown in Scheme 1.3
stabilization of the cytosine iminooxo tautomer has been further
investigated by preparing the complex cis-[(PMePh2)2Pt{1-MeTy(-
H)}(1-MeCy,N3)]+, 1b, containing the less hindered PMePh2
ligands. This simple change of the metal coordination sphere
strongly effects the relative stability of the tautomeric forms of
the cytosine ligand since the platination at the N(3) site appears
largely predominant in a DMSO solution of 1b.
Experimental
Synthesis and materials
cis-[(PMePh2)2Pt(m-OH)]2(NO3)2 and 1-MeCy6 were prepared as
5
previously reported. cis-[(PPh3)2Pt{1-MeTy(-H)}(1-MeCy,N4)]-
Scheme 1
NO3 was prepared as described in reference [4] and the sample
for X-ray analysis was obtained by slow diffusion of Et2O into a
DMF solution of the compound, at room temperature. 1-MeTy
and all the solvents (CH2Cl2, DMF, DMSO-d6, CDCl3, Et2O) were
Aldrich products.
aDipartimento di Scienze Chimiche, Universita` di Padova, Via Marzolo
1, 35131, Padova, Italy. E-mail: bruno.longato@unipd.it; Tel: +39 049
8275197; Fax: +39 049 8275161
bDipartimento di Scienze Chimiche, Universita` di Trieste, Via Giorgieri 1,
34127, Trieste, Italy
cis-(PMePh2)2Pt{1-MeTy(-H)}(ONO2). To a solution of cis-
[(PMePh2)2Pt(m-OH)]2(NO3)2 (48.6 mg, 3.6·10-2 mmol) in CH2Cl2
(3 cm3) 1-MeTy (11.0 mg, 7.8·10-2 mmol) was added, and the
suspension stirred at room temperature for ca. 12 h. Addition
of pentane (25 cm3) to the resulting solution afforded a white
† Electronic supplementary information (ESI) available: 15N–1H HMBC
spectrum of 1b. CCDC reference number 705857 for compound 2a. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/b818514j
2400 | Dalton Trans., 2009, 2400–2405
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solid which was isolated and dried under vacuum. Purification
of the solid from CHCl3, by vapour diffusion of Et2O at
room temperature, afforded crystals having the composition cis-
(PMePh2)2Pt{1-MeTy(-H)}(ONO2)·1H2O·1/4CH2Cl2. (50.4 mg,
85%). (Found: C, 46.46; H, 4.02; N, 5.00. C32.25H35.5N3Cl0.5O6P2Pt
requires C, 46.34; H, 4.29; N, 5.02). dH (300.13 MHz; CDCl3;
Me4Si) 7.69-7.23 (20H, cm, PPh2), 6.38 (1H, s, H6), 3.03 (3H, s,
NCH3), 1.89 (3H, d, 2JHP 11.6, PMe), 1.80 (3H, d, 2JHP 11.6, PMe),
1.65 (3H, s, CCH3); dH (300.13 MHz; DMSO-d6; Me4Si) 7.64-7.25
(20H, cm, PPh2), 6.84 (1H, s, H6), 2.96 (3H, s, NCH3), 2.09 (3H,
d, 2JHP 12, PMe), 1.75 (3H, d, 2JHP 12, PMe), 1.50 s (3H, s, CCH3).
ences were H3PO4 (85 w/w in D2O) for 31P, and CH3NO2 (in CDCl3
at 50% w/w) for 15N. Inverse detected spectra were obtained
through heteronuclear multiple bond correlation (HMBC) exper-
iments, using parameters similar to those previously reported.7
X-Ray structure determination
Diffraction data for compound 2a were collected at room tem-
perature on a Nonius DIP-1030H system with Mo-Ka radiation
˚
(l = 0.71073 A). Cell refinement, indexing and scaling of the data
set were carried out using programs Denzo8 and Scalepack.8 The
structure was solved by direct method and subsequent Fourier
analyses9 and refined by the full-matrix least-squares method
based on F2 with all observed reflections.9 A residual in the DF
map was interpreted as a lattice water oxygen (hydrogen atoms not
located). All the calculations were performed using the WinGX
System, Ver 1.70.01.10
1
dP (121.5 MHz; CDCl3) AB multiplet -5.32 (1P, d, JPPt 3284),
1
2
-13.74 (1P, d, JPPt 4062, JPP 23.1). dP (121.5 MHz; DMSO-d6)
1
1
AB multiplet -2.58 (1P, d, JPPt 3241), -11.17 (1P, d, JPPt 4108,
2JPP = 24.3).
cis-[(PMePh2)2Pt{1-MeTy(-H)}(1-MeCy,N3)]NO3 (1b). To a
solution of cis-(PMePh2)2Pt{1-MeTy(-H)}(ONO2) (31 mg,
3.8·10-2 mmol) in 3 cm3 of CH2Cl2 was added 1-MeCy (5.0 mg,
3.9·10-2 mmol) which, under stirring at room temperature,
dissolved in a few h. Addition of Et2O afforded a white
solid that after filtration, washing with Et2O and dried under
vacuum had the composition cis-[(PMePh2)2Pt{1-MeTy(-H)}(1-
MeCy,N3)]NO3·1H2O·1/4CH2Cl2 (24.9 mg, 71%). (Found: C,
46.12; H, 4.08; N, 8.44. C37,25H42,5N6O7Cl0,5P2Pt requires C, 46.55;
H, 4.47; N, 8.74). dH (300.13 MHz; CDCl3; Me4Si) 7.86-7.21 (20H,
cm, PPh2), 1.34 (3H, d, 2JHP 11, PCH3), 1.32 (3H, d, 2JHP 11, PCH3).
Main conformer (1-MeTy(-H) resonances): 6.31 (1H, s, H6), 2.99
(3H, s, NCH3), 1.56 (3H, s, CCH3); (1-MeCy resonances): 8.69
(1H, br s, NH), 8.60 (1H, br s, NH), 6.63 (1H, d, J 7.1, H6),
6.06 (1H, d, J 7.1, H5), 2.98 (3H, s, NCH3). Minor conformer:
(1-MeTy(-H) resonances): 6.27 (1H, s, H6), 2.85 (3H, s, NCH3),
1.58 (3H, s, CCH3), (1-MeCy resonances): 8.75 (1H, br s, NH),
8.63 (1H, br s, NH), 6.61 (1H, d, J 7.2, H6), 6.10 (1H, d, J 7.2,
H5), 3.02 (3H, s, NCH3). dH (300.13 MHz; DMSO-d6; Me4Si) 8.01-
7.39 (20H, cm, PPh2), 1.64 (3H, d, 2JHP 10.3, PCH3), 1.63 (3H, d,
2JHP 10.3, PCH3). Main conformer (1-MeTy(-H) resonances): 6.72
(1H, s, H6), 2.89 (3H, s, NCH3), 1.41 (3H, s, CCH3); (1-MeCy
resonances): 8.75 (1H, br s, NH), 8.37 (1H, br s, NH), 7.21 (1H,
d, J 7.1, H6), 5.42 (1H, d, J 7.1, H5), 2.93 (3H, s, NCH3). Minor
conformer: (1-MeTy(-H) resonances): 6.68 (1H, s, H6), 2.89 (3H,
s, NCH3), 1.38 (3H, s, CCH3), (1-MeCy resonances): 8.68 (1H, br
s, NH), 8.37 (1H, br s, NH), 7.21 (1H, d, J 7.1, H6), 5.42 (1H, d,
J 7.1, H5), 2.93 (3H, s, NCH3). dP (121.5 MHz; CDCl3), minor
conformer: AB multiplet -11.48 (1P, d, 1JPPt 3249), -13.17 (1P, d,
Crystal data of 2a·0.5(H2O): C47H45N6O6.50P2Pt, M = 1054.92,
¯
triclinic, space group P1, a = 14.868(4), b = 17.155(4), c =
◦
˚
22.120(5) A, a = 105.94(2), b = 96.01(3), g = 112.59(3) , V =
3
3
-1
˚
4866(2) A , Z = 4, Dc = 1.440 g/cm , m(Mo-Ka) = 3.002 mm ,
F(000) = 2116, q range = 1.53–25.35◦. Final R1 = 0.0521, wR2 =
0.1262, S = 0.768 for 1086 parameters and 58142 reflections, 15539
unique [R(int) = 0.0674], of which 6194 with I > 2s(I), max
-3
˚
positive and negative peaks in DF map 1.101, -0.776 e·A .
Results and discussion
Crystal and molecular structure of
cis-[(PPh3)2Pt{1-MeTy(-H)}(1-MeCy, N4)]NO3 (2a)
We have recently shown that cis-[(PPh3)2Pt(m-OH)]2(NO3)2 re-
acts with 1-MeTy, in CH2Cl2, DMF or CH3CN, affording the
neutral complex cis-(PPh3)2Pt{1-MeTy(-H)}(ONO2) in which
the thyminato ligand is N(3) platinated and the nitrato group
acts as monodentate ligand.4 Addition of one equivalent of 1-
MeCy leads to the mixed complex cis-[(PPh3)2Pt{1-MeTy(-H)}(1-
MeCy,N3)]NO3 (1a), resulting in the immediate replacement of
the nitrato ligand. The deprotonated 1-MeTy and the neutral 1-
MeCy, are both platinated at the N(3) atom. In a few days at room
temperature in chlorinated solvents, DMSO or DMF, 1a converts
into the isomer cis-[(PPh3)2Pt{1-MeTy(-H)}(1-MeCy,N4)]NO3,
2a, in which the cytosine is coordinated to the metal through
the exocyclic N(4) atom, as shown by multinuclear NMR studies
in solution and now confirmed in the solid state.
The X-ray structural determination of 2a shows the presence
in the unit cell of two crystallographic independent complexes
(A and B), disordered nitrate anions and a lattice water molecule.
The metal ion has a square planar coordination geometry achieved
through the phosphorous atoms and the nitrogen donors of the
nucleobases. The thyminato is bound through the endocyclic N(3)
atom, the cytosine through the deprotonated amino group N(4)
(Fig. 1 and 2 and Table 1).
2
1JPPt 3489, JPP 24.4); main conformer: AB multiplet -11.75 (1P,
d, 1JPPt 3249), -13.17 (1P, d, 1JPPt 3485, 2JPP 24.4). dP (121.5 MHz;
1
DMSO-d6), minor conformer: AB multiplet -9.83 (1P, d, JPPt
1
2
3224), -11.82 (1P, d, JPPt 3495, JPP 24.7); main conformer: AB
1
1
multiplet -10.11 (1P, d, JPPt 3224), -11.88 (1P, d, JPPt 3495,
2JPP 24.3).
NMR measurements
NMR spectra were obtained in solution of various solvents at
298 K, with a Bruker AVANCE 300 MHz for 1H and 31P (operating
at 300.13 and 121.5 MHz, respectively) and a Bruker 400
AMX-WB spectrometer for 15N (operating at 40.6 MHz). d are
in ppm and J in Hz. The 1H chemical shifts were referenced to the
residual impurity of the solvent and to Me4Si. The external refer-
The atoms of the coordination N2P2 plane are almost coplanar
˚
with max deviations of
0.02 A. The two complexes are
conformationally very similar, differing slightly in the orientation
of the nucleobase and phenyl rings. The position of the thymine
N(1) nitrogen atom was not definitely assigned, having a pseudo
two-fold axis that does not allow differentiation on the DFourier
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Fig. 1 ORTEP drawing of complex cation A (head–tail orientation of
the bases) with indication of the intramolecular H-bond and p–p stacking
interactions.
Scheme 2
The bond lengths and angles in the two independent complexes
(not highly accurate) fall in a wide range, comparable within
2–3s (Table 1). These data indicate that the Pt–N(3)(thyminato)
˚
bond distances (2.095(8) and 2.045(9) A) appear shorter with
˚
respect to the Pt–N(4)(cytosine) ones (2.110(9) and 2.089(8) A).
The coordination bond angle C(4c)–N(4c)–Pt(1) at cytosine is also
very similar in the two complexes with a mean value of 127.1(9)◦.
The N(3t)–Pt–N(4c) angle, 87.2(3) and 86.5(3)◦ in complexes A
and B, respectively, is narrower in contrast to the P(1)–Pt–P(2)
one that average to 96.3◦, likely induced by steric requirements.
The thymine base is oriented almost normal to the coordination
mean plane forming a dihedral angle of 82.88◦ (average value in
the two complexes), while the cytosine plane is more bent, and its
Table 1 Coordination bond lengths and angles for the two independent
complexes
Fig. 2 ORTEP drawing of complex cation B (head–head orientation, the
cytosine and thymine bases are labelled as “d” and “u”, respectively).
Complex A
Complex B
map of the endocyclic N(1) and C(5) atoms. Based on the thermal
parameters values and bond distances, we tentatively assigned
the N(1) thymine atom in the two complexes corresponding to
the two possible head-to-head and head-to-tail conformational
isomers present in solution. In the head-to-head (hh) and head-to-
tail (ht) conformations the methyl group on cytosine and thymine
N1 nitrogen atoms lie on the same and on the opposite side,
respectively, of the P2Pt plane (see Scheme 2). On the other hand,
in both molecules the hydrogen at the cytosine N(3) atom (close
to the metal) is indicative of a syn isomer.4
Pt(1)–N(3t)
Pt(1)–N(4c)
Pt(1)–P(1)
Pt(1)–P(2)
N(3t)–Pt(1)–N(4c)
N(3t)–Pt(1)–P(1)
N(3t)–Pt(1)–P(2)
N(4c)–Pt(1)–P(1)
N(4c)–Pt(1)–P(2)
P(1)–Pt(1)–P(2)
C(4c)–N(4c)–Pt(1)
2.095(8)
2.110(9)
2.274(3)
2.286(3)
87.2(3)
2.045(9)
2.089(8)
2.281(3)
2.272(3)
86.5(3)
88.9(2)
89.4(3)
174.8(3)
175.9(2)
87.9(2)
96.13(11)
127.2(9)
174.0(2)
175.9(2)
87.6(2)
96.48(12)
127.0(8)
2402 | Dalton Trans., 2009, 2400–2405
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ring forms a dihedral angle averaging to 78.6(2)◦. This favours the
formation of an intramolecular H bond between the N(3)–H and
the thymine oxygen (see Fig. 1, mean values of N ◊ ◊ ◊ O distances
and N–H ◊ ◊ ◊ O angles of ca. 2.86 A and 161 , respectively).
The species are stabilized by two intramolecular p–p interac-
tions given that two phosphine phenyl rings are oriented to stack
with the model nucleobases. The centroid-to-centroid distance
˚
bond length is significantly longer (2.100(3) A) than the Pt–N(4a)
˚
one (2.061(4) A). Similar to the structure reported here, a N(4)-
H ◊ ◊ ◊ N(3) hydrogen bond occurs between the bases along with
intramolecular p–p stacking, confirming that the PPh3 derivatives
appear stabilized by these interactions.
On the other hand the different crystal packing of cis-
[(PPh3)2Pt(1-MeCy){1-MeCy(-H),N4}]+ (a polymer built by a H-
bonding scheme) and of 2a (pair of molecules, see above) could
explain the differences observed in the coordination distance
values.
◦
˚
˚
is shorter for the phenyl–thymine coupling, ◦3.394(9) A with a
◦
˚
dihedral angle of 14.9 (3.413(9) A and 14.77 in complex B), in
comparison to the values measured for the phenyl–cytosine pair,
◦
◦
˚
˚
of 3.657(10) A and 26.46 (3.887(9) A, 30.39 in complex B). An
additional intramolecular p–p interaction (not indicated in Fig. 1
and 2) is realized between two, almost parallel, phenyl groups.
The crystal packing evidences both the complexes, located near
a center of symmetry, forming pairs of molecules connected by
H-bonds occurring between the N(4)–H cytosine donor with the
thymine oxygen O(2) of the symmetry related species, the N ◊ ◊ ◊ O
Characterisation of cis-[(PMePh2)2Pt{1-MeTy(-H)}-
(1-MeCy,N3)]NO3 (1b)
With a procedure analogous to that used for the PPh3 derivative,4
the thyminate complex cis-(PMePh2)2Pt{1-MeTy(-H)}(ONO2)
was prepared. The 1H and 31P NMR data of the new compound,
in CDCl3 and DMSO, strongly support a structure in which the
nitrate group acts as monodentate ligand, as found in the PPh3
analogue. Addition of one equivalent of 1-MeCy affords the mixed
complex cis-[(PMePh2)2Pt{1-MeTy(-H)}(1-MeCy,N3)]NO3 (1b).
The spectroscopic analysis of the isolated product is consistent
with the presence of the deprotonated 1-MeTy and the neutral
1-MeCy, both platinated at the N(3)atom. Due to the different
orientations of the nucleobases with respect to the metal coordi-
nation plane (Scheme 3), two conformers are expected.
˚
distance being 2.992 A (Fig. 3 and Table 2). A similar arrangement
is also detected for the other independent complex B involving
˚
N(4)–H with O(4) with a more labile interaction of 3.102 A.
Fig. 3 Crystal packing showing the pairing of complexes about an
inversion center with an indication of H-bonds (phosphine phenyl groups
not shown for clarity).
Most of the structural features in complexes A and B are similar
to those previously described in the complex cis-[(PPh3)2Pt(1-
MeCy){1-MeCy(-H),N4}]+,11 although in this species the coor-
dination distances showed a reverse trend, being the Pt–N(3)
Scheme 3
Table 2 Intra- and inter-molecular hydrogen bonds
D–H
d(D–H)
d(H ◊ ◊ ◊ A)
<DHA
d(D ◊ ◊ ◊ A)
A
Symmetry
Complex A
N(3c)–H3c
N(4c)–H4c
0.860
0.860
2.039
2.188
163.63
155.45
2.875
2.992
O(4t)
O(2t)
[-x,-y,-z + 1]
Complex B
N(3d)–H3d
N(4d)–H4d
0.860
0.860
2.020
2.310
158.68
153.18
2.838
3.102
O(2u)
O(4u)
[-x + 1,-y + 1,-z + 2]
Note: In molecule B the cytosine and thymine bases are labelled as “d” and “u”, respectively.
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Fig. 4 31P{ H} NMR spectrum of 1b (central part) in CDCl3 at 25 ◦C.
1
In this Scheme, the methyl group, bound to the N(1) atom in
1-MeCy and 1-MeTy, is oriented in opposite directions in the ht
conformer. Accordingly, the 31P NMR spectrum of 1b in CDCl3 is
characterised by two partially overlapped AB multiplets, of relative
intensities 1.5 : 1 (Fig. 4). Heterocorrelate 31P and 15N experiments
indicate that the two phosphorous resonances at lower field, having
1JPPt = 3249 Hz, are attributable to the phosphine in a transposition
to the thyminate ligand.
Since the formal migration of the metal from N(3) to the N(4)
site of the cytosine occurs quantitatively in 1a, but only to a minor
extent in 1b, the nature of the ancillary ligands plays an important
role. The stabilisation of the cytosine ligand in its unusual imooxo
tautomeric form is clearly favoured by the bulkier PPh3 molecules,
probably for steric reasons. The platination of the cytosine at the
N(4) position, in fact, permits less crowding around the metal
being one of the two pyrimidinic rings relatively further away
from the coordination center.
1
In the H NMR spectrum each nucleobase shows two sets of
resonances having relative intensities of 1.5 : 1, whose attribution
(see Experimental) was obtained through a COSY experiment.
The assignments of the cytosine NH2 protons were obtained
through inverse detected 1H,15N heteronuclear multiple bond
coherence experiments (HMBC). In CDCl3, the NH2 resonances
are observed at d 8.69 and 8.60 ppm for the major conformer, and
at d 8.75 and 8.63 ppm for the other. As shown in Figure A in the
ESI,† the couple of proton signals at lower field correlate with the
same N(4) nucleus (15N at d = -271 ppm), whereas the protons at
d 8.69 and 8.60 ppm correlate with the 15N signal at d = -270 ppm
(1JNH in the range 80–90 Hz).
In this context, it is interesting to note that the diphosphine
analogue, cis-[(dppf)Pt{1-MeTy(-H)}(1-MeCy,N3)]BF4 (dppf =
1,1¢-bis(diphenylphosphino)ferrocene),12 undergoes a similar re-
arrangement of the cytosine in DMF solution, with a large
predominance of the cis-[(dppf)Pt{1-MeTy(-H)}(1-MeCy,N4)]+
species present at the equilibrium. In that case, however, the solid
isolated from the mixture turned out to be the starting complex.
Conclusion
The NMR spectra of 1b in CDCl3, exhibit other very weak
resonances, whose attribution remains uncertain. Similar results
were obtained in DMSO-d6 in which, however, complex 1b appears
in equilibrium with the tautomeric species cis-[(PMePh2)2Pt{1-
MeTy(-H)}(1-MeCy,N4)]+ (2b) as clearly indicated by the presence
of a weak singlet at d 10.50 ppm, attributable to the N(3)H proton
of the N(4)-coordinated cytosine, and by a weak AX multiplet
(dP = -7.05 and -8.42 with 2JPP = 23.4 Hz) in the corresponding
31P NMR spectrum. The relative intensities of the signals indicate
that the iminooxo tautomer is ca. 7% of the isomeric mixture and
its concentration does not change after several weeks at room
temperature.
The X-ray structure of the compound cis-[(PPh3)2Pt{1-
MeTy(-H)}(1-MeCy,N4)]NO3 here reported represents the first
example of a phosphino complex in which the neutral 1-MeCy
exhibits N(4)-coordination to a metal centre. The two crystallo-
graphic independent molecules have been modelled taking into
account the conformational isomers previously characterised in
solution.4 These complexes appear stabilized by strong intramolec-
ular p–p interactions between the pyrimidinic rings and the
phosphine phenyl substituents, allowing the formation of intra-
and intermolecular hydrogen bonds.
The peculiar properties of the PPh3 ligands in the stabilisation
of the cytosine molecule in the iminooxo form stem from the
2404 | Dalton Trans., 2009, 2400–2405
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1998, 4, 397; P. J. Sanz Miguel, P. Lax, M. Willermann and B. Lippert,
Inorg. Chim. Acta, 2004, 357, 4552.
3 B. Lippert, Prog. Inorg. Chem., 1989, 1, 371.
4 D. Montagner, E. Zangrando and B. Longato, Inorg. Chim. Acta, 2009,
362, 725.
following observations: (i) the PMe3 complex, cis-[(PMe3)2Pt{1-
MeTy(-H)}(1-MeCy,N3)]+, in solution slowly rearranges into the
n+
polynuclear cytosinate species cis-[(PMe3)2Pt{1-MeCy(-H)}]n
(n = 2, 3) and free 1-MeTy;4 (ii) the tautomeric equilibrium is
largely shifted toward the usual N(3)-coordination for PMePh2
derivatives, in particular in chlorinated solvents; (iii) the opposite
holds for the dppf analogue of 1a. However, in spite of its
thermodynamic stability, we were unable to isolate the iminooxo
species, cis-[(dppf)Pt{1-MeTy(-H)}(1-MeCy,N4)]+.12
5 B. Longato, G. Bandoli and A. Dolmella, Eur. J. Inorg. Chem., 2004,
1092.
6 T. J. Kistenmacher, M. Rossi, J. P. Caradonna and L. G. Marzilli, Adv.
Mol. Relax. Interact. Processes, 1979, 15, 119.
7 L. Schenetti, A. Mucci and B. Longato, J. Chem. Soc., Dalton Trans.,
1996, 299.
8 Z. Otwinowski, and W. Minor, “Processing of X-ray Diffraction Data
Collected in Oscillation Mode”. in Methods in Enzymology, ed. C. W.
Carter, Jr. and R. M. Sweet, Volume 276, New York, Academic Press,
1997, 307.
9 SHELX97 Programs for Crystal Structure Analysis (Release 97–2), G.
M. Sheldrick, University of Go¨ttingen, Germany, 1998.
10 L. J. Farrugia, J. Appl. Crystallogr., 1999, 32, 837.
11 B. Longato, D. Montagner and E. Zangrando, Inorg. Chem., 2006, 45,
8179.
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