Synthesis and in solution and solid state structural study of intramolecular Pt...H interactions in pentafluorophenyl platinum(II) complexes containing the ligands triazene, formamidine, and 2-aminopyridine

Article abstract of DOI:10.1021/ic050800g

The preparation of the [NBu₄][Pt(C₆F ₅)₃L] complexes (L = triazene, 1; formamidine, 2; 2-aminopyridine, 3) have been carried out. These ligands contain a hydrogen atom, with more or less acidic character, in a position suitable for establishing an intramolecular hydrogen bonding interaction with the metal center. This interaction has been detected in solution for 1; its ¹H NMR spectrum shows that the resonance assignable to this hydrogen has platinum satellites. For 2, this coupling is not observed, and the interaction, if it exists, has to be weaker because of the less acidic character of the hydrogen atom. The 2-aminopyridine ligand is more flexible than the triazene or formamidine, and also in this case, no evidence of the interaction in solution is obtained. Nevertheless, if another potential proton acceptor is present, such as ClO₄^- in [NBu₄]₂[Pt(C ₆F₅)₃(C₅H₆N ₂)](ClO₄) (4), a conventional N-H...O-Cl hydrogen bond is formed. The crystal structures of complexes 1-4 have been determined by X-ray diffraction.

Full text of DOI:10.1021/ic050800g

Inorg. Chem. 2005, 44, 9444−9452
Synthesis and in Solution and Solid State Structural Study of
Intramolecular Pt‚‚‚H Interactions in Pentafluorophenyl Platinum(II)
Complexes Containing the Ligands Triazene, Formamidine, and
2-Aminopyridine
Jose´ M. Casas, Beatriz E. Diosdado, Larry R. Falvello, Juan Fornie´s,* and Antonio Mart´ın
Departamento de Qu´ımica Inorga´nica, Instituto de Ciencia de Materiales de Arago´n,
UniVersidad de Zaragoza-C.S.I.C., E-50009 Zaragoza, Spain
Received May 18, 2005
The preparation of the [NBu4][Pt(C6F5)3L] complexes (L
) triazene, 1; formamidine, 2; 2-aminopyridine, 3) have
been carried out. These ligands contain a hydrogen atom, with more or less acidic character, in a position suitable
for establishing an intramolecular hydrogen bonding interaction with the metal center. This interaction has been
1
detected in solution for 1; its H NMR spectrum shows that the resonance assignable to this hydrogen has platinum
satellites. For 2, this coupling is not observed, and the interaction, if it exists, has to be weaker because of the less
acidic character of the hydrogen atom. The 2-aminopyridine ligand is more flexible than the triazene or formamidine,
and also in this case, no evidence of the interaction in solution is obtained. Nevertheless, if another potential
-
proton acceptor is present, such as ClO4 in [NBu4]2[Pt(C6F5)3(C5H6N2)](ClO4) (4), a conventional N
−H‚‚‚
O
−Cl
hydrogen bond is formed. The crystal structures of complexes 1
−4 have been determined by X-ray diffraction.
Introduction
bond (D ) hydrogen donor atom), and (ii) hydrogen bonding,
in which the metal donates electron density to the hydrogen
atom. These hydrogen bonds are supported by electron-rich
metals, especially in low oxidation states. In fact, all of the
M‚‚‚H-D (D ) hydrogen donor) systems that have been
reported until now involve d8 or d10 metals.10-16
The basic character of the platinum(II) centers makes them
suitable starting materials for preparing complexes containing
such interactions because PtII is an electron-rich center and
its complexes are square planar which enables the close
location of a hydrogen atom above and below the coordina-
tion plane. In addition, the acidic nature of this hydrogen
also favors the interaction.
It is well-known that hydrogen bonding plays a very
important role in several different areas of chemistry such
as crystal engineering, molecular recognition,1,2 molecular
biology and catalysis,3,4 molecular aggregation in the solid
state, and host-guest exchange.5,6
Atoms involved in “traditional” hydrogen bonding belong
to the second and third periods of groups 14-17. However,
metals can also establish interactions with hydrogen atoms
in two different ways:7 (i) agostic interactions,8,9 in which
the metal center receives electron density from the D-H
* To whom correspondence should be addressed. E-mail: juan.fornies@
unizar.es.
(1) Navarro, J. A. R.; Freisinger, E.; Lippert, B. Inorg. Chem. 2000, 39,
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Most of the Pt f H interactions reported so far are
intramolecular.17,18 Van Koten reported a series of complexes
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1974, 96, 754.
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9444 Inorganic Chemistry, Vol. 44, No. 25, 2005
10.1021/ic050800g CCC: $30.25
© 2005 American Chemical Society
Published on Web 11/16/2005

Pentafluorophenyl Platinum(II) Complexes
Chart 1
Free triazene and formamidine ligands have the configura-
tions shown in Chart 1.23-28 The triazene or more frequently
the triazenide ligands are easily coordinated to transition
metals.29-32 However, examples of its coordination to Pt are
scarce.33,34 In addition to this, amidines have rarely been used
as ligands, despite the fact that the first example was prepared
with platinum in 1865,35 and very few crystal structures of
monodentate neutral formamidine complexes are known.35
The neutral 2-aminopyridine ligand is present in a great
number of complexes in which it acts as a monodentate
N-pyridine donor.36-45
containing N-H‚‚‚Pt interactions,17,19 among which, is the
shortest Pt‚‚‚H distance found for an intramolecular interac-
tion (Pt‚‚‚H distance ) 2.11(5) Å, Pt‚‚‚H-N ) 168(4)°). 17
In this case, the hydrogen atom belongs to a cationic
fragment, that enhances its acidic nature. In most of the cases,
the proton donor atom is found to be nitrogen, and there are
only a couple of examples of O-H‚‚‚Pt interactions.18 On
the other hand, fewer intermolecular Pt f H interactions
are known; a classic example is the one found in [NPr4]-
[PtCl4][cis-PtCl2(NH2Me)2].20 The structure of this complex
was determined by neutron diffraction, and a short Pt‚‚‚H
distance of 2.262(11) Å and an N-H‚‚‚Pt angle of 164.4-
(7)° were found. It has been proposed that the presence of
this D-H‚‚‚Pt interaction may facilitate proton transfer to
the platinum center followed by coordination of the conju-
gated base (D-) to the platinum in a oxidative addition
process.16,19,21,22
A Pt f H interaction can easily be understood in terms
similar to those for a classical hydrogen bond. The metal
center has a full orbital with an electron pair that can interact
with an electropositive hydrogen atom, or in other words,
the electron pair is donated to create a 3-center 4-electron
system.16,18
In addition to the electrostatic attraction between the
electropositive H atom and the electron pair on platinum, a
significant covalent contribution to this interaction has been
suggested by theoretical studies.19
The choice of suitable ligands in terms of their structural
features or the electronic properties of their donor atoms is
essential for the synthesis of complexes with Pt f H
interactions. Ligands containing aminic groups in suitable
positions usually yield this type of interaction. Triazene,
formamidine, and 2-aminopyridine (see Chart 1) are nitrogen
donors, and they contain NH groups which can act as proton
donors. The NdE-NHR skeletons of the ligands (circled
in the chart) are quite similar, but a small change can
influence the acidity or the final orientation of the hydrogen
atom and thus can be an important factor in establishing the
Pt f H interaction. They have been used to prepare
complexes of stoichiometry [PtR3L]- (R ) C6F5), and the
results are described in this paper.
Experimental Section
General Methods. C, H, and N analyses were carried out with
a Perkin-Elmer 240B microanalyzer. IR spectra were recorded over
the 4000-200 cm-1 range on a Perkin-Elmer 883 spectrophotometer
1
using Nujol mulls between polyethylene sheets. H and 19F NMR
spectra at room temperature were recorded on a Bru¨ker ARX-300,
AV-400, Unity-300, or Gemini-300 spectrometer in solution of
1
deuterated solvents. H and 19F NMR spectra at variable temper-
atures were recorded in acetone-d6 or CD2Cl2 solutions on a Bru¨ker
ARX-300 spectrometer. Positive and negative ion FAB mass spectra
were recorded on a VG-Autospec spectrometer operating at ca. 30
kV, using the standard cesium ion FAB gun and 3-nitrobenzyl
alcohol as the matrix. [NBu4]2[Pt(C6F5)3Cl],46 [NBu4]2[Pt2(µ-C6F5)2-
48
(C6F5)4],47 and AgClO4 were prepared as described elsewhere.
The triazene, formamidine, and 2-aminopyridine ligands were used
as purchased from Aldrich.
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Inorganic Chemistry, Vol. 44, No. 25, 2005 9445

Casas et al.
Caution: Perchlorate salts are potentially explosive. Only small
amounts have to be handled and with great caution.
Synthesis of [NBu4](x+1)[Pt(C6F5)3(C5H6N2)](ClO4)x [x ) 0 (3),
x ) 1 (4)]. Method A (x ) 0). 2-Aminopyridine (0.020 g 0.212
mmol) was added to a yellow solution of [NBu4]2[Pt2(µ-C6F5)2-
(C6F5)4] (0.200 g, 0.106 mmol) in 20 mL of CH2Cl2. The mixture
was stirred for 1 h. The solvent was evaporated to dryness, and the
residue was treated with n-hexane. The white solid was filtered
and washed with n-hexane. Yield: 80%. Anal. Found (calcd for
C39H42N3F15Pt): C, 45.47 (45.35); H, 4.28 (4.10); N, 4.06 (4.07).
IR (cm-1): C6F5 ν(C-F) 1054 vs, 952 vs; X-sensitive mode49 801
s, 788 m, 772 s; C5H6N2 1630 m, 1590 m, 772 m, ν(N-H) 3511
Synthesis of [NBu4][Pt(C6F5)3(PhNdNNHPh)] (1). AgClO4
(0.063 g 0.302 mmol) was added to a solution of [NBu4]2[Pt(C6F5)3-
Cl] (0.367 g, 0.302 mmol) in acetone (20 mL). The mixture was
stirred at room temperature for 30 min. The precipitated AgCl was
eliminated by filtration and 0.060 g (0.302 mmol) of PhNdNNHPh
were added to the resulting solution. The solvent was evaporated
to dryness; then, 4 mL of OEt2 were added to the oily residue, and
the mixture was evaporated to dryness. The addition of 10 mL of
iPrOH to the residue yielded complex 1 as a yellow solid that was
filtered off and washed with n-hexane. Yield: 75%. Anal. Found
(calcd for C46H47N4F15Pt): C, 48.59 (48.63); H, 4.45 (4.17); N,
4.72 (4.92). IR (cm-1): C6F5 ν(C-C) 1632 m, ν(C-F) 1054 vs,
955 vs; X-sensitive mode49 804 s, 788 w, 771 s; C12H11N3 1599 s,
1316 m, 1244 s, 1170 w, 1157 w, 762 s, 697 m, 686 m, 659 m,
+
m, 3339 m; NBu4 884 w
Method B (x ) 1). AgClO4 (0.256 g, 1.233 mmol) and 0.116 g
(1.233 mmol) of the solid ligand 2-aminopyridine were added to a
solution of [NBu4]2[Pt(C6F5)3Cl] (1.500 g, 1.233 mmol) in acetone
(15 mL). The mixture was stirred at room temperature for 30 min.
The precipitated AgCl was filtered off, and the resulting solution
was evaporated to dryness. The oily yellow residue was treated
+
1
537 m, 504 w, 433 w, ν(N-H) 3061 m; NBu4 883 m. H NMR
(acetone-d6, room temperature): δ (C12H11N3) 13.92 [s, 1H, JPt-H
) 48.3 Hz], 8.61 (d, 2H), 7.62 (m, 4H), 7.49 (t, 2H), 7.33 (m,
2H); (NBu4) 3.54 (m, 8H), 1.91 (m, 8H), 1.51 (sext, 8H), 1.05 (t,
12H). 19F NMR (acetone-d6, 293 K): δ -115.27 [d, 4F, o-F, 3JPt-Fo
i
i
with PrOH. The white solid was filtered and washed with PrOH
and n-hexane. Yield: 78%. Anal. Found (calcd for C55H78N4F15-
ClO4Pt): C, 47.97 (48.05); H, 5.68 (5.71); N, 3.90 (4.07). IR (cm-1):
C6F5 ν(C-F) 1054 vs, 952 vs; X-sensitive mode49 801 s, 788 m,
772 s; C5H6N2 1630 m, 1590 m, 772 m, ν(N-H) 3511 m, 3435 m,
3
) 359 Hz], -117.83 [d, 2F, o-F, JPt-Fo ) 523 Hz], -165.11 (t,
+
-
1
2F, p-F), -166.84 (t, 1F, p-F), -165.63 (m, 4F, m-F), -167.42
3339 m; NBu4 884 w; ClO4 1163 m, 625 s. H NMR (acetone-
d6, 293 K): δ C5H6N2 8.38 [d, 1H, JPt-Ho ) 33.3 Hz], 7.31 (t,
(m, 2F, m-F). 19F NMR (acetone-d6, 208 K): δ -114.98 (d, 2F,
3
3
o-F), -115.27 (d, 2F, o-F), -117.82 [d, 1F, o-F, JPt-Fo ) 593
1H), 6.77 (s, 2H), 6.54 (d, 1H), 6.44 (t, 1H); NBu4 3.45 (m, 16H),
1.83 (m, 16H), 1.43 (sext, 16H), 0.97 (t, 24H). 1H NMR (acetone-
d6, 213 K): δ C5H6N2 8.34 (d, 1H), 7.36 (t, 1H), 7.04 (s, 2H), 6.55
(d, 1H), 6.51 (t, 1H); NBu4 3.42 (m, 16H), 1.76 (m, 16H), 1.35
(sext, 16H), 0.92 (t, 24H). 1H NMR (acetone-d6, 193 K): δ C5H6N2
8.33 (d, 1H), 7.37 (t, 1H), 7.14 (s, 1H), 7.05 (s, 1H), 6.55 (d, 1H),
6.53 (t, 1H); NBu4 3.40 (m, 16H), 1.74 (m, 16H), 1.32 (sext, 16H),
0.90 (t, 24H). 1H, NMR, (CD2Cl2, 293 K): δ C5H6N2 8.40 [d, 1H,
3JPt-Ho ) 34.3 Hz], 7.28 (t, 1H), 6.43 (d + t, 2H), 5.97 (s, 2H);
NBu4 3.11 (m, 16H), 1.61 (m, 16H), 1.41 (sext, 16H), 1.00 (t, 24H).
1H NMR (CD2Cl2, 193 K): δ C5H6N2 8.26 (s, 1H), 7.27 (m, 1H),
6.42 (m, 2H); NBu4 2.99 (m, 16H), 1.48 (m, 16H), 1.27 (sext, 16H),
Hz], -118.02 [d, 1F, o-F, 3JPt-Fo ) 487 Hz], -163.65 (t, 2F, p-F),
-165.39 (t, 1F, p-F), -164.47 (m, 4F, m-F), -166.22 (m, 2F, m-F).
FAB- (m/z): [Pt(C6F5)3(C12H11N3)]- 893.
Complex 1 Isolated with Distilled Water. The preceding
i
reaction, precipitated with distilled water instead PrOH, renders
1
complex 1 containing water molecules that make the H NMR
different, especially for the signal of the NH hydrogen atom which
appears 1.5 ppm lower and does not show platinum satellites.
1H NMR (acetone-d6, room temperature): δ C12H11N3 12.48 (s,
1H), 8.04 (d, 2H), 7.58 (m, 6H), 7.41 (m, 2H); NBu4 3.54 (m, 8H),
1.91 (m, 8H), 1.51 (sext, 8H), 1.05 (t, 12H); H2O 2.95 (s).
Synthesis of [NBu4][Pt(C6F5)3(PhNdCHNHPh)] (2). AgClO4
(0.036 g, 0.173 mmol) was added to a solution of [NBu4]2[Pt(C6F5)3-
Cl] (0.211 g, 0.173 mmol) in thf (15 mL). The mixture was stirred
at room temperature for 30 min. The precipitated AgCl was
eliminated by filtration, and PhNdCHNHPh (0.034 g 0.173 mmol)
was added to the resulting solution. The solvent was evaporated to
1
0.88 (t, 24H). H NMR (CD2Cl2, 178 K): δ C5H6N2 8.21 (s, 1H),
7.24 (d, 1H), 6.93 (s, 1H), 6.39 (m, 2H), 4.95 (s, 1H); NBu4 2.97
(m, 16H), 1.44 (m, 16H), 1.23 (sext, 16H), 0.84 (t, 24H). 19F NMR
3
(acetone-d6, 323 K): δ -116.43 [d, 2F, o-F, JPt-Fo) 536 Hz],
3
-116.68 [d, 4F, o-F, JPt-Fo) 401 Hz], -166.85 (m, 6F, 4m-F +
2p-F), -168.79 (m, 3F, 2m-F + 1p-F). 19F NMR (acetone-d6, 193
3
i
K): δ -114.95 [d, 2F, o-F, JPt-Fo) 403 Hz], -166.27 (d, 1F,
dryness. The residue was treated with PrOH giving complex 2.
o-F), -116.50 (d, 1F, o-F), -117.98 [d, 2F, o-F, 3JPt-Fo) 355 Hz],
-164.99 (m, 6F, 4m-F + 2p-F), -166.85 (m, 3F, 2m-F + 1p-F).
Yield: 32%. Anal. Found (calcd for C47H48N3F15Pt): C, 49.30
(49.73); H, 4.13 (4.26); N, 3.72 (3.70). IR (cm-1): C6F5 ν(C-F)
1054 vs, 954 vs; X-sensitive mode49 803 s, 786 w, 768 s; C13H12N2
1651 vs, 1600 s, 1590 s, 1313 m, 1206 m, 742 w, 695 w, 530 w,
3
19F NMR (CD2Cl2, 313 K): δ -118.07 [d, 2F, o-F, JPt-Fo) 549
Hz], -118.31 [d, 4F, o-F, 3JPt-Fo) 395 Hz], -166.27 (m, 6F, 4m-F
+ 2p-F), -168.13 (m, 3F, 2m-F + 1p-F). 19F NMR (CD2Cl2, 253
K): δ -118.42 (m, 8F, o-F), -165.59 (m, 6F, 4m-F + 2p-F),
-167.42 (m, 3F, 2m-F + 1p-F). 19F NMR (CD2Cl2, 178 K): δ
+
1
504 w; NBu4 883 w. H NMR, (acetone-d6, room temperature):
δ C13H12N2 10.63 (d, 1H), 8.79 [d, 1H, 3JPt-H ) 31.7 Hz], 8.00 (d,
2H), 7.50 (m, 4H), 7.36 (m, 2H), 7.24 (t, 1H), 7.18 (t, 1H); NBu4
3.58 (m, 8H), 1.96 (m, 8H), 1.56 (sext, 8H), 1.09 (t, 12H). 1H NMR
(acetone-d6, 195 K): δ C13H12N2 10.71 (d, 1H), 8.93 (d, 1H), 8.12
(d, 2H), 7.45 (m, 4H), 7.28 (m, 2H), 7.16 (t, 1H), 7.08 (t, 1H);
NBu4 3.41 (m, 8H), 1.75 (m, 8H), 1.31 (sext., 8H), 0.90 (t, 12H).
19F NMR (acetone-d6, room temperature): δ -115.89 [d, 4F, o-F,
3JPt-Fo ) 386 Hz], -117.46 [d, 2F, o-F, 3JPt-Fo ) 560 Hz], -166.85
(m, 6F, 4m-F + 2p-F), -168.72 (m, 3F, 2m-F + 1p-F). 19F NMR
3
-116.85 [d, 2F, o-F, JPt-Fo) 398 Hz], -118.06 (d, 1F, o-F),
3
-119.00 (d, 1F, o-F), -120.05 [d, 2F, o-F, JPt-Fo)) 321 Hz],
-164.48 (m, 6F, 4m-F + 2p-F), -166.37 (m, 3F, 2m-F + 1p-F).
X-ray Structure Determination. Crystal data and other details
of the structural analyses are presented in Table 1. Suitable crystals
were obtained by slow diffusion of n-hexane into CH2Cl2 solutions
of the complexes. Crystals were mounted at the end of glass fibers.
For 1, the unit cell dimensions were initially determined from the
positions of 184 reflections in 60 intensity frames measured at 0.3°
intervals in ω and subsequently refined on the basis of the positions
of the 7687 reflections from the main dataset. For 2, the unit cell
dimensions were initially determined from the positions of 184
reflections in 90 intensity frames measured at 0.3° intervals in ω
3
(acetone-d6,195 K): δ -114.82 [d, 2F, o-F, JPt-Fo ) 347 Hz],
3
-115.36 [d, 2F, o-F, JPt-Fo ) 395 Hz], -116.84 (d, 1F, o-F),
-117.27 (d, 1F, o-F), -165.08 (m, 6F, 4m-F + 2p-F), -166.55
(m, 3F, 2m-F + 1p-F).
(49) Uso´n, R.; Fornie´s, J. AdV. Organomet. Chem. 1988, 28, 188.
9446 Inorganic Chemistry, Vol. 44, No. 25, 2005

Pentafluorophenyl Platinum(II) Complexes
Table 1. Crystal Data and Structure Refinement for [NBu4][Pt(C6F5)3(PhNdNNHPh)] (1), [NBu4][Pt(C6F5)3(PhNdCHNHPh)] (2),
[NBu4][Pt(C6F5)3(C5H6N2)] (3), and [NBu4]2[Pt(C6F5)3(C5H6N2)](ClO4) (4)
1
2
3
4
empirical formula
fw
C46H47F15N4Pt
1135.97
C47H48F15N3Pt
1134.97
C39H42F15N3Pt
1032.85
C55H78ClF15N4O4Pt
1374.75
temp
173(2) K
100 K
293(2) K
298(1) K
wavelength
0.71073 Å
0.71073 Å
0.71073 Å
0.71073 Å
cryst syst, space group
unit cell dimensions
monoclinic, P21/c
a ) 10.2376(6) Å
b ) 18.1822(11) Å
c ) 25.2453(16) Å
â ) 100.8950(10)°
monoclinic, P2(1)/n
a ) 10.2221(7) Å
b ) 18.3393(13) Å
c ) 25.027(3) Å
â ) 101.522(9)°
triclinic, P1h
monoclinic, P21/n
a ) 10.8046(12) Å
b ) 20.285(2) Å
c ) 28.824(3) Å
â ) 95.822(18)°
a ) 10.0918(12) Å
b ) 11.5285(17) Å
c ) 19.238(2) Å
R ) 72.960(12)°
â ) 86.775(13)°
γ ) 68.819(12)°
1992.3(4) Å3
2, 1.722 Mg/m3
3.625 mm-1
vol
4614.5(5) Å3
4, 1.635 Mg/m3
3.139 mm-1
4597.1(7) Å3
4, 1.640 Mg/m3
3.150 mm-1
6284.8(12) A3
4, 1.453 Mg/m3
2.364 mm-1
Z, calculated density
abs coeff
F(000)
2256
2256
1020
2792
cryst size
0.45 × 0.34 × 0.19 mm
0.26 × 0.20 × 0.14 mm
0.45 × 0.35 × 0.25 mm
0.35 × 0.30 × 0.28 mm
GOF on F2
1.008
0.870
1.038
1.112
Final Ra indices [I > 2σ(I)]
R1 ) 0.0417
wR2 ) 0.0968
R1 ) 0.0588
wR2 ) 0.1140
1.95 and -0.89 e‚Å-3
R1 ) 0.0627
wR2 ) 0.1494
R1 ) 0.1044
wR2 ) 0.1912
3.52 and -2.11 e‚Å-3
R1 ) 0.0253
wR2 ) 0.0693
R1 ) 0.0289
wR2 ) 0.0710
0.95 and -0.57 e‚Å-3
R1 ) 0.0721
wR2 ) 0.1299
R1 ) 0.2038
wR2 ) 0.1680
0.96 and -1.10 e‚ Å-3
Ra indices (all data)
largest diff. peak and hole
a R1 ) ∑(|Fo| - |Fc|)/∑ |Fo|; wR2 ) [∑w(Fo - Fc )2/∑w(Fc )2 ]0.5
.
2
2
2
and subsequently refined on the basis of the positions of 7687
reflections from the main data set. For these two structures, the
diffraction frames were integrated using the SAINT package50 and
corrected for absorption with SADABS.51 For 3, the unit cell
dimensions were determined from 25 centered reflections in the
range of 23.0 < 2θ < 31.8°. An absorption correction was applied
based on the 488 azimuthal scan data. For 4, the unit cell dimensions
were determined from 116 centered reflections, including symmetry
equivalents, in the range of 20.4 < 2θ < 28.8°. An absorption
correction was applied based on 440 azimuthal scan data. Lorentz
and polarization corrections were applied for all the structures.
The structures were solved by Patterson and Fourier methods.
All refinements were carried out using the program SHELXL-93
or SHELXL-97.52,53 All non-hydrogen atoms were assigned aniso-
tropic displacement parameters and refined without positional
constraints except as noted below. For 1, 2, and 4, all hydrogen
atoms were constrained to idealized geometries and assigned
isotropic displacement parameters 1.2 times the values of Uiso for
their attached carbon atoms (1.5 times for methyl hydrogen atoms).
For 4, the aminic hydrogen atoms were not included in the model.
For 3, the positions of the hydrogen atoms of the 2-aminopyridine
ligand were found in difference density maps and freely refined
with a common isotropic displacement parameter. In 1, one of the
methyl groups of the NBu4+ cation is disordered over two positions
which were refined with an occupancy of 0.5/0.5. The C-C
distances between these disordered carbon atoms and the adjacent
C atom were AFIX constrained to idealized values. No H atoms
were attached to the disordered C atoms and its adjacent C atom.
In 3, two carbon atoms of the NBu4+ cation (C(38) and C(39)) are
disordered over two positions, which were refined with occupancies
of 0.6/0.4, and their anisotropic displacement parameters were
constrained to be the same. Also, the interatomic distances for this
fragment were DFIX, SAME, SADI restrained to idealized values.
No attempts were made to include the hydrogen attached to these
+
carbon atoms. In 4, some C-C distances of one of the NBu4
cations and the Cl-O distances of the ClO4- anion were restrained
to idealized values. The anisotropic displacement parameters of the
-
four oxygen atoms of the ClO4 anion were constrained to be the
same. Full-matrix least-squares refinement of these models against
F2 converged to the final residual indices given in Table 1.
Results and Discussion
Synthesis and Characterization of [NBu4][Pt(C6F5)3-
(PhNdNNHPh)] (1). Complex 1 was obtained by substitu-
tion of the halide in [NBu4]2[Pt(C6F5)3Cl] for the neutral
triazene ligand using AgClO4 as the halide scavenger (eq
1).
[NBu4]2[Pt(C6F5)3Cl] + AgClO4 + PhNdNNHPh f
[NBu4][Pt(C6F5)3(PhNdNNHPh)] + AgCl + NBu4ClO4
(1)
The IR spectrum of 1 shows a signal at 3061 cm-1
assignable to ν(N-H) of the triazene ligand. The homologous
signal in the free ligand appears at 3203 cm-1. It is known
that hydrogen bonds can be detected by infrared and Raman
spectroscopy mainly because of a decrease of the ν(X-H)
frequency resulting from the weakening of the X-H bond
and the broadening of the band.54 Thus, the IR of 1 seems
to indicate the existence of a hydrogen bonding interaction
involving the amminic hydrogen of the triazene ligand.55
1
The H NMR room-temperature spectrum confirms the
presence of such an interaction with the Pt center (platinum
satellites). Thus, the resonance corresponding to the hydrogen
atom appears in the spectrum of 1 as a singlet at 13.92 ppm,
compared to 11.35 ppm for the free ligand. The displacement
(50) SAINT Integration Software; Siemens Analytical X-ray Instruments
Inc.: Madison, WI, 1994.
(51) Sheldrick, G. M. SADABS, version 2.03; University of Go¨ttingen:
Go¨ttingen, Germany.
(52) Sheldrick, G. M. SHELXL-97, Fortran Program for Crystal Structure
Refinement; University of Go¨ttingen: Go¨ttingen, Germany 1997.
(53) Sheldrick, G. M. SHELXL-93, Program for Crystal Structure Deter-
mination; University of Go¨ttingen: Go¨ttingen, Germany, 1993.
(54) Mart´ın, A. J. Chem. Educ. 1999, 76, 578.
(55) Del Bene, J. E.; Jordan, M. J. T. J. Am. Chem. Soc. 2000, 122, 4794-
4797.
Inorganic Chemistry, Vol. 44, No. 25, 2005 9447

Casas et al.
Scheme 1
cording to this, there are two different types of C6F5 groups,
cis and trans, as expected. The equivalence of the o-fluorine
atoms and the m-fluorine atoms of each C6F5 should be a
consequence of rotation around the Pt-C bond at that
temperature, since the presence of Pt‚‚‚H-N bonds excludes
rotation of the N.
In a variable-temperature 19F NMR study it has been found
that the -117.83 ppm signal from the room-temperature
spectrum, which corresponds to the o-fluorine atoms of the
C6F5 group trans to the triazene ligand, splits into two
doublets at 273 K, showing the inequivalence of the two
o-F at this temperature. The two mutually trans-C6F5 groups
show the same signal pattern as at room temperature, but at
223 K, the two o-fluorine atoms of the mutually trans-C6F5
groups also become inequivalent and generate two doublets
each.
of the signal of the interacting hydrogen to higher frequencies
(∆δ ) 2.57 ppm) is the result of the deshielding caused by
the electron density donated from the platinum center, which
2
is contained in the dz orbital, perpendicular to the coordina-
tion plane.16-18,54 This signal shows platinum satellites [JPt-H
) 48.30 Hz] which is unequivocal evidence of the existence
of a Pt-H interaction in solution.16,18,56,57 The magnitude of
the coupling constant has been related to the strength of the
Pt f H interaction. In the literature, some examples of higher
values of JPt-H can be found for this class of interaction.17,18,58
However, it is necessary to take into consideration that, in
addition to the acidic character of the interacting hydrogen,
another important factor that must be considered is the
rigidity of the skeleton of the ligand that contains the proton
donor fragment. Some ligands are able to force the hydrogen
into an optimal position, above the platinum coordination
Synthesis and Characterization of [NBu4][Pt(C6F5)3-
(PhNdCHNHPh)] (2). Complex 2 is obtained in a manner
similar to that for complex 1, but formamidine was used as
the neutral N-donor ligand.
The formamidine ligand is structurally very similar to
triazene, with a dCH- fragment (which is isolobal with N)
instead of the dN- fragment. Nevertheless, there are
differences in the acidity of the hydrogen atom of the N-H
fragment, which is stronger in the triazene ligand, as reflected
in the 1H NMR spectra of both free ligands. The N-H signal
for triazene (vide supra) appears at 11.35 ppm, while in the
formamidine ligand, it appears at lower frequencies, 9.23
ppm. This difference in the acidity of the hydrogen should
cause a different behavior in the NMR spectra of 2.
2
plane, to establish the interaction with the 5dz orbital. The
complexes for which larger JPt-H values have been reported
contain more rigid ligands than the triazene; thus, they can
force the Pt f H interaction more effectively (Scheme 1).
In some examples with larger JPt-H values, the interacting
hydrogen atom belongs to an OH fragment, which is more
polar than NH, and thus the H is more acidic, which again
favors the interaction.
Thus, the 1H NMR spectrum of 2 (acetone, room temper-
ature or 195 K) shows signals corresponding to the forma-
midine ligand and the NBu4+ cation. The signal for the NH
group appears as a doublet at 10.63 ppm, and no platinum
satellites are observed. This chemical shift represents a ∆δ
of 1.40 ppm toward higher frequencies than in the free ligand
spectrum (cf. ∆δ ) 2.57 ppm, JPt-H ) 48.3 Hz in 1). The
resonance corresponding to C-H appears as a doublet at
8.79 ppm with Pt-H coupling (31.7 Hz). This coupling is
transmitted through the ligand skeleton. The hydrogen atoms
of the phenyl groups appear at aromatic frequencies. The
ortho hydrogen atoms of the phenyl group closer to the
platinum atom show a doublet at 8.00 ppm. The other atoms
of the phenyl groups show signals between 7.50 and 7.08
ppm. These observations do not categorically show that a
Pt f H interaction exists for 2 in solution. The displacement
toward higher frequencies of the signal of the N-H hydrogen
atom is compatible with this interaction, despite the fact that
the magnitude of the displacement is less important than for
1. The most conclusive evidence, which would be the
existence of Pt-H coupling, is not observed. Thus, if a Pt
f H interaction exists in 2, it must be weaker than in 1.
If a solution of complex 1 in acetone-d6 is treated with
water and its H NMR recorded, the signal corresponding
1
to the hydrogen atom that interacts is shifted toward lower
frequency; no platinum satellites are observed in this case.
This experiment suggests that the Pt f H interaction is
disrupted and new hydrogen bonds are formed with water
molecules.
The two phenyl groups of the triazene ligand are inequiva-
1
1
lent in the H NMR spectrum. On the basis of a H-1H
COSY experiment all signals have been assigned, even if
some of them appear overlapped. The resonance correspond-
ing to the hydrogen atoms ortho to the nitrogen atom
coordinated to the platinum center appears at δ 8.61 as a
doublet. This signal appears at lower frequencies than the
other signals of the ligand because these hydrogen atoms
are closer to the platinum center.
The room-temperature 19F NMR spectrum of complex 1
shows two signals for the o-fluorine atoms of the three C6F5
groups in a 2:1 intensity ratio with Pt-Fo coupling constants
of 361 and 528 Hz. The p-fluorine atoms appear as two
triplets in a 2:1 intensity ratio, and two complex signals in
the same ratio also appear for the m-fluorine atoms. Ac-
The 19F NMR spectrum of complex 2 at room temperature
shows a pattern similar to that described for complex 1: two
doublets (ratio 2:1) corresponding to the o-fluorine atoms,
with Pt-H coupling constants of 386 and 560 Hz, respec-
tively. The p-fluorine and m-fluorine atoms appear to be
(56) Del Bene, J. E.; Perera, S. A.; Bartlett, R. J. J. Am. Chem. Soc. 2000,
12, 3560.
(57) Jordan, M. J. T.; Del Bene, J. E. J. Am. Chem. Soc. 2000, 122, 2101.
(58) Casas, J. M.; Fornie´s, J.; Mart´ın, A. J. Chem. Soc., Dalton Trans.
1997, 1559.
9448 Inorganic Chemistry, Vol. 44, No. 25, 2005

Pentafluorophenyl Platinum(II) Complexes
Table 3. Selected Bond Distances (Å) and Angles (deg) for
[NBu4][Pt(C6F5)3(PhNdCHNHPh)] (2)
Pt-C(20)
Pt-C(14)
Pt-C(26)
Pt-N(1)
2.004(8)
2.025(7)
2.052(7)
2.126(7)
N(1)-C(1)
N(1)-C(2)
N(2)-C(1)
N(2)-C(8)
1.285(13)
1.416(13)
1.355(13)
1.375(14)
C(20)-Pt-C(14)
C(20)-Pt-C(26)
C(14)-Pt-N(1)
C(26)-Pt-N(1)
C(1)-N(1)-C(2)
88.7(3)
C(1)-N(1)-Pt
C(2)-N(1)-Pt
C(1)-N(2)-C(8)
N(1)-C(1)-N(2)
118.2(6)
121.7(6)
126.7(7)
122.7(8)
90.1(3)
92.8(3)
88.6(3)
120.1(8)
As expected, in both complexes the platinum atoms lie at
the center of a square-planar environment. The Pt-C
distances are within the range found for other similar
compounds.59-61 The shortest Pt-C distance corresponds to
the Cipso of the C6F5 group trans to the neutral ligand because
of the trans influence of the N-donor ligands. The three C6F5
rings are not perpendicular to the molecular plane. The
dihedral angles for 1 are 74.7(2), 74.1(2), and 64.1(2)°, and
the dihedral angles for 2 are 78.2(4), 75.7(4) and 64.5(4)°.
For 1, the N-N bond lengths [N(1)-N(2) ) 1.276(7) Å,
N(2)-N(3) ) 1.301(7) Å] are equal within the experimental
error and indicate delocalization of the charge over the N3
framework. The dihedral angles between the N3 framework
and phenyl groups of the triazene ligand are 11.8(4) and 9.9-
(5)°.
Figure 1. Structure of the complex anion of [NBu4][Pt(C6F5)3(PhNd
NNHPh)] (1).
The hydrogen atoms were not observed in the difference
maps. Atom H(3), attached to N(3) of the triazene fragment,
was placed on the external bisector of the angle N(5)-N(3)-
C(25) with an N-H distance of 0.88 Å. The resulting Pt f
H contact parameters, Pt‚‚‚H(3) ) 2.45 Å and Pt‚‚‚H(3)-
N(3) ) 123°, are within the ranges found previously for this
type of interaction.17,20,62-66
Similar positioning of the analogous H atom in 2 [H(2)]
led to a Pt‚‚‚H(2) distance of 2.59 Å, longer than in 1 but
still within the expected range. The Pt‚‚‚H(2)-N(2) angle,
119°, is similar to those found in other intramolecular Pt f
H systems, including 1. One ortho-H atom in 1, namely that
attached to C(20) (Figure 1) could in principle have ap-
proached Pt in the same fashion as does H(3). But in fact,
the triazene ligand and this phenyl group are oriented in such
a way that the (C)H(20)‚‚‚Pt distance, 2.80 Å, is significantly
longer than the H(3)‚‚‚Pt distance formed by the same ligand.
So it appears that the latter contact is favored at the expense
of the former. Similarly in 2, the [C(7)]H(7)‚‚‚Pt distance
(Figure 2), 2.97 Å, is much longer than that of [N(2)]H(2)‚
‚‚Pt.
Figure 2. Structure of the complex anion of [NBu4][Pt(C6F5)3(PhNd
CHNHPh)] (2).
Table 2. Selected Bond Distances (Å) and Angles (deg) for
[NBu4][Pt(C6F5)3(PhNdNNHPh)] (1)
Pt-C(7)
Pt-C(1)
Pt-C(13)
Pt-N(1)
2.008(6)
2.063 (6)
2.076(6)
2.087 (5)
N(1)-N(2)
N(1)-C(19)
N(2)-N(3)
N(3)-C(25)
1.276(7)
1.459(9)
1.301(7)
1.370(9)
C(7)-Pt-C(1)
C(7)-Pt-C(13)
C(1)-Pt-N(1)
C(13)-Pt-N(1)
N(2)-N(3)-C(25)
89.1(2)
N(2)-N(1)-C(19)
N(2)-N(1)-Pt
C(19)-N(1)-Pt
N(1)-N(2)-N(3)
113.2(6)
123.7(4)
123.1(4)
113.9(5)
89.2(2)
92.7(2)
89.0(2)
120.9(6)
partially overlapped. Also, at low temperature, the spectra
is similar to the one described for 1.
A significant difference between the structures of 1 and 2
is the different orientation of the phenyl groups of the neutral
ligand. Where in 1 these phenyl rings are essentially coplanar
with the N3 skeleton, for 2, they are substantially rotated
with respect to the N-C-N plane, with dihedral angles of
36.5(8)° and 14.6(9)°. The environment of C(1) is planar,
The IR absorption ν(N-H) cannot be observed, in contrast
to the infrared spectrum of complex 1, which shows an
absorption at 3061 cm-1.
Crystal Structure of Complexes [NBu4][Pt(C6F5)3(PhNd
NNHPh)] (1) and [NBu4][Pt(C6F5)3(PhNdCHNHPh)] (2).
The structures of the anions of complexes 1 and 2 with the
atom numbering scheme are shown in Figures 1 and 2,
respectively. Important crystallographic data and data col-
lection parameters are summarized in Table 1. Selected bond
distances and angles are listed in Table 2 for 1 and Table 3
for 2.
(59) Uso´n, R.; Fornie´s, J.; Toma´s, M.; Ara, I. AdV. Organomet. Chem. 1988,
288, 219.
(60) Uso´n, R.; Fornie´s, J.; Toma´s, M.; Ara, I. J. Organomet. Chem. 1988,
358, 525.
(61) Uso´n, R.; Fornie´s, J.; Toma´s, M.; Ara, I. Inorg. Chim. Acta 1992,
198-200, 165.
Inorganic Chemistry, Vol. 44, No. 25, 2005 9449

Casas et al.
but the C(1)-N distances are different [C(1)-N(1) ) 1.285-
(13) Å and C(1)-N(2) ) 1.355(13) Å] which might indicate
that the double bond is more localized in 2 than for 1. All
of these observations point to the fact that while in 1 there
seems to be a significant degree of electron delocalization,
not only in the N3 system but also involving the phenyl
groups, in 2 there is no such delocalization or at least not to
such an extent. This would affect the electronic properties
of the hydrogen located in a suitable position to interact with
the metal center.
Synthesis and Characterization of [NBu4](x+1)[Pt(C6F5)3-
(C5H6N2)](ClO4)x [x ) 0 (3), x ) 1 (4)]. The reaction of
2-aminopyridine with [NBu4]2[Pt2(µ-C6F5)2(C6F5)4] (2:1 mo-
lar ratio) in CH2Cl2 produces cleavage of the C6F5 bridging
system and coordination of L, yielding [NBu4][Pt(C6F5)3-
(C5H6N2)] (3) (eq 2).
Figure 3. Structure of the complex anion of [NBu4][Pt(C6F5)3(C5H6N2)]
(3).
[NBu4]2[Pt2(µ-C6F5)2(C6F5)4] + 2(2-aminopyridine) f
one would render the mutually trans C6F5 equivalent. At 193
K, this signal becomes broader because the dynamic process
slows down.
2[NBu4][Pt(C6F5)3(C5H6N2)] (2)
On the other hand, if the preparation of 3 is attempted
starting from [NBu4]2[Pt(C6F5)3Cl] in a manner similar to
that described for the preparation of 1 and 2, removal of the
NBu4ClO4 is not possible, even by washing the resulting solid
with iPrOH, a solvent in which this salt is soluble. This solid
is better formulated as [NBu4]2[Pt(C6F5)3(C5H6N2)](ClO4) (4).
Thus, depending on the starting material, [NBu4](x+1)[Pt-
(C6F5)3(C5H6N2)](ClO4)x with x ) 0 (3) or x ) 1 (4) is
obtained. The presence of NBu4ClO4 cannot be considered
as an impurity, since it has an structural role that will be
discussed later.
The infrared spectra of 3 and 4 show three signals in the
X-sensitive zone49 corresponding to the presence of three
C6F5 groups. In 4, signals at 1163 and 625 cm-1 from the
ClO4 anion are also observed.
The H NMR spectra of 3 and 4 (CD2Cl2, 293 K) are
identical except for the relative intensities of the signals
corresponding to the NBu4+ cation and the 2-aminopyridine
ligand (1:1 for 3 and 2:1 for 4). These spectra confirm the
coordination of the ligand to the platinum center through
the pyridinic nitrogen atom since the resonance correspond-
ing to the hydrogen atom in the 6 position of the aromatic
ring shows platinum satellites (JPt-H ) 34.2 Hz, 8.40 ppm).
The rest of the aromatic hydrogen atoms appear at 7.28 ppm
(triplet) and 6.43 ppm (doublet + triplet). The aminic
hydrogen atoms give a single resonance at 5.97 ppm showing
their equivalence at room temperature, and no Pt satellites
are observed. The two mechanisms that can account for this
observation are a fast rotation of the NH2 fragment around
the C-N bond or a pyramidal inversion process,67 and either
The 19F NMR spectra of 3 and 4 are essentially identical
and show, at room temperature, two signals corresponding
to two types of C6F5 groups: the trans pentafluorophenyl
groups are equivalent. The o-F-Pt coupling constants are
401 and 536 Hz, respectively, corresponding to the two types
of C6F5 groups in the molecule, one group trans to the neutral
ligand and two equivalent mutually trans C6F5 groups. The
signal of the o-fluorine atoms of the mutually trans C6F5
rings begins to split at 233-253 K, indicating that at lower
temperatures the o-fluorine atoms belonging to the same C6F5
group become inequivalent. At 193 K, there are four signals
in the o-fluorine region with an integration corresponding
to 2, 1, 1, and 2 atoms. Within the C6F5 rings, the two
o-fluorine atoms are inequivalent, while the o-fluorine atoms
corresponding to the mutually trans C6F5 groups are equiva-
lent in pairs. This equivalence is not caused by the symmetry
of the molecule because the aminic hydrogen atoms would
be equivalent and that is not the case. At 193 K, the signal
corresponding to the aminic hydrogen atoms disappears. At
lower temperature (178 K), two new broad signals appear
at 6.93 and 4.95 ppm. One of these hydrogen atoms is more
deshielded than the other, but the frequency of the signal
does not suggest a Pt-H interaction. It seems more likely
that at low temperature the rotation around the Pt-C, Pt-
N, and C-NH2 bonds is slower, resulting in different
chemical environments for the two aminic hydrogen atoms.
-
1
Crystal Structures of [NBu4][Pt(C6F5)3(C5H6N2)] (3)
and [NBu4]2[Pt(C6F5)3(C5H6N2)](ClO4) (4). The structures
of the anions of complexes 3 and 4, with the atom numbering
scheme, are shown in Figures 3 and 4, respectively. Important
crystallographic data and data collection parameters are
summarized in Table 1. Selected bond distances and angles
are listed in Table 4 for 3 and Table 5 for 4. For the structure
of 3, H atoms were located on difference maps, and their
coordinates were refined. In the case of 4, the quality of the
diffraction data collected did not allow for the experimental
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Inorg. Chem. 1987, 26, 503.
(65) Albinati, A.; Arz, A.; Pregosin, P. S. Inorg. Chem. 1987, 26, 508.
(66) Albinati, A.; Pregosin, P. S.; Wombacher, F. Inorg. Chem. 1990, 29,
1812.
(67) Rauk, A.; Allen, L. C.; Mislow, K. Angew. Chem., Int. Ed. Engl. 1970,
9, 404.
9450 Inorganic Chemistry, Vol. 44, No. 25, 2005

Pentafluorophenyl Platinum(II) Complexes
because of our inability to locate the NH2 hydrogen atoms
for this structure. Nevertheless, the Pt-N(2) distance is
similar in both complexes (3.144(3) Å in 3 and 3.188(9) Å
in 4) which, from the merely geometric point of view, does
not rule out the existence of such an interaction.
The most remarkable features of the structure of 4 are the
presence of a perchlorate anion and the formation of an
N-H‚‚‚O-Cl hydrogen bond, which can be inferred from
the N(2)‚‚‚O(2) distance of 3.275(16) Å.
In square-planar complexes of Pt containing the 2-ami-
nopyridine ligand, it is commonly found that the pyridinic
ring lies nearly perpendicular to the square plane;41,45 but in
the anion of complex 3, the dihedral angle is 67.0(1)°. This
seems to indicate that the Pt‚‚‚H(2) interaction affects the
dihedral angle of the ligand containing the interacting
hydrogen. The C6F5 groups in 3 are twisted 53.1(1)° [C(1)],
61.7(1)° [C(7)], and 80.5(1)° [C(13)] with respect to the
square plane [C(1) is cis to the pyridinic ligand]. The small
value of the dihedral angle found for the C(1) ring is probably
the result of the steric effect caused by the relatively bulky
NH2 substituent on the pyridinic ligand.
In 4, the dihedral angle between the pyridinic ring and
the coordination plane is now 86.6(4)°; the two planes are
almost perpendicular, and the dihedral angles of the pen-
tafluorophenyl rings are 77.9(5), 85.8(4), and 86.6(4)°. These
values are consistent with the conclusion that the hydrogen
bonding takes place only to an oxygen atom of the perchlo-
rate anion.73
Figure 4. Structure of the anions of [NBu4]2[Pt(C6F5)3(C5H6N2)](ClO4)
(4).
Table 4. Selected Bond Distances (Å) and Angles (deg) for
[NBu4][Pt(C6F5)3(C5H6N2)] (3)
Pt-C(7)
Pt-C(13)
Pt-C(1)
Pt-N(1)
N(2)-H(1)
2.013(4)
2.051(4)
2.056(4)
2.096(3)
0.86(5)
N(1)-C(19)
N(1)-C(23)
N(2)-C(19)
N(2)-H(2)
1.350(5)
1.358(5)
1.339(5)
0.93(5)
C(7)-Pt-C(13)
C(7)-Pt-C(1)
C(13)-Pt-N(1)
C(1)-Pt-N(1)
C(19)-N(1)-Pt
94.04(14)
88.68(14)
87.12(13)
90.17(13)
122.1(3)
C(23)-N(1)-Pt
119.5(3)
C(19)-N(2)-H(2)
C(19)-N(2)-H(1)
H(2)-N(2)-H(1)
N(2)-C(19)-N(1)
118(3)
119(3)
120(4)
117.5(4)
Table 5. Selected Bond Distances (Å) and Angles (deg) for
[NBu4]2[Pt(C6F5)3(C5H6N2)](ClO4) (4)
The existence in 4 of this N-H‚‚‚O-Cl intermolecular
hydrogen bond explains the crystallization of the perchlorate
and complex anions together. This hydrogen bond has to
have an appreciable strength, since it is not broken even by
Pt-C(7)
Pt-C(13)
Pt-C(1)
Pt-N(1)
2.007(11)
2.072(13)
2.091(13)
2.085(9)
N(1)-C(19)
N(1)-C(23)
N(2)-C(23)
1.374(15)
1.367(15)
1.360(19)
i
donor solvents such as PrOH. Also, complex 4 illustrates
the fact that the presence of competing proton acceptors can
obviate the formation of weak Pt f H interactions.
A comparison of the values of the N-C distances
involving the pyridinic nitrogen atom in both complexes
[N(1)-C(19) ) 1.350(5) Å (3), 1.374(15) Å (4); N(1)-C(23)
) 1.358(5) Å (3), 1.367(15) Å (4)] and the aminic one
[N(2)-C(19) ) 1.339(5) Å (3); N(2)-C(23) ) 1.360(19)
Å (4)] reveals that all three are equal within the experimental
error. This observation is consistent with an electron density
delocalization of the π system of the ring and the free
electron pair of the aminic group also in the N-C-N
fragment, as previously reported.74
C(7)-Pt-C(13)
C(7)-Pt-C(1)
C(13)-Pt-N(1)
C(1)-Pt-N(1)
C(23)-N(1)-C(19)
91.3(4)
90.3(4)
90.4(4)
88.1(4)
121.9(12)
C(23)-N(1)-Pt
C(19)-N(1)-Pt
N(2)-C(23)-N(1)
N(2)-C(23)-C(22)
120.2(10)
117.8(9)
121.2(13)
126.8(14)
location of the hydrogen atoms, and therefore, the H atoms
for the aminic group were not included in the model.
In the two complexes, the platinum atoms lie at the centers
of square-planar environments with Pt-C and Pt-N dis-
tances within the ranges found in similar compounds.47,68-72
The 2-aminopyridine ligand is coordinated to the platinum
center through the pyridinic N atom. In the case of complex
3, the -NH2 fragment is located above the square plane in
such a way that the Pt‚‚‚H(2) distance is 2.58(3) Å and Pt‚
‚‚H(2)-N(2) ) 120(1)°. Both parameters are consistent with
the existence of a weak Pt-H interaction. However, nothing
can be inferred about a possible Pt‚‚‚H-N interaction in 4
Concluding Remaks
To understand the different behavior displayed by com-
plexes 1-4 in regard to their M f H hydrogen bonds, it is
important to consider the various factors that determine the
strengths of these interactions which are either moderate or
weak.75 The capacity of Pt(II) to donate electron density to
(68) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D.
G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1.
(69) Uso´n, R.; Fornie´s, J.; Toma´s, M.; Casas, J. M.; Navarro, R. J. Chem.
Soc., Dalton Trans. 1989, 169.
(70) Casas, J. M.; Fornie´s, J.; Mart´ın, A.; Menjo´n, B. Organometallics 1993,
12, 4376.
(71) Ara, I.; Casas, J. M.; Fornie´s, J.; Rueda, A. J. Inorg. Chem. 1996, 35,
7345.
(73) Jin, Z. M.; Pan, Y. J.; Li, X. F.; Hu, M. L.; Shen, L. J. Mol. Struct.
2003, 660, 67.
(74) Krizanovic, O.; Sabat, M.; Beyerle-Pfnur, R.; Lippert, B. J. Am. Chem.
Soc. 1993, 115, 5538.
(72) Casas, J. M.; Diosdado, B. E.; Falvello, L. R.; Fornie´s, J.; Mart´ın, A.;
Rueda, A. J. Dalton Trans. 2004, 2733.
(75) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University
Press: New York, 1997.
Inorganic Chemistry, Vol. 44, No. 25, 2005 9451

Casas et al.
an extent necessary to establish stable interactions has been
previously reported. In complexes 1-4, the topological and
electronic properties of the Pt donor sites are so similar that
we must look to other factors to explain the differences
among these systems.
nopyridine ligand of 3 and 4, namely, rotation of the donor
group about a C-N single bond, adds such flexibility to these
systems. Complexes 3 and 4 demonstrate the well-established
competitive nature of such interactions. The presence of an
energetically more favorable alternative for the donor group
in 4, the possibility of forming a hydrogen bond to perchlo-
rate, prevails over the weaker interaction to Pt that is
observed in 3.
The acidity of the hydrogen atom is an important dis-
criminant, the more acidic the hydrogen, the more favorable
the hydrogen bond. Complex 1, with the more acidic H atom
of triazene, forms a Pt f H interaction stable enough to be
detected in solution (where well-known factors render less
favorable interactions unobservable). In contrast, the less
acidic H atom of formamidine in 2 forms a Pt f H hydrogen
bond that can be observed in the crystal but not in solution.
The flexibility of the ligand carrying the proton donor is
a more obvious factor in the formation of these interactions.
The ligand must be able to bind to Pt and, at the same time,
position the hydrogen atom appropriately to permit hydrogen
bond formation. The extra degree of freedom in the ami-
Acknowledgment. This work has been suported by the
Spanish Ministerio de Educacio´n y Ciencia (Spain) y Fondos
FEDER (Projects BQU2002-03997-C02-02 and BQU2002-
00554). B.E.D. thanks the CICYT for a grant.
Supporting Information Available: Further details of the
structure determinations of 1-4 including atomic coordinates, bond
distances and angles, and thermal parameters. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC050800G
9452 Inorganic Chemistry, Vol. 44, No. 25, 2005