Cationic σ-phenylplatinum(II) complexes with carboxylic acid functionality: pKa determinations and x-ray structures

Article abstract of DOI:10.1021/ic025776a

The preparation and characterization of a novel series of cationic σ-phenylplatinum(II) complexes of the type trans-[Pt(σ-C₆H₅)(L)₂A]OTf (A = picolinic acid, L = PPh₃ (4) and PMePh₂ (7); A = nicotinic acid, L = PPh₃ (5) and PMePh₂ (8); A = isonicotinic acid, L = PPh₃ (6), PMePh₂ (9), and PEt₃ (10)) are described. The pK_a value for the carboxylic acid functionality in selected complexes was found to follow the order 7 (pK_a = 5.23 ± 0.09) > 8 (4.85 ± 0.10) > 9 (3.51 ± 0.08) > 6 (3.26 ± 0.07) ≈ 10 (3.21 ± 0.08) by means of potentiometric titration experiments in 50% (v/v) EtOH/H₂O solution at 295 K. The X-ray crystal structures of 9 and 10 were also determined. The asymmetric unit of each of 9 and 10 comprises a univalent complex cation, a triflate anion, and a solvent CH₂Cl₂ molecule of crystallization. Centrosymmetrically related pairs of complex cations in 9 associate via the familiar carboxylic acid dimer motif, whereas with 10, the carboxylic acid dimer motif is absent. Instead, the carboxylic acid residue forms both donor and acceptor interactions to the trifiate anion and CH₂Cl₂ solvent of crystallization, respectively, to afford a 10-membered ring structure. Possible reasons for the observed differences in the solid-state structures of 9 and 10 are presented.

Full text of DOI:10.1021/ic025776a

Inorg. Chem. 2003, 42, 1057−1063
Cationic σ-Phenylplatinum(II) Complexes with Carboxylic Acid
Functionality: pKa Determinations and X-ray Structures
Michael G. Crisp, Edward R. T. Tiekink,† and Louis M. Rendina*
Department of Chemistry, The UniVersity of Adelaide, Adelaide, South Australia 5005, Australia
Received June 7, 2002
The preparation and characterization of a novel series of cationic σ-phenylplatinum(II) complexes of the type trans-
[Pt(σ-C H )(L)2A]OTf (A ) picolinic acid, L ) PPh3 (4) and PMePh2 (7); A ) nicotinic acid, L ) PPh3 (5) and
6
5
PMePh2 (8); A ) isonicotinic acid, L ) PPh3 (6), PMePh2 (9), and PEt3 (10)) are described. The pK value for the
a
carboxylic acid functionality in selected complexes was found to follow the order 7 (pK ) 5.23 ± 0.09) > 8 (4.85
a
± 0.10) > 9 (3.51 ± 0.08) > 6 (3.26 ± 0.07) ≈ 10 (3.21 ± 0.08) by means of potentiometric titration experiments
in 50% (v/v) EtOH/H O solution at 295 K. The X-ray crystal structures of 9 and 10 were also determined. The
2
asymmetric unit of each of 9 and 10 comprises a univalent complex cation, a triflate anion, and a solvent CH Cl2
2
molecule of crystallization. Centrosymmetrically related pairs of complex cations in 9 associate via the familiar
carboxylic acid dimer motif, whereas with 10, the carboxylic acid dimer motif is absent. Instead, the carboxylic acid
residue forms both donor and acceptor interactions to the triflate anion and CH Cl2 solvent of crystallization,
2
respectively, to afford a 10-membered ring structure. Possible reasons for the observed differences in the solid-
state structures of 9 and 10 are presented.
Introduction
struction of large molecular polygons and other types of
supramolecular arrays,3 particularly when the H-bonding
interactions are enhanced via charge-assistance by the use
of ionic complexes.3a Diverse examples of the simultaneous
use of these bonding motifs in late-transition-metal chemistry
include the synthesis of a unique organoplatinum(IV) poly-
rotaxane with a remarkable architecture,4 the assembly of
infinite single- and double-stranded chains by isonicotina-
mide complexes of platinum(II) and rhodium(III),5 respec-
tively, a cyclic tetramer of platinum(II) complexes containing
5-aminoorotic acid,6 the self-assembly of sheet and polymer
structures by pyridine-carboxamide complexes of palladium-
(II),7 infinite 1D-, 2D-, and 3D-assemblies involving silver-
(I) complexes of nicotinamide and isonicotinamide,8 and the
Major advances have been made in the use of the
coordinate-covalent bond for the construction of large
molecular polygons and polyhedra by self-assembly.1 More
recently, the combination of the coordinate-covalent and
H-bonding motifs has been shown to expand the utility of
metal complexes as tectons2 (building blocks) in the con-
* To whom correspondence should be addressed. E-mail: lou.rendina@
adelaide.edu.au. Phone: 61 8 8303 4269. Fax: 61 8 8303 4358.
† Present address: Department of Chemistry, National University of
Singapore, Singapore 117543.
(1) For recent reviews, see: (a) Olenyuk, B.; Fechtenko¨tter, A.; Stang, P.
J. J. Chem. Soc., Dalton Trans. 1998, 1707. (b) Fujita, M.; Ogura, K.
Coord. Chem. ReV. 1996, 148, 249. (c) Fujita, M. Chem. Soc. ReV.
1998, 27, 417. (d) Stang, P. J.; Olenyuk, B. Acc. Chem. Res. 1997,
30, 502. (e) Jones, C. J. Chem. Soc. ReV. 1998, 27, 289. (f) Stang, P.
J. Chem. Eur. J. 1998, 4, 19. (g) Baxter, P. N. W. In ComprehensiVe
Supramolecular Chemistry; Lehn, J.-M., Chair, E., Atwood, J. L.,
Davies, J. E. D., MacNicol, D. D., Vo¨gtle, F., Eds.; Pergamon Press:
Oxford, 1996; Vol. 9, p 165. (h) Caulder, D. L.; Raymond, K. N. J.
Chem. Soc., Dalton Trans. 1999, 1185. (i) Caulder, D. L.; Raymond,
K. N. Acc. Chem. Res. 1999, 32, 975. (j) Chambron, J.-C.; Dietrich-
Buchecker, C.; Sauvage, J.-P. In ComprehensiVe Supramolecular
Chemistry; Lehn, J.-M., Chair, E., Atwood, J. L., Davies, J. E. D.,
MacNicol, D. D., Vo¨gtle, F., Eds.; Pergamon Press: Oxford, 1996;
Vol. 9, p 43. (k) Uller, E.; Demleitner, I.; Bernt, I.; Saalfrank, R. W.
In Structure and Bonding; Fujita, M., Ed.; Springer: Berlin, 2000;
Vol. 96, p 149. (l) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem.
ReV. 2000, 100, 853. (m) Halpern, J. Proc. Natl. Acad. Sci. U.S.A.
2002, 99, 4762 and references therein.
(2) Simard, M.; Su, D.; Wuest, J. D. J. Am. Chem. Soc. 1991, 113, 4696.
(3) For recent reviews, see: (a) Braga, D.; Grepioni, F. Acc. Chem. Res.
2000, 33, 601. (b) Braga, D.; Grepioni, F.; Desiraju, G. R. Chem ReV.
1998, 98, 1375. (c) Aakero¨y, C. B.; Beatty, A. M. Aust. J. Chem.
2001, 54, 409. (d) Burrows, A. D.; Chan, C.-W.; Chowdhry, M. M.;
McGrady, J. E.; Mingos, D. M. P. Chem. Soc. ReV. 1995, 329.
(4) Fraser, C. S. A.; Jennings, M. C.; Puddephatt, R. J. Chem. Commun.
2001, 1310.
(5) Kuehl, C. J.; Tabellion, F. M.; Arif, A. M.; Stang, P. J. Organometallics
2001, 20, 1956.
(6) Burrows, A. D.; Mingos, D. M. P.; White, A. J. P.; Williams, D. J. J.
Chem. Soc., Dalton Trans. 1996, 3805.
(7) Qin, Z.; Jennings, M. C.; Puddephatt, R. J. Inorg. Chem. 2001, 40,
6220.
10.1021/ic025776a CCC: $25.00 © 2003 American Chemical Society
Published on Web 01/17/2003
Inorganic Chemistry, Vol. 42, No. 4, 2003 1057

Crisp et al.
Scheme 1
construction of nanoscale cyclic arrays containing platinum-
(II) and nicotinic acid.9
Herein we describe the synthesis and characterization of
a series of σ-phenylplatinum(II) complexes with a carboxylic
acid functionality incorporated through the use of pyridine
monocarboxylic acid ligands. The ability of these model
compounds to act as tectons both in solution and in the solid
state is limited by the presence of only one carboxylic acid
functionality, but despite this limitation, their association in
the solid state is found to differ quite remarkably. The
σ-phenyl ligand simply serves to block one of the coordina-
tion sites in these model complexes, and it may also
participate in favorable π-π stacking interactions. We also
report the pKa values for a selected number of complexes in
order to determine the effect of platinum coordination on
the Brønsted acidity of the free CO2H group, the results of
which have particular relevance to studies of molecular
recognition involving these and related species in solution,
particularly in polar solvent systems.
Synthesis and Characterization of σ-Phenylplatinum-
(II) Complexes with Carboxylic Acid Functionality
The target σ-phenylplatinum(II) complexes were prepared
from the corresponding iodo complexes 1-3 by treatment
with 1 equiv of AgOTf in CH2Cl2 solution to afford the labile
σ-phenyl(triflato)platinum(II) species. The addition of iso-
meric pyridine monocarboxylic acids (picolinic acid, nicotinic
acid, and isonicotinic acid) readily afforded the target cationic
complexes 4-10 as air-stable, microcrystalline solids in good
yield and purity (Scheme 1). The products were fully
characterized by 1D (1H, 31P, and 19F)- and 2D (1H-1H
COSY) NMR and IR spectroscopy, microanalysis, and, in
the case of 9 and 10, by X-ray crystallography.
of the PPh3 proton signals. This aids in the assignment of
the other aromatic protons that are masked by the PPh3
protons in 5, for example. The characteristic AA′XX′ signals
at δ 8.17 and 8.27 are attributed to the H2 and H3 protons of
the isonicotinic acid ligand, respectively. As expected, the
1H NMR spectra of the picolinic acid complexes 4 and 7
were more complicated than their respective isomers. For
example, 7 shows a signal at δ 6.55 that is assigned to the
ortho σ-aryl protons, and the multiplet signal at δ 6.87 is
assigned to (overlapping) meta and para σ-aryl protons.
The 31P{1H} NMR spectra of all complexes prepared in
this work show a singlet flanked by 195Pt satellite signals,
consistent with mutually trans phosphine ligands. In contrast
to PPh3, the presence of the electron-rich phosphine ligands
PEt3 and PMePh2 leads to a significant upfield shift of the
signal and a decrease in the magnitude of the 195Pt-31P
coupling constant.11 The magnitudes of 1JPtP for all complexes
fall within the range that is typical for trans-substituted
σ-arylplatinum(II) species.11,12
1
The H NMR spectrum of 5 shows multiplet signals at δ
6.74, 6.37, and 6.53, which correspond to the ortho, meta,
and para σ-phenyl protons, respectively. As expected, the
two pyridine ring protons adjacent to the electronegative
nitrogen atom, H2 and H6, are shifted downfield compared
to the other aromatic protons. Complex 8 has a very similar
1H NMR spectrum to that of 5, the main difference being
the presence of a P-Me multiplet centered at δ 1.62, which
constitutes the X part of an AMM′X3X3′ spin system (where
1
A ) 195Pt, M ) 31P, and X ) H).10-12
The isonicotinic acid complexes 6, 9, and 10 have the
1
simplest H NMR spectra because of their high symmetry.
The IR spectra of 4-10 show the expected strong
ν(CdO) and broad ν(OsH) bands at ca. 1720 cm-1 and
2500-3200 cm-1, respectively, at frequencies that are typical
for aromatic carboxylic acids.
For example, 10 displays characteristic signals at δ 7.32,
6.87, and 7.05, assigned to the ortho, meta, and para σ-aryl
protons, respectively. As anticipated, the aromatic region of
the spectrum is significantly simplified without the presence
pKa Determinations
(8) (a) Aakero¨y, C. B.; Beatty, A. M. Chem. Commun. 1998, 1067. (b)
Aakero¨y, C. B.; Beatty, A. M.; Helfrich, B. A. J. Chem. Soc., Dalton
Trans. 1998, 1943.
(9) Gianneschi, N. C.; Tiekink, E. R. T.; Rendina, L. M. J. Am. Chem.
Soc. 2000, 122, 8474.
Potentiometric titration experiments were used to deter-
mine the pKa value for the carboxylic acid functionality in
complexes 6-10.13 All measurements were conducted in a
50% (v/v) EtOH/H2O solution at 295.0 ( 0.1 K as the
complexes were poorly soluble in water alone. Table 1
(10) Crisp, M. G.; Pyke, S. M.; Rendina, L. M. J. Organomet. Chem. 2000,
607, 222.
(11) Pregosin, P. S.; Kunz, R. W. 31P and 13C NMR of Transition Metal
Complexes; Springer: Berlin, 1979.
(12) Anderson, G. K.; Clark, H. C.; Davies, J. A. Organometallics 1982,
1, 64.
(13) The pKa values of 4 and 5 could not be determined owing to their
poor solubility in this solvent system.
1058 Inorganic Chemistry, Vol. 42, No. 4, 2003

Cationic σ-Phenylplatinum(II) Complexes
Table 1. pKa Values for 6-10a
complex
pKa
6
7
8
9
3.26 ( 0.07
5.23 ( 0.09
4.85 ( 0.10
3.51 ( 0.08
3.21 ( 0.08
10
a Measured in 50% (v/v) EtOH/H2O solution at 295.0 ( 0.1 K.
summarizes the pKa values that were determined for each
of the complexes.
Comparisons between the acidities of the complexes pre-
pared in this work and those of the free pyridine carboxylic
acids are not pertinent in this context owing to the basic
nature of the nitrogen atom present in the latter, which can
act as an internal base and form the corresponding zwitter-
ionic species. A more useful comparison of the acidities
reported in Table 1 is with simple aromatic acids such as
benzoic acid, the pKa of which has been determined pre-
viously in 50% (v/v) EtOH/H2O solution (5.67 ( 0.02 at
298 K).14 From the results presented in Table 1, it is clear
that the complexes are significantly stronger Brønsted acids
than benzoic acid, with 6 and 10 being approximately 300
times more acidic. This enhanced acidity is most likely
attributed to the significant polarization of the O-H bond
by the cationic platinum(II) center, thus promoting the facile
loss of the acidic proton. The acidity of the complexes
increases in the order 715 < 8 < 9 < 6 ≈ 10, and it does not
appear to be a function of the steric bulk or the basicity of
the phosphine ligands; for example, the acidities of 6 and
10 are equal within experimental error, despite the obvious
differences in the nature of the phosphine ligands. The
acidities for the PMePh2 series of complexes decrease in the
order 715 (R′′ ) 2-CO2H) < 8 (R′′ ) 3-CO2H) < 9 (R′′ )
4-CO2H); that is, the acidities appear to follow the same order
of the (estimated) pKa values for free pyridine monocar-
boxylic acids with respect to the position of the CO2H group
in the pyridine ring.16
Treatment of 6-10 with 1 equiv of base (e.g., KOH,
KOtBu, proton sponge) resulted in the isolation of mixtures
containing the zwitterionic platinum(II) species and the
corresponding triflate salt, for example, KOTf. Repeated
attempts at purifying the zwitterion triflate salt mixtures by
repeated washings, recrystallization from numerous solvent
systems, or column chromatography were unsuccessful. Only
minor differences were found upon comparison of the NMR
spectra of the pure cationic complexes with those of the
corresponding zwitterions. For example, in the 31P{1H} NMR
spectrum of 9 and its corresponding zwitterion (in the
presence of KOTf), the single resonance appears at δ 9.1
(1JPtP ) 2883 Hz) and δ 9.0 (1JPtP ) 2894 Hz), respectively,
Figure 1. Molecular structure and crystallographic numbering scheme
for 9.
1
in 50% (v/v) EtOH/H2O solution. H and 31P{1H} NMR
studies also confirmed the general robust nature of 6-10
and their corresponding zwitterionic species in various
solvent systems, including that used in the pKa experiments,
even after prolonged periods of time (>100 h) at room
temperature.
X-ray Crystallography
(14) Niazi, M. S. K.; Mollin, J. Bull. Chem. Soc. Jpn 1987, 60, 2605.
(15) Caution should be exercised in the interpretation of pKa phenomena
involving 7 as it proved to be somewhat unstable in solution,
particularly in the presence of polar solvents. This is most likely
attributed to the considerable steric interactions between the 2-CO2H
group and the PPh3 ligands that, in addition to the strong trans effect
exerted by the σ-phenyl ligand, would facilitate loss of the N-donor
ligand.
The molecular structures of 9 and 10 are shown in Figures
1 and 2, respectively. Crystal data and refinement details
are presented in Table 2, and selected geometric parameters
are shown in Table 3. The asymmetric unit of each of 9 and
10 comprises a univalent complex cation, a triflate anion,
and a solvent CH2Cl2 molecule of crystallization. In each
(16) Green, R. W.; Tong, H. K. J. Am. Chem. Soc. 1956, 78, 4896.
Inorganic Chemistry, Vol. 42, No. 4, 2003 1059

Crisp et al.
Table 2. Crystallographic Data for 9 and 10
9
10
formula
fw
C40H38Cl2F3NO5P2PtS
C26H42Cl2F3NO5P2PtS
865.6
0.16 × 0.26 × 0.45
triclinic
P1h
9.735(2)
10.839(4)
17.265(8)
92.47(4)
94.93(3)
105.19(4)
1748(1)
2
1029.7
0.13 × 0.16 × 0.32
monoclinic
P21/n
11.367(3)
20.614(7)
17.583(5)
90
95.83(2)
90
4099(2)
4
cryst size, mm3
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
D
calcd, g cm-3
1.669
1.645
F(000)
2040
37.27
10150
860
43.52
8439
µ, cm-1
no. data collected
θ
max, deg
27.5
9679
6471
27.5
8015
6714
no. unique data
no. obsd data
with I g 2.0σ(I)
R, Rw (obsd data)
R, Rw (all data)
0.030, 0.062
0.070, 0.072
0.95
0.026, 0.061
0.042, 0.066
0.90
F
max, e Å-3
Table 3. Selected Geometric Parameters (Å, deg) for 9 and 10
9
10
Pt-P1
2.3022(12)
2.3080(12)
2.134(3)
2.023(4)
1.276(6)
1.264(6)
173.63(4)
92.04(9)
88.45(11)
93.82(9)
85.72(11)
178.92(15)
2.3110(12)
2.3095(13)
2.118(3)
2.027(4)
1.193(5)
1.323(5)
174.38(4)
93.31(9)
86.92(12)
92.29(9)
87.50(12)
178.91(16)
Pt-P2
Pt-N1
Pt-C8
C7-O1
C7-O2
P1-Pt-P2
P1-Pt-N1
P1-Pt-C8
P2-Pt-N1
P2-Pt-C8
N1-Pt-C8
O2sH‚‚‚O1i of 1.64 Å, O2‚‚‚O1i of 2.709(5) Å, and an angle
subtended at H of 171°; symmetry operation i: -x, 1 - y,
1 - z. The crystal structure also features significant interac-
tions of the type C-H‚‚‚π (Figure 3). The closest of these
involves C17sH so that the distance between the H atom
and the ring centroid of a translationally related C27-C32
phenyl ring is 2.72 Å; symmetry operation ii: -1 + x, y, z.
The closest interaction involving the complex and triflate
anion occurs between the phenyl C19sH and O3ii atoms so
that H‚‚‚O3ii is 2.48 Å and the angle at H is 139° (Figure
3). Finally, the shortest interaction involving the dichlo-
romethane molecule involves the triflate so that C41sH is
2.30 Å from O5iii with an angle of 140° at H; symmetry
operation iii: 1 - x, 1 - y, -z. In contrast to that just
described for 9, the mode of intermolecular association in
10 is quite distinct.
Figure 2 shows the asymmetric unit for 10 and highlights
the different types of intermolecular interactions operating
in the crystal structure. Notable is the absence of the
carboxylic acid dimer motif as found in 9. Instead, the
carboxylic acid residue forms both donor and acceptor
interactions to the triflate anion and CH2Cl2 solvent of
crystallization, respectively, to afford a 10-membered ring
structure. The details of the association between the car-
boxylic acid and triflate anion are O2sH‚‚‚O3 of 1.88 Å,
Figure 2. Molecular structure and crystallographic numbering scheme
for 10.
complex, the platinum atom exists in the expected square
planar geometry defined by a C, N, P2 donor set indicating
that the isonicotinic acid is N-bound. The platinum atom lies
0.0102(1) and 0.0465(1) Å out of the respective coordination
planes in 9 and 10. In 10, the platinum-bound phenyl and
isonicotinic acid groups are effectively orthogonal to the
square plane as seen in the magnitude of the dihedral angles
of 88.6° and 89.4°, respectively. By contrast, in 9, the
respective dihedral angles are 74.5° and 91.5° with the large
deviation for the phenyl group ascribed to the reduced steric
hindrance imposed by the phosphine ligands on that side of
the molecule. This result indicates that there is no preferential
orientation for the platinum-bound phenyl group in 9 and
10. While there are no significant differences in the geometric
parameters defining the platinum atom interactions in the
two structures (see Table 3), there is a fascinating difference
in the mode of association between the molecules in the solid
state.
As shown in Figure 1, centrosymmetrically related pairs
of complex cations in 9 associate via the familiar car-
boxylic acid dimer motif. The details of this association are
1060 Inorganic Chemistry, Vol. 42, No. 4, 2003

Cationic σ-Phenylplatinum(II) Complexes
unexpected. However, as has been mentioned in a recent
review of hydrogen-bonding interactions operating in crystal
structures of organic molecules,18 the carboxylic acid dimer
motif is not as pervasive as one might anticipate. Indeed, in
this survey it was found that the classical dimer motif
occurred in approximately one-third of the structures owing
to the presence of competing hydrogen-bonding functional-
ities. This observation notwithstanding, it is curious that the
carboxylic acid dimer motif is found in 9 but not in 10. As
mentioned already, the geometric parameters defining the
two structures are essentially the same, and therefore, there
does not appear to be an electronic reason to account for
the disparate modes of association found in the solid state.
In the same way, as has been noted in Table 1, the pKa values
of the carboxylic acid groups in the two complexes are
similar, albeit these pertain to the solution phase. In the
absence of any obvious chemical or structural imperatives,
“subtle crystal packing effects” can be cited as the cause for
the different crystal structures. Using an analogous argument
cited for the appearance of polymorphs of tricyclohexyl-
phosphinegold(I) 2-mercaptobenzoate, Au(SC6H4-2-CO2H)-
PCy3, in which the carboxylic acid dimer motif was either
present or not, it is possible to derive a qualitative explanation
for these observations.19 Basically, there are two consider-
ations, namely, the formation of the carboxylic acid dimer
motif with the concomitant formation of rod-shaped ag-
gregates on one hand, and the absence of the dimer motif
and the formation of balls or spherical aggregates on the
other. Whereas the formation of the carboxylic dimer motif
might be favorable in 10, the consequence of this would be
the formation of less favorable packing of rods, compared
to the packing of spheres. Thus, in 9 the energy of
stabilization in the lattice favors the formation of the
carboxylic acid dimer motif and the global packing of the
resultant rodlike entities. In 10, the global packing consid-
erations dominate so that rods are not formed.
Figure 3. Diagram emphasizing the intermolecular associations formed
by the complex cation 9 with the exception of the carboxylic acid dimer
which is shown in Figure 1. Shown are the CsH‚‚‚π, involving the H17
atoms and the C27-C32 aromatic rings, and the CsH‚‚‚O(triflate) contacts.
Geometric parameters defining these associations are discussed in the text.
O2‚‚‚O3 of 2.665(4) Å, with an angle of 171° subtended at
the H atom. For the interaction involving the dichloromethane
molecule, C27sH‚‚‚O1 is 2.40 Å, C27‚‚‚O1 is 3.244(7) Å,
and angle at H is 143°. The second H atom of the
dichloromethane molecule also forms a weak hydrogen bond,
to O5 of the triflate, so that H‚‚‚O5 is 2.47 Å, C27‚‚‚O5 is
3.196(6) Å, and the angle at H is 130°. While it is noted
that the CsH‚‚‚O interactions involving the dichloromethane
molecule are not as well defined as are the O‚‚‚H associa-
tions, such CsH‚‚‚O are now well established in the
literature.17 The association between the molecules high-
lighted in Figure 2 has the result that a 10-membered ring
is formed. As found for 9, there are CsH‚‚‚π and
CsH‚‚‚O(triflate) interactions in the lattice of 10. The
distinct forms of intermolecular association involving the
carboxylic acid groups in 9 and 10 gives rise to chemically
important differences between the two sets of carboxylic acid
CsO bond distances as may be noted from Table 3. In 9,
where strong intermolecular association resulting in car-
boxylic acid dimer formation has been observed, there is
relatively little difference in the CsO bond distances; indeed,
they are found to be equal within experimental error, a point
emphasing the strong nature of these contacts. By contrast,
in 10, where a relatively strong interaction was noted between
the hydroxyl proton and the triflate anion and only a weak
interaction between the carbonyl oxygen atom and the weakly
acidic dichloromethane H-atom, there is a significant dispar-
ity in the CsO bond distances. This observation provides
additional evidence that the “dichloromethane bridge” be-
tween the complex cation and triflate anion is only weak.
Further, and consistent with this argument, is the systematic
variation in the SsO bond distances such that the S1sO3
bond distance of 1.451(3) Å is significantly longer than the
S1sO5 distance of 1.422(3) Å. It is interesting to speculate
upon the reasons for the different crystal structures found
for 9 and 10.
Conclusions
The considerable acidity of the CO2H functionality in
complexes such as 6 and 10 may complicate host-guest
studies involving these and related complexes, particularly
when polar solvents are employed or traces of adventitious
water are present in low polarity solvents. In the solid-state
association of 9 and 10, crystal packing effects appear to
dictate the assembly of these complexes in the crystal lattice,
and the expected carboxylic acid dimer motif is not neces-
sarily observed. Even in the absence of such effects, the
results of the X-ray study may have potential relevance to
the molecular recognition properties of these and related
cationic species in nonpolar solutions and, in certain cases,
demonstrate that the counterion does not necessarily play a
spectator role in the recognition process.
(17) Steiner, T. Angew. Chem., Int. Ed. 2002, 41, 48.
(18) Allen, F. H.; Motherwell, W. D. S.; Raithby, P. R.; Shields, G. P.;
Taylor, R. New. J. Chem. 1999, 25.
(19) Smyth, D. R.; Vincent, B. R.; Tiekink, E. R. T. Cryst. Growth Des.
2001, 1, 113.
In the context of the two structures reported herein, the
absence of the carboxylic acid dimer motif in 10 is, perhaps,
Inorganic Chemistry, Vol. 42, No. 4, 2003 1061

Crisp et al.
described for 4, complex 2 (0.400 g, 0.501 mmol) was treated with
AgOTf (0.122 g, 0.501 mmol), followed by picolinic acid (0.062
g, 0.504 mmol), to afford 7 as a white solid (0.200 g, 58%). IR
Experimental Section
General Methods. All procedures were performed under an inert
atmosphere of high purity N2 using standard Schlenk techniques.
CH2Cl2 was distilled from CaH2. Toluene was predried over CaSO4,
followed by distillation from sodium metal. Nicotinic, isonicotinic,
and picolinic acids were obtained commercially (Aldrich) and were
dried in a desiccator over P2O5 prior to use. All 1D NMR spectra
were recorded at 298 K by means of a Varian Gemini 2000 NMR
spectrometer (1H at 300.10 MHz, 19F at 282.23 MHz, and 31P at
121.50 MHz). 2D COSY NMR spectroscopy experiments were
performed on a Varian Unity INOVA 600 MHz NMR instrument.
1H and 19F{1H} chemical shifts are reported in ppm relative to TMS
(0 ppm) and C6H5CF3 (63.7 ppm), respectively. 31P{1H} spectra
were referenced to a sealed external standard of 85% H3PO4 (0
ppm). IR spectra were recorded as Nujol mulls (NaCl plates) on a
Perkin-Elmer Spectrum BX FT-IR spectrophotometer. Elemental
analyses were determined by Chemical and Micro Analytical
Services, Pty. Ltd., Victoria (Australia). trans-Iodophenylbis(tri-
phenylphosphine)platinum(II) (1), trans-iodobis(methyldiphen-
ylphosphine)phenylplatinum(II) (2), and trans-iodophenylbis(tri-
ethylphosphine)platinum(II) (3) were prepared from 1,5-cyclooc-
tadiene(iodo)phenylplatinum(II) according to the general procedure
outlined by Clark et al.12
(Nujol) 1725 ν(CdO), 2700-3200 ν(OsH) cm-1 1H NMR
.
(CDCl3) δ 6.55 (m, 2H, Ho), 6.87 (m, 2H, Hm), 6.87 (m, 1H, Hp),
3
7.60 (m, 1H, H3), 8.68 (d, 1H, JHH ) 4.5 Hz, H6), 7.99 (td, 1H,
3JHH ) 7.4 Hz, 4JHH ) 1.8 Hz, H5), 8.25 (dt, 1H, 3JHH ) 7.50 Hz,
4JHH ) 1.2 Hz, H4), 7.42-7.18 (m, 20H, PsPh), 1.42 (m, 6H, Ps
1
Me). 31P{1H} NMR δ 6.5 (s, JPtP ) 3089 Hz). Anal. Calcd for
C25H40F3NO5P2PtS: C, 49.58; H, 3.84; N, 1.48. Found: C, 49.53;
H, 3.63; N, 1.59.
trans-Bis(methyldiphenylphosphine)(nicotinic acid)phenyl-
platinum(II) Triflate (8). Following a similar procedure to that
described for 4, complex 2 (0.348 g, 0.436 mmol) was treated with
AgOTf (0.106 g, 0.414 mmol), followed by nicotinic acid (0.0537
g, 0.436 mmol), to afford 8 as a white solid (0.312 g, 88%). IR
(Nujol) 1720 ν(CdO), 2800-3200 ν(OsH) cm-1 1H NMR
.
(CDCl3) δ 8.37 (s, 1H, H2), 8.19 (d, 1H, 3JHH ) 4.8 Hz, H6), 6.89
(m, 1H, H5), 7.88 (d, 1H, 3JHH ) 8.1 Hz, H4), 7.52-7.16 (m, 20H,
P-Ph), 1.62 (m, 6H, PsMe). 31P{1H} NMR δ 9.4 (s, 1JPtP ) 2870
Hz).19F NMR δ 48.5. Anal. Calcd for C39H36F3NO5P2PtS: C, 49.58;
H, 3.84; N, 1.48. Found: C, 49.60; H, 3.74; N, 1.53.
trans-Bis(methyldiphenylphosphine)(isonicotinic acid)phenyl-
platinum(II) Triflate (9). Following a similar procedure to that
described for 4, complex 2 (0.132 g, 0.165 mmol) was treated with
AgOTf (0.040 g, 0.157 mmol), followed by isonicotinic acid (0.020
g, 0.162 mmol), to afford 9 as a white solid (0.132 g, 85%). IR
trans-Phenyl(picolinic acid)bis(triphenylphosphine)platinum-
(II) triflate (4). To a stirred solution of 1 (0.400 g, 0.433 mmol)
in CH2Cl2 (20 mL) was added AgOTf (0.106 g, 0.433 mmol). The
mixture was stirred in the absence of light for 16 h, and AgI was
removed by filtration through Celite filter aid. To the clear filtrate
was added picolinic acid (0.0533 g, 0.433 mmol), and the mixture
was stirred for 16 h at room temperature. The solvent was removed
in vacuo to yield 4 as a white solid (0.270 g, 68%). IR (Nujol)
(Nujol) 1712 ν(CdO), 2500-3000 ν(OsH) cm-1
.
1H NMR
3
(CDCl3) δ 8.17 (d, 2H, JHH ) 6.6 Hz, H2/6), 6.78 (m, 2H, H3/5),
7.45-7.23 (m, 20H, PsPh), 1.64 (m, 6H, PsMe). 31P{1H} NMR
1
δ 9.4 (s, JPtP ) 2914 Hz).19F NMR δ 48.5. Anal. Calcd for
1720 ν(CdO), 2800-3100 ν(OsH) cm-1. H NMR (CDCl3) δ
1
C39H36F3NO5P2PtS: C, 49.58; H, 3.84; N, 1.48. Found: C, 49.44;
H, 3.88; N, 1.42.
3
6.63 (d, 2H, JHH ) 7.5 Hz, Ho), 6.33 (m, 2H, Hm), 6.33 (m, 1H,
Hp), 8.69 (m, 1H, H3), 8.03 (d, 1H, JHH ) 7.8 Hz, H6), 8.28 (td,
3
trans-(Isonicotinic acid)phenylbis(triethylphosphine)platinum-
(II) Triflate (10). Following a similar procedure to that described
for 4, complex 3 (0.255 g, 0.402 mmol) was treated with AgOTf
(0.098 g, 0.381 mmol), followed by isonicotinic acid (0.049 g, 0.402
mmol), to afford 10 as a white solid (0.298 g, 95%). IR (Nujol)
4
3
1H, JHH ) 1.8 Hz, JHH ) 7.4 Hz, H5), 7.75 (m, 1H, H4), 7.52-
7.14 (m, 30H, P-Ph). 31P{1H} NMR δ 23.0 (s, 1JPtP ) 3212 Hz).
Anal. Calcd for C25H40F3NO5P2PtS‚CH2Cl2: C, 52.05; H, 3.67; N,
1.21. Found: C, 52.27; H, 3.86; N, 1.40.
1
1731 ν(CdO), 2700-3100 ν(OsH) cm-1. H NMR (CDCl3) δ
trans-(Nicotinic acid)phenylbis(triphenylphosphine)platinum-
(II) Triflate (5). Following a similar procedure to that described
for 4, complex 1 (0.300 g, 0.325 mmol) was treated with AgOTf
(0.079 g, 0.309 mmol), followed by nicotinic acid (0.040 g, 0.325
mmol), to afford 5 as a white solid (0.312 g, 90%). IR (Nujol)
7.32 (d, 2H, 3JHH ) 8.1 Hz, 3JPtH ) 29 Hz, Ho), 6.87 (m, 2H, Hm),
3
7.05 (m, 1H, Hp), 8.89 (d, 2H, JHH ) 6.3 Hz, H2/6), 8.27 (m, 2H,
3JHH ) 6.6 Hz, H3/5), 1.73-1.02 (m, 30H, PEt3). 31P{1H} NMR δ
1
13.4 (s, JPtP ) 2697 Hz). 19F NMR δ 48.2. Anal. Calcd for
1720 ν(CdO), 2800-3200 ν(OsH) cm-1. H NMR (CDCl3) δ
1
C25H40F3NO5P2PtS: C, 38.46; H, 5.16; N, 1.79. Found: C, 38.46;
H, 5.15; N, 1.72.
3
6.74 (d, 2H, JHH ) 7.5 Hz, Ho), 6.37 (m, 2H, Hm), 6.53 (m, 1H,
Hp), 8.57 (s, 1H, H2), 8.20 (d, 1H, JHH ) 5.7 Hz, H6), 6.82 (m,
3
Determination of pKa Values. pKa determinations were carried
out in triplicate by preparing a stock solution (0.01 mol dm-3) of
the complex of interest in degassed 50% (v/v) EtOH/H2O solution.
3
1H, H5), 7.84 (d, 1H, JHH ) 7.8 Hz, H4), 7.57-7.14 (m, 30H,
P-Ph). 31P{1H} NMR δ 20.8 (s, JPtP ) 3045 Hz). 19F NMR δ
1
The solution was then titrated with KOH solution (0.1 mol dm-3
)
48.4. Anal. Calcd for C49H40F3NO5P2PtS: C, 55.06; H, 3.77; N,
1.31. Found: C, 55.09; H, 3.78; N, 1.40.
prepared in the same solvent system. The pH of the solution was
measured using a calibrated glass electrode with a 0.1 mol dm-3
silver-silver chloride electrode at 295.0 ( 0.1 K. The pKa value
for each of the complexes was calculated by means of the procedure
reported by Albert and Serjeant.20
trans-(Isonicotinic acid)phenylbis(triphenylphosphine)platinum-
(II) Triflate (6). Following a similar procedure to that described
for 4, complex 1 (0.075 g, 0.081 mmol) was treated with AgOTf
(0.019 g, 0.077 mmol), followed by isonicotinic acid (0.010 g, 0.081
mmol), to afford 6 as a white solid (0.078 g, 90%). IR (Nujol)
X-ray Diffraction. Suitable colorless crystals of 9 and 10 were
grown from CH2Cl2/n-hexane by means of diffusion-layering
techniques. Data for the crystals were collected at 173 K employing
graphite monochromatized Mo KR radiation, λ ) 0.71069 Å, on a
Rigaku AFC7R diffractometer. Corrections were made for Lorentz
1726 ν(CdO), 2800-3100 ν(OsH) cm-1. H NMR (CDCl3) δ
1
6.75 (d, 2H, 3JHH ) 6.9 Hz, 3JPtH ) 24 Hz, Ho), 6.31 (m, 2H, Hm),
6.51 (m, 1H, Hp), 8.35 (d, 2H, JHH ) 8.1 Hz, H2/6), 7.20 (m, 2H,
3
H3/5), 7.43-7.27 (m, 30H, PsPh). 31P{1H} NMR δ 22.1 (s, JPtP
1
) 3054 Hz). Anal. Calcd for C49H40F3NO5P2PtS: C, 55.06; H, 3.77;
N, 1.31. Found: C, 54.91; H, 3.73; N, 1.37.
trans-Phenyl(picolinic acid)bis(methyldiphenylphosphine)-
platinum(II) Triflate (7). Following a similar procedure to that
(20) Albert, A.; Serjeant, E. P. The Determination of Ionization Con-
stants: A Laboratory Manual, 3rd ed.; Chapman and Hall: New York,
1984.
1062 Inorganic Chemistry, Vol. 42, No. 4, 2003

Cationic σ-Phenylplatinum(II) Complexes
and polarization effects,21 and for absorption using an empirical
procedure (DIFABS22). Crystal data and refinement details are given
in Table 2.
Acknowledgment. We thank Mr P. Clements for record-
ing the COSY NMR spectra, and Dr S. M. Pyke for useful
NMR discussions. We also thank Johnson Matthey for the
generous loan of K2[PtCl4]. We are grateful to the Australian
Research Council (ARC) and the University of Adelaide for
financial support.
The structures were solved by heavy-atom methods,23 and each
was refined by a full-matrix least-squares procedure based on F2.21
Non-hydrogen atoms were refined with anisotropic displacement
parameters, and hydrogen atoms were included in the model in their
calculated positions; the O-H atom was located from a dif-
ference map in each case. The refinements were continued until
convergence after the application of a weighting scheme of the form
w ) 1/[σ2(Fo)2 + (0.0243P)2 + 2.1531P] for 9 and w ) 1/[σ2(Fo)2
Supporting Information Available: X-ray crystallographic data
files for 9 and 10, including atomic coordinates, bond distances
and angles, and thermal parameters, in CIF format. This material
is available free of charge via the Internet at http://pubs.acs.org.
2
+ (0.0239P)2 + 2.7679P] for 10, where P ) (Fo + 2Fc2)/3. The
IC025776A
numbering schemes, shown in Figures 1-3, were drawn with
ORTEP24 at 50% displacement ellipsoids.
(23) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; Garc´ıa-
Granda, S.; Smits, J. M. M.; Smykalla, C. The DIRDIF Program
System; Technical Report of the Crystallography Laboratory; The
University of Nijmegen: Nijmegen, The Netherlands, 1992.
(24) Johnson, C. K. ORTEP; Report ORNL-5138; Oak Ridge National
Laboratory, TN, 1976.
(21) teXsan: Structure Analysis Software; Molecular Structure Corp.: The
Woodlands, TX, 1997.
(22) Walker, N.; Stuart, D. Acta Crystallogr., Sect. A 1983, 39, 158.
Inorganic Chemistry, Vol. 42, No. 4, 2003 1063