Tight-contact ion pairs involving Pt(II) dithiooxamide complexes: The acid-base reactions between hydrohalogenated ion-paired complexes and pyridine

Article abstract of DOI:10.1021/ic900875b

The equilibrium constants relative to HCl exchange between Pt(ll)-containing tight contact ion pairs (TCIP) and pyridine have been investigated in chloroform solution at 298 K. The general formulas of the metal species are: {[Pt(H₂-R₂

Full text of DOI:10.1021/ic900875b

Inorg. Chem. 2009, 48, 10397–10404 10397
DOI: 10.1021/ic900875b
Tight-Contact Ion Pairs Involving Pt(II) Dithiooxamide Complexes: the Acid-Base
Reactions between Hydrohalogenated Ion-Paired Complexes and Pyridine
Antonino Giannetto,*,† Fausto Puntoriero,† Anna Barattucci,‡ Santo Lanza,*,† and Sebastiano Campagna*,†
†
ꢀ
Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, Universita di Messina, Via Sperone
‡
ꢀ
31, 98166 Messina, Italy, and Dipartimento di Chimica Organica e Biologica, Universita di Messina,
Via Sperone 31, 98166 Messina, Italy
Received May 6, 2009
The equilibrium constants relative to HCl exchange between Pt(II)-containing tight contact ion pairs (TCIP) and
pyridine have been investigated in chloroform solution at 298 K. The general formulas of the metal species are:
{[Pt(H2-R2-dithiooxamide)2]2þ, 2Cl-} (a-type compounds; R=methyl (1a), ethyl (2a), n-propyl (3a), iso-propyl (4a),
n-butyl (5a), cyclohexyl (6a), benzyl (7a), β-phenyl-ethyl (8a), allyl (9a)) and {[(H-R2-dithiooxamidate)Pt(H2-R2-
dithiooxamide)]+, Cl-} (b-type compounds; R has the same meanings as before, given rise to 1b-9b species;
moreover, the mixed R compound 10b, containing R=benzyl on a DTO (dthiooxamidate/dithiooxamide) ligand and
R = ethyl on the other DTO ligand, has also been investigated). Moreover, the parent species [Pt(H-R2-
dithiooxamidate)2] (c-type compounds; 1c-10c) have also been prepared. Out of 29 compounds reported in the
paper, 19 compounds are here reported for the first time, and their synthesis and characterization data are also given.
Compounds of a-type exhibit two successive equilibrium constants, which are related to successive HCl transfer from
the TCIP to pyridine. By comparing the equilibrium constants Kc of the various b-type species, we have been able to
(i) obtain information on the relative stability of the TCIP and, by taking advantage of the two equilibrium constants Kc1
and Kc2 found for each a-type species, (ii) gain knowledge on the electronic interaction between the two basic sites of
the Pt(II) bis-dithiooxamide complexes, mediated by the metal center. Linear relationships are found between the pKc
of the compounds and the σ-Taft value (Σσ*) of the amine substituents of the DTO ligands. Interestingly, the slope of
such linear correlations is much steeper for pKc2 than for pKc1, indicating that the electronic interaction between the
basic sites increases with the electron donating ability of the R substituent. A parallel is proposed between the splitting
of HCl transfer equilibrium constants in a-type TCIP and oxidation potential splitting in dinuclear, bridge-linked metal
complexes.
Introduction
usually anionic in nature.3,4 An example of biological pro-
cesses involving TCIPs is potassium transport in biological
membranes, which is supposed to be mediated by the forma-
tion of weak ion pairs.5 The formation of TCIP also plays
important roles on processes involving artificial systems, like
the stabilization of some diacids6 and the anion-templated
assembly of catenanes and pseudorotaxanes.7 However, in
spite of the importance of TCIPs in the above-mentioned
fields, detailed investigations on the parameters which deter-
mine TCIP formation and stability are quite rare, in parti-
cular when multiple TCIP equilibria are involved.
Tight-contact ion pairs (TCIPs) play key roles in anion and
ion-pair receptor chemistry,1,2 and in general in the func-
tional behavior of most cofactors and substrates involved in
biological transformations and/or processes, which are
*To whom correspondence should be addressed. E-mail: giannettoa@
unime.it (A.G.), lanza@unime.it (S.L.), campagna@unime.it (S.C.).
(1) (a) Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001, 40, 486. (b)
Gale, P. A. Coord. Chem. Rev. 2003, 240, 191. (c) Sessler, J. L.; Gale, P. A.; Cho,
W.-S. Anion Receptor Chemistry; Royal Society of Chemistry: Cambridge,
2006.
(2) (a) Manabe, K.; Okamura, K.; Date, T.; Koga, K. J. Org. Chem. 1993,
58, 6692. (b) Motomura, T.; Aoyama, Y. J. Org. Chem. 1991, 56, 7224. (c) Reetz,
M. T.; Niemeyer, C. M.; Hermes, M.; Goddard, R. Angew. Chem., Int. Ed. 1992,
31, 1017. (d) Dietrich, B.; Hosseini, M. W.; Lehn, J.-M.; Sessions, R. B. J. Am.
Chem. Soc. 1981, 103, 1282. (e) Dixon, R. P.; Geib, S. J.; Hamilton, A. D. J. Am.
Chem. Soc. 1992, 114, 365. (f) Ariga, K.; Anslyn, E. V. J. Org. Chem. 1992, 57,
417. (g) Arduini, A.; Secchi, A.; Pochini, A. In Calyxarenes in the Nanoworld;
Vicens, J., Harrowfield, J., Eds.; Springer: Berlin, 2007; p 63, and refs. therein
(3) Yoon, D.-W.; Gross, D. E.; Lynch, V. M.; Lee, C.-H.; Bennett, P. C.;
Sessler, J. L. Chem. Commun. 2009, 1109, and refs. therein.
(4) (a) Jeffrey, G. A.; Saenger, W. Hydrogen Bonding in Biological
Structures; Springer-Verlag: Berlin, 1991. (b) Hughes, M. P.; Smith, B. D.
J. Org. Chem. 1997, 62, 4492.
ꢀ
(5) (a) Gadsby, D. C. Nature 2004, 427, 795–7. (b) Noskov, S. Y.; Berneche,
S.; Roux, B. Nature 2004, 431, 830.
(6) Karaman, R.; Bruice, T. C. Inorg. Chem. 1992, 31, 2455.
(7) (a) Sambrook, M. R.; Beer, P. D.; Wisner, J. A.; Paul, R. L; Cowley,
A. R. J. Am. Chem. Soc. 2004, 126, 15364. (b) Sambrook, M. R.; Beer, P. D.;
Wisner, J. A.; Paul, R. L; Cowley, A. R.; Szemes, F.; Drew, M. G. B. J. Am. Chem.
Soc. 2005, 127, 2292.
r
2009 American Chemical Society
Published on Web 10/02/2009
pubs.acs.org/IC

10398 Inorganic Chemistry, Vol. 48, No. 21, 2009
Giannetto et al.
involving sequential titration of the two HCl moieties in each
compound, (ii) gain knowledge on the electronic interaction
between the two basic sites of the Pt(II) bis-dithiooxamide
complexes, mediated by the metal center. The investigation
has been performed by analyzing the properties of 29 com-
plexes (see Figure 2 for structural formulas): 19 are new
species, whose synthesis and full characterization is here
reported. Among the compounds (see Figure 2), nine contain
two HCl “complexed” sites (type a species), 10 are neutral
species (type c species), and 10 are mixed systems (type b
species). Also two compounds containing different R sub-
stituents have been investigated (10b and 10c).
2 þ
f½PtðH2-R2-DTOÞ ꢀ , 2Cl-g
2
þ py h f½ðH-R2-DTOÞPtðH2-R2-DTOÞꢀþ, Cl-g
Figure 1. Schematization of the equilibrium between TCIP and dehy-
drohalogenated species.
þ py HCl
ð1Þ
ð2Þ
3
Recently, we have demonstrated the ability of [Pt(H-R2-
dithiooxamidate)2] complexes (R = alkyl groups) to form
f½ðH-R2-DTOÞPtðH2-R2-DTOÞꢀþ, Cl-g
TCIPs of general formula {[Pt(H2-R2-dithiooxamide)2]2þ
,
þ py h ½PtðH-R2-DTOÞ2ꢀ þ py HCl
2Cl-} (see Figure 1) by addition of HCl,8,9 where ion-pair
formation is driven by H-bonding. The process is common to
HX species, where X isa halogen or a pseudo-halogen anion.9
Moreover, we have also reported that new solid-state lumi-
nescent sensors for gaseous HCl or amines can be prepared
by taking advantage of the luminescent nature of the {[Pt(H2-
R2-dithiooxamide)2]2þ, 2Cl-} species as opposed to the
absence of luminescence of the corresponding neutral
[Pt(H-R2-dithiooxamidate)2] complexes. Such luminescent
sensors operate via switchable formation of TCIP species,10
an extension of our former solution studies on the photo-
physical properties of Pt(II) dithiooxamide/dithiooxamidate
compounds.9,11
3
Results and Discussion
Synthetic Approach and Structural Considerations.
TCIPs 1a-9a can be easily obtained in very high yields
by adding the 2-fold amount of the desired substituted
dithiooxamide species to cis-Pt(Me2SO)2Cl2 (Scheme 1,
step i) in chloroform.8,9 The neutral complexes 1c-9c are
prepared from 1a-9a ion pairs by adding sodium bicar-
bonate (step ii in Scheme 1). Although the formation of
TCIPs 1a-9a is a stepwise process, the monochelate ion
pairs {[(Me2SO)ClPt(H2-R2-DTO)]+,Cl-} are not stable
in solution.8 However, the addition of the stoichiometric
amount of the desired dithiooxamide to a chloroform
suspension of cis-Pt(Me2SO)2Cl2 in the presence of
sodium bicarbonate affords the stable monochelate com-
plexes [(Me2SO)ClPt(H-R2-DTO)] (Scheme 1, step iii).
Such monochelate complexes have been already synthe-
sized and fully characterized,12 but here they have not
been isolated in that they were used as intermediate
species toward the preparation of b-type species. Indeed,
after removing sodium bicarbonate, a further stoichio-
metric amount of substituted dithioxamide is added to the
solution obtained by step iii, to finally yield type-b com-
plex ion pairs {[(H-R2-DTO)Pt(H2-R2-DTO)]+,Cl-} in a
high yield (Scheme 1, step iv). Dehydrohalogenation of
b-type ion pairs provides c-type complexes (Scheme 1,
step v). If a different dithioxamide H2-R02-DTO is added
to [(Me2SO)ClPt(H-R2-DTO)] in step iv, compounds
having different R substituents can be obtained. This is
the route used for preparing 10b and 10c.
To gain information on the equilibrium between TCIPs
containing HCl “moieties” and parent dehydrohalogenated
systems in Pt(II) bis(R-dithiooxamide/amidate) complexes,
so inferring useful concepts for the design of more elaborate
luminescent sensors and, on a broader viewpoint, to learn
more on parameters capable to influence TCIP stability, we
started a systematic investigation on the properties which
govern the equilibrium shown in Figure 1. In particular, here
we investigate the equilibrium constants between {[Pt(H2-R2-
dithiooxamide)2]2þ, 2Cl-} and pyridine in chloroform solu-
tion. Indeed, pyridine can compete with {([Pt(H2-R2-
dithiooxamide)2])2þ, 2Cl-} species for protons, so driving
formation of the neutral parent complexes from the tight ion
pairs. The reactions studied are shown in eqs 1 and 2. In these
equations, py stands for pyridine and DTO stands for
dithiooxamidate or dithiooxamide ligands. By comparing
the equilibrium constants for such reactions in the various
compounds, we have been able to (i) obtain information
on the relative stability of the TCIP, and, by taking advantage
of the difference between the two equilibrium constants
Type c complexes exhibit a planar structure.8 On the
contrary, the bis-chelated a-type complexes cannot have a
planar asset of dithiooxamide ligands around the square
planar Pt(II) center since two coplanar amidic hydrogens
would exert a high reciprocal repulsion, and as a conse-
quence the dithooxamide ligands have to be distorted
ꢀ
ꢀ
(8) Rosace, G.; Bruno, G.; Monsu Scolaro, L.; Nicolo, F.; Sergi, S.;
Lanza, S. Inorg. Chim. Acta 1993, 208, 59.
(9) Rosace, G.; Giuffrida, G.; Guglielmo, G.; Campagna, S.; Lanza, S.
Inorg. Chem. 1996, 35, 6816.
(10) Nastasi, F.; Puntoriero, F.; Palmeri, N.; Cavallaro, S.; Campagna,
S.; Lanza, S. Chem. Commun. 2007, 4740.
(11) For other cases of photophysical properties of TCIPs, see for
example: McCosar, B. H.; Schanze, K. S. Inorg. Chem. 1996, 35, 6800.
(12) Lanza, S.; Loiseau, F.; Tresoldi, G.; Serroni, S.; Campagna, S. Inorg.
Chim. Acta 2007, 360, 1929.

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10399
Figure 2. Structural formulas and abbreviations of the investigated complexes.
Scheme 1. Synthetic Routes for a-, b-, and c-Type Complexes
from planarity. Actually, the crystal structures of neutral
dithiooxamide complexes of copper, zinc,13 and
bismuth14 show R(H)NC(=S)-C(=S)N(H)R torsional
angles of about 45°. Nevertheless, a-type complexes exhi-
bit sharp A2 or A2B2 signals for N-CH2- protons, and
this suggests the rapid twisting of R(H)NC(=S) groups
around the C-C pivot bond of the S,S-coordinated
dithiooxamide. Type b complexes are constituted by a
S,S-chelate dithiooxamidate and a S,S chelate dithio-
oxamide which retains a chloride ion as a contact ion
pair. However, their proton spectra in solution show only
one pattern of signals for identical R groups of both
S,S-chelate ligands; this is in line with a rapid intermole-
cular exchange of the HCl moiety between the nitrogen
systems of the ligands.
(13) Antolini, L.; Fabretti, G.; Franchini, L.; Menabue, G.; Pellacani, C.;
Desseyn, H. O.; Dommisse, R.; Hoffamans, H. C. J. Chem. Soc., Dalton
Trans. 1987, 1921.
(14) Drew, M. G. B.; Kisenyi, J. M.; Willey, G. R. J. Chem. Soc., Dalton
Trans. 1984, 1723.

10400 Inorganic Chemistry, Vol. 48, No. 21, 2009
Giannetto et al.
Figure 3. Changes of the absorption spectrum of 4a (1.39 ꢁ 10-4 M)
upon successive pyridine addition in chloroform. Panel (a) shows the
overall process, and panels (b) and (c) show the separate two successive
processes. See text for details.
Figure 4. Titration curves relative to the experiments in Figure 3(b)
(top) and Figure 3(c) (bottom). [complex] = 1.39 ꢁ 10-4 M. Top:
absorption values monitored at λ=513 nm; bottom: absorption values
monitored at λ=534 nm. [py] stands for molar concentration of added
pyridine. A stands for absorbance.
Acid-Base Behavior. Pyridine has been successfully
used to study proton transfer.15 Since proton transfer is
the first requisite for the occurrence of the equilibria
shown in eqs 1 and 2, we performed titration of such
equilibria by using pyridine as the titration agent for the
{[Pt(H2-R2-DTO)2]2þ, 2Cl-} complexes (1a-9a) in
chloroform, by following the evolution of the process
spectrophotometrically.
Table 1. Equilibrium Constants and σ-Taft Valuesa
com- R substi-
pound
tuent
Kc1
Kc2
Σσ*
pKc1 pKc2
A typical experiment is shown in Figure 3, reporting the
changes of the absorption spectrum of {[Pt(H2-(iso-
propyl)2-DTO)2]2þ, 2Cl-} (4a) upon pyridine addition.
Clearly, two successive reactions take place, and each one
is characterized by a set of isosbestic points. For 4a,
isosbestic points are at 533 and 446 nm for the first step,
whereas isosbestic points for the second step are at 471
and 401 nm. Figure 4 shows the titration curves connected
with the experiment in Figure 3. All the a-type complexes
exhibit similar absorption spectrum changes and titration
behavior, which corresponds to sequential HCl transfer
from the Pt(II) complexes to pyridines. This assignment is
confirmed by comparison of the spectra obtained upon
first and second titration steps of the a-type compounds
with those of the corresponding b-type and c-type com-
pounds, independently prepared (compare spectra in
Figure 3 with those shown in Supporting Information).
The absorption spectra changes upon pyridine titration
have already been interpreted9,10 and are due to a blue-
shift of the low-energy charge-transfer (CT) band, corre-
sponding to a CT transition from a Pt/S orbital to DTO-
centered orbitals, on passing from a-type to c-type
species.
1a
2a
3a
4a
5a
6a
7a
8a
methyl
ethyl
n-propyl
4.1 ((0.4) 0.066 ((0.004) -0.98 -0.613 1.180
2.3 ((0.4) 0.039 ((0.003) -0.88 -0.362 1.409
2.2 ((0.3) 0.077 ((0.05) -0.86 -0.342 1.113
iso-propyl 1.7 ((0.2) 0.028 ((0.003) -0.79 -0.230 1.796
cyclohexyl 2.6 ((0.3) 0.024 ((0.002) -0.83 -0.415 1.620
n-butile
benzyl
β-phenyl-
ethyl
2.1 ((0.3) 0.062 ((0.004) -0.85 -0.322 1.208
7.2 ((0.5) 1.280 ((0.05) -1.20 -0.857 -0.107
4.9 ((0.5) 0.250 ((0.05) -1.06 -0.690 0.602
9a
allyl
6.1 ((0.5) 1.40 ((0.05)
-1.11 -0.785 -0.146
0.824
10bb benzyl/ethyl
0.150 ((0.005) -1.03
a Data are in chloroform solution at 298 K. Σσ* values are taken from
ref 16b, and refer to the value derived from the amine species used to
prepare the a-type compounds (see ref 17). The Kc2 values of the a-type
compounds are identical, within experimental uncertainty, to the single
Kc value obtained for the corresponding b-type compounds. b The Kc2
value for 10b is the single Kc value obtained. The Σσ* value assigned to
this compound has a different meaning, explained in the text.
The equilibrium constants Kc are collected in Table 1,
which also reports the pKc values. The different Kc con-
stants (Kc1 and Kc2, see eqs 3 and 4, related to equilibria
in eqs 1 and 2, respectively) of the two titration equilibria
for each compound indicate that significant electro-
nic interaction takes place between the two sites involved
in equilibrium reactions. The equilibrium in eq 2 has also
been investigated by using the independently prepared
and characterized {[(H-R2-DTO)Pt(H2-R2-DTO)]+, Cl-}
(15) See for example: Menger, F. M.; Singh, T. D.; Bayer, F. L. J. Am.
Chem. Soc. 1976, 98, 5011.

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10401
ligands to coordinate protons (and, via hydrogen bond-
ing, the HCl “moiety”). It is therefore not unexpected that
the linear correlation in Figure 5 is found, with less
negative Σσ* values corresponding to higher effective
electron density on the nitrogen and as a consequence
to lower pKc values, that is, relatively more stable TCIPs.
The second equilibrium constants Kc2 values span a
much larger range for the series of compounds under
study, compared to the first equilibrium constants Kc1
values (Table 1, Figure 5), although the linear correlation
between Kc2 and the Taft constant is still kept. Differ-
ences between Kc1 and Kc2 for each species are related to
the electronic interaction between the two active sites.
Apparently, the difference in Kc1 and Kc2 increases with
the electronic density on the nitrogen of the amine
substituents of the DTO, as indicated by the Taft con-
stant. Such a behavior can be rationalized by considering
that upon the first step of the titration (that is, once b-type
species are formed from a-type compounds), the electron
density on the already titrated nitrogen, which is not
engaged in hydrogen bonding anymore, is now available
to be partially transferred, via the Pt(II) complex mole-
cular framework, to the other half of the molecule where
the hydrogen bonding is still present, reinforcing the still
existing TCIP. Such a reinforcement effect is therefore
more effective (so leading to a larger splitting in the Kc
values) for R substituents inducing more effective elec-
tron density on their nitrogens, according to the σ-Taft
values. The situation is reminiscent of the oxidation
splitting occurring in symmetric dinuclear metal com-
plexes where the metal centers, which undergo oxidation,
are connected by bridging ligands.18 In these latter
species, oxidation of the first metal center causes a shift
of electron density, via bridging ligand orbitals, making
second metal oxidation more difficult. When the bridging
ligand is kept constant, the metal-centered oxidation
splitting depends on the effective electron density which
is located on each metal center (in other words, from the
energy level of the metal-centered occupied orbitals), on
its turn a function of the nature of the peripheral ligands.
For strongly electron acceptors peripheral ligands, leav-
ing less electron density on the metal centers (i.e., strongly
stabilizing the metal orbitals, which have to couple with
the lowest unoccupied molecular orbital (LUMO)
centered in the bridging ligands, in the superexchange
electron-transfer mechanism mediating the metal-metal
coupling), oxidation splitting can not occur.19,20 In this
similitude, oxidation splitting is here represented by Kc1
and Kc2 splitting and the electronic factors, governed in
bridged dinuclear metal complexes by peripheral ligands,
are here determined by the σ-Taft values of the nitrogen
substituents of the dithiooxamide ligands. As a matter of
fact, Kc1 and Kc2 tend to become closer and closer with
decreasing σ-Taft values, and can be predicted that for
Figure 5. Plots of pKc vs Σσ* values given in Table 1; (1) and (2) refer to
pKc1 and pKc2, respectively.
b-type species. Obviously, for the b-type species (whichonly
contain a single acid site) a single titration step was found,
and the Kc values obtained (not shown) are identical,
within experimental uncertainty, to the Kc2 values of the
corresponding type-a species. An exception to this general
trend is represented by 10b, whose behavior will be dis-
cussed later.
½f½ðH-R2-DTOÞPtðH2-R2-DTOÞꢀþ, Cl-gꢀ½pyHClꢀ
Kc1 ¼
2 þ
½f½PtðH2-R2-DTOÞ2ꢀ , 2Cl-gꢀ½pyꢀ
ð3Þ
½PtðH-R2-DTOÞ2ꢀ½pyHClꢀ
Kc2 ¼
½f½ðH-R2-DTOÞPtðH2-R2-DTOÞ2ꢀþ, Cl-gꢀ½pyꢀ
ð4Þ
The data in Table 1 warrant extensive discussion. The
Kc values are related to the stability of the {[Pt(H2-R2-
DTO)2]2þ, 2Cl-} ion pairs: at a first approximation,
lower values indicate that the equilibria in eqs 1 and 2 are
displaced toward the reactants, suggesting an increased
relative stability of the {[Pt(R-DTO)2H2]2þ, 2Cl-} ion
pair. A very good linear correlation is found between the
pKc values and the so-called σ-Taft value (Σσ*)16,17 of the
amine substituents of the DTO ligands (Figure 5).
The σ-Taft value for an amine is a constant which
expresses the polar effect of the substituents, taking also
into account for steric effects on the nitrogen atom, so
determining its basicity. In other words, the σ-Taft value
reflects the effective electron density on the nitrogen
atom. In our case, it expresses the ability of the DTO
(16) (a) Taft, R. W. J. Am. Chem. Soc. 1953, 75, 4231. (b) Hall, H. K., Jr.
J. Am. Chem. Soc. 1957, 79, 5441. (c) De Tar, D. F. J. Am. Chem. Soc. 1980,
102, 7988.
(18) (a) Robin, M. B.; Day, P. Adv. Inorg. Radiochem. 1967, 10, 247. (b)
Richardson, D. E.; Taube, H. J. Am. Chem. Soc. 1983, 105, 40. (c) Creutz, C.
Prog. Inorg. Chem. 1983, 30, 1.
(17) As Σσ* values, we used the values reported in ref 16b for the various
amines. Note that we used the values for the amines employed to prepare the
DTO ligands, and not the Σσ* of the DTO ligands, which should be used
from a more rigorous point of view. However, we feel that this functional
approximation is acceptable, since the difference in the DTO properties are
dictated by the R substituents on the nitrogens.
(19) Giuffrida, G.; Campagna, S. Coord. Chem. Rev. 1994, 135-136, 517.
(20) The situation is even more general. This discussion applies indeed to
any symmetric species with two identical redox-active sites, and has its
explanation in the superexchange theory.21 The dinuclear metal complexes
oxidation splitting is taken as an example, and a simplified view is here used
for immediate qualitative comparison with the present systems.

10402 Inorganic Chemistry, Vol. 48, No. 21, 2009
Giannetto et al.
nitrogen substituents yielding σ-Taft values of about
-1.45, a single titration process, corresponding to simul-
taneous transfer of the two HCl moieties of a {[Pt(H2-R2-
DTO)2]2þ, 2Cl-} species to pyridines, can take place.
As already mentioned, 10b represents a unique species
in the series of compounds here studied: it is a mono-
hydrohalogenated species containing differently N-sub-
stituted DTO moieties (Figure 2). Whereas for the other
b-type species here studied, which all have identical
N-substituted DTO moieties on both sides of the metal
complex, the coordinated HCl molecule is very rapidly
exchanged between the two sites, the N-substituted DTO
species of 10b have quite different basic properties
(compare Kc values of 2a and 7a in Table 1), so that the
coordinated HCl can become prevalently localized on
the (more basic) ethyl-substituted DTO. The single Kc
value obtained for 10b (reported in Table 1 as Kc2, for
uniformity of presentation) is therefore most likely
referred to the titration of HCl moiety coordinated by
the ethyl-substituted DTO. Within such an assumption, if
no electronic interaction was active between the two DTO
moieties of the complex, the Kc value for 10b should
correspond to the Kc2 of 2a (0.039), which is indeed
equivalent to the single Kc value of 2b. The experimental
Kc value of 10b is instead quite larger (0.15, see Table 1),
confirming that extensive electronic interaction takes
place between the two DTO ligands of 10b. The HCl-free
N-benzyl substituted dithiooxamidate ligand of 10b has
in fact less effective electron density than the equivalent
HCl-free N-ethyl dithiooxamidate ligand which is present
in 2b, so a smaller electron density can be transferred via
the metal center to the HCl-coordinated, otherwise
equivalent, N-ethyl substituted dithiooxamide ligand in
10b compared to 2b. This makes the TCIP in 10b less
stable (and therefore the HCl moiety can be more easily
transferred to the pyridine, yielding a higher Kc value)
than in 2b. Interestingly, if the electronic interaction
between the two DTO moieties of 10b is taken into
account by considering a virtual, geometrically averaged
single Σσ* value to 10b (this is indeed the meaning of the
Σσ* value reported for this compound in Table 1), the
linear correlation in Figure 5 is followed also by 10b. This
result, indicating that R substituents which are not
directly involved in the ion pairing process (for 10b, the
benzyl substituent of the dithiooxamidate ligand) affect
the stability constant of remote ion-paired sites (the ethyl-
substituted N atom), suggests that long-range effects
can also be active in determining the stability of TCIP
species.
nature of the R substituents of the dithiooxamide (DTO)
ligands. Indeed the pKc values of the above-mentioned
equilibria for the variously R-substituted compounds are
linearly dependent on the σ Taft parameter of the R substit-
uents. Moreover, the splitting of the two Kc values related to
successive HCl transfer reactions occurring in each a-type
compound decreases with the σ Taft parameter, that is on the
effective electron density on the nitrogen atom of the DTO
ligand, involved in the transfer reaction. This effect indicates
that significant electronic interaction between the two
acid-base sites of the {[Pt(H2-R2-dithiooxamide)2]2þ, 2Cl-}
compounds takes place, most likely mediated by the metal
center. In agreement with such a conclusion, the single Kc
value obtained for the mixed-R compound {[(H-benzyl2-
DTO)Pt(H2-ethyl2-DTO)]+, Cl-} does not correspond to
the expected value for HCl exchange reaction involving an
ethyl-DTO ligand, but to a Kc value geometrically averaged
between ethyl- and benzyl-substituted DTO ligands. In
particular, this latter result also indicates a long-range effect
of the R substituent which is not directly involved in ion
pairing on the TCIP stability. Finally, we proposed a parallel
between splitting of Kc values in multiple TCIP systems and
oxidation splitting in symmetric dinuclear metal complexes.
Altogether, our data represent an approach for investigat-
ing the stability of TCIPs and demonstrates that the equili-
brium constants in the formation of multiple TCIP systems
can be significantly interconnected one another and governed
by parameters which can be easily tuned, such as electron
density on ion pairing sites. Further work is in progress in our
laboratory to investigate in detail the temperature depen-
dence of the equilibrium constants, with the aim of obtaining
thermodynamic parameters.
Experimental Section
Solventswerepurified bystandardproceduresanddistilled
before the use. Secondary dithioxamides22 and cis-Pt-
(Me2SO)2Cl2 were prepared according to the literature
23
methods. Electronic spectra were recorded with a PE
Lambda35 UV VIS spectrometer. 1H NMR and 13C-
{1H}NMR spectra were recorded at 298 K on a Bruker
ARX-300, equipped with a broadband probe operating at
300.13 and 75.56 MHz, respectively. Chemical shifts (δ, ppm)
were referred to SiMe4. Details on the procedure used to
determine the equilibrium constants Kc are given in the
Supporting Information.
Preparation of Compounds. {[Pt(H2R2DTO)2]++,2Cl-},
a-type 1a-9a compounds have been prepared according to
reported general procedures.8,9 Characterization of 1a, 5a, 6a,
and 7a have been already reported.8,9 Characterization of new
compounds is as follows:
2a. 1H NMR (300.13 MHz, CDCl3, room temperature):
3
δ 13.55 (bs, N-H); 3.81 (q, N-CH2-CH3, J=7.2); 1.38 (t,
Conclusions
3
N-CH2-CH3, J=7.2). 13C{1H}NMR (75.48 MHz, CDCl3,
The equilibrium constants of the HCl exchange reaction
between Pt(II)-containing tight contact ion pairs of general
formula {[Pt(H2-R2-dithiooxamide)2]2þ, 2Cl-} (a-type com-
pounds) and pyridine have been studied. The results show
that the stability of the TCIP is significantly influenced by the
room temperature) δ 184.00 (CS); 45.00 (N-CH2-CH3); 12.18
(N-CH2-CH3). Analysis: calculated for C12H24N4S4Cl2Pt
(MW 618.59): C, 23.30; H, 3.91; N, 9.06. Found: C, 23.23; H,
3.99; N, 8.92. Yield 89%.
3a. 1H NMR (300.13 MHz, CDCl3, room temperature):
δ 13,37 (bs, N-H); 3.70 (t, N-CH2-CH2-CH3, 3J = 7.3);
1.98 (m, N-CH2-CH2-CH3); 1.06 (t, N-CH2-CH2-CH3,
(21) (a) McConnell, H. H. J. Chem. Phys. 1961, 35, 508. (b) Lambert, C.;
::
Noll, G. J. Am. Chem. Soc. 1999, 121, 8434. (c) Demadis, K. D.; Hartshorn,
C. M.; Meyer, T. J. Chem. Rev 2001, 101, 2655. (d) Paddon-Row, M. N. In
Electron Transfer in Chemistry (Ed.: Balzani, V.), Wiley-VCH, Weinheim,
2001, Vol. 3, p. 179. (e) Brunschwig, B. S.; Creutz, C.; Sutin, N. Chem. Soc. Rev.
2002, 31, 168.
(22) Hurd, R. N.; De La Mater, G.; McElheny, C. G.; Turner, R. J.;
Vallingford, V. H. J. Org. Chem. 1961, 26, 3980.
(23) Kukushkin, Y. N.; Viaz’menskii, Y. E.; Zorina, L. I.; Pazhukina, Y.
L. Russ. J. Inorg. Chem. 1968, 13.

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10403
3J = 7.3). 13C{1H}NMR (75.48 MHz, CDCl3, room
temperature) δ 183.7 (CS); 51.65 (N-CH2-CH2-CH3); 20.53
(N-CH2-CH2-CH3); 11.77 (N-CH2-CH2-CH3). Analysis:
calculated for C16H32N4S4Cl2Pt (MW 674.70): C, 28.48; H, 3.91;
N, 9.06. Found: C, 28.39; H, 3.81; N, 9.16. Yield 85%.
C, 30.11; H, 4.89; N, 8.78. Found: C, 30.25; H, 4.72; N, 8.88.
Yield 84%.
6b. 1H NMR (300.13 MHz, CDCl3, room temperature):
δ 13.0 (N-H); 4.06 (m, N-CH2(CH2)5); 1.3-2.12 (m, N-CH2-
(CH2)5). 13C{1H}NMR (75.48 MHz, CDCl3, room
temperature): δ 59.40 (m, N-CH2(CH2)5); 24.80, 29.90, 31.50
(N-CH2(CH2)5). Analysis: calculated for C32H55N4S4ClPt
(MW 854.60): C, 44.97; H, 6.49; N, 6.55. Found: C, 45.11; H,
6.52; N, 6.43. Yield 90%.
4a. 1H NMR (300.13 MHz, CDCl3, room temperature):
δ 13.57 (bs, N-H); 4.45 (seven lines, N-CH(CH3)2 3J=6.45);
1.57 (d, N-CH(CH3)2, 3J=6.45). 13C{1H}NMR (75.48 MHz,
CDCl3, room temperature)
δ
185.00 (CS); 54.03
7b. 1H NMR (300.13 MHz, CDCl3, room temperature):
δ 13.50 (bs, N-H); 7.31-7.46 (m, N-CH2-C6H5); 4.86 (s,
N-CH2-C6H5). 13C{1H}NMR (75.48 MHz, CDCl3, room
temperature): δ 128.35, 128.90, 134.30 (N-CH2-C6H5); 52.90
(N-CH2-C6H5). Analysis: calculated for C32H31N4S4ClPt
(MW 830.41): C, 46.28; H, 3.76; N, 6.45. Found: C, 46.13; H,
3.70; N, 6.52. Yield 80%.
(N-CH(CH3)2); 20.17 (N-CH(CH3)2). Analysis: calculated
for C16H32N4S4Cl2Pt (MW 674.70): C, 28.48; H, 3.91; N, 9.06.
Found: C, 28.53; H, 3.82; N, 8.97. Yield 78%.
8a. 1H NMR (300.13 MHz, CDCl3, room temperature): δ
13,40 (bs, N-H); 7.20-7.33 (m, N-CH2-CH2-C6H5); 3.95 (t,
3
N-CH2-CH2-C6H5, J = 7.7); 3.25 (t, N-CH2-CH2-C6H5,
3J=7.7). 13C{1H}NMR (75.48 MHz, CDCl3, room temperature)
δ 184.2 (CS); 123.4, 131.0, 137.3, 146.9 (N-CH2-CH2-C6H5);
51.00 (N-CH2-CH2-); 33.28 (N-CH2-CH2-C6H5). Analy-
sis: calculated for C36H40N4S4Cl2Pt (MW 922.98): C, 46.84; H,
4.37; N, 6.07. Found: C, 46.61; H, 4.42; N, 6.14. Yield 86%.
9a. 1H NMR (300.13 MHz, CDCl3, room temperature): δ 13.3
(bs, N-H); 6.06 (m, N-CH2-CHdCH2); 5.45 (d, N-CH2-
CHdCHtransHcis, 3J=10.0); 5.54 (d, N-CH2-CHdCHtransHcis,
1
8b. H NMR (300.13 MHz, CDCl3, room temperature): δ
13,28 (bs, N-H); 7.206-7.332 (m, N-CH2-CH2-C6H5); 3.86
(t, N-CH2-CH2-C6H5, 3J = 7.0); 3.022 (t, N-CH2-
CH2-C6H5, 3J=7.0). 13C{1H}NMR (75.48 MHz, CDCl3, room
temperature): δ 126.61, 128.65, 128.67, 138.67 (N-CH2-
CH2-C6H5); 50.40 (N-CH2-CH2-C6H5); 34.90 (N-CH2-
CH2-C6H5). Analysis: calculated for C36H39N4S4ClPt (MW
886.52): C, 48.77; H, 4.43; N, 6.32. Found: C, 48.90; H, 4.33; N,
6.41. Yield 83%.
3
3J = 17.0); 4.39 (d, N-CH2-CHdCH2, J=6). 13C{1H}NMR
(75.48 MHz, CDCl3, room temperature) δ 184.00 (CS); 128.24
(N-CH2-CHdCH2); 121.42 (N-CH2-CH=CH2); 51.80
(N-CH2-CH=CH2). Analysis: calculated for C16H24N4S4Cl2-
Pt(MW 666.66): C, 28.83; H, 3.63; N, 8.40. Found: C, 28.72; H,
3.56; N, 9.51. Yield 91%.
9b. 1H NMR (300.13 MHz, CDCl3, room temperature):
δ 13.3 (bs, NH); 6.05 (m, N-CH2-CHdCH2); 5.39 (d,
N-CH2-CHdCHtransHcis
,
3J = 17.0); 5.34 (d, N-CH2-
3
3
CHdCHtransHcis, J=10.0); 4.31 (d, N-CH2-CHdCH2, J=
6). 13C{1H}NMR (75.48 MHz, CDCl3, room temperature):
δ 130.90 (N-CH2-CHdCH2); 119.60 (N-CH2-CH=CH2);
{[(HR2DTO)Pt(H2R2DTO)]+,Cl-}, b-Type Compounds.
General Method. One mmol of cis-Pt(Me2SO)2Cl2 was sus-
pended in chloroform (150 mL) together with 2 g of NaHCO3.
The equivalent quantity (1 mmol) of the desired dithiooxamide
was added in small amount to this mixture which becomes
orange within a few minutes. The mixture was allowed to stand
for 1 h. After this time NaHCO3 was removed by filtration. To
the orange filtrate 1 mmol of dithiooxamide was added. The
solution turned purple within few minutes. After standing (1 h)
the purple solution was concentrated to a small volume. Petro-
leum ether was added and {[(H-R2-DTO)Pt(H2-R2-DTO)2]+,
Cl-} precipitated as a dark purple powder.
51.70 (N-CH2-CH=CH2). Analysis: calculated for C16H234
-
N4S4ClPt (MW 630.17): C, 30.49; H, 3.68; N, 8.89. Found: C,
30.51; H, 3.77; N, 8.71. Yield 77%.
{[(H(ethyl)2DTO)Pt(H2(benzyl)2DTO)]+,Cl-} (10b). A 1 mmol
portion of cis-Pt(Me2SO)2Cl2 was suspended in chloroform (150
mL) together with 2 g of NaHCO3. The equivalent quantity
(1 mmol) of the diethyldithiooxamide was added in small
amount to this mixture which became orange within a few
minutes. The mixture was allowed to stand for 1 h. After this
time NaHCO3 was removed by filtration. To the orange filtrate
1 mmol of dibenzyldithiooxamide was added. The solution
turned purple within few minutes. After standing (1 h) the
purple solution was concentrated to a small volume. Petroleum
ether was added, and {[(H(ethyl)2DTO)Pt(H2(benzyl)2DTO)]+,
Compound 5b has been already reported.9 Characterization
data for the new compounds are as follows:
1b. 1H NMR (300.13 MHz, CDCl3, room temperature):
δ 13.4 (bs, N-H); 3.39 (s, N-CH2-). 13C{1H}NMR (75.48
MHz, CDCl3, room temperature) δ 35.80 (N-CH3). Analysis:
calculated for C8H15N4S4ClPt (MW 526.03): C, 18.27; H, 2.87;
N, 10.65. Found: C, 18.39; H, 2.78; N, 10.78. Yield 82%.
2b. 1H NMR (300.13 MHz, CDCl3, room temperature):
1
Cl-} precipitated as a dark purple powder. H NMR (300.13
MHz, CDCl3, room temperature): δ 13.23 (bs, NH); 7.29-7.36
(m, N-CH2-C6H5) 4.86 (s, N-CH2-C6H5); 3.77 (q,
N-CH2-CH3, 3J = 7.2); 1.56 (t. N-CH2-CH3, 3J = 7.2).
13C{1H}NMR (75.48 MHz, CDCl3, room temperature): δ
128.13, 128.30, 128.920 (N-CH2-C6H5); 53.50 (N-CH2-
C6H5); 44.88 (N-CH2-CH3); 12.90 (N-CH2-CH3). Analysis:
calculated for C22H27N4S4ClPt (MW 706.27): C, 37.41; H, 3.85;
N, 7.93. Found: C, 37.50; H, 3.72; N, 8.02. Yield 84%.
[Pt(HR2DTO)2], c-Type Compounds. [Pt(HR2DTO)2], c-type
compounds have been prepared according to reported proce-
dures.8 Characterization data of 1c, 4c, 5c, 6c, and 7c have been
already reported. Characterization data of the new compounds
are as follows:
3
δ 13.21 (bs, N-H): 3.76 (q, N-CH2-CH3, J=7.2); 1.38 (t,
3
N-CH2-CH3, J=7.2). 13C{1H}NMR (75.48 MHz, CDCl3,
room temperature):
δ
44.55 (N-CH2-CH3); 12.51
(N-CH2-CH3). Analysis: calculated for C12H23N4S4ClPt
(MW 582.13): C, 24.76; H, 3.98; N, 9.62. Found: C, 24.61; H,
4.05; N, 9.53. Yield 77%.
3b. 1H NMR (300.13 MHz, CDCl3, room temperature):
3
δ 13.25 (bs, NH); 3.70 (t, N-CH2-CH2-CH3, J=7.3); 1.98
3
(m, N-CH2-CH2-CH3); 1.06 (t, N-CH2-CH2-CH3, J =
7.3). 13C{1H}NMR (75.48 MHz, CDCl3, room temperature): δ
51.30 (N-CH2-CH2-CH3); 21.00 (N-CH2-CH2-CH3);
11.80 (N-CH2-CH2-CH3). Analysis: calculated for
C16H31N4S4ClPt (MW 638.24): C, 30.11; H, 4.89; N, 8.78.
Found: C, 29.06; H, 4.93; N, 8.68. Yield 88%.
2c. 1H NMR (300.13 MHz, CDCl3, room temperature):
δ 3.67 (q, N-CH2-CH3, 3J = 7.2); 1.38 (t, N-CH2-CH3,
3J = 7.2). 13C{1H}NMR (75.48 MHz, CDCl3, room temp-
erature):
δ 179.60 (CS); 44.15 (N-CH2-CH3); 13.90
4b. 1H NMR (300.13 MHz, CDCl3, room temperature): δ 13.5
(bs, N-H); 4.42 (seven lines, N-CH(CH3)2 3J= 6.7); 1.43 (d,
N-CH(CH3)2, 3J = 6.7). 13C{1H}NMR (75.48 MHz, CDCl3,
room temperature): δ 56.04 (N-CH(CH3)2); 21.80 (N-CH-
(CH3)2). Analysis: calculated for C16H31N4S4ClPt (MW 638.24):
(N-CH2-CH3). Analysis: calculated for C12H22N4S4Pt (MW
545.67): C, 26.41; H, 4.06; N, 10.27. Found: C, 26.38; H, 4.15; N,
10.11. Yield 81%.
3c. 1H NMR (300.13 MHz, CDCl3, room temperature): δ 3.59
(t, N-CH2-CH2-CH3, 3J=7.0): 1.78 (m, N-CH2-CH2-CH3);

10404 Inorganic Chemistry, Vol. 48, No. 21, 2009
Giannetto et al.
1.00 (t, N-CH2-CH2-CH3, 3J = 7.0). 13C{1H}NMR (75.48
were added, and the solution was kept under magnetic
stirring. From the initial purple color, the solution turned
red into few minutes. After removal of NaHCO3 by filtra-
tion, the solution was concentrated to a small volume.
Finally petroleum ether was added and [(H(ethyl)2-
MHz, CDCl3, room temperature):
δ 179.83 (CS); 51.10
(N-CH2-CH2-CH3); 22.09 (N-CH2-CH2-CH3); 11.88
(N-CH2-CH2-CH3). Analysis: calculated for C16H30N4S4Pt
(MW 601.78): C, 31.93; H, 5.02; N, 9.31. Found: C, 31.79; H,
5.12; N, 9.20. Yield 79%.
1
DTO)Pt(H(benzyl)2DTO)] precipitated as a red powder. H
8c. 1H NMR (300.13 MHz, CDCl3, room temperature):
δ 7.206-7.332 (m, N-CH2-CH2-C6H5); 3.86 (t, N-CH2-
CH2-C6H5, 3J=7.0); 3.022 (t, N-CH2-CH2-C6H5, 3J=7.0).
13C{1H}NMR (75.48 MHz, CDCl3, room temperature): δ
180.00 (CS); 126.61, 128.65, 128.67, 138.67 (N-CH2-
CH2-C6H5); 50.40 (N-CH2-CH2-C6H5); 34.90 (N-CH2-
CH2-C6H5). Analysis: calculated for C36H38N4S4Pt (MW
850.06): C, 50.86; H, 4.51; N, 6.59. Found: C, 51.01; H, 4.59;
N, 6.51. Yield 82%.
NMR (300.13 MHz, CDCl3, room temperature): δ 7.29-7.36
(m, N-CH2-C6H5); 4.80 (s, N-CH2-C6H5); 3.68 (q,
N-CH2-CH3, 3J = 7.3); 1.38 (t. N-CH2-CH3, 3J = 7.2).
13C{1H}NMR (75.48 MHz, CDCl3, room temperature): δ
180.70 (CS); 127.80, 128.22, 128.78, 136.70 (N-
CH2-C6H5); 53.45 (N-CH2-C6H5); 44.25 (N-CH2-
CH3); 13.77 (N-CH2-CH3). Analysis: calculated for
C
22H26N4S4Pt (MW 669.81): C, 39.45; H, 3.91; N, 8.36.
Found: C, 39.31; H, 4.05; N, 8.27. Yield 83%.
9c. 1H NMR (300.13 MHz, CDCl3, room temperature):
δ
6.04 (m, N-CH2-CHdCH2); 5.30 (d, N-CH2-
CHdCHtransHcis
3J = 17.1); 5.24 (d, N-CH2-CHd
CHtransHcis
3J = 10.2); 4.25 (d, N-CH2-CHdCH2, 3J = 6).
13C{1H}NMR (75.48 MHz, CDCl3, room temperature):
180.53 (CS); 132.34 (N-CH2-CHdCH2); 118.23
Acknowledgment. The authors thank The University of
Messina for financial support (Progetti di Ricerca di Ateneo,
PRA).
,
,
δ
Supporting Information Available: The absorption spectra of
independently prepared 4a, 4b, and 4c compounds. Absorption
spectra changes upon pyridine addition of representative a-type
and b-type compounds and relative titration curves. Description
of the method used to calculate the equilibrium constants. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(N-CH2-CH=CH2); 51.760 (N-CH2-CH=CH2). Analysis:
calculated for C16H22N4S4Pt (MW 593.71): C, 32.37; H, 3.74; N,
9.44. Found: C, 32.29; H, 3.68; N, 9.55. Yield 69%.
[(H(ethyl)2DTO)Pt(H(benzyl)2DTO)] (10c). To a chloro-
form solution (100 mL) of {[(H(ethyl)2DTO)Pt(H2(benzyl)2-
DTO)]+,Cl-} (compound 10b, 0.2 mmol), 2 g of NaHCO3