New surfactant phosphine ligands and platinum(II) metallosurfactants. influence of metal coordination on the critical micelle concentration and aggregation properties

Article abstract of DOI:10.1021/la902459f

We have prepared the first platinum(II) metallosurfactants from a new family of linear surfactant phosphines Ph₂P(CH₂) _nSO₃Na{1(n = 2),2(n = 6), and 3 (n = 10)}, which were synthesized by reaction between the halosulfonates X(CH₂) _nSO₃Na and sodium diphenylphosphide. The metallosurfactants Cw-[PtCl₂L₂] (L = 1-3) were obtained after reaction between the phosphines and PtCl₂ in dimethylsulfoxide. All compounds were fully characterized by the usual methods {NMR ( ¹H, ¹³C, ¹P, ¹⁹⁵Pt), IR, MS-ESI and HRMS}. By exploring the surfactant properties of phosphines 1-3 and their respective platinum metallosurfactants CW-[PtCl₂L₂] (L = 1-3) through surface tension measurements, dynamic light scattering spectroscopy, and cryo-TEM microscopy, we were able to analyze the influence of the metal coordination on the critical micelle concentration (cmc) and the aggregation properties. The cmc values of platinum metallosurfactants were considerably lower than those obtained for the free phosphines 1-3. This behavior could be understood by an analogy between the structure of cis-[PtCl₂L₂] complexes and bolaform surfactants. The calculated values of area per molecule also showed different tendencies between 1-3 and cis-[PtCl₂L₂] complexes, which could be explained on the basis of the possible conformations of these compounds in the air-water interface. The study of aggregates by dynamic light scattering spectroscopy and cryo-TEM microscopy showed the formation of spherical disperse medium size vesicles in all cases. However, substantial differences were observed between the three free phosphines (the population of micellar aggregates increased with long chain length) and also between phosphines and their respective metallosurfactants.

Full text of DOI:10.1021/la902459f

pubs.acs.org/Langmuir
© 2009 American Chemical Society
New Surfactant Phosphine Ligands and Platinum(II) Metallosurfactants.
Influence of Metal Coordination on the Critical Micelle Concentration and
Aggregation Properties
Elisabet Parera,† Francesc Comelles,‡ Ramon Barnadas,§ and Joan Suades*,†
†
´
Departament de Quımica, Universitat Autonoma de Barcelona, Edifici C, 08193 Bellaterra, Spain,
Institut de Quımica Avanc-ada de Catalunya, CSIC, Jordi Girona, 18-26, 08034 Barcelona, Spain, and
ꢀ
‡
´
§
´
Departament de Fisicoquımica, Facultat de Farmacia, Universitat de Barcelona, Avda. Joan XXIII s/n,
ꢀ
08028 Barcelona, Spain
Received July 8, 2009. Revised Manuscript Received October 5, 2009
We have prepared the first platinum(II) metallosurfactants from a new family of linear surfactant phosphines
Ph2P(CH2)nSO3Na {1 (n = 2), 2 (n = 6), and 3 (n = 10)}, which were synthesized by reaction between the halosulfonates
X(CH2)nSO3Na and sodium diphenylphosphide. The metallosurfactants cis-[PtCl2L2] (L = 1-3) were obtained after
reaction between the phosphines and PtCl2 in dimethylsulfoxide. All compounds were fully characterized by the usual
methods {NMR (1H, 13C, 31P, 195Pt), IR, MS-ESI and HRMS}. By exploring the surfactant properties of phosphines
1-3 and their respective platinum metallosurfactants cis-[PtCl2L2] (L = 1-3) through surface tension measurements,
dynamic light scattering spectroscopy, and cryo-TEM microscopy, we were able to analyze the influence of the metal
coordination on the critical micelle concentration (cmc) and the aggregation properties. The cmc values of platinum
metallosurfactants were considerably lower than those obtained for the free phosphines 1-3. This behavior could be
understood by an analogy between the structure of cis-[PtCl2L2] complexes and bolaform surfactants. The calculated
values of area per molecule also showed different tendencies between 1-3 and cis-[PtCl2L2] complexes, which could be
explained on the basis of the possible conformations of these compounds in the air-water interface. The study of
aggregates by dynamic light scattering spectroscopy and cryo-TEM microscopy showed the formation of spherical
disperse medium size vesicles in all cases. However, substantial differences were observed between the three free
phosphines (the population of micellar aggregates increased with long chain length) and also between phosphines and
their respective metallosurfactants.
Introduction
materials,4 metallomesogens,5 optoelectronic devices,6 sol-
vatochromic probes,7 homogeneous catalysis,8 medicine,9 and
nanoparticles.10 Other works have been devoted to metal-con-
taining soft materials11 or to the study of metallosurfactants
of different metals such as cadmium(II),12 chromium(III),13
Molecules that display surfactant properties and contain a
transition metal atom linked to the molecular structure are known
as metallosurfactants. Consequently, these metallic compounds
display the characteristic properties of a surfactant (surface ac-
tivity and self-assembly) and are a useful tool for achieving unique
arrangements with metallic compounds such as concentrating
metals in the interfaces or forming metal aggregates of different
sizes (micelles and vesicles). Although this is a relatively new
research field,1,2 the singular properties of these compounds
have been oriented to a wide range of potential applications in-
cluding magnetic resonance imaging,3 templates for mesoporous
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*Corresponding author. Fax þ 34 935813101. E-mail: Joan.Suades@
uab.es.
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Bushby, L.; Beeby, A. Adv. Colloid Interface Sci. 2008, 144, 13.
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Langmuir 2010, 26(2), 743–751
Published on Web 10/26/2009
DOI: 10.1021/la902459f 743

Article
Parera et al.
Scheme 1
cobalt(III),14,15 copper(II),15-18 iron(II),19 nickel(II),18 rhodium-
(I),20 ruthenium(II),20,21 and zinc(II).12 It should be pointed out
that in most of the reported metallosurfactants the polar head-
group contains the metal atom. Even the concept of metallosur-
factant has been related to a molecule in which the polar
headgroup of the surfactant contains a metal center.1 However,
there have also been reports of some surfactant complexes with a
polar headgroup that does not contain the metal center. These
compounds are usually prepared from surfactant ligands, mole-
cules that contain both a polar headgroup and a donor group with
the ability to form stable bonds with metals.22 This kind of ligand
has beengenerally used to prepare metallosurfactants for catalytic
processes and surface-active phosphines are common compounds
for this goal.22 Nevertheless, in these studies the metallosurfactant
complex is formed in the catalytic pool and frequently could not
be isolated and fully characterized. A particularly interesting
aspect of surface-active phosphines is that they offer the possibi-
lity of comparing the surfactant properties of the free ligands with
their respective metal complexes. Hence, the influence of the
formation of metal-phosphine bonds on the aggregation proper-
ties of surface-active phosphines can be studied. In a previous
paper,23 we reported that the critical micelle concentration (cmc)
values of a series of palladium(II) complexes prepared with two
different families of surface-active phosphines were significantly
lower than the values obtained from their respective free phos-
phines. This result was related to the hypothesis that a metallo-
surfactant prepared with a surface-active ligand could be
considered as a gemini surfactant, where the two ligands act as
conventional surfactant units and the phosphorus-metal bonds
as the rigid spacer group as shown in Scheme 1.
In the present paper, we have undertaken novel work about the
influence of the polar group position on the aggregation proper-
ties of surfactant ligands and metallosurfactants. Hence, if in a
previously reported work the phosphine ligands contained a
polar group linked nearby the phosphorus atom (Scheme 2, dia-
gram A), in the current study the polar group is linked on the
opposite side of the hydrophobic group (Scheme 2, diagram B).
The study of this new family of phosphines (diagram B) can
supply information about the influence of the position of the
diphenylphosphino group in surfactant properties and lead to
new metallosurfactants with very attractive properties. The polar
group is located furthest from the phosphorus donor atom and it
may induce interesting changes in the aggregation properties of
ligands and metallosurfactants. It could favor the formation of
arrangements in which the metal atoms are located inside the
aggregate, whereas previously reported phosphines (A) have a
preference for placing metal atoms on the surface as seen in
Scheme 2.
(14) (a) Jaeger, D. A.; Peacock, M. F.; Bohle, D. S. Langmuir 2003, 19, 4859.
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Metal Chem. 2006, 31, 62. (c) Jaeger, D. A.; Jose, R.; Mendoza, A.; Apkarian, R. P.
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1996, 12, 3695. (c) Ghosh, P.; Khan, T. K.; Bharadwaj, P. K. Chem. Commun. 1996,
189.
Materials and Methods
Synthetic and Characterization Methods. A complete
description of synthetic and characterization methods for com-
pounds 2, 3 and cis-[PtCl2L2] (L = 1-3) can be found in the Sup-
porting Information.
Surface Tension Measurements. The surface tension mea-
surements of the aqueous solutions were performed in the
Departament de Tecnologia de Tensioactius de l’Institut de
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M. B.; Winnington, A. L. Chem. Commun. 1998, 665.
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1995, 1769.
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Supramol. Sci. 1998, 5, 337. (b) Taniguchi, M.; Ueno, N.; Okamoto, K.; Karthaus, O.;
Shimomura, M.; Yamagishi, A. Langmuir 1999, 15, 7700. (c) Bowers, J.; Danks, M. J.;
Bruce, D. W.; Heenan, R. K. Langmuir 2003, 19, 292. (d) Bowers, J.; Danks, M. J.;
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Bruce, D. W.; Webster, J. R. P. Langmuir 2005, 21, 1346. (f) Bowers, J.; Amos, K. E.;
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Surtchev, M.; Eiser, E.; Elsevier, C. J. Nano Lett. 2006, 6, 145. (h) Domínguez-
Quı
´
mica Avanc-ada de Catalunya (IQAC-CSIC) at 25 °C with
€
a Kruss K-12 automatic tensiometer (Hamburg, Germany)
equipped with a Wilhelmy plate. All compounds were previously
recrystallized and lyophilized. The water solutions of amphiphiles
were prepared with degassed Milli-Q water. The different solu-
tions were prepared by dilution of a concentrated sample and then
aged for at least 30 min before the determinations. The stability
criterion for surface tension values was tuned to (0.1 mN/m for
five consecutive measurements. The cmc values were taken from
the intersection of two linear sections obtained in the graphical
plots of surface tension versus logarithm of the concentration.
The area occupied per molecule adsorbed at the water/air inter-
face, expressed in A2, was obtained from the equation A = 1016/
NAΓ, where NA is Avogadro’s number and Γ the surface excess
ꢁ
Gutierrez, D.; De Paoli, G.; Guerrero-Martínez, A.; Ginocchietti, G.; Ebeling, D.; Eiser,
E.; De Cola, L.; Elsevier, C. J. J. Mater. Chem. 2008, 18, 2762.
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Hanson, B. E.; Bakos, J. Angew. Chem., Int. Ed. 1995, 34, 1645. (d) Gulyas, H.; Arva,
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744 DOI: 10.1021/la902459f
Langmuir 2010, 26(2), 743–751

Parera et al.
Article
Scheme 2
concentration in mol/cm2, calculated according to the Gibbs
equation: Γ = -(dγ/d log C)/2.303nRT, where n is the number
of molecular species in solution (n = 2 for surfactant phosphine
ligands and n = 3 for the platinum metallosurfactants), and (dγ/d
log C) is the slope of the linear part of the graph obtained
immediately below the cmc.
Scheme 3
Dynamic Light Scattering (DLS). The DLS measurements
were performed in the Departament de Fisicoquımica de la
´
ꢀ
Facultat de Farmacia de la Universitat de Barcelona using a
Malvern Zetasizer ZS90 (Malvern Instruments Ltd., Malvern, U.K.)
equipped with an He-Ne laser. In this device scattered light is
detected at 90° and its intensity on the detector is automatically
adjusted in order to achieve an optimal range. This fact allows the
analysis of several orders of sample concentration, avoiding their
dilution and, consequently, changes in the phase equilibrium. All
compounds were previously recrystallized and lyophilized. The
water solutions of amphiphiles were prepared with degassed
Milli-Q water. The solutions were previously centrifuged for
2-3 min at 13000 rpm and then aged for at least 1 h before
measurements. For all DLS measurements, the temperature was
25 ( 0.5 °C. Each data acquisition was a mean of 10 consecutive
analyses and each experiment was repeated three times. The data
were analyzed by cumulant method using the software provided
by the manufacturer. The DLS instrument used for these experi-
ments can be used to characterize particles with diameters in the
range of 2 nm to 6 μm.
stable bonds with transition metals as well as being fairly stable to
oxidation. The different length of the alkyl chain with two (1), six
(2), or ten (3) methylene groups can supply information about the
influence of the chain length on the aggregation properties of
these compounds and their respective metallosurfactants. Ligand
1 was prepared by previously reported procedures24 but new
preparation methods had to be designed to obtain ligands 2 and 3
because, as far as we know, the synthesis of phosphines Ph2P-
(CH2)nSO3Na has been reported only for n = 2-4.25
Although ligands 2 and 3 are simple molecules, their prepara-
tion is dependent on the availability of the respective linear
halosulfonated compounds. Thus, compound Cl(CH2)6SO3Na
was synthesized by reaction between 1-bromo-6-chlorohexane
and sodium sulfite as shown in Scheme 4.
The different lability of bromine versus chlorine as the leaving
group under nucleophilic attack by sulfite assures the formation
of Cl(CH2)6SO3Na as the main product with respect to the
1
disulfonate NaO3S(CH2)6SO3Na. Hence, the analysis of the H
Cryo-Electron Microscopy. The microscopy studies were
ꢀ
NMR spectrum (integration of methylene group bonded to
chlorine versus methylene bonded to sulfonate) of the reaction
mixture evidenced that the disulfonate is formed in a ratio of
nearly 10%. However this compound has a low solubility in
ethanol and can be efficiently separated by recrystallization.
Phosphine 2 was prepared in good yield (76%) from the halo-
sulfonate by reaction with sodium diphenylphosphide in liquid
ammonia as shown in Scheme 5, using similar reaction conditions
to those previously reported for other sulfonated phosphines.23,24
The 31P{1H} NMR spectrum of 2 shows a sole signal at -15.5
ppm, a very similar position to that reported for 1 (-15.7 ppm),24
and in agreement with published data for other linear alkyldi-
phenylphosphines.26 The formation of the P-C bond by coupling
ꢀ
performed in the Servei de Microscopia Electronica de la Uni-
ꢀ
versitat Autonoma de Barcelona. Micrographs were obtained using
a Jeol JEM-1400 electron microscope operating at 120 kV and
equipped with a CCD multiscan camera (Gatan). The microscope
was equipped with a Gatan cryoholder, and the samples were
maintained at -177 °C during imaging. Micro drops (2 μL) of the
water solutions of amphiphiles were blotted onto holey carbon grids
(Quantifoil) previously glow discharged in an BAL-TEC MSC 010
glow discharger unit, which were immediately plugged into liquid
ethane at -180 °C using a Leica EM CPC cryoworkstation.
Results
Surfactant Ligands. Three linear alkylsulfonated phosphines
were chosen as candidates for surfactant ligands with the struc-
tural design displayed in diagram B (Scheme 2). These ligands
(1-3) are shown in Scheme 3 and all of them contain a
diphenylphosphine group, which has the ability to form very
(24) Ganguly, S.; Roundhill, D. M. Organometallics 1993, 12, 4825.
(25) Wedgwood, J. L.; Hunter, A. P.; Kresinski, R. A.; Roman, A.; Platt, A. W. G.;
Stein, B. K. Inorg. Chim. Acta 1999, 290, 189.
(26) Hill, W. E.; Minahan, D. M. A.; Taylor, J. G.; McAuliffe, C. A. J. Chem.
Soc. Perkin II 1982, 327.
Langmuir 2010, 26(2), 743–751
DOI: 10.1021/la902459f 745

Article
Parera et al.
Scheme 4
Scheme 5
between the alkylsulfonate and the diphenylphosphine fragments
is clearly revealed in the 13C{1H} NMR spectrum. In the alkyl
chain, the carbon atom in R position is shifted to higher field as a
result of the substitution of chlorine by phosphorus (from 44.0
ppm for Cl-CH2- to 30.9 for P-CH2-) and the signals of alpha,
beta, and gamma carbon atoms show the characteristic coupling
with the phosphorus nucleus, in complete concordance with the
reported JP-C values for linear alkyldiphenylphosphines.26
Ligand 3 was synthesized by a similar method to that employed
for 2, but inthis case the precursor Br(CH2)10Cl was not available.
However, Br(CH2)10SO3Na has been reported as an intermediate
product in the preparation of the thiol HS(CH2)10SO3Na.27 In
that work, the preparation of Br(CH2)10SO3Na was described as
the simple reaction between Br(CH2)10Br and sodium sulfite in a
water/ethanol reflux. However, working in identical reaction
conditions a mixture of three compounds was obtained: the
disulfonate NaO3S(CH2)10SO3Na (main product, ∼70%), the
bromosulfonate Br(CH2)10SO3Na (∼15%), and the correspond-
ing alcohol HO(CH2)10SO3Na (∼15%). All attempts to improve
the yield of bromosulfonate by addition of excess Br(CH2)10Br led
to a decrease in the amount of disulfonate in the reaction
products, but the Br(CH2)10SO3Na/HO(CH2)10SO3Na ratio
was nearly identical. Disulfonate can be easily separated from
the reaction mixture by recrystallization but the separation of
Br(CH2)10SO3Na from HO(CH2)10SO3Na turned out to be more
complicated. Thus, we chose the 1,10-dichlorodecane as an
alternative to the homologous dibromine compound to avoid
hydrolysis because the chlorine-carbon bond is more inert than
the bromine-carbon bond (Scheme 4). The slow addition of a
sodium sulfite solution to a large excess of Cl(CH2)10Cl in
ethanol/water reflux led to the formation of a mixture of the
desired chlorosulfonate and the disulfonate with anapproximated
ratio of Cl(CH2)10SO3Na/NaO3S(CH2)10SO3Na = 2.5. This
mixture was nearly free of the alcohol HO(CH2)10SO3Na (it
could not be detected by 1H NMR spectroscopy). The pure
Cl(CH2)10SO3Na was easily obtained from this mixture by simple
recrystallization in ethanol.
Table 1
reaction products
cis-[PtCl2(2)2] trans-[PtCl2(2)2] [PtCl(2)3]Cl
Pt(II) precursor
a
K2PtCl4
[PtCl2(PhCN)2]b
[PtCl2(PhCN)2]c
80%
33%
52%
43%
100%
20%
67%
48%
34%
trans-[PtCl2(PhCN)2]d
PtCl2
PtCl2
23%
e
f
100%
Reaction conditions: a H2O, 25 °C, 8 days reaction time.25 b THF, 25 °C,
30 min reaction time, metal solution was slowly added to ligand solution.
c THF, 25 °C, 30 min reaction time, ligand solution was slowly added to
metal solution. d THF, 0 °C, 30 min reaction time, metal solution was
slowly added to ligand solution. e DMSO, 80 °C, 12 h reaction time.30
f H2O, 25 °C, 12 h reaction time.30
beta, and gamma carbon atoms in agreement with the proposed
structure.
Platinum(II) Metallosurfactants. Straightforward proce-
dures for the preparation of [PtCl2L2] complexes with sulfonated
phosphines have been reported based on the simple reaction
between K2PtCl4 and two equivalents of phosphine ligand in
water medium.28,29 However, the use of this method has been
questioned by other authors who claim that no pure products
could be isolated using this approach.30 A common problem is the
presence of mixtures of cis-[PtCl2L2] and trans-[PtCl2L2] com-
plexes, but other species have also been identified as [PtClL3]þ.30
Since our aim was to study the aggregation properties of
platinum(II) metallosurfactants with ligands 1-3, we required a
synthetic procedure that led to pure complexes owing to the fact
that the separation of cis-[PtCl2L2] and trans-[PtCl2L2] metallo-
surfactants would probably be quite difficult due to their amphi-
philic nature and similar solubilities. Consequently, we performed
a preliminary study with different platinum(II) precursors and
reaction conditions in order to find an optimum procedure.
Table 1 shows the results obtained with ligand 2 on the basis of
the analysis of the reaction mixture by means of 31P{1H} NMR
spectroscopy.
The reaction between the chlorosulfonate Cl(CH2)10SO3Na
and sodium diphenylphosphide in liquid ammonia led to phos-
phine 3 (Scheme 5), which was separated from NaCl by recrys-
tallization and isolated as a white solid in quite good yield (64%).
The significant spectroscopic data for this compound were similar
to those found for 2, a unique signal in the 31P NMR spectrum
(-15.4 ppm) and the characteristic P-C couplings for alpha,
The preparation of complex cis-[PtCl2(1)2] from K2PtCl4 has
been reported in the literature.25 However, the same procedure
applied to the preparation of the homologous cis-[PtCl2(2)2] did
not lead to the pure complex because [PtCl(2)3]Cl was also
obtained in a significant quantity (Table 1). Similar behavior
has been reported for the preparation of Pt(II) complexes with
(28) Larpent, C.; Patin, H. Appl. Organomet. Chem. 1987, 1, 529.
(29) Hermann, W. A.; Kellner, J.; Riepl, H. J. Organomet. Chem. 1990, 389, 103.
(30) Francisco, L. W.; Moreno, D. A.; Atwood, J. D. Organometallics 2001, 20,
4237.
(27) Jacobson, K. A.; Nikodijevic, O.; Ji, X. D.; Berkich, D. A.; Eveleth, D.;
Dean, R. L.; Hiramatsu, K.; Kassell, N. F.; van Galen, P. J.; Lee, K. S. J. Med.
Chem. 1992, 35, 4143.
746 DOI: 10.1021/la902459f
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Article
Figure 1. 31P{1H} NMR spectrum of complexes cis-[PtCl2(1)2] (left) and cis-[PtCl2(2)2] (right) showing the signals assigned to the species A
and B.
phosphines Ph2P(CH2)nSO3Na (n = 3, 4).25 In contrast, the reac-
tion starting from [PtCl2(PhCN)2] in THF is very fast but a mix-
ture of complexes is obtained. In this case, the order of addition of
reactants or the use of pure trans-[PtCl2(PhCN)2] gave rise to
different results, but no pure complexes could be obtained
(Table 1). However, the reaction between PtCl2 and the sulfonated
phosphine in DMSO solutions allowed the preparation of pure cis
complexes using the procedure described for the preparation of
cis-[PtCl2(TPPTS)2].30 The cis-[PtCl2L2] (L = 1, 2, 3) platinum
metallosurfactants were characterized by NMR (1H, 31P, 13C, and
195Pt), IR, MS-ESI, and HRMS. The 31P{1H} NMR spectrum of
all complexes shows a singlet in the 6-8 ppm region accompanied
by the satellite-peaks of the 195Pt atom (Figure 1). The shift of this
signal with respect to the free ligand (22-23 ppm) is consistent
with the coordination of phosphine to platinum.25,30,31 The values
found for 1JPt-P coupling constants (3652-3666 Hz) concord with
the proposed structure since they fall in the range reported for
cis-[PtCl2L2] complexes and are significantly higher than the
typical values for trans-[PtCl2L2] complexes with tertiary phos-
phines (2300-2600 Hz).25,30-32 The 31P{1H} NMR spectrum of
complex cis-[PtCl2(1)2] shows small peaks around the main signals
as displayed in Figure 1. Since recrystallization of this complex
does not modify this pattern, a plausible explanation of this result
could be the coexistence of the main complex with small quantities
of the species resulting from the substitution of one chloride ligand
by a sulfonate. Therefore, in addition to the signals of the main
complex (A), the signals of complex (B) are observed as the
characteristic pattern of two different phosphorus atoms coupled
to a single platinum nucleus. The coexistence of this kind of species
has been previously claimed in the literature,25 and it is evident that
its existence is favored in ligand 1 by the high stability of six-
member chelated rings. The 195Pt{1H} NMR spectra show a triplet
for all complexes, in agreement with the coupling of the platinum
cis-[PtCl2(TPPTS)2] (-4437 ppm) and far from the trans-[PtCl2-
(TPPTS)2] complex (-4072 ppm).30 The 1H NMR spectra show
the signals of methylene groups. The most significant data is the
downfield shift of methylene bound to phosphorus with respect to
the position in the free phosphine as a result of the coordination to
the metal atom. The assignment of these signals has been corro-
borated by a HMBC (195Pt-1H) experiment which shows an
unique correlation signal in agreement with the position of this
methylene group closer to the metal.
Finally, accurate mass measurements of the cis-[PtCl2L2] (L =
1-3) platinum metallosurfactants by HRMS in the negative
region provided exact masses of the main peaks ([M-2Na-Cl])
for all complexes, which corresponded to the proposed molecular
formulas.
Surface Tension Measurements. The interfacial behavior of
surfactant ligands 1-3 and their respective metallosurfactants
cis-[PtCl2L2] (L = 1-3) were studied by surface tension measure-
ments of aqueous solutions using the Wilhelmy plate method. The
graphic representations of surface tension vs concentration are
shown in Figure 2. We should highlight that the three ligands and
the three respective metal complexes exhibit the characteristic
behavior of surfactants with a significant reduction in the
surface tension to values of 50-30 mN/m in the cmc. These
results evidence the surfactant properties of ligands 1-3 and the
behavior as metallosurfactants of their respective complexes
cis-[PtCl2L2] (L = 1-3). Although ligand 1 was chosen with
the aim of having a ligand with a structure similar to 2-3 but with
a very short alkyl chain for comparison purposes, it is evident that
the two phenyl groups provide a hydrophobic character to
this ligand and its platinum complex surfactant properties are
not so different from those of their homologous compounds
with 2-3.
Some relevant numerical parameters obtained from the surface
tension measurements are displayed in Table 2 and their compar-
ison can shed some light on the influence of ligand coordination in
surfactant properties. The comparison between the cmc values of
ligands and platinum complexes shows a regular decrease of cmc
values with the length of the hydrocarbon chain. The diminution
of log(cmc) versus the number of carbon atoms in the hydro-
carbon chain has a near linear behavior for ligands and complexes
1
nucleus with two equivalent phosphorus atoms. The JPt-P
coupling constants are nearly identical to those found in 31P{1H}
NMR spectroscopy within experimental error and the position of
the signals (-4399/-4412 ppm) is similar to the reported value for
(31) Lucey, D. W.; Atwood, J. D. Organometallics 2002, 21, 2481.
(32) Grim, S. O.; Keiter, R. L.; McFarlane, W. Inorg. Chem. 1967, 6, 1133.
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Parera et al.
have shown that these compounds are much less efficient at
reducing surface tension than a corresponding conventional
surfactant.34,39
The area occupied per molecule adsorbed in the water/air
interface was calculated from the slope of the linear decrease of
surface tension below the cmc via the Gibbs equation (Γ = -(dγ/d
log C)/2.303nRT; Γ = surface excess concentration; n = number
of species in solution, it is n = 2 for ligands and n = 3 for platinum
metallosurfactants).33 The results (Table 2) evidence that the area
per molecule is very similar for ligands 1-3 and all values are
notably lower than those obtained for platinum complexes. Hence,
packing at the interface should be very similar for all ligands,
probably with the hydrophilic sulfonate group on water and the
lipophilic group oriented in the air in extended chain conforma-
tions, since the area per molecule remains nearly constant with
changes in the length of the hydrocarbon chain from phosphine 1
to 3. The higher values of area per molecule for ligands 1-3 with
respect to conventional linear alkylsulfonates may be related to the
presence of the bulky diphenylphosphino group in the alkyl chain.
Indeed, previous studies with trisubstituted alkyl benzene sulfo-
nates have also shown changes in the area per molecule at the
interface that have been associated with the molecular sizes.40
As mentioned above, all platinum complexes have a larger area
per molecule than free phosphines. This behavior is similar to that
observed for organic bolaform disulfonates, which also exhibit
largerareas thanconventional monomeric sulfonates.41 However,
the most remarkable fact regarding the area per molecule data
reported in Table 2, is the different trend in results obtained for
platinum complexes with respect to free ligands. Whereas all
phosphines (1-3) exhibit similar values, data for platinum com-
plexes cis-[PtCl2L2] (L = 1 and 2) are very similar within the
experimental error but this value is considerably larger for
complex cis-[PtCl2(3)2]. This result suggests a substantially dif-
ferent packing of cis-[PtCl2(3)2] compared to cis-[PtCl2L2] (L = 1
and 2) complexes. Large values of area per molecule for bolaform
surfactants have been associated with flatted structures.41 How-
ever, the similarity of values obtained for platinum complexes
with phosphines 1 and 2 (which contain a different number of
methylene groups) compared to the very large value obtained for
3, lead us to consider that this complex may form a double-loop
(Figure 3), a conformation that has been postulated for some
bolaform surfactants.38 This organization could be stabilized in
this case by hydrogen bonds between the chlorine atoms bonded
to the metal and water molecules.
Figure 2. Surface tension measurements for phosphines 1-3 and
metallosurfactants cis-[PtCl2L2] (L = 1-3).
as is observed in common surfactants.33,34 The cmc value for
ligand 3 could only be established in the 0.5-1.2 mM range
because the representation of surface tension vs concentration
(Figure 2) reached a minimum around the cmc value. This
singular behavior may be related to the presence of an impurity
in this compound,35,36 possibly very small quantities of the
disulfonate [O3S(CH2)10SO3]2- not detectable by NMR. The
comparison between free phosphines and their respective plati-
num complexes in Table 2 demonstrate that cmc values are sub-
stantially lower for cis-[PtCl2L2] (L=1-3) complexes and γcmc
are significantly higher. The ratio between the cmc of platinum
metallosurfactants and their free ligands is almost five for
compounds with 1-2, and it is nearly ten for 3. This behavior is
reminiscent of bolaform surfactants because the decrease in the
cmc is comparable to that reported for these compounds with
respect to conventional monomeric surfactant.34,37,38 Thus, if we
consider that {PtCl2} fragment acts essentially as a linker between
the two hydrophobic chains, the metallosurfactacts [PtCl2L2]
(L=1-3) can be seen as bolaform surfactants (Scheme 6). Fur-
Light Scattering Analysis. Aqueous solutions of ligands 1-3
and platinum complexes cis-[PtCl2L2] (L = 1-3) were studied by
dynamic light scattering spectroscopy (DLS) in order to compare
the aggregate size of surfactant phosphines and their platinum
complexes. All the water solutions prepared were completely clear
and no turbid solution was obtained even at the highest concen-
trations. Ligands 1-3 were analyzed at different concentrations
above the cmc and the results show a different behavior between
the three surfactant phosphines. The average hydrodynamic
diameter of aggregates calculated for ligands 1 and 3 remained
nearly invariable with phosphine concentration although they are
substantially different in size. The value of 150 ( 20 nm obtained
for ligand 1 is consistent with the presence of vesicles of medium
thermore, the limited surface tension diminution at the cmc (γcmc
)
for these platinum metallosurfactants is also consistent with this
analogy because previous reports about bolaform surfactants
(33) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley: New
York, 1989; Chapters 2 and 3.
(34) Mao, G.; Tsao, Y.; Tirrell, M.; Davis, H. T. Langmuir 1994, 10, 4174.
(35) Rosen, M. J. J. Colloids Interface Sci. 1981, 79, 587.
(36) Lin, S. Y.; Lin, Y. Y.; Chen, E. M.; Hsu, C. T.; Kwan, C. C. Langmuir 1999,
15, 4370.
(40) Zhang, L.; Wanga, X.; Gong, Q.; Zhanga, L.; Luoa, L.; Zhao, S.; Yua, J.
(37) (a) Jong, L. I.; Abbott, N. L. Langmuir 1998, 14, 2235. (b) Yan, Y.; Xiong, W.;
Huang, J.; Li, Z.; Li, X.; Li, N.; Fu, H. J. Phys. Chem. B 2005, 109, 357.
(38) Abid, S. K.; Hamid, S. M.; Sherrington, D. C. J. Colloid Interface Sci. 1987,
120, 245.
(39) Eastoe, J.; Rogers, S. E.; Martin, L. J.; Paul, A.; Guittard, F.; Guittard, E.;
Heenan, R. K.; Webster, J. R. P. Langmuir 2006, 22, 2034.
J. Colloid Interface Sci. 2008, 327, 451.
(41) (a) Wilkinson, T. S.; Boonstra, A.; Montoya-Goni, A.; van Es, S.;
Monteiro, M. J.; German, A. L. J. Colloid Interface Sci. 2001, 237, 21. (b) Gouzy,
~
ꢁ
M. F.; Guidetti, B.; Andre-Barres, C.; Rico-Lattes, I.; Lattes, A.; Vidal, C. J. Colloid
Interface Sci. 2001, 239, 517. (c) Han, F.; Huang, J.; Zheng, B.; Li, Z. Colloids Surf., A
2004, 242, 115.
748 DOI: 10.1021/la902459f
Langmuir 2010, 26(2), 743–751

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Article
Scheme 6
Table 2
γcmc
(mN/m)
surfactants. Hence, these conically shaped molecules can be
efficiently packed in micelles.42 It is evident that the shape of ligand
1 is very far from a conventional surfactant and it should be better
described as a polar sulfonate group attached to a large tridimen-
sional lipophilic group instead of a linear chain. Consequently, the
packing of this ligand can hardly lead to small micelles due to its
bulky hydrophobic group. Rather it is logical that this molecular
arrangement would lead to the formation of vesicles; similar to the
behavior of surfactants with large hydrophobic groups, such as
fullerenes.43 This result opens up the possibility of using ligands like
1-3 to control the size of aggregates, an idea that could be very
useful in the future in order to design new nanostructures on the
basis of supramolecular arrangements with these molecules.
Aqueous solutions of platinum complexes cis-[PtCl2L2] (L =
1-3) could be analyzed by DLS at a sole concentration above the
cmc due to the lower solubility of metal complexes with respect to
free phosphines. In spite of this experimental limitation, results
demonstrate a significantly different behavior of platinum com-
plexes in comparison to their respective free phosphines. The ave-
rage hydrodynamic diameters of aggregates in metal complexes
are very similar for all metal complexes (cis-[PtCl2(1)2], 178 (
2 nm; cis-[PtCl2(2)2], 203 ( 1 nm; cis-[PtCl2(3)2], 176 ( 7 nm)
indicating the formation of middle size vesicles in all cases. A
subpopulation analysis (Figure 5) shows that a short-range of
medium size vesicles (200-300 nm) is responsible for the main
signal (89-95% of scattered light) for the three platinum com-
plexes. A small percentage of very large vesicles (4000-5000 nm)
are also observed in all cases and the complex cis-[PtCl2(3)2] is the
only species that evidenced the presence of a relevant population
of micellar aggregates. It should be borne in mind that, because
big particles are more efficient scatterers than small ones, intensity
distribution enhances larger aggregates when the vesicle popula-
tion is polymodal. Consequently, these results indicate that the
observed tendency of free ligand 3 to form micelles is maintained
to a considerable extent in its platinum complex. Previous works
have shown a transition from micelles to vesicles upon coordina-
tion of different chelating amphiphiles to transition metals.16,44,45
cmc
(mM)
Γ (mol/cm2)
A (A2)
99 ( 6
1
2
3
14
4.0
0.5-1.2
2.9
0.94
29.2
40.0
43.8
51.2
47.0
54.0
(1.7 ( 0.1) ꢀ 10-10
(1.64 ( 0.03) ꢀ 10-10 101 ( 2
(1.6 ( 0.2) ꢀ 10-10 100 ( 10
(6.9 ( 0.6) ꢀ 10-11 240 ( 20
(7.7 ( 0.9) ꢀ 10-11 220 ( 30
(4.7 ( 0.4) ꢀ 10-11 350 ( 30
cis-[PtCl2(1)2]
cis-[PtCl2(2)2]
cis-[PtCl2(3)2]
0.062
Figure 3. Schematic illustration of presumed different conforma-
tion of cis-[PtCl2(3)2] with respect to cis-[PtCl2L2] (L = 1 and 2) in
the air/water interface.
size, whereas the diameter of 16 ( 1 nm obtained for ligand 3
clearly points to the formation of smaller aggregates. On the
contrary, phosphine 2 demonstrated an unequivocal dependence
between the hydrodynamic diameter and the ligand concentration.
At low concentration the average size was comparable to that of
phosphine 1 whereas at high concentration it decreased to values
closer to that of phosphine 3. A subpopulation analysis (Figure 4)
revealed that medium size vesicles (150-250 nm) were responsible
for the main signal for ligand 1 (91-98% of scattered light). This
was also the main signal for ligand 2 (74-83% of scattered light)
but it decreased at low concentrations at the same time that the
intensity of the micellar peak increased. The main signal for ligand
3 corresponded to a micellar peak (population: 5-15 nm, intensity:
69-88%) in all cases. Consequently, these results are in agreement
with a tendency of surfactant phosphines Ph2P(CH2)nSO3Na to
mainly form vesicles with short alkyl chains and micelles with long
alkyl ones. A plausible explanation of this result could be proposed
on the basis of the different shapes of the three ligands. Predis-
position of surfactants to form micelles has been related to the size
of large polar groups with respect to nonpolar tails in conventional
(43) (a) Zhou, S.; Burger, C.; Chu, B.; Sawamura, M.; Nagahama, N.; Toganoh,
M.; Hackler, U. E.; Isobe, H.; Nakamura, E. Science 2001, 291, 1944. (b) Burger, C.;
Hao, J.; Ying, Q.; Isobe, H.; Sawamura, M.; Nakamura, E.; Chu, B. J. Colloid Interface
Sci. 2004, 275, 632.
(44) (a) Martinez, J. S.; Zhang, G. P.; Holt, P. D.; Jung, H.-T.; Carrano, C. J.;
Haygood, M. G.; Butler, A. Science 2000, 287, 1245. (b) Owen, T.; Pynn, R.;
Martinez, J. S.; Butler, A. Langmuir 2005, 21, 12109. (c) Owen, T.; Pynn, R.;
Hammouda, B.; Butler, A. Langmuir 2007, 23, 9393. (d) Owen, T.; Webb, S. M.;
Butler, A. Langmuir 2008, 24, 4999.
(45) (a) Sommerdijk, N. A. J. M.; Booy, K. J.; Pistorius, A. M. A.; Feiters, M.
C.; Nolte, R. J. M.; Zwanenburg, B. Langmuir 1999, 15, 7008. (b) Luo, X.; Wu, S.;
Liang, Y. Chem. Commun. 2002, 5, 492. (c) Luo, X.; Miao, W.; Wu, S.; Liang, Y.
ꢀ
Langmuir 2002, 18, 9611. (d) Apostol, M.; Baret, P.; Serratrice, G.; Desbrieres, J.;
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(42) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday
Trans. 2 1976, 72, 1525.
Putaux, J. L.; Stebe, M. J.; Expert, D.; Pierre, J.-L. Angew. Chem., Int. Ed. 2005, 44,
2580.
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Figure 4. Dynamic light scattering spectroscopy. Subpopulation analysis for phosphines 1-3. Percentages values correspond to results of
scattered light. (Sample concentrations: (a) 40 g/L; (b) 4 g/L; (c) 40 g/L; (d) 9.6 g/L).
the area per molecule found in the surface tension measurements.
The larger area permits the molecule to adopt a more conical
shape that enables the formation of a significant population of
metallomicelles.
Cryo-Transmission Electron Microscopy (cryo-TEM).
Cryo-electron microscopy of water solutions of ligands 1-3
confirmed the formation of polydisperse spherical vesicles in all
cases with some significant differences in size and morphology
between the different ligands. For phosphine 1 two main types of
aggregates were observed, medium size vesicles (100-300 nm)
and large vesicles with dimensions of around 1 μm (Figure 6). This
result is consistent with the light scattering analysis, which also
indicated the formation of medium size vesicles. Micrographs
obtained from phosphine 2 show vesicles with a broad range of
sizes (50 nm to 1 μm), a result that it is also coherent with the light
scattering analysis, which evidenced the higher polydispersity of
this ligand. Finally, although micelles cannot be observed using
this technique, the analysis of phosphine 3 solutions showed
Figure 5. Dynamic light scattering spectroscopy. Subpopulation
analysis for metallosurfactants cis-[PtCl2L2] (L = 1-3). Percen-
substantial morphological differences with respect to the other
two compounds. All observed vesicles are of similar size (100-
tages values correspond to results of scattered light. (Sample
250 nm) and multilamellar vesicles are often observed as can be
concentrations: 4.5 g/L for cis-[PtCl2(1)2]; 3.3 g/L for cis-[PtCl2-
seen in Figure 6.
(2)2]; 1.0 g/L for cis-[PtCl2(3)2]).
Using the same methodology, the analysis of water solutions of
platinum complexes cis-[PtCl2L2] (L = 1-3) also demonstrated
the formation in all cases of polydisperse spherical vesicles with
This behavior has been related to changes in the shape of
molecules after coordination to metals. The bonding between
some differences between the three compounds. Micrographs of
amphiphiles and metal leads to a new surfactant with multiple
complex cis-[PtCl2(1)2] mainly show unilamelar vesicles with a
alkyl tails, which has a more rodlike geometry favoring a vesicle
size range of 140-240 nm (Figure 6) although smaller vesicles of
40-80 nm could also be observed. Complex cis-[PtCl2(2)2] also
showed the formation of spherical vesicles but with a significantly
smaller rang size (20-100 nm) than those with ligand 1. Hence in
this case aggregates of greater than a hundred nanometers were
not be observed. Finally, micrographs with complex cis-[PtCl2-
(3)2] (Figure 6) exhibit the presence of unilamelar vesicles with
a broad size range (20-250 nm). Thus, with this compound it
is common to observe the presence of very small aggregates
(∼20 nm) coexisting with large vesicles. We should highlight that
in general terms, cryo-TEM results are consistent with DLS
analysis. Middle size vesicles (200-300 nm) are responsible for
the main signal of scattered light for the three metallosurfactants
and polydisperse spherical vesicles of this size range have been
observed for the three platinum metallosurfactants by means of
arrangement.45 In contrast, the monomeric amphiphiles display a
more cone-shaped geometry which have a preference to form
micelles.42,45 It should be emphasized that in all these previous
studies, metal coordination took place around the polar head-
group of the amphiphilic molecule, whereas in ligands 1-3 the
metal is covalently linked to the amphiphilic ligand at one extreme
of the hydrophobic chain and the polar group is placed in the
other side of the chain. In spite of this important difference,
our results also exhibit a substantial preference of cis-[PtCl2L2]
(L = 1-3) metallosurfactants to form vesicles of similar sizes.
Probably, these metallosurfactants attain a looplike conforma-
tion, as shown in Figure 3, which is also a more cylindrical shaped
molecule that promotes vesicle formation. The singular behavior
of cis-[PtCl2(3)2] (it is the sole compound that demonstrably
formed metallomicelles) could be related to the large value of
750 DOI: 10.1021/la902459f
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Article
Figure 6. Cryo-TEM micrographs of prepared samples. Top row: Different morphologies of phosphine 3 aggregates; multilamellar vesicle
(A), bilamellar vesicle (B), and unilamellar vesicle (C). Bottom row: Large (D) and small (E) unilamellar vesicle of cis-[PtCl2(1)2]. Two small
unilamellar vesicles of cis-[PtCl2(3)2] (F).
cryo-TEM. Furthermore, the cryo-TEM study suggests that the
smaller aggregates are more frequent with the complex cis-[PtCl2-
(3)2]. This is in agreement with the dynamic light scattering
analysis, since this complex is the sole platinum compound that
showed a significant population of micellar size.
were only observed with ligand 3, an attractive result that could be
useful in future studies. Metallosurfactants cis-[PtCl2L2] (L =
1-3) mainly form medium size vesicles. This behavior could be
related to the shape of these molecules, an hypothesis that is
consistent with previously reported studies about the influence
of metal coordination in the formation of aggregates.44,45
Finally, the study of aggregate morphology, that has been
performed by cryo-TEM, showed in the majority of cases the
formation of polydisperse spherical vesicles with sizes that in
general terms are consistent with the DLS analysis.
Conclusions
The set of linear alkylsulfonated phosphines 1-3 are convenient
ligands to prepare new metallosurfactants by means of simple
complexation with a metallic fragment. In addition, since the free
phosphines 1-3 behave as surfactants, these compounds provide a
useful tool for studying the influence of metal coordination in the
surfactant and aggregation properties of these compounds.
On the basis of surface tension measurement results, we
propose an analogy between metal complexes cis-ML2 (L =
1-3) and bolaform surfactants since these complexes can be
visualized as two conventional surfactants linked by the metallic
fragment. The analysis of values of the area occupiedper molecule
adsorbed in the surface gave a singular result for the complex
cis-[PtCl2(3)2] that could beexplained bya different packing in the
interface. This behavior evidence that the length of the alkyl chain
in ligands 1-3 can lead to relevant changes in the supramolecular
arrangements of their metallosurfactants.
Acknowledgment. This research was supported by the Di-
ꢁ
ꢁ
reccion General de Investigacion, Ministerio de Ciencia y Tecno-
logıa (Project CTQ2007-63913). We thank Dr. Emma Rossinyol
´
ꢀ
ꢀ
(Servei de Microscopia Electronica de la UAB) for valuable
assistance with the cryo-TEM studies and to Prof. Joan Estelrich
ꢀ
´
(Departament de Fisicoquımica, Facultat de Farmacia, Univer-
sitat de Barcelona) for DLS measurements.
Supporting Information Available: Complete description
of synthetic and characterization methods for compounds 2
and 3 and cis-[PtCl2L2] (L = 1-3). 195Pt{1H} NMR spectra
of cis-[PtCl2L2] complexes (L = 1-3). Negative-ion HRMS
spectra of cis-[PtCl2L2] complexes (L = 1-3). Dynamic light
scattering spectroscopy: Average hydrodynamic diameters
obtained for aggregates of phosphines 1-3 at different
concentrations. This material is available free of charge via
the Internet at http://pubs.acs.org.
DLS spectroscopy has shown important differences in the
size of aggregates formed by the three phosphines (1-3) at
concentrations above the cmc, indicating that changes in the
number of methylene groups in the alkyl chain can involve
significant modification in the size of aggregates. Thus, micelles
Langmuir 2010, 26(2), 743–751
DOI: 10.1021/la902459f 751