A metal complex that imitates a micelle

Article abstract of DOI:10.1039/c0cc00648c

A metal complex with a micelle-like, core-shell structure adopts higher nuclearity in water than in organic solvents, thereby imitating also the growth of a micelle, but through covalent rather than non-covalent aggregation.

Full text of DOI:10.1039/c0cc00648c

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A metal complex that imitates a micellewz
Nicola Giri and Stuart L. James
Received 29th March 2010, Accepted 26th April 2010
DOI: 10.1039/c0cc00648c
A metal complex with a micelle-like, core–shell structure adopts
higher nuclearity in water than in organic solvents, thereby
imitating also the growth of a micelle, but through covalent
rather than non-covalent aggregation.
The concept of ‘metal-complex-as-amphiphile’, in which metal
complexes act as components of metallo-micelles, is well
established.1,2 However, an alternative concept, that of
‘metal-complex-as-micelle’, in which a single metal complex
imitates the structure and behaviour of a complete micelle, has
not yet been described. Here we report an example of such a
metal complex. The structure is shown generically in Fig. 1a,
and can be seen as a hybrid of a metal complex and a
micelle, since it is based on amphiphilic ligands which form
a hydrophobic interior surrounded by hydrophilic head
groups. Importantly, the stability of this type of complex in
water is therefore not only due to metal coordination but also
hydrophobic interactions. For the complex reported here,
we show that the structure it adopts is indeed directed by
hydrophobic interactions. In particular, in pure water the
complex is dinuclear with bridging ligands. However, if the
hydrophobic interactions are negated or overcome by lowering
the solvent polarity or pH, the complex switches to a
mononuclear state, with its ligands changing from bridging
to chelating. In this way the complex therefore imitates a true
micelle by growing or shrinking in response to its medium, but
by a non-micelle-like mechanism which requires breaking and
remaking of coordination bonds.
The system described here is based around types of silver
complex formed by bidentate phosphines such as dppe,
bis(diphenylphosphino)ethane. The complex [Ag(dppe)2]+ is
normally mononuclear (i.e. both ligands chelating) but
can become dinuclear for certain ligand variants or upon
crystallisation (Fig. 1b).3 Therefore the monomer–dimer
equilibrium appears to be finely balanced for this type of
complex. In the present work, the complex was given a
pseudo-micellar structure by using an amphiphilic variant of
dppe. Specifically, hydrophilic piperazine groups were added
to the para positions of the diphosphine’s hydrophobic aryl
groups. The amphiphilic ligand (L1, Fig. 1c) can therefore
generate complexes with hydrophobic cores surrounded by
hydrophilic head groups.
Fig. 1 (a) Schematic of micelles, metal complexes and hybrid
structures, (b) equilibrium between mono-silver and di-silver complexes
of diphosphines and (c) structures of dppe and L1.
The behaviours of free, uncoordinated L1 and its silver(I)
complex are summarised in Fig. 2. Free L1 forms micelles in
water, behaving as a non-ionic surfactant,3 as revealed by the
following observations: (i) a critical micelle concentration
(CMC) of 5 mM was determined using the dye inclusion
technique,5 corroborated by DLS (dynamic light scattering),
(ii) DLS shows particles of hydrodynamic diameter 5.0 nm in
D2O (20 mM, pH 9.0, Fig. 3b) with low polydispersity
whereas chloroform solutions showed no aggregation,
(iii) the size of the micelles in water was not influenced by
ionic strength since 10 mM NaCl solutions gave identical
results to those in deionized water, (iv) concentration-
dependent line broadening is seen in 1H and 31P-NMR spectra
in D2O with upfield 31P shifts relative to those in CDCl3
(Fig. 3a), and (v) a cloud point (33 1C) was observed in water
which is a phenomenon well known for related non-ionic
surfactants.6
Centre for the Theory and Application of Catalysis,
School of Chemistry and Chemical Engineering,
Queen’s University Belfast, David Keir Building, Stranmillis Road,
Belfast BT9 5AG, UK. E-mail: s.james@qub.ac.uk
w This article is part of the ‘Emerging Investigators’ themed issue for
ChemComm.
z Electronic supplementary information (ESI) available: Synthesis of
L1, experimental details, details of modelling. See DOI: 10.1039/
c0cc00648c
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For the metal complex, in pure D2O a 2 : 1 mixture of L1
and AgOTf was found to adopt the dinuclear structure
[Ag2(L1)4]2+ as revealed by 31P-NMR spectroscopy. The
spectrum consists of two pairs of multiplets at d = ꢁ1.5 (PA)
and 1.9 (PB) which can be assigned to the chelating and
bridging ligands respectively (Fig. 3c). PA consists of a pair
of triplets of doublets from which both P–Ag and PA–PB
coupling constants could be extracted (1J(109Ag–31PA) = 225 Hz
and 2J(PA–PB) = 42 Hz). The PB signal is complicated by
coupling to both Ag centres and the pattern was not readily
analysable. However, based on the separation between the two
symmetrical halves of the multiplet, 1J(109Ag–31PB) was estimated
to be 290 Hz. The magnitudes of the silver–phosphorus
couplings are diagnostic of AgP4 coordination centres.7 The
assignments were consistent with a 31P COSY NMR spectrum
which showed PA and PB to be mutually coupled. Further
substantiation came from literature values for the analogous
disilver complex formed by 1,2-bis(di-4-pyridylphosphino)-
ethane.3c The 1H-NMR spectrum also showed the expected
two ligand environments.
Addition of methanol or acid to the disilver complex
converted it to the mononuclear bis-chelate [Ag(L1)2]+
(Fig. 2, right) as indicated by simpler NMR spectra and
DLS (Fig. 3c, d). At pH 1.0 with four equivalents of HO3SCF3
per L1 (one for each piperazine group) a single 31P environment
coupled to 107Ag and 109Ag was seen (dP = 3.8 ppm,
Fig.
2 Schematic diagram to show the structures adopted by
the free ligand and its silver complex change in response to the
medium.
1J
= 263 Hz). The data are similar to those for the
109Agꢁ31P
analogous dppe complex [Ag(dppe)2]+ (4.4 ppm, 266 Hz3a).
Molecular models of the mono- and di-silver complexes are
shown in Fig. 4. They exhibit micelle-like core–shell structures,
each complex having a hydrophobic interior surrounded by
hydrophilic head groups. Importantly, the models also show
that some of the hydrophobic interior of the monosilver
complex remains exposed to the solvent and that in the disilver
complex more of this hydrophobic surface is shielded from the
solvent. This helps to understand adoption of the dinuclear
structure in pure water as being driven by hydrophobic
interactions.
Overall, as shown in Fig. 2, there are similarities between the
behaviour of the free ligand and its Ag complex. In pure water,
where hydrophobic interactions are present, free L1 forms
micelles and its silver complex is large (dinuclear). When
hydrophobic interactions are absent (in organic solvents or
low pH) free L1 is monomeric and its silver complex is
correspondingly small (mononuclear). The driving forces for
the changes in structure are the same for the free ligand and
metal complex. For both species methanol decreases the
hydrophobic interactions. Similarly, protonation destabilises
both the micelle and the disilver complex by causing head
group–head group repulsions between the piperazine groups.
However, an important difference is that whereas the micelle is a
non-covalent assembly of individual molecules, the disilver
complex is a single, covalently bonded molecule. Changing
between mono- and di-nuclear complexes requires breaking
and then remaking of coordination bonds. Therefore, the silver
complex can be seen to imitate the growth of a true micelle but
through covalent rather than non-covalent aggregation.
Fig. 3 (a) 31P NMR spectra for L1, (b) dynamic light scattering of L1
micelles in water, (c) 31P NMR spectra showing interconversion
between mono- and di-nuclear silver complexes with addition of
HOTf, and (d) dynamic light scattering for the mono- and di-nuclear
complexes.
Addition of methanol (to 30% v/v) or acid (HO3SCF3) to
aqueous L1 disrupted the micelles as shown by DLS and
sharpening of NMR signals. Micelle disruption by methanol
is expected because it lowers the hydrophobicity of the
medium. Micelle disruption by acid is also expected since
protonation generates positive charges on the piperazine
groups, increasing the water-solubility of L1 as well as electro-
static repulsion between the piperazine head groups in the
micelles.4 The free ligand therefore serves to illustrate typical
micellar behaviour.
In summary, we report that hydrophobic interactions can
be important in directing the structures adopted by metal
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is currently underway with further discrete and polymeric
systems.
We are grateful to A. P. de Silva for valuable discussions.
Notes and references
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4 For overviews of micellar assembly containing the points raised here
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Membrane Mimetic Chemistry, Wiley-Interscience, 1982.
Fig. 4 Modelled structures of [Ag(L1)2]+ (above) and [Ag2(L1)4]2+
(below) showing their pseudo-micellar structures. Piperazine
substituents = blue, Ag = white, and P = yellow. For clarity
H-atoms are not shown.
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complexes which have micelle-like core–shell configurations.
Previous work on the aggregation of metal complexes
by hydrophobic interactions can be described as ‘metal-
complex-as-amphiphile’, because of the head-group-and-tail
structure of the complexes and the fact that non-covalent
aggregation preserves the original molecular species. However,
the situation here is better described as ‘metal-complex-
as-micelle’.8 This description relates to the structural
similarity of the complexes to micelles (core–shell) as well as
the fact that the larger species stabilised by hydrophobic
interactions is a single metal complex and not a non-covalent
aggregate.
7 Methods in Stereochemical Analysis, in Phosphorus-31 NMR
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L. D. Quin, VCH Publishers Inc, Deerfield Beach, FL, 1987, vol. 8.
8 The complexes should not be confused with actual micelles
however, the latter being held together entirely by non-covalent
interactions with different thermodynamic implications. The
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micelle’, see C. N. Moorefield and G. R. Newkome, C. R. Chim.,
2003, 6, 715, but which are constitutionally dynamic and responsive.
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The concept may also be relevant to other metal complexes
based on hydrophobic ligands to which hydrophilic substituents
have been added (to render them water soluble). Examples of
such complexes can be found in homogeneous catalysis9
and medicinal chemistry.3c,10 It also points to a general
way of making coordination systems which respond to their
medium in ways typically associated with micelles. Work
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Ramirez, R. J. Aguilera, R. Ovalle and M. Contel, Eur. J. Inorg.
Chem., 2009, 3421; J. J. Liu, P. Galettis, A. Farr, A. Maharaj,
H. Samarasiha, A. C. McGechan, B. C. Baguley, R. J. Bowen,
S. J. Berners-Price and M. J. McKeage, J. Inorg. Biochem., 2008,
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Chem. Commun., 2011, 47, 245–247 | 247