SELF-ASSEMBLY IN SOLUTION OF METAL COMPLEXES BASED ON
5.3 POLAR–NONPOLAR COMPOUNDS AS TEMPLATES FOR METAL-CONTAINING NANOSTRUCTURES
Currently, the main focus is on exploit of polar–nonpolar compounds, such as organosiloxanes as templates for developing supramolecular compos- ites with metal complexes designed to function as flexible nanostructural materials. Hydrogen-bonding and metal – ligand interactions are among the most powerful directing forces for the self-assembly processes, which allow the construction of large suprastructures. In particular, such interac- tions led to the development of discrete metal-containing nanostructures, which are promising candidates in fields such as molecular recognition, photophysics, and catalysis (Company et al., 2006).
Amphiphilic supermolecules can be designed by employing nonco- valent interactions, electrostatic, and complementary hydrogen bonding, toward hierarchically self-assemblies in different solvent media. To date,
TABLE 5.1 Several Siloxane Amphiphiles and Their Applicability.
Structure of various siloxane amphiphiles Applicability References
• Biocompatible nonionic polymer surfactant
Racles and Hamaide (2005)
• Colloidal stabilization of nanoparticles
Racles and Cozan (2012)
• Nonionic and
ionic surfactants Racles et al.
(2006)
• Polymer nanopar-
ticles formulations Racles et al.
(2009)
• Biocompatible nonionic surfactant
Racles et al. (2014a, 2014b, 2014c)
• Polymer nanopar- ticles formulations
• Stabilization of nanoparticles
• Aqueous disper- sion of metal oxide nanoparticle
Structure of various siloxane amphiphiles Applicability References
• Biocompatible
compounds Racles (2010)
• Stabilization of
nanoparticles Racles et al.
(2011)
• Ligand for 4f
metals Racles et
al. (2014a, 2014b, 2014c)
• Aqueous disper- sion of metal oxide nanoparticle
• Micellar solubili- zation of nonsol- uble drugs TABLE 5.1 (Continued)
metal–ligand coordination bonds have been largely used for the synthesis of discrete molecules and extended networks, coordination polymers, or bulk organogels (Liu et al., 2016).
Metal-directed self-assembly is one of the most useful approaches for generation of complex and highly elaborated molecular architectures. In this case, the advantage of metal-directed self-assembly is conferred by the control over different geometries and bond strengths depending on the metallic centers used in the system. The rational concepts regarding the building blocks: the number of ligands, their relative orientation in space, and the geometry of metal ions are key factors which predict what species are able to self-assemble in solution. A broad variety of nano- objects such as nanotubes, micelles, or vesicles has been obtained from various molecules: synthetic amphiphilic block copolymers, amphiphilic perylene bisimides, porphyrins, terpyridines, inclusion complexes of β-cyclodextrin, or metal clusters (Baytekin et al., 2009).
The template method toward metallosupramolecular structures consists in using preorganized building blocks to predict and build up molecular structures via hierarchical self-assembly (Menozzi et al., 2006).
Metalloproteins and enzymes can be easily designed starting from complex amino acids and metal centers found in biological systems.
Diblock copolypeptide amphiphiles are representative synthetic materials, which can be used in this purpose due to their ability to form double-walled
vesicles or fibrillar nanostructures based on their self-assembly capacity, which recommend them as templates for inorganic compounds or for their transformation via dynamic tuning of the electronic state of the materials (Lutolf et al., 2005).
For the construction of such structures, Schiff bases have been often used especially for their directionality, thermodynamic stability, and coor- dination ability of nitrogen atom (Nakamura et al., 2016). Pyridyl-imine based ligands have been used to design a wide variety of self-assembled structures: from macrocycles, helicates, and polymers to metal–organic capsules as well as interlocked structures (Castilla et al., 2014).
Polyazomethines, known as Schiff base polymers or polyimines represent one of the most known classes of macromolecular ligands, and are synthesized from a wide variety of carbonyl and amine precursors (Calligaris et al., 1972; Garnovskii et al., 1993). It is well known that the properties of the polymers can be radically modified by minor changes in chemical structure, the possibility to use metal ions for the preparation of polymeric structures with interesting and useful properties being very attractive. The most known strategy for the design of such compounds is the use of polydentate linkers containing donor atoms or groups of atoms with ability to coordinate metal ions to form polymeric structures with different architectures. The presence of the lone electron pair in a sp2 hybridized orbitals of the nitrogen atom of the imine (CH═N) group (Kianfara et al., 2010) in the structure of polyazomethines gives them a remarkable ability to coordinate various metals ions from s, p, d, or f blocks with different oxidation state (Andruh et al., 2009; Sui et al., 2010;
Hazra et al., 2009; Dolai et al., 2013).
Design of molecular structures with targeted properties (optical, magnetic, or electrical) based on these types of compounds having different number of coordination sites is an innovative direction regarding the development of new metal complexes that have proven their applica- bility in various fields, such as solar cells, electrochromic devices, organic field-effect transistors (OFET) (Hindson et al., 2010; Petrus et al., 2015;
Isik et al., 2012; Sicard et al., 2013) analytical chemistry (optical, elec- trochemical, and chromatographic sensors) (Jungreis and Thabet, 1969;
Ibrahim and Sharif, 2007), and material science (emitting diode OLED and PLED) (Katsuki, 1995). Their valuable properties recommend poly- azomethines as thermostable polymers with good hydrolytic stability and enhanced mechanical properties (Saugusa et al., 1992; Cooper et al., 2009).
However, one of the highest limitations of this class of compounds is their insolubility and infusibility due to the rigidity of the macromolecular chains. These limitations can be overcome by few approaches: the intro- duction of the alkyl or alkoxy groups in ortho position of the aromatic ring, the presence of some irregularities in the polymeric chain, inserting flexible groups in the main chain or pendant in order to disturb the well- organized packing of the polymer chains, or by inclusion complexes of rotaxanes (Samal et al., 1999; Shulpin, 2002; Venegas-Yazigi et al., 2010).
One alternative to these modifications, toward improved solubility and transition temperatures, is the introduction of siloxane segments in the structure of polyazomethines (Racles, 2008; Racles et al., 2007).
Given the various possibilities of obtaining such structures, our goal was directed toward the preparation of metal complexes derived from a polyazomethine having flexible siloxane sequences in the structure, by using metal ions with different coordination geometries, namely copper(II), cobalt(II), and zinc(II). The use of flexible ligands having siloxane sequence as building blocks for the coordination compounds is an attractive direction for obtaining new metal complexes with interesting properties (Vasiliu et al., 2005; Cazacu et al., 2003).
The right choice of polar-nonpolar diblock composition, the flexibility of the backbone chains, and experimental parameters allowed obtaining various micellar morphologies, polymeric hollow structures (Li et al., 2015), nanoporous thin films (Wang et al., 2008), NPs, nanocrystals, nano- tubes, nanowires, etc. (Liu et al., 2013; Perez-Pagea et al., 2016).
5.4 METAL COMPLEXES CONTAINING SILOXANE LIGANDS