Self-assembly of methylzinc-polyethylene glycol amphiphiles and their application to materials synthesis.

Amphiphilic molecules have proven to be of extraordinary value because they bear the potential to bridge the molecular scale and the nanoscale. They are characterized by two linked molecular parts, which have different solvent compatibilities. When used with a solvent that is compatible with only one of the molecular regions, self-assembly into higher organized structures, such as micelles and lyotropic phases, takes place. Amphiphiles are used in numerous ways: as detergents, for the stabilization of colloidal particles, and even for the preparation of nanoporous materials, to name only a few. Classical examples of amphiphiles are surfactants containing a hydrophilic head group (for instance, cationic ammonium) and a long hydrophobic alkyl chain. As a result of the chemical “simplicity” of the head group, classical amphiphiles are rather restricted in expressing additional functional properties. A tempting strategy to extend the functional spectrum of amphiphiles is to equip them with a metal. Some examples of metal-containing amphiphiles exist already in the emerging field of metallomesogens. Metals were either introduced as counterions to the head group, or the head group itself contained a multidentate ligand capable of binding to certain metal cations (Werner complexes). Amphiphiles with organometallic head groups and their selfassembly behavior have not yet been reported. A fascinating perspective of metallomesogens is that the metal could represent a source for the synthesis of an inorganic material. Generally, two sources are used (materials precursor plus structure-directing agent) for the preparation of many nanostructured inorganic materials, but metalcontaining amphiphiles bear the potential that their selforganization properties could be combined with precursor chemistry in a one-pot approach. Metallomesogens have been used for the synthesis of nanostructured inorganic materials only rarely. There are reports about the covalent anchoring of an alkoxysilane fragment ( Si(OR)3) to a long alkyl chain through hydrolytically stable Si C bonds. The alkoxysilane fragment fulfills two functions: it becomes the hydrophilic head group after hydrolysis of the silyl ethers, thus inducing the self-organization of a liquid-crystalline phase, and it forms the silicate as a result of polycondensation. To the best of our knowledge, other reports describing the preparation of nanoporous metal oxides from metallic amphiphiles do not exist. We became interested in the subject of organometallic amphiphiles in the course of our recent studies of a versatile precursor system for the preparation of various zinc oxide (ZnO) materials. These ZnO precursors are organometallic oxo cluster compounds containing ZnCH3 groups and a central “Zn4O4” heterocubane core (see Supporting Information). Our initial idea was to prepare a new metallomesogen by attaching a long organic chain to the zinc-oxo cluster core (Supporting Information). As a result of the shielding of the zinc-oxo core with alkyl groups, polar polyethylene glycol (PEG) was selected as a side chain. At the same time the amphiphile should be able to afford a nanostructured ZnO material. The synthesis of the Zn4O4 heterocubane is simple: 7] dimethylzinc (ZnMe2) is reacted with an equimolar amount of a low-molecular-weight alcohol ROH (R: e.g., isopropyl) in anhydrous toluene to afford [MeZnOR]4 tetramers in quantitative yield. Therefore, the first attempt to prepare the desired amphiphiles [(MeZn)4(OR)3OPEG] was to conduct the reaction ZnMe2+ 0.25PEGn+ 0.75ROH!?, with PEGn=HOEt(OEt)nOX (X: end groups, OMe, or H; n= 400, average molecular mass in gmol ). However, the workup of the reaction showed that the latter approach was too simple. Instead of [(MeZn)4(OR)3OPEG], two compounds were isolated: the “normal” heterocubane [MeZnOR]4 containing no PEGn and a gel-like substance. In further experiments it was found that the gel results from the reaction of ZnMe2 solely with PEGn (Figure 1). In the following, we concentrate on this gel, termed [MeZnOPEG400]. Its molecular-scale structure was characterized, the self-organization on different length scales was studied, and its ability to form ZnO materials was examined. The gel (Figure 1) was isolated and dried in vacuo. The drying led to a significant shrinkage, which indicates that the material was highly swollen in the initial state. The molecular structure of the solvent-free material was characterized by using a combination of FTIR spectroscopy, NMR spectroscopy, and MALDI-TOF mass spectrometry (MS). The solidstate C NMR spectrum (Figure 2a) is characterized by only two signal groups: besides the ether groups from the PEG [*] Dr. S. Polarz, Dipl.-Chem. R. Regenspurger Department of Chemistry University of Konstanz 78547 Konstanz (Germany) Fax: (+49)7531-884406 E-mail: [email protected]