42575-G3
Ligand Design and Geometry Control in Electroactive Heterospin Precursors for Magnetic Switching
The goals of this research included (i) the development of comprehensive synthetic methodology for precursors aiming at molecular switching, (ii) an approach using five-coordinate environments to enable cycling of radicals, (iii) the investigation of electroactive ligands for modulation of redox properties or and (iv) use of these as building blocks for multimetallic clusters. PRF support has yielded ten publications in these areas.
For Goals 1 and 2 (“Investigation of properties of five-coordinate complexes” and “Influence of aromatic groups on the electronic properties of five-coordinate complexes”) the Verani Group has used ligand design to foster unusual geometries in trivalent metal centers and to enhance stable switching mechanisms (Inorg. Chem. 2006, 45, 955). Adding to the complex [FeIIIL1], described previously, ongoing research focuses on newly developed systems with improved redox cyclability allowing up to 100 oxidation/reduction cycles without decomposition. A manuscript to Angewandte Chemie including mechanisms of controlled oxidation (single vs. multiple electrons) and broken symmetry DFT calculations is planned for 2009.
Expanding on these observations, current research on molecules able to behave as switches when deposited onto surfaces was emphasized. Efforts have been directed to functionalizing further these species in order to attain amphiphilic properties that allow for surface deposition and film formation. We have reported previously on progress related to Goal 3 (“Induction of redox changes by means of terminal ligands”), where considerable effort has been devoted to the development of metal-containing soft materials containing NN'O ligands that can later act as terminal ligands (Inorg. Chem. 2005, 44, 7414, Dalton Trans. 2006, 2517, Inorg. Chem. 2007, 46, 9808). A new article was published (Inorg. Chem. 2008, 47, 3119) discussing how the hydrolysis of a similar NN'O-containing ligand led to the use of bis-(tert-butyl-salycilaldehyde)copper(II), [CuII(LSAL)2] (1) as a precursor for the amphiphilic species [CuII(L2I)2] (3), [CuII(L2A)2] (3′), and [CuII(L3)2] (4). These complexes exhibit hydrophilic copper-containing headgroups, hydrophobic alkyl or alkoxo tails, and present potential as precursors for redox-responsive capping groups in Langmuir–Blodgett films. All systems were characterized by electrochemical techniques, compression isotherms and Brewster angle microscopy. Good redox activity was observed for 3 with two phenoxyl radical processes between, but this complex lacks amphiphilic behavior. To attain good balance between redox response and amphiphilicity, increased core flexibility in 3′ and incorporation of alkoxy chains in 4 were attempted and core flexibility improved Langmuir film formation with a higher formal collapse and showed excellent cyclability of the ligand-based processes. Another important development will appear (Chem.-Eur. J. 2008 early view) where we describe the synthesis and characterization of a novel series of single-tail amphiphiles LPyCn, (C10-18) and their copper(II)-containing complexes of relevance for patterned films. The N-(pyridine-2-ylmethyl)alkylamine ligands and their complexes [LPyC18CuIICl2] (1), [LPyC16CuIICl2] (2), [LPyC14CuIICl2] (3), [LPyC18CuIIBr2] (4), [LPyC16CuIIBr2] (5), and [LPyC10CuIIBr2] (6) were synthesized, isolated, and characterized. Species 1, 2, 3, and 6 had their molecular structure solved by X-ray diffraction methods. Aiming at the use of these species as precursors for redox-responsive films, an assessment of their electrochemical properties involved voltammetry in different solvents, electrolytes, and scan rates. DFT calculations in bulk and at interfaces were used to evaluate their electronic structure and dipole moments. Morphology and order of the resulting films at the air/water interface were studied by isothermal compression and Brewster angle microscopy ad biphasic patterned Langmuir films were observed for several species This information is pivotal for ongoing research aiming the development of (i) second-generation ligands containing distinctive rigidity to enhance the stability of monolayers at higher surface pressures, thus leading to highly ordered assemblies, (ii) precursors with different metal ions leading to specific optic (e.g. colorless Cu+ to green Cu2+) or redox (e.g. Fe2+/Fe3+) properties, as well as (iii) the manufacture of responsive Langmuir-Blodgett films on solid substrates with these materials.
While developing Goal 4 (“Homo/heterospin clusters with [FeIIIL2]), we have observed and pursued synthetic routes that allow for the incorporation of metal clusters into soft materials. This progress was published recently (Chem. Eur. J. 2007, 13, 9948 ) in which a general approach toward amphiphilic systems bearing multimetallic clusters, and their ability to form Langmuir-Blodgett films, is presented. Similarly, the publication of thermotropic mesomorphism on such metal clusters (Inorg. Chem. 2006, 45, 7587) lead to another report on the liquid crystalline behavior of copper amphiphiles with the abovementioned LPyCn ligands (Inorg. Chem. 2008, 47, 7225) were we demonstrate that small changes in the geometry of cationic mesogens can be imposed by the presence of apically coordinated anions, allowing for tuning in the properties of the resulting mesophases.
This research will lead to ordered responsive films composed of coordination complexes, thus pivotal for device fabrication in molecular electronics. The PRF support has made possible for the PI and one graduate student to engage in this program and will be fundamental for further development of the PI's career. All publications acknowledge the agency.
