Single-molecule magnets, their oligomers and polymers: their structure, magnetic properties at ambient and high pressures, and high-field EPR spectroscopy
Department of Chemistry, University of North Florida, Jacksonville FL 32226
Single molecule magnets (SMMs) are molecular materials, namely zero-dimensional, which exhibit both classical (magnetization hysteresis) and quantum properties (i.e. quantum tunneling, quantum phase interference, entanglement and others). SMMs are typically studied as the monomers, but it is unclear what happens if we have polymeric arrays of them or oligomers. In this work, the archetypical Mn12 SMM was used as a building block in order to synthesize polymers and oligomers. In this talk, our work on 1D / 2D / 3D polymers as well as oligomers of Mn12 SMMs will be discussed. These materials were thoroughly characterized with single-crystal x-ray crystallography, SQUID magnetometry at both ambient and elevated pressures, along with high-field high-frequency EPR spectroscopy.
Cobalt-Manganese-Oxide Clusters as Potential Water Oxidation Catalysts (WOCs)
Preet Mahalay, Khalil A. Abboud and George Christou
Department of Chemistry, University of Florida, Gainesville, FL 32611
The ability of Nature to achieve high-efficiency catalytic water oxidation in plants and cyanobacteria using the earth-abundant metals Mn and Ca is unmatched in any of the artificial systems known to date. Most of the latter rely on 4d and 5d metals such as Ru, Ir, etc. Thus, efficient catalytic oxidation of water using 3d metals such as Mn, Co and Cu remains a long-standing challenge for synthetic chemists. Homometallic, high oxidation state Mn and Co systems have been extensively researched as WOCs over the past 20 years, but only recently has an efficient water oxidation catalyst of Mn been reported – the very recent report of water oxidation electrocatalysis with a remarkably low overpotential of only 0.33 V by a member of the well-known [Mn12O12(O2CR)16(H2O)4] family of clusters. While very active, the catalyst begins to decay within a few hours and is assigned to slow decomposition of the Mn12 cluster. Since several robust Co/O clusters containing kinetically-inert CoIII have also been reported in the literature and shown to be good WOCs, we have initiated a program seeking to amalgamate the benefits of the two areas by developing mixed-metal Co/Mn/O/RCO2- clusters. In this presentation will be described the development of a synthetic route to a new family of high oxidation state CoIII/MnIV/O clusters and their characterization by X-ray crystallography, magnetic studies, and other techniques.
Conducting Charge Transfer Salts of Fe(II) Complexes with Organic TCNQ Radicals
Ökten Üngör, Hoa Phan, Michael Shatruk
Florida State University, Department of Chemistry & Biochemistry
Fe(II) complexes with ligands of an intermediate ligand field strength often show magnetic bistability, i.e. switching between the high-spin (HS) and low-spin electronic configurations driven by changes in temperature, pressure, or photoexcitation. This control by external stimuli makes spin crossover (SCO) complexes promising materials for applications. We are interested in designing multifunctional materials that exhibit both SCO and conductivity by combining Fe(II) centers and organic TCNQ-type acceptors. In such complexes, TCNQ radical anions are arranged in stacks that provide conducting pathways. The stacking distance can be affected by structural changes induced by SCO, and thus the synergy between the SCO and conductivity can be achieved. The synthesis of such materials can be achieved in two ways: first, by coordinating TCNQ ligands directly to the Fe(II) center, which is partially protected by blocking ligands that limit the growth of extended structures; second, by co-crystallizing completely blocked Fe(II) centers with free TCNQ radicals. We will discuss several complexes, in which Fe(II) cations have been co-crystallized with fractionally charged TCNQ radical anions to result in hybrid semiconducting materials.
Resolution of the chiral octanuclear Iron-Oxo-Pyrazolate to its P and M enantiomers
Konstantinos Lazarou[a], Karilys Gonzales[b] and Raphael R. Raptis[a]
[a] Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199.
[b] University of Puerto Rico-Carolina, Carolina, PR 00984.
Enantioselective transition-metal-based catalysis is a well-developed field with numerous applications. However, efficient chiral catalysts based on cheap and abundant first-row metals like Fe are, so far, scarce. In our group we have characterized a family of octanuclear Fe3+-complexes of the general formula [Fe8(μ4-O)4(μ-4-R-pz)12X4], [Fe8], where pz = pyrazolato anion, for a variety of terminal X-ligands and 4-R-substituents on the bridging pyrazolates. These complexes contain a central Fe4(μ4-O)4-cubane surrounded by four more Fe-centers, the X-ligands coordinated to the latter, forming all together a Fe8(μ4-O)4X4-core with tetrahedral geometry. However, the twelve bridging pyrazolato ligands (each bridging one cubane-Fe to one outer-Fe) adopt a propeller-like rotation in order to adjust to the steric requirements of the iron oxide core, and in so doing violate the mirror symmetry of the Td point group. This reduces the symmetry of the complex to that of the chiral T point group resulting in its crystallization as a racemic mixture of M and P enantiomers. Here we demonstrate that substitution of the terminal Cl, in the case of [Fe8(μ4-O)4(μ-4-Cl-pz)12Cl4, by rac-4-sec-Bu-C6H4O, results in spontaneous resolution upon crystallization of the P/S and M/R enantiomers, as shown by single-crystal X-ray crystallography. Further proof of the selectivity of each complex enantiomer towards the R or S enantiomer of the phenol, is given by single-crystal Circular Dichroism experiments.
Photochemistry of (η3-allyl)Ru halide precursors for photo assisted chemical vapor deposition
Christopher R. Brewer,1 Olivia M. Hawkins,1 Bryan Salazar,2 Amy V. Walker2 and Lisa McElwee-White1
1 - Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200, USA
2 - Department of Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas 75080, USA
Chemical vapor deposition (CVD) is a potentially attractive technique for the metallization of organic thin films. However, thermal CVD processes often require high temperatures which are incompatible with organic substrates. Photochemistry provides an alternative means of initiating precursor decomposition without heating the substrate. Readily available Ru precursors, such as (η3-allyl)Ru(CO)3X (X = Cl, Br, I), have been used to deposit Ru on functionalized self-assembled monolayers by means of photochemical CVD as a model system for deposition of metal on a thermally sensitive substrate. Recent work has been conducted to investigate the influence of the halogen of the decomposition of the precursor during the deposition process. Quantum yields for carbonyl loss and luminescence results are discussed in the context of precursor design for photochemical deposition techniques.
Effects of Cl- and NO3- Ions on Cerium Dioxide Nanocluster Structures
Bradley Russell-Webster, Khalil A. Abboud & George Christou.
Department of Chemistry, University of Florida, Gainesville FL 32611-7200, USA
Metal oxide nanoparticles provide exciting prospects for various applications as they exhibit much greater catalytic activities than their bulk counterparts. Of tremendous importance are cerium dioxide nanoparticles (CNPs) owing to their widespread use as catalysts in many industrial and medical processes. Their activity is found to vary according to the surface facets present. It has been determined, both experimentally and theoretically, that the activity of the facets increases in the order (111) < (110) < (100), making synthesis of CNPs with many (100) facets highly desirable. The standard ‘top-down’ synthetic approach provides CNPs with mixtures of sizes and shapes, making it extremely difficult to obtain structural information to atomic resolution, especially of the exact identity of the high-activity (100) facets. Recently the Christou group has worked to shed light into the mysteries of CNPs using a bottom-up synthetic procedure to synthesize molecular analogues of CNPs, so-called ‘Ce/O nanoclusters’. Synthesis of these molecular clusters enables structural characterization to atomic resolution using X-ray crystallography, allowing identification of Ce3+ ions and location of H+ binding sites. In the Ce/O nanoclusters that have synthesized to date, the most thermodynamically stable facets have all been observed, (111), (110) and (100). In CNP synthesis, the use of Cl- or NO3- ions has been reported to control the growth of selected facets by altering of surface free energy by adsorption. This use of these ions has therefore been explored in the synthesis of our Ce/O nanoclusters to compare the effects of Cl- and NO3- ions on facet formation. One important result of this work is that Cl- ions produce an unprecedented amount of surface Ce3+ ions in the resulting Ce/O nanocluster.