Friday May 4th – Presentations

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A Reassessment of the Electronic Structure of Cr(VI) Sites Supported on Amorphous Silica and Implications for Cr Coordination Number.

Nathan Peek, Albert Stiegman, David B. Jeffcoat, Susannah L. Scott, §, Cristina Moisii, Lambertus van de Burgt, Salvatore Profeta

Florida State University

08:45 AM
Inorganic Chemistry

The electronic structure of isolated Cr(VI) sites supported on silica was re-investigated using multiple, complementary electronic spectroscopies applied to transparent xerogel monoliths. The absorption spectrum exhibits three previously reported peaks, at 22,800, 29,100 and 41,500 cm-1, as well as a previously unresolved band at ca. 36,900 cm-1. The emission is a long-lived red luminescence with λmax = 13,600 cm-1, emanating from the lowest excited state. Assignment of the excited states was facilitated using time-dependent density functional theory (TD-DFT) calculations performed on cluster models. All of the observed electronic transitions and their energies are accounted for by dioxoCr(VI) sites. Small variations in site symmetry at Cr result principally in inhomogeneous broadening of the spectral bands, as well as a red-edge effect in the photoemission spectrum. Recently, it has been proposed that there are two monomeric sites present on the surface: the well-established four coordinate dioxo site and a five-coordinate mono-oxo site. The results of this study find no compelling evidence for a significant contribution from a mono-oxoCr(VI) sites.

The Preparation of High-Nuclearity Heterometallic Pb-Mn Clusters

Elly Earlywine, Khalil A. Abboud, and George Christou

Department of Chemistry, University of Florida, Gainesville FL 32611-7200

09:05 AM
Inorganic Chemistry

Intense research activity continues in the development of high-nuclearity metal-oxo clusters because of their relevance to various areas of chemistry, physics, and material science. Recently, some of these clusters have also attracted great interest because they can structurally be considered small molecular pieces of the bulk metal oxides. In our group, the use of manganese with its various stable paramagnetic oxidation states (II-IV) in metal-oxo-carboxylate chemistry has led to numerous clusters with fascinating structural features and intriguing physical properties (e.g., magnetism). Although considerable effort has also been made in the development of heterometallic manganese clusters, few clusters have been reported that contain both manganese and a heavy main group element such as bismuth or lead. Additionally, several of them are low-nuclearity (eight or fewer metal centers). New Pb/Mn clusters also have the possibility of showing their own interesting physical properties, such as multiferroism. Hence, we have initiated a study into the preparation of high-nuclearity, heterometallic heavy metal Mn-oxo clusters, with the focus of this talk being on the discovery of new Pb-Mn-oxo clusters.

AlFe2B2 as Water Oxidation Catalyst

Dallas K. Mann,a Yury V. Kolen’ko,b and Michael Shatruka

a Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, FL 32306, United States
b International Iberian Nanotechnology Laboratory, Avenida Mestre José Veiga s/n, 4715-330 Braga, Portugal

09:25 AM
Inorganic Chemistry

Currently, many highly important catalytic processes depend critically on compounds that contain noble metals such as platinum or rhenium. Such catalysts, however, are non-sustainable in the long run, due to the low abundance and high cost of such metals. To counter this issue, we have developed a guided approach to the discovery of new sustainable catalysts.  This approach utilizes machine-learning analsyis (deep neural networks, or DNN) to identify potentially promising catalysts, in particular, among complex metallic alloys (CMAs). The DNN analyzes atomic fingerprints, i.e. radial distribution functions, around crystallographically unique sites to effectively screen through a vast database of intermetallic structures and identify materials with transition metal sites similar in their coordination topology to the active sites in known state-of-the-art catalyst. The approach also builds on the active site isolation concept, which has proven to be useful for the design of efficient heterogeneous catalysts. Guided by the DNN analysis, we began synthesizing several promising CMAs, which are characterized by extremely large unit cell volumes. CMAs are known to contain unique structural defects which might contribute to unconventional catalytic behavior. Our initial investigation has begun with AlFe2B2, a ternary boride known for its remarkably high magnetocaloric effect near room temperature with a composition consisting of light earth-abundant elements.1 In this contribution, we will demonstrate that AlFe2B2 acts as excellent catalyst for the oxygen evolution reaction (OER). In addition to the good catalytic activity, this material has remained remarkably stable under catalytic conditions, providing nearly constant OER rate for over 300 hours. The nature of our neural network approach and the catalytic performance of AlFe2B2 will be discussed in this presentation.

References

Tan, X.; Chai, P.; Thompson, C. M.; Shatruk, M. J. Am. Chem. Soc.2013, 117, 17399-17411.

Terbium(III) doped nano-spinels as green emitters for solid state lighting.

David A. Hardy, Geoffrey F. Strouse

Florida State University

10:15 AM
Inorganic Chemistry

Down-shifting phosphors are routinely used in solid state lighting to convert higher energy UV or blue light to visible light. However, color quality of commercial LEDs are modest and new RGB phosphors are needed to tune the color of light. Commercial LEDs are composed of bulk semiconductors that typically contain materials on the DOE high risk index. For improving performance of down-shifting phosphors, size of the phosphor must be reduced to sub 10nm to reduce scattering of the pump led, the host lattice must be composed of earth abundant materials, and must absorb pump LED irradiation and convert to pure red, green, and blue emission for optimal color quality. Here we present the microwave synthesis of nanophosphors doped with Terbium(III) to accomplish green emission. Lanthanide emission is accomplished by energy transfer from surface ligands to lanthanide metal centers by using the molecular antenna effect. Optical measurements of the nano-spinel will be discussed including absorption, emission, lifetime, and quantum yields. Ligand exchanges were completed in efforts to improve quantum efficiencies and the nanospinels were characterized by pXRD, TEM, EPR, and FT-IR.

Giant Molecules: Exploring Synthetic Parameters on the Path to New Mn-O Torus-like Clusters

Ashlyn Hale, Khalil A. Abboud, George Christou*

Department of Chemistry, University of Florida, PO Box 117200 Gainesville, FL 32611-7200, USA

10:35 AM
Inorganic Chemistry

The discovery of fascinating giant molecular clusters of paramagnetic metal ions (metal nuclearity of 30 or greater, and diameters up to 4.3 nm) has garnered immense interest due to their relevance to chemistry, physics, and materials science. The allure of diverse giant clusters is rooted in their fantastic features, such as their aesthetically-pleasing architectures, nanoscale size regime, and phenomenal physical properties. These features uniquely position giant clusters at the interface between the classical and quantum realms. Mn-O clusters are at the forefront of 3d transition metal giant clusters, owing to the relevance of Mn to molecular magnetism, especially single-molecule magnets (SMMs). The largest homometallic Mn-O cluster and SMM to date was synthesized by our group over a decade ago: the Mn84­ torus is a SMM and with a diameter of ~4.3 nm, it truly straddles the classical-quantum interface. Over a decade later, our group reported a second member in the family, the Mn­70 torus, the second largest SMM and Mn-O cluster to date. Herein, we present the revitalized and systematic investigation into this family of giant torus-like clusters, in order to further deduce which synthetic and structural parameters dictate the curvature and thus the nuclearity of the product, paving the way for targeted syntheses and the discovery of new giant Mn-O clusters.

Creating New Coordination Environments for Spin-Crossover Complexes: Data Mining and Chemical Intuition

Jeremy J. Hrudka, Hoa Phan, Alina Dragulescu-Andrasi, Sandugash Yergeshbayeva, Michael Shatruk

Florida State University, Department of Chemistry and Biochemistry

10:55 AM
Inorganic Chemistry

The design of transition metal complexes that exhibit switching between the low-spin and high-spin electronic configurations (spin crossover, SCO) usually involves some a priori knowledge of typical ligand coordination environments which are conducive to the occurrence of SCO for a specific metal ion. For example, the octahedral coordination of the Fe(II) ions by six N-donor atoms is known to often furnish SCO behavior. In this contribution, we demonstrate that effective prediction of the possibility of spin-state switching in homoleptic Fe(II) complexes with chelating diimines can be achieved on a very simple principle that takes into account the separate between the N-donor sites of the chelating ligand. We further use this principle to analyze the ligand field strength and modify the SCO temperature of heteroleptic complexes. Among these, especially noteworthy are the complexes with the {N4S2} ligand set, which provides a new coordination environment for the design of SCO Fe(II) complexes.

Single-molecule magnets, their oligomers and polymers: their structure, magnetic properties at ambient and high pressures, and high-field EPR spectroscopy

Christos Lampropoulos

Department of Chemistry, University of North Florida, Jacksonville FL 32226

04:00 PM
Inorganic Chemistry

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

04:35 PM
Inorganic Chemistry

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

04:55 PM
Inorganic Chemistry

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.

05:45 PM
Inorganic Chemistry

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 [Fe84-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 Fe44-O)4-cubane surrounded by four more Fe-centers, the X-ligands coordinated to the latter, forming all together a Fe84-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 [Fe84-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

06:05 PM
Inorganic Chemistry

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

06:25 PM
Inorganic Chemistry

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.