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.