(34) Jeon, I.-R.; Negru, B.; Van Duyne, R. P.; Harris, T. D. “A 2D Semiquinone Radical-Containing Microporous Magnet with Solvent-Induced Switching from Tc = 26 to 80 K” submitted for publication.
(33) Gaudette, A. I.; Jeon, I.-R.; Anderson, J. S.; Grandjean, F.; Long, G. J.; Harris, T. D. “Electron Hopping through Double-Exchange Coupling in a Mixed-Valence Diiminobenzoquinone-Bridged Fe2 Complex” J. Am. Chem. Soc. 2015, Article ASAP (10.1021/jacs.5b07251).
(32) DeGayner, J. A.; Jeon, I.-R.; Harris, T. D. “A Series of Tetraazalene Radical-Bridged M2 (M = CrIII, MnII, FeII, CoII) Complexes with Strong Magnetic Exchange Coupling” Chem. Sci. 2015, Advance Article (10.1039/C5SC02725J).
(31) Demir, S.; Jeon, I.-R.; Long, J. R.; Harris, T. D. “Radical Ligand-Containing Single-Molecule Magnets” Coord. Chem. Rev. 2015, 289, 149-176.
(30) Park, J. G.; Jeon, I.-R.; Harris, T. D. “Electronic Effects of Ligand Substitution on Spin Crossover in a Series of Diiminoquinonoid-Bridged FeII2 Complexes” Inorg. Chem. 2015, 54, 359-369.
(29) Anderson, J. S.; Gallagher, A. T.; Mason, J. A.; Harris, T. D. “A Five-Coordinate Heme Dioxygen Adduct Isolated within a Metal-Organic Framework” J. Am. Chem. Soc. 2014, 136, 16489-16492.
(28) Jeon, I.-R.; Park, J. G.; Haney, C. R.; Harris, T. D. “Spin Crossover Iron(II) Complexes as PARACEST MRI Thermometers” Chem. Sci. 2014, 5, 2461-2465.
(27) Jeon, I.-R.; Park, J. G.; Xiao, D. J.; Harris, T. D. “An Azophenine Radical-Bridged Fe2 Single-Molecule Magnet with Record Magnetic Exchange Coupling” J. Am. Chem. Soc. 2013, 135, 16845-16848.
(26) Forshaw, A. P.; Smith, J. M.; Ozarowski, A.; Krzystek, J.; Smirnov, D.; Zvyagin, S. A.; Harris, T. D.; Karunadasa, H. I.; Zadrozny, J. M.; Schnegg, A.; Holldack, K.; Jackson, T. A.; Alamiri, A.; Barnes, D. M.; Telser, J. “Low-Spin Hexa-Coordinate Mn(III): Synthesis and Spectroscopic Investigation of Homoleptic Tris(pyrazolyl)borate and Tris(carbene)borate Complexes” Inorg. Chem. 2013, 52, 144-159.
(25) Fout, A. R.; Xiao, D. J.; Zhao, Q.; Harris, T. D.; King, E. R.; Eames, E. V.; Zheng, S.-L.; Betley, T. A. “Trigonal Mn3 and Co3 Clusters Supported by Weak-Field Ligands: A Structural, Spectroscopic, Magnetic, and Computational Investigation into the Correlation of Molecular and Electronic Structure” Inorg. Chem. 2012, 51, 10290-10299.
(24) Bhowmick, I.; Dechambenoit, P.; Hillard, E. A.; Coulon, C.; Harris, T. D.; Clérac, R. “A Canted Antiferromagnetic Ordered Phase of Cyanido-Bridged MnIII2ReIV Single-Chain Magnets” Chem. Commun. 2012, 48, 9717-9719.
(23) Bhowmick, I.; Harris, T. D.; Dechambenoit, P.; Hillard, E.; Pichon, C.; Jeon, I.-R.; Clérac, R. “Cyanido-Bridged One-Dimensional Systems Assembled from [ReIVCl4(CN)2]2- and [MII(cyclam)]2+ (M = Ni, Cu) Precursors” Science China Chem. 2012, 55, 1004-1012.
(22) Feng, X.; Liu, J.; Harris, T. D.; Hill, S.; Long, J. R. “Slow Magnetic Relaxation Induced by a Large Transverse Zero-Field Splitting in a MnIIReIV(CN)2 Single-Chain Magnet” J. Am. Chem. Soc. 2012, 134, 7521-7529.
(21) Eames, E. V.; Harris, T. D.; Betley, T. A. “Modulation of Magnetic Behavior via Ligand-Field Effects in the Trigonal Clusters (PhL)Fe3L*3 (L* = thf, py, PMe2Ph)” Chem. Sci. 2012, 3, 407-415.
(20) Harris, T. D.; Betley, T. A. “Multi-Site Reactivity: Reduction of Six Equivalents of Nitrite to Give an Fe6(NO)6 Cluster with a Dramatically Expanded Octahedral Core” J. Am. Chem. Soc. 2011, 133, 13852-13855.
(19) Feng, X.; Harris, T. D.; Long, J. R. “Influence of Structure on Exchange Strength and Relaxation Barrier in a Series of FeIIReIV(CN)2 Single-Chain Magnets” Chem. Sci. 2011, 2, 1688-1694.
(18) Harris, T. D.; Zhao, Q.; Hernández Sánchez, R.; Betley, T. A. “Expanded Redox Accessibility via Ligand Substitution in an Octahedral Fe6Br6 Cluster” Chem. Commun. 2011, 47, 6344-6346.
(17) Zhao, Q.; Harris, T. D.; Betley, T. A. “[(HL)2Fe6(NCMe)m]n+ (m = 0, 2, 4, 6; n = −1, 0, 2, 3, 4, 6): An Electron-Transfer Series Featuring Octahedral Fe6 Clusters Supported by a Weak-Field Hexaamide Ligand Platform” J. Am. Chem. Soc. 2011, 133, 8293–8306.
(16) Hazra, S.; Sasmal, S.; Fleck, M.; Grandjean, F.; Sougrati, M. T.; Ghosh, M.; Harris, T. D.; Bonville, P.; Long, G. J.; Mohanta, S. “Slow Magnetic Relaxation and Electron Delocalization in an S = 9/2 Iron(II/III) Complex with Two Crystallographically Inequivalent Iron Sites” J. Chem. Phys. 2011, 134, 174507/1-13.
(15) Harris, T. D.; Soo, H. S.; Chang, C. J.; Long, J. R. “A Cyano-Bridged FeIIReIV Cluster Incorporating Two High-Magnetic Anisotropy Building Units” Inorg. Chim. Acta 2011, 369, 91-96.
(14) Scepaniak, J. J.; Harris, T. D.; Vogel, C. S.; Sutter, J.; Meyer, K.; Smith, J. M. “Spin Crossover in a Four-Coordinate Iron(II) Complex” J. Am. Chem. Soc. 2011, 133, 3824-3827.
(13) Harris, T. D.; Coulon, C.; Clérac, R.; Long, J. R. “Record Ferromagnetic Exchange Through Cyanide and Elucidation of the Magnetic Phase Diagram for a CuIIReIV Chain Compound” J. Am. Chem. Soc. 2011, 133, 123-130.
(12) Harman, W. H.; Harris, T. D.; Freedman, D. E.; Fong, H.; Chang, A.; Rinehart, J. R.; Ozarowski, A.; Sougrati, M. T.; Grandjean, F.; Long, G. J.; Long, J. R.; Chang, C. J. “Slow Magnetic Relaxation in a Family of Trigonal Pyramidal Iron(II) Pyrrolide Complexes” J. Am. Chem. Soc. 2010, 132, 18115-18126.
(11) Zadrozny, J. M.; Freedman, D. E.; Jenkins, D. M.; Harris, T. D.; Iavarone, A. T.; Mathonière, C.; Clérac, R.; Long, J. R. “Slow Magnetic Relaxation and Charge Transfer in Cyano-Bridged Coordination Clusters Incorporating [Re(CN)7]3-/4-” Inorg. Chem. 2010, 49, 8886-8896.
(10) Ley, A. N.; Dunaway, L. E.; Brewster, T. P.; Dembo, M. D.; Harris, T. D.; Baril-Robert, F.; Patterson, H. H.; Pike, R. D. “Reversible Luminescent Surface Reaction of Amines with Copper(I) Cyanide” Chem. Commun. 2010, 46, 4565-4567.
(9) Harris, T. D.; Bennett, M. V.; Clérac, R.; Long, J. R. “[ReCl4(CN)2]2–: A High Magnetic Anisotropy Building Unit Giving Rise to the Single-Chain Magnets (DMF)4MReCl4(CN)2 (M = Mn, Fe, Co, Ni)” J. Am. Chem. Soc. 2010, 132, 3980-3988.
(8) Freedman, D. E.; Harman, W. H.; Harris, T. D.; Long, G. J.; Chang, C. J.; Long, J. R. “Slow Magnetic Relaxation in a High-Spin Iron(II) Complex” J. Am. Chem. Soc. 2010, 132, 1224-1225.
(7) Kong, X.-J.; Long, L.-S.; Huang, R.-B.; Zheng, L.-S.; Harris, T. D.; Zheng, Z. “A Four-Shell 136-Metal 3d-4f Heterometallic Cluster Approximating a Rectangular Parallelepiped” Chem. Commun. 2009, 4354-4356.
(6) Rinehart, J. D.; Harris, T. D.; Kozimor, S. A.; Bartlett, B. M.; Long. J. R. “Magnetic Exchange Coupling in Actinide-Containing Molecules” Inorg. Chem. 2009, 48, 3382-3395.
(5) Dincă, M.; Harris, T. D.; Iavarone, A. T.; Long, J. R. “Synthesis and Characterization of the Cubic Coordination Cluster [CoIII6CoII2(IBT)12]14- (H3IBT = 4,5-bis(tetrazol-5-yl)imidazole)” J. Mol. Struct. 2008, 890, 139-143.
(4) Bartlett, B. M.; Harris, T. D.; DeGroot, M. W.; Long, J. R. “High-Spin Ni3Fe2(CN)6 and Cu3Cr2(CN)6 Clusters Based on a Trigonal Bipyramidal Geometry” Z. Anorg. Allg. Chem. 2007, 2380-2385.
(3) Davis, K. B.; Harris, T. D.; Castellani, M. P.; Golen, J. A.; Rheingold, A. L. “Synthesis and X-ray Crystal Structure of [(C5Ph5)CrCl(µ-Cl)2Tl]2: An Example of the Rare M-X-TlI Linkage (X = Halide)” Organometallics 2007, 26, 4843-4845.
(2) Harris, T. D.; Long, J. R. “Linkage Isomerism in a Face-Centered Cubic Cu6Cr8(CN)24 Cluster with an S = 15 Ground State” Chem. Commun. 2007, 1360-1362.
(1) Harris, T. D.; Castellani, M. P.; Rheingold, A. L.; Reiff, W. M.; Yee, G. T. “1,1′-Diethyl-2,2′,3,3′,4,4′,5,5′-Octamethylferrocenium Tetracyanoethylenide, [Fe(C5EtMe4)2]+[TCNE]‑, A Charge-Transfer Salt Magnetic Solid with a Novel Structural Motif” Inorg. Chim. Acta 2006, 359, 4651-4654.
MRI contrast agents are evaluated largely by their relaxivity, a quantitative measure of how much the agent accelerates proton spin relaxation of water molecules. Among the many factors that contribute to overall relaxivity, the spin state (S) of the contrast agent is perhaps the most important, as relaxivity increases as S2. As a result, complexes of the S = 7/2 Gd3+ ion are by far the most commonly used commercial contrast agents. Nevertheless, Gd3+ complexes face a number of inherent limitations, such as an upper spin limit of S = 7/2. In addition, gadolinium is toxic in its free ion form, necessitating the use of multidentate chelating ligands to sequester the metal ion. Consequently, only one or zero coordination sites remain open for water binding at the metal centers, thereby limiting relaxivity. As an alternative to Gd3+ complexes, we seek to develop multinuclear transition metal-containing coordination clusters as MRI contrast agents. Most importantly, these clusters will feature paramagnetic centers that are very strongly magnetically coupled, such that the high-spin ground state remains well isolated even up to 37 °C. In addition to larger values of S and multiple open coordination sites for water exchange, use of redox-active transition metal centers will enable these clusters to be employed as redox-sensitive contrast agents, where internal stimuli, such as the local reducing environment of tumor cells, trigger a redox event and thus change in relaxivity in the cluster.
The activation of ubiquitous small molecules, such as H2O, O2, N2, CH4, and CO2, and their subsequent transformation into useful chemical fuels, is among the most important and urgent challenges currently facing science. In Nature, these processes are carried out within proteins, whose inorganic cores serve to stabilize reactive intermediates. Crucially, metalloprotein active sites are embedded within a protein superstructure, prohibiting the highly-reactive intermediates from engaging in side reactions such as bimolecular condensation. In the absence of a protein superstructure, we seek to embed reaction centers within the porous structure of metal-organic frameworks (MOFs). This work will aim to (1) stabilize and isolate reactive species, such as metal-ligand multiple bonds and metal-O2 adducts, and to (2) employ these species in catalysis, such as atom insertion into olefins or C−H bonds.
In 2010, U.S. data processing centers consumed approximately 100 billion kWh of energy, representing nearly 2% of the total national electricity consumption, and this number is expected to rapidly grow. One key method of reducing this energy expenditure is to reduce the physical size of magnetic bits used for information storage and processing. Toward that end, two decades ago, researchers discovered that some molecules, known as single-molecule magnets, display an inherent energy barrier to spin reversal and could therefore, in principle, be used as bits for information storage. Indeed, considering a molecular bit with a diameter of 1 nm, a close-packed array of molecules could represent an improvement of nearly three orders of magnitude over the current best magnetic alloy-based storage. However, single-molecule magnets face a critical limitation – their spin relaxation barrier (ΔA) is proportional to only two parameters, spin ground state (S) and axial zero-field splitting (D), and these two parameters cannot be increased simultaneously. As an alternative, moving to one-dimensional single-chain magnets offers a distinct advantage – their relaxation barrier (Δτ) depends on S, D, but also on the strength of coupling between spin units, J. Thus, we seek to design strongly-coupled single-chain magnets. More specifically, we will target chain compounds featuring redox-active ligands, such that strong direct exchange between paramagnetic metal centers and bridging ligands will provide strong magnetic coupling and thus high spin relaxation barriers.
We also seek to employ redox-active ligands, such as relatives of those used for chain formation, as bridges in metal-organic frameworks that function as microporous magnets at room temperature. These materials may find use in a number of applications, ranging from the room-temperature magnetic separation of oxygen from air to the design of lightweight permanent magnets. Currently, no porous magnet has been shown to exhibit an ordering temperature above 219 K, with the vast majority ordering well below 100 K. These low ordering temperatures result from the fact that porous magnetic materials generally feature paramagnetic inorganic nodes connected to one another by large diamagnetic bridging ligands. As such, the paramagnetic inorganic centers can then only couple with one another through the bridging ligand via weak, indirect superexchange. Since magnetic ordering temperature is directly correlated to the strength of magnetic coupling between spin centers, the resulting compound only behaves as a magnet at low temperature. As an alternative, the use of paramagnetic organic linkers will lead to strong, direct coupling pathways between inorganic and organic units. Additional work is focused on imparting tertiary function to the magnets. In particular, photo-induced electron-transfer is being targeted, enabling the magnetic order to be switched on and off upon irradiation of different wavelengths of light.
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