Facile synthesis of mononuclear early transition-metal complexes of κ3cyclo-tetrametaphosphate ([P4O12]4−) and cyclo-trimetaphosphate ([P3O9]3−)

We herein report the preparation of several mononuclear-metaphosphate complexes using simple techniques and mild conditions with yields ranging from 56% to 78%. Treatment of cyclo-tetrametaphosphate ([TBA]4[P4O12]·5H2O, TBA = tetra-n-butylammonium) with various metal sources including (CH3CN)3Mo(CO)3, (CH3CN)2Mo(CO)2(η3-C3H5)Cl, MoO2Cl2(OSMe2)2, and VOF3, leads to the clean and rapid formation of [TBA]4[(P4O12)Mo(CO)3]·2H2O, [TBA]3[(P4O12)Mo(CO)2(η3-C3H5)], [TBA]3[(P4O12)MoO2Cl] and [TBA]3[(P4O12)VOF2]·Et2O salts in isolated yields of 69, 56, 68, and 56% respectively. NMR spectroscopy, NMR simulations and single crystal X-ray studies reveal that the [P4O12]4− anion behaves as a tridentate ligand wherein one of the metaphosphate groups is not directly bound to the metal. cyclo-Trimetaphosphate-metal complexes were prepared using a similar procedure i.e., treatment of [PPN]3[P3O9]·H2O (PPN = bis(triphenylphosphine)iminium) with the metal sources (CH3CN)2Mo(CO)2(η3-C3H5)Cl, MoO2Cl2(OSMe2)2, MoOCl3, VOF3, WOCl4, and WO2Cl2(CH3CN)2 to produce the corresponding salts, [PPN]2[(P3O9)Mo(CO)2(η3-C3H5)], [PPN]2[(P3O9)MoO2Cl], [PPN]2[(P3O9)MoOCl2], [PPN]2[(P3O9)VOF2]·2CH2Cl2, and [PPN]2[(P3O9)WO2Cl] in isolated yields of 78, 56, 75, 59, and 77% respectively. NMR spectroscopy, NMR simulations and single-crystal X-ray studies indicate that the trianionic ligand [P3O9]3− in these complexes also has κ3 connectivity.Eni S.p.A. (Firm)Eni-MIT Solar Frontiers Center (Program

Synthesis, Physicochemical Characterization, and Catalytic Evaluation of Fe\u3csup\u3e3+\u3c/sup\u3e-Containing SSZ-70 Zeolite

Whereas one-dimensional, 10-membered ring zeolites are typically used for hydroisomerization, Fe3+-containing SSZ-70 (Fe-SSZ-70) shows remarkable isomerization selectivity for a zeolite containing 12- and partially blocked 14-membered rings, in addition to 10-membered rings. Fe-SSZ-70 was compared to Al3+-containing SSZ-70 (Al-SSZ-70) in constraint index and n-decane hydrocracking tests. Fe-SSZ-70 exhibited a 74% total isomer yield (64% yield of monobranched isomers and 10% cracking yield) at 85% conversion compared to 49% total isomer yield (41% yield of monobranched isomers and 36% cracking yield) for Al-SSZ-70 at the same conversion. The selectivity to isomerization is attributed to the weaker acid strength of Fe-SSZ-70 over Al-SSZ-70. Fe-SSZ-70 was directly synthesized with Fe3+ isomorphously substituted in tetrahedral positions. The coordination environment of the Fe3+ was characterized using Mössbauer, electron paramagnetic resonance, and diffuse reflectance UV-vis spectroscopies. The physicochemical properties were further probed with inductively coupled plasma atomic emission spectroscopy, temperature-programmed desorption of isopropylamine, and nitrogen adsorption-desorption. The Fe3+ was tetrahedrally coordinated in the as-made materials and became partially octahedrally coordinated upon calcination; enough Fe3+ remained in the framework after calcination for Fe-SSZ-70 to remain catalytically active

Magnetic Direct Exchange and Heavy Atom Effects on Slow Magnetic Relaxation in Transition Metal Single-Molecule Magnets

This dissertation describes the design, synthesis, and characterization of a wide array of transition metal complexes for application in single-molecule magnets. Magnetic relaxation in paramagnetic metal complexes proceed through several processes. This work investigates how synthetic chemistry tools can lead to a better understanding of and control over these relaxation processes, with the goal to slow down all possible relaxation pathways so that single-molecule magnets can retain their magnetization as long as possible. Chapter One provides a fundamental background of single-molecule magnet behavior in transition metal complexes and delineates important criteria to consider in designing single-molecule magnets. Two major strategies are explored. First, large spin clusters with strong magnetic exchange are explored in Chapters Two, Three, and Four with the goal to minimize through-barrier relaxation processes. Second, the effect of heavy atoms are investigated in Chapters Five and Six, with the aim to understand spin-vibronic coupling which undermines the spin-reversal barrier.Chapter Two describes the synthesis and characterization of semiquinone-bridged dinuclear transition metal complexes of iron(II), cobalt(II), and nickel(II). In this system, tris(2-dimethylaminoethyl)amine (Me6tren) ligand scaffold enforces trigonal pyramidal geometry on the metal centers, engendering large magnetic anisotropy. This work also showcases the first magnetometry measurement of a semiquinone radical, which subsequently shows exceptionally strong magnetic direct exchange. This results in the first example of a thermally isolated large spin ground state in a multinuclear Ni complex realized in [(Me6tren)2Ni2(µ2-C6H4O2)]3+. Slow magnetic relaxation is observed in [(Me6tren)2Co2(µ2-C6H4O2)]3+ and [(Me6tren)2Ni2(µ2-C6H4O2)]3+ with an evident Orbach relaxation barrier of 22 and 46 cm–1, respectively. Through-barrier relaxation processes are partially suppressed as a result of magnetic coupling in this system.Chapter Three describes the work on tetranuclear metal clusters [M4(NPtBu3)4]+/0 (M = Co, Ni, Cu; tBu = tert-butyl) featuring low-coordinate metal centers engaged in direct metal–metal orbital overlap. These clusters show thermally isolated large spin ground states as a result of extremely strong ferromagnetic direct exchange from magnetic orbital direct overlap and delocalized, itinerant electrons. This exchange mechanism is analogous to magnetic interactions in ferromagnetic metals. Unusually large magnetic anisotropy is observed, which is attributed to the low-coordinate environment around the cobalt and nickel centers. The [Ni4(NPtBu3)4]+ complex exhibits the first example of zero-field slow magnetic relaxation in easy-plane molecular magnets, and slow magnetic relaxation under a small applied field solely follows Orbach process. Magnetic characterization of the copper analog [Cu4(NPtBu3)4]+ reveals that the alternative spin-vibronic relaxation is sufficiently slow, thus enabling the observation of the sole Orbach barrier in [Ni4(NPtBu3)4]+ with a spin reversal barrier of 16 cm–1. The [Co4(NPtBu3)4]+ complex exhibits a thermally isolated S = 9/2 ground state and a large magnetic anisotropy D = –12.34 cm–1. The molecule exhibits a spin reversal barrier of 87 cm–1, the largest value reported for transition metal clusters.Chapter Four describes the design, synthesis, and characterization of a series of dinuclear trigonal nickel paddlewheel complexes, Ni2DArF3 (DArF– = N,N′-diarylformamidinate). This work is motivated by the need to rationally control magnetic anisotropy in a system featuring direct metal–metal orbital overlap. The synthesis of the trigonal nickel paddlewheel complexes are described, showing the first examples of high spin nickel paddlewheel complexes. Due to the direct metal orbital overlap, the compounds exhibit thermally isolated S = 3/2 ground state. By changing the aryl substituents of the ligands, the trigonal symmetry around the metal centers can be tuned. Consequently, magnetic anisotropy of the complexes can be adjusted from D = –13 to –29 cm–1. Spin reversal barriers of 26 to 55 cm–1 are observed, with partially suppressed through-barrier relaxations.Chapter Five describes a mononuclear triad M(CNDipp)6 (M = V, Nb, Ta; Dipp = 2,6-diisopropylphenyl) as an experimental validation of the newly proposed spin-vibronic relaxation model. These low-spin S = 1/2 isocyanide complexes exhibit slow magnetic relaxation via spin-vibronic coupling. Analysis of relaxation dynamics in this series indicates that spin-orbit coupling of the metal center facilitates spin-vibronic relaxation, as is evident by the observation that the spin relaxation rate of the tantalum complex is the fastest, followed by niobium, and vanadium complexes, respectively.Chapter Six describes the study of heavy ligand effect on slow magnetic relaxation in two-coordinate nickel(I) complexes, (IPr)NiE(SiMe3)3 (IPr = IPr = 1,3-bis(2,6-diisopropylphenyl)-imidazoline-2-ylidene; E = C, Si, Ge, Sn). As in previous chapter, these nickel complexes feature S = 1/2 ground state and slow magnetic relaxation arising from spin-vibronic coupling. However, partially unquenched orbital angular momentum due to the linear geometry significantly adds anisotropy to the system. The alkyl complex (IPr)NiC(SiMe3)3 exhibits notable quantum tunneling of magnetization at low temperature, and Raman relaxation at higher temperature. Upon moving to heavier ligands in (IPr)NiE(SiMe3)3 (E = Si, Ge, Sn), the complexes show significantly slower quantum tunneling and approximately five times slower Raman relaxation. This work provides an experimental evidence of the effect of heavy ligand on slow magnetic relaxation, a much-debated topic at the current time

Studies of metaphosphate acids and metaphosphate anhydrides in aprotic media

Thesis: S.B., Massachusetts Institute of Technology, Department of Chemistry, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 59-61).The chemistry of metaphosphate acids has historically been studied in aqueous media, where acid-catalyzed hydrolysis and solvent leveling effects of these strong acids have prevented their observations and rigorous characterization. Solubilization of tri-, tetra-, and hexametaphosphates in aprotic media using the IPPN + cation ([PPNI+ bis(triphenylphosphine)imninium) has revealed the rich acid chemistry of metaphosphates that has previously been elusive in aqueous media. Protonation of imetaphosphates in organic media has resulted in six metaphosphate acids. X-ray diffraction studies display that the structural configurations of metaphosphate acids are dictated by strong hydrogen bonding interactions. As a consequence of anti-cooperative effect, intramolecular hydrogen bonds are preferred at low degrees of protonation, and intermolecular hydrogen bonds are preferred at high degrees of protonation, resulting in oligomeric and polymeric structures. Because of the symmetry of the hydrogen bonds in metaphosphate acids, Low-Barrier Hydrogen Bonds (LBHB) are formed if the conformation of the metaphosphate ring allows. Metaphosphate anhydrides result from the dehydration of metaphosphate acids. They can undergo hydrolysis to regenerate metaphosphate acids, or alternatively alcoholysis to generate metaphosphate esters. Alcoholysis of metaphosphiate anhydrides presents a novel method to quantitatively phosphorylate organic substrates, of particular interest are substrates of biological significance such as nucleosides. The phosphorylating ability of metaphosphate anhydrides makes them promising candidates for biological phosphorylation.by Khetpakorn Chakarawet.S.B

Semiquinone radical-bridged M2 (M = Fe, Co, Ni) complexes with strong magnetic exchange giving rise to slow magnetic relaxation.

The use of radical bridging ligands to facilitate strong magnetic exchange between paramagnetic metal centers represents a key step toward the realization of single-molecule magnets with high operating temperatures. Moreover, bridging ligands that allow the incorporation of high-anisotropy metal ions are particularly advantageous. Toward these ends, we report the synthesis and detailed characterization of the dinuclear hydroquinone-bridged complexes [(Me6tren)2MII 2(C6H4O2 2-)]2+ (Me6tren = tris(2-dimethylaminoethyl)amine; M = Fe, Co, Ni) and their one-electron-oxidized, semiquinone-bridged analogues [(Me6tren)2MII 2(C6H4O2 -˙)]3+. Single-crystal X-ray diffraction shows that the Me6tren ligand restrains the metal centers in a trigonal bipyramidal geometry, and coordination of the bridging hydro- or semiquinone ligand results in a parallel alignment of the three-fold axes. We quantify the p-benzosemiquinone-transition metal magnetic exchange coupling for the first time and find that the nickel(ii) complex exhibits a substantial J < -600 cm-1, resulting in a well-isolated S = 3/2 ground state even as high as 300 K. The iron and cobalt complexes feature metal-semiquinone exchange constants of J = -144(1) and -252(2) cm-1, respectively, which are substantially larger in magnitude than those reported for related bis(bidentate) semiquinoid complexes. Finally, the semiquinone-bridged cobalt and nickel complexes exhibit field-induced slow magnetic relaxation, with relaxation barriers of U eff = 22 and 46 cm-1, respectively. Remarkably, the Orbach relaxation observed for the Ni complex is in stark contrast to the fast processes that dominate relaxation in related mononuclear NiII complexes, thus demonstrating that strong magnetic coupling can engender slow magnetic relaxation

Semiquinone radical-bridged M2 (M = Fe, Co, Ni) complexes with strong magnetic exchange giving rise to slow magnetic relaxation.

The use of radical bridging ligands to facilitate strong magnetic exchange between paramagnetic metal centers represents a key step toward the realization of single-molecule magnets with high operating temperatures. Moreover, bridging ligands that allow the incorporation of high-anisotropy metal ions are particularly advantageous. Toward these ends, we report the synthesis and detailed characterization of the dinuclear hydroquinone-bridged complexes [(Me6tren)2MII 2(C6H4O2 2-)]2+ (Me6tren = tris(2-dimethylaminoethyl)amine; M = Fe, Co, Ni) and their one-electron-oxidized, semiquinone-bridged analogues [(Me6tren)2MII 2(C6H4O2 -˙)]3+. Single-crystal X-ray diffraction shows that the Me6tren ligand restrains the metal centers in a trigonal bipyramidal geometry, and coordination of the bridging hydro- or semiquinone ligand results in a parallel alignment of the three-fold axes. We quantify the p-benzosemiquinone-transition metal magnetic exchange coupling for the first time and find that the nickel(ii) complex exhibits a substantial J < -600 cm-1, resulting in a well-isolated S = 3/2 ground state even as high as 300 K. The iron and cobalt complexes feature metal-semiquinone exchange constants of J = -144(1) and -252(2) cm-1, respectively, which are substantially larger in magnitude than those reported for related bis(bidentate) semiquinoid complexes. Finally, the semiquinone-bridged cobalt and nickel complexes exhibit field-induced slow magnetic relaxation, with relaxation barriers of U eff = 22 and 46 cm-1, respectively. Remarkably, the Orbach relaxation observed for the Ni complex is in stark contrast to the fast processes that dominate relaxation in related mononuclear NiII complexes, thus demonstrating that strong magnetic coupling can engender slow magnetic relaxation

Large Anisotropy Barrier in a Tetranuclear Single-Molecule Magnet Featuring Low-Coordinate Cobalt Centers

The tetranuclear cobalt cluster compound [Co₄(μ-NP^tBu₃)₄]­[B­(C₆F₅)₄] (^tBu = <i>tert</i>-butyl) was synthesized by chemical oxidation of Co₄(NP^tBu₃)₄ with [FeCp₂]­[B­(C₆F₅)₄] and magnetically characterized to study the effect of electronic communication between low-coordinate metal centers on slow magnetic relaxation in a transition metal cluster. The dc magnetic susceptibility data reveal that the complex exhibits a well-isolated <i>S</i> = ⁹/₂ ground state, which persists even to 300 K and is attributed to the existence of direct metal–metal orbital overlap. The ac magnetic susceptibility data further reveals that the complex exhibits slow magnetic relaxation in the absence of an applied field, and that the relaxation dynamics can be fit with a combination of Orbach, quantum tunneling, and Raman relaxation processes. The effective spin reversal barrier for this molecule is 87 cm^–1, the largest reported to date for a transition metal cluster, and arises due to the presence of a large easy-axis magnetic anisotropy. The complex additionally exhibits waist-restricted magnetic hysteresis and magnetic blocking below 3.6 K. Taken together, these results indicate that coupling of low-coordinate metal centers is a promising strategy to enhance magnetic anisotropy and slow magnetic relaxation in transition metal cluster compounds