William W. Shum

The tetracyanopyrazinide dimer dianion, [TCNP]2(2-). 2-Electron 8-center bonding.

The reaction of Cr(C(6)H(6))(2) and 1,2,4,5-tetracyanopyrazine (TCNP) forms [Cr(C(6)H(6))(2)][TCNP], and TCNP is reduced and forms the eclipsed pi-[TCNP](2)(2-) dimer. Diamagnetic [TCNP](2)(2-) has an intradimer separation of 3.14(2) A. The intradimer C...C and N...N separations are 3.29(2) and 3.42(2) A, respectively, and increase with the distance from the center of the molecule, due to nitriles bending away from the plane of the molecule by 5 +/- 1 degrees. [TCNP](2)(2-) is best described by an atoms-in-molecules analysis as having a 2e(-)/8c C-C bond involving the four C atoms from each six-member ring. The results of B3LYP/6-31+G(d)-computed interactions indicate that the [TCNP](*-)...[TCNP](*-) interactions in an isolated [TCNP](2)(2-) are repulsive by 58.9 kcal/mol, and that the stability of [TCNP](2)(2-) primarily originates from [TCNP](*-)...cation(+) electrostatic interactions, whose sum (-209.8 kcal/mol) exceeds the sum of the repulsive [TCNP](*-)...[TCNP](*-) and cation(+)...cation(+) interactions (140.3 kcal/mol).

Structure and Magnetic Interactions in the Organic-Based Ferromagnet Decamethylferrocenium Tetracyanoethenide, [FeCp*2]•+[TCNE]•−

The structures of three temperature-dependent polymorphs of solvent-free decamethylferrocenium tetracyanoethenide, [FeCp*(2)][TCNE], are determined from high-resolution synchrotron powder diffraction data. [FeCp*(2)][TCNE] is the first organic-based ferromagnetic material to be synthesized and is known to have two structural phase transitions at 249 and 282 K. The low-temperature phase, which exhibits spontaneous ferromagnetic order below 4.8 K, was determined at 12 K. At that temperature, it has monoclinic space group P2(1)/c [a = 9.6637(4) A, b = 14.1217(5) A, c = 18.6256(7) A, beta = 113.231(2) degrees, Z = 4] and consists of parallel chains of alternating [Fe(C(5)Me(5))(2)](*+) and [TCNE](*-) ions, with an intrachain Fe...Fe distance of 10.45 A. Structures of the intermediate and ambient temperature phases, also studied here, are characterized by increasing disorder. At 250 K, the unit cell space group is P2(1)/m [a = 9.7100(3) A, b = 14.4926(4) A, c = 9.4997(3) A, beta = 113.153(1) degrees, Z = 2]. At ambient temperature, the lattice, albeit quite disordered, belongs to the orthorhombic space group Cmcm [a = 10.629(1) A, b = 16.128(1) A, c = 14.593(1) A, Z = 4]. Nearest-neighbor magnetic interactions were evaluated for the 12 K structure by CASSCF and CASSCF/MCQDPT calculations (a methodology similar to the CASPT2 method). Similar trends are observed in computations with and without inclusion of spin-orbit coupling. The strongest are two intrachain [FeCp*(2)](*-)...[TCNE](*-) interactions (ferromagnetic with values of approximately 45 and approximately 29 cm(-1)), although weaker, nonnegligible, ferro- or antiferromagnetic interchain interactions of less than +/-0.2 cm(-1) are also present. Magnetic interactions that lead to ordering are therefore three-dimensional, despite the vastly different intra- and interchain coupling strengths.

Giant antiferromagnetically coupled moments in a molecule-based magnet with interpenetrating lattices

The competing energy scales found in molecule-based magnets frequently give rise to novel magnetic behavior that is rarely seen in solid-state systems. [Ru2(O2CMe)4]3[Cr(CN)6] is an intriguing molecule-based magnet with weakly-coupled, interpenetrating lattices. The field-dependence of the magnetization of this compound is reminiscent of a metamagnetic transition from an antiferromagnet to a paramagnet, but unlike typical metamagnets like FeCl2 it also exhibits a magnetic remanance and coercive field. We demonstrate that this compound behaves like two giant moments with a weak antiferromagnetic coupling and a large energy barrier between the orientations of each moment.

Spintronic detection of interfacial magnetic switching in a paramagnetic thin film of tris(8-hydroxyquinoline)iron(III)

Dali Sun,1,* Christopher M. Kareis,2 Kipp J. van Schooten,1 Wei Jiang,3 Gene Siegel,3 Marzieh Kavand,1 Royce A. Davidson,2 William W. Shum,2 Chuang Zhang,1 Haoliang Liu,1 Ashutosh Tiwari,3 Christoph Boehme,1 Feng Liu,3 Peter W. Stephens,4 Joel S. Miller,2 and Z. Valy Vardeny1,† 1Department of Physics & Astronomy, University of Utah, Salt Lake City, Utah 84112, USA 2Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, USA 3Department of Material Science & Engineering, University of Utah, Salt Lake City, Utah 84112, USA 4Department of Physics & Astronomy, Stony Brook University, Stony Brook, New York 11794, USA (Received 26 May 2016; revised manuscript received 11 November 2016; published 17 February 2017)