Theoretical Studies on the Magnetic Switching Controlled by Stacking Patterns of Bis(maleonitriledithiolato) Nickelate(III) Dimers

1-(2-Fluorobenzylideneamino)pyridinium bis(1,2-dicyanoethene-1,2-dithiolato)nickelate(II)

In the title complex, (C₁₂H₁₀FN₂)₂[Ni(C₄N₂S₂)₂], the anion lies on an inversion center with the Ni^II ion coordinated by four S atoms in a slightly distorted square-planar environment. In the unique cation, the dihedral angle between the benzene and pyridine rings is 7.1 (2) Å.

Bis[1-(2,6-dichlorobenzyl)-3-methylpyrazin-1-ium] bis(maleonitriledithiolato)nickelate(II)

In the crystal structure of the title compound, (C₁₂H₁₁Cl₂N₂)₂[Ni(C₄N₂S₂)₂], the Ni^II complex dianion is located on an inversion centre. The Ni^II atom is coordinated by four S atoms in a square-planar geometry. In the cation, the dihedral angle between the benzene and pyrazine rings is 85.2 (2)°.

Observation of intermolecular charge transfer in a quasi-one-dimensional molecular alloy system

X-band single-crystal electron paramagnetic resonance (EPR) studies of the molecular alloy [NO 2BzPy][Au 0.57Ni 0.43(mnt) 2] are presented in this paper. At room temperature, EPR spectra show both intense resonance signals (main signals) and weak satellite quartet lines. The characteristics of both intense and weak EPR signals depend on the magnetic field orientation. The main signals arise from two magnetically nonequivalent [Ni(mnt) 2] (-) anions, and their corresponding principal values of the g tensor are ( g x ') 1 = 2.04653, ( g y ') 1 = 2.00096, and ( g z ') 1 = 2.15319 and ( g x ') 2 = 2.04520, ( g y ') 2 = 1.99734, and ( g z ') 2 = 2.15361, respectively. The weak satellite lines, whose patterns strongly depend on the magnetic field direction, can be attributed to the hyperfine coupling of the electron spin with the (197)Au nucleus of the [Au(mnt) 2] (-) species. Density functional theory calculations for the spin and charge distributions of the dimer {[Ni(mnt) 2][Au(mnt) 2]} (2-) indicate that the hyperfine interaction of the electron spin with the (197)Au nuclear spins is caused, in part, by the charge transfer between the [Ni(mnt) 2] (-) and the [Au(mnt) 2] (-) species.

A New Quasi-1D Spin System with Spin Transition Exhibiting Novel CN···π Interactions

A new molecular tecton with a flexible conformation, 1-methyl-ethyldeneaminopyridinium, was synthesized, and its corresponding compound with [Ni(mnt)(2)](-) exhibits novel CN...pi interactions and spin transition.

Hexagonal Supramolecular Architectures from Ferrocenium Cations Incorporating [Ni(mnt)2]− Columns: Structures and Properties of [Alkylferrocene][Ni(mnt)2]− Charge-Transfer Complexes (mnt=maleonitriledithiolate)

The crystal architecture, magnetic properties, and thermodynamic properties of [n-butylferrocene][Ni(mnt)2] (1), [tert-butylferrocene][Ni(mnt)2] (2), [1,1'-diethylferrocene][Ni(mnt)2] (3), and [1,1'-diisopropylferrocene][Ni(mnt)2] (4) were investigated (mnt=maleonitriledithiolate). These complexes exhibit a unique supramolecular structure in which the ferrocenium cations constitute honeycomb-like assembled structures surrounding columns of the anions. For 1, the cations form a dimer through a very short intermolecular ferrocene-ferrocene distance of 3.28 A, which mediates an antiferromagnetic interaction with a singlet-triplet energy gap of 5 K. First-order phase transitions occur in 1-3 at 364, 361, and 350 K, respectively, accompanied by thermal hysteresis.

Order−Disorder Transition Coupled with Magnetic Bistability in the Ferricinium Salt of a Radical Nickel Dithiolene Complex

The ferricinium salt of the anionic, S = (1)/(2), dithiolene complex [Ni(tfadt)(2)](-) (where tfadt is the asymmetrically substituted 2-(trifluoromethyl)acrylonitrile-1,2-dithiolate) crystallizes at room temperature (rt) into uniform chains of dithiolene complexes, separated by ferricinium cations. Both ionic entities are characterized by disorder affecting one CF(3) and one Cp moiety. Above 250 K, this compound displays a Curie-type behavior. At lower temperatures, two first-order transitions around 249 and 137 K are revealed by susceptibility measurements and the observation of hysteresis effects. Crystal structure determinations performed at 230 and 120 K show that the high-temperature transition is associated with an ordering of the CF(3) and Cp groups together with a dimerization of the anionic stacks that thus induces a drop of the susceptibility. The second, low-temperature transition leads to a tetramerization of these nickel dithiolene stacks now in a complete diamagnetic state while the remaining susceptibility originates from the sole ferricinium contribution. The first order character of the 249 K transition with its associated bistable behavior is likely correlated with the structural order-disorder transition, an original behavior in this class of materials where bistability is most often associated with a strengthening of interstack intermolecular interactions (hydrogen bonding, pi-pi interactions ...).

Quasi-one-dimensional molecular magnets based on derivatives of (fluorobenzyl)pyridinium with the [M(mnt)2] monoanion (M = Ni, Pd or Pt; mnt2- = maleonitriledithiolate): syntheses, crystal structures and magnetic properties

The syntheses, structural characterizations and magnetic behaviors of three new complexes, 1-(3',4',5'-trifluorobenzyl)pyridinium [M(mnt)2]- [M = Ni (1), Pd (2) or Pt (3)], are reported. These complexes are isomorphous and their prominent structural character is that the [M(mnt)2]- anions form columnar stacks, in which the dimerization was observed. Complexes 2 and 3 are diamagnetic, while 1 possesses an energy gap of 2474 K. For crystal 4, 1-(4'-fluorobenzyl)pyridinium [Ni(mnt)2] (its structure and magnetic susceptibility were briefly reported earlier), the magnetic behavior can be divided into two regimes, namely, weakly ferromagnetic coupling above 93 K and strongly antiferromagnetic coupling below 93 K. A transition occurs at 93 K which switches the magnetic exchange nature from ferromagnetic to antiferromagnetic. A sharp thermal abnormality with lambda-shape, associated with the transition, appears from its heat capacity measurement to indicate that the transition is first order. The temperature dependences of the superlattice diffractions revealed the existence of the pretransitional phenomena up to at least 140 K. The unusual magnetic behavior of 4, such as the origin of the ferromagnetic interaction in the high temperature phase and what causes the spin transition, are discussed further.

Design of a Magnetic Bistability Molecular System Constructed by H-Bonding and π···π-Stacking Interactions

Crystal structures and magnetic properties were determined for two novel polymorphs of the complex [H2DABCO][Ni(mnt)2] [(H2DABCO)2+ = diprotonated 1,4-diazabicyclo[2.2.2]octane; mnt2- = maleonitriledithiolate]. For each polymorph, anions form a layered structure in which two kinds of dimers were observed. The adjacent anionic sheets are held together by cations via H-bonding interactions between protons of cations and CN groups of anions. Two polymorphs possess spin bistability; namely, upon cooling, a magnetic transition happens at around 120 K with about 1 K hysteresis on heating for the alpha phase and at 112 K with about 10 K hysteresis for the beta phase. Above the transition, the magnetic behaviors of two polymorphs can be approximately interpreted by a singlet-triplet model of an antiferromagnetically coupled S = 1/2 dimer, which is supported by the crystal structures and spin dimer analyses based on extended Hückel molecular orbital calculations.