Inducing Ferrimagnetic Exchange in 1D-FeSe2 Chains Using Heteroleptic Amine Complexes: [Fe(en)(tren)][FeSe2]2.

[Fe(en)(tren)][FeSe2]2 (en = ethylenediamine, C2H8N2, tren = tris(2-aminoethyl)amine, C6H18N4) has been synthesized by a mixed-ligand solvothermal method. Its crystal structure contains heteroleptic [Fe(en)(tren)]2+ complexes with distorted octahedral coordination, incorporated between 1D-FeSe2 chains composed of edge-sharing FeSe4 tetrahedra. The twisted octahedral coordination environment of the Fe-amine complex leads to partial dimerization of Fe-Fe distances in the FeSe2 chains so that the FeSe4 polyhedra deviate strongly from the regular tetrahedral geometry. 57Fe Mössbauer spectroscopy reveals oxidation states of +3 for the Fechain atoms and +2 for the Fecomplex atoms. The close proximity of Fe atoms in the chains promotes ferromagnetic nearest neighbor interactions, as indicated by a positive Weiss constant, θ = +53.8(6) K, derived from the Curie-Weiss fitting. Magnetometry and heat capacity reveal two consecutive magnetic transitions below 10 K. DFT calculations suggest that the ordering observed at 4 K is due to antiferromagnetic intrachain interactions in the 1D-FeSe2 chains. The combination of two different ligands creates an asymmetric coordination environment that induces changes in the structure of the Fe-Se fragments. This synthetic strategy opens new ways to explore the effects of ligand field strength on the structure of both Fe-amine complexes and surrounding Fe-Se chains.

As-Se Pentagonal Linkers to Induce Chirality and Polarity in Mixed-Valent Fe-Se Tetrahedral Chains Resulting in Hidden Magnetic Ordering.

A novel mixed-valent hybrid chiral and polar compound, Fe7As3Se12(en)6(H2O), has been synthesized by a single-step solvothermal method. The crystal structure consists of 1D [Fe5Se9] chains connected via [As3Se2]-Se pentagonal linkers and charge-balancing interstitial [Fe(en)3]2+ complexes (en = ethylenediamine). Neutron powder diffraction verified that interstitial water molecules participate in the crystal packing. Magnetic polarizability of the produced compound was confirmed by X-ray magnetic circular dichroism (XMCD) spectroscopy. X-ray absorption spectroscopy (XAS) and 57Fe Mössbauer spectroscopy showed the presence of mixed-valent Fe2+/Fe3+ in the Fe-Se chains. Magnetic susceptibility measurements reveal strong antiferromagnetic nearest neighbor interactions within the chains with no apparent magnetic ordering down to 2 K. Hidden short-range magnetic ordering below 70 K was found by 57Fe Mössbauer spectroscopy, showing that a fraction of the Fe3+/Fe2+ in the chains are magnetically ordered. Nevertheless, complete magnetic ordering is not achieved even at 6 K. Analysis of XAS spectra demonstrates that the fraction of Fe3+ in the chain increases with decreasing temperature. Computational analysis points out several competing ferrimagnetic ordered models within a single chain. This competition, together with variation in the Fe oxidation state and additional weak intrachain interactions, is hypothesized to prevent long-range magnetic ordering.

Solvothermal Synthesis of [Cr7 S8 (en)8 Cl2 ]Cl3 ⋠2H2 O with Magnetically Frustrated [Cr7 S8 ]5+ Double-Cubes.

A novel transition metal chalcohalide [Cr7 S8 (en)8 Cl2 ]Cl3 ⋅ 2H2 O, with [Cr7 S8 ]5+ dicubane cationic clusters, has been synthesized by a low temperature solvothermal method, using dimethyl sulfoxide (DMSO) and ethylenediamine (en) solvents. Ethylenediamine ligand exhibits bi- and monodentate coordination modes; in the latter case ethylenediamine coordinates to Cr atoms of adjacent clusters, giving rise to a 2D polymeric structure. Although magnetic susceptibility shows no magnetic ordering down to 1.8 K, a highly negative Weiss constant, θ=-224(2) K, obtained from Curie-Weiss fit of inverse susceptibility, suggests strong antiferromagnetic (AFM) interactions between S=3/2 Cr(III) centers. Due to the complexity of the system with (2S+1)7 =16384 microstates from seven Cr3+ centers, a simplified model with only two exchange constants was used for simulations. Density-functional theory (DFT) calculations yielded the two exchange constants to be J1 =-21.4 cm-1 and J2 =-30.2 cm-1 , confirming competing AFM coupling between the shared Cr3+ center and the peripheral Cr3+ ions of the dicubane cluster. The best simulation of the experimental data was obtained with J1 =-20.0 cm-1 and J2 =-21.0 cm-1 , in agreement with the slightly stronger AFM exchange within the triangles of the peripheral Cr3+ ions as compared to the AFM exchange between the central and peripheral Cr3+ ions. This compound is proposed as a synthon towards magnetically frustrated systems assembled by linking dicubane transition metal-chalcogenide clusters into polymeric networks.

Tuning Fe-Se Tetrahedral Frameworks by a Combination of [Fe(en)3]2+ Cations and Cl- Anions.

A one-dimensional (1D) chain compound [Fe(en)3]3(FeSe2)4Cl2 (en = ethylenediamine), featuring tetrahedral FeSe2 chains separated by [Fe(en)3]2+ cations and Cl- anions, has been synthesized by a low temperature solvothermal method using simple starting materials. The degree of distortion in the Fe-Se backbone is similar to previously reported compounds with isolated 1D FeSe2 chains. 57Fe Mössbauer spectroscopy reveals the mixed-valent nature of [Fe(en)3]3(FeSe2)4Cl2 with Fe3+ centers in the [FeSe2]- chains and Fe2+ centers in the [Fe(en)3]2+ complexes. SQUID magnetometry indicates that [Fe(en)3]3(FeSe2)4Cl2 is paramagnetic with a reduced average effective magnetic moment, µeff = 9.51 µB per formula unit, and a negative Weiss constant, θ = -10.9(4) K, indicating antiferromagnetic (AFM) nearest neighbor interactions within the [FeSe2]- chains. Weak antiferromagnetic coupling between chains, combined with rather strong intrachain AFM coupling, leads to spin-glass behavior at low temperatures, as indicated by a frequency shift of the peak observed at 3 K in AC magnetic measurements. A combination of [Fe(en)3]2+ and Cl- ions is also capable of stabilizing mixed-valent 2D Fe-Se puckered layers in the crystal structure of [Fe(en)3]4(Fe14Se21)Cl2, where Fe14Se21 layers have a unique topology with large open pores. Property measurements of [Fe(en)3]4(Fe14Se21)Cl2 could not be performed due to the inability to either grow large crystals or synthesize this material in single-phase form.