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A new paramagnetic ligand, 4-(2'-4-(2''-furyl)-pyrimidyl)-1,2,3,5-dithiadiazolyl (furylpymDTDA) and three transition metal coordination complexes, M(hfac) 2 (furylpymDTDA) M=Mn, Co, Ni; hfac=1,1,1,5,5,5-hexafluoroacetylacetonato-), are reported. The solid-state structures are influenced by the geometry of the coordination sphere of the M(II) centers: trigonal (Mn) vs. octahedral (Co and Ni). While the hs-Mn(II) complex exhibits pairwise multi-centre 2-electron bonds (i. e., pancake bonds) between the planar π radical DTDA moieties, the hs-Co(II) and Ni(II) complexes crystallize with close contacts between coordinated furylpymDTDA radical ligands that define linear 1D arrays of molecular units. The magnetic data for the hs-Co(II) and Ni(II) complexes indicate ferromagnetic (FM) interactions between molecular units, apparently mediated by radical-radical contacts along the supramolecular chains. Computational analysis suggests proximity between regions of large α- and β-spin density on neighbouring molecular sites enabling FM exchange, in accordance with the McConnell I mechanism. The magnetic data for the Ni(II) complex are consistent with a Heisenberg spin chain, whereas the hs-Co(II) complex exhibits Ising-like spin chain behaviour and a magnetic phase transition to an FM ordered state at 4.6 K., (© 2024 The Author(s). Chemistry - A European Journal published by Wiley-VCH GmbH.)

We report two new cyanido-bridged Fe(II)-Ag(I) coordination polymers using different acetylpyridine isomers, {Fe(4acpy) 2 [Ag(CN) 2 ] 2 } 1 and {Fe(3acpy)[Ag(CN) 2 ] 2 } 2 (4acpy = 4-acetylpyridine; 3acpy = 3-acetylpyridine) displaying thermally and photoinduced spin crossover (SCO). In both cases, the acetylpyridine ligand directs the coordination polymer structure and the SCO of the materials. Using 4-acetylpyridine, a two-dimensional (2D) structure is observed in 1 made of layers stacked on each other by silver-ketone interactions leading to a complete SCO and reversible thermally and photoswitching of the magnetic and optical properties. Changing the acetyl group to a 3-position, a completely different structure is obtained for 2 . The unexpected coordination of the carbonyl group to the Fe(II) centers induces a three-dimensional (3D) structure, leading to statistical disorder around the Fe(II) with three different coordination spheres, [N 6 ], [N 4 O 2 ], and [N 5 O]. This disorder gives rise to an incomplete thermally induced SCO with a poor photoswitchability. These results demonstrate that the choice of the acetyl position on the pyridine dictates the structural characteristics of the compounds with a direct impact on the SCO behavior. Remarkably, this work opens interesting perspectives for the future design of Fe-Ag cyanido coordination polymers with judiciously substituted pyridine ligands to tune the thermally and photoinduced SCO properties.

The attractive electronic properties of metal-pyrazine materials─electrical conductivity, magnetic order, and strong magnetic coupling─can be tuned in a wide range depending on the metal employed, as well as its ligand-imposed redox environment. Using solvent-directed synthesis to control the dimensionality of such systems, a discrete tetranuclear chromium(III) complex, exhibiting a rare example of bridging radical pyrazine, has been prepared from chromium(II) triflate and neutral pyrazine. The strong antiferromagnetic interaction between Cr III ( S = 3/2) and radical pyrazine ( S = 1/2) spins, theoretically estimated at about -932 K, leads to a thermally isolated S T = 4 ground state, which remains the only populated state observable even at room temperature.

The impact that the anion and alkyl group has on the electronic structures and magnetic properties of four mononuclear Mn(III) complexes is explored in [Mn(salEen-Br) 2 ]Y (salEen-Br = 2-{[2-(ethylamino)ethylimino]methyl}-4-Br-phenol; Y = ClO 4 - 1 and BF 4 - ·1/3CH 2 Cl 2 2 ) and [Mn(salBzen-Br) 2 ]Y (salBzen-Br = 2-{[2-(benzylamino)ethylimino]methyl}-4-Br-phenol; Y = ClO 4 - 3 and BF 4 - 4 ). X-ray structures of [Mn(salEen-Br) 2 ]ClO 4 ·0.45C 6 H 14 1-hexane , [Mn(salEen-Br) 2 ]BF 4 ·0.33CH 2 Cl 2 ·0.15C 6 H 14 2-dcm-hexane , and 3 - 4 reveal that they crystallize in ambient conditions in the monoclinic P 2 1 / c space group. Lowering the temperature, 2-dcm-hexane uniquely exhibits a structural phase transition toward a monoclinic P 2 1 / n crystal structure determined at 100 K with the unit cell trebling in size. Remarkably, at room temperature, the axially elongated Jahn-Teller axis in 2-dcm-hexane is poorly defined but becomes clearer at low temperature after the phase transition. Magnetic susceptibility measurements of 1 - 4 reveal that only 3 and 4 show slow relaxation of magnetization with Δ eff / k B = 27.9 and 20.7 K, implying that the benzyl group is important for observing single-molecule magnet (SMM) properties. Theoretical calculations demonstrate that the alkyl group subtly influences the orbital levels and therefore very likely the observed SMM properties.

Single-ion anisotropy is vital for the observation of Single-Molecule Magnet (SMM) properties (i.e., a slow dynamics of the magnetization) in lanthanide-based systems. In the case of europium, the occurrence of this phenomenon has been inhibited by the spin and orbital quantum numbers that give way to J = 0 in the trivalent state and the half-filled population of the 4f orbitals in the divalent state. Herein, by optimizing the local crystal field of a quasi-linear bis(silylamido) Eu II complex, the [Eu II (N{SiMePh 2 } 2 ) 2 ] SMM is described, providing an example of a europium complex exhibiting slow relaxation of its magnetization. This behavior is dominated by a thermally activated (Orbach-like) mechanism, with an effective energy barrier of approximately 8 K, determined by bulk magnetometry and electron paramagnetic resonance. Ab initio calculations confirm second-order spin-orbit coupling effects lead to non-negligible axial magnetic anisotropy, splitting the ground state multiplet into four Kramers doublets, thereby allowing for the observation of an Orbach-like relaxation at low temperatures., (© 2024. The Author(s).)