Thermally and Photoinduced Spin-Crossover Behavior in Iron(II)-Silver(I) Cyanido-Bridged Coordination Polymers Bearing Acetylpyridine Ligands.

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, [N6], [N4O2], and [N5O]. 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.

Alkyl chain length influence of the functionalized diethanolamine ligand on the slow relaxation of the magnetization in {CoIII3DyIII3} complexes.

We report the synthesis, structural characterization and magnetic properties of a series of Co(III)/Dy(III) complexes built up from an N-alkyl derivatized amino-alcohol ligand diethanolamine, functionalized with CnH2n+1 alkyl chains (n = 3, 4, 6, 8 and 10). While n = 3 afforded a butterfly {CoIII2DyIII2} core, the other alkyl chains (n = 4, 6, 8) afforded hexanuclear triangle-in-triangle {CoIII3DyIII3} complexes as shown by single-crystal X-ray determinations. Infrared spectroscopy allows us to characterize the C10 derivative complex, which did not crystallize. Magnetic ac susceptibility data collected below 10 K at driving frequencies up to 10 kHz under zero-dc field and up to 1 T provide insight into the SMM behaviour of the studied complexes. The characteristic time of the magnetization dynamics can be understood in terms of the competing Raman, Direct and Orbach-like mechanisms. A relationship between the magnetization dynamics within the family of complexes and the increasing alkyl chain length is discussed.

Slow magnetization dynamics in Co(ii)/Co(iii) triethanolamine/pivalate complexes.

We report the synthesis, structural characterization and a combined computational and experimental study of the magnetic properties of two pivalate cobalt complexes, a mononuclear Co(ii) one and a tetranuclear Co(ii)3Co(iii) mixed valence polynuclear one. The latter shows SMM behaviour revealed under an applied DC field with a thermal barrier of ca. 30 cm-1 competing with direct and Raman relaxation processes. The Orbach thermal barrier can be understood from the doublets energy ladder arising from the anisotropic exchange interaction among ground Seff = 1/2 of each Co(ii) sites. The strong local zero-field splitting of the S = 3/2 Co(ii) states affords these well isolated ground Kramers doublets. DC and AC magnetic susceptibility measurements as well as HF-EPR spectra support this interpretation. CASSCF quantum chemical computations have been also performed in order to aid the overall comprehension of the magnetic behaviour in the reported complexes.

New mu(4)-oxido-bridged copper benzoate quasi-tetrahedron and bis-mu(3)-hydroxido-bridged copper azide and copper thiocyanate stepped cubanes: core conversion, structural diversity, and magnetic properties.

[Cu(2)(mu(4)-O)Cu(2)] and [Cu(2)(mu(3)-OH)(2)Cu(2)] geometrical arrangements are found in a new family of tetranuclear complexes: [Cu(4)(mu(4)-O)(mu-bip)(2)(mu-O(2)CPh)(4)].0.5CH(2)Cl(2) (1.0.5CH(2)Cl(2)), [Cu(4)(mu(3)-OH)(2)(mu-bip)(2)(N(3))(4)] (2), and [Cu(4)(mu(3)-OH)(2)(mu-bip)(2)(NCS)(4)(DMF)(2)] (3.2DMF) [Hbip = 2,6-bis(benzyliminomethyl)-4-methylphenol; DMF = dimethylformamide]. These complexes have been characterized by X-ray crystallography, and their magnetic properties have been studied. Complex 1 reacts with azide and thiocyanate anions, leading to 2 and 3 with a change of the [Cu(4)(mu(4)-O)] core into [Cu(4)(mu(3)-OH)(2)] units. These compounds are new examples of [Cu(4)] complexes where Cu(II) ions are connected by two types of water-derived ligands: oxide and hydroxide. Formation of these [Cu(4)] complexes can be controlled by changing the bridging ligands, which allows an effective tuning of the self-assembly. The study of the magnetic properties reveals that these complexes exhibit strong intramolecular antiferromagnetic interactions to yield a S(T) = 0 ground state. For the three complexes, the temperature dependence of the magnetic susceptibility was fitted using a model with two isolated S = 1/2 dimers based on the H = -2J{S(Cu,1).S(Cu,2)} spin Hamiltonian with J/k(B) = -289 K for 1; J/k(B) = -464 and -405 K for 2 and 3, respectively (where J is the exchange constant through the oxido-phenoxido or hydroxido-phenoxido bridges, respectively).

Two-dimensional networks of lanthanide cubane-shaped dumbbells.

The syntheses, structures, and magnetic properties are reported for three new lanthanide complexes, [Ln(III)(4)(mu(3)-OH)(2)(mu(3)-O)(2)(cpt)(6)(MeOH)(6)(H(2)O)](2) (Ln = Dy (1.15MeOH), Ho (2.14MeOH), and Tb (3.18MeOH)), based on 4-(4-carboxyphenyl)-1,2,4-triazole ligand (Hcpt). The three complexes were confirmed to be isomorphous by infrared spectroscopy and single-crystal X-ray diffraction. The crystal structure of 1 reveals that the eight-coordinate metal centers are organized in two cubane-shaped moieties composed of four Dy(III) ions each. All metal centers in the cubane core are bridged by two mu(3)-oxide and two mu(3)-hydroxide asymmetrical units. Moreover, each cubane is linked to its neighbor by two externally coordinating ligands, forming the dumbbell {Dy(III)(4)}(2) moiety. Electrostatic interactions between the ligands of the triazole-bridged dimers form an extended supramolecular two-dimensional arrangement analogous to a metal-organic framework with quadrilateral spaces occupied by ligands from axial sheets and by four solvent molecules. The magnetic properties of the three compounds have been investigated using dc and ac susceptibility measurements. For 1, the static and dynamic data corroborate the fact that the {Dy(III)(4)} cubane-shaped core exhibits slow relaxation of its magnetization below 5 K associated with a single-molecule magnet behavior.