[1,2,4]Triazolo[4,3‑<i>a</i>]Pyridines as Bridging LigandsMagnetism of Azole-Bridged Dinuclear Copper(II) Complexes Influenced by the Trigonal Distortion Parameter τ

The dinuclear doubly azole-bridged copper­(II) complexes [Cu^II₂(<b>L</b>)₂(MeCN)₄]­(ClO₄)₄·3.73MeCN·0.80H₂O and [Cu^II₂(<b>L</b>)₆]­(ClO₄)₄·solvent (solvent = 2MeCN·H₂O; 2MeCN·2H₂O; 1.5MeOH·3.5H₂O) were prepared [<b>L</b> = 3-(6-methyl-2-pyridyl)-[1,2,4]­triazolo­[4,3-<i>a</i>]­pyridine]. Structural characterizations revealed very different local geometries about the copper­(II) ions, being trigonal bipyramidal for the former (τ = 0.76) and square pyramidal for the latter (τ = 0.07, 0.15, 0.07) complex. Magnetic measurements of bulk material [Cu^II₂(<b>L</b>)₂­(H₂O)₄]­(ClO₄)₄ and [Cu^II₂(<b>L</b>)₆]­(ClO₄)₄·2H₂O revealed antiferromagnetic coupling in both complexes, however, of very different strengths. Electron paramagnetic resonance (EPR) spectroscopy was applied to investigate magnetic properties of the complexes in detail. These experimental findings were supported by broken-symmetry DFT calculations. Systematic magneto-structural correlations are discussed

Thermal Spin Crossover and LIESST Effect Observed in Complexes [Fe(L^Ch)₂(NCX)₂] [L^Ch = 2,5-Di(2-Pyridyl)-1,3,4-Chalcadiazole; Ch = O, S, Se; X = S, Se, BH₃]

Two new complexes belonging to the family [Fe^II(<b>L</b>)₂(NCX)₂] have been synthesized and structurally characterized. Complexation of the ligand <b>L</b>^<b>O</b> = 2,5-di­(2-pyridyl)-1,3,4-oxadiazole results in the formation of two derivatives of [Fe^II(<b>L</b>^<b>O</b>)₂(NCS)₂] (<b>1</b>) with the common s-trans and the rarely observed s-cis conformation. No thermal spin crossover (SCO) was observed for the amorphous bulk material of the mixture. Using the new ligand <b>L</b>^<b>Se</b> = 2,5-di­(2-pyridyl)-1,3,4-selenadiazole, compound [Fe^II(<b>L</b>^<b>Se</b>)₂(NCS)₂]·1.4DCM·0.6MeOH (<b>2</b>·1.4DCM·0.6MeOH) was structurally characterized by single crystal X-ray diffraction. Bulk material of [Fe^II(<b>L</b>^<b>Se</b>)₂(NCS)₂]·MeOH (<b>2</b>·MeOH) exhibits a thermally induced SCO with small hysteresis [<i>T</i>_1/2(↓) = 91 K, <i>T</i>_1/2(↑) = 96 K]. LIESST and reflectivity studies have been performed on the SCO complexes [Fe^II(<b>L</b>^<b>S</b>)₂(NCS)₂], [Fe^II(<b>L</b>^<b>S</b>)₂(NCSe)₂], [Fe^II(<b>L</b>^<b>S</b>)₂(NCBH₃)₂]·H₂O [<b>L</b>^<b>S</b> = 2,5-di­(2-pyridyl)-1,3,4-thiadiazole], and <b>2</b>·MeOH. All complexes belong to the <i>T</i>₀ = 90 K line [<i>T</i>(LIESST) = <i>T</i>₀ – 0.3<i>T</i>_1/2]. [Fe^II(<b>L</b>^<b>S</b>)₂(NCS)₂], that exhibits a two-stepped thermal SCO process, has been found to also exhibit two well-separated <i>T</i>(LIESST) temperatures [<i>T</i>(LIESST, 1) = 44 K; <i>T</i>(LIESST, 2) = 53 K]