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We observed by optical microscopy the propagation of the thermal spin transition in single crystals of the archetype compound [Fe(btr)2(NCS)2]·H2O (btr = 4,4′-bis-1,2,4-triazole). In fresh crystals we could follow well-defined transformation fronts of various shapes. The propagation conditions revealed to depend very sensitively on the structural quality of the crystals. The best observations were obtained on cooling selected fresh crystals, rather thin, protected by an oil layer, under isothermal conditions. They evidenced the progression of a well-defined transformation front through the entire crystal. This is typical for a nucleation and growth process governed by inhomogeneous nucleation. The front shape was simple (circular or linear) and progression was regular in the best cases. Crystals of lesser quality (or already cycled through the transition) exhibited a more complex behaviour, assigned to the impact of internal stresses, either genuine or generated by the previous transformation of the crystals. These observations provide new insights into the physics of like-spin domains, which are briefly discussed. [ABSTRACT FROM AUTHOR]

Abstract: Structural analysis of a previously reported half-sandwich complex having three-legged “piano-stool” geometry [(η6-C6H6)RuII(L1)Cl][PF6] (1) (L1 = 2-(pyrazol-1-ylmethyl)pyridine) is described. Treatment of 1 with (i) Ag(CF3SO3) in CH3CN and (ii) NaN3 in CH3OH, and (iii) the reaction between [(η6-C6H6)Ru(L2)Cl]-[PF6] (2) (previously reported) and NaCN in C2H5OH led to the isolation of [(η6-C6H6)Ru(L1)(CH3CN)][PF6]2 (3), [(η6-C6H6)Ru(L1)(N3)][PF6] (4), and [(η6-C6H6)Ru(L2)(CN)][PF6] (5), respectively (L2 = 2-(3,5-dimethyl-pyrazol-1-ylmethyl)pyridine). The complex [(η6-C6H6)Ru(L4)Cl][PF6] (6) with a new ligand (L4 = 2-[3-(4-fluorophenyl)pyrazol-1-ylmethyl]pyridine) has also been synthesized. The structures of 3–6 have been elucidated (1H NMR spectra; CD3CN). The molecular structures of 1, 4, and 6·C6H5CH3 have been determined. Notably, the crystal-packing in these structures is governed by C–H⋯X (X = Cl, N) interactions, generating helical architectures. [Copyright &y& Elsevier]

Abstract: Four half-sandwich ruthenium(II) complexes [(η6-C6H6)Ru(L1-O)][PF6] (1), [(η6-C6H6)Ru(L2-O)][PF6] (2), [(η6-C6H6)Ru(L3-O)][PF6] (3), [(η6-C6H6)Ru(L4-O)][PF6] (4a), and [(η6-C6H6)Ru(L4-O)][BPh4] (4b) [L1-OH, 4-nitro-6-{[(2′-(pyridin-2-yl)ethyl)methylamino]methyl}-phenol; L2-OH, 2,4-di-tert-butyl-6-{[(2′-(pyridin-2-yl)ethyl)methylamino]methyl}-phenol; L3-OH, 2,4-di-tert-butyl-6-{[2′-((pyridin-2-yl)benzylamino)methyl}-phenol; L4-OH, 2,4-di-tert-butyl-6-{[(2′-imethylaminoethyl)methylamino]methyl}-phenol (L4-OH)], supported by a systematically varied series of tridentate phenolate-based pyridylalkylamine and alkylamine ligands are reported. The molecular structures of 1–3, 4a, and 4b have been elucidated in solution using 1H NMR spectroscopy and of 1, 3, and 4b in the solid state by X-ray crystallography. Notably, due to coordination by the ligands the Ru center assumes a chiral center and in turn the central amine nitrogen also becomes chiral. The 1H NMR spectra exhibit only one set of signals, suggesting that the reaction is completely diastereoselective [1: S Ru,S N/R Ru,R N; 2: R Ru,R N/S Ru,S N; 3: S Ru,R N/R Ru,S N; 4b: S Ru,R N/R Ru,S N]. The crystal packing in 1 and 3 is stabilized by C–H…O interactions, in 4b no meaningful secondary interactions are observed. From the standpoint of generating phenoxyl radical, as investigated by cyclic voltammetry (CV), complex 1 is redox-inactive in MeCN solution. However, 2, 3, and 4a generate a one-electron oxidized phenoxyl radical coordinated species [2]2+ , [3]2+ , and [4a]2+ , respectively. The radical species are characterized by CV, UV–Vis, and EPR spectroscopy. The stability of the radical species has been determined by measuring the decay constant (UV–Vis spectroscopy). [Copyright &y& Elsevier]