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The following iron(II) complexes of 2,6- bis(oxazolinyl)pyridine (PyBox; LH) derivatives are reported: [Fe( LH)2][ClO4]2 ( 1); [Fe(( R)- LMe)2][ClO4]2 (( R)- 2; LMe=2,6- bis{4-methyloxazolinyl}pyridine); [Fe(( R)- LPh)2][ClO4]2 (( R)- 3) and [Fe(( R)- LPh)(( S)- LPh)][ClO4]2 (( RS)- 3; LPh=2,6- bis{4-phenyloxazolinyl}pyridine); and [Fe(( R)- L iPr)2][ClO4]2 (( R)- 4) and [Fe(( R)- L iPr)(( S)- L iPr)][ClO4]2 (( RS)- 4; L iPr=2,6- bis{4- isopropyloxazolinyl}pyridine). Solid ( R)- 3⋅MeNO2 exhibits an unusual very gradual, but discontinuous thermal spin-crossover with an approximate T [ABSTRACT FROM AUTHOR]

Abstract: Reaction of 2 equiv of the sodium salt of ethyl pyrazole-4-carboxylate, with 1 equiv of 2,6-dibromopyridine, in diglyme at 130°C for 5days yields 2,6-di[4-(ethylcarboxy)pyrazol-1-yl]pyridine (L1), with 2-bromo-6-[4-(ethylcarboxy)pyrazol-1-yl]pyridine (L2) as a significant byproduct. Reduction of L1 with excess NaBH4 in thf affords 2,6-di[4-(hydroxymethyl)pyrazol-1-yl]pyridine (L3) in low yield. The crystalline complex [Fe(L1)2][BF4]2 ·2CF3CH2OH is low-spin at 150K, while bulk samples with this formula are approximately 10% high-spin and 90% low-spin at room temperature. This ratio does not vary significantly on cooling from its magnetic susceptibility, suggesting that the material might be contaminated by a second, minor high-spin phase. Single crystals of [Fe(L3)2][BF4]2·1.4CH3CN have a mixed spin-state population, with the low-spin state predominating at 150K. The [Fe(L3)2(BF4)]+ moieties in the lattice associate into 1-D chains through intermolecular O–H⋯O and O–H⋯F hydrogen bonding. Bulk samples of [Fe(L3)2][BF4]2 ·H2O are fully low-spin below 200K, but the magnetic data imply the onset of a gradual thermal spin-transition centred above room temperature. DSC and TGA measurements imply that this transition is centred at 322K, and involves loss of lattice water. Both complexes undergo spin-crossover in (CD3)2CO solution, with transition midpoints near 250K. [Copyright &y& Elsevier]

Reaction of 2,6-difluoropyridine with 2 equiv of indazole and NaH at room temperature affords a mixture of 2,6-bis(indazol-1-yl)pyridine (1-bip), 2-(indazol-1-yl)-6-(indazol-2-yl)pyridine (1,2-bip), and 2,6-bis(indazol-2-yl)pyridine (2-bip), which can be separated by solvent extraction. A two-step procedure using the same conditions also affords both 2-(indazol-1-yl)-6-(pyrazol-1-yl)pyridine (1-ipp) and 2-(indazol-2-yl)-6-(pyrazol-1-yl)pyridine (2-ipp). These are all annelated analogues of 2,6-di(pyrazol-1-yl)pyridine, an important ligand for spin-crossover complexes. Iron(II) complexes [Fe(1-bip)2]2+, [Fe(1,2-bip)2]2+, and [Fe(1-ipp)2]2+ are low-spin at room temperature, reflecting sterically imposed conformational rigidity of the 1-indazolyl ligands. In contrast, the 2-indazolyl complexes [Fe(2-bip)2]2+ and [Fe(2-ipp)2]2+ are high-spin in solution at room temperature, whereas salts of [Fe(2-bip)2]2+ exhibit thermal spin transitions in the solid state. Notably, [Fe(2-bip)2][BF4]2·2MeNO2 adopts a terpyridine embrace lattice structure and undergoes a spin transition near room temperature after annealing, resulting in thermal hysteresis that is wider than previously observed for this structure type (T1/2 = 266 K, ΔT = 16-20 K). This reflects enhanced mechanical coupling between the cations in the lattice through interdigitation of their ligand arms, which supports a previously proposed structure/function relationship for spin-crossover materials with this form of crystal packing. All of the compounds in this work exhibit blue fluorescence in solution under ambient conditions. In most cases, the ligand-based emission maxima are slightly red shifted upon complexation, but there is no detectable correlation between the emission maximum and the spin state of the iron centers. [ABSTRACT FROM AUTHOR]

Abstract: 2,6-Di(1-methyl-pyrazol-3-yl)pyridine (LMe), 2,6-di(1-allyl-pyrazol-3-yl)pyridine (LAll), 2,6-di(1-benzyl-pyrazol-3-yl)pyridine (LBz) and di(1-isopropyl-pyrazol-3-yl)pyridine (L i Pr) have been synthesized by alkylation of deprotonated di{1H-pyrazol-3-yl}pyridine (3-bpp), and converted to salts of the corresponding [Fe(LR)2]2+ complexes (R=Me, All, Bz and iPr). Crystal structures of [Fe(LMe)2]X2 (X− =BF4 −, ClO4 − and PF6 −), [Fe(LAll)2][BF4]2, [Fe(LBz)2][BF4]2 and [Fe(L i Pr)2][PF6]2 have been determined at 150K. All of these contain high-spin iron centres except [Fe(LMe)2][BF4]2·xH2O, which is predominantly low-spin at that temperature. All the complexes are high-spin between 5 and 300K as solvent-free bulk powders, and are also high-spin in (CD3)2CO solution between 193 and 293K. This was unexpected, since the parent complex [Fe(3-bpp)2]2+ undergoes spin-crossover in the same solvent with T ½ =247K [40]. The high-spin nature of the [Fe(LR)2]2+ complexes in solution must reflect a subtle balance of steric and electronic factors involving the ligand ‘R’ substituents. [Copyright &y& Elsevier]