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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]

Chlorination of 2,6-bis(pyrazol-1-yl)pyrazine (bppz) with NaClO in acetic acid afforded 2,6-bis(4-chloropyrazol-1-yl)pyrazine ( L2Cl). 2,6-Bis(4-bromopyrazol-1-yl)pyrazine ( L2Br), 2,6-bis(4-iodopyrazol-1-yl)pyrazine ( L2I), 2,6-bis(4-methylpyrazol-1-yl)pyrazine ( L2Me), and 2,6-bis(4-nitropyrazol-1-yl)pyrazine ( L2NO2) were also prepared by reactions of the preformed 4-substituted pyrazoles with 2,6-dichloropyrazine. The reduction of L2NO2 with iron powder gave 2,6-bis(4-aminopyrazol-1-yl)pyrazine ( L2NH2) and L2I was converted into 2,6-bis[4-(phenylethynyl)pyrazol-1-yl]pyrazine ( L2CCPh) by a Sonogashira coupling reaction. The salts [Fe( L2Me)2]X2 (X- = BF4- and ClO4-) underwent thermal spin-crossover abruptly at around 200 K in one and two steps, respectively. The [Fe( L2Me)2]X2 salts exhibited different light-induced excited spin-state trapping (LIESST) behavior; the BF4- salt behaves classically [ T(LIESST) = 93 K], but the ClO4- salt undergoes a multistep LIESST relaxation. In contrast, solid [Fe( L2Cl)2][BF4]2 adopts a fixed 2:1 high/low-spin-state population that does not change with temperature below 300 K, whereas [Fe( L2Br)2][BF4]2 and [Fe( L2I)2][BF4]2 form low-spin solvated crystals that are transformed into high-spin powders on drying. The pyrazinyl group in the L2R ligands slightly stabilizes the low-spin state of the complexes, as determined by solution-phase magnetic measurements. The crystal structure of [Fe( L2CCPh)(OH2) z][BF4]2 contains a disordered mixture of six- ( z = 3) and seven-coordinate ( z = 4) iron centers. [ABSTRACT FROM AUTHOR]

The complexes [Fe(L1R)2]X2 (L1R = 2,6-bis-{3-iso-propylpyrazol-1-yl}pyridine[L1Pri], X−=BF4− or PF6−; L1R = 2,6-bis-{3-phenylpyrazol-1-yl}pyridine [L1Ph], X−=BF4−) are high-spin between 5 and 300 K in the solid, and between 180 and 300 K in acetone solution. In contrast, [Fe(L1Mes)2]X2 (L1Mes = 2,6-bis-{3-[2,4,6-trimethylphenyl]pyrazol-1-yl}pyridine, X−=BF4− or PF6−) are fully low-spin at 300 K in the solid and solution phases. This shows that the spin state of the Fe ion in these compounds depends on the inductive properties of the ligand 3,3″-substituents only, as long as those substituents are aromatic. [Copyright &y& Elsevier]

The cover image compares the molecular spin states of two diastereomers of a spin‐crossover iron(II) complex. Solution‐phase magnetic measurements show the low‐spin state of the heterochiral (meso) diastereomer is stabilized relative to its homochiral (rac) congener. This reflects a steric clash between phenyl substituents in the rac isomer, which occupies two diagonal quadrants in the molecule. The resultant twisting of the ligand conformation and metal‐coordination sphere explains the observed reduction in the ligand field. More information can be found in the Full Paper by M. A. Halcrow et al. on page 9067. [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]

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]