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Following a published synthesis of 2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine (L1), treatment of α,α′-dibromo-2,6-diacetylpyridine with 2 equiv. 2-aminopyrimidine or 2-aminoquinoline in refluxing acetonitrile respectively gives 2,6-bis(imidazo[1,2-a]pyrimidin-2-yl)pyridine (L2) and 2,6-bis(imidazo[1,2-a]quinolin-2-yl)pyridine (L3). Solvated crystals of [Fe(L1)2][BF4]2 (1[BF4]2) and [Fe(L2)2][BF4]2 (2[BF4]2) are mostly high-spin, although one solvate of 1[BF4]2 undergoes thermal spin-crossover on cooling. The iron coordination geometry is consistently distorted in crystals of 2[BF4]2 which may reflect the influence of intramolecular, inter-ligand N⋯π interactions on the molecular conformation. Only 1 : 1 Fe : L3 complexes were observed in solution, or isolated in the solid state; a crystal structure of [FeBr(py)2L3]Br·0.5H2O (py = pyridine) is presented. A solvate crystal structure of high-spin [Fe(L4)2][BF4]2 (L4 = 2,6-di{quinolin-2-yl}pyridine; 4[BF4]2) is also described, which exhibits a highly distorted six-coordinate geometry with a helical ligand conformation. The iron(II) complexes are high-spin in solution at room temperature, but 1[BF4]2 and 2[BF4]2 undergo thermal spin-crossover equilibria on cooling. All the compounds exhibit a ligand-based emission in solution at room temperature. Gas phase DFT calculations mostly reproduce the spin state properties of the complexes, but show small anomalies attributed to intramolecular, inter-ligand dispersion interactions in the sterically crowded molecules. [ABSTRACT FROM AUTHOR]

Four bis[2‐{pyrazol‐1‐yl}‐6‐{pyrazol‐3‐yl}pyridine] ligands have been synthesized, with butane‐1,4‐diyl (L1), pyrid‐2,6‐diyl (L2), benzene‐1,2‐dimethylenyl (L3) and propane‐1,3‐diyl (L4) linkers between the tridentate metal‐binding domains. L1 and L2 form [Fe2(μ−L)2]X4 (X−=BF4− or ClO4−) helicate complexes when treated with the appropriate iron(II) precursor. Solvate crystals of [Fe2(μ−L1)2][BF4]4 exhibit three different helicate conformations, which differ in the torsions of their butanediyl linker groups. The solvates exhibit gradual thermal spin‐crossover, with examples of stepwise switching and partial spin‐crossover to a low‐temperature mixed‐spin form. Salts of [Fe2(μ−L2)2]4+ are high‐spin, which reflects their highly twisted iron coordination geometry. The composition and dynamics of assembly structures formed by iron(II) with L1−L3 vary with the ligand linker group, by mass spectrometry and 1H NMR spectroscopy. Gas‐phase DFT calculations imply the butanediyl linker conformation in [Fe2(μ−L1)2]4+ influences its spin state properties, but show anomalies attributed to intramolecular electrostatic repulsion between the iron atoms. [ABSTRACT FROM AUTHOR]

4‐(tert‐Butylsulfanyl)‐2,6‐di(pyrazol‐1‐yl)pyridine (L) was obtained in low yield from a one‐pot reaction of 2,4,6‐trifluoropyridine with 2‐methylpropane‐2‐thiolate and sodium pyrazolate in a 1:1:2 ratio. The materials [FeL2][BF4]2⋅solv (1[BF4]2⋅solv) and [FeL2][ClO4]2⋅solv (1[ClO4]2⋅solv; solv=MeNO2, MeCN or Me2CO) exhibit a variety of structures and spin‐state behaviors including thermal spin‐crossover (SCO). Solvent loss on heating 1[BF4]2⋅x MeNO2 (x≈2.3) occurs in two steps. The intermediate phase exhibits hysteretic SCO around 250 K, involving a "reverse‐SCO" step in its warming cycle at a scan rate of 5 K min−1. The reverse‐SCO is not observed in a slower 1 K min−1 measurement, however, confirming its kinetic nature. The final product [FeL2][BF4]2⋅0.75 MeNO2 was crystallographically characterized, and shows abrupt but incomplete SCO at 172 K which correlates with disorder of an L ligand. The asymmetric unit of 1[BF4]2⋅y Me2CO (y≈1.6) contains five unique complex molecules, four of which undergo gradual SCO in at least two discrete steps. Low‐spin 1[ClO4]2⋅0.5 Me2CO is not isostructural with its BF4− congener, and undergoes single‐crystal‐to‐single‐crystal solvent loss with a tripling of the crystallographic unit cell volume, while retaining the P1‾ space group. Three other solvate salts undergo gradual thermal SCO. Two of these are isomorphous at room temperature, but transform to different low‐temperature phases when the materials are fully low‐spin. [ABSTRACT FROM AUTHOR]

Treatment of 2,4,6‐trifluoropyridine with sodium imidazolate, then 2 equiv. sodium pyrazolate, in two reaction steps yields the title ligand L in 40 % overall yield. Crystalline [FeL2][BF4]2 and [FeL2][ClO4]2 are isostructural high‐spin complexes, with highly twisted six‐coordinate geometries and pendant imidazolyl groups. Conversely, [Ag(µ‐L)]BF4·0.5MeCN is a linear coordination polymer of distorted square planar silver ions linked by κ1:κ3,µ‐L ligands. Some samples of [FeL2][BF4]2 were contaminated by [FeIII3(µ‐F)2F6L8]BF4, which was prepared in pure form following reaction of Fe[BF4]2·6H2O, Na3[FeF6] and L in appropriate ratios. The linear [{trans‐FeFL4}2(µ‐F)2FeF4]+ assembly is supported by monodentate L ligands coordinated through their imidazolyl donors, and donating C–H····F interactions to the central [FeF6]3– moiety. The cluster exhibits moderate antiferromagnetic coupling between its iron(III) centres. [ABSTRACT FROM AUTHOR]

Abstract: Crystalline [FeL2][BF4]2⋅Me2CO (L=N‐[2,6‐di{pyrazol‐1‐yl}pyrid‐4‐yl]acetamide) is high‐spin at room temperature, and undergoes an abrupt, hysteretic spin‐crossover at T 1 / 2 =137 K (ΔT 1 / 2 =14 K) that proceeds to about 50 % completeness. This is associated with a crystallographic phase transition, from phase 1 (P21/c, Z=4) to phase 2 (P21, Z=48). The cations associate into chains in the crystal through weak intermolecular π⋅⋅⋅π interactions. Phase 2 contains a mixture of high‐spin and low‐spin molecules, which are grouped into triads along these chains. The perchlorate salt [FeL2][ClO4]2⋅Me2CO also adopts phase 1 at room temperature but undergoes a different phase transition near 135 K to phase 3 (P21/c, Z=8) without a change in spin state. [ABSTRACT FROM AUTHOR]

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]

The phase behaviour and spin states of [Fe(NCS)2(dpq)2] (1; dpq = dipyrido[3,2-f:2′,3′-h]quinoxaline), [Fe(NCS)2(dppz)2] (2; dppz = dipyrido[3,2-a:2′3′-c]phenazine) and [Fe(NCS)2(dppn)2] (3; dppn = dipyrido[3,2-a:2′3′-c]benzophenazine) have been investigated. Solvent-free 1 and 2 are isostructural and low-spin in the crystalline state, in contrast to previously published 2·py (py = pyridine) which exhibits a hysteretic spin-crossover (SCO) transition near 140 K. The inactivity of 1 and 2 towards SCO may relate to their more crowded intermolecular lattice environment, particularly two very short intermolecular anion…π contacts involving the NCS− ligands. Two solvate phases of 1 are also described, including 1·2py which undergoes gradual SCO with T½ca. 188 K. Bulk samples of 2 and 3 are predominantly low-spin and isostructural with the crystals of 2 by powder diffraction, but bulk samples of 1 contain an extra phase that exhibits hysteretic SCO, but was not crystallographically characterised. Crystal structures of low-spin [Fe(dppz)3][ClO4]2 (4) and a solvate of [Fe(dppn)3][BF4]2 (5) are also described, which are the first homoleptic complexes of these ligands to be crystallographically characterised. [ABSTRACT FROM AUTHOR]

The complex [FeL2][BF4]2 ( 1; L=4-(isopropylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine) forms solvate crystals 1⋅solv (solv=MeNO2, MeCN, EtCN, or Me2CO). Most of these materials lose their solvent sluggishly on heating. However, heating 1⋅MeNO2 at 450 K, or storing 1⋅EtCN under ambient conditions, leads to single-crystal to single-crystal exchange of the organic solvent for atmospheric moisture, forming 1⋅H2O. Solvent-free 1 ( 1⋅sf) can be generated in situ by annealing 1⋅H2O at 370 K in the diffractometer or magnetometer. The different forms of 1 are isostructural ( P21/ c, Z=4) and most of them exhibit spin-crossover (SCO) at 141≤ T [ABSTRACT FROM AUTHOR]