Di-Iron(II) [2+2] Helicates of Bis-(Dipyrazolylpyridine) Ligands: The Influence of the Ligand Linker Group on Spin State Properties.

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 1 H 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.

Ligand-Directed Metalation of a Gold Pyrazolate Cluster.

Solid "[AuL]" (HL = 3-[pyrid-2-yl]-5-tertbutyl-1H-pyrazole) can be crystallized as cyclic [Au3(µ-L)3] and [Au4(µ-L)4] clusters from different solvents. The crystalline tetramer contains a square Au4 core with an HT:TH:TH:HT arrangement of ligand substituents, which preorganizes the cluster to chelate to additional metal ions via its pendant pyridyl groups. The addition of 0.5 equiv of AgBF4 to [AuL] yields [Ag2Au4(µ3-L)4][BF4]2, where two edges of the Au4 square are spanned by Ag+ ions via metallophilic Ag···Au contacts. Treatment of [AuL] with [Cu(NCMe)4]PF6 affords the metalloligand helicate [Cu2Au2(µ-L)4][PF6]2, via oxidation of the copper and partial fragmentation of the cluster.

Structural Transformations and Spin-Crossover in [FeL2 ]2+ Salts (L=4-{tert-Butylsulfanyl}-2,6-di{pyrazol-1-yl}pyridine): The Influence of Bulky Ligand Substituents.

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 Me2 CO) 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 Me2 CO (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 Me2 CO 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 P 1 ‾ 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.

Structures and Spin States of Iron(II) Complexes of Isomeric 2,6-Di(1,2,3-triazolyl)pyridine Ligands.

Iron(II) complex salts of 2,6-di(1,2,3-triazol-1-yl)pyridine (L1) are unexpectedly unstable in undried solvent. This is explained by the isolation of [Fe(L1)4(H2O)2][ClO4]2 and [Fe(NCS)2(L1)2(H2O)2]·L1, containing L1 bound as a monodentate ligand rather than in the expected tridentate fashion. These complexes associate into 44 grid structures through O-H···N hydrogen bonding; a solvate of a related 44 coordination framework, catena-[Cu(µ-L1)2(H2O)2][BF4]2, is also presented. The isomeric ligands 2,6-di(1,2,3-triazol-2-yl)pyridine (L2) and 2,6-di(1H-1,2,3-triazol-4-yl)pyridine (L3) bind to iron(II) in a more typical tridentate fashion. Solvates of [Fe(L3)2][ClO4]2 are low-spin and diamagnetic in the solid state and in solution, while [Fe(L2)2][ClO4]2 and [Co(L3)2][BF4]2 are fully high-spin. Treatment of L3 with methyl iodide affords 2,6-di(2-methyl-1,2,3-triazol-4-yl)pyridine (L4) and 2-(1-methyl-1,2,3-triazol-4-yl)-6-(2-methyl-1,2,3-triazol-4-yl)pyridine (L5). While salts of [Fe(L5)2]2+ are low-spin in the solid state, [Fe(L4)2][ClO4]2·H2O is high-spin, and [Fe(L4)2][ClO4]2·3MeNO2 exhibits a hysteretic spin transition to 50% completeness at T1/2 = 128 K (ΔT1/2 = 6 K). This transition proceeds via a symmetry-breaking phase transition to an unusual low-temperature phase containing three unique cation sites with high-spin, low-spin, and 1:1 mixed-spin populations. The unusual distribution of the spin states in the low-temperature phase reflects "spin-state frustration" of the mixed-spin cation site by an equal number of high-spin and low-spin nearest neighbors. Gas-phase density functional theory calculations reproduce the spin-state preferences of these and some related complexes. These highlight the interplay between the σ-basicity and π-acidity of the heterocyclic donors in this ligand type, which have opposing influences on the molecular ligand field. The Brønsted basicities of L1-L3 are very sensitive to the linkage isomerism of their triazolyl donors, which explains why their iron complex spin states show more variation than the better-known iron(II)/2,6-dipyrazolylpyridine system.

Elucidating the Structural Chemistry of a Hysteretic Iron(II) Spin-Crossover Compound From its Copper(II) and Zinc(II) Congeners.

Annealing [FeL2 ][BF4 ]2 ⋅2 H2 O (L=2,6-bis-[5-methyl-1H-pyrazol-3-yl]pyridine) affords an anhydrous material, which undergoes a spin transition at T1/2 =205 K with a 65 K thermal hysteresis loop. This occurs through a sequence of phase changes, which were monitored by powder diffraction in an earlier study. [CuL2 ][BF4 ]2 ⋅2 H2 O and [ZnL2 ][BF4 ]2 ⋅2 H2 O are not perfectly isostructural but, unlike the iron compound, they undergo single-crystal-to-single-crystal dehydration upon annealing. All the annealed compounds initially adopt the same tetragonal phase but undergo a phase change near room temperature upon re-cooling. The low-temperature phase of [CuL2 ][BF4 ]2 involves ordering of its Jahn-Teller distortion, to a monoclinic lattice with three unique cation sites. The zinc compound adopts a different, triclinic low-temperature phase with significant twisting of its coordination sphere, which unexpectedly becomes more pronounced as the crystal is cooled. Synchrotron powder diffraction data confirm that the structural changes in the anhydrous zinc complex are reproduced in the high-spin iron compound, before the onset of spin-crossover. This will contribute to the wide hysteresis in the spin transition of the iron complex. EPR spectra of copper-doped [Fe0.97 Cu0.03 L2 ][BF4 ]2 imply its low-spin phase contains two distinct cation environments in a 2:1 ratio.

Modulating the Magnetic Properties of Copper(II)/Nitroxyl Heterospin Complexes by Suppression of the Jahn-Teller Distortion.

A series of six-coordinate [Cu(L)L1][BF4]2 (L1 = 2,6-bis{1-oxyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-2-yl}pyridine) complexes are reported. Ferromagnetic coupling between the Cu and L1 ligand spins is enhanced by an L coligand with distal methyl substituents, which is attributed to a sterically induced suppression of its Jahn-Teller distortion.

Supramolecular Iron Metallocubanes Exhibiting Site-Selective Thermal and Light-Induced Spin-Crossover.

Treatment of Fe[BF4]2·6H2O with 4,6-di(pyrazol-1-yl)-1H-pyrimid-2-one (HL1) or 4,6-di(4-methylpyrazol-1-yl)-1H-pyrimid-2-one (HL2) affords solvated crystals of [{FeIII(OH2)6}⊂FeII8(µ-L)12][BF4]7 (1, HL = HL1; 2, HL = HL2). The centrosymmetric complexes contain a cubic arrangement of iron(II) centers, with bis-bidentate [L]- ligands bridging the edges of the cube. The encapsulated [Fe(OH2)6]3+ moiety templates the assembly through 12 O-H···O hydrogen bonds to the [L]- hydroxylate groups. All four unique iron(II) ions in the cages are crystallographically high-spin at 250 K, but they undergo a gradual high → low spin-crossover on cooling, which is predominantly centered on one iron(II) site and its symmetry-related congener. This was confirmed by magnetic susceptibility data, light-induced excited spin state trapping (LIESST) effect measurements, and, for 1, Mössbauer spectroscopy and diffuse reflectance data. The clusters are stable in MeCN solution, and 1 remains high-spin above 240 K in that solvent. The cubane assembly was not obtained from reactions using other iron(II) salts or 4,6-di(pyrazol-1-yl)pyrimidine ligands, highlighting the importance of hydrogen bonding in templating the cubane assembly.

Relationship between the Molecular Structure and Switching Temperature in a Library of Spin-Crossover Molecular Materials.

Structure-function relationships relating the spin-crossover (SCO) midpoint temperature (T1/2) in the solid state are surveyed for 43 members of the iron(II) dipyrazolylpyridine family of SCO compounds. The difference between T1/2 in the solid state and in solution [ΔT(latt)] is proposed as a measure of the lattice contribution to the transition temperature. Negative linear correlations between the SCO temperature and the magnitude of the rearrangement of the coordination sphere during SCO are evident among isostructural or near-isostructural subsets of compounds; that is, a larger change in the molecular structure during SCO stabilizes the high-spin state of a material. Improved correlations are often obtained when ΔT(latt), rather than the raw T1/2 value, is considered as the measure of the SCO temperature. Different lattice types show different tendencies to stabilize the high-spin or low-spin state of the molecules they contain, which correlates with the structural changes that most influence ΔT(latt) in each case. These relationships are mostly unaffected by the SCO cooperativity in the compounds or by the involvement of any crystallographic phase changes. One or two materials within each subset are outliers in some or all of these correlations, however, which, in some cases, can be attributed to small differences in their ligand geometry or unusual phase behavior during SCO. A reinvestigation of the structural chemistry of [Fe(3-bpp)2][NCS]2·nH2O [3-bpp = bis(1H-pyrazol-3-yl)pyridine; n = 0 or 2], undertaken as part of this study, is also presented.

An Incomplete Spin Transition Associated with a Z'=1→Z'=24†Crystallographic Symmetry Breaking.

Crystalline [FeL2 ][BF4 ]2 ⋅Me2 CO (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 T1/2 =137 K (ΔT1/2 =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 ⋅Me2 CO 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.

2,6-Bis(pyrazol-1-yl)pyridine-4-carboxylate Esters with Alkyl Chain Substituents and Their Iron(II) Complexes.

Two series of 4-(alkoxyphenyl) 2,6-bis{pyrazol-1-yl}pyridine-4-carboxyate (L3R) or alkyl 2,6-bis{pyrazol-1-yl}pyridine-4-carboxyate (L4R) esters have been synthesized and complexed to iron(II), where R = C nH2 n+1 ( n = 6, 12, 14, 16, 18); two other derivatives related to L3R are also reported. While the solid [Fe(L4R)2][BF4]2 compounds are isostructural by powder diffraction and show similar spin state behaviors, the [Fe(L3R)2][BF4]2 series shows more varied structures and magnetic properties. This was confirmed by solvated crystal structures of [Fe(L3R)2][BF4]2 with n = 6, 14, 16, which all adopt the P1̅ space group but show significantly different side-chain conformations and/or crystal packing. The solid complexes are mostly low spin at room temperature, with many exhibiting the onset of thermal spin crossover (SCO) upon warming. Heating the complexes with n ≥ 14 significantly above their SCO temperature transforms them irreversibly into a predominantly high spin state, which is accompanied by structure changes and loss of crystallinity by powder diffraction. These transformations do not coincide with lattice solvent loss and may reflect melting and refreezing of their alkyl chain conformations during the thermal cycle. Four of the complexes exhibit SCO in CD3CN solution with T1/2 = 273-277 K, which is apparently unaffected by their alkyl chain substituents.

Spin States of Homochiral and Heterochiral Isomers of [Fe(PyBox)2 ]2+ Derivatives.

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)-LiPr )2 ][ClO4 ]2 ((R)-4) and [Fe((R)-LiPr )((S)-LiPr )][ClO4 ]2 ((RS)-4; LiPr =2,6-bis{4-isopropyloxazolinyl}pyridine). Solid (R)-3⋅MeNO2 exhibits an unusual very gradual, but discontinuous thermal spin-crossover with an approximate T1/2 of 350 K. The discontinuity around 240 K lies well below T1/2 , and is unconnected to a crystallographic phase change occurring at 170 K. Rather, it can be correlated with a gradual ordering of the ligand conformation as the temperature is raised. The other solid compounds either exhibit spin-crossover above room temperature (1 and (RS)-3), or remain high-spin between 5-300 K [(R)-2, (R)-4 and (RS)-4]. Homochiral (R)-3 and (R)-4 exhibit more twisted ligand conformations and coordination geometries than their heterochiral isomers, which can be attributed to steric clashes between ligand substituents [(R)-3]; or, between the isopropyl substituents of one ligand and the backbone of the other ((R)-4). In solution, (RS)-3 retains its structural integrity but (RS)-4 undergoes significant racemization through ligand redistribution by 1 H NMR. (R)-4 and (RS)-4 remain high-spin in solution, whereas the other compounds all undergo spin-crossover equilibria. Importantly, T1/2 for (R)-3 (244 K) is 34 K lower than for (RS)-3 (278 K) in CD3 CN, which is the first demonstration of chiral discrimination between metal ion spin states in a molecular complex.

Gradual Thermal Spin-Crossover Mediated by a Reentrant Z' = 1 → Z' = 6 → Z' = 1 Phase Transition.

The Fe[BF4]2 complex of the Schiff base podand tris[4-(thiazol-4-yl)-3-aza-3-butenyl]amine exhibits gradual thermal spin-crossover with T1/2 ≈ 208 K in the solid state. A weak discontinuity in the magnetic susceptibility curve at 190 K is associated with a reentrant symmetry-breaking transition involving a trebling of the unit cell volume (from P21/c, Z = 4, to P21, Z = 12). The intermediate phase contains six independent cations in puckered layers of low-spin, and high-spin or mixed-spin, molecules with an overall 30% high-spin population at 175 K.

Iron(II) Complexes of 2,4-Dipyrazolyl-1,3,5-triazine Derivatives-The Influence of Ligand Geometry on Metal Ion Spin State.

Seven [FeL2][BF4]2 complex salts were prepared, where L is a 6-substituted 2,4-di(pyrazol-1-yl)-1,3,5-triazine (bpt) derivative. The complexes are all crystallographically high-spin, and exhibit significant distortions from an ideal D2d-symmetric coordination geometry. In one case, an unusual type of metal ion disorder was observed among a cubic array of ligands in the crystal lattice. The complexes are also high-spin between 3 and 300 K in the solid state and, where measured, between 239 and 333 K in CD3CN solution. This result is unexpected, since homoleptic iron(II) complexes of related 2,6-di(pyrazol-1-yl)pyridine, 2,6-di(pyrazol-1-yl)pyrazine, and 2,6-di(pyrazol-1-yl)pyrimidine derivatives often exhibit thermal spin-crossover behavior. Gas-phase density functional theory calculations confirm the high-spin form of [Fe(bpt)2]2+ and its derivatives is stabilized relative to iron(II) complexes of the other ligand types. This reflects a weaker Fe/pyrazolyl σ-bonding interaction, which we attribute to a small narrowing of the chelate ligand bite angle associated with the geometry of the 1,3,5-triazinyl ring. Hence, the high-spin state of [Fe(bpt)2]2+ centers does not reflect the electronic properties of its heterocyclic ligand donors but is imposed by the bpt ligand conformation. A high-spin homoleptic iron(III) complex of one of the bpt derivatives was also synthesized.

Different Spin-State Behaviors in Isostructural Solvates of a Molecular Iron(II) Complex.

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 Me2 CO). 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 ≤ T1/2 ≤ 212 K, depending on their solvent content. The exception is the EtCN solvate, whose pristine crystals remain high-spin between 3-300 K. The cooperativity of the spin-transitions depends on the solvent, ranging from gradual and incomplete when solv=acetone to abrupt with 17 K hysteresis when solv=MeCN. Our previously proposed relationship between molecular structure and SCO explains some of these observations, but there is no single structural feature that correlates with SCO in all the 1⋅solv materials. However, changes to the unit cell dimensions during SCO differ significantly between the solvates, and correlate with the SCO cooperativity. In particular, the percentage change in unit cell volume during SCO for the most cooperative material, 1⋅MeCN, is 10 times smaller than for the other 1⋅solv crystals.

A Unified Treatment of the Relationship Between Ligand Substituents and Spin State in a Family of Iron(II) Complexes.

The influence of ligands on the spin state of a metal ion is of central importance for bioinorganic chemistry, and the production of base-metal catalysts for synthesis applications. Complexes derived from [Fe(bpp)2 ](2+) (bpp=2,6-di{pyrazol-1-yl}pyridine) can be high-spin, low-spin, or spin-crossover (SCO) active depending on the ligand substituents. Plots of the SCO midpoint temperature (T1/2 ) in solution vs. the relevant Hammett parameter show that the low-spin state of the complex is stabilized by electron-withdrawing pyridyl ("X") substituents, but also by electron-donating pyrazolyl ("Y") substituents. Moreover, when a subset of complexes with halogeno X or Y substituents is considered, the two sets of compounds instead show identical trends of a small reduction in T1/2 for increasing substituent electronegativity. DFT calculations reproduce these disparate trends, which arise from competing influences of pyridyl and pyrazolyl ligand substituents on Fe-L σ and π bonding.

Iron(II) complexes of tridentate indazolylpyridine ligands: enhanced spin-crossover hysteresis and ligand-based fluorescence.

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.

Remarkable scan rate dependence for a highly constrained dinuclear iron(II) spin crossover complex with a wide thermal hysteresis loop.

The abrupt [HS-HS] ↔ localized [HS-LS] spin crossovers of a new triazole-based diiron(II) complex result in a record-equaling thermal hysteresis loop width for a dinuclear complex (ΔT = 22 K by SQUID magnetometer in "settle" mode) and show a remarkable scan rate dependence of only the cooling branch, as revealed by detailed magnetic, DSC, and Mössbauer studies.

A homologous series of [Fe(H2Bpz2)2(L)] spin-crossover complexes with annelated bipyridyl co-ligands.

Four new iron(II) complexes [Fe(H2Bpz2)2(L)] were prepared (pz = pyrazolyl), where L is dipyrido[3,2-f:2',3'-h]quinoxaline (dpq), dipyrido[3,2-a:2'3'-c]phenazine (dppz), dipyrido[3,2-a:2'3'-c]benzo[i]-phenazine (dppn), and dipyrido[3,2-a:2',3'-c](6,7,8,9-tetrahydro)phenazine (dppc). Crystal structures of [Fe(H2Bpz2)2(dpq)], [Fe(H2Bpz2)2(dppz)], and [Fe(H2Bpz2)2(dppn)] all reveal stacks of complex molecules formed through π-π stacking between interdigitated bipyridyl chelate ligands, often with additional intercalated toluene or uncoordinated bipyridyl ligand (dpq). Molecules of [Fe(H2Bpz2)2(dppc)] form a different stacking motif in the crystal, with weaker contacts between individual molecules. Many of the structures also contain channels of disordered solvent, running between the molecular stacks. Despite their different stacking motifs, all these compounds exhibit very gradual thermal spin-crossover (SCO) on cooling, which occur over different temperature ranges but are otherwise quite similar in form. Weak thermal hysteresis in one of these spin equilibria can be attributed to the effects of a change in bipyridyl ligand conformation in the molecular stacks around 150 K, which was observed crystallographically. These results demonstrate that strong mechanical coupling between molecules in a crystal is not sufficient to engineer cooperative SCO switching, if other regions of the lattice are less densely packed.

Effect of N4-substituent choice on spin crossover in dinuclear iron(II) complexes of bis-terdentate 1,2,4-triazole-based ligands.

Seven new dinuclear iron(II) complexes of the general formula [Fe(II)2(PMRT)2](BF4)4·solvent, where PMRT is a 4-substituted-3,5-bis{[(2-pyridylmethyl)-amino]methyl}-4H-1,2,4-triazole, have been prepared in order to investigate the substituent effect on the spin crossover event. Variable temperature magnetic susceptibility and (57)Fe Mössbauer spectroscopy studies show that two of the complexes, [Fe(II)2(PMPT)2](BF4)4·H2O (N(4) substituent is pyrrolyl) and [Fe(II)2(PM(Ph)AT)2](BF4)4 (N(4) is N,N-diphenylamine), are stabilized in the [HS-HS] state between 300 and 2 K with weak antiferromagnetic interactions between the iron(II) centers. Five of the complexes showed gradual half spin crossover, from [HS-HS] to [HS-LS], with the following T(1/2) (K) values: 234 for [Fe(II)2(PMibT)2](BF4)4·3H2O (N(4) is isobutyl), 147 for [Fe(II)2(PMBzT)2](BF4)4 (N(4) is benzyl), 133 for [Fe(II)2(PM(CF3)PhT)2](BF4)4·DMF·H2O (N(4) is 3,5-bis(trifluoromethyl)phenyl), 187 for [Fe(II)2(PMPhT)2](BF4)4 (N(4) is phenyl), and 224 for [Fe(II)2(PMC16T)2](BF4)4 (N(4) is hexadecyl). Structure determinations carried out for three complexes, [Fe(II)2(PMPT)2](BF4)4·4DMF, [Fe(II)2(PMBzT)2](BF4)4·CH3CN, and [Fe(II)2(PM(Ph)AT)2](BF4)4·solvent, revealed that in all three complexes both iron(II) centers are stabilized in the high spin state at 90 K. A general and reliable 4-step route to PMRT ligands is also detailed.

Iron(II) complexes of 2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine and related ligands with annelated distal heterocyclic donors.

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.