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

Manipulating metal spin states for biomimetic, catalytic and molecular materials chemistry

This article surveys the relationship between ligand type, coordination geometry and metal spin state in complexes of iron and other metal ions. Compounds and materials containing high-, intermediate- and low-spin metal ions differ in their molecular structures, their physical properties and their chemical reactivity. Implications and applications of these variations are summarised, including the use of base metals in light-harvesting dyes and in different forms of catalysis. Recent studies of the electronic influence of ligand substituents, or ligand conformational constraints, on metal ion spin states are described, which have revealed unexpected complexities

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

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 Tmath formula of 350 K. The discontinuity around 240 K lies well below Tmath formula , 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 1H NMR. (R)-4 and (RS)-4 remain high-spin in solution, whereas the other compounds all undergo spin-crossover equilibria. Importantly, Tmath formula for (R)-3 (244 K) is 34 K lower than for (RS)-3 (278 K) in CD3CN, which is the first demonstration of chiral discrimination between metal ion spin states in a molecular complex

Ligand-Directed Metallation 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

Five 2,6-Di(pyrazol-1-yl)pyridine-4-carboxylate Esters, and the Spin States of their Iron(II) Complexes

Two phenyl ester and three benzyl ester derivatives have been synthesized from 2,6- di(pyrazol-1-yl)pyridine-4-carboxylic acid and the appropriate phenyl or benzyl alcohol using N,N’- dicyclohexylcarbodiimide as the coupling reagent. Complexation of the ligands with Fe[BF₄]₂·6H₂O in acetone yielded the corresponding [FeL₂][BF₄]₂ complex salts. Four of the new ligands and four of the complexes have been crystallographically characterised. Particularly noteworthy are two polymorphs of [Fe(L³)₂][BF₄]₂·2MeN₂2 (L³ = 3,4-dimethoxyphenyl 2,6-di{pyrazol-1-yl}pyridine-4- carboxylate), one of which is crystallographically characterised as high-spin while the other exhibits the onset of spin-crossover above room temperature. The other complexes are similarly low-spin at low temperature but exhibit gradual spin-crossover on heating, except for an acetone solvate of [Fe(L⁵)₂][BF₄]₂ (L⁵ = benzyl 2,6-di{pyrazol-1-yl}pyridine-4-carboxylate), which exhibits a more abrupt spin-transition at T½ = 273 K with 9 K thermal hysteresis

Effect of the Coulomb energy on Skyrmions

The Coulomb effect, an essential ingredient in nuclear systems, is quantitatively investigated for certain Skyrmions with charge greater than 1. This is the first time such a calculation has been done. To do this we calculate the Coulomb energy from numerically generated multi-Skyrmions and, simultaneously, introduce an effective alpha-like particle approximation for large Skyrmions with baryon number B, where B is a multiple of 4. The calculated Coulomb energies and the corresponding fitted curve from this approximation match well with the results from the numerical method, as well as the behavior of the Coulomb energy in the semiempirical mass formula. The Skyrme model correctly reproduces the well known results for the Coulomb energy of nuclei. This suggests that the alpha particles can be used as the fundamental degrees of freedom in the Skyrme model, like the proposals in many other models. The Coulomb effect on the binding energy of the Skyrmions increases with baryon number but has a small effect overall. However, the effect could be significantly more pronounced for loosely bound Skyrme models

Rigidification of a macrocyclic tris-catecholate scaffold leads to electronic localisation of its mixed valent redox product

The catecholate groups in [{Pt(L)}3(μ3-tctq)] (H6tctq = 2,3,6,7,10,11-hexahydroxy-4b,8b,12b,12d-tetramethyltribenzotriquinacene; L = a diphosphine chelate) undergo sequential oxidation to their semiquinonate forms by voltammetry, with ΔE½ = 160–170 mV. The monoradical [{Pt(dppb)}3(μ3-tctq•)]+ is valence-localised, with no evidence for intervalence charge transfer in its near-IR spectrum. This contrasts with previously reported [{Pt(dppb)}3(μ3-ctc•)]+ (H6ctc = cyclotricatechylene), based on the same macrocyclic tris-dioxolene scaffold, which exhibits partly delocalised (class II) mixed valency

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

Silver(i) complexes of bis- and tris-(pyrazolyl)azine derivatives - dimers, coordination polymers and a pentametallic assembly.

Silver(I) complexes of 2,4,6-tri(pyrazol-1-yl)pyridine (tpp), 2,4,6-tri(pyrazol-1-yl)pyrimidine (tpym), 2,4,6-tri(pyrazol-1-yl)-1,3,5-triazine (tpt) and 2,4-di(pyrazol-1-yl)-1,3,5-triazine (bpt) are reported. Dinuclear [Ag₂(μ-tpp)₂(MeCN)₂][BF₄]₂·2MeCN and [Ag₂(μ-tpym)₂(MeCN)₂][BF₄]₂ are formed from approximately planar [AgL(NCMe)]⁺ (L = tpp or bpym) centres, which dimerise via apical interactions to the pendant pyrazolyl donor on each ligand. Two polymeric solvatomorph phases [Ag₂(μ-tpp)₂][BF₄]₂·nMeNO₂ were obtained by crystallising AgBF₄ and tpp from nitromethane solution. One is composed of the same dimeric [Ag₂(μ-tpp)₂]²⁺ motif as the MeCN solvates, but with semibridging pyrazolyl substitutents replacing the solvent ligands. The other form has helicate [Ag₂(μ-tpp)₂]²⁺ dimers linked into chains by unsupported Ag⋯Ag interactions. In contrast, [Ag₅(μ₃-tpym)₄][BF₄]₅·2MeNO₂ contains discrete pentametallic assemblies, of a flattened [Ag₄(μ-tpym)₄]⁴⁺ molecular square centred by the fifth silver ion. Three helical or linear 1D coordination polymer topologies are described for [Ag(μ-tpt)]X (X¯ = BF₄¯ or ClO₄¯), where ditopic tpt ligands bind one silver ion in a [2 + 1] geometry and the other in bidentate, [1 + 1] or monodentate fashion. Finally, [Ag(bpt)]BF₄ is a polymer of square planar silver ions linked by bis-bidentate bpt ligands. Most of the tpt and bpt structures include short anion⋯π contacts to the ligand triazinyl rings. Electrospray mass spectra confirm the oligomeric nature of the Ag/tpym and tpt complexes in MeNO₂ solution