Mechanisms of formation of pseudocalixarene Schiff base macrocycles investigated by ESI-MS

Starting from 4-substituted phenols, three dialdehydes were synthesised as Schiff base pseudocalixarene macrocyclic precursors. Two of the dialdehydes, 2,2’–methylene-bis- [(6-hydroxymethyl)-4-methylphenol] and 2,2’–methylene-bis-[(6-hydroxymethyl)-4- phenylphenol] were structurally characterised. For the phenyl substituted compound condensation with 1,3-diaminopropan-2-ol, with transition metal ions as template, was investigated and a series of dinuclear complexes was synthesised and characterised by IR, FAB-MS and elemental analysis. The analytical data implied that the complexes have the same saddle shape conformation controlled by hydrogen bonds resulting from mono-deprotonation of the methylendiphenol units as was observed in previous work. A range of TM2M trinuclear complexes [TM = Cu(II), Ni(II) and M = Li(I), Na(I), Mg(II), Ca(II)] of (2+2) macrocycles was synthesised and characterised by IR, MS (FAB, ESI) and elemental analysis. Additionally [Cu2Ca(2+2)(NO3)2](MeOH)2 was characterised by X-ray crystallography. An ESI-MS was used to follow condensation reactions between 2,2’–methylene-bis-[(6- hydroxymethyl)-4-tert-butylphenol] and 1,3-diaminopropan-2-ol in solution with various templates. It was found that, when a transition metal is used alone, the reaction produces only the (2+2) macrocycle. Cu(II) produced equilibrium mixtures containing dicopper(II) and tricopper(II) species but Ni(II) and Zn(II) yielded only dinuclear complexes. When transition metal ions were used in combination with group 1 or group 2 metal ions, the size of the macrocycle and nuclearity of the complex depended on the synthetic route and nature of alkali or alkaline earth metal. Among the products identified in the ESI-MS spectra were trinuclear complexes of the (2+2) macrocycle, pentanuclear sandwich complexes of two (2+2) macrocycles, tetranuclear (3+3) complexes, pentanuclear (4+4) and hexanuclear (6+6) species. One of the routes resulted in formation of [BaCu4(4+4)]2+ ion via a [BaCu4(dialdehyde)4]2+ cluster which was established to be a double template process where both metals are necessary for formation of the macrocycle. The central Ba(II) ion holds the dialdehydes together and the Cu(II) ion orients the carbonyl groups for Schiff base condensation

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

Data to support study of Iron(II) Complexes of 2,4-Dipyrazolyl-1,3,5-Triazine Derivatives ‒ the Influence of Ligand Geometry on Metal Ion Spin State

Seven derivatives of [FeL2]2+ (L = 2,4-di{pyrazol-1-yl}-1,3,5-triazine) are all high-spin. DFT calculations imply this can be attributed to the geometry of the L ligand

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

Data to Support Iron(II) Complexes of 2,6-Bis(imidazo[1,2?a]pyridin-2-yl)pyridine and Related Ligands with Annelated Distal Heterocyclic Donors

Complexes of the title ligand and two of its derivatives are mostly high-spin in the solid state, but exhibit thermal spin-crossover equilibria in solution with a ligand-centred room-temperature emission

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

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

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

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

Data to support study of Spin States of Homochiral and Heterochiral Isomers of [Fe(PyBox)2]2+ Derivatives

Homochiral [Fe((R)-LPh)2]2+ (LPh = 2,6-bis{4-phenyloxazolinyl}pyridine) undergoes spin-crossover in CD3CN at 34 K lower temperature than its heterochiral diastereomer [Fe((R)-LPh)((S)-LPh)]2+