Simon A. Barrett

The spin-states and spin-crossover behaviour of iron(II) complexes of 2,6-dipyrazol-1-ylpyrazine derivatives

The syntheses of [FeL2]X2 (L = 2,6-dipyrazol-1-ylpyrazine [L2H], 2,6-bis{3-methylpyrazol-1-yl}pyrazine [L2Me], 2,6-bis{3,5-dimethylpyrazol-1-yl}pyrazine [L2Me2] or 2,6-bis{3-[2,4,6-trimethylphenyl]pyrazol-1-yl}pyrazine [L2Mes]; X− = BF4− or ClO4−) are described. Solvent-free [Fe(L2H)2][BF4]2 and [Fe(L2H)2][ClO4]2 exhibit very similar abrupt spin-state transitions at 223 K and 208 K respectively, which show hysteresis loops of 3–5 K. Powder diffraction measurements afforded related, but not identical, unit cells for these two compounds, and imply that [Fe(L2H)2][ClO4]2 is isomorphous with [Fe(L1H)2][BF4]2 (L1H = 2,6-dipyrazol-1-ylpyridine). The single crystalline solvate [Fe(L2H)2][BF4]2·3CH3NO2 undergoes a similarly abrupt spin-state transition at 198 K. Polycrystalline [Fe(L2Me)2][BF4]2 and [Fe(L2Me)2][ClO4]2 are isomorphous with each other and also exhibit spin-state transitions at low temperature, although these are very different in form. In contrast, both salts of [Fe(L2Me2)2]2+ and [Fe(L2Mes)2]2+ are fully low-spin at 295 K. Single crystal structures of [Fe(L2Me2)2][BF4]2·0.5{CH3}2CO·0.1H2O and [Fe(L2Mes)2][BF4]2·5CH3NO2 show low-spin complex dications, and imply that [Fe(L2Me2)2][BF4]2 is low-spin as a result of intra-ligand steric repulsions involving the pyrazole 5-methyl substituents. NMR and UV/vis data in MeCN and MeNO2 show that the spin states of all four complex dications are similar in solution and the solid state except for [Fe(L2Me2)2]2+, which exists as a mixture of high- and low-spin species in these solvents.

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

Abstract 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 (T 1/2 ) 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 T 1/2 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.

Anion-dependent spin crossover in solution for an iron(II) complex of a 1H-pyrazolyl ligand

The spin-crossover equilibrium midpoint temperature (T1/2) in [Fe(3-bpp)2]X2 (3-bpp = 2,6-di{pyrazol-3-yl}pyridine) varies from 259 K when X− = BPh4− to 277 K when X− = Br−, at 10 mM concentrations in an acetone–water solvent mixture.

Macropolyhedral boron-containing cluster chemistry. Isolation and characterisation of the 27-vertex contiguous triple-cluster species [(PMe2Ph)PtB26H26(PMe2Ph)]

Abstract Reaction between [(PMe2Ph)2PtB8H12] and anti-B18H22 in benzene results in intimate cluster fusion to generate [(PMe2Ph)PtB26H26-(PMe2Ph)] which consists of a nido eleven-vertex {PtB10} subcluster fused, with a {PtB2} triangular face in common, to a second nido eleven-vertex {PtB10} subcluster, which is fused in turn to a nido ten-vertex {B10} subcluster with a {B2} edge in common.

Syntheses of new hydroxy-[3.3]orthocyclophanes as models for the galactose oxidase Tyr-Cys cofactor

Abstract The syntheses of 3,4-benzo-8,9-(3′-hydroxybenzo)bicyclo[4,4,1]undeca-3,8-dien-11-one, 3,4-benzo-8,9-(3′-hydroxy-4′-methylsulfanylbenzo)bicyclo[4,4,1]undeca-3,8-dien-11-one and their ethylene acetals have been achieved. Crystallographic, UV/Vis and NMR data show that the two ketones adopt boat/chair conformations that are fluxional in solution, while the acetals exhibit chair/chair conformations with layered benzo rings. Comparison of the oxidation potentials of the four compounds suggests that an ortho-methylsulfanyl substituent and a π–π interaction both thermodynamically stabilise the phenoxonium radical species derived from these compounds, by approximately equal amounts.

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.

Iron(II) complexes of 4-sulfanyl-, 4-sulfinyl- and 4-sulfonyl-2,6-dipyrazolylpyridine ligands. A subtle interplay between spin-crossover and crystallographic phase changes

Oxidation of 4-(methylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine (LSMe) with hydrogen peroxide or mCPBA yields 4-(methylsulfinyl)-2,6-di(pyrazol-1-yl)pyridine (LSOMe) and 4-(methylsulfonyl)-2,6-di(pyrazol-1-yl)pyridine (LSO2Me), respectively. Solid [Fe(LSMe)2][ClO4]2 (1[ClO4]2) is high-spin at room temperature, and exhibits an abrupt spin-transition at T1/2 = 256 K. A shoulder on the cooling side of the χMT vs. T curve is associated with a hysteretic crystallographic phase change, occurring around T↓ = 245 K and T↑ = 258 K. The phase change involves a 180° rotation of around half the methylsulfanyl substituents in the crystal. This contrasts with the previously reported BF4− salt of the same compound, which is isostructural to 1[ClO4]2 at room temperature but transforms to a different crystal phase in its low-spin state. Solid [Fe(LSOMe)2][BF4]2 (2[BF4]2) and [Fe(LSO2Me)2][BF4]2 (3[BF4]2) both exhibit gradual spin-crossover equilibria centred significantly above room temperature. Solution measurements show that the oxidised sulfur centers in 2[BF4]2 and 3[BF4]2 stabilise the low spin states of those complexes.

Polyhedral palladaborane chemistry: isolation and structural characterization of ten-vertex [(PMe2Ph)2PdB9H12(PMe2Ph)] and eleven-vertex [(PMe2Ph)2PdB10H12]

AbstractTen-vertex [6,6-(PMe2Ph)2-arachno-6-PdB9H12-9-(PMe2Ph)] 1a and eleven-vertex [7,7-(PMe2Ph)2-nido-7-PdB10H12] 2a have been isolated as occasional by-products from an extension to palladaborane chemistry of the one-pot route for the preparation of the nine-vertex platinaborane [(PMe2Ph)2PtB8H12] 3b from [PtCl2(PMe2Ph)2] and B10H14, but using [PdCl2(PMe2Ph)2] instead of [PtCl2(PMe2Ph)2] to generate [4,4-(PMe2Ph)2-arachno-4-PdB8H12] 3a in yields of upto 75%. The two by-products 1a and 2a are each characterized by single-crystal X-ray diffraction analysis. Space group and cell parameters are as follows: for 1a, triclinic,