Colin A. Kilner

Antisymmetric exchange in two tricopper(II) complexes containing a [Cu3(μ3-OMe)]5+ core

Reaction of CuX2(X-=Cl- or Br-) with 2 molar equivalents of 3[5]-(2,4,6-trimethylphenyl)pyrazole (HpzMes) in MeOH in the presence of NaOH yields [Cu3X(HpzMes)2(micro-pzMes)3(micro3-OMe)]X (X-=Cl- or Br-). Crystal structures of these compounds show almost identical triangles of Cu(II) ions, centred by a triply bridging methoxide ligand and with three edge-bridging pyrazolide groups. The mesityl substituents on the bridging pyrazolide ligands are arranged in HT, HH, TT fashion. chi(M)T for both compounds decreases steadily with decreasing temperature, reaching 0.40 cm(3) mol(-1) K at 70 K before decreasing further below 40 K. This low temperature behaviour could not be interpreted using conventional superexchange Hamiltonians, but was reproduced by an alternative model that incorporated an additional antisymmetric exchange term. This interpretation was confirmed by the Q-band EPR spectra of the two compounds. NMR experiments show that the structures of these compounds are not retained in solution, in contrast to other closely related tricopper compounds. These are the first examples of triangular Cu(II) compounds bearing a [Cu3micro3-OR)]5+(R is not equal to H) core motif, and the first triangular compounds showing antisymmetric exchange to have been analysed by both susceptibility and EPR measurements.

Iron(II) complexes with a terpyridine embrace packing motif show remarkably consistent cooperative spin-transitions

Six structurally related iron(II) complexes show remarkably similar abrupt thermal spin-transitions.

Sequential one-pot bimetallic Ir(III)/Pd(0) catalysed mono-/bis-alkylation and spirocyclisation processes of 1,3-dimethylbarbituric acid and allenes

Microwave assisted indirect functionalization of alcohols with 1,3-dimethylbarbituric acid followed by spirocyclisation employing a sequential one-pot Ir(III)/Pd(0) catalysed process, involving the formation of three new C-C bonds, one spirocyclic ring and one di- or tri-substituted exocyclic alkene, is described.

An iron(II) complex exhibiting five anhydrous phases, two of which interconvert by spin-crossover with wide hysteresis

[FeL2][BF4]2·2H2O (L = 2,6-di{5-methylpyrazol-3-yl}pyridine) adopts a 1 : 1 high : low spin state population, and can be converted into different high-spin anhydrous phases by recrystallisation (phase 1AA) or by thermal dehydration (phase 1BB). Upon cooling in vacuo, the latter undergoes a thermal spin-state transition centred near T1/2 = 205 K. The transition has a thermal hysteresis width of 65 K in freshly prepared samples, although this gradually narrows to 37 K on repeated scanning. X-Ray powder diffraction measurements performed in vacuo show that 1BB, initially formed at 375 K, exhibits two consecutive crystallographic phase changes near 300 and 270 K, before undergoing a third phase change concomitant with its spin-state transition. None of these new phases is isostructural with 1AA, which itself undergoes a thermal spin-crossover on cooling without a change in crystal symmetry.

Synthesis of functionalised aromatic oligamide rods

A current goal in synthetic chemistry is the design and synthesis of molecules that adopt well defined conformations-so called foldamers. In this manuscript we describe a modular approach for construction of rod shaped para-oligobenzamide molecules. Our approach permits regiospecific incorporation of side chains through a phenolic ether linkage on the scaffold; a feature that partly restricts the conformation of the rod through intramolecular hydrogen-bonding.

Structural diversity in iron(II) complexes of 2,6-di(pyrazol-1-yl)pyridine and 2,6-di(3-methylpyrazol-1-yl)pyridine

The syntheses, magnetochemistry and crystallography of [Fe(L1)2]I0.5[I3]1.5 (1), [Fe(L1)2][Co(C2B9H11)2]2 (2) and [Fe(L2)2][SbF6]2 (3) (L1 = 2,6-di(pyrazol-1-yl)pyridine; L2 = 2,6-di(3-methylpyrazol-1-yl)pyridine) are described. Compounds 1 and 3 are high-spin between 5-300 K. For 1, this reflects a novel variation of an angular Jahn-Teller distortion at the iron centre, which traps the molecule in its high-spin state. No such distortion is present in 3; rather, the high-spin nature of this compound may reflect ligand conformational strain caused by an intermolecular steric contact in the crystal lattice. Compound 2 exhibits a gradual high --> low spin transition upon cooling with T(1/2) = 318 +/- 3 K, that is only 50% complete. This reflects the presence of two distinct, equally populated iron environments in the solid. One of these unique iron centres adopts the same angular structural distortion shown by 1 and so is trapped in its high-spin state, while the other, which undergoes the spin-crossover, has a more regular coordination geometry. In contrast with 3, the solvated salts [Fe(L2)2][BF4]2 x 4 CH3CN and [Fe(L2)2][ClO4]2 x (CH3)2CO both undergo gradual thermal spin-transitions centred at 175 +/- 3 K.

Steric effects on the electronic and molecular structures of nickel(II) and cobalt(II) 2,6-dipyrazol-1-ylpyridine complexes

Abstract The complexes [M(L1R)2](BF4)2 (M=Ni, Co; L1R=2,6-dipyrazol-1-ylpyridine [L1H], 2,6-bis-{3-iso-propylpyrazol-1-yl}pyridine [L1Pri], 2,6-bis-{3-phenylpyrazol-1-yl}pyridine [L1Ph], 2,6-bis-{3-[2,4,6-trimethylphenyl]pyrazol-1-yl}pyridine [L1Mes]) and [M(L2)2](BF4)2 (M=Ni, Co; L2=2-{3-[2,4,6-trimethylphenyl]pyrazol-1-yl}-6-{5-[2,4,6-trimethylphenyl]pyrazol-1-yl}pyridine) have been prepared. Single crystal structure determinations of [M(L1H)2](BF4)2 (M=Ni, Co) and solvates of [Ni(L1Mes)2](BF4)2, [Co(L1Mes)2](ClO4)2 and [Co(L2)2](BF4)2 all show six-coordinate metal centres with local near-D2d symmetry. The L1Mes and L2 mesityl substituents have only a small effect on the MN{pyrazole} (M=Ni, Co) bond lengths in these compounds. The d–d spectra of the complexes show that L1Mes is a significantly better donor ligand than L1H, L1Pri or L1Ph, and that L1Pri is a weaker ligand than might be expected purely on inductive grounds. A combination of UV–Vis/NIR, EPR, NMR and magnetic measurements have demonstrated that all the Co(II) compounds are high-spin in the solid state and in solution at 290 K.

Helical geometry and liquid crystalline properties of 2,3,6,7,10,11-hexaalkoxy-1-nitrotriphenylenes

The single-crystal X-ray structure of 2,3,6,7,10,11-hexaethoxy-1-nitrotriphenylene confirms earlier calculations of the molecular geometry and shows that the α-nitro substituent imparts a helical twist to the triphenylene nucleus. Within the crystal, the molecules are arranged in tilted columns on an oblique lattice, and along the columns the molecular dipoles are arranged antiferroelectrically, with a molecule–molecule dipole–dipole interaction of ca. −5.5 kJ mol−1. It is suggested that this interaction may be important in stabilising the mesophase. Eleven other 2,3,6,7,10,11-hexaalkoxytriphenylenes and their mononitro derivatives (with side chains ranging from –OC2H5 to –OC16H33) have been prepared and characterised. Most of the α-nitrated compounds give enantiotropic columnar mesophases that show wider mesophase ranges than the precursor aryl ethers. For the systems with side chains from –OC4H9 to –OC11H23, α-nitration increases the mesophase range (relative to the simple aryl ether), and for the systems with –OC12H25, –OC14H29, and –OC16H33, α-nitration induces liquid crystal behaviour (the simple aryl ethers analogues being non-mesogenic). Although it had previously been claimed that 2,3,6,7,10,11-hexapropoxytriphenylene is not mesogenic, we found that it exhibits a plastic columnar phase between 100 and 175 °C.

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.

Do Glycosyl Sulfonium Ions Engage in Neighbouring-Group Participation? A Study of Oxathiane Glycosyl Donors and the Basis for their Stereoselectivity

Neighbouring-group participation has long been used to control the synthesis of 1,2-trans-glycosides. More recently there has been a growing interest in the development of similar strategies for the synthesis of 1,2-cis-glycosides, in particular the use of auxiliary groups that generate sulfonium ion intermediates. However, there has been some debate over the role of sulfonium ion intermediates in these reactions: do sulfonium ions actually engage in neighbouring-group participation, or are they a resting state of the system prior to reaction through an oxacarbenium ion intermediate? Herein, we describe the reactivities and stereoselectivities of a family of bicyclic thioglycosides in which an oxathiane ring is fused to the sugar to form a trans-decalin-like structure. A methyl sulfonium ion derived from one such glycosyl donor is so stable that it can be crystallised from ethanol, yet it reacts with complete stereoselectivity at high temperature. The importance of a ketal group in the oxathiane ring for maintaining this high stereoselectivity is investigated using a combination of experiment and ab initio calculations. The data are discussed in terms of S(N)1 and S(N)2 type mechanisms. Trends in stereoselectivity across a series of compounds are more consistent with selective addition to oxacarbenium ions rather than a shift between S(N)1 and S(N)2 mechanisms.