Reece G. Miller

Spin crossover with thermal hysteresis in cobalt(II) complexes and the importance of scan rate

In this perspective article, firstly the handful of cobalt(II) complexes reported to date that show abrupt spin crossover (SCO) with thermal hysteresis is reviewed, focusing on the synthesis, structures and magnetic behaviour (which includes examples of multi-step SCO and of reverse-SCO). Secondly, the abrupt and hysteretic SCO complex [CoII(dpzca)2] is used as a simple case study to illustrate some of the effects of scan-rate on hysteresis loop shape, position and width. The key message is that, whilst often overlooked, kinetic considerations are important to SCO behaviour, so the mode (settle or sweep) and scan rate used for such studies should always be reported, ideally alongside a scan rate study of the hysteresis loop region, and relaxation rate studies within it.

Selective gas adsorption in a pair of robust isostructural MOFs differing in framework charge and anion loading.

Activation of the secondary assembly instructions in the mononuclear pyrazine imide complexes [Co(III)(dpzca)2](BF4) or [Co(II)(dpzca)2] and [Ni(II)(dpzca)2] has facilitated the construction of two robust nanoporous three-dimensional coordination polymers, [Co(III)(dpzca)2Ag](BF4)2·2(H2O) [1·2(H2O)] and [Ni(II)(dpzca)2Ag]BF4·0.5(acetone) [2·0.5(acetone)]. Despite the difference in charge distribution and anion loading, the framework structures of 1·2(H2O) and 2·0.5(acetone) are isostructural. One dimensional channels along the b-axis permeate the structures and contain the tetrafluoroborate counterions (the Co(III)-based MOF has twice as many BF4(-) anions as the Ni(II)-based MOF) and guest solvent molecules. These anions are not readily exchanged whereas the solvent molecules can be reversibly removed and replaced. The H2, N2, CO2, CH4, H2O, CH3OH, and CH3CN sorption behaviors of the evacuated frameworks 1 and 2 at 298 K have been studied, and modeled, and both show very high selectivity for CO2 over N2. The increased anion loading in the channels of Co(III)-based MOF 1 relative to Ni(II)-based MOF 2 results in increased selectivity for CO2 over N2 but a decrease in the sorption kinetics and storage capacity of the framework.

Commensurate CO2 Capture, and Shape Selectivity for HCCH over H2CCH2, in Zigzag Channels of a Robust CuI(CN)(L) Metal–Organic Framework

A novel copper(I) metal-organic framework (MOF), {[Cu(I)2(py-pzpypz)2(μ-CN)2]·MeCN}n (1·MeCN), with an unusual topology is shown to be robust, retaining crystallinity during desolvation to give 1, which has also been structurally characterized [py-pzpypz is 4-(4-pyridyl)-2,5-dipyrazylpyridine)]. Zigzag-shaped channels, which in 1·MeCN were occupied by disordered MeCN molecules, run along the c axis of 1, resulting in a significant solvent-accessible void space (9.6% of the unit cell volume). These tight zigzags, bordered by (Cu(I)CN)n chains, make 1 an ideal candidate for investigations into shape-based selectivity. MOF 1 shows a moderate enthalpy of adsorption for binding CO2 (-32 kJ mol(-1) at moderate loadings), which results in a good selectivity for CO2 over N2 of 4.8:1 under real-world operating conditions of a 15:85 CO2/N2 mixture at 1 bar. Furthermore, 1 was investigated for shape-based selectivity of small hydrocarbons, revealing preferential uptake of linear acetylene gas over ethylene and methane, partially due to kinetic trapping of the guests with larger kinetic diameters.

Synthesis and characterisation of two high spin iron(II) complexes of 3,4-diphenyl-5-(2-pyridyl)-1,2,4-triazole

The stepwise synthesis of the new ligand 4,5-diphenyl-3-pyridyl-1,2,4-triazole (phppt) from N-phenyl-pyridine-2-thiocarboxamide and benzohydrazide is reported. Two iron(II) complexes, [Fe(phppt)2(SCN)2]·2MeOH and [Fe(phppt)2(SeCN)2]·Et2O, have been prepared and structurally characterised at 90 K, showing that both complexes are high spin (HS). The Mössbauer spectrum of [Fe(phppt)2(SeCN)2]·1.5MeOH is consistent with the iron(II) ion being HS at 5.2 K.

Smaller is smarter in a new cobalt(II) imide: intermolecular interactions involving pyrazine versus the larger aromatic quinoxaline

The synthesis of one symmetric and one non-symmetric ligand based on imide, quinoxaline and pyrazine moieties are reported. The symmetric ligand, N-(2-quinoxalylcarbonyl)-2-quinoxalinecarboxamide (Hquinoxquinox), is structurally characterised as Hquinoxquinox·d6DMSO. The non-symmetric ligand, N-(2-pyrazylcarbonyl)-2-quinoxalinecarboxamide (HLquinoxpz), was used to prepare the cobalt(II) complex [CoII(quinoxpz)2]·CH3OH, which was observed to remain high spin at 90 K. The structures of HLquinoxquinox·d6DMSO and [CoII(quinoxpz)2]·CH3OH are evaluated with regard to their potential to produce spin crossover (SCO) behaviour and for the construction of three-dimensional coordination polymers.