Spin Crossover, Reversible Redox, and Supramolecular Interactions in 3d Complexes of 4‑(4-Pyridyl)-2,5-dipyrazyl-pyridine

A new terpyridine-inspired terdentate ligand, 4-(4-pyridyl)-2,5-dipyrazyl-pyridine (<b>py-pzpypz</b>), featuring three “spare” nitrogen donors “out the back”, has been used to synthesize five bis-ligand complexes, [M^II(<b>py-pzpypz</b>)₂]­X₂, where M = Mn with X = ClO₄, or M = Fe, Co, Ni, and Zn with X = BF₄. In contrast, when M = Cu^II, regardless of the M:L ratio employed, 1:1 M:L products were obtained: for X = BF₄ a 1D chain {[Cu^II(<b>py-pzpypz</b>)­(DMF)₂]­(BF₄)₂}<i>_n</i>, and for X = Cl a monometallic complex [Cu­(<b>py-pzpypz</b>)­Cl₂]. All seven complexes were structurally characterized, confirming the expected N₆ coordination of the M^II centers in all cases except Cu^II. Notably, a Jahn–Teller elongation is observed in the Co^II complex, consistent with it being low spin at 100 K. The Cu^II 1D chain complex has an N₄O₂ coordination sphere as in this case the “spare” pyridine donor out the back of the <b>py-pzpypz</b> ligand bridges to the next Cu^II center in the chain, hence providing both a terdentate site and a monodentate pyridine to the next Cu^II center, and the coordination sphere is completed by weak axial coordination by two DMF solvent molecules. The Cu^II center in the monometallic complex has an N₃Cl₂ square pyramidal coordination sphere. In all cases, the noncoordinating, “spare”, pyrazine nitrogen atoms are involved in interesting intermolecular interactions, including N_Pz–π interactions and nonclassical C–H···N_Pz hydrogen bonding. The Fe^II complex is low spin as expected. Two polymorphs of the Co^II complex were obtained, both of which showed gradual spin crossover, with a room temperature <i>T</i>_1/2. Two reversible redox processes are observed for [Co^II(<b>py-pzpypz</b>)₂]­(BF₄)₂, with <i>E</i>_m(M^I/M^II) = −0.63 V and <i>E</i>_m(M^II/M^III) = +0.37 V, and a quasireversible redox process for [Fe^II(<b>py-pzpypz</b>)₂]­(BF₄)₂, with <i>E</i>_m(M^II/M^III) = +1.26 V, versus 0.01 M AgNO₃/Ag in MeCN. These potentials are shifted to significantly higher potentials (by ∼0.45 V) than the literature values for the corresponding Fe^II and Co^II complexes of the equivalent all-pyridine ligand, consistent with replacement of the two pyridine rings by two pyrazine rings significantly stabilizing the lower oxidation states

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

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

Ligand Modifications on a Cp(quinolate)Ru Catalyst to Improve Its Stability in a Bio-orthogonal Deprotection Reaction

The deprotection or activation of substances in biological systems is of particular interest as this method can be used to activate prodrugs in a site- and time-specific manner, thus minimizing possible side effects. Investigations of the literature-known Ru catalyst [RuCp(QL)(η3-allyl)PF6] (with Cp = η-cyclopentadienide, QL= 5-(methoxycarboyl)-8-quinolinolate, 5c) revealed stability issues of the dissolved catalyst in air. We surmised that a more stable catalyst would perform better under biologically relevant conditions and that classical modifications in the ligand set would affect such improved properties. In this work, a systematic study is reported to modify the Cp ligand by using Cp* (Cp* = η-pentamethyl-cyclopentadienide), trimethylsilyl Cp, or t-butyl Cp instead and on the allyl ligand by introducing a methyl group at the middle carbon of 5c. Periodical 1H NMR measurements in DMSO-d6 were performed to monitor the stability of the complexes for longer periods in air, and the catalytic activity of the synthesized compounds was investigated by the deprotection of an alloxycarbonyl (alloc)-protected fluorescent coumarin dye, as monitored by an increase in fluorescence intensity. Modification of the allyl ligand had no effect on the stability, but modification of the Cp ligand was shown to affect the stability of the dissolved complex and, in the case of Cp*, significantly prolong it. As expected, the more stable catalysts are catalytically active for a longer period, but as the reaction rate is not as fast, slightly lower or similar overall yields as compared to the original complex were achieved. Preliminary MTT testing of the obtained complexes revealed IC50 values in the low micromolar range

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

A novel copper­(I) metal–organic framework (MOF), {[Cu^I₂(py-pzpypz)₂(μ-CN)₂]·MeCN}_<i>n</i> (<b>1</b>·MeCN), with an unusual topology is shown to be robust, retaining crystallinity during desolvation to give <b>1</b>, which has also been structurally characterized [py-pzpypz is 4-(4-pyridyl)-2,5-dipyrazylpyridine)]. Zigzag-shaped channels, which in <b>1</b>·MeCN were occupied by disordered MeCN molecules, run along the <i>c</i> axis of <b>1</b>, resulting in a significant solvent-accessible void space (9.6% of the unit cell volume). These tight zigzags, bordered by (Cu^ICN)_<i>n</i> chains, make <b>1</b> an ideal candidate for investigations into shape-based selectivity. MOF <b>1</b> 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, <b>1</b> 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

Spin Crossover in Dinuclear N₄S₂ Iron(II) Thioether–Triazole Complexes: Access to [HS-HS], [HS-LS], and [LS-LS] States

Access to a new family of <i>thioether</i>-linked <b>PSRT</b> ligands, 4-substituted-3,5-bis­{[(2-pyridylmethyl)­sulfanyl]­methyl}-4<i>H</i>-1,2,4-triazoles (analogues of the previously studied amino-linked <b>PMRT</b> ligands), has been established. Four such ligands have been prepared, <b>PSPhT</b>, <b>PS</b>^{<i><b>i</b></i>}<b>BuT</b>, <b>PS</b>^{<i><b>t</b></i><b>‑Bu</b>}<b>PhT</b>, and <b>PS</b>^<b>Me</b><b>PhT</b>, with <b>R</b> = Ph, ^<i>i</i>Bu, ^{<i>t</i>‑Bu}Ph, and ^MePh, respectively. Three dinuclear colorless to pale green iron­(II) complexes, [Fe^II₂(<b>PSRT</b>)₂]­(BF₄)₄·solvent, featuring N₄S₂ donor sets, were prepared. Single-crystal structure determinations on [Fe^II₂(<b>PSPhT</b>)₂]­(BF₄)₄·2MeCN·H₂O, [Fe^II₂(<b>PSPhT</b>)₂]­(BF₄)₄·2¹/₂MeCN·¹/₂H₂O·THF, [Fe^II₂­(<b>PS</b>^<b>Me</b><b>PhT</b>)₂]­(BF₄)₄·2MeCN, and [Fe^II₂(<b>PS</b>^{<i><b>i</b></i>}<b>BuT</b>)₂]­(BF₄)₄·4MeCN reveal that all four are stabilized in the [HS-HS] state to 100 K and that both possible binding modes of the bis-terdentate ligands, <i>cis</i>- and <i>trans</i>-axial, are observed. Variable-temperature magnetic susceptibility studies of air-dried crystals (solvatomorphs of the single crystal samples) reveal the first examples of spin crossover (SCO) for a dinuclear iron­(II) complex with N₄S₂ coordination. Specifically, [Fe^II₂(<b>PSPhT</b>)₂]­(BF₄)₄·2¹/₂H₂O undergoes a multistep but complete SCO from [HS-HS] to [LS-LS], whereas [Fe^II₂(<b>PS</b>^<b>Me</b><b>PhT</b>)₂]­(BF₄)₄·1¹/₂MeCN·2H₂O exhibits a half-SCO from [HS-HS] to [HS-LS]. In contrast, [Fe^II₂(<b>PS</b>^{<i><b>i</b></i>}<b>BuT</b>)₂]­(BF₄)₄·MeCN·H₂O remains [HS-HS] down to 50 K. The reflectance spectrum of pale green [Fe^II₂(<b>PSPhT</b>)₂]­(BF₄)₄·¹/₂CHCl₃·2¹/₂H₂O (solvatomorph A) reveals a trace of LS character (572 nm band ¹A_1g → ¹T_1g). Evans’ ¹H NMR method and UV–vis spectroscopy studies revealed that on cooling dark green acetonitrile solutions of these complexes from 313 to 233 K, all three undergo SCO centered at or near room temperature. The tendency of the complexes to go LS in solution reflects the electronic impact of <b>R</b> on the σ-donor strength of the <b>PSRT</b> ligand, whereas the opposite trend in stabilization of the LS state is seen in the solid state, where crystal packing effects, of the <b>R</b> group and solvent content, dominate the SCO behavior

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) [<b>1</b>·2­(H₂O)] and [Ni^II(dpzca)₂Ag]­BF₄·0.5­(acetone) [<b>2</b>·0.5­(acetone)]. Despite the difference in charge distribution and anion loading, the framework structures of <b>1</b>·2­(H₂O) and <b>2</b>·0.5­(acetone) are isostructural. One dimensional channels along the <i>b</i>-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 <b>1</b> and <b>2</b> 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 <b>1</b> relative to Ni^II-based MOF <b>2</b> results in increased selectivity for CO₂ over N₂ but a decrease in the sorption kinetics and storage capacity of the framework