Crossing the Traditional Boundaries: Salen-Based Schiff Bases for Thermal Protective Applications

A broad spectrum of applications of “Salen”-based Schiff bases tagged them as versatile multifunctional materials. However, their applicability is often bounded by a temperature threshold and, thus, they have rarely been used for high temperature applications. Our investigation of a classical Schiff base, <i>N</i>,<i>N</i>′-bis­(4-hydroxysalicylidene)­ethylenediamine (L2), reveals that it displays an intriguingly combative response to an elevated temperature/fire scenario. L2 resists and regulates thermal degradation by forming an ablative surface, and acts as a thermal shield. A polycondensation via covalent cross-linking, which forms a hyperbranched cross-linked resin is found to constitute the origin of the ablative surface. This is a unique example of a resin formation produced with a Schiff base, that mimicks the operational strategy of a high-heat resistant phenolic resin. Further applicability of L2, as a flame retardant, was tested in an engineering polymer, polyamide-6. It was found that it reinforces the polymer against fire risks by the formation of an intumescent coating. This paves the way for a new strategic avenue in safeguarding polymeric materials toward fire risks. Further, this material represents a promising start for thermal protective applications

Fe^II Spin Transition Materials Including an Amino–Ester 1,2,4-Triazole Derivative, Operating at, below, and above Room Temperature

A new family of one-dimensional Fe^II 1,2,4-triazole spin transition coordination polymers for which a modification of anion and crystallization solvent can tune the switching temperature over a wide range, including the room temperature region, is reported. This series of materials was prepared as powders after reaction of ethyl-4<i>H</i>-1,2,4-triazol-4-yl-acetate (αEtGlytrz) with an iron salt from a MeOH/H₂O medium affording: [Fe­(αEtGlytrz)₃]­(ClO₄)₂ (<b>1</b>); [Fe­(αEtGlytrz)₃]­(ClO₄)₂·CH₃OH (<b>2</b>); [Fe­(αEtGlytrz)₃]­(NO₃)₂·H₂O (<b>3</b>); [Fe­(αEtGlytrz)₃]­(NO₃)₂ (<b>4</b>); [Fe­(αEtGlytrz)₃]­(BF₄)₂·0.5H₂O (<b>5</b>); [Fe­(αEtGlytrz)₃]­(BF₄)₂ (<b>6</b>); and [Fe­(αEtGlytrz)₃]­(CF₃SO₃)₂·2H₂O (<b>7</b>). Their spin transition properties were investigated by ⁵⁷Fe Mossbauer spectroscopy, superconducting quantum interference device (SQUID) magnetometry, and differential scanning calorimetry (DSC). The temperature dependence of the high-spin molar fraction derived from ⁵⁷Fe Mössbauer spectroscopy in <b>1</b> reveals an abrupt single step transition between low-spin and high-spin states with a hysteresis loop of width 5 K (<i>T</i>_c^↑ = 296 K and <i>T</i>_c^↓ = 291 K). The properties drastically change with modification of anion and/or lattice solvent. The transition temperatures, deduced by SQUID magnetometry, shift to <i>T</i>_c^↑ = 273 K and <i>T</i>_c^↓ = 263 K for (<b>2</b>), <i>T</i>_c^↑ = 353 K and <i>T</i>_c^↓ = 333 K for (<b>3</b>), <i>T</i>_c^↑ = 338 K and <i>T</i>_c^↓ = 278 K for (<b>4</b>), <i>T</i>^↑ = 320 K and <i>T</i>^↓ = 305 K for (<b>5</b>), <i>T</i>_c^↑ = 106 K and <i>T</i>_c^↓ = 92 K for (<b>6</b>), and <i>T</i>^↑ = 325 K and <i>T</i>^↓ = 322 K for (<b>7</b>). Annealing experiments of <b>3</b> lead to a change of the morphology, texture, and magnetic properties of the sample. A dehydration/rehydration process associated with a spin state change was analyzed by a mean-field macroscopic master equation using a two-level Hamiltonian Ising-like model for <b>3</b>. A new structural-property relationship was also identified for this series of materials [Fe­(αEtGlytrz)₃]­(anion)₂·<i>n</i>Solvent based on Mössbauer and DSC measurements. The entropy gap associated with the spin transition and the volume of the inserted counteranion shows a linear trend, with decrease in entropy with increasing the size of the counteranion. The first materials of this substance class to display a complete spin transition in both spin states are also presented

Salen Complexes as Fire Protective Agents for Thermoplastic Polyurethane: Deep Electron Paramagnetic Resonance Spectroscopy Investigation

The contribution of copper complexes of <i>salen</i>-based Schiff bases <i>N</i>,<i>N</i>′-bis­(salicylidene)­ethylenediamine (C1), <i>N</i>,<i>N</i>′-bis­(4-hydroxysalicylidene)­ethylenediamine (C2), and <i>N</i>,<i>N</i>′-bis­(5-hydroxysalicylidene)­ethylenediamine (C3) to the flame retardancy of thermoplastic polyurethane (TPU) is investigated in the context of minimizing the inherent flammability of TPU. Thermal and fire properties of TPU are evaluated. It is observed that fire performances vary depending upon the substitution of the salen framework. Cone calorimetry [mass loss calorimetry (MLC)] results show that, in TPU at 10 wt % loading, C2 and C3 reduce the peak of heat release rate by 46 and 50%, respectively. At high temperature, these copper complexes undergo polycondensation leading to resorcinol-type resin in the condensed phase and thus acting as intumescence reinforcing agents. C3 in TPU is particularly interesting because it delays significantly the time to ignition (MLC experiment). In addition, pyrolysis combustion flow calorimetry shows reduction in the heat release rate curve, suggesting its involvement in gas-phase action. Structural changes of copper complexes and radical formation during thermal treatment as well as their influence on fire retardancy of TPU in the condensed phase are investigated by spectroscopic studies supported by microscopic and powder diffraction studies. Electron paramagnetic resonance (EPR) spectroscopy was fully used to follow the redox changes of Cu­(II) ions as well as radical formation of copper complexes/TPU formulations in their degradation pathways. Pulsed EPR technique of hyperfine sublevel correlation spectroscopy reveals evolution of the local surrounding of copper and radicals with a strong contribution of nitrogen fragments in the degradation products. Further, the spin state of radicals was investigated by the two-dimensional technique of phase-inverted echo-amplitude detected nutation experiment. Two different radicals were detected, that is, one monocarbon radical and an oxygen biradical. Thus, the EPR study permits to deeply investigate the mode of action of copper salen complexes in TPU

Selective and Reusable Iron(II)-Based Molecular Sensor for the Vapor-Phase Detection of Alcohols

A mononuclear iron­(II) neutral complex (<b>1</b>) is screened for sensing abilities for a wide spectrum of chemicals and to evaluate the response function toward physical perturbation like temperature and mechanical stress. Interestingly, <b>1</b> precisely detects methanol among an alcohol series. The sensing process is visually detectable, fatigue-resistant, highly selective, and reusable. The sensing ability is attributed to molecular sieving and subsequent spin-state change of iron centers, after a crystal-to-crystal transformation