Proton-Coupled Electron Transfer in Ferrocenium–Phenolate Radicals

Electron and proton transfer (ET, PT) can be intimately coupled, provided suitable redox and acid/base sites are available. The amide-linked ferrocene–phenol <b>H-1</b> is deprotonated to the phenolate <b>[1]</b>^<b>–</b> by phosphazene bases and oxidized to the ferrocenium ion <b>[H-1]</b>^<b>+</b> by silver hexafluoroantimonate. Concomitant oxidation and deprotonation yields the radical <b>[1]</b>^<b>•</b>, featuring a characteristic near-IR absorption band. The ground state of <b>[1]</b>^<b>•</b> is best described as the ferrocenium–phenolate zwitterion <b>[1b]</b>^<b>•</b> with a dynamic dissymmetric N···H···O hydrogen bond (PT). The ferrocenium–iminolate N···H–O tautomer <b>[1b]</b>^<b>•</b><b>-NHO′</b> can undergo a thermal structural rearrangement to the high-energy OH···O tautomer <b>[1b]</b>^<b>•</b><b>-OHO</b>, while the amide–phenolate N–H···O tautomer <b>[1b]</b>^<b>•</b><b>-NHO</b> is poised to optical electron transfer to yield the ferrocene–phenoxyl valence isomer <b>[1a]</b>^<b>•</b><b>-NHO</b> (<i>E</i>_op = 1.18–1.19 eV)

Novel Iron(II) Microporous Spin-Crossover Coordination Polymers with Enhanced Pore Size

In this Communication, we report the synthesis and characterization of novel Hofmann-like spin-crossover porous coordination polymers of composition {Fe­(L)­[M­(CN)₄]}·G [L = 1,4-bis­(4-pyridylethynyl)­benzene and M^II = Ni, Pd, and Pt]. The spin-crossover properties of the framework are closely related to the number and nature of the guest molecules included in the pores

Novel Iron(II) Microporous Spin-Crossover Coordination Polymers with Enhanced Pore Size

In this Communication, we report the synthesis and characterization of novel Hofmann-like spin-crossover porous coordination polymers of composition {Fe­(L)­[M­(CN)₄]}·G [L = 1,4-bis­(4-pyridylethynyl)­benzene and M^II = Ni, Pd, and Pt]. The spin-crossover properties of the framework are closely related to the number and nature of the guest molecules included in the pores

Synthesis of Nanocrystals and Particle Size Effects Studies on the Thermally Induced Spin Transition of the Model Spin Crossover Compound [Fe(phen)₂(NCS)₂]

Surfactant-free nanocrystals of the model spin-crossover compound [Fe­(phen)₂(NCS)₂] (phen: 1,10-phenanthroline) have been synthesized applying the reverse micelle technique. The morphology of the nanocrystals, characterized by scanning electronic microscopy, corresponds to rhombohedric platelets with dimensions ranging from 203 × 203 × 106 nm to 142 × 142 × 74 nm. Variation of the concentration of the Fe­(BF₄)₂·6H₂O salt in the synthesis has been found to have little influence on the crystallite size. In contrast, the solvent–surfactant ratio (ω) is critical for a good particle growth. The spin transition of the nanocrystals has been characterized by magnetic susceptibility measurements and Mössbauer spectroscopy. The nanocrystals undergo an abrupt and more cooperative spin transition in comparison with the bulk compound. The spin transition is centered in the interval of temperature of 175–185 K and is accompanied by 8 K of thermal hysteresis width. The crystallite quality more than the crystallite size is responsible for the higher cooperativity. The magnetic properties of the nanocrystals embedded in organic polymers such as polyethylene glycol, nujol, glycerol, and triton have been studied as well. The spin transition in the nanocrystals is affected by the polymer coating. The abrupt and first-order spin transition transforms into a more continuous spin transition as a result of the chemical pressure asserted by the organic polymers on the Fe­(II) centers

Spin Crossover Star-Shaped Metallomesogens of Iron(II)

Three new types of spin crossover (SCO) metallomesogens of Fe^II based on symmetric tripod ligands and their magnetic and structural properties are reported here. These were obtained by condensation of tris­(2-aminoethyl)­amin (tren) with the aldehyde derived from 3-alkoxy-6-methylpyridine (<b>mpyN</b>, N (number of carbon atoms in <i>n</i>-alkyl chains) = 8, 18), 1-alkyl-1<i>H</i>-imidazole (<b>imN</b>, N = 4, 16, 18, 20, 22), or 1-alkyl-1<i>H</i>-benzimidazole (<b>bimN</b>, N = 6, 14, 16, 18, 20). A complex derived from 1-octadecyl-1<i>H</i>-naphtho­[2,3-<i>d</i>]­imidazole (<b>nim18</b>) retains the high spin state at any temperature. Single crystals of the short-chain complexes were investigated by a combination of X-ray crystallography, magnetic measurements and Mössbauer spectroscopy. Generally, in comparison with the short-chain complexes the long-chain complexes display more gradual SCO and undergo a phase transition crystal–liquid crystal that is reflected in their magnetic properties. Characterization by X-ray powder diffractometry and differential calorimetry reveal formation of a smectic mesophase upon melting

<i>Screw</i>-<i>Type</i> Motion and Its Impact on Cooperativity in BaNa₂Fe[VO₄]₂

BaNa₂Fe­[VO₄]₂ contains a Jahn–Teller active ion (Fe^II, 3d⁶, high-spin) in an octahedral coordination. On the basis of a combination of temperature-dependent X-ray diffraction and Mössbauer and Raman spectroscopies, we demonstrate the coupling of lattice dynamics with the electronic ground state of Fe^II. We identify three lattice modes combined to an effective canted <i>screw</i>-<i>type</i> motion that drives the structural transition around room temperature from the high-temperature (<i>P</i>3̅) via intermediate phases to the low-temperature phase (<i>C</i>2/<i>c</i>). The dynamics of the electronic ground state of Fe­(II) are evident from Mössbauer data with signatures of a motion-narrowed doublet above 320 K, a gradual evolution of the ⁵E_g electronic state below 293 K, and finally the signature of the thermodynamically preferred orbitally nondegenerate ground state (⁵A_g) of Fe­(II) below 100 K. The continuous nature of the transition is associated with the temperature-dependent phonon parameters derived from Raman spectroscopy, which point out the presence of strong electron–phonon coupling in this compound. We present a microscopic mechanism and evaluate the collective component leading to the structural phase transition

Spin Crossover in Fe(II)–M(II) Cyanoheterobimetallic Frameworks (M = Ni, Pd, Pt) with 2‑Substituted Pyrazines

Discovery of spin-crossover (SCO) behavior in the family of Fe^II-based Hofmann clathrates has led to a “new rush” in the field of bistable molecular materials. To date this class of SCO complexes is represented by several dozens of individual compounds, and areas of their potential application steadily increase. Starting from Fe²⁺, square planar tetracyanometalates M^II(CN)₄^2– (M^II = Ni, Pd, Pt) and 2-substituted pyrazines Xpz (X = Cl, Me, I) as coligands we obtained a series of nine new Hofmann clathrate-like coordination frameworks. X-ray diffraction reveals that in these complexes Fe^II ion has a pseudo-octahedral coordination environment supported by four μ₄-tetracyanometallates forming its equatorial coordination environment. Depending on the nature of X and M, axial positions are occupied by two 2X-pyrazines (X = Cl and M^II = Ni (<b>1</b>), Pd (<b>2</b>), Pt (<b>3</b>); X = Me and M^II = Ni (<b>4</b>), Pd (<b>5</b>)) or one 2X-pyrazine and one water molecule (X = I and M^II = Ni (<b>7</b>), Pd (<b>8</b>), Pt (<b>9</b>)), or, alternatively, two distinct Fe^II positions with either two pyrazines or two water molecules (X = Me and M^II = Pt (<b>6</b>)) are observed. Temperature behavior of magnetic susceptibility indicates that all compounds bearing FeN₆ units (<b>1</b>–<b>6</b>) display cooperative spin transition, while Fe^II ions in N₅O or N₄O₂ surrounding are high spin (HS). Structural changes in the nearest Fe^II environment upon low-spin (LS) to HS transition, which include ca. 10% Fe–N distance increase, lead to the cell expansion. Mössbauer spectroscopy is used to characterize the spin state of all HS, LS, and intermediate phases of <b>1</b>–<b>9</b> (see abstract figure). Effects of a pyrazine substituent and M^II nature on the hyperfine parameters in both spin states are established

Spin Crossover in Fe(II)–M(II) Cyanoheterobimetallic Frameworks (M = Ni, Pd, Pt) with 2‑Substituted Pyrazines

Discovery of spin-crossover (SCO) behavior in the family of Fe^II-based Hofmann clathrates has led to a “new rush” in the field of bistable molecular materials. To date this class of SCO complexes is represented by several dozens of individual compounds, and areas of their potential application steadily increase. Starting from Fe²⁺, square planar tetracyanometalates M^II(CN)₄^2– (M^II = Ni, Pd, Pt) and 2-substituted pyrazines Xpz (X = Cl, Me, I) as coligands we obtained a series of nine new Hofmann clathrate-like coordination frameworks. X-ray diffraction reveals that in these complexes Fe^II ion has a pseudo-octahedral coordination environment supported by four μ₄-tetracyanometallates forming its equatorial coordination environment. Depending on the nature of X and M, axial positions are occupied by two 2X-pyrazines (X = Cl and M^II = Ni (<b>1</b>), Pd (<b>2</b>), Pt (<b>3</b>); X = Me and M^II = Ni (<b>4</b>), Pd (<b>5</b>)) or one 2X-pyrazine and one water molecule (X = I and M^II = Ni (<b>7</b>), Pd (<b>8</b>), Pt (<b>9</b>)), or, alternatively, two distinct Fe^II positions with either two pyrazines or two water molecules (X = Me and M^II = Pt (<b>6</b>)) are observed. Temperature behavior of magnetic susceptibility indicates that all compounds bearing FeN₆ units (<b>1</b>–<b>6</b>) display cooperative spin transition, while Fe^II ions in N₅O or N₄O₂ surrounding are high spin (HS). Structural changes in the nearest Fe^II environment upon low-spin (LS) to HS transition, which include ca. 10% Fe–N distance increase, lead to the cell expansion. Mössbauer spectroscopy is used to characterize the spin state of all HS, LS, and intermediate phases of <b>1</b>–<b>9</b> (see abstract figure). Effects of a pyrazine substituent and M^II nature on the hyperfine parameters in both spin states are established

Spin Crossover in Fe(II)–M(II) Cyanoheterobimetallic Frameworks (M = Ni, Pd, Pt) with 2‑Substituted Pyrazines

Discovery of spin-crossover (SCO) behavior in the family of Fe^II-based Hofmann clathrates has led to a “new rush” in the field of bistable molecular materials. To date this class of SCO complexes is represented by several dozens of individual compounds, and areas of their potential application steadily increase. Starting from Fe²⁺, square planar tetracyanometalates M^II(CN)₄^2– (M^II = Ni, Pd, Pt) and 2-substituted pyrazines Xpz (X = Cl, Me, I) as coligands we obtained a series of nine new Hofmann clathrate-like coordination frameworks. X-ray diffraction reveals that in these complexes Fe^II ion has a pseudo-octahedral coordination environment supported by four μ₄-tetracyanometallates forming its equatorial coordination environment. Depending on the nature of X and M, axial positions are occupied by two 2X-pyrazines (X = Cl and M^II = Ni (<b>1</b>), Pd (<b>2</b>), Pt (<b>3</b>); X = Me and M^II = Ni (<b>4</b>), Pd (<b>5</b>)) or one 2X-pyrazine and one water molecule (X = I and M^II = Ni (<b>7</b>), Pd (<b>8</b>), Pt (<b>9</b>)), or, alternatively, two distinct Fe^II positions with either two pyrazines or two water molecules (X = Me and M^II = Pt (<b>6</b>)) are observed. Temperature behavior of magnetic susceptibility indicates that all compounds bearing FeN₆ units (<b>1</b>–<b>6</b>) display cooperative spin transition, while Fe^II ions in N₅O or N₄O₂ surrounding are high spin (HS). Structural changes in the nearest Fe^II environment upon low-spin (LS) to HS transition, which include ca. 10% Fe–N distance increase, lead to the cell expansion. Mössbauer spectroscopy is used to characterize the spin state of all HS, LS, and intermediate phases of <b>1</b>–<b>9</b> (see abstract figure). Effects of a pyrazine substituent and M^II nature on the hyperfine parameters in both spin states are established

Spin Crossover in Fe(II)–M(II) Cyanoheterobimetallic Frameworks (M = Ni, Pd, Pt) with 2‑Substituted Pyrazines

Discovery of spin-crossover (SCO) behavior in the family of Fe^II-based Hofmann clathrates has led to a “new rush” in the field of bistable molecular materials. To date this class of SCO complexes is represented by several dozens of individual compounds, and areas of their potential application steadily increase. Starting from Fe²⁺, square planar tetracyanometalates M^II(CN)₄^2– (M^II = Ni, Pd, Pt) and 2-substituted pyrazines Xpz (X = Cl, Me, I) as coligands we obtained a series of nine new Hofmann clathrate-like coordination frameworks. X-ray diffraction reveals that in these complexes Fe^II ion has a pseudo-octahedral coordination environment supported by four μ₄-tetracyanometallates forming its equatorial coordination environment. Depending on the nature of X and M, axial positions are occupied by two 2X-pyrazines (X = Cl and M^II = Ni (<b>1</b>), Pd (<b>2</b>), Pt (<b>3</b>); X = Me and M^II = Ni (<b>4</b>), Pd (<b>5</b>)) or one 2X-pyrazine and one water molecule (X = I and M^II = Ni (<b>7</b>), Pd (<b>8</b>), Pt (<b>9</b>)), or, alternatively, two distinct Fe^II positions with either two pyrazines or two water molecules (X = Me and M^II = Pt (<b>6</b>)) are observed. Temperature behavior of magnetic susceptibility indicates that all compounds bearing FeN₆ units (<b>1</b>–<b>6</b>) display cooperative spin transition, while Fe^II ions in N₅O or N₄O₂ surrounding are high spin (HS). Structural changes in the nearest Fe^II environment upon low-spin (LS) to HS transition, which include ca. 10% Fe–N distance increase, lead to the cell expansion. Mössbauer spectroscopy is used to characterize the spin state of all HS, LS, and intermediate phases of <b>1</b>–<b>9</b> (see abstract figure). Effects of a pyrazine substituent and M^II nature on the hyperfine parameters in both spin states are established