Detailed EPR Study of Spin Crossover Dendrimeric Iron(III) Complex

The unusual magnetic behavior of the first dendritic Fe³⁺ complex with general formula [Fe­(L)₂]⁺Cl^–·H₂O based on a branched Schiff base has been investigated by electron paramagnetic resonance (EPR) and Mössbauer spectroscopy. EPR displays that complex consists of the three types of magnetically active iron centers: one <i>S</i> = 1/2 low-spin (LS) and two <i>S</i> = 5/2 high-spin (HS) centers with strong low-symmetry and weak distorted octahedral crystal fields. Analysis of the magnetic behavior reflected by <i>I</i> versus <i>T</i> (where <i>I</i> is the EPR lines integrated intensity of the spectrum) demonstrates that the dendritic Fe³⁺ complex has sufficiently different behavior in three temperature intervals. The first (4.2–50 K) interval corresponds to the antiferromagnetic exchange interactions between LS–LS, LS–HS, and HS–HS centers. The appearance of a presumable magnetoelectric effect is registered in the second (50–200 K) temperature interval, whereas a spin transition process between LS and HS centers occurs in the third (200–330 K) one. The coexistence of the magnetic ordering, presumable magnetoelectric effect, and spin crossover in one and the same material has been detected for the first time. The Mössbauer spectroscopy data completely confirm the EPR results

Ytterbium and Europium Complexes of Redox-Active Ligands: Searching for Redox Isomerism

The reaction of (dpp-Bian)­Eu^II(dme)₂ (<b>3</b>) (dpp-Bian is dianion of 1,2-bis­[(2,6-diisopropylphenyl)­imino]­acenaphthene; dme is 1,2-dimethoxyethane) with 2,2′-bipyridine (bipy) in toluene proceeds with replacement of the coordinated solvent molecules with neutral bipy ligands and affords europium­(II) complex (dpp-Bian)­Eu^II(bipy)₂ (<b>9</b>). In contrast the reaction of related ytterbium complex (dpp-Bian)­Yb^II(dme)₂ (<b>4</b>) with bipy in dme proceeds with the electron transfer from the metal to bipy and results in (dpp-Bian)­Yb^III(bipy)­(bipy^–̇) (<b>10</b>) – ytterbium­(III) derivative containing both neutral and radical-anionic bipy ligands. Noteworthy, in both cases dianionic dpp-Bian ligands retain its reduction state. The ligand-centered redox-process occurs when complex <b>3</b> reacts with <i>N</i>,<i>N</i>′-bis­[2,4,6-trimethylphenyl]-1,4-diaza-1,3-butadiene (mes-dad). The reaction product (dpp-Bian)­Eu^II­(mes-dad)­(dme) (<b>11</b>) consists of two different redox-active ligands both in the radical-anionic state. The reduction of 3,6-di-<i>tert</i>-butyl-4-(3,6-di-<i>tert</i>-butyl-2-ethoxyphenoxy)-2-ethoxycyclohexa-2,5-dienone (the dimer of 2-ethoxy-3,6-di-<i>tert</i>-butylphenoxy radical) with (dpp-Bian)­Eu^II(dme)₂ (<b>3</b>) caused oxidation of the dpp-Bian ligand to radical-anion to afford (dpp-Bian)­(ArO)­Eu^II(dme) (ArO = OC₆H₂-3,6-<i>t</i>Bu₂-2-OEt) (<b>12</b>). The molecular structures of complexes <b>9</b>–<b>12</b> have been established by the single crystal X-ray analysis. The magnetic behavior of newly prepared compounds has been investigated by the SQUID technique in the range 2–310 K. The isotropic exchange model has been adopted to describe quantitatively the magnetic properties of the exchange-coupled europium­(II) complexes (<b>11</b> and <b>12</b>). The best-fit isotropic exchange parameters are in good agreement with their density functional theory-computed counterparts

Crucial Role of Paramagnetic Ligands for Magnetostructural Anomalies in “Breathing Crystals”

Breathing crystals based on polymer-chain complexes of Cu­(hfac)₂ with nitroxides exhibit thermally and light-induced magnetostructural anomalies in many aspects similar to a spin crossover. In the present work, we report the synthesis and investigation of a new family of Cu­(hfac)₂ complexes with <i>tert</i>-butylpyrazolylnitroxides and their nonradical structural analogues. The complexes with paramagnetic ligands clearly exhibit structural rearrangements in the copper­(II) coordination units and accompanying magnetic phenomena characteristic for breathing crystals. Contrary to that, their structural analogues with diamagnetic ligands do not undergo rearrangements in the copper­(II) coordination environments. This confirms experimentally the crucial role of paramagnetic ligands and exchange interactions between them and copper­(II) ions for the origin of magnetostructural anomalies in this family of molecular magnets

Crucial Role of Paramagnetic Ligands for Magnetostructural Anomalies in “Breathing Crystals”

Breathing crystals based on polymer-chain complexes of Cu­(hfac)₂ with nitroxides exhibit thermally and light-induced magnetostructural anomalies in many aspects similar to a spin crossover. In the present work, we report the synthesis and investigation of a new family of Cu­(hfac)₂ complexes with <i>tert</i>-butylpyrazolylnitroxides and their nonradical structural analogues. The complexes with paramagnetic ligands clearly exhibit structural rearrangements in the copper­(II) coordination units and accompanying magnetic phenomena characteristic for breathing crystals. Contrary to that, their structural analogues with diamagnetic ligands do not undergo rearrangements in the copper­(II) coordination environments. This confirms experimentally the crucial role of paramagnetic ligands and exchange interactions between them and copper­(II) ions for the origin of magnetostructural anomalies in this family of molecular magnets

Crucial Role of Paramagnetic Ligands for Magnetostructural Anomalies in “Breathing Crystals”

Breathing crystals based on polymer-chain complexes of Cu­(hfac)₂ with nitroxides exhibit thermally and light-induced magnetostructural anomalies in many aspects similar to a spin crossover. In the present work, we report the synthesis and investigation of a new family of Cu­(hfac)₂ complexes with <i>tert</i>-butylpyrazolylnitroxides and their nonradical structural analogues. The complexes with paramagnetic ligands clearly exhibit structural rearrangements in the copper­(II) coordination units and accompanying magnetic phenomena characteristic for breathing crystals. Contrary to that, their structural analogues with diamagnetic ligands do not undergo rearrangements in the copper­(II) coordination environments. This confirms experimentally the crucial role of paramagnetic ligands and exchange interactions between them and copper­(II) ions for the origin of magnetostructural anomalies in this family of molecular magnets