5 research outputs found
Detailed EPR Study of Spin Crossover Dendrimeric Iron(III) Complex
The
unusual magnetic behavior of the first dendritic Fe<sup>3+</sup> complex
with general formula [FeÂ(L)<sub>2</sub>]<sup>+</sup>Cl<sup>â</sup>¡H<sub>2</sub>O based on a branched Schiff base has been investigated
by electron paramagnetic resonance (EPR) and MoĚssbauer 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 Fe<sup>3+</sup> complex
has sufficiently different behavior in three temperature intervals.
The first (4.2â50 K) interval corresponds to the antiferromagnetic
exchange interactions between LSâLS, LSâHS, and HSâHS
centers. The appearance of a presumable magnetoelectric effect is
registered in the second (50â200 K) temperature interval, whereas
a spin transition process between LS and HS centers occurs in the
third (200â330 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 MoĚssbauer
spectroscopy data completely confirm the EPR results
Ytterbium and Europium Complexes of Redox-Active Ligands: Searching for Redox Isomerism
The reaction of (dpp-Bian)ÂEu<sup>II</sup>(dme)<sub>2</sub> (<b>3</b>) (dpp-Bian is dianion of
1,2-bisÂ[(2,6-diisopropylphenyl)Âimino]Âacenaphthene; dme is 1,2-dimethoxyethane)
with 2,2â˛-bipyridine (bipy) in toluene proceeds with replacement
of the coordinated solvent molecules with neutral bipy ligands and
affords europiumÂ(II) complex (dpp-Bian)ÂEu<sup>II</sup>(bipy)<sub>2</sub> (<b>9</b>). In contrast the reaction of related ytterbium
complex (dpp-Bian)ÂYb<sup>II</sup>(dme)<sub>2</sub> (<b>4</b>) with bipy in dme proceeds with the electron transfer from the metal
to bipy and results in (dpp-Bian)ÂYb<sup>III</sup>(bipy)Â(bipy<sup>â</sup>Ě) (<b>10</b>) â ytterbiumÂ(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>â˛-bisÂ[2,4,6-trimethylphenyl]-1,4-diaza-1,3-butadiene
(mes-dad). The reaction product (dpp-Bian)ÂEu<sup>II</sup>Â(mes-dad)Â(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)ÂEu<sup>II</sup>(dme)<sub>2</sub> (<b>3</b>) caused oxidation of the dpp-Bian ligand to radical-anion to afford
(dpp-Bian)Â(ArO)ÂEu<sup>II</sup>(dme) (ArO = OC<sub>6</sub>H<sub>2</sub>-3,6-<i>t</i>Bu<sub>2</sub>-2-OEt) (<b>12</b>). The molecular structures of complexes <b>9</b>â<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 2â310 K. The isotropic
exchange model has been adopted to describe quantitatively the magnetic
properties of the exchange-coupled europiumÂ(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 âBreathing Crystalsâ
Breathing crystals based on polymer-chain complexes of
CuÂ(hfac)<sub>2</sub> 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
CuÂ(hfac)<sub>2</sub> complexes with <i>tert</i>-butylpyrazolylnitroxides
and their nonradical structural analogues. The complexes with paramagnetic
ligands clearly exhibit structural rearrangements in the copperÂ(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 copperÂ(II)
coordination environments. This confirms experimentally the crucial
role of paramagnetic ligands and exchange interactions between them
and copperÂ(II) ions for the origin of magnetostructural anomalies
in this family of molecular magnets
Crucial Role of Paramagnetic Ligands for Magnetostructural Anomalies in âBreathing Crystalsâ
Breathing crystals based on polymer-chain complexes of
CuÂ(hfac)<sub>2</sub> 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
CuÂ(hfac)<sub>2</sub> complexes with <i>tert</i>-butylpyrazolylnitroxides
and their nonradical structural analogues. The complexes with paramagnetic
ligands clearly exhibit structural rearrangements in the copperÂ(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 copperÂ(II)
coordination environments. This confirms experimentally the crucial
role of paramagnetic ligands and exchange interactions between them
and copperÂ(II) ions for the origin of magnetostructural anomalies
in this family of molecular magnets
Crucial Role of Paramagnetic Ligands for Magnetostructural Anomalies in âBreathing Crystalsâ
Breathing crystals based on polymer-chain complexes of
CuÂ(hfac)<sub>2</sub> 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
CuÂ(hfac)<sub>2</sub> complexes with <i>tert</i>-butylpyrazolylnitroxides
and their nonradical structural analogues. The complexes with paramagnetic
ligands clearly exhibit structural rearrangements in the copperÂ(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 copperÂ(II)
coordination environments. This confirms experimentally the crucial
role of paramagnetic ligands and exchange interactions between them
and copperÂ(II) ions for the origin of magnetostructural anomalies
in this family of molecular magnets