18 research outputs found
Selective Activation of Chalcogen Bonding: An Efficient Structuring Tool toward Crystal Engineering Strategies
ConspectusAmong the noncovalent interactions available
in the toolbox of
crystal engineering, chalcogen bonding (ChB) has
recently entered the growing family of Ļ-hole interactions,
following the strong developments based on the halogen bonding (XB)
interaction over the last 30 years. The monovalent character of halogens
provides halogen bonding directionality and strength. Combined with
the extensive organic chemistry of Br and I derivatives, it has led
to many applications of XB, in solution (organo-catalysis, anion recognition
and transport), in the solid state (cocrystals, conducting materials,
fluorescent materials, topochemical reactions, ...), in soft matter
(liquid crystals, gels,Ā·Ā·Ā·), and in biochemistry. The
recognition of the presence of two Ļ-holes on divalent chalcogens
and the ability to activate them, as in XB, with electron-withdrawing
groups (EWG) has fueled more recent interest in chalcogen bonding.
However, despite being identified for many years, ChB still struggles
to make a mark due to (i) the underdeveloped synthetic chemistry of
heavier Se and Te; (ii) the limited stability of organic chalcogenides,
especially tellurides; and (iii) the poor predictability of ChB associated
with the presence of two Ļ-holes. It therefore invites a great
deal of attention of molecular chemists to design and develop selected
ChB donors, for the scrutiny of fundamentals of ChB and their successful
use in different applications. This Account aims to summarize our
own contributions in this direction that extend from fundamental studies
focused on addressing the lack of directionality/predictability in
ChB to a systematic demonstration of its potential, specifically in crystal engineering, and particularly toward anionic networks
on the one hand, topochemical reactions on the other hand.In
this Account, we share our recent results aimed at recovering
with ChB the same degree of strength and predictability found with
XB, by focusing on divalent Se and Te systems with two different substituents,
one of them with an EWG, to strongly unbalance both Ļ-holes.
For that purpose, we explored this dissymmetrization concept within
three chemical families, selenocyanates RāSeCN, alkynyl derivatives
RāCCā(Se/Te)Me, and o-carborane
derivatives. Such compounds were systematically engaged in cocrystals
with either halides or neutral bipyridines as ChB acceptors, revealing
their strong potential to chelate halides as well as their ability
to organize reactive molecules such as alkenes and butadiynes toward
[2+2] cycloadditions and polydiacetylene formation, respectively.
This selective activation concept is not limited to ChB but can be
effectively used on all other Ļ-hole interactions (pnictogen
bond, tetrel bond, etc.) where one needs to control the directionality
of the interaction
Electronic Effects of Ligand Substitution on Spin Crossover in a Series of Diiminoquinonoid-Bridged Fe<sup>II</sup><sub>2</sub> Complexes
A series of four isostructural Fe<sup>II</sup><sub>2</sub> complexes,
[(TPyA)<sub>2</sub>Fe<sub>2</sub>(<sup>X</sup>L)]<sup>2+</sup> (TPyA
= trisĀ(2-pyridylmethyl)Āamine; <sup>X</sup>L<sup>2ā</sup> =
doubly deprotonated form of 3,6-disubstituted-2,5-dianilino-1,4-benzoquinone;
X = H, Br, Cl, and F), were synthesized to enable a systematic study
of electronic effects on spin crossover behavior. Comparison of X-ray
diffraction data for these complexes reveals the sole presence of
high-spin Fe<sup>II</sup> at 225 K and mixtures of high-spin and low-spin
Fe<sup>II</sup> at 100 K, which is indicative of incomplete spin crossover.
In addition, crystal packing diagrams show that these complexes are
well-isolated from one another in the solid state, owing primarily
to the presence of bulky tetraĀ(aryl)Āborate counteranions, such that
spin crossover is likely not significantly affected by intermolecular
interactions. Variable-temperature dc magnetic susceptibility data
confirm the structural observations and reveal that 54(1), 56(1),
62(1), and 84(1)% of Fe<sup>II</sup> centers remain high-spin even
below 65 K. Moreover, fits to magnetic data provide crossover temperatures
of <i>T</i><sub>1/2</sub> = 160(1), 124(1), 121(1), and
110(1) K for X = H, Br, Cl, and F, respectively, along with enthalpies
of Ī<i>H</i> = 11.4(3), 8.5(3), 8.3(3), and 7.5(2)
kJ/mol, respectively. These parameters decrease with increasing electronegativity
of X and thus increasing electron-withdrawing character of <sup>X</sup>L<sup>2ā</sup>, suggesting that the observed trends originate
primarily from inductive effects of X. Moreover, when plotted as a
function of the Pauling electronegativity of X, both <i>T</i><sub>1/2</sub> and Ī<i>H</i> undergo a linear decrease.
Further analyses of the low-temperature magnetic data and variable-temperature
MoĢssbauer spectroscopy suggest that the incomplete spin crossover
behavior in [(TPyA)<sub>2</sub>Fe<sub>2</sub>(<sup>X</sup>L)]<sup>2+</sup> is best described as a transition from purely [Fe<sub>HS</sub>-Fe<sub>HS</sub>] (HS = high-spin) complexes at high temperature
to a mixture of [Fe<sub>HS</sub>-Fe<sub>HS</sub>] and [Fe<sub>HS</sub>-Fe<sub>LS</sub>] (LS = low-spin) complexes at low temperature, with
the number of [Fe<sub>HS</sub>-Fe<sub>HS</sub>] species increasing
with decreasing electron-withdrawing character of <sup>X</sup>L<sup>2ā</sup>
Electronic Effects of Ligand Substitution on Spin Crossover in a Series of Diiminoquinonoid-Bridged Fe<sup>II</sup><sub>2</sub> Complexes
A series of four isostructural Fe<sup>II</sup><sub>2</sub> complexes,
[(TPyA)<sub>2</sub>Fe<sub>2</sub>(<sup>X</sup>L)]<sup>2+</sup> (TPyA
= trisĀ(2-pyridylmethyl)Āamine; <sup>X</sup>L<sup>2ā</sup> =
doubly deprotonated form of 3,6-disubstituted-2,5-dianilino-1,4-benzoquinone;
X = H, Br, Cl, and F), were synthesized to enable a systematic study
of electronic effects on spin crossover behavior. Comparison of X-ray
diffraction data for these complexes reveals the sole presence of
high-spin Fe<sup>II</sup> at 225 K and mixtures of high-spin and low-spin
Fe<sup>II</sup> at 100 K, which is indicative of incomplete spin crossover.
In addition, crystal packing diagrams show that these complexes are
well-isolated from one another in the solid state, owing primarily
to the presence of bulky tetraĀ(aryl)Āborate counteranions, such that
spin crossover is likely not significantly affected by intermolecular
interactions. Variable-temperature dc magnetic susceptibility data
confirm the structural observations and reveal that 54(1), 56(1),
62(1), and 84(1)% of Fe<sup>II</sup> centers remain high-spin even
below 65 K. Moreover, fits to magnetic data provide crossover temperatures
of <i>T</i><sub>1/2</sub> = 160(1), 124(1), 121(1), and
110(1) K for X = H, Br, Cl, and F, respectively, along with enthalpies
of Ī<i>H</i> = 11.4(3), 8.5(3), 8.3(3), and 7.5(2)
kJ/mol, respectively. These parameters decrease with increasing electronegativity
of X and thus increasing electron-withdrawing character of <sup>X</sup>L<sup>2ā</sup>, suggesting that the observed trends originate
primarily from inductive effects of X. Moreover, when plotted as a
function of the Pauling electronegativity of X, both <i>T</i><sub>1/2</sub> and Ī<i>H</i> undergo a linear decrease.
Further analyses of the low-temperature magnetic data and variable-temperature
MoĢssbauer spectroscopy suggest that the incomplete spin crossover
behavior in [(TPyA)<sub>2</sub>Fe<sub>2</sub>(<sup>X</sup>L)]<sup>2+</sup> is best described as a transition from purely [Fe<sub>HS</sub>-Fe<sub>HS</sub>] (HS = high-spin) complexes at high temperature
to a mixture of [Fe<sub>HS</sub>-Fe<sub>HS</sub>] and [Fe<sub>HS</sub>-Fe<sub>LS</sub>] (LS = low-spin) complexes at low temperature, with
the number of [Fe<sub>HS</sub>-Fe<sub>HS</sub>] species increasing
with decreasing electron-withdrawing character of <sup>X</sup>L<sup>2ā</sup>
Mn<sup>III</sup>āFe<sup>III</sup> Heterometallic Compounds within Hydrogen-Bonded Supramolecular Networks Promoted by an [Fe(CN)<sub>5</sub>(CNH)]<sup>2ā</sup> Building Block: Structural and Magnetic Properties
The reaction of [FeĀ(CN)<sub>6</sub>]<sup>3ā</sup> and [MnĀ(acacen)]<sup>+</sup> (H<sub>2</sub>acacen = <i>N</i>,<i>N</i>ā²-bisĀ(acetylacetone)Āethylenediamine)
building units in the presence of supramolecular cations, [(F-Anil)Ā(18-crown-6)]<sup>+</sup> (F-Anil<sup>+</sup> = 3-fluoroanilinium) or [(Me-F-Anil)Ā(18-crown-6)]<sup>+</sup> (Me-F-Anil<sup>+</sup> = 3-fluoro-4-methylanilinium), affords
two new bimetallic compounds, [(F-Anil)Ā(18-crown-6)]Ā[MnĀ(acacen)ĀFeĀ(CN)<sub>5</sub>(CNH)]Ā·MeOH (<b>1</b>) and [(Me-F-Anil)Ā(18-crown-6)]Ā[MnĀ(acacen)Ā(MeOH)ĀFeĀ(CN)<sub>5</sub>(CNH)]Ā·MeOH (<b>2</b>), respectively. Compound <b>1</b> exhibits a one-dimensional topology, while compound <b>2</b> is a dinuclear discrete system due to the coordination of
a MeOH molecule at the axial position of the [MnĀ(acacen)]<sup>ā</sup> unit. For both systems, the acidity of the corresponding supramolecular
cation triggers the protonation of the Fe<sup>III</sup> moiety as
[FeĀ(CN)<sub>5</sub>(CNH)]<sup>2ā</sup>. Moreover, the resulting
āCNH ligand induces hydrogen bonding interactions connecting
the chains for <b>1</b> or the molecules for <b>2</b> into
higher dimensional supramolecular networks. Magnetic properties of
compounds incorporating these [FeĀ(CN)<sub>5</sub>(CNH)]<sup>2ā</sup> building blocks were, for the first time, thoroughly investigated,
indicating a three-dimensional antiferromagnetic order of single-chain
magnets for <b>1</b> and an antiferromagnetically interacting <i>S</i> = 3/2 spin ground state for <b>2</b>
An Azophenine Radical-Bridged Fe<sub>2</sub> Single-Molecule Magnet with Record Magnetic Exchange Coupling
One-electron reduction of the complex
[(TPyA)<sub>2</sub>Fe<sup>II</sup><sub>2</sub>(<sup>NPh</sup>L<sup>2ā</sup>)]<sup>2+</sup> (TPyA = trisĀ(2-pyridylmethyl)Āamine, <sup>NPh</sup>LH<sub>2</sub> = azophenine = <i>N</i>,<i>N</i>ā²,<i>N</i>ā³,<i>N</i>ā“-tetraphenyl-2,5-diamino-1,4-diiminobenzoquinone)
affords the complex [(TPyA)<sub>2</sub>Fe<sup>II</sup><sub>2</sub>(<sup>NPh</sup>L<sup>3āā¢</sup>)]<sup>+</sup>. X-ray
diffraction and MoĢssbauer spectroscopy confirm that the reduction
occurs on <sup>NPh</sup>L<sup>2ā</sup> to give an <i>S</i> = 1/2 radical bridging ligand. Dc magnetic susceptibility measurements
demonstrate the presence of extremely strong direct antiferromagnetic
exchange between <i>S</i> = 2 Fe<sup>II</sup> centers and <sup>NPh</sup>L<sup>3āā¢</sup> in the reduced complex, giving
an <i>S</i> = 7/2 ground state with an estimated coupling
constant magnitude of |<i>J</i>| ā„ 900 cm<sup>ā1</sup>. MoĢssbauer spectroscopy and ac magnetic susceptibility reveal
that this complex behaves as a single-molecule magnet with a spin
relaxation barrier of <i>U</i><sub>eff</sub> = 50(1) cm<sup>ā1</sup>. To our knowledge, this complex exhibits by far the
strongest magnetic exchange coupling ever to be observed in a single-molecule
magnet
An Azophenine Radical-Bridged Fe<sub>2</sub> Single-Molecule Magnet with Record Magnetic Exchange Coupling
One-electron reduction of the complex
[(TPyA)<sub>2</sub>Fe<sup>II</sup><sub>2</sub>(<sup>NPh</sup>L<sup>2ā</sup>)]<sup>2+</sup> (TPyA = trisĀ(2-pyridylmethyl)Āamine, <sup>NPh</sup>LH<sub>2</sub> = azophenine = <i>N</i>,<i>N</i>ā²,<i>N</i>ā³,<i>N</i>ā“-tetraphenyl-2,5-diamino-1,4-diiminobenzoquinone)
affords the complex [(TPyA)<sub>2</sub>Fe<sup>II</sup><sub>2</sub>(<sup>NPh</sup>L<sup>3āā¢</sup>)]<sup>+</sup>. X-ray
diffraction and MoĢssbauer spectroscopy confirm that the reduction
occurs on <sup>NPh</sup>L<sup>2ā</sup> to give an <i>S</i> = 1/2 radical bridging ligand. Dc magnetic susceptibility measurements
demonstrate the presence of extremely strong direct antiferromagnetic
exchange between <i>S</i> = 2 Fe<sup>II</sup> centers and <sup>NPh</sup>L<sup>3āā¢</sup> in the reduced complex, giving
an <i>S</i> = 7/2 ground state with an estimated coupling
constant magnitude of |<i>J</i>| ā„ 900 cm<sup>ā1</sup>. MoĢssbauer spectroscopy and ac magnetic susceptibility reveal
that this complex behaves as a single-molecule magnet with a spin
relaxation barrier of <i>U</i><sub>eff</sub> = 50(1) cm<sup>ā1</sup>. To our knowledge, this complex exhibits by far the
strongest magnetic exchange coupling ever to be observed in a single-molecule
magnet
A 2D Semiquinone Radical-Containing Microporous Magnet with Solvent-Induced Switching from <i>T</i><sub>c</sub> = 26 to 80 K
The
incorporation of tetraoxolene radical bridging ligands into
a microporous magnetic solid is demonstrated. Metalation of the redox-active
bridging ligand 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (LH<sub>2</sub>) with Fe<sup>II</sup> affords the solid (Me<sub>2</sub>NH<sub>2</sub>)<sub>2</sub>Ā[Fe<sub>2</sub>L<sub>3</sub>]Ā·ā2H<sub>2</sub>OĀ·6DMFā. Analysis of X-ray diffraction, Raman
spectra, and MoĢssbauer spectra confirm the presence of Fe<sup>III</sup> centers with mixed-valence ligands of the form (L<sub>3</sub>)<sup>8ā</sup> that result from a spontaneous electron transfer
from Fe<sup>II</sup> to L<sup>2ā</sup>. Upon removal of DMF
and H<sub>2</sub>O solvent molecules, the compound undergoes a slight
structural distortion to give the desolvated phase (Me<sub>2</sub>NH<sub>2</sub>)<sub>2</sub>Ā[Fe<sub>2</sub>L<sub>3</sub>], and
a fit to N<sub>2</sub> adsorption data of this activated compound
gives a BET surface area of 885(105) m<sup>2</sup>/g. Dc magnetic
susceptibility measurements reveal a spontaneous magnetization below
80 and 26 K for the solvated and the activated solids, respectively,
with magnetic hysteresis up to 60 and 20 K. These results highlight
the ability of redox-active tetraoxolene ligands to support the formation
of a microporous magnet and provide the first example of a structurally
characterized extended solid that contains tetraoxolene radical ligands
A 2D Semiquinone Radical-Containing Microporous Magnet with Solvent-Induced Switching from <i>T</i><sub>c</sub> = 26 to 80 K
The
incorporation of tetraoxolene radical bridging ligands into
a microporous magnetic solid is demonstrated. Metalation of the redox-active
bridging ligand 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (LH<sub>2</sub>) with Fe<sup>II</sup> affords the solid (Me<sub>2</sub>NH<sub>2</sub>)<sub>2</sub>Ā[Fe<sub>2</sub>L<sub>3</sub>]Ā·ā2H<sub>2</sub>OĀ·6DMFā. Analysis of X-ray diffraction, Raman
spectra, and MoĢssbauer spectra confirm the presence of Fe<sup>III</sup> centers with mixed-valence ligands of the form (L<sub>3</sub>)<sup>8ā</sup> that result from a spontaneous electron transfer
from Fe<sup>II</sup> to L<sup>2ā</sup>. Upon removal of DMF
and H<sub>2</sub>O solvent molecules, the compound undergoes a slight
structural distortion to give the desolvated phase (Me<sub>2</sub>NH<sub>2</sub>)<sub>2</sub>Ā[Fe<sub>2</sub>L<sub>3</sub>], and
a fit to N<sub>2</sub> adsorption data of this activated compound
gives a BET surface area of 885(105) m<sup>2</sup>/g. Dc magnetic
susceptibility measurements reveal a spontaneous magnetization below
80 and 26 K for the solvated and the activated solids, respectively,
with magnetic hysteresis up to 60 and 20 K. These results highlight
the ability of redox-active tetraoxolene ligands to support the formation
of a microporous magnet and provide the first example of a structurally
characterized extended solid that contains tetraoxolene radical ligands
Low-Coordinate Iron(II) Complexes of a Bulky Bis(carbene)borate Ligand
The
bulky bisĀ(carbene)Āborate ligand H<sub>2</sub>BĀ(<sup>t</sup>BuIm)<sub>2</sub><sup>ā</sup> allows for the synthesis of
three- and four-coordinate ironĀ(II) complexes, including heteroleptic
H<sub>2</sub>BĀ(<sup>t</sup>BuIm)<sub>2</sub>FeNĀ(TMS)<sub>2</sub> and
homoleptic [H<sub>2</sub>BĀ(<sup>t</sup>BuIm)<sub>2</sub>]<sub>2</sub>Fe. The magnetic properties of these coordinatively unsaturated complexes
have been characterized by SQUID magnetometry, but no evidence of
single-molecule magnet behavior is observed, despite large negative
uniaxial zero field splitting. The three-coordinate complex H<sub>2</sub>BĀ(<sup>t</sup>BuIm)<sub>2</sub>FeNĀ(TMS)<sub>2</sub> serves
as a precursor for the synthesis of the four-coordinate mixed carbene
complex H<sub>2</sub>BĀ(<sup>t</sup>BuIm)<sub>2</sub>(<sup>i</sup>Pr<sub>2</sub>Im)ĀFeCl, which has a coordination environment similar to that
found in trisĀ(carbene)Āborate ironĀ(II) chloride complexes. Despite
this similarity, attempts to prepare the corresponding ironĀ(IV) nitride
were unsuccessful, suggesting that subtle structural factors are critical
to stabilizing this species
2D Conductive Iron-Quinoid Magnets Ordering up to <i>T</i><sub>c</sub> = 105 K via Heterogenous Redox Chemistry
We report the magnetism
and conductivity for a redox pair of iron-quinoid
metalāorganic frameworks (MOFs). The oxidized compound, (Me<sub>2</sub>NH<sub>2</sub>)<sub>2</sub>[Fe<sub>2</sub>L<sub>3</sub>]Ā·2H<sub>2</sub>OĀ·6DMF (LH<sub>2</sub> = 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinone)
was previously shown to magnetically order below 80 K in its solvated
form, with the ordering temperature decreasing to 26 K upon desolvation.
Here, we demonstrate this compound to exhibit electrical conductivity
values up to Ļ = 1.4(7) Ć 10<sup>ā2</sup> S/cm (<i>E</i><sub>a</sub> = 0.26(1) cm<sup>ā1</sup>) and 1.0(3) Ć 10<sup>ā3</sup> S/cm (<i>E</i><sub>a</sub> = 0.19(1) cm<sup>ā1</sup>) in its solvated
and desolvated forms, respectively. Upon soaking in a DMF solution
of Cp<sub>2</sub>Co, the compound undergoes a single-crystal-to-single-crystal
one-electron reduction to give (Cp<sub>2</sub>Co)<sub>1.43</sub>(Me<sub>2</sub>NH<sub>2</sub>)<sub>1.57</sub>[Fe<sub>2</sub>L<sub>3</sub>]Ā·4.9DMF. Structural and spectroscopic analysis confirms this
reduction to be ligand-based, and as such the trianionic framework
is formulated as [Fe<sup>III</sup><sub>2</sub>(L<sup>3āā¢</sup>)<sub>3</sub>]<sup>3ā</sup>. Magnetic measurements for this
reduced compound reveal the presence of dominant intralayer metalāorganic
radical coupling to give a magnetically ordered phase below <i>T</i><sub>c</sub> = 105 K, one of the highest reported ordering
temperatures for a MOF. This high ordering temperature is significantly
increased relative to the oxidized compound, and stems from the overall
increase in coupling strength afforded by an additional organic radical.
In line with the high critical temperature, the new MOF exhibits magnetic
hysteresis up to 100 K, as revealed by variable-field measurements.
Finally, this compound is electrically conductive, with values up
to Ļ = 5.1(3) Ć 10<sup>ā4</sup> S/cm with <i>E</i><sub>a</sub> = 0.34(1) eV. Taken together, these results
demonstrate the unique ability of metal-quinoid MOFs to simultaneously
exhibit both high magnetic ordering temperatures and high electrical
conductivity