13 research outputs found
Hard Single-Molecule Magnet Behavior by a Linear Trinuclear Lanthanideâ[1]Metallocenophane Complex
A synthetic protocol
was developed that involves the transmetalation
of a mono-dysprosiumâ[1]ÂferroÂcenophane complex with DyX<sub>3</sub> (X = Cl<sup>â</sup> or I<sup>â</sup>) to afford
[Dy<sub>3</sub>Fc<sub>6</sub>Li<sub>2</sub>Â(THF)<sub>2</sub>]<sup>â</sup>, featuring a rare linear arrangement of magnetically
anisotropic Dy<sup>3+</sup> ions. The close spatial inter-lanthanide
proximity, in combination with Îź<sub>2</sub>-bridging sp<sup>2</sup>-hybridized C<sub>Cp</sub> groups, enforces significant magnetic
coupling and results in hard single-molecule magnet (SMM) behavior,
with an effective barrier to magnetization reversal of up to 268 cm<sup>â1</sup>. Our results highlight the versatility of lanthanide
metalloÂcenophane architectures toward the development of novel
multiÂnuclear SMM frameworks
Slow Magnetic Relaxation in a Lanthanide-[1]Metallocenophane Complex
The first example
of a lanthanide metallocenophane complex has
been isolated as [LiÂ(THF)<sub>4</sub>]Â[DyFc<sub>3</sub>Li<sub>2</sub>(THF)<sub>2</sub>] (<b>1</b>). The molecular structure of complex <b>1</b> differs dramatically from those of main group and transition
metal ferrocenophane complexes and features a distorted trigonal prismatic
geometry around the DyÂ(III) ion and close intramolecular Dy¡¡¡Fe
distances. Furthermore, complex <b>1</b> exhibits all characteristics
of a soft single-molecule magnet
Slow Magnetic Relaxation in a Lanthanide-[1]Metallocenophane Complex
The first example
of a lanthanide metallocenophane complex has
been isolated as [LiÂ(THF)<sub>4</sub>]Â[DyFc<sub>3</sub>Li<sub>2</sub>(THF)<sub>2</sub>] (<b>1</b>). The molecular structure of complex <b>1</b> differs dramatically from those of main group and transition
metal ferrocenophane complexes and features a distorted trigonal prismatic
geometry around the DyÂ(III) ion and close intramolecular Dy¡¡¡Fe
distances. Furthermore, complex <b>1</b> exhibits all characteristics
of a soft single-molecule magnet
Electrocatalytic CO<sub>2</sub> Reduction by Imidazolium-Functionalized Molecular Catalysts
We present the first
examples of CO<sub>2</sub> electro-reduction
catalysts that feature charged imidazolium groups in the secondary
coordination sphere. The functionalized Lehn-type catalysts display
significant differences in their redox properties and improved catalytic
activities as compared to the conventional reference catalyst. Our
results suggest that the incorporated imidazolium moieties do not
solely function as a charged tag but also alter mechanistic aspects
of catalysis
Exchange Coupling and Magnetic Blocking in Bipyrimidyl Radical-Bridged Dilanthanide Complexes
The synthesis and magnetic properties of three new bipyrimidyl
radical-bridged dilanthanide complexes, [(Cp*<sub>2</sub>Ln)<sub>2</sub>(Îź-bpym<sup>â˘</sup>)]<sup>+</sup> (Ln = Gd, Tb, Dy),
are reported. Strong Ln<sup>III</sup>-bpym<sup>â˘â</sup> exchange coupling is observed for all species, as indicated by the
increases in Ď<sub>M</sub><i>T</i> at low temperatures.
For the Gd<sup>III</sup>-containing complex, a fit to the data reveals
antiferromagnetic coupling with <i>J</i> = â10 cm<sup>â1</sup> to give an <i>S</i> = <sup>13</sup>/<sub>2</sub> ground state. The Tb<sup>III</sup> and Dy<sup>III</sup> congeners
show single-molecule magnet behavior with relaxation barriers of <i>U</i><sub>eff</sub> = 44(2) and 87.8(3) cm<sup>â1</sup>, respectively, a consequence of the large magnetic anisotropies
imparted by these ions. Significantly, the latter complex exhibits
a divergence of the field-cooled and zero-field-cooled dc susceptibility
data at 6.5 K and magnetic hysteresis below this temperature
Exchange Coupling and Magnetic Blocking in Bipyrimidyl Radical-Bridged Dilanthanide Complexes
The synthesis and magnetic properties of three new bipyrimidyl
radical-bridged dilanthanide complexes, [(Cp*<sub>2</sub>Ln)<sub>2</sub>(Îź-bpym<sup>â˘</sup>)]<sup>+</sup> (Ln = Gd, Tb, Dy),
are reported. Strong Ln<sup>III</sup>-bpym<sup>â˘â</sup> exchange coupling is observed for all species, as indicated by the
increases in Ď<sub>M</sub><i>T</i> at low temperatures.
For the Gd<sup>III</sup>-containing complex, a fit to the data reveals
antiferromagnetic coupling with <i>J</i> = â10 cm<sup>â1</sup> to give an <i>S</i> = <sup>13</sup>/<sub>2</sub> ground state. The Tb<sup>III</sup> and Dy<sup>III</sup> congeners
show single-molecule magnet behavior with relaxation barriers of <i>U</i><sub>eff</sub> = 44(2) and 87.8(3) cm<sup>â1</sup>, respectively, a consequence of the large magnetic anisotropies
imparted by these ions. Significantly, the latter complex exhibits
a divergence of the field-cooled and zero-field-cooled dc susceptibility
data at 6.5 K and magnetic hysteresis below this temperature
Exchange Coupling and Magnetic Blocking in Bipyrimidyl Radical-Bridged Dilanthanide Complexes
The synthesis and magnetic properties of three new bipyrimidyl
radical-bridged dilanthanide complexes, [(Cp*<sub>2</sub>Ln)<sub>2</sub>(Îź-bpym<sup>â˘</sup>)]<sup>+</sup> (Ln = Gd, Tb, Dy),
are reported. Strong Ln<sup>III</sup>-bpym<sup>â˘â</sup> exchange coupling is observed for all species, as indicated by the
increases in Ď<sub>M</sub><i>T</i> at low temperatures.
For the Gd<sup>III</sup>-containing complex, a fit to the data reveals
antiferromagnetic coupling with <i>J</i> = â10 cm<sup>â1</sup> to give an <i>S</i> = <sup>13</sup>/<sub>2</sub> ground state. The Tb<sup>III</sup> and Dy<sup>III</sup> congeners
show single-molecule magnet behavior with relaxation barriers of <i>U</i><sub>eff</sub> = 44(2) and 87.8(3) cm<sup>â1</sup>, respectively, a consequence of the large magnetic anisotropies
imparted by these ions. Significantly, the latter complex exhibits
a divergence of the field-cooled and zero-field-cooled dc susceptibility
data at 6.5 K and magnetic hysteresis below this temperature
Exchange Coupling and Magnetic Blocking in Bipyrimidyl Radical-Bridged Dilanthanide Complexes
The synthesis and magnetic properties of three new bipyrimidyl
radical-bridged dilanthanide complexes, [(Cp*<sub>2</sub>Ln)<sub>2</sub>(Îź-bpym<sup>â˘</sup>)]<sup>+</sup> (Ln = Gd, Tb, Dy),
are reported. Strong Ln<sup>III</sup>-bpym<sup>â˘â</sup> exchange coupling is observed for all species, as indicated by the
increases in Ď<sub>M</sub><i>T</i> at low temperatures.
For the Gd<sup>III</sup>-containing complex, a fit to the data reveals
antiferromagnetic coupling with <i>J</i> = â10 cm<sup>â1</sup> to give an <i>S</i> = <sup>13</sup>/<sub>2</sub> ground state. The Tb<sup>III</sup> and Dy<sup>III</sup> congeners
show single-molecule magnet behavior with relaxation barriers of <i>U</i><sub>eff</sub> = 44(2) and 87.8(3) cm<sup>â1</sup>, respectively, a consequence of the large magnetic anisotropies
imparted by these ions. Significantly, the latter complex exhibits
a divergence of the field-cooled and zero-field-cooled dc susceptibility
data at 6.5 K and magnetic hysteresis below this temperature
Oxidative Stretching of MetalâMetal Bonds to Their Limits
Oxidation of quadruply bonded Cr<sub>2</sub>(dpa)<sub>4</sub>, Mo<sub>2</sub>(dpa)<sub>4</sub>, MoWÂ(dpa)<sub>4</sub>, and W<sub>2</sub>(dpa)<sub>4</sub> (dpa = 2,2â˛-dipyridylamido)
with 2 equiv of silverÂ(I) triflate or ferrocenium triflate results
in the formation of the two-electron-oxidized products [Cr<sub>2</sub>(dpa)<sub>4</sub>]<sup>2+</sup> (<b>1</b>), [Mo<sub>2</sub>(dpa)<sub>4</sub>]<sup>2+</sup> (<b>2</b>), [MoWÂ(dpa)<sub>4</sub>]<sup>2+</sup> (<b>3</b>), and [W<sub>2</sub>(dpa)<sub>4</sub>]<sup>2+</sup> (<b>4</b>). Additional two-electron oxidation
and oxygen atom transfer by <i>m</i>-chloroperoxybenzoic
acid results in the formation of the corresponding metalâoxo
compounds [Mo<sub>2</sub>OÂ(dpa)<sub>4</sub>]<sup>2+</sup> (<b>5</b>), [WMoOÂ(dpa)<sub>4</sub>]<sup>2+</sup> (<b>6</b>), and [W<sub>2</sub>OÂ(dpa)<sub>4</sub>]<sup>2+</sup> (<b>7</b>), which feature
an unusual linear M¡¡¡MîźO structure. Crystallographic
studies of the two-electron-oxidized products <b>2</b>, <b>3</b>, and <b>4</b>, which have the appropriate number of
orbitals and electrons to form metalâmetal triple bonds, show
bond distances much longer (by >0.5 Ă
) than those in established
triply bonded compounds, but these compounds are nonetheless diamagnetic.
In contrast, the CrâCr bond is completely severed in <b>1</b>, and the resulting two isolated Cr<sup>3+</sup> magnetic
centers couple antiferromagnetically with <i>J</i>/<i>k</i><sub>B</sub>= â108(3) K [â75(2) cm<sup>â1</sup>], as determined by modeling of the temperature dependence of the
magnetic susceptibility. Density functional theory (DFT) and multiconfigurational
methods (CASSCF/CASPT2) provide support for âstretchedâ
and weak metalâmetal triple bonds in <b>2</b>, <b>3</b>, and <b>4</b>. The metalâmetal distances in
the metalâoxo compounds <b>5</b>, <b>6</b>, and <b>7</b> are elongated beyond the single-bond covalent radii of the
metal atoms. DFT and CASSCF/CASPT2 calculations suggest that the metal
atoms have minimal interaction; the electronic structure of these
complexes is used to rationalize their multielectron redox reactivity
A Well-Defined Terminal Vanadium(III) Oxo Complex
The ubiquity of vanadium oxo complexes
in the V+ and IV+ oxidation states has contributed to a comprehensive
understanding of their electronic structure and reactivity. However,
despite being predicted to be stable by ligand-field theory, the isolation
and characterization of a well-defined terminal mononuclear vanadiumÂ(III)
oxo complex has remained elusive. We present the synthesis and characterization
of a unique terminal mononuclear vanadiumÂ(III) oxo species supported
by the pentadentate polypyridyl ligand 2,6-bisÂ[1,1-bisÂ(2-pyridyl)Âethyl]Âpyridine
(PY5Me<sub>2</sub>). Exposure of [V<sup>II</sup>(NCCH<sub>3</sub>)Â(PY5Me<sub>2</sub>)]<sup>2+</sup> (<b>1</b>) to either dioxygen or selected
O-atom-transfer reagents yields [V<sup>IV</sup>(O)Â(PY5Me<sub>2</sub>)]<sup>2+</sup> (<b>2</b>). The metal-centered one-electron
reduction of this vanadiumÂ(IV) oxo complex furnishes a stable, diamagnetic
[V<sup>III</sup>(O)Â(PY5Me<sub>2</sub>)]<sup>+</sup> (<b>3</b>) species. The vanadiumÂ(III) oxo species is unreactive toward H-
and O-atom transfer but readily reacts with protons to form a putative
vanadium hydroxo complex. Computational results predict that further
one-electron reduction of the vanadiumÂ(III) oxo species will result
in ligand-based reduction, even though pyridine is generally considered
to be a poor Ď-accepting ligand. These results have implications
for future efforts toward low-valent vanadyl chemistry, particularly
with regard to the isolation and study of formal vanadiumÂ(II) oxo
species