5 research outputs found
Synthesis and Characterization of Divalent Manganese, Iron, and Cobalt Complexes in Tripodal Phenolate/N-Heterocyclic Carbene Ligand Environments
Two
novel tripodal ligands, (BIMPN<sup>Mes,Ad,Me</sup>)<sup>â</sup> and (MIMPN<sup>Mes,Ad,Me</sup>)<sup>2â</sup>, combining two
types of donor atoms, namely, NHC and phenolate donors, were synthesized
to complete the series of N-anchored ligands, ranging from chelating
species with trisÂ(carbene) to trisÂ(phenolate) chelating
arms. The complete ligand series offers a convenient way of tuning
the electronic and steric environment around the metal center, thus,
allowing for control of the complexâs reactivity. This series
of divalent complexes of Mn, Fe, and Co was synthesized and characterized
by <sup>1</sup>H NMR, IR, and UV/vis spectroscopy as well as by single-crystal
X-ray diffraction studies. Variable-temperature SQUID magnetization
measurements in the range from 2 to 300 K confirmed <i>high-spin</i> ground states for all divalent complexes and revealed a trend of
increasing zero-field splitting |<i>D</i>| from MnÂ(II),
to FeÂ(II), to CoÂ(II) complexes. Zero-field <sup>57</sup>Fe MoĚssbauer
spectroscopy of the FeÂ(II) complexes <b>3</b>, <b>4</b>, <b>8</b>, and <b>11</b> shows isomer shifts δ
that increase gradually as carbenes are substituted for phenolates
in the series of ligands. From the single-crystal structure determinations
of the complexes, the different steric demand of the ligands is evident.
Particularly, the molecular structure of <b>1</b>î¸in
which a pyridine molecule is situated next to the MnâCl bondî¸and
those of azide complexes <b>2</b>, <b>4</b>, and <b>6</b> demonstrate the flexibility of these mixed-ligand derivatives,
which, in contrast to the corresponding symmetrical TIMEN<sup>R</sup> ligands, allow for side access of, e.g., organic substrates, to
the reactive metal center
A Copper(II) Thiolate from Reductive Cleavage of an <i>S</i>âNitrosothiol
<i>S</i>-Nitrosothiols RSNO represent circulating
reservoirs
of nitric oxide activity in the plasma and play intricate roles in
protein function control in health and disease. While nitric oxide
has been shown to reductively nitrosylate copperÂ(II) centers to form
copperÂ(I) complexes and ENO species (E = R<sub>2</sub>N, RO), well-characterized
examples of the reverse reaction are rare. Employing the copperÂ(I)
β-diketiminate [Me<sub>2</sub>NN]ÂCu, we illustrate a clear example
in which an RSâNO bond is cleaved to release NO<sub>gas</sub> with formation of a discrete copperÂ(II) thiolate. The addition of
Ph<sub>3</sub>CSNO to [Me<sub>2</sub>NN]Cu generates the three-coordinate
copperÂ(II) thiolate [Me<sub>2</sub>NN]ÂCuSCPh<sub>3</sub>, which is
unstable toward free NO
A Copper(II) Thiolate from Reductive Cleavage of an <i>S</i>âNitrosothiol
<i>S</i>-Nitrosothiols RSNO represent circulating
reservoirs
of nitric oxide activity in the plasma and play intricate roles in
protein function control in health and disease. While nitric oxide
has been shown to reductively nitrosylate copperÂ(II) centers to form
copperÂ(I) complexes and ENO species (E = R<sub>2</sub>N, RO), well-characterized
examples of the reverse reaction are rare. Employing the copperÂ(I)
β-diketiminate [Me<sub>2</sub>NN]ÂCu, we illustrate a clear example
in which an RSâNO bond is cleaved to release NO<sub>gas</sub> with formation of a discrete copperÂ(II) thiolate. The addition of
Ph<sub>3</sub>CSNO to [Me<sub>2</sub>NN]Cu generates the three-coordinate
copperÂ(II) thiolate [Me<sub>2</sub>NN]ÂCuSCPh<sub>3</sub>, which is
unstable toward free NO
A Mononuclear Fe(III) Single Molecule Magnet with a 3/2â5/2 Spin Crossover
The air stable complex [(PNP)ÂFeCl<sub>2</sub>] (<b>1</b>)
(PNP = <i>N</i>[2-PÂ(CHMe<sub>2</sub>)<sub>2</sub>-4-methylphenyl]<sub>2</sub><sup>â</sup>), prepared from one-electron oxidation
of [(PNP)ÂFeCl] with ClCPh<sub>3</sub>, displays an unexpected <i>S</i> = 3/2 to <i>S</i> = 5/2 transition above 80
K as inferred by the dc SQUID magnetic susceptibility measurement.
The ac SQUID magnetization data, at zero field and between frequencies
10 and 1042 Hz, clearly reveal complex <b>1</b> to have frequency
dependence on the out-of-phase signal and thus being a single molecular
magnet with a thermally activated barrier of <i>U</i><sub>eff</sub> = 32â36 cm<sup>â1</sup> (47â52 K).
Variable-temperature MoĚssbauer data also corroborate a significant
temperature dependence in δ and Î<i>E</i><sub>Q</sub> values for <b>1</b>, which is in agreement with the
system undergoing a change in spin state. Likewise, variable-temperature
X-band EPR spectra of <b>1</b> reveals the <i>S</i> = 3/2 to be likely the ground state with the <i>S</i> =
5/2 being close in energy. Multiedge XAS absorption spectra suggest
the electronic structure of <b>1</b> to be highly covalent with
an effective iron oxidation state that is more reduced than the typical
ferric complexes due to the significant interaction of the phosphine
groups in PNP and Cl ligands with iron. A variable-temperature single
crystal X-ray diffraction study of <b>1</b> collected between
30 and 300 K also reveals elongation of the FeâP bond lengths
and increment in the ClâFeâCl angle as the <i>S</i> = 5/2 state is populated. Theoretical studies show overall similar
orbital pictures except for the dÂ(<i>z</i><sup>2</sup>)
orbital, which has the most sensitivity to change in the geometry
and bonding, where the quartet (<sup>4</sup>B) and the sextet (<sup>6</sup>A) states are close in energy
Fluoride Bridges as Structure-Directing Motifs in 3d-4f Cluster Chemistry
The use of kinetically robust chromiumÂ(III) fluorido
complexes
as synthons for mixed 3d-4f clusters is reported. The tendency toward
linear {Cr<sup>III</sup>âFâLn<sup>III</sup>} units dictates
the cluster topology. Specifically, we show that reaction of <i>cis</i>-[Cr<sup>III</sup>F<sub>2</sub>(NN)<sub>2</sub>]ÂNO<sub>3</sub> (NN = 1,10-phenanthroline (âphenâ) or 2,2â˛-bipyridine
(âbpyâ)) with LnÂ(NO<sub>3</sub>)<sub>3</sub>¡<i>x</i>H<sub>2</sub>O produces isostructural series of molecular
{Ln<sub>2</sub>Cr<sub>2</sub>} squares (<b>1</b>â<b>9</b>) with linear fluoride bridges. In a parallel fashion, <i>fac</i>-[Cr<sup>III</sup>F<sub>3</sub>L], where L = <i>N</i>,<i>Nâ˛</i>,<i>N</i>âł-trimethyl-1,4,7-triazacyclononane
(âMe<sub>3</sub>tacnâ), reacts with NdÂ(NO<sub>3</sub>)<sub>3</sub>¡6H<sub>2</sub>O to form a fluoride-centered penta-nuclear
complex and <i>fac</i>-[Cr<sup>III</sup>F<sub>3</sub>Lâ˛],
with LⲠ= 1,1,1-tris-((methylamino)Âmethylethane) (âMe<sub>3</sub>tameâ), reacts with [LnÂ(hfac)<sub>3</sub>(H<sub>2</sub>O)<sub>2</sub>] (hfacH = 1,1,1,5,5,5-hexafluoroacetylacetone) to
yield an isostructural series of {Ln<sub>3</sub>Cr<sub>2</sub>} (<b>10</b>â<b>14</b>) trigonal bipyramids with no central
ligand. The formation of the latter is accompanied by a partial solvolysis
of the CrÂ(III) precursor but without formation of insoluble LnF<sub>3</sub>. The magnetic properties of the gadolinium containing clusters
allow quantification of fluoride-mediated, antiferromagnetic GdâCr
exchange interactions of magnitude between 0.14 cm<sup>â1</sup> and 0.71 cm<sup>â1</sup> (<i>HĚ</i> = <i>J</i><sub>12</sub><b>SĚ</b><sub>1</sub>¡<b>SĚ</b><sub>2</sub> formalism) and vanishingly small <i>J</i><sub>GdâGd</sub> of 0.06(0) cm<sup>â1</sup>. The large spin and small anisotropy together with weak exchange
interactions in the {Gd<sub>3</sub>Cr<sub>2</sub>} (<b>11</b>) cluster give rise to a very large magneto-caloric effect of âÎ<i>S</i><sub>m</sub> = 28.7 J kg<sup>â1</sup> K<sup>â1</sup> (Îź<sub>0</sub><i>H</i> = 90 to 0 kOe)