11 research outputs found
Die Sammlung Simone Collinet. Simone Breton als leidenschaftliche Sammlerin des Surrealismus
A series of highly active yttrium phosphasalen initiators
for the
heteroselective ring-opening polymerization of <i>rac</i>-lactide are reported. The initiators are yttrium alkoxide complexes
ligated by iminophosphorane analogues of the popular “salen”
ligand, termed “phosphasalens”. A series of novel phosphasalens
have been synthesized, with varying substituents on the phenoxide
rings and ethylene, propylene, <i>rac</i>-cyclohexylene, <i>R</i>,<i>R</i>-cyclohexylene, phenylene, and 2,2-dimethylpropylene
groups linking the iminophosphorane moieties. Changing the substituents
on the phosphasalen ligands results in changes to the rates of polymerization
(<i>k</i><sub>obs</sub>) and to the PLA heterotacticity
(<i>P</i><sub>s</sub> = 0.87). Generally, the initiators
have high rates, excellent polymerization control, and a tolerance
to low loadings
The word as a unit of meaning. The role of context in words meaning
A unit of meaning is a word plus all those words within its contextual context that are needed to disambiguate this word to make it monosemous. A lot of research were made to study the influence of the context. They testify that there is usually in each word a hard core of relatively stable meaning and can be modified by the context within certain limits
Branched Redox-Active Complexes for the Study of Novel Charge Transport Processes
The syntheses and electrochemical/optical
properties of some branched and linear 1,1′-substituted ferrocene
complexes for molecular electronics are described. Metal centers were
extended (and where relevant, connected) by arylethynyl spacers functionalized
with <i>m-</i>pyridyl, <i>tert-</i>butylthiol
(S<sup><i>t</i></sup>Bu), and trimethylsilyl (TMS) moieties.
Such systems provide two well-defined molecular pathways for electron
transfer and hold interesting prospects for the study of new charge
transport processes, such as quantum interference, local gating, and
correlated hopping events
Catalytic Transformation of Levulinic Acid to 2‑Methyltetrahydrofuran Using Ruthenium–<i>N</i>‑Triphos Complexes
A series of pre-
or in situ-formed ruthenium complexes were assessed
for the stepwise catalytic hydrogenation of levulinic acid (LA) to
2-methyltetrahydrofuran (2-MTHF) via γ-valerolactone (γVL)
and 1,4-pentanediol (1,4-PDO). Two different catalytic systems based
on the branched triphosphine ligands Triphos (CH<sub>3</sub>C(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>) and <i>N</i>-triphos
(N(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>) were investigated.
The most active catalyst was the preformed ruthenium species [RuH<sub>2</sub>(PPh<sub>3</sub>){N(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-κ<sup>3</sup><i>P</i>}] (<b>5</b>), which
gave near quantitative conversion of LA to 1,4-PDO when no acidic
additives were present, and 87% 2-MTHF when used in conjunction with
HN(Tf)<sub>2</sub>. Various acidic additives were assessed to promote
the final transformation of 1,4-PDO to 2-MTHF; however, only HN(Tf)<sub>2</sub> was found to be effective, and NH<sub>4</sub>PF<sub>6</sub> and <i>para</i>-toluenesulfonic acid (<i>p</i>-TsOH) were found to be detrimental. Mechanistic investigations were
carried out to explain the observed catalytic trends and importantly
showed that PPh<sub>3</sub> dissociation from <b>5</b> resulted
in its improved catalytic reactivity. The presence of acidic additives
removes catalytically necessary hydride ligands and may also compete
with the substrate for binding to the catalytic metal center, explaining
why only an acid with a noncoordinating conjugate base was effective.
Crystals suitable for X-ray diffraction experiments were grown for
two complexes: [Ru(NCMe)<sub>3</sub>{N(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-κ<sup>3</sup><i>P</i>}] (<b>14</b>) and [Ru<sub>2</sub>(μ-Cl)<sub>3</sub>{N(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-κ<sup>3</sup><i>P</i>}<sub>2</sub>][BPh<sub>4</sub>] (<b>16</b>)
Scandium and Yttrium Phosphasalen Complexes as Initiators for Ring-Opening Polymerization of Cyclic Esters
The synthesis and
characterization of novel scandium and yttrium
phosphasalen complexes is reported, where phosphasalen refers to two
different bis(iminophosphorane) derivatives of the more ubiquitous
salen ligands. The activity of the complexes as initiators for the
ring-opening polymerization of cyclic esters is presented. The scandium
complexes are inactive for lactide polymerization but slow and controlled
initiators for ε-caprolactone polymerization. The lack of activity
toward lactide exhibited by these compounds is probed, and a rare
example of single-monomer insertion product, unable to undergo further
reactions with lactide, is identified. In contrast, the analogous
yttrium phosphasalen complex is a very active initiator for the ring-opening
polymerization of <i>rac</i>-lactide (<i>k</i><sub>obs</sub> = 1.5 × 10<sup>–3</sup> s<sup>–1</sup> at 1:500 [yttrium initiator]:[<i>rac</i>-lactide], 1 M
overall concentration of lactide in THF at 298 K). In addition to
being a very fast initiator, the yttrium complex also maintains excellent
levels of polymerization control and a high degree of isoselectivity,
with the probability of isotactic enchainment being <i>P</i><sub>i</sub> = 0.78 at 298 K
8‑Quinolinolato Gallium Complexes: Iso-selective Initiators for <i>rac</i>-Lactide Polymerization
The
synthesis and characterization of a series of 8-quinolinolato gallium
complexes is presented, and the complexes are analogous to a series
of aluminum complexes previously reported. The complexes have been
shown to be active initiators for the ring-opening polymerization
of <i>rac</i>-lactide. High degrees of polymerization control
are demonstrated, as exemplified by the linear evolution of molecular
weight as the polymerization progresses, narrow polydispersity indices,
and molecular weights corresponding to those predicted on the basis
of initiator concentration. Some of the initiators show iso-selective
polymerization of <i>rac</i>-lactide, with <i>P</i><sub>i</sub> = 0.70. The polymerization rates have been monitored,
and the pseudo first-order rate constants are compared to those of
analogous aluminum compounds. The 8-quinolinolato gallium initiators
show rates approximately 3 times higher than those of the series of
aluminum compounds, while maintaining equivalently high iso-selectivity
(<i>P</i><sub>i</sub> = 0.70) and polymerization control
Synthesis, Characterization, and Reactivity of Ruthenium Hydride Complexes of N‑Centered Triphosphine Ligands
The
reactivity of the novel tridentate phosphine ligand N(CH<sub>2</sub>PCyp<sub>2</sub>)<sub>3</sub> (N-triphos<sup>Cyp</sup>, <b>2</b>; Cyp = cyclopentyl) with various ruthenium complexes was investigated
and compared that of to the less sterically bulky and less electron
donating phenyl derivative N(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub> (N-triphos<sup>Ph</sup>, <b>1</b>). One of these complexes
was subsequently investigated for reactivity toward levulinic acid,
a potentially important biorenewable feedstock. Reaction of ligands <b>1</b> and <b>2</b> with the precursors [Ru(COD)(methylallyl)<sub>2</sub>] (COD = 1,5-cycloocatadiene) and [RuH<sub>2</sub>(PPh<sub>3</sub>)<sub>4</sub>] gave the tridentate coordination complexes
[Ru(tmm){N(CH<sub>2</sub>PR<sub>2</sub>)<sub>3</sub>-κ<sup>3</sup><i>P</i>}] (R = Ph (<b>3</b>), Cyp (<b>4</b>); tmm = trimethylenemethane) and [RuH<sub>2</sub>(PPh<sub>3</sub>){N(CH<sub>2</sub>PR<sub>2</sub>)<sub>3</sub>-κ<sup>3</sup><i>P</i>}] (R = Ph (<b>5</b>), Cyp (<b>6</b>)), respectively. Ligands <b>1</b> and <b>2</b> displayed
different reactivities with [Ru<sub>3</sub>(CO)<sub>12</sub>]. Ligand <b>1</b> gave the tridentate dicarbonyl complex [Ru(CO)<sub>2</sub>{N(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-κ<sup>3</sup><i>P</i>}] (<b>7</b>), while <b>2</b> gave
the bidentate, tricarbonyl [Ru(CO)<sub>3</sub>{N(CH<sub>2</sub>PCyp<sub>2</sub>)<sub>3</sub>-κ<sup>2</sup><i>P</i>}] (<b>8</b>). This was attributed to the greater electron-donating characteristics
of <b>2</b>, requiring further stabilization on coordination
to the electron-rich Ru(0) center by more CO ligands. Complex <b>7</b> was activated via oxidation using AgOTf and O<sub>2</sub>, giving the Ru(II) complexes [Ru(CO)<sub>2</sub>(OTf){N(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-κ<sup>3</sup><i>P</i>}](OTf) (<b>9</b>) and [Ru(CO<sub>3</sub>)(CO){N(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-κ<sup>3</sup><i>P</i>}] (<b>11</b>), respectively. Hydrogenation of these complexes
under hydrogen pressures of 3–15 bar gave the monohydride and
dihydride complexes [RuH(CO)<sub>2</sub>{N(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-κ<sup>3</sup><i>P</i>}] (<b>10</b>) and [RuH<sub>2</sub>(CO){N(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-κ<sup>3</sup><i>P</i>}] (<b>12</b>), respectively.
Complex <b>12</b> was found to be unreactive toward levulinic
acid (LA) unless activated by reaction with NH<sub>4</sub>PF<sub>6</sub> in acetonitrile, forming [RuH(CO)(MeCN){N(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-κ<sup>3</sup><i>P</i>}](PF<sub>6</sub>) (<b>13</b>), which reacted cleanly with LA to form
[Ru(CO){N(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>-κ<sup>3</sup><i>P</i>}{CH<sub>3</sub>CO(CH<sub>2</sub>)<sub>2</sub>CO<sub>2</sub>H-κ<sup>2</sup><i>O</i>}](PF<sub>6</sub>)
(<b>14</b>). Complexes <b>3</b>, <b>5</b>, <b>7</b>, <b>8</b>, <b>11</b>, and <b>12</b> were
characterized by single-crystal X-ray crystallography
Lanthanide(III) Complexes of Rhodamine–DO3A Conjugates as Agents for Dual-Modal Imaging
Two
novel dual-modal MRI/optical probes based on a rhodamine–DO3A
conjugate have been prepared. The bis(aqua)gadolinium(III) complex <b>Gd.L1</b> and mono(aqua)gadolinium(III) complex <b>Gd.L2</b> behave as dual-modal imaging probes (<i>r</i><sub>1</sub> = 8.5 and 3.8 mM<sup>–1</sup> s<sup>–1</sup> for <b>Gd.L1</b> and <b>Gd.L2</b>, respectively; λ<sub>ex</sub> = 560 nm and λ<sub>em</sub> = 580 nm for both complexes).
The rhodamine fragment is pH-sensitive, and upon lowering of the pH,
an increase in fluorescence intensity is observed as the spirolactam
ring opens to give the highly fluorescent form of the molecule. The
ligands are bimodal when coordinated to Tb(III) ions, inducing fluorescence
from both the lanthanide center and the rhodamine fluorophore, on
two independent time frames. Confocal imaging experiments were carried
out to establish the localization of <b>Gd.L2</b> in HEK293
cells and primary mouse islet cells (∼70% insulin-containing
β cells). Colocalization with MitoTracker Green demonstrated <b>Gd.L2</b>’s ability to distinguish between tumor and healthy
cells, with compartmentalization believed to be in the mitochondria. <b>Gd.L2</b> was also evaluated as an MRI probe for imaging of tumors
in BALB/c nude mice bearing M21 xenografts. A 36.5% decrease in <i>T</i><sub>1</sub> within the tumor was observed 30 min post
injection, showing that <b>Gd.L2</b> is preferentially up taken
in the tumor. <b>Gd.L2</b> is the first small-molecule MR/fluorescent
dual-modal imaging agent to display an off–on pH switch upon
its preferential uptake within the more acidic microenvironment of
tumor cells
The Unusual Redox Properties of Fluoroferrocenes Revealed through a Comprehensive Study of the Haloferrocenes
We
report the synthesis and full characterization of the entire
haloferrocene (FcX) and 1,1′-dihaloferrocene (fcX<sub>2</sub>) series (X = I, Br, Cl, F; Fc = ferrocenyl, fc = ferrocene-1,1′-diyl).
Finalization of this simple, yet intriguing set of compounds has been
delayed by synthetic challenges associated with the incorporation
of fluorine substituents. Successful preparation of fluoroferrocene
(<b>FcF</b>) and 1,1′-difluoroferrocene (<b>fcF</b><sub><b>2</b></sub>) were ultimately achieved using reactions
between the appropriate lithiated ferrocene species and <i>N-</i>fluorobenzenesulfonimide (NFSI). The crude reaction products, in
addition to those resulting from analogous preparations of chloroferrocene
(<b>FcCl</b>) and 1,1′-dichloroferrocene (<b>fcCl</b><sub><b>2</b></sub>), were utilized as model systems to probe
the limits of a previously reported “oxidative purification”
methodology. From this investigation and careful solution voltammetry
studies, we find that the fluorinated derivatives exhibit the <i>lowest</i> redox potentials of each of the FcX and fcX<sub>2</sub> series. This counterintuitive result is discussed with reference
to the spectroscopic, structural, and first-principles calculations
of these and related materials
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jig nFEB 03 2008Used I and SupUsed I and SupNot UsedJIGGER, TOMMY NOGGIN, between the jigs and the reelsChecked by Adrian Young on Tue 09 Jun 2015; Checked by Cathy Wiseman on Mon 03 Aug 201