4 research outputs found
Metathesis Activity Encoded in the Metallacyclobutane Carbon-13 NMR Chemical Shift Tensors
Metallacyclobutanes are an important
class of organometallic intermediates,
due to their role in olefin metathesis. They can have either planar
or puckered rings associated with characteristic chemical and physical
properties. Metathesis active metallacyclobutanes have short MāC<sub>Ī±/Ī±ā²</sub> and MĀ·Ā·Ā·C<sub>Ī²</sub> distances, long C<sub>Ī±/Ī±ā²</sub>āC<sub>Ī²</sub> bond length, and isotropic <sup>13</sup>C chemical
shifts for both early d<sup>0</sup> and late d<sup>4</sup> transition
metal compounds for the Ī±- and Ī²-carbons appearing at
ca. 100 and 0 ppm, respectively. Metallacyclobutanes that do not show
metathesis activity have <sup>13</sup>C chemical shifts of the Ī±-
and Ī²-carbons at typically 40 and 30 ppm, respectively, for
d<sup>0</sup> systems, with upfield shifts to ca. ā30 ppm for
the Ī±-carbon of metallacycles with higher d<sup><i>n</i></sup> electron counts (<i>n</i> = 2 and 6). Measurements
of the chemical shift tensor by solid-state NMR combined with an orbital
(natural chemical shift, NCS) analysis of its principal components
(Ī“<sub>11</sub> ā„ Ī“<sub>22</sub> ā„ Ī“<sub>33</sub>) with two-component calculations show that the specific
chemical shift of metathesis active metallacyclobutanes originates
from a low-lying empty orbital lying in the plane of the metallacyclobutane
with local Ļ*Ā(MāC<sub>Ī±/Ī±ā²</sub>)
character. Thus, in the metathesis active metallacyclobutanes, the
Ī±-carbons retain some residual alkylidene character, while their
Ī²-carbon is shielded, especially in the direction perpendicular
to the ring. Overall, the chemical shift tensors directly provide
information on the predictive value about the ability of metallacyclobutanes
to be olefin metathesis intermediates
CāH Activation on Co,O Sites: Isolated Surface Sites versus Molecular Analogs
The activation and
conversion of hydrocarbons is one of the most important challenges
in chemistry. Transition-metal ions (V, Cr, Fe, Co, etc.) isolated
on silica surfaces are known to catalyze such processes. The mechanisms
of these processes are currently unknown but are thought to involve
CāH activation as the rate-determining step. Here, we synthesize
well-defined CoĀ(II) ions on a silica surface using a metal siloxide
precursor followed by thermal treatment under vacuum at 500 Ā°C.
We show that these isolated CoĀ(II) sites are catalysts for a number
of hydrocarbon conversion reactions, such as the dehydrogenation of
propane, the hydrogenation of propene, and the trimerization of terminal
alkynes. We then investigate the mechanisms of these processes using
kinetics, kinetic isotope effects, isotopic labeling experiments,
parahydrogen induced polarization (PHIP) NMR, and comparison with
a molecular analog. The data are consistent with all of these reactions
occurring by a common mechanism, involving heterolytic CāH
or HāH activation via a 1,2 addition across a CoāO bond
CāH Activation on Co,O Sites: Isolated Surface Sites versus Molecular Analogs
The activation and
conversion of hydrocarbons is one of the most important challenges
in chemistry. Transition-metal ions (V, Cr, Fe, Co, etc.) isolated
on silica surfaces are known to catalyze such processes. The mechanisms
of these processes are currently unknown but are thought to involve
CāH activation as the rate-determining step. Here, we synthesize
well-defined CoĀ(II) ions on a silica surface using a metal siloxide
precursor followed by thermal treatment under vacuum at 500 Ā°C.
We show that these isolated CoĀ(II) sites are catalysts for a number
of hydrocarbon conversion reactions, such as the dehydrogenation of
propane, the hydrogenation of propene, and the trimerization of terminal
alkynes. We then investigate the mechanisms of these processes using
kinetics, kinetic isotope effects, isotopic labeling experiments,
parahydrogen induced polarization (PHIP) NMR, and comparison with
a molecular analog. The data are consistent with all of these reactions
occurring by a common mechanism, involving heterolytic CāH
or HāH activation via a 1,2 addition across a CoāO bond
Near-IR Two Photon Microscopy Imaging of Silica Nanoparticles Functionalized with Isolated Sensitized Yb(III) Centers
Bright nano-objects
emitting in the near-infrared with a maximal
cross section of 41.4 Ć 10<sup>3</sup> GM (Goppert Mayer) were
prepared by implanting ca. 180 4,4ā²-diethylaminostyryl-2,2ā²-bipyridine
(DEAS) YbĀ(III) complexes on the surface of 12-nm silica nanoparticles.
The surface complexes ([DEASĀ·Ln@SiO<sub>2</sub>], Ln = Y, Yb)
were characterized using IR, solid-state NMR, UV-vis, and EXAFS spectroscopies
in combination with the preparation and characterization of similar
molecular analogues by analytical techniques (IR, solution NMR, UVāvis,
X-ray crystallography) as well as DFT calculations. Starting from
the partial dehydroxylation of the silica at 700 Ā°C under a high
vacuum having 0.8 OHĀ·nm<sup>ā2</sup>, the grafting of
LnĀ(NĀ(SiMe<sub>3</sub>)<sub>2</sub>)<sub>3</sub> generates ī¼SiOāLnĀ(NĀ(SiMe<sub>3</sub>)<sub>2</sub>)<sub>2</sub>, which upon thermal step and coordination
of the DEAS chromophore yields (ī¼SiO)<sub>3</sub>LnĀ(DEAS).
Surface and molecular analogues display similar properties, in terms
of DEAS binding constants absorption maxima and luminescence properties
(intense emission band assigned to a ligand centered CT fluorescence
and lifetime) in the solid state, consistent with the molecular nature
of the surface species. The densely functionalized nanoparticles can
be dispersed via ultrasonication in small 15ā20 nm aggregates
(one to six elementary particles) that were detected using two-photon
microscopy imaging at 720 nm excitation, making them promising nano-objects
for bioimaging