“…All materialsÐ sodium meta periodate (GFS), manganese sulfate m onohydrate (Mallinckrodt), 1,10-phenanthroline (Aldrich), sodium acetate (Aldrich), The test chosen for the stoppedow device interfaced to the FT-IR spectrom eter was the substitution reaction between W (CO) 5 (T HF) and 1,10-phenanthroline in tetrahydrofuran. This reaction has been previously studied 10 and is well characterized. The m aterialsÐ tetrahydrofuran (Fisher), hexacarbonyltungsten (Johnson M atthey), and 1,10-phenanthroline (Aldrich)Ð were used as received.…”
Section: Experim Ental S To P P Ed -F Lo W a Tt Ach M En Tmentioning
A stopped-flow rapid-mixing device interfaced with a rapid-scan FT-IR spectrometer and a diode-array UV-visible spectrophotometer permits the observation of reaction transient intermediates over a broad range of wavelengths at minimal cost. The system has been evaluated for both spectral regions with the use of two different chemical reaction systems. The presence of a transient intermediate is clearly indicated in one case. Unexpected reactivity was observed in the other case. This approach will allow the study of chemical reactions even when no spectral changes occur in the UV-visible region.
“…All materialsÐ sodium meta periodate (GFS), manganese sulfate m onohydrate (Mallinckrodt), 1,10-phenanthroline (Aldrich), sodium acetate (Aldrich), The test chosen for the stoppedow device interfaced to the FT-IR spectrom eter was the substitution reaction between W (CO) 5 (T HF) and 1,10-phenanthroline in tetrahydrofuran. This reaction has been previously studied 10 and is well characterized. The m aterialsÐ tetrahydrofuran (Fisher), hexacarbonyltungsten (Johnson M atthey), and 1,10-phenanthroline (Aldrich)Ð were used as received.…”
Section: Experim Ental S To P P Ed -F Lo W a Tt Ach M En Tmentioning
A stopped-flow rapid-mixing device interfaced with a rapid-scan FT-IR spectrometer and a diode-array UV-visible spectrophotometer permits the observation of reaction transient intermediates over a broad range of wavelengths at minimal cost. The system has been evaluated for both spectral regions with the use of two different chemical reaction systems. The presence of a transient intermediate is clearly indicated in one case. Unexpected reactivity was observed in the other case. This approach will allow the study of chemical reactions even when no spectral changes occur in the UV-visible region.
“…The volume of activation for these reactions, estimated from the effect of high pressure on k 1 and k 2 , is a very sensitive parameter in the absence of any significant changes in electrostriction (neutral complexes and neutral L) and gives direct information regarding the extent of bond formation/bond breakage in the transition states of reactions 1 and 2. These data and their mechanistic interpretation have been discussed in detail in the literature. − …”
Section: Introductionmentioning
confidence: 95%
“…Over the past few years, we have developed an interest in the importance of solvent effects in photoinduced ligand substitution reactions of hexacarbonyl group 6 metal complexes. − In these studies we have systematically varied the nature of the metal (Cr, Mo, W), the entering ligand (mono- versus bidentate), and the solvent. It is especially the coordination ability and solvation properties of the solvent that affect the observed CO displacement reactions and the nature of the underlying reaction mechanism.…”
Section: Introductionmentioning
confidence: 99%
“…Both reaction steps 1 and 2 have been studied in considerable detail in order to investigate the influence of the selected solvents. 10 Reaction 2 depends strongly on the nature of the bidentate ligand, it is very sensitive to the influence of steric hindrance, and the values of k 2 can vary over many orders of magnitude for different ligands L.…”
The ring-closure reaction of M(CO)5L−L (M = W,
L−L = bipyridine) to produce M(CO)4(L−L) in
nonpolar
supercritical (sc) CO2/benzene mixtures was selected to
investigate the repulsive (or intrinsic) part of the
activation volume associated with this reaction. The activation
volumes over the range between 7.5 and 24
MPa at different temperatures were higher than those in liquid benzene
and liquid CO2/benzene, but much
lower than those in pure sc CO2. The UV/vis spectral
studies of this reaction showed that the addition of
small amounts of cosolvent can increase the solubility of the
ring-closure species. The kinetic traces recorded
for the reaction also demonstrated this solubility
enhancement.
“…Recent kinetic studies on the thermal ring closure reactions of complexes of the type M(CO) 5 L (M = Cr, Mo, W; L = bidentate ligand) indicated that the mechanism of such processes is controlled to a large extent by the size of the metal center and the nature of the chelating ligand. − In these investigations the M(CO) 5 L species were prepared in solution by photolysis of M(CO) 6 in the presence of L. The reaction involves the initial photodissociation of one CO ligand (eq 1), leading to the intermediate pentacarbonyl with a monocoordinated bidentate ligand (eq 2), which subsequently undergoes ring closure by loss of another CO (eq 3). If reaction 3 is too rapid, it is necessary to use flash photolysis techniques in order to study its kinetics. ,− …”
Thermal ring-closure reactions of pentacarbonylmetal complexes
with bidentate N−N
ligands were studied in detail in the past using photochemical
techniques to produce these
species in solution prior to the actual reaction. Some of the
monodentate pentacarbonylchromium complexes have now been isolated and enable a detailed
reinvestigation of the
thermal chelation reaction of Cr(CO)5(N−N)
(N−N = 1,4-diisopropyl-1,4-diazabutadiene, 4,4‘-dimethyl-2,2‘-bipyridine). The reactions were studied as a function
of [N−N], temperature,
and pressure. The unexpected ligand concentration dependence, viz.
a decrease in observed
rate constant with increasing [N−N], can be accounted for in terms
of competitive reactions
leading to the formation of Cr(CO)6 and chelated
Cr(CO)4(N−N). At high [N−N] the
limiting
rate constant represents only the chelation reaction, for which the
observed activation
volumes are between +13 and +18 cm3
mol-1. These strongly support the
operation of a
dissociatively activated chelation mechanism.
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