Thermal ring closure in Mo(CO)5L (L = bpy, dmbpy, dpbpy) transients generated by pulsed laser flash photolysis. Mechanistic information from high-pressure effects
“…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%
“…These data and their mechanistic interpretation have been discussed in detail in the literature. [1][2][3][4][5][6][7][8][9][10] In our previous study, 11 we used the W(CO) 6 /1,10-phenanthroline system to investigate the above-outlined reactions in supercritical CO 2 and ethane. Activation volumes as high as +7000 cm 3 mol -1 were found just above the critical point in each of these fluids for the ring-closure reaction 2.…”
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.
“…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%
“…These data and their mechanistic interpretation have been discussed in detail in the literature. [1][2][3][4][5][6][7][8][9][10] In our previous study, 11 we used the W(CO) 6 /1,10-phenanthroline system to investigate the above-outlined reactions in supercritical CO 2 and ethane. Activation volumes as high as +7000 cm 3 mol -1 were found just above the critical point in each of these fluids for the ring-closure reaction 2.…”
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. ,− …”
Section: Introductionmentioning
confidence: 99%
“…The intimate nature of the ring-closure process was especially revealed by the more recently reported volumes of activation. − It could be demonstrated that the volume of activation is the best parameter to describe the nature of the different ring-closure mechanisms. The pressure dependence of these reactions was usually studied under conditions similar to those adopted for the ambient-pressure studies. − ,, In fact for L = phen, en (ethylenediamine), and dab (1,4-diisopropyl-1,4-diazabutadiene), the volumes of activation for M = Mo and W were all significantly negative and supported the operation of an associatively activated chelation mechanism. ,− For M = Cr, however, the volume of activation suggested a fine balance between the size of the metal center and steric hindrance on the free end of the bidentate ligand as the transition state is approached, which controlled the intimate nature of the chelation mechanism in terms of an associative or dissociative process.…”
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.
Complexes of the type cis-[PtPh 2 (CO)(η 1 -P−N)] and cis-[PtPh 2 (CO)(η 1 -P−S)], where bidentate phosphorus−nitrogen and phosphorus−sulfur ligands are bound to the metal centre in a monodentate fashion [P−N = Ph 2 PC 5 H 4 N (Ph 2 PPy), Ph 2 P(CH 2 ) 2 C 5 H 4 N (ppye), Ph 2 P(o-C 6 H 4 )NMe 2 (PNMe 2 ), Ph 2 P(CH 2 ) n NMe 2 (n = 2, 3, i.e., peNMe 2 and ppNMe 2 ) and P−S = Ph 2 P(CH 2 ) 2 SEt (P−SEt), Ph 2 P(CH 2 ) n SPh (n = 1, 2, i.e., P−CH 2 SPh and P−SPh)], were prepared in situ by reaction of the hybrid ligands with cis-[PtPh 2 (CO)(SEt 2 )]. In each case, the first observed process was the fast substitution of diethyl sulfide by the phosphanyl group leading to the monosubstituted ring-open cis-[PtPh 2 (CO)(η 1 -P−X)] (X = N or S) complexes, which were characterised in solution by 1 H and 31 P{ 1 H} NMR spectroscopy. These initially formed species undergo a slow ring closure process with extrusion of carbon monoxide and formation of the chelate [PtPh 2 (P−X)] products, except in the case of the short-bite Ph 2 PPy and P−CH 2 SPh ligands and of ppNMe 2 , where ring closure was not observed. The chelate complexes were isolated as solids
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