Motional heterogeneity in single-site silica-supported species revealed by deuteron NMR.

The molecular dynamics of [-SiDMe(2)] grafted on two amorphous silica materials, mesoporous SBA and non-porous Aerosil, was investigated by deuteron ((2)H) solid-state NMR spectroscopy. Quadrupole echo (QE), quadrupole Carr-Purcell-Meiboom-Gill (QCPMG) and magic angle spinning (MAS) spectra were recorded as a function of temperature. These were analyzed to determine the rates and trajectories of molecular motion of the surface species. The dynamics were modelled as a composite two frame motion with independent rotations around the two Si-O bonds. In the first frame there are fast three-site jumps of the -SiDMe(2) group described by a single rate (k(1)) and unequal populations of the tetrahedral sites, such that the ratio D : Me : Me is around 1 : 4 : 4. In the second frame, the Si-O axis makes small step, nearest-neighbour jumps at a rate k(2) along an arc defined by the rim of a cone with a fixed half-angle. Both rates were found to be in the fast motional regime (k(1,2) > 10(10) s(-1)) throughout the experimentally accessible temperature range, 190-350 K. The experimental data are compatible only with models that include a distribution of arc lengths, lambda, in the second frame. The best fit of the simulations to the experimental data yields the distributions of the arc length. The results unequivocally demonstrate that even though the sites all have the same average environment, as reported by the isotropic chemical shifts, the dynamics of the grafted species are microscopically spatially heterogeneous with different molecules on the surface having different ranges of motional trajectories and populations. Furthermore, a clear difference in dynamic behavior is observed between the two silica supports, the motion being more constrained on the mesoporous SBA. This differential mobility is possibly due to differences in surface roughness and to the pore structure of SBA compared with the smoother surface of Aerosil.

Metal fragment isomerisation upon grafting a d(2) ML4 perhydrocarbyl Os complex on a silica surface: origin and consequence.

Grafting of Os bisalkylidene complexes, [Os(=CHtBu)(2)(CH(2)tBu)(2)], on a silica partially dehydroxylated at 700 degrees C selectively yields an alkylidyne complex [([triple bond]SiO)Os([triple bond]CtBu)(CH(2)tBu)(2)] according to mass balance analysis, IR and solid state NMR spectroscopies, but 70% of the silanols remains unreacted. Grafting corresponds to a replacement of one alkyl by a siloxy ligand and induces the isomerisation of the metal fragment from a bis-alkylidene to an alkyl alkylidyne. Molecular (B3PW91) and periodic DFT-calculations show that the bis-alkylidene is the energetically favoured isomer for the perhydrocarbyl complex, while the alkyl alkylidyne isomer is more stable when one of the alkyl ligands is replaced by a siloxy ligand. The change of the nature of the ligand is accompanied with a change of geometry: from a distorted tetrahedral structure for [Os(=CHtBu)(2)(CH(2)tBu)(2)] to a butterfly-geometry with apical siloxy and alkylidyne ligands for [([triple bond]SiO)Os([triple bond]CtBu)(CH(2)tBu)(2)]. Finally, DFT calculations show that grafting occurs via a sigma-bond metathesis between the silanol and a metal-alkyl bond and not through the typical addition of the silanol onto the alkylidene ligand.

Well-defined silica-supported Mo-alkylidene catalyst precursors containing one or substituent: methods of preparation and structure-reactivity relationship in alkene metathesis.

The monosiloxy surface complexes [([triple bond]SiO)Mo([triple bond]NAr)(=CHCMe(2)R')(OR)] (R' = Me or Ph; OR = OtBu, OCMe(CF(3))(2) or OAr) are obtained by grafting onto SiO(2-(700)) either symmetric Mo-alkylidene derivatives [Mo([triple bond]NAr)(=CHCMe(2)R')(OR)(2)] or asymmetric derivatives, that is, with two different pendant ligands, one amido and one alkoxy/aryloxy, [Mo([triple bond]NAr)(=CHCMe(2)R')(OR)(NC(6)H(8))]. The formation of these complexes was confirmed by mass-balance analysis, and infrared (IR) and NMR spectroscopies. These systems are highly efficient catalyst precursors for the metathesis of acyclic alkenes; the best results were seen when OR=OCMe(CF(3))(2). Nevertheless, they display poor performances in ring-closing metathesis, possibly due to the rigidity of the metal center (as evidenced by NMR spectroscopy), which therefore slows the rate of the metathesis.

Direct observation of reaction intermediates for a well defined heterogeneous alkene metathesis catalyst.

Grafting of [W([triple bond]NAr)(=CHtBu)(2,5-Me(2)NC(4)H(2))(2)] on a silica partially dehydroxylated at 700 degrees C (SiO(2- (700))) generates the corresponding monosiloxy complex [([triple bond]SiO)W([triple bond]NAr)(=CHtBu)(2,5-Me(2)NC(4)H(2))] as the major species (approximately 90%) along with [([triple bond]SiO)W([triple bond]NAr)(CH(2)tBu)(2,5-Me(2)NC(4)H(2))(2)], according to mass balance analysis, IR, and NMR studies. This heterogeneous catalyst displays good activity and stability in the metathesis of propene. Very importantly, solid state NMR spectroscopy allows observation of the propagating alkylidene as well as stable metallacyclobutane intermediates. These species have the same reactivity as the initial surface complex [([triple bond]SiO)W([triple bond]NAr)(=CHtBu)(2,5-Me(2)NC(4)H(2))], which shows that they are the key intermediates of alkene metathesis.

Dramatic enhancement of the alkene metathesis activity of Mo imido alkylidene complexes upon replacement of one tBuO by a surface siloxy ligand.

[(triple bond SiO)Mo(triple bond NAr)(=CHCMe2R)(OtBu)], a well-defined silica supported alkene metathesis catalyst precursor, shows a dramatic enhancement of activity and selectivity compared to [Mo(triple bond NAr)(=CHCMe2R)(OtBu)2] and [(triple bond SiO)Mo(triple bond NAr)(=CHCMe2R)(CH2tBu)], respectively.