Molecular and Silica‐Supported Mo and W d⁰ Imido‐Methoxybenzylidene Complexes: Structure and Metathesis Activity

Abstract

5‐Coordinated methoxybenzylidene complexes M(=NAr)(=CH−C6H4−o‐OMe)(OtBuF3)2 (Ar=2,6‐iPr2C6H3; tBuF3=CMe2(CF3)) of Mo (1mMo) and W (1mW) were synthesized by cross‐metathesis from the corresponding neophylidene/neopentylidene precursors and o‐methoxystyrene. 1mMo and 1mW were grafted onto the surface of silica partially dehydroxylated at 700 °C to give well‐defined silica‐supported alkylidenes (≡SiO)M(=NAr)(=CH−C6H4−o‐OMe)(OtBuF3) (M=Mo (1Mo), W (1W)). Supported methoxybenzylidene complexes were tested in metathesis of cis‐4‐nonene, 1‐nonene, and ethyl oleate, and compared to their molecular precursors and supported classical analogs (≡SiO)M(=NAr)(=CHCMe2R)(OtBuF3) (M=Mo, R=Ph (2Mo), M=W, R=Me (2W)). Both grafted complexes 1Mo and 1W show significantly better performance as compared to their molecular precursors 1mMo and 1mW but are less efficient than the classical 4‐coordinated alkylidenes 2Mo and 2W. Noteworthy, both 1Mo and 1W can reach equilibrium conversion in metathesis of cis‐4‐nonene at catalyst loadings as low as 50 ppm.

Olefin metathesis: what have we learned about homogeneous and heterogeneous catalysts from surface organometallic chemistry?

“…77,78,80,[131][132][133] To probe the applicability of SOMC catalysts towards this type of olens, ethyl (or methyl) oleate was typically used as a test substrate in some of the studies cited above. While it is very early to draw any structure-activity relationships in this area, these results have generally proven that well-dened silicasupported catalysts based on Re, 134 Mo, 59,60,63,64,108,135 and W 101,106,107,122,123,135 alkylidenes are sufficiently functional group tolerant and can easily reach TONs of 500-1000, with the best record so far being the cationic W oxo NHC complex on silica reaching TON of ca. 12 000 at 10 ppm catalyst loading.…”

Metal‐Surface Interactions and Surface Heterogeneity in ‘Well‐Defined’ Silica‐Supported Alkene Metathesis Catalysts: Evidences and Consequences

“…We then further investigated the structure of both molecular and grafted species by measuring and computing the chemical shift anisotropy (CSA) of their alkylidene carbons, as CSA can provide information about dynamics or small changes in the electronic structure of a metal site . The measured CSA parameters for the molecular complex 1 at 100 K are as follows: δ iso =300 ppm; δ 11 =621 ppm, δ 22 =161 ppm, δ 33 =118 ppm, which yields a span (Ω)=503 ppm and a skew ( κ )=−0.83 (see Supporting Information ).…”

“…The plateau is reached when the first species are deactivated, possibly through a more facile β ‐H transfer because the metal site is low‐coordinate. Stabilization by additional coordinating ligand is found in several olefin metathesis catalysts, such as Grubbs–Hoveyda catalysts and supported Ru‐based catalysts, as well as in group 6 d 0 systems . During the second active phase, the previously dormant sites, that still needs to be activated and requires first to react with a terminal olefin, catalyze the metathesis reaction.…”

“…We then further investigated the structure of both molecular and grafted species by measuring and computing the chemical shift anisotropy (CSA) of their alkylidene carbons, as CSA can provide information about dynamics or small changes in the electronic structure of a metal site. [32,33] The measured CSA parameters for the molecular complex 1 at 100 K are as follows: δ iso = 300 ppm; δ 11 = 621 ppm, δ 22 = 161 ppm, δ 33 = 118 ppm, which yields a span (Ω) = 503 ppm and a skew (к) = À 0.83 (see Supporting Information). Experimental values are reproduced well by DFT calculation (δ iso = 319 ppm with δ 11 = 693 ppm, δ 22 = 149 ppm, δ 33 = 114 ppm) giving confidence in the DFT modeling using two component zeroth order regular approximation (ZORA) calculations including spin-orbit coupling.…”

“…Stabilization by additional coordinating ligand is found in several olefin metathesis catalysts, such as Grubbs -Hoveyda catalysts [42,43] and supported Ru-based catalysts, [44] as well as in group 6 d 0 systems. [33,45,46] During the second active phase, the previously dormant sites, that still needs to be activated and requires first to react with a terminal olefin, catalyze the metathesis reaction. Since no plateau is observed in the presence of 1-nonene additive, it is likely that, during the slow intermediate regime, terminal olefins are formed through isomerization of internal olefins, and serve to initiate the second, less reactive site.…”