Origin and Use of Hydroxyl Group Tolerance in Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Catalysts

Abstract

The origin of hydroxyl group tolerance in neutral and especially cationic molybdenum imido alkylidene N‐heterocyclic carbene (NHC) complexes has been investigated. A wide range of catalysts was prepared and tested. Most cationic complexes can be handled in air without difficulty and display an unprecedented stability towards water and alcohols. NHC complexes were successfully used with substrates containing the hydroxyl functionality in acyclic diene metathesis polymerization, homo‐, cross and ring‐opening cross metathesis reactions. The catalysts remain active even in 2‐PrOH and are applicable in ring‐opening metathesis polymerization and alkene homometathesis using alcohols as solvent. The use of weakly basic bidentate, hemilabile anionic ligands such as triflate or pentafluorobenzoate and weakly basic aromatic imido ligands in combination with a sterically demanding 1,3‐dimesitylimidazol‐2‐ylidene NHC ligand was found essential for reactive and yet robust catalysts.

Origin and Use of Hydroxyl Group Tolerance in Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Catalysts

“…This can be observed in the single crystal X-ray structure of 4 d (Figure 4). Coordination of acetonitrile trans to the NHC is also observed with mono- [30] dentate ligands, as is seen in the structures of 5 d and its acetonitrile adduct ( Figure S128,129, Supporting Information). 4 d crystallizes in the triclinic space group P-1 with a = 1279.17(5) pm, b = 1815.54 (7) pm, c = 1860.54(8) pm, a = 64.059(2)8, b = 70.333(2)8, g = 73.654(2)8 (Z = 2).…”

“…To finally prove the inherent stability of these catalysts against moisture, we were able to crystallize 4 a as the dihydrate complex ( Figure 5). [30] the NHC causing the stronger trans-effect, the MoÀO(water) bond is significantly longer than the Mo À O(water) bond trans to the alkylidene (232 vs. 219 pm).…”

“…However, the monotriflate-monocarboxylate spe- Thermal ellipsoids are set at a 50 % probability level. [30] Scheme 2. Synthesis of cationic monocarboxylate species 11-15 from bistriflates 9 a-d [12,15] via mixed monotriflate/monocarboxylate complex 10 or in situ generation of cationic monotriflate species and subsequent salt metathesis.…”

“…Thermal ellipsoids are set at a 50 % probability level. The B(Ar F ) 4 anion and hydrogens except for H on C37 have been omitted for clarity [30].…”

Silica‐Supported Cationic Tungsten Imido Alkylidene Stabilized by an N‐Heterocyclic Carbene Ligand Boosts Activity and Selectivity in the Metathesis of α‐Olefins

“…Alkene metathesis is a robust and atom‐economic synthetic method for the creation of carbon‐carbon double bonds; it has thus found ample use in both academia and industry [1–3] . Over the last decades, substantial efforts have been put in developing efficient molecular catalysts, [4–15] but immobilized analogs have received increasing attention because of their benefits for process intensification [16,17] and their improved performance with the development of well‐defined supported systems, mostly prepared through surface organometallic chemistry (SOMC) [18–23] . SOMC, a molecular approach to generate well‐defined heterogeneous catalysts, in which the surface is exploited as a ligand to covalently anchor molecular complexes, has been particularly successfully employed for the preparation of silica‐supported Schrock‐type metathesis catalysts of the general formula [(≡SiO)M(E)(=CHR)(X)] (M=Mo or W, E=NR or O), that display high activity, selectivity and stability with performances often surpassing those of their homogeneous counterparts ( Scheme 1).…”