Oxo 2-Adamantylidene Complexes of Mo(VI) and W(VI)

Abstract

Molybdenum and tungsten oxo 2-adamantylidene (Adene) complexes that contain two nonafluoro-tert-butoxide (ORF9) ligands have been prepared through addition of 2-methylene- or 2-ethylideneadamantane to neophylidene or neopentylidene complexes. The isolated oxo complexes include W­(O)­(Adene)­(ORF9)2(PPh2Me) (1 W ), W­(O)­(Adene)­(ORF9)2 (2 W ), W­(O)­(Adene)­(2,5-dimethylpyrrolide)2 (3 W ), W­(O)­(Adene)­(2,5-dimethylpyrrolide)­(2,6-dimesitylphenoxide) (4 W ), Mo­(O)­(Adene)­(ORF9)2(PPhMe2) (1 Mo ), and Mo­(O)­(Adene)­(ORF9)2 (2 Mo ). Compound 2 W is a dimer that contains unsymmetrically bridging oxo ligands; it dissociates readily and reversibly into monomers, especially in the presence of a donor such as THF. In contrast, 2 Mo is a monomer. Both 2 W and 2 Mo are remarkably stable thermally. The pale-blue complexes Mo­(Adene)­(ORF9)2Cl2 (5 Mo ) and W­(Adene)­(ORF9)2Cl2 (5 W ) are formed upon addition of PCl5 to 2 Mo and 2 W , respectively. The oxo complexes are reactive olefin metathesis initiators, while 5 Mo and 5 W are relatively poor initiators. We ascribe the thermal stability of 2 Mo and 2 W to resistance of the 2-adamantylidene ligands to couple bimolecularly, to the absence of an α-hydrogen in the alkylidene, or both.

Effect of Lewis Acids on the Catalyst Activity for Alkene Metathesis, Z-/E- Selectivity and Stability of Tungsten Oxo Alkylidenes

“…In the subsequent years, the Schrock group reported other tungsten oxo alkylidenes and the role of ancillary ligands on the catalytic activity and product selectivity were deeply discussed (i.e. 15 and 16) [36][37][38][39][40][41][42]. Moreover, Buchmeiser and co-workers synthetized a series of very active cationic tungsten oxo alkoxy alkylidene bearing an N-heterocyclic carbene (NHC) as fourth ligand (17)(18) [43][44][45].…”

“…The knowledge acquired on tungsten oxo alkylidenes allowed the synthesis of molybdenum and vanadium oxo analogues as well as other molybdenum precursors that are also active in metathesis (22,23) [40,[52][53][54][55][56][57][58]. The number of existing molybdenum oxo alkylidenes is limited and their activity for olefin metathesis is usually lower than that of tungsten species.…”

W‐oxo Adamantylidenes: Stable Molecular Precursors for Efficient Silica‐Supported Metathesis Catalysts

“…[25 -27] However, recent synthetic developments in molecular chemistry have shown that W oxo adamantylidene complexes can be readily prepared (Scheme 1,c), possibly as a consequence of the sterically demanding adamantylidene, which prevents decomposition by homocoupling. [28] In this work, we investigate the grafting and reactivity of W-oxo adamantylidenes containing strongly donating X/Y ligands (pyrrolidine) 1 and weakly donating X/Y ligands (-OR F9 ) 2 (Scheme 1,c). We show that both species can be grafted to silica, with the activity of the grafted species following the established trends of activity with electron donation from the ligands.…”

“…Most such investigations have been performed on the larger family of imido complexes, [16,24] because isolable oxo complexes have been more difficult to prepare ( Scheme 1 ,b ) [25–27] . However, recent synthetic developments in molecular chemistry have shown that W oxo adamantylidene complexes can be readily prepared ( Scheme 1 ,c ), possibly as a consequence of the sterically demanding adamantylidene, which prevents decomposition by homocoupling [28] . In this work, we investigate the grafting and reactivity of W‐oxo adamantylidenes containing strongly donating X/Y ligands (pyrrolidine) 1 and weakly donating X/Y ligands (‐OR F9 ) 2 ( Scheme 1 ,c ).…”

Tungsten Sulfido Alkylidene and Cationic Tungsten Sulfido Alkylidene N-Heterocyclic Carbene Complexes

“…In the early 1980s, Schrock et al developed tungsten oxo alkylidene complexes, which were among the first well-defined high oxidation state complexes for olefin metathesis reactions. , The focus soon shifted to tungsten and molybdenum imido alkylidene complexes. − Recently, molybdenum oxo alkylidene complexes were added to the arsenal of olefin metathesis catalysts. − While the imido ligand in molybdenum and tungsten imido alkylidene complexes allows for a broad range of variations, the variability in the corresponding molybdenum and tungsten oxo alkylidene complexes is intrinsically lower. So far, reports on olefin metathesis-active tungsten alkylidene complexes with other ligands than oxo or imido ligands are scarce. − …”