Cationic Tungsten Oxo Alkylidene N-Heterocyclic Carbene Complexes via Hydrolysis of Cationic Alkylidyne Progenitors

Abstract

A route to cationic tungsten alkylidyne N-heterocyclic carbene (NHC) complexes of the general formula [W­(CR)­X2(NHC)­(tBuCN) y +B­(ArF)4 –] (R = tBu, C6H4-4-OMe; X = Br, Cl, OTf; NHC = 1,3-dimesityl-1,3-dihydro-2H-imidazol-2-ylidene, IMes; 1,3-bis­(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene, IDipp; 1,3-dimesityl-4,5-dichloro-1,3-dihydro-2H-imidazol-2-ylidene, IMesCl2; y = 1, 2; ArF = 3,5-bis­(trifluoro­methyl)­phenyl) is presented. Addition of 1 equiv of water to cationic tungsten benzylidyne and neopentylidyne NHC complexes results in the formation of the corresponding cationic tungsten oxo alkylidene complexes; however, only the resulting neopentylidene complexes were isolated as stable compounds and tested in selected olefin metathesis reactions. Interestingly, even though a strongly basic ligand in the form of the NHC is coordinated to the metal center, this bond is unaffected by the reaction with 1 equiv of water. The hydrolysis of cationic tungsten alkylidyne NHC complexes represents one pathway to cationic tungsten oxo alkylidene NHC complexes and enables the synthesis of complexes, which were so far not easily accessible by previously published procedures.

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

“…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]. Both Schrock and Buchmeiser tungsten oxo alkylidenes were heterogenized by grafting on silica (19)(20) [46][47][48].…”

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

“…Gratifyingly, the reaction of [W(CCMe 3 )-Cl 2 (tBuCN) 2 (IDipp) + B(Ar F ) 4 − ] with H 2 S proceeded without complication, while the analogous oxo complex could not be isolated via the reaction with H 2 O. 22 With Schrock catalysts as well as with cationic metal alkylidene NHC complexes, productivities can often be increased by variation of the anionic ligands from halides to fluorinated and/or bulky alkoxides or phenolates, 4,25−27,30−32 which is why the chlorides in W03−W05 were reacted with lithium pentafluorophenoxide and 2,6-diphenylphenoxide, respectively. Reaction with lithium pentafluorophenoxide in CH 2 Cl 2 resulted in the formation of W06, W07, and W08 in 63, 63, and 68% isolated yield, respectively.…”

“…One exception was allylbenzene, for which W09 showed the highest productivity with a turnover number (TON) of 1070. The TONs for 1- octene were all in a similar range and comparable to the productivities of the analogous tungsten oxo alkylidene 22 and tungsten imido alkylidene complexes. 26 With allyl sulfide, complexes W06 and W07 containing the pentafluorophenoxide ligand showed high productivities of 1310 and 1240, respectively.…”

“…To gain further insights into the charge distribution in cationic tungsten sulfido alkylidene NHC complexes, the atomic charges were calculated for W06−W08 and W10 and the analogous tungsten oxo and tungsten imido alkylidene NHC pentafluorophenoxide complexes 22,26 Calculations were carried out at the PBE0-D3BJ/def2-TZVP level for PBE0-D3BJ/def2-SVP optimized geometries with the charge model 5 (CM5) analogously to previous calculations for cationic molybdenum imido alkylidene NHC complexes and the Schrock catalyst [Mo(N-2,6-dimethylphenyl)(CHCMe 2 Ph)-(OC(CF 3 ) 3 ) 2 ]. 39 It should be mentioned that these calculated charges are not physical observables and should not be interpreted as approximations for oxidation states.…”

“…7,8,10,20,21 We recently outlined the synthesis of cationic tungsten oxo NHC complexes via the hydrolysis of cationic tungsten alkylidyne NHC complexes. 22 Since the hydrolysis of alkylidyne complexes proved to be viable in the synthesis of molybdenum and tungsten oxo alkylidene complexes, we were interested in if tungsten alkylidyne complexes would react in a similar fashion with hydrogen sulfide, which is in view of the much higher acidity of H 2 S (pK a = 6.92) compared to water an anything but trivial undertaking.…”