Molybdenum and Tungsten Alkylidyne Complexes Containing Mono-, Bi-, and Tridentate N-Heterocyclic Carbenes

Abstract

New tungsten and molybdenum alkylidyne complexes bearing mono-, bi-, and tridentate N-heterocyclic carbenes (NHCs) have been synthesized. Formation of unprecedented structures in complexes bearing N-tert-butyl substituents on the imidazol(in)-2-ylidene was observed, leading to molybdenum complexes containing an abnormal carbene (Mo-4) and a bridging O,C,C-pincer ligand (Mo-10) and to a tungsten complex containing a cationic imidazolinium-tagged alkoxide forming an inner salt with an anionic tungsten center (W-5). Both the abnormal carbene binding in Mo-4 and the O,C,C-pincer-type structure of Mo-10 were confirmed by singlecrystal X-ray analysis, and the proposed structure of W-5 is supported by the single-crystal X-ray structure of a minor byproduct (W-8) formed during the synthesis of W-4, displaying the aforementioned inner-salt-like structure. The novel alkylidyne complexes were also investigated for their capability to form a previously postulated quasi-cationic species with a weakly coordinating anion (WCA) during the alkyne homometathesis of 1phenyl-1-propyne. Overall, incorporation of bidentate and strongly σ donating NHCs as well as introduction of better leaving groups did not lead to the expected increase in catalytic activity. Despite identical ligand spheres, changing from molybdenum to tungsten led to complete loss of activity in the bidentate systems.

Recent Advances in Catalysis Involving Bidentate N-Heterocyclic Carbene Ligands

“…Research areas where metal mediated organic molecule transformations were conducted using bidentate NHC ligands to a lesser degree include reduction of carbonyl compounds using hydrosilylation process [ 209 , 214 , 215 , 216 , 217 , 218 , 219 ], isomerization of allylic alcohols [ 220 ], cross metathesis [ 221 ], arene borylation [ 222 , 223 ], amination [ 224 ], reduction of sulfoxides [ 225 ], alcohol oxidation [ 132 , 226 , 227 ], CO 2 reduction [ 228 , 229 , 230 , 231 ], norbornene polymerization [ 232 , 233 , 234 ], alkylation of amines [ 235 ], C-H borylation [ 236 ], CO 2 activation [ 237 ], hydroamination [ 238 ], functionalization of propane [ 239 , 240 ], ylide cycloaddition [ 74 ], three component coupling [ 241 ], hydroxylation of benzene [ 242 ], hydroaminoethlyation [ 243 ], water oxidation [ 244 ], amine alkylation [ 245 ], homopolymerization of ethylene [ 246 ], destruction of chemical warfare [ 247 ], Friedel-Crafts alkylation [ 248 ], C–H arylation [ 249 ], C–H oxidation [ 250 ], SABRE catalyst [ 251 ], and alcohol amidation reactions [ 211 ]. S...…”

“…Due to their strong σ donating properties, bidentate N-heterocyclic carbene ligands were recently utilized to stabilize alkylidyne molybdenum and tungsten cations [ 255 ]. While there are numerous examples of metal complexes of these ligands, there are few well-defined complexes which are structurally characterized and for which the mechanistic pathways for the metathesis reactions are well understood [ 246 ]. The general structure of the complex, consists of a five coordinate 14-electron molybdenum alkylidyne complex bearing alkoxy and carbene ligands which is active in alkyne metathesis reactions.…”

“…Depending on the noncoordinating substituent on NHC ligand, different yields from substitution of one alkoxy and dimethylethoxy ligands were obtained. In the case of N -tert-butyl, a C-H bond activation and subsequent elimination of alkoxide was also observed [ 246 ].…”

Catalytic Alkyne and Diyne Metathesis with Mixed Fluoroalkoxy-Siloxy Molybdenum Alkylidyne Complexes

“…16,19 Convenient access to these complexes was provided by the development of reliable protocols for their preparation from Mo(CO)6 or W(CO)6, respectively, 20,21 which also furnished MoF6-type initiators for ring-opening alkyne metathesis polymerization (ROAMP) [22][23][24] as well as N-heterocyclic carbene (NHC) adducts of related molybdenum and tungsten benzylidyne complexes. [25][26][27][28] Recently, related molybdenum benzylidyne complexes were even used for olefin metathesis reactions. 29 Siloxide-supported alkylidyne complexes are another important class of alkyne metathesis catalysts, 30 and the triphenylsiloxy molybdenum complex I is probably the most frequently used species, 31,32 in particular for natural product synthesis through ring-closing alkyne metathesis (RCAM).…”

183W NMR Spectroscopy Guides the Search for Tungsten Alkylidyne Catalysts for Alkyne Metathesis

“…[16][17][18][19] Related tungsten alkylidynes, most notably complexes of types 5 and 6 carrying more electron-withdrawing (partly) fluorinated alkoxide ligands, have also been thoroughly studied from the organometallic viewpoint, [20][21][22][23] but the relatively few available reactivity data do not convincingly prove that their scope is significantly broader than that of 4a. [24] Results and Discussion Tungsten Alkylidynes with aT ripodal Silanolate Ligand Architecture When seen against this backdrop,i ti sp erhaps surprising that tungsten alkylidynes with silanolates as ancillary ligands are conspicuously underrepresented in the literature (Scheme 1). Then eopentylidyne complex 8a is accessible from 7aby salt metathesis but was found to degrade rapidly in solution as well as in the solid state;the derived phenanthroline adduct [8a•(phen)] is stable but kinetically inert and hence no adequate precatalyst either.…”