Simple Molybdenum(IV) Olefin Complexes of the Type Mo(NR)(X)(Y)(olefin)

Syntheses of Tungsten Imido Cyclohexylidene Complexes Using Perfluoro-t-Butanol or Hexafluoro-t-Butanol as Acids

The fluorinated alcohols, (CF3)3COH (RF9OH) and (CF3)2MeCOH (RF6OH), react with W(NR)2Cy2 (Cy=Cyclohexyl; R=2,6-diisopropylphenyl or 1-adamantyl) in C6D6 at 55 °C to give cyclohexylidene complexes. Traditional routes to terminal alkylidene complexes (neopentylidene or neophylidene) have used either triflic acid or HCl (rarely), but relatively weak fluorinated acids are sufficient and active bisalkoxide catalysts are therefore prepared directly. An α hydrogen abstraction reaction to give a cyclohexylidene complex from a biscyclohexyl complex appears to be as facile as α hydrogen abstraction to give a neopentylidene or neophylidene ligand, but isomerization of a cyclohexene formed through β hydrogen abstraction is also a possibility. The ORF9 ligands can be replaced readily with dimethylpyrrolide (Me2Pyr) or other more basic alkoxides. Single crystal X-ray studies were carried out on W(NAr)2Cy2, W(NAr)(ORF9)2(C6H10)(ArNH2), W(NAr)(ORF6)2(C6H10)(ArNH2), W(NAd)(ORF9)2(C6H10)(AdNH2), W(NAr)(O-i-PrF6)3Cy, and W(NAd)(η1-Me2Pyr)2(C6H10) (C6H10=cyclohexylidene).

Thermal Formation of Metathesis-Active Tungsten Alkylidene Complexes from Cyclohexene

A 7-tungstabicyclo[4.3.0]nonane complex forms slowly upon addition of cyclohexene to the ethylene complex, W(NAr)(OSiPh3)2(C2H4), at 22 °C. A single-crystal X-ray study showed its structure to be closest to a square pyramid (τ = 0.23). At 22 °C, loss of cyclohexene or ring contraction of the 7-tungstabicyclo[4.3.0]nonane complex is slow. Above ∼80 °C, cyclohexene is ejected to give W(NAr)(OSiPh3)2(C2H4), but a sufficient amount of 7-tungstabicyclo[4.3.0]nonane complex remains in the presence of cyclohexene and the ring contracts to yield methylenecyclohexane and a methylidene complex or ethylene and a cyclohexylidene complex. Other complexes that have been observed include an 8-tungstabicyclo[4.3.0]nonane complex formed from 1,7-octadiene, a 7-tungstabicyclo[4.2.0]octane complex (formed from a methylidene complex and cyclohexene), and a methylenecyclohexane complex. 13C-Labeling studies show that the exo-methylene group in methylenecyclohexane and the α positions in the 8-tungstabicyclo[4.3.0]nonane come from ethylene. An alternative ring contraction of a tungstacyclopentane made from two molecules of cyclohexene cannot be excluded when concentrations of ethylene are low. A cyclohexylidene complex could also form from two cyclohexenes via a newly proposed "alkyl/allyl" mechanism. The results reported here are the first experimental confirmations that a tungstacyclopentane can ring-contract thermally at a substituted WCα position to form a tungstacyclobutane and therefore metathesis-active alkylidenes.

Reductive Retrocyclization of a Mangana(II)cyclopentasilane to Form Manganese(0) Bis(η2-disilene) Complexes

Ligand-exchange reactions on a mangana(II)cyclopentasilane complex that contains two THF ligands with aryl isocyanides led to the formation of manganese(0) bis(η2-disilene) complexes via a retrocyclization. In stark contrast, ligand-exchange reactions with CNtBu, an N-heterocyclic carbene, or pyridine-based ligands furnished manganese(II) complexes wherein the manganacyclopentasilane framework remained intact. The thermolysis of the obtained bis(η2-disilene) complex in the presence of mesityl isocyanide led to the formation of a cyclotetrasilane via the formal dimerization of the two η2-disilene moieties. The insertion of a mesityl isocyanide into the Mn-Siβ bond results in the formation of a manganese(II) complex supported by a [SiCSi]-type tridentate ligand scaffold.

Ring Contraction of a Tungstacyclopentane Supported on Silica: Direct Conversion of Ethylene to Propylene

The reaction of W(NAr)(¹³C₄H₈)(OSiPh₃)₂ (1) (NAr = 2,6-diisopropylphenylimido) with silica partially dehydroxylated at 700 °C (SiO_2-700) is highly dependent on the reaction conditions. The primary product of this reaction is W(NAr)(¹³C₄H₈)(OSiPh₃)(OSi(O-)₃) (2) when the reaction is carried out in the dark. Grafting 1 onto SiO_2-700 in ambient lab light results in the formation of 2, W(NAr)(¹³CH₂¹³CH₂)(OSiPh₃)(OSi(O-)₃) (4), and one isomer of square-pyramidal W(NAr)(¹³CH₂¹³CH(¹³Me)¹³CH₂)(OSiPh₃)(OSi(O-)₃) (3). Heating 2 to 85 °C for 6 h results in the formation of 3, 4, W(NAr)(¹³CH(¹³Me)¹³CH₂¹³CH₂)(OSiPh₃)(OSi(O-)₃) (5), and W(NAr)((¹³CH₂)₂¹³CH(¹³Me)(¹³CH₂)₂)(OSiPh₃)(OSi(O-)₃) (6). Photolysis of 2 with blue LEDs (λ_max = 450 nm) produces 4, both isomers of 3, 5, and free ethylene. In the presence of excess ethylene and blue LED irradiation at 85 °C, 1/SiO_2-700 catalyzes the direct conversion of ethylene to propylene.

Cationic molybdenum oxo alkylidenes stabilized by N-heterocyclic carbenes: from molecular systems to efficient supported metathesis catalysts

Cationic d0 group 6 olefin metathesis catalysts have been recently shown to display in most instances superior activity in comparison to their neutral congeners. Furthermore, their catalytic performance is greatly improved upon immobilization on silica. In this context, we have developed the new family of molecular cationic molybdenum oxo alkylidene complexes stabilized by N-heterocyclic carbenes of the general formula [Mo(O)(CHCMe3)(IMes)(OR)[X-]] (IMes = 1,3-dimesitylimidazol-2-ylidene; R = 1,3-dimesityl-C6H3, C6F5; X- = B(3,5-(CF3)2C6H3)4 -, B(ArF)4, tetrakis(perfluoro-t-butoxy)aluminate (PFTA)). Immobilization of [Mo(O)(CHCMe3)(IMes)(O-1,3-dimesityl-C6H3)+B(ArF)4 -] on silica via surface organometallic chemistry yields an active alkene metathesis catalyst that shows the highest productivity towards terminal olefins amongst all existing molybdenum oxo alkylidene catalysts.

Tungstacyclopentane Ring Contraction Yields Olefin Metathesis Catalysts

Exposure of a solution of the square pyramidal tungstacyclopentane complex W(NAr)(OSiPh₃)₂(C₄H₈) (Ar = 2,6-i-Pr₂C₆H₃) to ethylene at 22 °C in ambient (fluorescent) light slowly leads to the formation of propylene and the square pyramidal tungstacyclobutane complex W(NAr)(OSiPh₃)₂(C₃H₆). No reaction takes place in the dark, but the reaction is >90% complete in ∼15 min under blue LED light (∼450 nm λ_max). The intermediates are proposed to be (first) an α methyl tungstacyclobutane complex (W(NAr)(OSiPh₃)₂(αMeC₃H₅)), and then from it, a β methyl version. The TBP versions of each can lose propylene and form a methylene complex, and in the presence of ethylene, the unsubstituted tungstacyclobutane complex W(NAr)(OSiPh₃)₂(C₃H₆). The W-C_α bond in an unobservable TBP W(NAr)(OSiPh₃)₂(C₄H₈) isomer in which the C₄H₈ ring is equatorial is proposed to be cleaved homolytically by light. A hydrogen atom moves or is moved from C3 to the terminal C4 carbon in the butyl chain as the bond between W and C3 forms to give the TBP α methyl tungstacyclobutane complex. Essentially, the same behavior is observed for W(NCPh₃)(OSiPh₃)₂(C₄H₈) as for W(NAr)(OSiPh₃)₂(C₄H₈), except that the rate of consumption of W(NCPh₃)(OSiPh₃)₂(C₄H₈) is about half that of W(NAr)(OSiPh₃)₂(C₄H₈). In this case, an α methyl-substituted tungstacyclobutane intermediate is observed, and the overall rate of formation of W(NCPh₃)(OSiPh₃)₂(C₃H₆) and propylene from W(NCPh₃)(OSiPh₃)₂(C₄H₈) is ∼20 times slower than in the NAr system. These results constitute the first experimentally documented examples of forming a metallacyclobutane ring from a metallacyclopentane ring (ring contraction) and establish how metathesis-active methylene and metallacyclobutane complexes can be formed and reformed in the presence of ethylene. They also raise the possibility that ambient light could play a role in some metathesis reactions that involve ethylene and tungsten-based imido alkylidene olefin metathesis catalysts, if not others.

Synthesis and structure of a pyridine-stabilized silanone molybdenum complex and its reactions with PMe3 and acetone

The synthesis, structure and reactivity of a pyridine-stabilized silanone molybdenum complex are described.

Interconversion of Molybdenum or Tungsten d2 Styrene Complexes with d0 1-Phenethylidene Analogues

Upon addition of 5-15% PhNMe₂H⁺X^- (X = B(3,5-(CF₃)₂C₆H₃)₄ or B(C₆F₅)₄) to Mo(NAr)(styrene)(OSiPh₃)₂ (Ar = N-2,6-i-Pr₂C₆H₃) in C₆D₆ an equilibrium mixture of Mo(NAr)(styrene)(OSiPh₃)₂ and Mo(NAr)(CMePh)(OSiPh₃)₂ is formed over 36 h at 45 °C (K_eq = 0.36). A plausible intermediate in the interconversion of the styrene and 1-phenethylidene complexes is the 1-phenethyl cation, [Mo(NAr)(CHMePh)(OSiPh₃)₂]⁺, which can be generated using [(Et₂O)₂H][B(C₆F₅)₄] as the acid. The interconversion can be modeled as two equilibria involving protonation of Mo(NAr)(styrene)(OSiPh₃)₂ or Mo(NAr)(CMePh)(OSiPh₃)₂ and deprotonation of the α or β phenethyl carbon atom in [Mo(NAr)(CHMePh)(OSiPh₃)₂]⁺. The ratio of the rate of deprotonation of [Mo(NAr)(CHMePh)(OSiPh₃)₂]⁺ by PhNMe₂ in the α position versus the β position is ∼10, or ∼30 per H_β. The slow step is protonation of Mo(NAr)(styrene)(OSiPh₃)₂ (k₁ = 0.158(4) L/(mol·min)). Proton sources such as (CF₃)₃COH or Ph₃SiOH do not catalyze the interconversion of Mo(NAr)(styrene)(OSiPh₃)₂ and Mo(NAr)(CMePh)(OSiPh₃)₂, while the reaction of Mo(NAr)(styrene)(OSiPh₃)₂ with pyridinium salts generates only a trace (∼2%) of Mo(NAr)(CMePh)(OSiPh₃)₂ and forms a monopyridine adduct, [Mo(NAr)(CHMePh)(OSiPh₃)₂(py)]⁺ (two diastereomers). The structure of [Mo(NAr)(CHMePh)(OSiPh₃)₂]⁺ has been confirmed in an X-ray study; there is no structural indication that a β proton is activated through a CH_β interaction with the metal. W(NAr)(CMePh)(OSiPh₃)₂ is also converted into a mixture of W(NAr)(CMePh)(OSiPh₃)₂ and W(NAr)(styrene)(OSiPh₃)₂ (K_eq = 0.47 at 45 °C in favor of the styrene complex) with 10% [PhNMe₂H][B(C₆F₅)₄] as the catalyst; the time required to reach equilibrium is approximately the same as in the Mo system.

Silica-Supported Molybdenum Oxo Alkylidenes: Bridging the Gap between Internal and Terminal Olefin Metathesis

Grafting a molybdenum oxo alkylidene on silica (partially dehydroxylated at 700 °C) affords the first example of a well-defined silica-supported Mo oxo alkylidene, which is an analogue of the putative active sites in heterogeneous Mo-based metathesis catalysts. In contrast to its tungsten analogue, which shows poor activity towards terminal olefins because of the formation of a stable off-cycle metallacyclobutane intermediate, the Mo catalyst shows high metathesis activity for both terminal and internal olefins that is consistent with the lower stability of Mo metallacyclobutane intermediates. This Mo oxo metathesis catalyst also outperforms its corresponding neutral silica-supported Mo and W imido analogues.

C-H Activation and Proton Transfer Initiate Alkene Metathesis Activity of the Tungsten(IV)-Oxo Complex

In alkene metathesis, while group 6 (Mo or W) high-oxidation state alkylidenes are accepted to be key reaction intermediates for both homogeneous and heterogeneous catalysts, it has been proposed that low valent species in their +4 oxidation state can serve as precatalysts. However, the activation mechanism for these latter species-generating alkylidenes-is still an open question. Here, we report the syntheses of tungsten(IV)-oxo bisalkoxide molecular complexes stabilized by pyridine ligands, WO(OR)₂py₃ (R = CMe(CF₃)₂ (2a), R = Si(O tBu)₃ (2b), and R = C(CF₃)₃ (2c); py = pyridine), and show that upon activation with B(C₆F₅)₃ they display alkene metathesis activities comparable to W(VI)-oxo alkylidenes. The initiation mechanism is examined by kinetic, isotope labeling and computational studies. Experimental evidence reveals that the presence of an allylic CH group in the alkene reactant is crucial for initiating alkene metathesis. Deuterium labeling of the allylic C-H group shows a primary kinetic isotope effect on the rate of initiation. DFT calculations support the formation of an allyl hydride intermediate via activation of the allylic C-H bond and show that formation of the metallacyclobutane from the allyl "hydride" involves a proton transfer facilitated by the coordination of a Lewis acid (B(C₆F₅)₃) and assisted by a Lewis base (pyridine). This proton transfer step is rate determining and yields the metathesis active species.

Synthesis, structures, and reactivity of the base-stabilized silanone molybdenum complexes

Base-stabilized silanone molybdenum complexes were synthesized by the oxygenation of the MSi bond in the silyl(silylene)molybdenum complex with 1 eq. of PNO in the presence of Lewis base L.

Z-selective metathesis homocoupling of 1,3-dienes by molybdenum and tungsten monoaryloxide pyrrolide (MAP) complexes

Molybdenum or tungsten monoaryloxide pyrrolide (MAP) complexes that contain OHIPT as the aryloxide (hexaisopropylterphenoxide) are effective catalysts for homocoupling of simple (E)-1,3-dienes to give (E,Z,E)-trienes in high yield and with high Z selectivities. A vinylalkylidene MAP species was shown to have the expected syn structure in an X-ray study. MAP catalysts that contain OHMT (hexamethylterphenoxide) are relatively inefficient.

Mechanistic aspects of hydrosilylation catalyzed by (ArN=)Mo(H)(Cl)(PMe3)3

The reaction of (ArN=)MoCl(2)(PMe(3))(3) (Ar = 2,6-diisopropylphenyl) with L-Selectride gives the hydrido-chloride complex (ArN=)Mo(H)(Cl)(PMe(3))(3) (2). Complex 2 was found to catalyze the hydrosilylation of carbonyls and nitriles as well as the dehydrogenative silylation of alcohols and water. Compound 2 does not show any productive reaction with PhSiH(3); however, a slow H/D exchange and formation of (ArN=)Mo(D)(Cl)(PMe(3))(3) (2(D)) was observed upon addition of PhSiD(3). Reactivity of 2 toward organic substrates was studied. Stoichiometric reactions of 2 with benzaldehyde and cyclohexanone start with dissociation of the trans-to-hydride PMe(3) ligand followed by coordination and insertion of carbonyls into the Mo-H bond to form alkoxy derivatives (ArN=)Mo(Cl)(OR)(PMe(2))L(2) (3: R = OCH(2)Ph, L(2) = 2 PMe(3); 5: R = OCH(2)Ph, L(2) = η(2)-PhC(O)H; 6: R = OCy, L(2) = 2 PMe(3)). The latter species reacts with PhSiH(3) to furnish the corresponding silyl ethers and to recover the hydride 2. An analogous mechanism was suggested for the dehydrogenative ethanolysis with PhSiH(3), with the key intermediate being the ethoxy complex (ArN=)Mo(Cl)(OEt)(PMe(3))(3) (7). In the case of hydrosilylation of acetophenone, a D-labeling experiment, i.e., a reaction of 2 with acetophenone and PhSiD(3) in the 1:1:1 ratio, suggests an alternative mechanism that does not involve the intermediacy of an alkoxy complex. In this particular case, the reaction presumably proceeds via Lewis acid catalysis. Similar to the case of benzaldehyde, treatment of 2 with styrene gives trans-(ArN=)Mo(H)(η(2)-CH(2)═CHPh)(PMe(3))(2) (8). Complex 8 slowly decomposes via the release of ethylbenzene, indicating only a slow insertion of styrene ligand into the Mo-H bond of 8.