Rapid Access to an Oxido-Alkylidene Complex of Mo(VI)

Varjas, Christopher J.; Powell, Douglas R.; Thomson, Robert K.

doi:10.1021/acs.organomet.5b00686

Cited by 14 publications

(19 citation statements)

References 51 publications

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“…The Mo=O bond length in 2 is substantially elongated in comparison to the parent complex 1 , 1.8221(9) versus 1.7198(18) Å . Previously reported molybdenum(VI) oxido borane adducts showed a similar bond elongation . The bonds from all other donor atoms to the metal center are slightly shortened, causing a decreased flexibility around the metal center, thereby most likely preventing isomerization (vide supra).…”

Section: Resultsmentioning

confidence: 99%

Heterolytic Si−H Bond Cleavage at a Molybdenum‐Oxido‐Based Lewis Pair

Zwettler

Walg

Belaj

et al. 2018

Chemistry A European J

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The reaction of a molybdenum(VI) oxido imido complex with the strong Lewis acid B(C6F5)3 gave access to the Lewis adduct [Mo{OB(C6F5)3}(NtBu)L2] featuring reversible B−O bonding in solution. The resulting frustrated Lewis pair (FLP)‐like reactivity is reflected by the compound's ability to heterolytically cleave Si−H bonds, leading to a clean formation of the novel cationic MoVI species 3 a (R=Et) and 3 b (R=Ph) of the general formula [Mo(OSiR3)(NtBu)L2][HB(C6F5)3]. These compounds possess properties highly unusual for molybdenum d0 species such as an intensive, charge‐transfer‐based color as well as a reversible redox couple at very low potentials, both dependent on the silane used. Single‐crystal X‐ray diffraction analyses of 2 and 4 b, a derivative of 3 b featuring the [FB(C6F5)3]− anion, picture the stepwise elongation of the Mo=O bond, leading to a large increase in the electrophilicity of the metal center. The reaction of 3 a and 3 b with benzaldehyde allowed for the regeneration of compound 2 by hydrosilylation of the benzaldehyde. NMR spectroscopy suggested an unusual mechanism for the transformation, involving a substrate insertion in the B−H bond of the borohydride anion.

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Section: Resultsmentioning

confidence: 99%

Heterolytic Si−H Bond Cleavage at a Molybdenum‐Oxido‐Based Lewis Pair

Zwettler

Walg

Belaj

et al. 2018

Chemistry A European J

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“…It was shown that upon using sterically demanding alkoxido, amido, and other ligands, bimolecular deactivationm ay be prevented but such compounds tend to be lessr eactive in metathesis reactions. [83][84][85][86][87][88] The investigation of Lewis acid-base interaction in olefin metathesis dates back to Osborn's description of ab ridge between catalyst and cocatalyst as described above (section 2), however, B(C 6 F 5 ) 3 was at that time not considered. [29] Significant reactivity enhancemento ft he oxido alkylidene catalysts used in metathesis reactions was achieved by addition of the borane Lewis acid.…”

Section: The Role Of Lewis Acids In Metathesis Reactionsmentioning

confidence: 99%

“…The adduct [Mo{O-B(C 6 F 5 ) 3 }(CHSiMe 3 )(N = PtBu 3 ) 2 ] (50)w as easily accessible by reactiono ft he parent complex with the borane. [87] The molecular structure, determined by single-crystal X-ray diffraction analysis,r eveals as ignificant elongation of the MoÀOb ond in the adduct compared with the one in the parent oxido complex [MoO(CHSiMe 3 )(N = PtBu 3 ) 2 ]( 0.15 longer, Figure 9). However,t he phosphinimido ligandss eem electronically deactivating as the parentc omplex is inactive towards 1-olefins and is ap oorR OMP catalystf or norbornene.…”

Section: The Role Of Lewis Acids In Metathesis Reactionsmentioning

confidence: 99%

“…However, as the oxido alkylidene moiety might be involved in heterogeneous catalytic systems, interest was revived in recent years. It was shown that upon using sterically demanding alkoxido, amido, and other ligands, bimolecular deactivation may be prevented but such compounds tend to be less reactive in metathesis reactions …”

Section: The Role Of Lewis Acids In Metathesis Reactionsmentioning

confidence: 99%

“…Molecular structures of the tungsten oxido alkylidene borane adduct [W{O-B(C 6 F 5 ) 3 }(CHtBu)(OHMT)(Me 2 Pyr)] [83] (49,left) and the molybdenum alkylideneb oranea dduct [Mo{O-B(C 6 F 5 ) 3 }(CHSiMe 3 )(N = PtBu 3 ) 2 ][87] (50, right).…”

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Interaction of Metal Oxido Compounds with B(C₆F₅)₃

Zwettler

Mösch‐Zanetti

2019

Chemistry A European J

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Lewis acid-base pair chemistry has been placed on an ew level with the discovery that adduct formationb etween an electron donor (Lewis base) and acceptor (Lewis acid) can be inhibited by the introductiono fs teric demand, thus preserving the reactivityo fb oth Lewis centers, resulting in highly unusual chemistry.S ome of these highly versatile frustrated Lewis pairs (FLP) are capable of splitting avariety of small molecules, such as dihydrogen, in ah eterolytic and even catalytic manner.T his is in sharp contrastt oc lassical reactions where the inert substrate must be activated by am etal-basedc atalyst. Very recently,r esearch has emerged combining the two concepts,n amely the formationo fF LPs in whichametal compound represents the Lewis base, allowing for novel chemistry by using the heterolytic splitting power of both together with the redox reactivity of the metal. Such reactivity is not restricted to the metal center itself being aL ewis acid or base, also ancillary ligandsc an be used as part of the Lewis pair,s till with the benefit of the redox-active metal centern earby. This Minireview is designed to highlight the novel reactions arising from the combination of metal oxido transition-metal or rare-earthmetal compounds with the Lewis acid B(C 6 F 5 ) 3 .I tc overs a wide area of chemistry including small molecule activation, hydrogenation and hydrosilylation catalysis, and olefin metathesis, substantiating the broad influence of the novel concept. Futureg oals of this young and excitinga rea are briefly discussed.

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

et al. 2019

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Grafting am olybdenum oxoa lkylidene on silica (partially dehydroxylated at 700 8 8C) affords the first example of awell-defined silica-supported Mo oxoalkylidene,whichisan analogue of the putative active sites in heterogeneous Mobased metathesis catalysts.Incontrast to its tungsten analogue, which shows poor activity towards terminal olefins because of the formation of as table off-cycle metallacyclobutane intermediate,t he 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 Wi mido analogues.Olefin metathesis is an atom-economical catalytic reaction that is used both in academia and in industry for the synthesis of ab road range of organic compounds that contain carbon = carbon double bonds.T his reaction is catalyzed by both early (molybdenum or tungsten) and late (ruthenium) transition-metal-based alkylidene complexes that perform well for aw ide range of organic substrates, which now include (for Mo) olefins that are directly functionalized with Cl, CF 3 ,orCNgroups. [1] Most of the early-transition-metal (d 0 )c atalysts share the same structural motif:(X)(Y)M( = CHR)E, [2] where X and Yare anionic ligands,Eis an imido or oxo ligand, and MisMoorW(Scheme 1). Early studies focused on generating metal oxo complexes-the proposed active sites of the industrial supported metal oxide catalysts. [3] Six-coordinate W-based oxo alkylidene complexes of the type W(O)(CHR)L 2 Cl 2 (L = ap hosphine ligand) were the first high oxidation state Wa lkylidene complexes to be prepared that metathesize both terminal and internal olefins (in the presence of at race amount of AlCl 3 ) and produce an ew alkylidene that could be observed as ac onsequence of olefin metathesis. [4] However,t he vast majority of well-defined Mo and Wc atalysts that have been developed over the last three decades contain an imido ligand that stabilizes the catalyst toward bimolecular decomposition and can be sterically and electronically modulated. Welldefined and catalytically active oxo alkylidenes based on W were first reported in 1996, [5] but it was not until 2012 that ap hosphine-free complex was developed. [6] Thec orresponding supported systems have shown unprecedented activity and stability, [7] especially in the form of cationic compounds. [8] Molecular oxo complexes contain bulky phenoxy ligands that prevent bimolecular decomposition reactions at the expense of poor metathesis activity.However, upon grafting on silica, one of the large phenoxides is replaced by ar elatively small surface siloxy ligand, thereby yielding highly active and stable supported catalysts with activities that are several orders of magnitude greater than the molecular precursor,demonstrating the beneficial effect of site isolation on silica surfaces. Scheme 1. General structures of well-defined metathesis catalysts based on a) Mo and Wimido, and b) Woxo alky...

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Rapid Access to an Oxido-Alkylidene Complex of Mo(VI)

Cited by 14 publications

References 51 publications

Heterolytic Si−H Bond Cleavage at a Molybdenum‐Oxido‐Based Lewis Pair

Heterolytic Si−H Bond Cleavage at a Molybdenum‐Oxido‐Based Lewis Pair

Interaction of Metal Oxido Compounds with B(C₆F₅)₃

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

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Rapid Access to an Oxido-Alkylidene Complex of Mo(VI)

Cited by 14 publications

References 51 publications

Heterolytic Si−H Bond Cleavage at a Molybdenum‐Oxido‐Based Lewis Pair

Heterolytic Si−H Bond Cleavage at a Molybdenum‐Oxido‐Based Lewis Pair

Interaction of Metal Oxido Compounds with B(C6F5)3

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

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Interaction of Metal Oxido Compounds with B(C₆F₅)₃