Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Schrock, Richard R.; Buchmeiser, Michael R.; Groos, Jonas; Benedikter, Mathis

doi:10.1016/b978-0-12-820206-7.00062-7

Cited by 13 publications

(14 citation statements)

References 458 publications

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“…We have outlined syntheses of tritylimido Mo complexes that complement those of W reported recently, and have shown that the tritylimido ligand is a robust, sterically demanding ligand that is worth further study in the context of olefin metathesis chemistry, and likely other imido chemistry. For example, tritylimido Mo and W alkylidene complexes may prove to be relatively stable toward imido-assisted bimolecular coupling of alkylidenes, which is a significant decomposition pathway for Mo and W d 0 imido alkylidene complexes . The trityl imido ligand also complements other relatively unexplored sterically demanding arylimido ligands, where the aryl group is hexamethylterphenyl, hexaisopropylterphenyl, or hexa- t -butyl-terphenyl…”

Section: Discussionmentioning

confidence: 99%

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Synthesis of Molybdenum(VI) Tritylimido Alkylidene Complexes

2022

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Mo(NCPh 3 )(CHCMe 2 Ph)(OTf) 2 (DME) (1) can be prepared in 46% yield through addition of 3 equiv of triflic acid to Mo(NCPh 3 ) 2 (CH 2 CMe 2 Ph) 2 in the presence of dimethoxyethane. Other tritylimido complexes that were prepared include Mo(NCPh 6), and [(Ph 3 SiO) 2 (NCPh 3 )Mo] 2 (μ-O)(μ-Cl) 2 (7). Single crystal X-ray diffraction studies were carried out for 2, 4, 5, 6, and 7. These results demonstrate that tritylimido is a robust, sterically demanding alkylimido ligand that complements less sterically demanding t-butylimido and 1-adamantylimido ligands.

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Section: Discussionmentioning

confidence: 99%

“…Mo(VI) and W(VI) imido alkylidene complexes have proven to be useful and highly tunable initiators in olefin metathesis reactions . Many NR variations of M(NR)(alkylidene)(X)(Y) complexes (where X and Y are monoanionic ligands such as an alkoxide or pyrrolide) have been explored.…”

Section: Introductionmentioning

confidence: 99%

Synthesis of Molybdenum(VI) Tritylimido Alkylidene Complexes

2022

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“…Within a few years, closely related alkylidenes featuring chloride, cyclopentadienyl or alkoxide ligands, such as (thf) 2 Cl 3 M=CH t Bu, Cp 2 ClM=CH t Bu, or (Me 3 P)(O t Bu)Cl 2 M=CH t Bu (M = Nb, Ta) have been prepared and employed as well-defined catalysts for olefin metathesis reactions in homogeneous solution . On the basis of these findings, numerous other coligands (e.g., phenoxides, amidophosphines, or nacnac derivatives) were explored for the synthesis of group 5 and other early metal alkylidenes. The groups of Fyrzuk, Ozerov, and Mindiola have found that monoanionic [PNP] scaffolds are particularly suited for the stabilization of Nb(V) and Ta(V) alkylidenes and bis(alkylidenes), also in the form of their parent methylidenes. , The yet known diamagnetic closed-shell [PNP]Nb and [PNP]Ta alkylidenes, which emerged from these studies, are summarized in Chart (rows A and B).…”

Section: Introductionmentioning

confidence: 99%

[PNP]-Stabilized Niobium(IV) and Tantalum(IV) Complexes: Synthesis and Characterization of an Open-Shell Tantalum Alkylidene

et al. 2023

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A flexible [PNP] ligand (forming six-membered chelates upon coordination) was employed for the synthesis of niobium(IV) and tantalum(IV) complexes. Starting from the protioligand (H[PNP]) and the tetrachlorides NbCl4(thf)2 or TaCl4, the trichloro complexes [PNP]MCl3 (1-M, M = Nb, Ta) were prepared via in situ deprotonation using LiN(SiMe3)2. Attempts to alkylate complexes 1-M (M = Nb, Ta), however, were impeded due to their limited solubility in nonchlorinated solvents. Hence, the lithiated ligand was reacted with (thf)2Cl3M=CHCMe2Ph (M = Nb, Ta) to afford the d 0-configured alkylidene complexes 2-Nb and 2-Ta, respectively. These meridionally coordinated and freely soluble complexes were found to be stable in nonchlorinated and chlorinated solvents, even at elevated temperature. Reduction of compounds 2-M with magnesium anthracene (2-Nb and 2-Ta) or sodium naphthalide (2-Ta) afforded the corresponding d 1-configured alkylidenes (3-Nb and 3-Ta), although the reduced niobium derivative was found to decompose rapidly. A 10-line and an 8-line EPR pattern was detected for 3-Nb (I(93Nb) = 9/2) and 3-Ta (I(181Ta) = 7/2), respectively, confirming the metalloradical character of each complex. For 3-Ta, the molecular structure was elucidated by single crystal X-ray diffraction. With the molecular structure of this d 1-configured (open-shell) tantalum alkylidene ascertained, its electronic structure was examined by DFT and CASSCF calculations.

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“…Several decades of solution studies that employ well-characterized d 0 Mo or W alkylidene initiators of homogeneous olefin metathesis reactions that are formed through α hydrogen abstraction in a dialkyl (usually dineopentyl or dineophyl) complex have produced abundant evidence that 14 electron TBP (trigonal bipyramidal) metallacyclobutane complexes with the MC 3 ring in equatorial positions and an axial oxo or imido ligand are the required intermediates. − Solution studies have also documented how alkylidenes decompose, with bimolecular coupling of alkylidenes (to give monometallic or bimetallic d 2 metal complexes) in sterically less demanding circumstances or rearrangement of a metallacyclobutane intermediate to an olefin being the most common (in the absence of water and oxygen or other destructive impurities). The first examples of how a d 2 olefin complex can be transformed into an alkylidene complex in a high-oxidation state Mo or W olefin metathesis system were published only recently; a protonation/deprotonation mechanism (with dimethylanilinium as the acid) allows an equilibrium to be established between Mo or W styrene complexes and phenethylidene complexes (∼2:1) with a cationic phenethyl complex being the intermediate .…”

Section: Introductionmentioning

confidence: 99%

Tungstacyclopentane Ring Contraction Yields Olefin Metathesis Catalysts

et al. 2022

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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 λmax). 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–Cα 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.

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Group 6 High Oxidation State Alkylidene and Alkylidyne Complexes

Cited by 13 publications

References 458 publications

Synthesis of Molybdenum(VI) Tritylimido Alkylidene Complexes

Synthesis of Molybdenum(VI) Tritylimido Alkylidene Complexes

[PNP]-Stabilized Niobium(IV) and Tantalum(IV) Complexes: Synthesis and Characterization of an Open-Shell Tantalum Alkylidene

Tungstacyclopentane Ring Contraction Yields Olefin Metathesis Catalysts

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