A Terminal Rh Methylidene from Activation of CH₂Cl₂

Abstract

Oxidative addition of carbon−halogen bonds at transition metals typically follows either a two-electron pathway (concerted M−R/M−X formation) or a radical chain pathway (stepwise M−R/M−X formation). When the reactive metal species is generated slowly, however, both mechanisms can compete to yield unexpected reactivity paths. The present report highlights the synthesis of rhodium methylidenes from chloroalkanes (e.g., CH 2 Cl 2 and CHCl 3 ) at POP-pincer frameworks (e.g., POP = 4,6-bis(ditert-butylphosphino)dibenzo[b,d]furan) via a cascade of halide abstraction and electron transfer steps. Experimental and computational studies are reported that support the proposed mechanism, including characterization of important reaction intermediates. The overall transformation represents a route toward reactive metal alkylidenes using milder and less-reactive carbenoid precursors than what is presently used.

Mechanism of Coupling of Methylidene to Ethylene Ligands in Dimetallic Assemblies; Computational Investigation of a Model for a Key Step in Catalytic C1 Chemistry

“…In general, we anticipate that a wider range of coupling mechanisms may be available to coordinatively unsaturated methylidene complexes such as 25 or those with easily dissociable ligands. This was one motivation for giving a separate series of citations for complexes with less than 18 valence electrons in the Introduction section. − In any case, we primarily generalize from our results to other coordinatively saturated systems.…”

“…Isolable methylidene complexes were relative latecomers to organometallic chemistry but are now well-represented. − The first, the nucleophilic tantalum complex (η 5 -C 5 H 5 ) 2 Ta­(CH 2 )­(CH 3 ) ( 1 ; Figure ), was described by Schrock in 1975 . This was followed by our reports of electrophilic rhenium complexes [(η 5 -C 5 R 5 )­Re­(NO)­(PX 3 )­(CH 2 )] + PF 6 – [R/X = Me/OPh or Me/ p -tol ( 4a , b + PF 6 – ); H/Ph ( 5 + PF 6 – )] .…”

Oxidative Addition of C–Cl Bonds to a Rh(PONOP) Pincer Complex

“…Despite economic and environmental imperatives for the use of chlorocarbons as substrates, the robust nature of C–Cl bonds remains a significant practical impediment, conferring attenuated or divergent reactivity compared to heavier halide counterparts. , With respect to well-defined rhodium complexes, only a limited number of examples of C–Cl bond activation can be found in the literature, but the use of rigid mer -tridentate “pincer” ligands is an emerging trend (Scheme ). − These versatile ancillary ligands are evidently well-suited to supporting the reactive rhodium centers required to bring about cleavage of a C–Cl bond …”

Structure and Methylene Transfer Reactivity of Thiolate-Bridged Dichromium Methylene Complexes Derived from Dihalomethane via Cleavage of Two Carbon–Halogen Bonds

“…The activation of two carbon–halogen bonds of dihalomethanes under mild conditions, especially those of the less reactive CH 2 Cl 2 or CH 2 Br 2 , is a fascinating and challenging subject . So far, a large number of transition-metal complexes have been reported to serve as good promoters for the relatively inert C–X bond activation of dihalomethane. − However, most of these complexes usually involve electron-rich transition-metal centers supported by strong-field phosphines, N-heterocyclic carbenes, nitrogen, or hybrid donor ligands . In sharp contrast, there are only a few transition-metal complexes bearing weak-field sulfur donors with similar reactivity .…”