On the Origin of Reactivity Enhancement/Suppression upon Sequential Ligation: [Re(CO)_x]⁺/CH₄ (x=0–3) Couples

Abstract

The thermal gas-phase reactions of rhenium carbonyl complexes [Re(CO) ] (x=0-3) with methane have been explored by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculation. While it had been concluded in previous studies that addition of closed-shell ligands in general decreases the reactivity of metal ions, the current work provides an exception: the previously demonstrated inertness of atomic Re towards methane is completely changed upon ligation with CO. Both [Re(CO)] and [Re(CO) ] bring about efficient dehydrogenation of the hydrocarbon under ambient conditions. However, addition of a third ligand to form [Re(CO) ] completely quenches the reactivity.

A comprehensive overview of directing groups applied in metal-catalysed C–H functionalisation chemistry

“…Li showed an alkyne insertion/intramolecular Diels–Alder process between N -pyrimidylindoles and benzoquinonic 1,6-enynes leading to fused and bridged polycyclic compounds ( Scheme 139A and B ). 817 While with Rh catalysis the fused polycyclic product was obtained almost exclusively, the use of Co allowed preferential formation of the bridged one, although not exclusively. The product switch arises from the different regioselectivities of the initial olefination step (Rh: alkyne 2,1-insertion; Co: 1,2-insertion), in turn due to different steric sensitivities (the Co catalyst is suggested to be more sterically sensitive, giving preferentially the least hindered insertion intermediate).…”

“…It is worth mentioning that the use of the classical pyrimidine cleavage protocol (NaOEt in DMSO, see Scheme 137 ) on the fused product did not result in DG cleavage, but in the cleavage of the oxa-cycle, giving the corresponding phenol. 817 …”

The Electric Field as a “Smart” Ligand in Controlling the Thermal Activation of Methane and Molecular Hydrogen

“…Regarding the role of closed‐shell ligands, while they generally decrease the reactivity of metal clusters there is a gain in selectivity, provided the reactivity is not quenched completely ,,. Quite a few studies suggest that these ligands change the electronic structures of the metal center such that spin‐selective processes are initiated . Remarkably, in some cases OEFs have been demonstrated to mimic the electronic effects that a ligand exert on the reactive center in the course of catalytic transformations …”

“…[4e,g,h, 6] Quite af ew studies suggest that these ligands change the electronic structures of the metal center such that spinselective processes are initiated. [7] Remarkably,insome cases OEFs have been demonstrated to mimic the electronic effects that al igand exert on the reactive center in the course of catalytic transformations. [5d, 6c, 8] CO 2 has been used widely in both small and large-scale synthesis as as olvent or ar eactant and is sometimes employed as ac oordinating ligand.…”

Elektrisches Feld als “smarter” Ligandenersatz zur kontrollierten thermischen Aktivierung von Methan und molekularem Wasserstoff

“…Was die Rolle von geschlossenschaligen Liganden betrifft, so verringern diese im Allgemeinen die Reaktivität von Metallclustern, führen dafür aber häufig zu einer verbesserten Selektivität, solange die Reaktivität nicht ganz ausbleibt ,,. Einige Studien lassen darauf schließen, dass Liganden die elektronische Struktur des Metallzentrums dahingehend ändern, dass spinselektive Prozesse ermöglicht werden . Erstaunlicherweise haben OEFs in einigen Fällen die gleiche Wirkung auf die Elektronenstruktur, die ein Ligand im Verlauf einer katalytischen Umwandlung auf das reaktive Zentrum ausübt …”