Ring contraction of six-membered metallabenzynes to five-membered metal–carbene complexes: a comparison with organic analogues

Abstract

The ring contraction of six-membered metallabenzynes to five-membered metal-carbene complexes in comparison with the corresponding rearrangement of benzyne to cyclopentadienylidene carbene have been studied by computational quantum mechanical calculations. The rearrangement of benzyne to a cyclopentadienylidene carbene is endothermic by 27.8 kcal mol(-1). The instability of the cyclopentadienylidene carbene is attributed to the electron deficient singlet carbene carbon atom. However, the isolobal replacement of a sp-hybridized carbon atom in benzyne by a 14 valence electron transition metal fragment (M(PH3)2Cl2, M = Fe, Ru and Os) makes the corresponding rearrangement feasible. The first row transition metal iron shows thermodynamic (exothermic by 19.1 kcal mol(-1)) and kinetic preference (energy barrier of 1.8 kcal mol(-1)) towards the carbene complex. However, the preference reduces down the group. The coordination number of the metal as well as the low-lying in-plane M-C π*-MO play a crucial role for this rearrangement. The conversion of metallabenzynes to metal-carbene complexes is also associated with a ring plane rotation to attain an effective overlap between the metal d-orbital and the p-orbital on the carbene carbon atom.

Recent Advances in Metallaaromatic Chemistry

“…Rearrangement is particularly favored for metallabenzynes of first-and secondrow transitionm etals, which may make isolation of compoundso ft his type particularly challenging. [155] Heterometallabenzenes An umber of heterometallabenzenes, where aC Hu nit in ah eterobenzene has been formally replaced by at ransition metal and its ancillaryl igands, have been reported over the past few decades. While other isomersa re theoretically possible, examples almost exclusively involveadirect metal-heteroatom bond.…”

“…DFT calculations suggest that carbene formation is a low‐energy decomposition route for metallabenzynes, and the thermodynamic favorability is influenced by the steric properties of the ligands and the substituents. Rearrangement is particularly favored for metallabenzynes of first‐ and second‐row transition metals, which may make isolation of compounds of this type particularly challenging …”

Constraint of a ruthenium-carbon triple bond to a five-membered ring

“…Traditionally, the strength of transition metal–carbon bonds decreases with the ascending of a column in the periodic table ( 15 ), which results in relative lability of the first- and second-row organometallic compounds in comparison with their third-row analogs ( 16 , 17 ). In particular, cyclic metal carbyne complexes with first- and second-row transition metals are considerably less stable than their third-row counterparts both thermodynamically and kinetically, as demonstrated by computational analysis ( 18 , 19 ). Thus, intrinsic instability of the non–third-row transition metal carbynes is responsible for their absence, which greatly restrained their synthesis and further exploration.…”

Reactions of Metal–Carbon Bonds within Six‐Membered Metallaaromatic Rings

“…The most likely reaction pathway has been proposedt op roceedv ia the initial migratory insertion reactiono ft he osmabenzyne complex 57 to ac arbene intermediate, af ollowing migratory insertion of metal carbene to OsÀCl bonds, and final transformation to chlorocyclopentadienyl complex 58.D FT calculations suggested that the p-acceptingl igands, such as carbonyl group and acetonitrile, could facilitate the migratory insertion reactions of osmabenzynest o form the corresponding cyclopentadienyl complexes.T he metal effect on this migratory insertionreactions has also been studied computationally. [37] The calculation results show that the metallabenzynes with lighter metal centers are easier to transform to the corresponding cyclopentadienyl complexes for both thermodynamic and kinetic rea.…”

“…DFT calculations suggested that the π‐accepting ligands, such as carbonyl group and acetonitrile, could facilitate the migratory insertion reactions of osmabenzynes to form the corresponding cyclopentadienyl complexes. The metal effect on this migratory insertion reactions has also been studied computationally . The calculation results show that the metallabenzynes with lighter metal centers are easier to transform to the corresponding cyclopentadienyl complexes for both thermodynamic and kinetic rea.…”