Synthesis of five-membered osmacycloallenes and conversion into six-membered osmacycloallenes

Isomerization reactions of metal vinylidene units

Isomerization reactions of unsaturated molecules offer an efficient strategy in atom-economical synthesis. Although isomerization reactions of unsaturated organic and organometallic compounds, such as alkenes, alkynes, and metal carbynes, have been achieved, those of metal vinylidene units that contain cumulated double bonds have never been reported. Herein, we inaugurally discovered isomerization reactions of metal vinylidene units via protonation and deprotonation reactions of metal carbenes. Experimental and theoretical investigations indicate that the electrical characteristics of substituents on the rings play a crucial role in controlling the formation of metal vinylidene units. The isomerization reactions of metal vinylidene units were driven by thermodynamic forces. Moreover, one of the angles at metal vinylidenes was found as 126.9°, representing the smallest angle in metal vinylidenes and the first cyclic 4d transition metal (Ru) vinylidene complex was successfully isolated. These investigations unveil novel structures and reactivity for metal vinylidenes, offering a fresh perspective on the isomerization reactions of unsaturated molecules containing cumulative unsaturated bonds.

Isolation, Reactivity, and Tunable Properties of a Strained Antiaromatic Osmacycle

Strain and antiaromaticity in compounds are recognized as two substantial destabilizing features, and consequently, realization of dual destabilizing features in a single molecule is challenging and far more difficult in a single ring. Moreover, transformation of an antiaromatic framework to different antiaromatic or aromatic species is a significant subject in antiaromatic chemistry and has attracted increasing interest. In this work, we isolated a highly strained antiaromatic metallacycle in which a cyclic metal vinylidene unit is embedded. Computational studies revealed its ring strain energies and antiaromatic character and showed that the metal incorporation and the phosphonium substituents play a crucial role in its stabilization. The mechanism of its formation has been illustrated by density functional theory (DFT) calculations and the isolation of a key intermediate. We further discovered diverse reactivities and structural reshuffling of this unusual strained antiaromatic complex according to its two destabilizing characters. We obtained two isomers of metallaindenes fused with oxiranes from the direct oxidation of the metal vinylidene or by nucleophilic addition to an isolated metallacyclocumulene formed by the reaction of metal vinylidene with hydroxide ion, achieving a reconfiguration of the antiaromatic framework. Transformations of the antiaromatic metallacycle by electrophiles to various aromatic metallaindynes have been achieved, and that a condensed Fukui function was employed to confirm the regioselectivity of the electrophilic additions, and the acid/base-induced aromaticity switch along with tunable photophysical properties were investigated. These interesting transformations not only enrich the chemistry of metal vinylidenes and antiaromatics and could also perform potentially as switchable optical materials.

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

The incorporation of a metal-carbon triple bond into a ring system is challenging because of the linear nature of triple bonds. To date, the synthesis of these complexes has been limited to those containing third-row transition metal centers, namely, osmium and rhenium. We report the synthesis and full characterization of the first cyclic metal carbyne complex with a second-row transition metal center, ruthenapentalyne. It shows a bond angle of 130.2(3)° around the sp-hybridized carbyne carbon, which represents the recorded smallest angle of second-row transition metal carbyne complexes, as it deviates nearly 50° from the original angle (180°). Density functional theory calculations suggest that the inherent aromatic nature of these metallacycles with bent Ru≡C-C moieties enhances their stability. Reactivity studies showed striking observations, such as ambiphilic reactivity, a metal-carbon triple bond shift, and a [2 + 2] cycloaddition reaction with alkyne and cascade cyclization reactions with ambident nucleophiles.

Synthesis of aromatic ruthenabenzothiophenes via C–H activation of thiophenes

The first ruthenabenzothiophenes have been achievedviathe C–H activation of thiophene.

Five-membered cyclic metal carbyne: synthesis of osmapentalynes by the reactions of osmapentalene with allene, alkyne, and alkene

The synthesis of small cyclic metal carbynes is challenging due to the large angle strain associated with the highly distorted nonlinear triple bonds. Herein, we report a general route for the synthesis of five-membered cyclic metal carbyne complexes, osmapentalynes, by the reactions of an osmapentalene derivative with allene, alkyne, and alkene. Experimental observations and theoretical calculations document the aromaticity in the fused five-membered rings of osmapentalynes. The realization of transforming osmapentalene to osmapentalyne through this general route would not only allow further exploration of metallapentalyne chemistry but also show promising applications of this novel aromatic system with broad absorption band and high molar absorption coefficient.

Reactions of osmapyridinium with terminal alkynes

The reactions of osmapyridinium with terminal alkynes are presented, which lead to the formation of ten-membered osmacycles and η⁴-coordinated cyclopentadiene complexes.