Construction of Multiple M–C Bonds: Chelation with Conjugated Carbon Chains

Abstract

The formation of metal–carbon (M–C) bonds represents a fundamental process in organometallic chemistry. A central question arises: what is the maximum number of M–C bonds that can be formed on a single metal center? Over the past decade, Xia and co-workers have been dedicated to addressing this challenge. They developed a strategy based on chelating metal centers with conjugated carbon chains, which allows for the formation of multiple M–C bonds whose number increases with the length of the carbon chain. A landmark achievement came in 2013 with the first synthesis of metallapentalynes, a new class of metal-bridged fused-ring metallacycles with planar Craig aromaticity. These complexes are formally constructed by chelating a metal center with a seven- atom conjugated carbon ligand via three M–C σ bonds. This breakthrough established a new research area termed “carbolong chemistry”, which focuses on the interactions between transition metals and conjugated carbon ligands. Since then, the Xia group has synthesized a diverse family of carbon-based polydentate chelates involving three to five coplanar M–C σ bonds, including the first metal-centered [15]annulene, collectively known as “carbolong complexes”. Moreover, carbolong motifs have been successfully incorporated into polymer backbones, yielding a new class of materials with numerous M–C bonds, referred to as “polycarbolongs”. Both carbolong complexes and polycarbolongs exhibit not only unique structures but also remarkable properties, showing considerable potential in applications such as catalysis, biomedicine, luminescent materials, and photovoltaics. This review summarizes recent advances in the synthesis and applications of carbolong complexes and polycarbolongs.

Keywords

Metal–carbon bond / Conjugated carbon chain / Carbolong chemistry / Polydentate chelate / Aromaticity / Alkynes / Cycloaddition / Metallacycles

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