1,3-Dipolar Cycloadduct of Organometallic Allene with Dipolarophile-An Apparent Huckel-Mobius Aromatic Five-Membered Organometallic Carbene with Ambiphilic Reactivity

1,3-Dipolar cycloaddition reaction is a powerful synthetic reaction for the generation of heterocycles in organic chemistry. However, similar reactions in organometallic chemistry are limited. Here, we report the structure and bonding of A-frame mu-carbido compounds [M-2(mu-C)Cl-2(mu-dppm)(2)] (1M, M = Co, Rh, and Ir), their cycloaddition reaction with a dipolarophile, dimethylacetylenediacetate (DMAD), and the structure and bonding of the resulting cycloadduct five-membered bimetallic carbene complexes [M-2(mu-C)(mu-DMAD)Cl-2(mu-dppm)(2)] (2M). The geometrical analysis revealed that carbido complex 1M exhibits a linear M-C-M spine with a planar environment around the metal atom. The bonding analysis indicated that the linear M-C-M spine has a bisallylic anionic type bonding scenario. Accordingly, the carbido carbon atom is highly electrophilic as indicated by the very high values of hydride affinities. The allylic anionic type delocalization enforces compound 1M to act as 1,3-dipole. Hence, these compounds undergo a stepwise cycloaddition reaction with DMAD resulting in five-membered bimetallic carbene 2M. The carbene center shows ambiphilic reactivity as compared to the nucleophilic reactivity of Arduengo type N-heterocyclic carbene. The five-membered bimetalla cyclic ring has Huckel-Mobius type aromatic delocalization having 10 pi electrons, which is in contrast to Arduengo type N-heterocyclic carbene which has 6 pi Huckel-type aromatic delocalization.