Synthesis and Characterization of Hypervalent Pentacoordinate Carbon Compounds Bearing a 7‐6‐7‐Ring Skeleton

Abstract

Dedicated to Prof. Masaaki Yoshifuji for the occasion of his 80th birthday.

Gas‐Phase Characterization of Hypervalent Carbon Compounds Bearing 7‐6‐7‐Ring Skeleton: Penta‐ versus Tetra‐Coordinate Isomers

“…Single crystal structures of compounds 1 and 2 were reported in the preceding paper [11] and are shown in Figure S1. Both compounds are present as cations ([ 1 ] + and [ 2 ] + ) with [BF 4 ] − as a counter anion.…”

“…X‐ray analyses of its single‐crystal revealed a tetra‐coordinate form, whereas UV‐vis absorption spectroscopy in solution suggested the presence of the penta‐coordinate isomer. These results implied an equilibrium between penta‐ and tetra‐coordinate isomers as shown in Scheme 1b, although clear spectroscopic assignment could not be given [11] …”

“…Although the experimental characterization of the penta‐coordinate carbons has been a great challenge due to difficulty in isolation of the TS, Yamamoto and co‐workers have succeeded in synthesizing “stable” penta‐coordinate carbon compounds and isolating them as single crystals [4] . The latest examples, reported in the preceding paper, [11] bear a semi‐flexible framework of 7‐6‐7‐ring (Scheme 1a, compound 1 ). Owing to the semi‐flexible skeletons, the penta‐coordinate structures in crystals seem to be in a delicate balance between covalent and electrostatic interactions.…”

“…Indeed, the structures of these compounds are sensitively dependent on substituent on ligands. Highly electron‐donating ligands such as thioxanthene ( 1 ) give a penta‐coordinate form, whereas other ligands with low electron‐donating ability (or with electron withdrawing ability) yield a conventional tetra‐coordinate form [11] . Among them, a compound with moderately electron donating ligand, bearing p ‐methylthiophenyl substituents (Scheme 1b, compound 2 ), [11] exhibited dual property.…”

“…Highly electron‐donating ligands such as thioxanthene ( 1 ) give a penta‐coordinate form, whereas other ligands with low electron‐donating ability (or with electron withdrawing ability) yield a conventional tetra‐coordinate form [11] . Among them, a compound with moderately electron donating ligand, bearing p ‐methylthiophenyl substituents (Scheme 1b, compound 2 ), [11] exhibited dual property. X‐ray analyses of its single‐crystal revealed a tetra‐coordinate form, whereas UV‐vis absorption spectroscopy in solution suggested the presence of the penta‐coordinate isomer.…”