Preparation of Novel Rhodonocenium Complexes via [2 + 2 + 1] Cyclotrimerization of Terminal Aryl Alkynes

Reactions of [Cp*Rh(η2-NO3)(OTf)] (1), which bears two labile ligands (NO3- and OTf-),
with a terminal aryl alkyne (phenylacetylene (HC⋮CPh) or 4-ethynyltoluene (HC⋮CC6H4CH3)) in alcohol (EtOH or n-BuOH) gave the substituted-rhodocenium cation [Cp*Rh(η5-C5H2Ar2−CH(Ar)OR)]+(OTf)- (2a−d:  Ar = Ph, p-tolyl; R = Et, n-Bu). Treatment of 1 with
4-ethynyltoluene in ethanol followed by the addition of a pseudo-halide (HSPh, Me3SiN3, or
Me3SiNCS) in dichloromethane also produced the substituted-rhodocenium cation [Cp*Rh(η5-C5H2(p-tolyl)2−CH(p-tolyl)Nu)]+(OTf)- (Nu- = SPh (3a), N3 (3b), NCS (3c)). All products
appeared to have been formed by the formal [2 + 2 + 1] cyclotrimerization of the terminal
aryl alkynes and the subsequent nucleophilic addition of the alcohol or pseudo-halide at the
exocyclic double bond of a fulvene intermediate. Molecular structures of 2b,c and 3a,c were
determined by X-ray diffraction. All complexes are isostructural and show sandwich-type
structures composed of one Cp* ligand and one substituted-Cp ligand.