Rhodium complexes of 3,4-di(β-naphthyl)-2,5-diarylcyclopentadienone: indirect detection of slowed naphthyl rotation

In the solid state, [(acetylacetonate)-3,4-di(β-naphthyl)-2,5-diarylcyclopentadienone]rhodium(I) (4a, aryl=phenyl, 4b, aryl=m-xylyl) exist as head-to-tail dimers in which each rhodium is bonded to the γ-carbon of the acetylacetonate ligand in the other half of the molecule. These dimers are also favoured in solution at low temperature, and restricted naphthyl rotation results in the formation of numerous conformers whereby the naphthyls can adopt either proximal or distal orientations. These rotamers can be detected by observation of the 1H- and 13C-NMR methyl resonances in the acetylacetonate moiety. Triphenylphosphine cleaves the dimers (4a and 4b) to give the corresponding monomers, (C4Ar2(naphthyl)2CO)Rh(acac)PPh3 (5a and 5b), and the crystal structure of 5a exhibits a proximal/distal disorder of one naphthyl substituent. The rotamers of 5a and 5b are also present in solution, and the variable-temperature 31P-NMR spectra yield activation enthalpies of 8.2±0.5 kcal mol−1 for naphthyl rotation in each compound.