Synthesis, structure, and fluxional behavior of 1,2,3,4-tetramethylfulvene-bridged diruthenium complexes

A new diruthenium complex (η1,η5-CH2C5Me4)Ru2(CO)6 (1) was synthesized by the reaction of 1,2,3,4-tetramethylfulvene and Ru3(CO)12 in 49% yield. The X-ray crystal structure of 1 revealed that the fulvene ligand bridges over two ruthenium atoms in an η1,η5-fashion. Fluxionality of the carbonyl ligands of 1 was studied in detail by line-shape analysis for the signals of the carbonyl ligands in the variable-temperature 13C NMR spectra (150 MHz), which revealed the existence of two processes. Mechanisms of the two processes were suggested based on the activation parameters, ΔH‡, ΔS‡, and ΔG‡298: process I (fast process) is an intramolecular and global exchange of five carbonyl ligands between two Ru atoms probably by way of a carbonyl-bridged structure, and process II (slow process) corresponds to the participation of the last carbonyl ligand to the exchange. Interestingly, the carbonyl exchange was completely suppressed in the case of (η1,η5-CH2C5Me4)Ru2(CO)5(PMe3) (2) which was obtained by the reaction of 1 with PMe3.