The structure and dynamic behaviour of disubstituted derivatives of [Rh6(CO)16] containing bidentate phosphorus ligands

The solution structure and dynamic behaviour of [Rh6(CO)14(μ,η2-dppm)] (1), [Rh6(CO)14(μ,η2-dppe)] (2) and [Rh6(CO)14(μ,η2-dppef)] (3) containing bridging diphosphine ligands have been examined using 1D 13C, 31P and 2D 13C–{103Rh}, 31P–{103Rh} HMQC and 13C EXSY NMR techniques. It has been shown that the solid state structure of these clusters remains unchanged in solution, including the close non-bonding intramolecular interactions of the perfluorinated phenyl rings with adjacent terminal carbonyls. In solution, two different dynamic processes have been found: for 2 and 3, the P–CH2–CH2–P chain of the coordinated dppe and dppef is non-rigid and this leads to the interchange of the two enantiomeric forms of the clusters through a ‘rocking’ motion of the bridging diphosphine; the rate of this exchange depends strongly on the non-bonding van der Waal's interactions between the phosphorus substituents and adjacent carbonyl ligands which results in substantially slower dynamics for 3 because the steric requirements of the fluorinated phenyl rings considerably hinder this racemization. The second type of dynamics found in 1–3 involves exchange of terminal/face-bridging CO's associated with the unsubstituted rhodium atoms. Regioselectivity of these exchanges is essentially similar for all three clusters, whereas the rates of these CO-exchanges are substantially higher in 3, which may be due to the lower net donicity of the per-fluorinated diphosphine.