Multinuclear NMR, X-ray, and DFT Studies on RhCl(diene)(phosphoramidite) Complexes

NMR, X-ray, and DFT studies on several [RhCl(diene)(phosphoramidite)] complexes suggest that both electronic and steric effects affect the nature of the olefin, chloride, and P-donor bonding. The X-ray study of [RhCl(1,5-COD)(phosphoramidite)] (phosphoramidite = (Binol)PN(CH(CH 3)Ph)2) reveals an intramolecular selectivity in the back-bonding in which the C-C bond of the olefin trans to the Cl ligand is preferentially elongated. Two types of dynamic processes have been detected at ambient temperature in CD2Cl2 solution using 2-D NOESY methods: presumed phosphoramidite dissociation and diolefin rotation. The former is observed with both 1,5-COD and NBD. The latter is more selective in that the intramolecular dynamics for the NBD analogues are relatively fast, but those for the 1,5-COD compounds, barely detectable. DFT calculations suggest that the diolefin rotation proceeds over a tetrahedral transition state and that there is a smaller energy barrier for the NBD complexes relative to the analogous 1,5-COD species. Two bis phosphoramidite salts of the form [Rh(1,5-COD) (phosphoramidite)2]BF4 are reported.