103Rh-NMR spectroscopy of some hydridorhodiumbis(carbonyl)diphosphine compounds

The 103Rh chemical shifts of a series of hydridorhodiumbis(carbonyl)diphosphine compounds 1–17 and one phosphine–phosphite analogue 18, containing chelating bidentate P-ligands, have been obtained by inverse HMQC detection sequences 1H-{103Rh} and 31P-{103Rh,1H}. These active hydroformylation catalysts are stable only under pressure of H2/CO (synthesis gas) and hence 1H-, 31P- and 103Rh-NMR spectra have been recorded in a sapphire high-pressure NMR tube. The compounds HRh(CO)2(PP) exist as a mixture of equatorial–equatorial and equatorial–axial five-coordinate isomers for 1–17, which are in a dynamic equilibrium that could not be brought into the slow exchange regime on the proton, the phosphorus or the rhodium NMR time scales. A correlation (R=0.980) was found between δ(103Rh) and the Hammett σp-constant of the para-substituents Y of the P(C6H4Y-p)2 groups in the thixantphos ligand for the series of compounds 4, 8–13 (lower δ(103Rh) for electron withdrawing substituents). Correlation of δ(103Rh) with Tolman basicity parameters (higher δ(103Rh) with higher basicity) also gave a good fit (R=0.955). The finding that correlations exist for this series between the ligand basicity, the ratio of equatorial–equatorial/equatorial–axial trigonal bipyramidal isomers and the δ(103Rh) indicates that small structural and electronic changes in the ligands in the vicinity of transition metal nuclei have a small but significant influence on δ(103Rh). This, together with other knowledge, may in the future serve to use Rh-NMR as an analytical tool in coordination chemistry.