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

The Rh-103 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 H-1-{Rh-103} and P-31-{Rh-103,H-1}. These active hydroformylation catalysts are stable only under pressure of H-2/CO (synthesis gas) and hence H-1-, P-31- and Rh-103-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 delta(Rh-103) and the Hammett sigma(p)-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 delta(Rh-103) for electron withdrawing substituents). Correlation of delta(Rh-103) With Tolman basicity parameters (higher delta(Rh-103) 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 delta(Rh-103) indicates that small structural and electronic changes in the ligands in the vicinity of transition metal nuclei have a small but significant influence on delta(Rh-103). This, together with other knowledge, may in the future serve to use Rh-NMR as an analytical tool in coordination chemistry. (C) 1999 Elsevier Science S.A. All rights reserved.