Multinuclear NMR studies on substituted derivatives of Rh 6(CO) 16 in solution

Abstract

Multinuclear NMR data ( 13C, 31P, 13C–{ 31P}, 13C–{ 103Rh} and 31P–{ 103Rh}) for a series of mono- and di-substituted derivatives of Rh 6(CO) 16 containing neutral two electron donor ligands [Rh 6(CO) 15L, (L=NCMe, py, cyclooctene, PPh 3, P(OPh) 3,1/2( μ 2, η 1: η 1-dppe)); Rh 6(CO) 14(LL), (LL= cis-CH 2CMe-CMeCH 2, dppm, dppe, (P(OPh) 3) 2)] are reported; these data show that the solid state structure is maintained in solution. Detailed assignments of the 13CO NMR spectra of Rh 6(CO) 15(PPh 3) and Rh 6(CO) 14(dppm) clusters have been made on the basis 13C–{ 103Rh} double resonance measurements and the specific stereochemical features of the observed long range couplings in these clusters have been studied. The stereochemical dependence of 3J(P–C) for terminal carbonyl ligands is discussed and the values of 3J(P–C) are found to be mainly dependent on the bond angles in the P–Rh–Rh–C fragment; these data enable the fine structure of the complex multiplets in the 13C–{ 1H} and 31P–{ 1H} NMR spectra of Rh 6(CO) 14 (dppm) to be simulated. Variable temperature 13C–{ 1H} NMR measurements on Rh 6(CO) 15(PPh 3) reveal the carbonyl ligands in this complex to be fluxional. The fluxional process involves exchange of all the CO ligands except the two terminal CO's associated with the rhodium trans to the substituted rhodium and can be explained by a simple oscillation of the PPh 3 on the substituted rhodium atom aided by concomitant exchange of the unique terminal CO on this rhodium with adjacent μ 3-CO's.