Synthesis and characterisation of ferrocene-containing β-diketonato complexes of rhodium(I) and rhodium(III)

The synthesis of new β-diketonato rhodium(I) complexes of the type [Rh(FcCOCHCOR)(CO)2] and [Rh(FcCOCHCOR)(CO)(PPh3)] with Fc=ferrocenyl and R=Fc, C6H5, CH3 and CF3 are described. 1H, 13C and 31P NMR data showed that for each of the non-symmetric β-diketonato mono-carbonyl rhodium(I) complexes, two isomers exist in solution. The equilibrium constant, Kc, which relates these two isomers in an equilibrium reaction, are concentration independent but temperature and solvent dependent. ΔrG, ΔrH and ΔrS values for this equilibrium have been determined and a linear relationship between solvent polarity on the Dimroth scale and Kc exists. The relationship between RhP bond lengths, d(RhP), and 31P NMR peak positions as well as coupling constants 1J(31P103Rh) has been quantified to allow calculation of approximate d(RhP) values. Variations in d(RhP) for [Rh(RCOCHCOR′)(CO)(PPh3)] complexes have also been related to the group electronegativities (Gordy scale) of the terminal β-diketonato R groups trans to PPh3. A measure of the electron density on the rhodium centre of [Rh(RCOCHCOR′)(CO)(PPh3)] may be expressed in terms of the IR carbonyl stretching wave number, ν(CO), the sum of the group electronegativities of the R and R′ groups, (χR+χR′), or the observed pKa′ values of the free β-diketones RCOCH2COR′. An empirical relationship between ν(CO) and either pKa′ or (χR+χR′) has also been quantified.