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

The synthesis of new beta-diketonato rhodium(l) 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. H-1, C-13 and P-31 NMR data showed that for each of the non-symmetric beta-diketonato mono-carbonyl rhodium(I) complexes, two isomers exist in solution. The equilibrium constant, K-c, which relates these two isomers in an equilibrium reaction, are concentration independent but temperature and solvent dependent. Delta(r)G, Delta(r)H and Delta(r)S values for this equilibrium have been determined and a linear relationship between solvent polarity on the Dimroth scale and K-c exists. The relationship between Rh-P bond lengths, d(Rh-P), and P-31 NMR peak positions as well as coupling constants (1)J(P-31-Rh-103) 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 beta-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, v(CO), the sum of the group electronegativities of the R and R' groups, (chi(R) + chi(R')), or the observed pK(a)' values of the free beta-diketones RCOCH2COR'. An empirical relationship between v(CO) and either pK(a)' or (chi(R) + chi(R')) has also been quantified. (C) 2002 Elsevier Science B.V. All rights reserved.