Kinetics of substitution of ferrocenyl-containing β-diketonato ligands by phenanthroline from β-diketonato-1,5-cyclooctadienerhodium(I) complexes
                                
                                    
                                        作者:Theunis G. Vosloo、W.C. (Ina) du Plessis、Jannie C. Swarts                                    
                                    
                                        DOI:10.1016/s0020-1693(01)00805-2
                                    
                                    
                                        日期:2002.3
                                    
                                    Second-order rate constants, k(2), for the substitution of the ferrocene-containing beta-diketonato ligands FeCOCHCOR- with R = CF3 (ferrocenoyltrifluroacetonato, fctfa, pK(a) 6.56), CCl3 (ferrocenoyltrichloroacetonato. fetca, 7.13), CH3 (ferrocenoylacatonato, fca, 10.01), Ph (anion of benzoylferrocenoylmethane, bfcm. 10.41) and Fe (anion of diferrocenoylmethane, dfcm, 13.1) (Ph = phenyl, Fc = ferrocenyl, values in brackets are the pK(a) values of the free beta-diketones) from the complexes [Rh(cod)(FcCOCHCOR)] with 1,10-phenanthroline (phen, cod = 1,5-cyclooctadiene) at 25 degreesC were found to be 560 (R = CF3), 1370 (CCl3), 30 (Ph), 18 (CH3) and 7.0 dm(3) mol(-1) s(-1) (Fc), respectively. The temperature dependence of each reaction was determined and the large negative values obtained for activation, DeltaS* < -100 J K (1) mol (1) for all but R = CCl3 (DeltaS(CCl3)* = -81 J K-1 mol(-1)), suggests an associative substitution mechanism. The rate law of the reaction was found to be R = k(s) + k(2)[phen]}[Rh(cod)(FcCOCHCOR)]. Since the solvent-associated rate constant k(s) approximate to 0 for all R except Ph (k(s,R Ph) = 0.06 s(-1)) the solvent. methanol. plays a limited role in the reaction. Results are interpreted to imply that the rate-determining step during substitution is breaking of an Rh-O bond and not the formation of an Rh-N bond. The role of beta-diketone pK(a), and group electronegativity, chi, of each R group on the rate of substitution are also discussed. (C) 2002 Elsevier Science B.V. All rights reserved.