Synthesis, kinetic and computational chemistry of thiopene-containing beta-diketonato complexes of rhodium(I) and rhodium(III)

[摘要] English: Thienyl-containing β-diketones of the type ThCOCH2COR, with R = Th (Hdtm), C6H5 (Hbth), have been prepared. The group electronegativity of the thienyl group is 2.107 (Gordy scale) as inferred from a linear methyl ester IR carbonyl stretching frequency - group electronegativity relationship. A single crystal X-ray determination of the structure of ThCOCH2COPh indicates, that the enolization in a direction away from the thienyl group, always dominates. Assymetric enolisation, in the direction furthest from the thienyl group, was also observed for ThCOCH2COR, with R = Th or CF3 (Htta). This finding is considered to be the result of resonance driving forces rather than inductive electronic effects of substituents on the pseudo-aromatic β-diketone core. By increasing the temperature of the solvent (CDCI3) from 0 to 50°C, the percentage keto isomer at equilibrium of Htta (4.2 - 7.1%), Hbth (5.3 - 9.5%) and Hdtm (14.8 - 21.9%) increased. Slow conversion kinetics from the keto to the dominant enol isomer was monitored in CDCI3 solution for Hbth and Hdtm. The keto-enol conversion for Htta was found to be too fast to measure. pKa' values of the thienyl-containing β-diketones ThCOCH2COR were determined to 6.49 (Htta), 9.01 (Hbth) and 8.89 (Hdtm).Synthetic routes to prepare new thienyl-containing β-diketones rhodium(I) complexes of the type [RhI(ThCOCHCOR)(CO)2], [RHI(ThCOCHCOR)(CO)(PPh3)] and [RhIII(β-diketonato)(CO)CPPh3)(Me)(I)] with Th = thienyl and R = C6H5 (Ph), Th and CF3 have been developed. The crystal structure of [Rh(dtm)(CO)2] was also solved. From 1H and 31P NMR studies, it is clear that for complexes of the type [RhI(β-diketonato)(CO)(PPh3)] and [RhIII(β-diketonato)(CO)(PPh3)(Me)(I)], with an unsymmetrical β-diketonato ligand, at least two main isomers exist in solution. The equilibrium constant, Kc, which relates these two isomers in an equilibrium reaction, is concentration independent but temperature dependent for [RhI(tta)(CO)(PPh3)]. ∆rG, ∆rH and ∆rS values for this equilibrium have been determined. However, changes in the temperature (-30 to 55°C in CDCl3) or concentration had little influence on the equilibrium constant of [RhI(bth)(CO)(PPh3)] and the [RhIII(β-diketonato)(CO)(PPh3)(Me)(I)] complexes of tta and bth.The chemical kinetics of the oxidative addition reaction of CH3I to [Rh(ThCOCHCOR)(CO)(PPh3)] have been studied in detail utilizing IR, UV/vis, 1H NMR, 19F NMR and 31P NMR techniques. The NMR studies revealed that the rate of oxidative addition of iodomethane to the different [Rh(ThCOCHCOR)(CO)(PPh3)] isomers were the same. Three definite sets of reactions, involving at least two Rh(III)-alkyl (two isomers each) and two Rh(III)-acyl species (two isomers each), as shown in the reaction sequence below, were observed.Action sequence in PDF full text.A quantum computational chemistry study, by means of Density Functional Theory (DFT) calculations was done on the thienyl-containing rhodium(I) and rhodium(III) complexes involved in the above reaction scheme. Excellent results, in agreement with experimental results were obtained on the geometry optimization of Hdtm and [Rh(ThCOCHCOCF3)(CO)(PPh3)]. By comparng the relative energies of the optimized geometries of the 12 possible rhodium(III)-alkyl and 6 possible rhodium(III)-acyl products,, the stereochemistry of the different alkyl1, alkyl2, acyl1 and acyl2 products in the above reaction scheme was proposed. Experimental results obtained on the stereochemistry of the selected alkyl1 and alkyl2 reaction products, by means of 1H NOESY, were consistent with the stereochemistry proposed by quantum chemistry calculations.

[发布机构] University of the Free State