Karakterisering en oksidatiewe addisiereaksies van dimeriese iridium(I) pirasoolkomplekse met jodometaan

[摘要] English: The aim of this study was to synthesise and characterise different binuclear iridium(I) pyrazolecomplexes and to investigate the oxidative addition reactions between this type of complexes andiodomethane (CH3I). The iridium(I) complexes used in this study, (Bu4N)[Ir2(-Dcbp)(cod)2]and (Bu4N)[Ir2(-Dcbp )(C0)2(PCY3)2], were characterised by physical methods such as NMR, IRand element analysis.A product of the reaction between (Bu4N)[Ir2(µ-Dcbp)(CO)2(PCY3)2] and l,2-dichloroethane wasisolated and characterised by X-ray crystallography. The crystals of the product,trans-[IrCI(CO)(PCY3)2], are triclinic with space group Pï. The importance of this structuredetermination centres on the fact that (Bu4N)[Ir2(µ-Dcbp)(COh(PCY3)2] as well as(BU4N)[Ir2(µ-Dcbp )(cod)2] react slowly with different solvents.The oxidative addition of iodomethane to (BU4N)[Ir2(µ-Dcbp )(cod)2] takes place according to thefollowing scheme: (see the scheme 1 on full text) The kinetic results of the oxidative addition of CH3I to (Bu4N)[Ir2(µ-Dcbp)(cod)2] show that theoxidative addition can occur via a direct pathway (K1-equilibrium) or a solvent-assisted pathway(K2, k3). The oxidative addition occurs mainly along the direct pathway, which is a factor 10 - 40faster than the solvent-assisted pathway. The observed solvent effect can not be attributed to thepolarity or donosity of the solvents.The positive ∆H�?values and fairly negative ∆S�?values of the oxidative addition step areindicative of an associative process. The fairly negative ∆S�?values together with the solventeffect indicates a three-center mechanism for oxidative addition.In the case of the (Bu4N)[Ir2(µ-Dcbp )(C0)2(PCY3)2] complex an iridium(III)alkyl complex isformed during oxidative addition (k1/k.1 pathway, Scheme 2) which is followed by the slowformation of the corresponding acyl complex (k2 pathway, Scheme 2). The equilibrium betweenthe starting complex and the alkyl complex is maintained during the formation of the acylcomplex. A very fast oxidative addition step (k'1/k'-1)-pathway, Scheme 2) leads only to theformation of an iridium(III)alkyl complex probably because of a trans configuration of thecarbonyl and methylligands which inhibits carbonyl insertion or methyl migration. The firstreaction, the formation of the alkyl complex, is reversible and dependent on the iodomethaneconcentration. (see the scheme 2 on full text) The activation parameters, ∆H�?and ∆S�?or the alkyl formation indicate associative activation forboth the forward (k1) and the reverse (k-1) steps. This is in contrast to the expected associativeforward (k1) and dissociative reverse (k1) reactions for oxidative addition. Athree-center mechanism, similar to that for the (Bu4N)[Ir2(µ-Dcbp)(cod)2] complex, is proposed.

[发布机构] University of the Free State