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

Abstract

English: The aim of this study was to synthesise and characterise different binuclear iridium(I) pyrazole complexes and to investigate the oxidative addition reactions between this type of complexes and iodomethane (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, IR and element analysis. A product of the reaction between (Bu4N)[Ir2(µ-Dcbp)(CO)2(PCY3)2] and l,2-dichloroethane was isolated 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 structure determination 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 the following 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 the oxidative 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 - 40 faster than the solvent-assisted pathway. The observed solvent effect can not be attributed to the polarity or donosity of the solvents. The positive ∆H≠ values and fairly negative ∆S≠ values of the oxidative addition step are indicative of an associative process. The fairly negative ∆S≠ values together with the solvent effect 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 is formed during oxidative addition (k1/k.1 pathway, Scheme 2) which is followed by the slow formation of the corresponding acyl complex (k2 pathway, Scheme 2). The equilibrium between the starting complex and the alkyl complex is maintained during the formation of the acyl complex. A very fast oxidative addition step (k'1/k'-1)-pathway, Scheme 2) leads only to the formation of an iridium(III)alkyl complex probably because of a trans configuration of the carbonyl and methylligands which inhibits carbonyl insertion or methyl migration. The first reaction, the formation of the alkyl complex, is reversible and dependent on the iodomethane concentration. (see the scheme 2 on full text) The activation parameters, ∆H≠ and ∆S≠,or the alkyl formation indicate associative activation for both the forward (k1) and the reverse (k-1) steps. This is in contrast to the expected associative forward (k1) and dissociative reverse (k1) reactions for oxidative addition. A three-center mechanism, similar to that for the (Bu4N)[Ir2(µ-Dcbp)(cod)2] complex, is proposed.

Afrikaans: Afrikaans: Die doel van hierdie ondersoek was om meer duidelikheid aangaande die oksidatiewe addisiereaksies van dimeriese iridium(I)pirasolaatkomplekse te verkry, asook om die meganisme waarvolgens oksidatiewe addisie plaasvind, te bepaal. Die oksidatiewe addisiereaksies tussen jodometaan (CH3I) en (Bu4N)[Ir2(µ-Dcbp)(LL')2] (LL' = cod; L = CO en L' = PCY3) is tydens hierdie studie bestudeer. 'n Produk van die reaksie tussen (BU4N)[Ir2(µ-Dcbp)(CO)2(PCY3)2] en 1,2-dichlooretaan IS geïsoleer en met behulp van X-straalkristallografie gekarakteriseer. Die produk, trans-[IrCl(CO)(PCY3)2], kristalliseer in die trikliene kristalstelsel met ruimtegroep PT. 'n Wanordelike pakkingspatroon is vir die chloried- en karbonielligande in die kristal waargeneem. Hierdie struktuurbepaling was van groot belang aangesien solvolisereaksies vir beide die dimeriese iridium(I)pirasolaatkomplekse waargeneem is. Die oksidatiewe addisiereaksie tussen CH31en (Bu4N)[Ir2(µ-Dcbp)(cod)2] kan as volg voorgestel word: (sien skema 1 op volledige teks) Die kinetiese resultate toon dat bogenoemde oksidatiewe addisiereaksie via 'n direkte pad (K1-ewewig) sowel as 'n oplosrniddelondersteunde pad (K2, k3) plaasvind. Die oksidatiewe addisie van CH31 aan (Bu4N)[Ir2(µ-Dcbp)(cod)2] geskied egter hoofsaaklik via die direkte pad wat ongeveer 'n faktor 10 - 40 vinniger as die oplosmiddelondersteunde pad is. 'n Klein

oplosmiddeleffek is tydens bogenoemde studie waargeneem en kan me In terme van die donositeite of polariteite van die onderskeie oplosmiddels verklaar word nie. Die matig positiewe ∆H≠ - en redelike groot negatiewe ∆S≠ "-waardes wat vir die oksidatiewe addisiereaksie (k.) bepaal is, dui op 'n assosiatief geaktiveerde meganisme. 'n Konserte driesentermeganisme kan vir die oksidatiewe addisiereaksie op grond van die klein oplosmiddeleffek en die groot negatiewe ∆S≠ waarde voorgestel word. Infrarooistudies dui daarop dat die oksidatiewe addisie van CH31 aan (Bu4N)[Ir2(µ-Dcbp)(C0)2(PCY3)2] via 'n reaktiewe iridium(III)alkielintermediêr verloop om 'n iridium(III)asielproduk te vorm (k1/k1- en krpaaie, Skema 2). Asielvorming vind plaas met die behoud van die ewewigstoestand in die oksidatiewe addisiestap. 'n Baie vinnige oksidatiewe addisiereaksie (k'1k'-1-pad, Skema 2) lei slegs tot die vorming van 'n iridium(III)alkielkompleks karbonielinlassing/metielmigrasie inhibeer. Die oksidatiewe addisiereaksie (alkielvorming) is vanwee die trans-konfigurasie van die karboniel- en metielligande wat omkeerbaar en toon 'n eersteorde-afhanklikheid ten opsigte van die jodometaankonsentrasie. (sien skema 2 op volledige teks) Die aktiveringsparameters, matig positiewe ∆H≠ - en redelike groot negatiewe ∆S≠ -waardes vir die oksidatiewe addisie- en reduktiewe eliminasiereaksies dui op assosiatiewe aktivering. 'n Konserte driesentermeganisme word soos in die geval van (Bu4N)[Ir2(µ-Dcbp)(cod)2] vir die alkielvorming voorgestel.