Iridium carbonyl complexes as model homogeneous catalysts

[摘要] English: The aim of this study was to investigate model iridium carbonyl complexes as homogeneouscatalyst precursors for processes such as olefin hydroformylation. The hydroformylation ofalkenes is one of the most important applications of transition metal based homogeneouscatalysis. The coordination chemistry of rhodium and iridium phosphine complexes plays amajor role in the understanding of basic organometallic reactions and homogenous catalyticprocesses.1 The diversity of tertiary phosphines in terms of their Lewis basicity andbulkiness render them excellent candidates to tune the reactivity of square-planar complexestowards a variety of chemical processes, such as oxidative addition and substitutionreactions.2Iridium(I) complexes of the type trans-[Ir(acac)(CO)(PR3)2] (acac = acetylacetonate,PR3 = PPh3, PPh2Cy, PPhCy2, PCy3) were synthesized and characterized by infrared (IR) andnuclear magnetic resonance spectroscopy (NMR). The X-ray crystallographic determinationsof trans-[Ir(acac-κO)(CO)(PPhCy2)2] and trans-[Ir(acac-κ2O,O)(CO)(PCy3)2] weresuccessfully completed and are compared with literature. Both complexes crystallize inmonoclinic crystal systems, C2/c. Only trans-[Ir(acac-κO)(CO)(PPhCy2)2] co-crystallizedwith solvent molecules as part of the basic molecular unit cell, though these solventmolecules show no apparent impact on the steric packing of the basic organometallic group.This delivered information as to the identification of products formed during the kineticstudies and increased the available information of these rare compounds in literature.3Two reactions were observed when rapid substitution of CO for PPh3 in [Ir(acac)(CO)2] wasinvestigated in methanol as solvent by use of cryo temperature photo-multiplier Stopped-flowspectrophotometry. The first reaction followed the general rate law for square planarsubstitution reactions where rate = (ks + k1[L])([substrate]) with pseudo first-order rateconstant kobs1 = ks + k1[L] and k1 the second-order rate constant for the substitution reaction.This indicated that the first step involves the substitution of one carbonyl group forming[Ir(acac)(CO)(PPh3)]. Linear plots of kobs against concentration of the incoming PPh3 ligandpassed through the origin implying that ks �?0, signifying that the solvent does notsignificantly contribute to the reaction rate and the rate law simplifies to kobs1 = k1[L], withk1 = 92.5(3) x 103, 77(3) x 103, 66(1) x 103 and 58(2) x 103 M-1 s-1 at -10, -20, -30 and-40 °C, respectively. The temperature dependence was determined with ΔHk1 = 5.8(6) kJmol-1 and the large negative values obtained for standard entropy change of activation,ΔSk1 = -127(2) J K-1 mol-1, suggests an associative substitution mechanism.The second reaction is defined by limiting kinetic behaviour and is indicative of a two-stepprocess involving the stepwise rapid formation of trans-[Ir(acac)(CO)(PPh3)2] with preequilibriumK2 = 1(3) x 102, 4(1) x 102, 7(2) x 102 M-1 at -20, -30 and -40 °C, respectivelyand rate-determining second step being the ring opening of the acac- ligand to yieldtrans-[Ir(acac-κO)(CO)(PPh3)2] with k3 = 18(5) x 101, 10(1) x 101, 4.7(4) x 101 M-1 s-1 at-20, -30 and -40 °C, respectively. The temperature dependence for the second reaction was determined with ΔHk3 = 30.8(3) kJ mol-1 and ΔSk3 = -79(1) J K-1 mol-1.

[发布机构] University of the Free State