Iridium carbonyl complexes as model homogeneous catalysts

English: The aim of this study was to investigate model iridium carbonyl complexes as homogeneous catalyst precursors for processes such as olefin hydroformylation. The hydroformylation of alkenes is one of the most important applications of transition metal based homogeneous catalysis. The coordination chemistry of rhodium and iridium phosphine complexes plays a major role in the understanding of basic organometallic reactions and homogenous catalytic processes.1 The diversity of tertiary phosphines in terms of their Lewis basicity and bulkiness render them excellent candidates to tune the reactivity of square-planar complexes towards a variety of chemical processes, such as oxidative addition and substitution reactions.2 Iridium(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) and nuclear magnetic resonance spectroscopy (NMR). The X-ray crystallographic determinations of trans-[Ir(acac-κO)(CO)(PPhCy2)2] and trans-[Ir(acac-κ2O,O)(CO)(PCy3)2] were successfully completed and are compared with literature. Both complexes crystallize in monoclinic crystal systems, C2/c. Only trans-[Ir(acac-κO)(CO)(PPhCy2)2] co-crystallized with solvent molecules as part of the basic molecular unit cell, though these solvent molecules 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 kinetic studies and increased the available information of these rare compounds in literature.3 Two reactions were observed when rapid substitution of CO for PPh3 in [Ir(acac)(CO)2] was investigated in methanol as solvent by use of cryo temperature photo-multiplier Stopped-flow spectrophotometry. The first reaction followed the general rate law for square planar substitution reactions where rate = (ks + k1[L])([substrate]) with pseudo first-order rate constant 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 ligand passed through the origin implying that ks ≈ 0, signifying that the solvent does not significantly contribute to the reaction rate and the rate law simplifies to kobs1 = k1[L], with k1 = 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) kJ mol-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-step process involving the stepwise rapid formation of trans-[Ir(acac)(CO)(PPh3)2] with preequilibrium K2 = 1(3) x 102, 4(1) x 102, 7(2) x 102 M-1 at -20, -30 and -40 °C, respectively and rate-determining second step being the ring opening of the acac- ligand to yield trans-[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.