Methanol carbonylation via platinum group metal complexes

[摘要] The aim of this study, firstly, involved the synthesis of a range of S,O-functionalized thiourealigands with systematically changing electro-steric properties and investigate these ligandscoordination modes to rhodium complexes in an attempt to primarily synthesize a range of[Rh(S,O-thioureato)(CO)2] and [Rh(S,O-thioureato)(CO)(PR1R2R3)] complexes. Moreover, theaim also included the synthesis of [Rh(diphosphine)(CO)2]+ complexes using a range ofdiphosphine ligands. These complexes were then to be used to synthesize the correspondingRh(III)-acyl complexes via iodomethane oxidative addition and study thecarbonylation/hydrogenation of methanol to ethanol by investigating the kinetic and activationparameters of the iodomethane oxidative addition as well as reductive elimination/hydrogenationof acyl iodide/acyl species.Several S,O-functionalized thiourea ligands were successfully synthesized and characterizedfrom which the X-ray crystallographic structures for several of the ligand systems are reported:N-benzoyl-N' -(2,4,6-trimethylphenyl)thiourea (Triclinic P 1, R1 = 5.60 %), N-benzoyl-N' -(2,6-dibromo-4-fluorophenyl)thiourea (Triclinic P 1, R1 = 3.76 %), N-benzoyl-N' -(pentafluorophenyl)thiourea (Monoclinic C2/c, R1 = 3.69 %), N-benzoyl-N -(phenethyl)thiourea(Monoclinic P21/n, R1 = 3.91 %), N-benzoyl-N' -(naphthalene-1-ylmethyl)thiourea (MonoclinicC2/c, R1 = 5.37 %), N-benzoyl-N '-(cyclohexyl)thiourea (Triclinic P, R1 = 2.10 %) and Nbenzoyl-N' -(isopentyl)thiourea (Triclinic P, R1 = 5.06 %). It was established that these ligandsexhibit a keto conformation in the solid state, where the carbonyl oxygen is trans to the sulphuratom and is stabilized by a hydrogen bond interaction with the terminal nitrogen atom of thethiourea moiety. The keto conformation was also confirmed in solution by NMR spectroscopy.Furthermore, hydrogen bond interactions exist between neighbouring molecules in the solidstate, which leads to either dimer or polymer formation in the crystal packing of these thioureacompounds.The [Rh(diphosphine)(CO)2]+ complexes could not be successfully synthesized, however, duringseveral attempts one synthetic route led to the formation of a cationic A-frame complex of thetype [Rh2(µ -Cl)(diphosphine)2(CO)2]BF4. The X-ray crystallographic structure of[Rh2( µ-Cl)(dppm)2(CO)2]BF4 (Monoclinic P21/n, R1 = 8.84 %) is reported.Several [Rh(S,O-thioureato)(CO)2] complexes were synthesized and characterized, however,these complexes were unstable outside of solution. Several attempts were made to synthesize[Rh(S,O-thioureato)(CO)(PPh3)] complexes, which led to the isolation of rhodium thioureacomplexes where the thiourea ligands exhibit S,O-, S- and N,S-coordination modes. The X-raycrystallographic structures of the following complexes are reported: [Rh(N,S-(N-4h2mPT))(CO)(PPh3)2] (Triclinic P, R1 = 2.75 %), [Rh(N,S-(N-PT))(S,O-(N-PT))(PPh3)2](Triclinic P, R1 = 4.44 %), [Rh(COD)(Cl)(S-(N-PTH))] (Triclinic P, R1 = 3.18 %),[Rh(COD)(Cl)(S-(N-tmPTH))] (Monoclinic C2/c, R1 = 6.74 %). [Rh(N,S-(N-4h2mPT))(CO)(PPh3)2] is analogous to typical Vaska-type complexes, where the coordinatedthiourea ligand is trans to the carbonyl ligand and the two PPh3 are trans to each other on therhodium centre. [Rh(N,S-(N-PT))(S,O-(N-PT))(PPh3)2] is a Rh(III) species with an octahedralarrangement around the rhodium centre, where one of thiourea ligands coordinated in its enolconformation. In both [Rh(COD)(Cl)(S-(N-PTH))] and [Rh(COD)(Cl)(S-(N-tmPTH))] thepreferred orientation of the free ligands translated to the orientation of the coordinated ligands.These complexes were also stabilized by hydrogen bond interactions between the chlorido ligandand the internal nitrogen atom of the thiourea moiety.A range of [Rh(S,O-(N-diPT))(CO)(PR1R2R3)] complexes were successfully synthesized usingN-benzoyl-N' ,N' -(diphenyl)thiourea and a range of phosphine ligands with systematicallychanging electro-steric properties (PPh3, PPh2Cy, PPhCy2, PCy3). The X-ray crystallographicstructures of the following complexes are reported: [Rh(S,O-(N-diPT))(CO)(PPh3)] (MonoclinicP21/c, R1 = 6.86 %), [Rh(S,O-(N-diPT))(CO)(PPh2Cy)] (Monoclinic P21/c, R1 = 6.32 %),[Rh(S,O-(N-diPT))(CO)(PCy3)] (Monoclinic P21/c, R1 = 6.86 %). The respective first ordercoupling constants (1JRh-P) and the carbonyl stretching frequencies ( VCO) were obtained, fromwhich the expected order of electronic effects of the phosphine ligands was established. The effective cone angles ( E) for the different phosphine ligands were also calculated, whichcorrelated well with the expected steric congestion of the ligands on the rhodium centre.The reactivity of the [Rh(S,O-(N-diPT))(CO)(PR1R2R3)] complexes towards the iodomethaneoxidative addition was investigated. In general the reaction rate of the individual reactionsincreased in the order of [Rh(N-diPT)(CO)(PPhCy2)] < [Rh(N-diPT)(CO)(PCy3)] <[Rh(N-diPT)(CO)(PPh3)] < [Rh(N-diPT)(CO)(PPh2Cy)]. This order of reactivity was ascribed toa combinative effect of both the steric and electronic properties of the phosphine ligands. Theactivation parameters calculated for the individual reactions were found to be similar. Theproposed mechanism for the iodomethane oxidative addition to complexes of the type[Rh(S,O-thioureato)(CO)(L)], where L = CO/PR1R2R3, is depicted in Scheme I.(Find scheme 1 on full text)The electro-steric effects of phosphine ligands in catalytic processes were further investigated bystudying these effects in the phosphine exchange reactions of Vaska-type complexes[Rh(Cl)(CO)(PR1R2R3)2] with the corresponding PR1R2R3 ligand via NMR techniques. Thereaction rate for the exchange reaction was almost two orders of magnitude faster for PPh3 thanfor PPh2Cy. Both exchange processes exhibited a large negative �?S�? and a small ∆H�?, whichsuggested an associative activation, where a stable 5-coordinated transition state is formed.

[发布机构] University of the Free State