Ruthenium and iridium complexes supported by tetradentate salen ligands : synthesis, structure, property and catalytic applications

Abstract

Abstract of thesis entitled RUTHENIUM AND IRIDIUM COMPLEXES SUPPORTED BY TETRADENTATE SALEN LIGANDS: SYNTHESIS, STRUCTURE, PROPERTY AND CATALYTIC APPLICATIONS

Submitted by Chi Lun LEE for the degree of Doctor of Philosophy at The University of Hong Kong in October 2017

Transition metal-catalyzed carbene transfer reaction represents an appealing and effective methodology for C–C, C–Si, C–N, and C–O bond formation in synthetic organic chemistry. In particular, ruthenium and iridium salen complexes have shown distinctive catalytic activity towards carbene transfer reactions. In this thesis, novel catalysts for these reactions were developed and the studies of metal-carbene intermediates for probing the underlying reaction mechanisms were described. A series of cis-β-(carbonyl) ruthenium complexes were synthesized by treatment of various sterically bulky H2salen ligands with Ru3(CO)12 in 1,2,4-trichlorobenzene at 185 °C. X-ray crystallography studies showed that the two phenyl planes of the salen ligand form dihedral angles of 74.02° and 63.47° in complexes with -CPh3 at the C2 position on the H2salen ligand which are larger than the corresponding angle of 50.70° in a complex with -tBu at the C2 position. My studies revealed that cis-β-[RuII(salen)(CO)2] (R)-3f represents an effective catalyst for the asymmetric intermolecular Si–H bond insertion of tert-butyl aryldiazoacetates under light irradiation, with the Si–H bond insertion products obtained in up to 96% yield and up to 84% ee. The active intermediates could be the respective cis-β-ruthenium salen monocarbene complex cis-β-[Ru(salen)(CO)(=CPh2)] 3g. The reaction of cis-β-[RuII(salen)(CO)2] 3b with Ph2CN2 gave the stable six-coordinate complex 3g whose characterization showed that 3b undergoes decarbonylation under light irradiation to give a mono(carbonyl) species and the resultant species reacts with diazo compound to generate metal-carbene intermediate. A series of trans-dichlororuthenium(IV) salen complexes were synthesized by reacting RuCl3 with H2salen ligands in refluxing ethanol solution. Cyclic voltammetry studies revealed that the E1/2 of Ru(IV)/Ru(III) couples follow the trend 4b < 4d < 4a < 4c which increases with electron-donating strength of the substituent on the salen ligand Cl < tBu < OMe. High-valent dichlororuthenium(IV) salen complex 4a was shown to catalyze C–H functionalization of indoles with methyl aryldiazoacetates to afford desired products in high product yields (up to 86%) and moderate enantioselectivities (up to 67% ee). The crude reaction mixture obtained from the treatment of complex 4a and Ph2CN2 in DCE was studied by a high-resolution ESI mass spectrometry experiment and [RuIII(L4.1)Cl(CPh2)] is proposed to be the reaction intermediate. This involves the detachment of one Cl ligand from precatalyst 4a to form the mono(chloro) species and the resultant species reacts with the diazo compound to generate metal-carbene intermediate. A series of trans-dichloroiridium(IV) salen complexes were synthesized. High-valent trans-dichloroiridium(IV) salen complex 5a showed high catalytic activity towards carbene C–H, Si–H, N–H and O–H bond insertion reactions. Complex 5a-catalyzed intermolecular carbene C–H bond insertion of 1-methylindole and 1,4-cyclohexadiene with methyl aryldiazoacetates to afford desired products in 86–96% and 89–98% yields respectively. Si–H insertion reaction of dimethylphenylsilane with methyl aryldiazoacetates proceeded smoothly with complex 5a as catalyst, giving the desired products in 87–93% yields. The same complex was also catalytically active towards intermolecular carbene N–H and O–H insertions to give the desired products in 85–93% and 52% yields respectively.