Lewis acidic metals as ligands for homogeneous palladium catalysis

  • Lewis Saure Liganden für die homogene Palladium-Katalyse

Steinhoff, Patrick; Okuda, Jun (Thesis advisor); Oppel, Iris Marga (Thesis advisor)

Aachen (2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2019

Abstract

This doctoral thesis investigates the influence of Lewis acidic metals as ligands for homogeneous Palladium catalysis. Low valent palladium complexes featuring a diphosphinometal-ligand (PMP) are synthesized. The metalloligand contains different Lewis acidic s-orbital acceptors (M = LiI, CuI, ZnII) and is based on a diphosphorylated trispyridylamine (TPA) pre-ligand. The synthesis of two pre-ligand derivates, bis(diphenylphosphine)-trispyridylamine (DPPTPA) and bis(dicyclohexylphosphine)trispyridylamine (DCPTPA) is described in this work.Based on these pre-ligands different heterobimetallic complexes featuring different Lewis acidic metals are synthesized and isolated. A stabilizing co-ligand is needed for a heterobimetallic Li-Pd(0)-complex using pre-ligand DPPTPA while no stabilizing co-ligand is needed for complex [(dpptpa)ZnPd]2+ as well as for heterobimetallic M-Pd(0)-complexes (M = LiI, CuI, ZnII) featuring pre-ligand DCPTPA. Different heterobimetallic Li-Pd(II)-complexes were isolated as well in this work. All feature a trans conformation stabilized by a Li-X-interaction (X = Br-, Cl-).The M-Pd-interaction is quantified by NBO/NLMO-analyses. These analyses show an increasing interaction between the donor dx2-y2-Pd-orbital and the acceptor s-orbital in the order LiI < CuI < ZnII. The influence of the M-Pd-interaction on the reactivity is furthermore investigated in this work. An oxidative addition is observed when 4-bromoanisole is reacted with [(dcptpa)LiPd]+ whereas no reaction takes place when adding 4-bromoanisole to complex [(dcptpa)ZnPd]2+. A reductive elimination of 4-methoxy-biphenyl is proven by GCMS after reacting the isolated oxidative addition product of 4-bromoanisole and [(dcptpa)LiPd]+ with LiPh. The catalytic activity of the heterobimetallic complexes is also tested in different reactions. A similar catalytic activity of heterobimetallic complexes [(dcptpa)ZnPd]2+ and [(dcptpa)LiPd]2+ is observed in the allylic amination reaction. The catalytic activity decreases with an increased steric demand of the substrates.Complex [(dcptpa)ZnPd]2+, in contrary to complexes [(dcptpa)MPd]+ (M = Li, Cu), shows an excellent catalytic activity (TOF1/2 = 3000 1/h) in the hydrosilylation of CO2 with HSiMe2Ph yielding a silyl formate. Preliminary mechanistic investigations suggest the formation of a hydride complex which is stabilized by the Zn-Pd-interaction.

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