Synthesis and Reactivity of Poly(propyleneimine) Dendrimers Functionalized with Cyclopentadienone N-Heterocyclic-Carbene Ruthenium(0) Complexes

Cristiana Cesari,Maria Cristina Cassani,Rita Mazzoni,Luca Rigamonti,Riccardo Conti,Valerio Zanotti,Andrea Cingolani

doi:10.3390/catal10020264

Abstract

Ligand design in metal chemistry is a fundamental step when pursuing compounds with specific reactivity. In this paper, the functionalization of the OH group in the lateral chain of the N-heterocyclic-carbene (NHC) ligand bound to a bis-carbonyl cyclopentadienone NHC ruthenium(0) complex allowed the decoration of five generations of poly(propyleneimine) (PPIs) dendrimers with up to 64 organometallic moieties. The coupling was achieved by employing carbonyldiimidazole and the formation of carbamate linkages between dendritic peripheral NH2 and lateral OH groups on ruthenium complexes. The synthetic procedure, chemical purification, and spectroscopic characterization of the five generations of dendrimers (3g1–5) are here described. The ruthenium-modified dendrimers were activated as catalysts in the transfer hydrogenation of the model compound 4-fluoroacetophenone in the presence of cerium ammonium nitrate as their mononuclear congeners. The catalytic activity, being similar for the five generations, shows a decrease if compared to mononuclear complexes. This detrimental effect might be ascribed to the –CH2NH– functionalization, largely present in dendrimer skeleton and that can compete with the hydrogen transfer mechanism, but also partially to a dendritic effect caused by steric encumbrance.

Highlights

Dendritic catalysts are functional macromolecules with precise and unambiguous structures, which, thanks to their monodisperse nature, maintain the advantages of homogeneous catalysts showing fast kinetic responses and easy tunability [1,2,3,4,5,6,7,8,9,10,11,12]
Functionalization, largely present in dendrimer skeleton and that can compete with the hydrogen transfer mechanism, and partially to a dendritic effect caused by steric encumbrance
Several reviews report on the use of dendrimers in catalysis [1,2,3,4,5,6,7,8,9,10,11], but few of them are dedicated to the dendrimer effect [14,15,16]

Summary

Introduction

Dendritic catalysts are functional macromolecules with precise and unambiguous structures, which, thanks to their monodisperse nature, maintain the advantages of homogeneous catalysts showing fast kinetic responses and easy tunability [1,2,3,4,5,6,7,8,9,10,11,12]. They can be removed from the reaction mixture by membrane or nanofiltration techniques and precipitation, exploiting their bigger sizes compared to products, and this confers the advantages of heterogeneous catalysts [13]. A dendrimer brings together a large number of catalytically active species in a nanoobject, while, on the Catalysts 2020, 10, 264; doi:10.3390/catal10020264 www.mdpi.com/journal/catalysts

Methods

Results

Conclusion