A Bimetallic Ruthenium Complex as a Catalyst Precursor for the Atom Transfer Radical Polymerization of Methacrylates at Ambient Temperature

Abstract

Abstract:The bimetallic ruthenium complex [(1,3,5-i-Pr 3 C 6 H 3 )Ru(m-Cl) 3 RuCl(C 2 H 4 )(PCy 3 )] has been synthesized by reaction of [(1,3,5-i-Pr 3 C 6 H 3 )RuCl 2 )] 2 with one equivalent of PCy 3 in the presence of ethylene. It can be used as a catalyst precursor for the controlled atom transfer radical polymerization of methacrylates at 35 8C. The resulting polymers show low polydispersities.Keywords: acrylates; bimetallic complex; polymerization; radical reaction; ruthenium Starting with seminal publications in the mid 1990s, [1] transition metal-catalyzed atom transfer radical polymerizations (ATRP) have become ubiquitous in modern macromolecular chemistry.[2] For most applications, Cu(I) complexes with amine-based ligands are used as the catalysts.[3] Nevertheless, there are a number of other transition metals which are known to catalyze ATRP reactions.[2] Among those, ruthenium complexes are of special interest because some of them were found to display very high activities. [4,5] In continuation of our efforts to develop new catalysts for ruthenium-catalyzed radical reactions, [5a,6] we have recently described the homobimetallic complex [(cymene)Ru(m-Cl) 3 RuCl(C 2 H 4 )(PCy 3 )] (1). [7] This complex shows an outstanding catalytic activity in atom transfer radical addition (ATRA) reactions. Since ATRP and ATRA are mechanistically closely related, [2] we were interested to see whether complex 1 can also be used for the controlled polymerization of acrylates. Unfortunately, complex 1 showed a very limited solubility in the monomer-toluene starting mixtures. We therefore investigated possibilities to increase the solubility of complex 1. A modification of the p-ligand appeared to be a suitable strategy because the catalytically relevant RuCl 2 (C 2 H 4 )(PCy 3 ) fragment would not be directly affected. Consequently, the well-soluble dimer [(1,3,5-i-Pr 3 C 6 H 3 )RuCl 2 ] 2 (2) [8] was chosen as the starting material. When a solution of complex 2 in isooctane was heated to 50 8C with one equivalent of PCy 3 in the presence of an atmosphere of ethylene, the desired bimetallic complex 3 was formed (Scheme 1). The product precipitates from solution and can be isolated by filtration (yield: 90%).The new complex was comprehensively characterized by 1 H and 13 C NMR spectroscopy, elemental analysis and single-crystal X-ray crystallography. The 13 C NMR spectrum of 3 in CD 2 Cl 2 showed two signals of equal intensity for the methyl groups of the i-Pr 3 C 6 H 3 p-ligand. This can be explained by the fact that complex 3 is chiral and configurationally stable on the NMR time scale. The methyl groups are thus diastereotopic. In the 1 H NMR spectrum, however, the difference between the two CH 3 groups was not resolved and only one doublet was observed at d ¼ 1.39 ppm. In order to avoid partial dissociation of the labile ethylene ligand, all spectra were recorded under an atmosphere of C 2 H 4 . Analysis of complex 3 by X-ray crystallography confirmed the expected dinuclear structure with three chloro bridges (Fi...

Homobimetallic Ruthenium Vinylidene, Allenylidene, and Indenylidene Complexes: Synthesis, Characterization, and Catalytic Studies

“…All these measurements are in line with those recorded previously by Severin et al for various other homobimetallic ruthenium-arene complexes bearing three m-chloro bridges. [22,35] Based on the data acquired by the Swiss group, the trans effect of the indenylidene unit in complex 11 may be ranked slightly superior to that of a vinylidene group and considerably higher than the one exerted by the ethylene ligand in complex 7a ( Figure 2). …”

Homobimetallic Ruthenium–N‐Heterocyclic Carbene Complexes: Synthesis, Characterization, and Catalytic Applications

“…This behavior sharply contrasts with the results previously reported for complex 2b at 35 8C that met all the criteria of controlled polymerization. [16] With the NHC-based complex 3a, the decrease of temperature most likely induced a switch of mechanism, from controlled ATRP to a redox-initiated free-radical process, [14] although further investigations are needed to fully clarify this point.…”

“…[15] By varying the nature of the arene ligand, the same group also synthesized the related homobimetallic complex 2b and successfully employed it as catalyst precursor for the ATRP of methacrylate monomers at 35 8C. [16] In view of the enhancements brought by the replacement of phosphines by NHCs in monometallic ruthenium-arene catalyst precursors of type 1, we decided to investigate the effect of similar modifications on the catalytic activity of complexes of type 2. In this contribution, we disclose the synthesis, characterization, and catalytic applications of two new homobimetallic ruthenium-arene complexes 3a and 3b bearing, respectively, the IMes and IMesCl 2 ligands.…”

Electron transfer reactions relevant to atom transfer radical polymerization

“…Much research has focused on developing novel nitrogen-based ligands for Cu complexes in an effort to adjust the redox potential of the catalysts, and in turn, affect the position of the ATRP equilibrium (vide infra) [14][15][16][17]. Additionally, the range of transition metals employed in ATRP continues to be expanded in the search for more powerful catalysts and now includes Ti, [18] Mo, [19][20][21][22] Re, [23][24][25] Ru, [2,[26][27][28][29][30][31][32] Fe, [33][34][35][36][37][38][39][40][41] Rh [26,[42][43][44][45], Ni, [43,[46][47][48][49][50][51][52][53] Pd, [54] Co, [55] Os, [56] and Cu [1,6,…”