Sterically Bulky NHC Adducts of GaMe3 and InMe3 for H2 Activation and Lactide Polymerization

Anaëlle Bolley,Samuel Dagorne,Béatrice Jacques,Lucas Thévenin,Gilles Schnee

doi:10.3390/inorganics6010023

Abstract

The sterically bulky Ga(III) and In(III) (IPr*)MMe3 adducts (1 and 2) and (SItBu)MMe3 adducts (3 and 4) (M = Ga, In; IPr* = 1,3-bis{2,6-bis(diphenylmethyl)-4-methylphenyl}-1,3-dihydro- imidazol-2-ylidene; SItBu = 1,3-bis(1,1-dimethylethyl)-imidazolidin-2-ylidene) were prepared and structurally characterized, allowing an estimation of the steric hindrance of such Lewis pairs (yields in 1–4: 92%, 90%, 73%, and 42%, respectively). While the IPr* adducts 1 and 2 are robust species, the more severely congested SItBu adducts 3 and 4 are more reactive and exhibit a limited stability in solution. Adduct (SItBu)GaMe3 (3) reacts quickly with H2 at room temperature to afford the corresponding aminal product, 1,3-di-tert-butylimidazolidine (5), along with free GaMe3. Such Frustrated Lewis Pair (FLP) reactivity constitutes the first instance of a H2 activation involving a simple trialkyl GaR3 species. Adduct 3 also mediates the ring-opening polymerization (ROP) of rac-lactide at room temperature to afford cyclic polylactide (PLA).

Highlights

Thanks to their exceptional σ-donating properties and steric tunability, N-heterocyclic carbenes (NHCs) constitute a privileged class of supporting ligands for the stabilization of various metal complexes [1,2,3,4,5]
The coordination of NHCs to oxophilic and high-oxidation-state metal centers such as group 13 metals M(III) (M = Al, Ga, In) is currently attracting growing attention [8]. This is due to the potential usefulness of the derived Lewis adducts for novel reactivity and/or small molecule activation, in particular through Frustrated Lewis Pair (FLP) reactivity [9]
The sterically bulky NHC–GaMe3 and NHC–InMe3 adducts (NHC = IPr*, SItBu) were prepared and characterized, and their level of steric congestion was assessed through structural analysis

Summary

Introduction

Thanks to their exceptional σ-donating properties and steric tunability, N-heterocyclic carbenes (NHCs) constitute a privileged class of supporting ligands for the stabilization of various metal complexes [1,2,3,4,5]. This has led to the widespread use of NHC-supported metal species in numerous research fields, notably in metal-based homogeneous catalysis and for the characterization of unusual organometallic motifs [6,7]. In the area of group 13 metal species, we and others have shown that sterically bulky group 13 metal Lewis adducts may display limited stability and readily isomerize to their “abnormal” counterparts for steric relief of (NHC)MMe3 (Scheme 1, taking the example of adducts (ItBu)MMe3 ) [12,13,14]

Methods

Results

Conclusion