Normal‐to‐Abnormal NHC Rearrangement of Al^III, Ga^III, and In^III Trialkyl Complexes: Scope, Mechanism, Reactivity Studies, and H₂ Activation

Abstract

The present contribution reports experimental and theoretical mechanistic investigations on a normal-to-abnormal (C2-to-C4-bonded) NHC rearrangement processes occurring with bulky group 13 metal NHC adducts, including the scope of such a reactivity for Al compounds. The sterically congested adducts (nItBu)MMe3 (nItBu=1,3-di-tert-butylimidazol-2-ylidene; M=Al, Ga, In; 1 a-c) readily rearrange to quantitatively afford the corresponding C4-bonded complexes (aItBu)MMe3 (4 a-c), a reaction that may be promoted by THF. Thorough experimental data and DFT calculations were performed on the nNHC-to-aNHC process converting the Al-nNHC (1 a) to its aNHC analogue 4 a. A nItBu/aItBu isomerization is proposed to account for the formation of the thermodynamic product 4 a through reaction of transient aItBu with THF-AlMe3 . The reaction of benzophenone with (nItBu)AlMe3 afforded the zwitterionic species (aItBu)(CPh2 -O-AlMe3 ) (6), reflecting the unusual reactivity that such bulky adducts may display. Interestingly, the nItBu/Al(iBu)3 Lewis pair behaves like a frustrated Lewis pair (FLP) since it readily reacts with H2 under mild conditions. This may open the way to future reactivity developments involving commonly used trialkylaluminum precursors.

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

“…The NMR data for 1 and 2 are consistent with the coordination of IPr* to the M(III) centers. In particular, the 13 As determined through X-ray crystallographic studies (XRD), the solid-state molecular structure of 2 confirmed the effective coordination of IPr* to InMe3, resulting in a four-coordinate In(III) adopting a distorted tetrahedral geometry and coplanar to the NHC heterocyclic ring (Figure 2). The In-Ccarbene bond distance (2.330(5) Å) in 2 is a bit longer than those in (IMes)InMe3 and (IPr)InMe3 (2.292(6) and 2.309(2) Å, respectively; IMes = 1,3-bis(2,4,6-trimethylphenyl)-1,3-dihydro-imidazol-2-ylidene; IPr = 1,3-bis{2,6-bis(1-methylethyl)phenyl})-1,3-dihydro-imidazol-2-ylidene), likely reflecting greater steric hindrance between the NHC and InMe3 in 2 [20,21].…”

“…While stable for days in benzene/toluene at room temperature (under inert atmosphere), they quickly decompose to unknown species in CH2Cl2 and THF at room temperature, in line with severely hindered Lewis pairs. The NMR data for 3 and 4 only display sharp resonances As earlier mentioned, the sterically hindered Lewis pairs (ItBu)MMe 3 (M = Ga, In) are unstable and rearrange to the corresponding C4-bonded isomers (aItBu)MMe 3 (Scheme 1) [12][13][14]. To avoid such isomerization, the backbone-saturated carbene StBu, prepared according to a known procedure [23], was coordinated to GaMe 3 and InMe 3 .…”

“…As earlier mentioned, the sterically hindered Lewis pairs (ItBu)MMe3 (M = Ga, In) are unstable and rearrange to the corresponding C4-bonded isomers (aItBu)MMe3 (Scheme 1) [12][13][14]. To avoid such isomerization, the backbone-saturated carbene StBu, prepared according to a known procedure [23], was coordinated to GaMe3 and InMe3.…”

“…FLP reactivity has attracted tremendous attention over the past few years thanks to the ability of sterically hindered Lewis pairs and FLPs to activate numerous polar/unsaturated substrates, including H 2 and CO 2 [9][10][11]. 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)MMe 3 (Scheme 1, taking the example of adducts (ItBu)MMe 3 ) [12][13][14].…”

“…The ItBu/Al(iBu) 3 FLP system was also shown to activate H 2 at room temperature [13]. The reactivity of lower group 13 metal Ga(III)-/In(III)-NHC Lewis pairs has been less studied, though recent studies have detailed FLP activation of various polar substrates with an NHC/GaR 3 mixture (R = alkyl, carbyl) [16,17].…”