A tridentate and dianionic N-heterocyclic olefin (NHO) with two unsymmetrically-tethered aryloxide sidearms on bismuth: a fortuitous followed by systematic discovery

Abstract

Herein, we report our previously developed bifunctional imidaozlium-phenol LH₂Br ([HO-4,6-^tBu₂-C₆H₂-2-CH₂{CH(NCH = CHNAr)}]Br; Ar = Dipp = 2,6-ⁱPr₂-C₆H₃), leading to three distinct outcomes with bismuth by simply altering the relative ratios of the reacting components. Treating LH₂Br and Bi(HMDS)₃ (HMDS = N(SiMe₃)₂) in a 1 : 1 ratio gives the NHC-Bi complex [(L)BiBr(HMDS)] (1). In contrast, LH₂Br, Bi(HMDS)₃, and KHMDS in a 2 : 1 : 1 ratio serendipitously result in an unprecedented NHO-Bi complex [(^DippNHO^2ArO)BiBr] (2), featuring a novel dianoinic and tridentate NHO ligand with two unsymmetric aryloxide sidearms. A tri-acidic imidazolium salt L′H₃I ([HO-4,6-^tBu₂-C₆H₂-2-CH₂{C(HO-4,6-^tBu₂-C₆H₂-2-CH₂)(NCH = CHNMe)}]I) is synthesized independently as a potential precursor to such an NHO framework, and the corresponding NHO-BiI complex [(^MeNHO^2ArO)Bi(μ-I)]₂ (3₂) is made as proof of concept. Lastly, treating LH₂Br and Bi(HMDS)₃ in a 2 : 1 ratio also unexpectedly leads to an ‘abnormal’ NHC-Bi complex [(^aL)BiBr₂(^DippImd)] (4; ^DippImd = Dipp-imidazole). The multi-component and essentially multi-step reactions leading to 2 and 4 are challenging to fully elucidate mechanistically. Still, control experiments indicate 1 as a possible intermediate in both cases. Based on these results and prior insights into LH₂Br and an intermediate LH, plausible routes for both 2 and 4 are hypothesized. DFT calculations are also performed to analyze the bonding in 2 and 3₂ and to justify an NHC to aNHC isomerization towards the formation of 4.