Stereoelectronic Profiling of Neutral and Monoanionic Biimidazoles and Mixed Diimines

Abstract

14 mono-, di-, and tetranuclear palladium complexes were prepared to study the coordination chemistry of symmetrical and unsymmetrical azole-derived diimines and their anions. The diverse range of complexes obtained highlights the structural and electronic diversities imposed by these ligands. Using the monopalladium species, the electronic properties of selected bidentate ligands were determined, ranked, and compared by 13 C NMR spectroscopy, extending the scope of the HEP2 (Huynh electronic parameter 2) scale, which can detect even subtle differences. Moreover, the %V bur (percentage volume buried) values as an estimate for the steric bulk of some ligands were determined using the solid-state molecular structures of their complexes, and a preliminary stereoelectronic map was established.

Electronic Profiling of N-Phosphine Oxide-Substituted Imidazolin-2-ylidenes (PoxIms) and Imidazolidin-2-ylidenes (SPoxIms)

“…Overall, the bidentate κ 2 -(S)PoxIm ligands are weaker donors than diNHCs (177−180 ppm), 38 dithiocarbamates/xanthates (174−179 ppm), 42 and monoanionic C^N chelators (176− 185 ppm), 43,44 but stronger donors than neutral diimines (157−164 ppm). 15,40 Their donicities are similar to those of N^O β-ketiminate chelators (NacAcs) (∼170 ppm) 43 and lie in the higher donor range of monoanionic N^N chelators (165−173 ppm). 40,41 Selenium Adducts of (S)PoxIms.…”

“…15,40 Their donicities are similar to those of N^O β-ketiminate chelators (NacAcs) (∼170 ppm) 43 and lie in the higher donor range of monoanionic N^N chelators (165−173 ppm). 40,41 Selenium Adducts of (S)PoxIms. Using the 77 Se NMR chemical shifts of NHC-selenium adducts, the relative πacidities of the respective NHCs can be estimated.…”

“…The HEP concept has also been extended for the donor strength assessment of popular bidentate ligands (HEP2) such as dicarbenes, neutral/monoanionic diimines, − dithiocarbamates, β-ketiminates, and monoanionic C^N chelators . Hence, we sought to evaluate the donor strength abilities of the charge-neutral bidentate κ 2 -(S)­PoxIm ligands using HEP2 complex probes of the type [PdBr­( i Pr 2 -bimy)­(L 1 ^L 2 )]­PF 6 {L 1 ^L 2 = charge-neutral and unsymmetrical chelator for evaluation}.…”

“…The difference is more pronounced (ΔHEP2 = 0.7–0.9 ppm) compared to the monodentate ligands (ΔHEP = 0.3–0.4 ppm), presumably due to the electronic distribution across two donor sites in a bidentate coordination mode. Overall, the bidentate κ 2 -(S)­PoxIm ligands are weaker donors than diNHCs (177–180 ppm), dithiocarbamates/xanthates (174–179 ppm), and monoanionic C^N chelators (176–185 ppm), , but stronger donors than neutral diimines (157–164 ppm). , Their donicities are similar to those of N^O β-ketiminate chelators (NacAcs) (∼170 ppm) and lie in the higher donor range of monoanionic N^N chelators (165–173 ppm). , …”

Donor Strength Determination of Anionic Ligands

“…In contribution to this area, we have developed an alternative methodology for the evaluation and comparison of donor strengths that is simple and can be applied across many different ligand families. , Using palladium NHC complexes of the type trans -[PdBr 2 ( i Pr 2 -bimy)­L], [PdBr­( i Pr 2 -bimy)­(L^L)] + , [PdBr­( i Pr 2 -bimy)­(L^L′)] + and [PdBr­( i Pr 2 -bimy)­(L^X)], monodentate (HEP, L) , as well as bidentate ligands (HEP2: L^L, L^L′, L^X) − can be probed with the Huynh Electronic Parameter (HEP) by comparing the 13 C carbene NMR signals (HEP/HEP2 values) of the i Pr 2 -bimy (1,3-diisopropyl-benzimidazolin-2-ylidene) reporter ligand.…”