Recent Applications of the Huynh Electronic Parameter (HEP)

Abstract

The Huynh electronic parameter (HEP) is a modern alternative method to determine ligand donor strengths by 13C NMR spectroscopy of metal NHC complexes containing the ligand of interest. Using the HEP, the electronic properties of numerous classical Werner-type and organometallic ligands have been evaluated thus far. Moreover, it has been extended to the HEP2, which provides a means to measure bidentate chelators. This Highlight Review covers recent applications of the HEP and its variants from 2016 onwards.

Versatile halogenation via a CNHC^Csp3 palladacycle intermediate

“…Complexes 3a–d and 4a–d all contain the i Pr 2 -bimy carbene ligand making them suitable probes to measure the electron-donating strengths of the transoid L and L 2 ligands using Huynh's electronic parameters (HEP & HEP2). 11 The detailed results and discussions are given in the ESI †. Here, a summary of the HEP2 scale was drawn to showcase that the current C NHC ^C sp3 chelators are stronger donors than the charge-neutral N^N (158.2–163.4 ppm), C NHC ^C NHC (177.1–180.3 ppm) 12 and monoanionic N^N (164.5–172.6 ppm), 13 or N^C sp2 (183.6–184.7 ppm) 14 ligands (Fig.…”

Gauging the donor strength of iron(0) complexesviatheir N-heterocyclic carbene gold(i) adducts

“…1(iii)). 9 The 13 C NMR signal of the benzimidazolinylidene carbon donor in [( i Pr 2 -bimy)PdBr 2 (L)] corresponds to the Huynh electronic parameter (HEP) (where better donor ligands lead to lower field shifts), and Huynh has shown that the 13 C carbenic NHC signal in [( i Pr 2 -bimy)Au(L)] + complexes can be directly correlated to the HEP for carbene ligands (this has yet to be expanded to other ligand classes). Given that Fe(0) has previously been reported to form complexes with gold fragments, 4 a , b we envisioned that the synthesis of bimetallic compounds of the type [( i Pr 2 -bimy)Au–Fe(CO) 3 (PR 3 ) 2 ][BAr F 4 ] {BAr F 4 = tetrakis(pentafluorophenyl)borate} might allow an accurate ordering and comparison of the donor strengths of Fe(0) complexes as ligands.…”

Synthesis of CAAC Complexes by Oxidative Addition of 2-Chloro-3,3-dimethyl-N-ethylindoleninium Cations to Carbonylmetalates and Subsequent Transmetalation

“…In addition, the selenium adduct of C (R = H) has been prepared and the π-acceptor strength of the CAAC has been determined by 77 Se NMR spectroscopy using the Ganters method . To complete the characterization of the benzannulated CAACs of type C , we now determined their σ-donor properties using the Huynh electronic parameter (HEP). , To this end, complex [ 5 ] was reacted with 1,3-diisopropyl-benzimidazolium tetrafluoroborate (bimy·HBF 4 ) in the presence of a base in acetone to give the CAAC–Au–NHC complex [ 6 ]­BF 4 (Scheme , see the SI). The HEP can then be calculated from the chemical shift δ­(C bimy ) in the 13 C NMR spectrum.…”