Electronic Properties Of N-heterocyclic Carbene Research Articles

A simply accessible organometallic system to gauge electronic properties of N-heterocyclic carbenes.

The intricate σ and π-bonding of N-heterocyclic carbenes (NHCs) to metals and the need to quantify their electronic properties to rationalize reactivity of complexes have resulted in the creation of numerous methodologies to understand the NHC-metal interaction which are, as we now show, flawed. Our search for a unified, easily accessible system to gauge these fundamental properties has resulted in the discovery of two systems that highlight the flaws present in existing systems and provide a more accurate measure of the NHC ligand electronic properties.

Quantifying and understanding the electronic properties of N-heterocyclic carbenes

The use of N-heterocyclic carbenes (NHCs) in chemistry has developed rapidly over the past twenty years. These interesting compounds are predominantly employed in organometallic chemistry as ligands for various metal centres, and as organocatalysts able to mediate an exciting range of reactions. However, the sheer number of NHCs known in the literature can make the appropriate choice of NHC for a given application difficult. A number of metrics have been explored that allow the electronic properties of NHCs to be quantified and compared. In this review, we discuss these various metrics and what they can teach about the electronic properties of NHCs. Data for approximately three hundred NHCs are presented, obtained from a detailed survey of the literature.

Redox-Active N-Heterocyclic Carbenes: Design, Synthesis, and Evaluation of Their Electronic Properties

To investigate effects of redox-active functional groups on the coordination chemistry and electronic properties of N-heterocyclic carbenes (NHCs), we prepared a series of complexes comprising 1,3-diferrocenylimidazolylidene and -benzimidazolylidene (1 and 2, respectively), 1-ferrocenyl-3-methyl- and 1,3-diphenyl-5-ferrocenylbenzimidazolylidene (3 and 4, respectively), N,N′-diisobutyldiaminocarbene[3]ferrocenophane (FcDAC), and 1,3-dimesitylnaphthoquinoimidazolylidene (NqMes) ligands and coordinated [Ir(COD)Cl] (COD = 1,5-cyclooctadiene), [Ir(CO)2Cl], and [M(CO)5] (M = Cr, Mo, W) units. The coordination chemistry of the aforementioned NHCs was investigated by X-ray crystallography, and their electronic properties were studied by NMR and IR spectroscopy, as well as electrochemistry. No significant variation in νCO was observed among metal carbonyl complexes supported by 2−4 and FcDAC, indicating that the number (one vs two) of redox-active groups, the location (N atom vs backbone) of the redox-active group, and carbene ring identities (strained six-membered, nonaromatic vs five-membered, heteroaromatic) did not have a significant effect on ligand electron-donating ability. Because the shifts in νCO upon oxidation of 1−3 and FcDAC were similar in magnitude but opposite in sign to NqMes, we conclude that the enhancement or attenuation of ligand donating is primarily Coulombic in origin (i.e., due to the molecule acquiring a positive or negative charge).

N-Heterocyclic carbenes: deducing σ- and π-contributions in Rh-catalyzed hydroboration and Pd-catalyzed coupling reactions

The effect of tuning the electronic properties of N-heterocyclic carbene (NHC) ligands was evaluated in multiple, mechanistically distinct, metal-mediated reactions. Hydroboration and Heck reactions, catalyzed by Rh–NHC and Pd–NHC complexes, respectively, were found to result in yields that were up to ten times lower when π-withdrawing substituents were incorporated into the NHC backbone relative to analogues bearing σ-withdrawing groups.

Steric and Electronic Properties of N-Heterocyclic Carbenes (NHC): A Detailed Study on Their Interaction with Ni(CO)4

N-heterocyclic carbene ligands IMes (1), SIMes (2), IPr (3), SIPr (4), and ICy (5) react with Ni(CO)(4) to give the saturated tricarbonyl complexes Ni(CO)(3)(IMes) (8), Ni(CO)(3)(SIMes) (9), Ni(CO)(3)(IPr) (10), Ni(CO)(3)(SIPr) (11), and Ni(CO)(3)(ICy) (12), respectively. The electronic properties of these complexes have been compared to their phosphine analogues of general formula Ni(CO)(3)(PR(3)) by recording their nu(CO) stretching frequencies. While all of these NHCs are better donors than tertiary phosphines, the differences in donor properties between ligands 1-5 are surprisingly small. Novel, unsaturated Ni(CO)(2)(IAd) (13) and Ni(CO)(2)(I(t)()Bu) (14) compounds are obtained from the reaction of Ni(CO)(4) with IAd (6) and I(t)()Bu (7). Complexes 13 and 14 are highly active toward substitution of the NHC as well as the carbonyl ligands. This has allowed the determination of Ni-C(NHC) bond dissociation energies and the synthesis of various unsaturated Ni(0) and Ni(II) complexes. Computational studies on compounds 8-14 are in line with the experimental findings and show that IAd (6) and I(t)()Bu (7) are more bulky than IMes (1), SIMes (2), IPr (3), SIPr (4), and ICy (5). Furthermore, a method based on %V(bur) values has been developed for the direct comparison of steric requirements of NHCs and tertiary phosphines. Complexes 8-14, as well as NiCl(C(3)H(5))(I(t)()Bu) (16) and NiBr(C(3)H(5))(I(t)()Bu) (17), have been characterized by X-ray crystallography.