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

Abstract

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 0 -diisobutyldiaminocarbene[3]ferrocenophane (FcDAC), and 1,3-dimesitylnaphthoquinoimidazolylidene (NqMes) ligands and coordinated [Ir(COD)Cl] (COD = 1,5-cyclooctadiene), [Ir(CO) 2 Cl], 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 ν CO was observed among metal carbonyl complexes supported by 2-4 and FcDAC, indicating that the number (one vs two) of redoxactive 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 ν CO 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).

Synthesis and study of olefin metathesis catalysts supported by redox-switchable diaminocarbene[3]ferrocenophanes

“…S5 †). These values were comparable to those previously reported for 14 (E 1/2 = 0.76 and 1.02 V), 20 although the Ir centered oxidation measured for 13 occurred at a potential higher than those recorded for other Ir(COD)Cl complexes supported by NHCs (E 1/2 = 0.65-0.97 V). 20,27c,52 Upon ligand exchange of the cyclooctadiene ligand for two π-acidic CO ligands (i.e., 15), a significant anodic shift was observed in the redox couple attributed to the Fe center (E 1/2 = 0.96 V, ΔE 1/2 = 150 mV) and the Ir oxidation process was no longer observed within the solvent window (see Fig.…”

“…S6 †), consistent with our prior report for the oxidation of 14 versus 16 (ΔE 1/2 = 180 mV) and related complexes. 20,22,52,53 The 150 mV shift observed upon ligand exchange (i.e., 13 → 15) suggested to us that the electronic communication b Values are reported relative to SCE through the addition of Fc* as an internal standard adjusted to −0.057 V. 73 c The Ir-centered oxidation was not observed within the solvent window. between the Ir and Fe centers was significant and that the decrease of electron density on the Ir center was due to the π-acidic CO ligands, which consequently raised the oxidation potential of the ferrocene moiety.…”

“…20 To quantify the ligand donating abilities of 5 and 5 + , the aforementioned ν CO s were converted to their corresponding the tentatively attributed to the formation of the desired FcDAC indenylidene complex 17 (Scheme 3). Signals assigned to a second product were also observed at δ 10.47 (d, J = 7.6, α-CH indenylidene ) and 47.07 ppm.…”

“…18 In light of these advantages, we have launched a program to explore redox-active NHCs as a general class of ligands for bestowing redox-switchable functions onto a broad range of transition metals. [19][20][21][22][23][24] For example, we recently disclosed a series of redox-switchable Ru-based olefin metathesis catalysts bearing N-ferrocenylated NHCs (Fig. 1).…”

“…As mentioned above, complexes of the type (L)M(COD)Cl and (L)M(CO) 2 Cl (L = NHC or phosphine, M = Rh, Ir) have proven to be useful for such purposes. 20,27 Furthermore, analogous complexes containing 1 have been previously reported, enabling subtle differences in the steric and electronic influences of the N-substituents to be deconvoluted. 19,20 As shown in Scheme 2, in situ deprotonation of 12 using NaHMDS followed by the addition of [Ir(COD)Cl] 2 afforded 13 in 55% yield after The solid state structures of 13 and 15 were elucidated after growing X-ray quality crystals via slow evaporation of concentrated CH 2 Cl 2 solutions ( Collectively, the structural similarities found in the Ir complexes supported by 1 or 5 suggested to us that the N-substituents bestowed similar steric influences on the coordinated metal centers.…”