The ability of N-heterocyclic carbenes (NHCs) to participate in π-back-bonding interactions was evaluated in a range of transition metal complexes. Rh chloride complexes containing a systematic series of various 1,3-dimethyl-4,5-disubstituted-imidazol-2-ylidenes and either 1,5-cyclooctadiene (cod) or two carbon monoxide ligands were synthesized (i.e., (NHC)RhCl(cod) and (NHC)RhCl(CO)2, respectively) and studied using 1H NMR and IR spectroscopies. In the former series, the 1H NMR chemical shifts of the signals attributable to the olefin trans to the NHC ligand were found to shift downfield by up to 0.17 ppm as the π-acidity of the substituents on the 4,5-positions increased (i.e., H → Cl → CN). Similarly, in the latter series, the IR stretching frequencies of the carbonyl groups trans to the NHC ligands were found to increase by 11 ± 0.5 cm-1 as π-acidity increased over the same series. Using the nitrile group as a diagnostic handle, the CN stretching frequency of (1,3-dimethyl-4,5-dicyanoimidazol-2-ylidene)(cod)RhCl was found to be 4 ± 0.5 cm-1 higher than 1,3-dimethyl-4,5-dicyanoimidazol-2-ylidene)(CO)2RhCl, a more π-acidic analogue. X-ray analysis of the aforementioned series of (NHC)(cod)RhCl complexes indicated changes in N−Ccarbene bond lengths that were consistent with greater π-donation from complexes containing 4,5-dihydroimidazol-2-ylidene relative to the their 4,5-dicyano analogues. Collectively, these results suggest not only that imidazol-2-ylidenes are capable of π-back-bonding but that this interaction may be tuned by changing the π-acidity of the substituents on the imidazole ring.

Deuces wild: From catalysis through materials science to biology, N‐heterocyclic carbenes (NHCs) can truly be regarded as a wild card of ligands. A new series of “two‐faced” (Janus‐type) ligands composed of two linearly opposed NHCs fused to an arene linker has been synthesized and can be used to prepare discrete bimetallic complexes.

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).

A new class of 1,3-disubstituted-triazenes were synthesized by coupling functionalized benzimidazol-2-ylidenes, as their free N-heterocyclic carbenes or generated in situ from their respective benzimidazolium precursors, to various aryl azides in modest to excellent isolated yields (36-99%). Electron delocalization between the two coupled components was studied using UV-vis spectroscopy, NMR spectroscopy, and X-ray crystallography. Depending on the complementarity of the functional groups on the N-heterocyclic carbenes and the organic azides, the respective triazenes were found to exhibit lambda(max) values ranging between 364 and 450 nm. X-ray crystallography revealed bond alteration patterns in a series of triazenes characteristic of donor-acceptor compounds. Triazene thermal stabilities were studied using thermogravimetric analysis and found to be strongly dependent on the sterics of the benzimidazol-2-ylidene component and the electronics of the azide component. Triazenes possessing bulky N-substituents (e.g., neo-pentyl, tert-butyl, etc.) were stable in the solid-state to temperatures exceeding 150 degrees C, whereas analogues with small N-substituents (e.g., methyl) were found to slowly decompose at room temperature. Triazenes featuring electron-rich phenyl azide components decomposed at higher temperatures than their electron-deficient analogues. Products of the thermally induced triazene decomposition reaction were identified as molecular nitrogen and the respective guanidine. Using an isotopically labeled triazene, the mechanism of the decomposition reaction was found to be analogous to the Staudinger reaction.

Jones'n for electrons: Various Rh complexes (see general structure) were formed with diaminocarbenes that contain 1,1′‐disubstituted ferrocenes in their backbones. Electronic communication between the Fe centers in the redox‐active carbenes and the coordinated transition metals was observed. For example, the oxidation potential of the Fe center in a bimetallic complex was found to depend on the electron density of the ligated Rh atom.