Synthesis, structure and electronic properties of N-dialkylamino- and N-alkoxy-1,2,4-triazol-3-ylidene ligands

1,2,4‐Triazol‐3‐ylidenes with an N‐2,4‐Dinitrophenyl Substituent as Strongly π‐Accepting N‐Heterocyclic Carbenes

“…Since the TEP value of 3 a (2064 cm −1 ) was larger than that of C (2058 cm −1 ), it appears that the incorporation of the N atom into the NHC framework reduced the σ‐donating ability of the NHC. Notably, the TEP values of 3 c and 3 b (2069 and 2068 cm −1 , respectively) are the highest reported so far, even higher than those of A 2e,f and B 2m (Figure 1). In this work, as the TEP value increased, we observed a downfield shift in the 13 C NMR signal of the carbenic carbon atom in the Rh I dicarbonyl complexes (Table 1).…”

Synthesis of monodentate bis(N-heterocyclic carbene) complexes of iridium: Mixed complexes of abnormal NHCs, normal NHCs, and triazole NHCs

“…One exception is the limited number of Rh and Ir NHC complexes with two dissimilar NHC ligands bound to the metal, that is, complexes in which the two NHC ligands differ in their steric or electronic properties. While many examples of chelating [5] and monodentate bis(NHC) or tris(NHC) [6][7][8][9][10][11][12][13][14][15][16][17][18] complexes of Rh and Ir are known, to the best of our knowledge only a few have been reported in which electronically or sterically distinct types of NHC ligands are present [17,19]. Given the importance of both steric and electronic dissymmetry in many catalytic reactions [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36], we sought a facile method to reliably synthesize monodentate bis(NHC) complexes in which the two NHC ligands are sterically and/or electronically distinct.…”

“…This procedure is based on the common method of Ag-NHC transmetallation [41][42][43][44], which is widely used in the synthesis of Ir and Rh NHC complexes [45], and has been previously employed by several groups to synthesize monodentate bis(NHC) complexes with identical NHC ligands [12,[14][15][16]46]. We demonstrate that the introduction of a second type of NHC ligand, in the form of a Ag-NHC complex, reliably yields monodentate bis(NHC) complexes with widely varying steric and electronic characteristics.…”

First N‐Heterocyclic Carbenes Relying on the Triazolone Structural Motif: Syntheses, Modifications and Reactivity

“…However,t riazolylidene 2 is significantly less p-acidic than the five-membered ring monoamido carbene 1,3-di(mesityl)imidiazolidin-4one-2-ylidene (E)w ith d( 77 Se) = 295 ppm. [22] In this compound, the carbonyl group withdraws electron density only from the Na tom adjacentt ot he carbene-C atom, whilei nt he triazole [8]: S1ÀC2 1.6578(18), O1ÀC1 1.214(2), N2ÀC1 1.361(2), N1ÀC2 1.326(2), N2ÀN1 1.392(2), N3ÀC2 1.382(2), N3ÀC1 1.382(2);N 1-C2-N3 104.97 (14), N2-C1-N3104.18 (14). (17), N1ÀC2 1.325(2), N3ÀC2 1.371(2);N1-C2-N31 04.6(2).…”

“…[4] Biscarbenes relyingo nt he 1,2,4-triazole core have been reported by the groups of Bertrand [5] and Peris. [6] 4-Amino-substituted triazolylidenes B were investigated by Schottenberger and co-workers [7] and Lassaletta, [8] while Siemeling and co-workers [9] demonstrated that Nitron, ac ommercially availablet riazole-based zwitterionicc ompound bearinga na mido group at C3, providesa ccess to carbene C by at automeric protons hift. Likewise, the carbon analogue of Nitron, D',r eported by CØsar,L avignee ta l., [10] also represents a" crypto-NHC", as it features the reactivityo fi ts carbenet automer.T riazolylidenes are particularly successful as organocatalysts, [11] while when coordinated as ligands to metal fragments they are less donating than structurally related imidazolylidenes.…”