Probing Cyclic π‐Electron Delocalization in an Imidazol‐2‐ylidene and a Corresponding Imidazolium Salt

Abstract

The extent of cyclic π-electron delocalization in the N-heterocyclic ring of an imidazol-2-ylidene (i.e., 1,3,4,5-tetramethylimidazol-2-ylidene) and its corresponding imidazolium salt (i.e., 1,3,4,5-tetramethylimidazolium chloride) has been investigated theoretically by using Bader's quantum theory of atoms in molecules (QTAIM) descriptors, delocalization indices, electron localizability indicators (ELI-Ds), and the source function tool. In addition, the experimental electron density distribution for the imidazolium salt has been obtained and analyzed from 100 K X-ray diffraction data. A significant drop is found in the ellipticity of the electron density along the C -N bond path in the imidazol-2-ylidene. This is shown to be a natural consequence of the σ lone pair of the C atom, which overwhelms the π-electron density, rather than a sign of a significantly diminished degree of π-electron delocalization in the imidazol-2-ylidene compared to its imidazolium salt. In fact, the source functions, the ELI-Ds, and the delocalization indices all probe a quite similar extent of cyclic π-electron delocalization in the N-heterocyclic rings of the two compounds.

Group 6 Carbonyl Complexes of C-Ylidic Cp Ligands

“…In comparison to the free ligand 1 (C1–C6 1.430(3) Å) the exocyclic imidazolium (imi) Cp bonds in the complexes ( 4 , C1–C6 1.453(2) Å; 5 , C1–C6 1.454(2) Å; 6 , C1–C6 1.453(5) Å) are about 0.023 Å longer. This is due to a reduced Coulombic attraction between the rings after coordination to the metal fragment and in addition by a slightly reduced s character in the C imi –Cp bond, which can be derived from the increasing angle of the imidazolium moiety toward that of tetramethylimidazolium chloride (TMI-Cl) (N1–C6–N2 ( 1 ) 105.2(2)° vs 4 106.9(1)°, 5 106.7(2)°, 6 106.6(3)°, TMI-Cl 108.4° , ). The C–O bond lengths of complexes 4 – 6 (1.168–1.173 Å) are about 0.04 Å longer than in free CO (1.128 Å) due to π back-donation and polarization with the metal fragment .…”

“…19 F{ 1 H} NMR (376 MHz, CD 2 Cl 2 ): δ −73.9 (d, 1 J PF = 711 Hz, PF 6 ). 31 Synthesis of Tricarbonyl(1,3-bis(1,3,4,5-tetramethylimidazolium)-η 5 -cyclopentadienyl)molybdenum(0) Hexafluorophosphate ([(2)Mo(CO) 3 ](PF 6 ); 8). Ligand 2 (50.0 mg, 110 μmol, 1.0 equiv) and [Mo(CO) 3 (MeCN) 3 ] (36.5 mg, 121 μmol, 1.1 equiv) were dissolved in 2 mL of dichloromethane.…”

“…19 F{ 1 H} NMR (376 MHz, CD 3 CN): δ −72.9 (d, 1 J PF = 707 Hz, PF 6 ). 31 7.56 (br s, 4H, 4-H), 3.51 (q, 3 J HH = 7.0 Hz, 4H, Et 2 O), 1.18 (t, 3 J HH = 7.0 Hz, 6H, Et 2 O). 13 C{ 1 H} NMR (176 MHz, CD 2 Cl 2 ): δ 162.3 (q + sept, 1 J 11BC = 50.0 Hz, 1 J 10BC = 16.6 Hz, C1), 135.4 (br s, C2/6), 129.4 (qq, 2 J CF = 31.5 Hz, 4 J CF = 2.7 Hz, C3/5), 125.1 (q, 1 J CF = 272.5 Hz, CF 3 ), 118.1 (sept, 3 J CF = 3.9 Hz, C4), 66.7 (CH 2 ), 15.3 (CH 3 ).…”

“…NCH 3 ), 9.5 (CCH 3 ).19 F{1 H} NMR (376 MHz, CD 2 Cl 2 ): δ −73.9 (d,1 J PF = 711 Hz, PF 6 ) 31. P{ 1 H} NMR (162 MHz, CD 2 Cl 2 ): δ −144.8 (spt, 1 J PF = 711 Hz, PF 6 ).…”

Revealing electronic features governing hydrolysis of cephalosporins in the active site of the L1 metallo-β-lactamase

“…Analysis of electron density 5 is able to locate specic inter-and intramolecular interactions in the chemical systems while various bonding descriptors allow one to analyze and quantify the features of particular chemical interactions. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] Recently we have demonstrated [24][25][26] that there are certain key interactions among a great number of all interatomic interactions in the active site of enzyme that mainly determine the observed macroscopic property, the steady-state rate constant. We pointed out that these key interactions can be dened at the stationary points of the reaction path on the potential energy surface (PES) from quantum mechanics/molecular mechanic (QM/MM) approach 27,28 combined with subsequent electrondensity analysis.…”