Tuning the Electronic and Ligand Properties of Remote Carbenes: A Theoretical Study

Abstract

The effect of annulation and carbonylation on the electronic and ligating properties of remote N-heterocyclic carbenes (rNHCs) has been studied quantum-chemically. The thermodynamic stability of these complexes has been assessed on the basis of their hydrogenation and stabilization energies, while HOMO-LUMO gaps are used to measure the kinetic stabilities. Annulated/carbonylated rNHCs are found to be weaker σ donors but better π acceptors compared with the parent rNHCs. The reactivity of these rNHCs has been studied by evaluating their nucleophilicity and electrophilicity indices. The nucleophilicity values are in good agreement with the σ basicities of all of the rNHCs. The (31)P NMR chemical shifts of the corresponding rNHC-phosphinidene adducts have been calculated and found to correlate well with the π acidities of these rNHCs.

Computational Ligand Descriptors for Catalyst Design

“…Phukan and collaborators, [155][156][157][178][179][180][181][182][183][184] have again used a range of descriptors to assess carbenes; while the exact selection varied with application and no single coherent database has been presented, capturing carbene properties across a wide range of structural modifications, most of these studies have included calculated singlet-triplet gaps as a measure of the thermodynamic stability of carbenes, along with HOMO-LUMO gaps as a measure of the kinetic stability (PBE1PBE/6-31G*, SDD on metals). Studies have included comparison of carbenes with silylenes, germylenes and abnormal carbenes, 178,182 analysis of ring size and heterocycles on carbene properties, 179 the introduction of boron substituents to carbene (NHC and PHC) backbones, 156 the effect of additional rings and carbonyl substituents on normal and abnormal NHCs, 157 analysis of remote carbenes, 181,183 consideration of adduct formation for normal and abnormal NHCs, 180 and small molecule activation by cyclic (alkyl)(amino) carbenes (CAACs). 184 Overlap with our core set of carbenes (Table 1c) is quite poor as these studies have been focussed on novel/unusual structures, so data have not been compiled in this case.…”

Formation of N‐Heterocyclic Carbenes by Tautomerization of Mesomeric Betaines: Cyclic Boron Adducts and Palladium Complexes From 2‐(Imidazolium‐1‐yl)phenolates

“…Compound 20 a:Y ellow crystals;C 30 H 29 Br 0.12 Cl 1.88 N 2 PPd; M r = 647.38;c rystal size 0.22 0.12 0.08 mm;m onoclinic;s pace group P2 1 /n (No. 14); a = 11.4229 (7), b = 17.2889(10), c = 14.0836(9) ; b = 99.959(2)8; V = 2739.5(3) 3 ; Z = 4; 1 calcd = 1.570 mg m À3 ; F(000) = 1313; m = 1.128 mm À1 ;4 9732 reflections (2V max = 55.28); 6315 unique reflections [R int = 0.030];3 41 parameters;1restraint; R1( for 5586 I > 2s(I)) = 0.022; wR2 (all data) = 0.058; S = 1.06;l argest diff. peak and hole 0.613 and À0.775 e À3 .T here is ad isorder of the halogen atom located trans to the P1 atom:C l/Br ratio = 88:12 (see the cif file in the Supporting Information for details).…”

“…[1] Structural variations of the substitution pattern,aswell as variation of the heterocyclic ring system,t uning of the electronic properties by the substituents, and investigation of steric factors yielded ab road variety of distinct classes of NHCs with interesting properties. Meanwhile, classical or "normal" NHCs (nNHCs), remote NHCs (rNHCs), [2][3][4][5][6][7] and abnormal NHCs (aNHCs) [8][9][10][11][12][13][14] which are also sometimes called mesoionic carbenes (MIC) [15][16] -are distinguished. Ar eview of complexes of the distinct types of NHCs has appeared recently.…”

CO2 Sequestration by Triazolylidene-Derived N-Heterocyclic Olefins: A Computational Study

“…In fact, these trNHOs were found to be promising CO 2 sequestrating agents, as good as the original NHOs. Motivated by the influence that structural changes and substitutions at N‐ and C‐ positions of the imidazolium and triazolium rings exert on the reactivity and stability of the corresponding NHCs, NHOs and trNHOs, in this work, we propose further modifications in the trNHO class.…”