Effect of substituents at the heteroatom on the structure and ligating properties of heterocyclic carbene, silylene, germylene and abnormal carbene: A theoretical study

Abstract

The effect of substituents at the heteroatom on the electronic structures of different N-heterocyclic carbenes (1, 2 and 3), silylene (4) and germylene (5) are examined using Density Functional Theory. The kinetic and thermodynamic stabilities of these molecules are assessed by examining the HOMO-LUMO gap and hydrogenation energies, respectively. The extent of cyclic electron delocalization present in these five-membered ring systems are quantified with the help of NICS calculations. The ligating properties of 1-5 and the recently synthesized free abnormal carbene 6 (Bertrand et al., Science, 2009, 326, 556-559) are examined by looking at the energies of the sigma symmetric electron-donating orbital of the respective molecules. Among the systems considered, 6 is found to have the strongest sigma-donating ability. A comparative study of the ligating properties between the two isomeric carbenes 1 and 6 is performed by calculating the carbonyl stretching frequencies of some iridium carbonyl complexes of these two tautomeric carbenes.

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.…”

Theoretical Prediction of a Neutral Zero‐Valent Beryllium Compound Isoelectronic with Singlet Carbenes

“…[14] Thus, the proposed compound is predicted to be one of the strongest neutral Lewis base and its basicity is even higher than highly basic N-heterocyclic carbenes. [12,35,42] The protonated derivatives are shown in Figure 5. The protonated derivatives are all planar.…”

“…[38] The reactivity of compound 1 with proton, Lewis acidic BH 3 and BeCl 2 is explored. The calculated eigen value of the lone pair orbital and gas phase proton affinity indicate that compound 1 should be considered as "super basic" and its basicity exceeds that of highly basic carbenes [12,35,42,44] and other Be(0) compounds. [16] The complexation of 1 with BeCl 2 molecule is highly favourable as suggested by the high complexation energy (71.5 kcal/mol).…”

N-heterocyclic carbone: computational prediction of a divalent carbon (0) compound

“…In summary, quantum chemical calculations provide a hint toward possible isolation of new five-membered neutral divalent carbon (0) compound within the familiar N-heterocyclic framework. The proposed compound contains two other NHC units whose electronic character can be modified in many ways, 51 and thus, the proposed compound may serve as an important perspective for transition metal catalysis. 44…”