A Computational and Structural Database Study of the Metal-Carbene Bond in Groups IA, IIA, and IIIA Imidazol-2-Ylidene Complexes

Samuel Tetteh

doi:10.1155/2019/5675870

Abstract

Imidazol-2-ylidenes are important N-heterocyclic carbenes which have become universal ligands in organometallic and coordination chemistry. Generally classified as σ-donor ligands, these compounds have been used to stabilize various metal complexes which hitherto were less stable in their catalytic processes. Herein, the number and distribution of group IA, group IIA, and group IIIA metal-imidazol-2-ylidene complexes retrieved from the Cambridge Structural Database (CSD) are assessed. The data showed that the mean M-Ccarbene bond length increases with increasing ionic size but is similar across each diagonal. Dominant factors such as Lewis acidity and electrostatic attractions were found to control the bonding modes of the respective ions. Generally, the metal ions show preference for tetrahedral coordination with larger cations forming complexes with higher coordination numbers. For their high number of entries (101), tetrahedrally coordinated boron complexes with various electron withdrawing and electron donating groups were studied computationally at the DFT/B3LYP level of theory. The strength of the B-Ccarbene bond was found to depend on steric interactions between bulky groups on the borenium atom and substituents on the N-positions of the imidazol-2-ylidene ligand. This observation was further confirmed by estimation of the binding energy, natural charge, and the electron distribution in the B-Ccarbene bond.

Highlights

N-heterocyclic carbenes (NHCs) are important ligands used in the synthesis and study of catalytic systems [1,2,3]
Ylidene complexes. e geometry around each metal ion was defined by restricting the number of bonded atoms in each search. e search was further refined by imposing the following secondary search criteria: 3D coordinated determined; crystallographic R factor ≤0.075; no disorder in the crystal structure; no errors; no polymeric structures; and no powder structures and only organometallic structures. e number of hits and the respective coordination numbers are shown in Table 1. e reference codes of geometries with only one hit are given in parenthesis. e Cambridge Structural Database (CSD) program Mercury [21] was used for the structure visualization while the associated Data Analysis Module was used for the statistical analyses of the retrieved data
All calculations were performed with the Gaussian 09 package [22]. e input and output files were visualized using the GaussView 5.0.8 [23] molecular viewer. e gas phase structures of the boron complexes were fully optimized at the density functional theory (DFT) using Becke’s three parameter hybrid method and the Lee–Yang– Parr correlation functional (B3LYP) [24] with the 6–311++G∗∗ basis set to include polarization and diffuse functions on all atoms. ese were done without symmetry constraints

Summary

Samuel Tetteh

E data showed that the mean M-Ccarbene bond length increases with increasing ionic size but is similar across each diagonal. Dominant factors such as Lewis acidity and electrostatic attractions were found to control the bonding modes of the respective ions. The metal ions show preference for tetrahedral coordination with larger cations forming complexes with higher coordination numbers. For their high number of entries (101), tetrahedrally coordinated boron complexes with various electron withdrawing and electron donating groups were studied computationally at the DFT/B3LYP level of theory.

Introduction

Methods

No of hits

Number of hits

St dev

Bond angle