Enhanced π-back-donation resulting in the trans labilization of a pyridine ligand in an N-heterocyclic carbene (NHC) PdII precatalyst: a case study

The molecular structure of the benzimidazol-2-ylidene–PdCl2–pyridine-type PEPPSI (pyridine-enhanced precatalyst, preparation, stabilization and initiation) complex {1,3-bis[2-(diisopropylamino)ethyl]benzimidazol-2-ylidene-κC 2}dichlorido(pyridine-κN)palladium(II), [PdCl2(C5H5N)(C23H40N4)], has been characterized by elemental analysis, IR and NMR spectroscopy, and natural bond orbital (NBO) and charge decomposition analysis (CDA). Cambridge Structural Database (CSD) searches were used to understand the structural characteristics of the PEPPSI complexes in comparison with the usual N-heterocyclic carbene (NHC) complexes. The presence of weak C—H...Cl-type hydrogen-bond and π–π stacking interactions between benzene rings were verified using NCI plots and Hirshfeld surface analysis. The preferred method in the CDA of PEPPSI complexes is to separate their geometries into only two fragments, i.e. the bulky NHC ligand and the remaining fragment. In this study, the geometry of the PEPPSI complex is separated into five fragments, namely benzimidazol-2-ylidene (Bimy), two chlorides, pyridine (Py) and the PdII ion. Thus, the individual roles of the Pd atom and the Py ligand in the donation and back-donation mechanisms have been clearly revealed. The NHC ligand in the PEPPSI complex in this study acts as a strong σ-donor with a considerable amount of π-back-donation from Pd to Ccarbene. The electron-poor character of PdII is supported by π-back-donation from the Pd centre and the weakness of the Pd—N(Py) bond. According to CSD searches, Bimy ligands in PEPPSI complexes have a stronger σ-donating ability than imidazol-2-ylidene ligands in PEPPSI complexes.

A theoretical study of some hydrogen-bonded complexes using the theory of atoms in molecules

π-Type hydrogen-bonded complexes consisting of hydrogen halide HX (X = Cl, F) and the carbon–carbon triple or double bond of vinyl acetylene (1-buten-3-yne, HC≡C-CH=CH2) have been studied. The vinyl acetylene molecule contains two possible π-bonding sites (C≡C and C=C). It offers three possible structures of [Formula: see text] that comprise two T-type bonds to C≡C (endo and exo approaches) and one T-type bond to C=C (perpendicular approach). The optimized geometries and the hydrogen-bond stabilization energies, based on MP2(FULL)/6-311 ++ G(d,p)//6-31G(d,p) calculations, indicate that the π-type hydrogen bond to a C≡C triple bond leads to a more stable complex than for an analogous bond to C=C. The calculated global minima for the complexes with HF and HCl correspond to the H—X moiety lying along a bisector of the C≡C triple bond in the endo approach, predictions that are in good agreement with the reported FTMS results. The topological properties of the electron density distributions of these two systems have been analyzed in terms of the theory of atoms in molecules. The nature of π-type hydrogen bonds has also been discussed using the Laplacian of the electron density, [Formula: see text] The complexes [Formula: see text] and [Formula: see text] as well as the hydrogen-bonded complex consisting of 2-butyne (CH3-C≡C-CH3) and HCl were also studied for comparison. Key words: ab initio calculation, hydrogen bonding, topological analysis of electron density, vinyl acetylene, 2-butyne.