A new π-acidity scale for several N-donor heterocycles as ligands in neutral gold(III) complexes
Graphical abstract
A computational DFT approach for the comparison of the π-acceptor character of some N-donor heterocycles L {L = pyridines, pyrimidines, imidazoles, pyrazoles and isoxazoles} in neutral AuCl3L complexes is reported. The π-acceptor ability scale pyridines ≈ pyrimidines < imidazoles < pyrazoles ≈ isoxazoles has been derived. Moreover, the minimum proton affinity value for a meaningful interaction between the ligands and the AuCl3 fragment has been estimated.
Introduction
A great number of monodentate N-donor heterocyclic d8 transition metal complexes have been extensively studied in past years [1], amongst other things, to determine the relative strength of the M–N bond on varying the nature of both the metal centre and the ancillary ligands. In many cases the relative reactivity of isosteric nitrogen ligands in substitution reactions can be correlated to their conjugate acids pKa values [2], [3], but it is still a demanding objective to discover a simple parameter to describe the relative π-acidity of different coordinated heterocycles. It is to be highlighted that a number of kinetic measurements on the reactivity of ligands such as pyridines, thiazoles, oxazoles and imidazoles in Pt(II) and Au(III) complexes has been published [3], and that the reported results strongly support the idea that the ground-state π-interactions of five-membered N-donors with these metal centres are stronger than those of pyridines of the same basicity.
An improved knowledge about the π-acceptor ability of different classes of nitrogen heterocycles could be of interest for the synthesis of new coordination compounds with well-tuned electronic properties and for their application in homogeneous catalysis. For example, Hashmi et al. recently reported the use of pyridine-based Au(III) complexes as efficient catalysts in organic syntheses [4].
From a theoretical point of view, the development of relativistic effective-core potentials [5], [6] and of efficient DFT exchange-correlation functionals [6], [7] caused, in recent years, an explosive diffusion of computational inorganic chemistry as a powerful tool to give insight into both new and old aspects of coordination chemistry of transition elements [8]. The aim of our recent researches was the development of a new ab initio approach for the comparison of the π-acceptor character of different N-donor heterocycles in d8 complexes and in the present work we propose a π-acidity scale for several groups of N-donor heterocycles L {L = pyridines (py), pyrimidines (pm), imidazoles (im), pyrazoles (pz) and isoxazoles (io)} coordinated to the AuCl3 fragment. The computational results agree with the greater π-acceptor ability of the five-membered N-donor heterocycles, already observed by kinetic measurements. Finally, it has also been possible to estimate the minimum proton affinity value for a meaningful interaction between AuCl3 and the considered N-donor heterocycles.
Section snippets
Proton affinities
In Fig. 1 the considered ligands, i.e. pyridines (py), pyrimidines (pm), imidazoles (im), pyrazoles (pz) and isoxazoles (io), are depicted. The electronic properties have been tuned by adding various methyl and/or trifluoromethyl groups to cover a range of proton affinities as wide as possible. The common numbering scheme for these heterocycles has been used to indicate the substituents positions. The choice of groups having only inductive electronic effects, such as –CH3 and –CF3, is justified
Conclusions
On the basis of the linear relationships between the AuCl3 Mulliken charges in AuCl3L complexes and the proton affinities of several groups of N-donor heterocycles L, it has been possible to build a new theoretical π-acidity scale for pyridines, pyrimidines, imidazoles, pyrazoles and isoxazoles referred to a specific metal fragment. A different π-back-bonding ability between five- and six-membered ligands has been highlighted, in perfect agreement with already reported experimental evidences.
Computational details
The ground-state computational geometry optimisations of the AuCl3L complexes {L = pyridines (py), pyrimidines (pm), imidazoles (im), pyrazoles (pz) and isoxazoles (io)} were carried out using the hybrid DFT B3PW91 method [12] without symmetry constrains, in combination with the Dunning’s split-valence double-ζ D95V basis set on the light atoms [13] and the ECP-based SDD basis set on Au and Cl [14]. The geometry optimisations of the free ligands L and of their conjugate acids [LH]+ were carried
Acknowledgements
Financial support of Ca’ Foscari University of Venice (Ateneo Fund 2007) is gratefully acknowledged. We sincerely thank CINECA (Centro Italiano di Supercalcolo, Bologna, Italy) for technical support.
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