DFT study on the reactivity of mono-substituted pyridine ligands

The global and local quantum chemical reactivity descriptors of 34 pyridine ligands are calculated using the DFT/B3LYP/6-311++G** method. These reactivity descriptors are used to describe the HSAB interaction of the studied pyridine ligands as N-nucleophile with the hard H+, the soft Cu+ and the border line Cu2+ Lewis acids as electrophile. The calculations predicted higher electron density on the ring N-atom and higher EHOMO values in case of electron releasing substituents compared to electron attracting groups which indicate harder N-nucleophiles in case of the former compared to the latter. It is found that, the global chemical hardness (η) does not offer acceptable prediction for the ligand hardness. On the other hand, good correlations are obtained between the calculated nucleophilicity index (N) with the Mayr’s experimental nucleophilicity values and the inverse of electrophilicity index (1/ω). The calculated Fukui function f k - values at the N-site are small in case of electron releasing substituents indicating a preferred N-site for hard reaction. In contrast, large f k - values in case of electron attracting groups indicate a preferred N-site for soft reaction. The Cu(I) affinities of the studied ligands decrease with increase their experimental ionization potentials in agreement with the HSAB principle. Similarly, the proton affinities decrease with increase the ligand ionization potentials, a contradiction to the HSAB principle as H+ is a hard acid and Cu+ is a soft acid. These results are explained in terms of electrostatic interaction which is the dominant processes in gas phase ion–molecule reactions.