A computational study on Π and σ modes of metal ion binding to heteroaromatics (CH)_5-mX_m and (CH)_6-mX_m (X = N and P): contrasting preferences between nitrogen- and phosphorous-substituted rings

Abstract

The π and σ complexation energy of various heteroaromatic systems which include mono-, di-, and trisubstituted azoles, phospholes, azines and phosphinines with various metal ions, viz. Li⁺, Na⁺, K⁺, Mg²⁺, and Ca²⁺, was calculated at the post Hartree−Fock MP2 level, MP2(FULL)/6-311+G(2d,2p)//MP2/6-31G*. The azoles and azines were found to form stronger σ complexes than the corresponding π complexes, whereas the phospholes and phosphinines had higher π complexation energy with Li⁺, Mg²⁺, and Ca²⁺ while their π and σ complexation energies were very comparable with Na+ and K+. The strongest π complex among the five-membered heteroaromatic system was that of pyrrole with all the metals except with Mg²⁺, while benzene formed the strongest π complex among the six-membered heterocyclic systems. The nitrogen heterocyclic system 4H-[1,2,4] triazole and pyridazine formed the strongest σ complex among the five- and six-membered heteroaromatic systems considered. The complexation energy of the π and σ complexes of the azoles and azines was found to decrease with the increase in the heteroatom substitution in the ring, while that of phospholes and phosphinines did not vary significantly. The azoles and azines preferred to form σ complexes wherein the metal had bidentate linkage, while the phospholes and phosphinines did not show binding mode preference. In the σ complexes of both azoles and phospholes, the metal binds away form the electron-deficient nitrogen or phosphorus center.