A study of the Group 1 metal tetra-aza macrocyclic complexes [M(Mecyclen)(L)] using electronic structure calculations.

Metal-cyclen complexes have a number of important applications. However, the coordination chemistry between metal ions and cyclen-based macrocycles is much less well studied compared to their metal ion-crown ether analogues. This work, which makes a contribution to address this imbalance by studying complex ions of the type [M(Mecyclen)(L)], was initiated by results of an experimental study which prepared some Group 1 metal cyclen complexes, namely [Li(Mecyclen)(HO)][BAr] and [Na(Mecyclen)(THF)][BAr] and obtained their X-ray crystal structures [J. M. Dyke, W. Levason, M. E. Light, D. Pugh, G. Reid, H. Bhakhoa, P. Ramasami, and L. Rhyman, Dalton Trans., 2015, 44, 13853]. The lowest [M(Mecyclen)(L)] minimum energy structures (M = Li, Na, K, and L = HO, THF, DEE, MeOH, DCM) are studied using density functional theory (DFT) calculations. The geometry of each [M(Mecyclen)(L)] structure and, in particular, the conformation of L are found to be mainly governed by steric hindrance which decreases as the size of the ionic radius increases from Li → Na → K. Good agreement of computed geometrical parameters of [Li(Mecyclen)(HO)] and [Na(Mecyclen)(THF)] with the corresponding geometrical parameters derived from the crystal structures [Li(Mecyclen)(HO)][BAr] and [Na(Mecyclen)(THF)][BAr] is obtained. Bonding analysis indicates that the stability of the [M(Mecyclen)(L)] structures originates mainly from ionic interaction between the Mecyclen/L ligands and the M centres. The experimental observation that [M(Mecyclen)(L)][BAr] complexes could be prepared in crystalline form for M = Li and Na, but that experiments aimed at synthesising the corresponding K, Rb, and Cs complexes failed resulting in formation of [MecyclenH][BAr] is investigated using DFT and explicitly correlated calculations, and explained by considering production of [MecyclenH] by a hydrolysis reaction, involving traces of water, which competes with [M(Mecyclen)(L)] formation. [MecyclenH] formation dominates for M = K, Rb, and Cs whereas formation of [M(Mecyclen)(L)] is energetically favoured for M = Li and Na. The results indicate that the number and type of ligands, play a key role in stabilising the [M(Mecyclen)] complexes and it is hoped that this work will encourage experimentalists to prepare and characterise other [M(Mecyclen)(L)] complexes.