A study of the Group 1 metal tetra-aza macrocyclic complexes [M(Me₄cyclen)(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(Me4cyclen)(L)]+, was initiated by results of an experimental study which prepared some Group 1 metal cyclen complexes, namely [Li(Me4cyclen)(H2O)][BArF] and [Na(Me4cyclen)(THF)][BArF] 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(Me4cyclen)(L)]+minimum energy structures (M = Li, Na, K, and L = H2O, THF, DEE, MeOH, DCM) are studied using density functional theory (DFT) calculations. The geometry of each [M(Me4cyclen)(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(Me4cyclen)(H2O)]+and [Na(Me4cyclen)(THF)]+with the corresponding geometrical parameters derived from the crystal structures [Li(Me4cyclen)(H2O)]+[BArF]-and [Na(Me4cyclen)(THF)]+[BArF]-is obtained. Bonding analysis indicates that the stability of the [M(Me4cyclen)(L)]+structures originates mainly from ionic interaction between the Me4cyclen/L ligands and the M+centres. The experimental observation that [M(Me4cyclen)(L)]+[BArF]-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 [Me4cyclenH][BArF] is investigated using DFT and explicitly correlated calculations, and explained by considering production of [Me4cyclenH]+by a hydrolysis reaction, involving traces of water, which competes with [M(Me4cyclen)(L)]+formation. [Me4cyclenH]+formation dominates for M+= K+, Rb+, and Cs+whereas formation of [M(Me4cyclen)(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(Me4cyclen)]+complexes and it is hoped that this work will encourage experimentalists to prepare and characterise other [M(Me4cyclen)(L)]+complexes.