Thermochromism of Cu(I) Tetrakisguanidine Complexes: Reversible Activation of Metal-to-Ligand Charge-Transfer Bands.

Tetranuclear, intensely blue-coloured Cu(I) complexes were synthesised in which two Cu2 X3 (-) units (X=Br or I) are bridged by a dicationic GFA (guanidino-functionalised aromatic) ligand. The UV/Vis spectra show a large metal-to-ligand charge-transfer (MLCT) band around 638 nm. The tetranuclear "low-temperature" complexes are in a temperature-dependent equilibrium with dinuclear Cu(I) "high-temperature" complexes, which result from the reversible elimination of two CuX groups. A massive thermochromism effect results from the extinction of the strong MLCT band upon CuX elimination with increasing temperature. For all complexes, quantum chemical calculations predict a small and method-dependent energy difference between the possible electronic structures, namely Cu(I) and dicationic GFA ligand (closed-shell singlet) versus Cu(II) and neutral GFA ligand (triplet or broken-symmetry state). The closed-shell singlet state is disfavoured by hybrid-DFT functionals, which mix in exact Hartree-Fock exchange, and is favoured by larger basis sets and consideration of a polar medium.