Copper(II) solvatochromic complexes [Cu(acac)(N^N)(ligand)]BPh4 with various axial ligands. Correlation between coordination geometries and d-d transition energies (acac = acetylacetonato, N^N = 1,10-phenanthoroline, 2,2′-bipyridyl)

A series of copper(II) solvatochromic complexes [Cu(acac)(N̂N)(ligand) ]BPh4 (acac = acetylacetonato; N̂N = 1,10-phenanthoroline (1), 2,2′-bipyridyl (2); ligand = HMPA, pyridine, DMSO, DMF, MeOH, acetone, and MeCN) have been synthesized and their coordination geometries were crystallographically investigated. The solvent-coordinated cations, adopting a five-coordinate square-pyramidal structure, formed head-to-tail dimers via π⋯π interactions. Solid-state absorption studies revealed that their d-d transition energies are correlated with the donor number of the axial ligands. A linear correlation was found between the d-d transition energies and the Cu-O (axial ligands) distances in the solid-state, revealing the role of the coordination environment on the d-d transition energies in the copper(II) solvatochromic complexes. © 2012 Elsevier Ltd. All rights reserved.