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The new, sterically encumbered phenanthroline ligands 1a,b, both characterized by the presence of bulky aryl substituents (3,5-di-tert-butyl-4-methoxyphenyl, 2,4,6-trimethylphenyl) in the 2,9-position, were prepared along with their homoleptic [Cu(1a,b)₂]⁺ and heteroleptic complexes [Cu(1a,b)(phen)]⁺ (phen = parent 1,10-phenanthroline). Due to the pronounced steric shielding, particularly effective in ligand 1a, the formation of the homoleptic complex [Cu(1a)₂]⁺ becomes very slow (5 days). Once formed, the homoleptic complexes [Cu(1a,b)₂]⁺ do not exchange ligands even with phen added in excess because they are kinetically locked due to the large tert-butylphenyl substituents at the phenanthroline unit. The electronic absorption spectra of the homoleptic complexes [Cu(1a)₂]⁺ and [Cu(1b)₂]⁺ evidence a strongly different ground state geometry of the two compounds, the former being substantially more …