Intramolecular π-stacking in copper(I) diketiminate phenanthroline complexes

Diketimines N,N'-dibenzyl-2-amino-4-imino-pent-2-ene (1), S,S-N,N'-di(phenylethyl)-2-amino-4-imino-pent-2-ene (2), N,N'-bis(3,4,5-trimethoxyphenylmethyl)-2-amino-4-imino-pent-2-ene (3), N, N'-bis(pentafluorophenylmethyl)-2-amino-4-imino-pent-2-ene (4) and N,N'-diisobutyl-2-amino-4-imino-pent-2-ene (5) react with CuOtBu in the presence of 2,9-R-2-1,10-phenanthroline to give the respective neutral, tetracoordinated diketiminate copper(I) phenanthroline complexes 1a+2a (R = H), 1b, 3b-5b (R = Me) and 1c+3c (R = Ph). Crystal structures were obtained for all complexes except 5b and intramolecular pi-stacking between the phenanthroline ligand and one or two N-benzyl substituents were observed in 1a, 2a, 1b and 1c, or 3b and 4b, respectively. UV/vis absorption spectra show two transitions in the visible region, a diketiminate-based transition at 373-386 nm and a transition at 600-666 nm, tentatively assigned as an MLCT to phenanthroline. All complexes were weakly luminescent in the solid state at room temperature with lifetimes of less than 60 ns. Weak luminescence was also observed in solution with lambda(max) = 720-830 nm for 1b, 1c, 3b, 4b and 5b and short luminescence lifetimes. Intramolecular pi-stacking interactions, which prevent flattening distortions in the solid state, appear to have advantageous effects on luminescence