Extended π-Systems in Diimine Ligands in [Cu(P^P)(N^N)][PF6] Complexes: From 2,2′-Bipyridine to 2-(Pyridin-2-yl)Quinoline

Abstract

We describe the synthesis and characterization of [Cu(POP)(1)][PF6], [Cu(POP)(2)][PF6], [Cu(xantphos)(1)][PF6], and [Cu(xantphos)(2)][PF6] in which ligands 1 and 2 are 2-(pyridin-2-yl)quinoline and 2-(6-methylpyridin-2-yl)quinoline, respectively. With 2,2'-bipyridine (bpy) as a benchmark, we assess the impact of the extended π-system on structural and solid-state photophysical properties. The single crystal structures of [Cu(POP)(2)][PF6], [Cu(xantphos)(1)][PF6], and [Cu(xantphos)(2)][PF6] were determined and confirmed a distorted tetrahedral copper(I) coordination environment in each [Cu(P^P)(N^N)]+ cation. The xanthene unit in [Cu(xantphos)(1)][PF6] and [Cu(xantphos)(2)][PF6] hosts the quinoline unit of 1, and the 6-methylpyridine group of 2. 1H NMR spectroscopic data indicate that these different ligand orientations are also observed in acetone solution. In their crystal structures, the [Cu(POP)(2)]+, [Cu(xantphos)(1)]+, and [Cu(xantphos)(2)]+ cations exhibit different edge-to-face and face-to-face π-interactions, but in all cases, the copper(I) centre is effectively protected by a ligand sheath. In [Cu(POP)(2)][PF6], pairs of cations engage in an efficient face-to-face π-stacking embrace, and we suggest that this may contribute to this compound having the highest photoluminescence quantum yield (PLQY = 21%) of the series. With reference to data from the Cambridge Structural Database, we compare packing effects and PLQY data for the complexes incorporating 2-(pyridin-2-yl)quinoline and 2-(6-methylpyridin-2-yl)quinoline, with those of the benchmark bpy-containing compounds. We also assess the effect that Cu⋯O distances in the {Cu(POP)} and {Cu(xantphos)} domains of [Cu(P^P)(N^N)][X] compounds have on solid-state PLQY values.