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

• Main Authors: Sarah Keller, Murat Alkan-Zambada, Alessandro Prescimone, Edwin C. Constable, Catherine E. Housecroft
• Format: Article
• Language: English
• Published: MDPI AG 2020-03-01
• Series: Crystals
• Subjects: single crystal structure; copper; 2-(pyridin-2-yl)quinoline; bis(phosphane); heteroleptic complex; photoluminescence
• Online Access: https://www.mdpi.com/2073-4352/10/4/255

We describe the synthesis and characterization of [Cu(POP)(<b>1</b>)][PF₆], [Cu(POP)(<b>2</b>)][PF₆], [Cu(xantphos)(<b>1</b>)][PF₆], and [Cu(xantphos)(<b>2</b>)][PF₆] in which ligands <b>1</b> and <b>2</b> 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)(<b>2</b>)][PF₆], [Cu(xantphos)(<b>1</b>)][PF₆], and [Cu(xantphos)(<b>2</b>)][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)(<b>1</b>)][PF₆] and [Cu(xantphos)(<b>2</b>)][PF₆] hosts the quinoline unit of <b>1</b>, and the 6-methylpyridine group of <b>2</b>. ¹H NMR spectroscopic data indicate that these different ligand orientations are also observed in acetone solution. In their crystal structures, the [Cu(POP)(<b>2</b>)]⁺, [Cu(xantphos)(<b>1</b>)]⁺, and [Cu(xantphos)(<b>2</b>)]⁺ 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)(<b>2</b>)][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.