Positional Isomerism in the N^N Ligand: How Much Difference Does a Methyl Group Make in [Cu(P^P)(N^N)]⁺ Complexes?

The synthesis and structural characterization of 5,6'-dimethyl-2,2'-bipyridine (5,6'-Me₂bpy) are reported, along with the preparations and characterizations of [Cu(POP)(5,6'-Me₂bpy)][PF₆] and [Cu(xantphos)(5,6'-Me₂bpy)][PF₆] (POP = bis(2-(diphenylphosphanyl)phenyl)ether, xantphos = 4,5-bis(diphenylphosphanyl)-9,9-dimethyl-9H-xanthene). Single-crystal X-ray structure determinations of [Cu(POP)(5,6'-Me₂bpy)][PF₆] and [Cu(xantphos)(5,6'-Me₂bpy)][PF₆] confirmed distorted tetrahedral copper(I) coordination environments with the 5-methylpyridine ring of 5,6'-Me₂bpy directed towards the (C₆H₄)₂O unit of POP or the xanthene unit of xantphos. In the xantphos case, this preference may be attributed to C-H…π interactions involving both the 6-CH unit and the 5-methyl substituent in the 5-methylpyridine ring and the arene rings of the xanthene unit. ¹H NMR spectroscopic data indicate that this ligand orientation is also preferred in solution. In solution and the solid state, [Cu(POP)(5,6'-Me₂bpy)][PF₆] and [Cu(xantphos)(5,6'-Me₂bpy)][PF₆] are yellow emitters, and, for powdered samples, photoluminescence quantum yields (PLQYs) are 12 and 11%, respectively, and excited-state lifetimes are 5 and 6 μs, respectively. These values are lower than PLQY and τ values for [Cu(POP)(6,6'-Me₂bpy)][PF₆] and [Cu(xantphos)(6,6'-Me₂bpy)][PF₆], and the investigation points to the 6,6'-dimethyl substitution pattern in the bpy ligand being critical for enhancement of the PLQY.