Pentafluorophenyl Copper–Pyridine Complexes: Synthesis, Supramolecular Structures via Cuprophilic and π-Stacking Interactions, and Solid-State Luminescence

Abstract

The effect of the binding of pyridine ligands to pentafluorophenyl copper, [C6F5Cu]4 (1), on structural features and photophysical properties has been investigated through a combined multinuclear NMR, X-ray crystallography, and photoluminescence study. Reaction of 1 with 2 equiv of pyridine yields a novel pyridine complex, 3, in which the tetranuclear framework of 1 is retained. Complex 3 features a rhombus-shaped tetracopper core with a short diagonal Cu···Cu distance of 2.5941(6) Å between the dicoordinate copper centers and a longer distance between the pyridine-coordinated copper centers of 4.178(1) Å. In contrast, treatment of 1 with 4 equiv of pyridine results in complete breakdown of the tetranuclear aggregate to give the formally dicoordinate species C6F5Cu(py) (4-H). Reaction of 1 with 2,2′-bipyridine results in formation of the tricoordinate complex C6F5Cu(2,2′-bipy) (5). Aggregate breakdown in species 4 and 5 is reflected in a significantly reduced chemical shift difference Δδ(19Fmeta/para) and a strong downfield shift of the copper-bound carbon atoms in the 13C NMR spectra in comparison to 3. A dynamic equilibrium is established at ratios of py/C6F5Cu ranging from 0 to 2. The solid-state structures of all compounds have been determined by single-crystal X-ray crystallography. The supramolecular assembly of complex 3 via arene–arene π-interactions leads to a network structure with solvent-filled channels propagating through the lattice along the crystallographic c axis. The 2,2′-bipyridine complex 5 also shows π-stacking as the dominant feature in the extended solid-state structure. In contrast, a different mode of supramolecular assembly is found for 4-H in that cuprophilic interactions lead to assembly into one-dimensional copper chains with equidistant Cu···Cu contacts of 2.8924(3) Å. However, the closely related complexes 4-R with methyl or chloro substituents in either the ortho or the para position form supramolecular stacks with structural features that, again, are dominated by offset perfluoroarene–arene interactions with intermolecular plane-to-plane separations of ca. 3.3–3.6 Å. The dicoordinate copper atoms are aligned in one-dimensional chains with alternating short and long Cu(I)···Cu(I) distances of 3.531(1)/3.698(1) Å in 4-pMe, 3.2454(5)/4.2970(5) Å in 4-oMe, 3.521(1)/3.784(1) Å in 4-pCl, and 3.4797(6)/3.8363(6) Å in 4-oCl. Compound 4-H is strongly blue fluorescent at 460 nm in the solid state, but yellow-green fluorescent at 77 K, resulting in an interesting example of luminescence thermochromism. In contrast, the substituted compounds 4-R display strong luminescence only at liquid nitrogen temperature. In all cases, the fluorescence emission band is in the range of ca. 410–425 nm and thus at significantly different energy from that in 4-H, which strongly suggests that the short Cu···Cu contacts in 4-H give rise to unique luminescence properties.