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

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.