Copper(II) Carboxylate Dimers Prepared from Ligands Designed to Form a Robust π···π Stacking Synthon: Supramolecular Structures and Molecular Properties

The reactions of bifunctional carboxylate ligands (1,8-naphthalimido)­propanoate,
(LC2–), (1,8-naphthalimido)­ethanoate, (LC1–), and (1,8-naphthalimido)­benzoate,
(LC4–) with Cu2(O2CCH3)4(H2O)2 in methanol or ethanol at room temperature lead to the formation
of novel dimeric [Cu2(LC2)4(MeOH)2] (1), [Cu2(LC1)4(MeOH)2]·2­(CH2Cl2) (2), [Cu2(LC4)4(EtOH)2]·2­(CH2Cl2) (3) complexes.
When the reaction of LC1– with Cu2(O2CCH3)4(H2O)2 was carried out
at −20 °C in the presence of pyridine, [Cu2(LC1)4(py)4]·2­(CH2Cl2) (4) was produced.
At the core of complexes 1–3 lies
the square Cu2(O2CR)4 “paddlewheel”
secondary building unit, where the two copper centers have a nearly
square pyramidal geometry with methanol or ethanol occupying the axial
coordination sites. Complex 4 contains a different type
of dimeric core generated by two κ1-bridging carboxylate
ligands. Additionally, two terminal carboxylates and four trans situated
pyridine molecules complete the coordination environment of the five-coordinate
copper­(II) centers. In all four compounds, robust π···π stacking interactions of the naphthalimide rings organize
the dimeric units into two-dimensional sheets. These two-dimensional
networks are organized into a three-dimensional architecture by two
different noncovalent interactions: strong π···π
stacking of the naphthalimide rings (also the pyridine rings for 4) in 1, 3, and 4,
and intermolecular hydrogen bonding of the coordinated methanol or
ethanol molecules in 1–3. Magnetic
measurements show that the copper ions in the paddlewheel complexes 1–3 are strongly antiferromagnetically
coupled with –J values ranging from 255 to
325 cm–1, whereas the copper ions in 4 are only weakly antiferromagnetically coupled. Typical values of
the zero-field splitting parameter D were found from
EPR studies of 1–3 and the related
known complexes [Cu2(LC2)4(py)2]·2­(CH2Cl2)·(CH3OH), [Cu2(LC3)4(py)2]·2­(CH2Cl2) and [Cu2(LC3)4(bipy)]·(CH3OH)2·(CH2Cl2)3.37 (LC3– = (1,8-naphthalimido)­butanoate)),
while its abnormal magnitude in [Cu2(LC2)4(bipy)] was qualitatively rationalized
by structural analysis and DFT calculations.