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, (L C2 – ), (1,8-naphthalimido)­ethanoate, (L C1 – ), 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(L C2 )4(MeOH)2] (1), [Cu2(L C1 )4(MeOH)2]·2­(CH2Cl2) (2), [Cu2(L C4 )4(EtOH)2]·2­(CH2Cl2) (3) complexes. When the reaction of L C1 – with Cu2(O2CCH3)4(H2O)2 was carried out at −20 °C in the presence of pyridine, [Cu2(L C1 )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(L C2 )4(py)2]·2­(CH2Cl2)·(CH3OH), [Cu2(L C3 )4(py)2]·2­(CH2Cl2) and [Cu2(L C3 )4(bipy)]·(CH3OH)2 ·(CH2Cl2)3.37 (L C3 – = (1,8-naphthalimido)­butanoate)), while its abnormal magnitude in [Cu2(L C2 )4(bipy)] was qualitatively rationalized by structural analysis and DFT calculations.