Highly Organized Structures and Unusual Magnetic Properties of Paddlewheel Copper(II) Carboxylate Dimers Containing the π−π Stacking, 1,8-Naphthalimide Synthon

The substituted carboxylate compounds N-(3-propanoic acid)-1,8-naphthalimide (HLC2) and N-(4-butanoic acid)-1,8-naphthalimide (HLC3) react with Cu2(O2CCH3)4(H2O)2 in the presence of either pyridine (py) or 4,4′-bipyridine (bipy) to produce the dimeric complexes [Cu2(LC2)4(py)2]·2(CH2Cl2)·(CH3OH) (1), [Cu2(LC3)4(py)2]·2(CH2Cl2) (2), [Cu2(LC2)4(bipy)]·unknown solvent (3), and [Cu2(LC3)4(bipy)]·(CH3OH)2·(CH2Cl2)3.37 (4). The core of these four compounds contains the square Cu2(O2CR)4 “paddlewheel” secondary building unit (SBU) structural motif with nonbonding Cu···Cu distances that average 2.66 Å, with each copper in a nearly square pyramidal geometry. Strong π−π stacking interactions of the 1,8-naphthalimide groups organize the structures of 1 and 2 into sheets and into a three-dimensional structure for 1. The propylene connector in the LC3 ligand allows an arrangement of the 1,8-naphthalimide groups that is different from the square shape of the SBU core. Use of the 4,4′-bipyridine linking ligand produces a three-dimensional structure for 4 organized by both covalent bonds and noncovalent forces where the 1,8-naphthalimide groups organize into a sheet structure and the 4,4′-bipyridine ligands link the sheets. In contrast, in 3, the 1,8-naphthalimide groups overlap to form only one-dimensional ribbons, with the second dimension formed by the 4,4′-bipyridine ligands and the third dimension linked by mechanical interlocking of these two-dimensional units. Although many of the π−π stacking interactions of the 1,8-naphthalimide groups are made with the dipole vectors of this group oriented at 180° (head-to-tail arrangement), skewed arrangements are observed in many cases. Ab initio calculations show that the interaction is relatively insensitive to this angle of rotation, apart from the region of steric repulsion when the rotation angle of the dipoles approaches 0°. Structural results also demonstrate that the rings can slip with respect to each other and maintain substantial interactions. These highly organized, extended structures influence the magnetic properties where all four compounds are strongly antiferromagnetically coupled, leading to diamagnetic Cu(II) solids at and above room temperature with J values that must be more negative than −600 cm−1.