Dicopper(II) Metallacyclophanes with Electroswitchable Polymethyl‐Substituted para ‐Phenylene Spacers

Double‐stranded anionic dinuclear copper(II) metallacyclic complexes of the paracyclophane type [Cu 2 L 2 ] 4− have been prepared by the Cu II ‐mediated self‐assembly of different para ‐phenylenebis(oxamato) bridging ligands with either zero‐, one‐, or four‐electron‐donating methyl substituents (L= N , N ′‐ para ‐phenylenebis(oxamate) (ppba; 1 ), 2‐methyl‐ N , N ′‐ para ‐phenylenebis(oxamate) (Meppba; 2 ), and 2,3,5,6‐tetramethyl‐ N , N ′‐ para ‐phenylenebis(oxamate) (Me 4 ppba; 3 )). These complexes have been isolated as their tetra‐ n ‐butylammonium ( 1 a – 3 a ), lithium(I) ( 1 b – 3 b ), and tetraphenylphosphonium salts ( 1 c – 3 c ). The X‐ray crystal structures of 1 a and 3 c show a parallel‐displaced π‐stacked conformation with a smaller deviation from perpendicularity between the two benzene rings and the basal planes of the square planar Cu II ions when increasing the number of methyl substituents (average dihedral angles ( ϕ ) of 58.72(7) and 73.67(5)° for 1 a and 3 c , respectively). Variable‐temperature (2.0–300 K) magnetic‐susceptibility measurements show an overall increase of the intramolecular antiferromagnetic coupling with the number of methyl substituents onto the para ‐phenylene spacers (− J =75–95, 100–124, and 128–144 cm −1 for 1 a – c , 2 a – c , and 3 a – c , respectively; H =− J S 1 × S 2 ). Cyclic voltammetry (CV) measurements show a reversible one‐electron oxidation of the double polymethyl‐substituted para ‐phenylenediamidate bridging skeleton at a relatively low formal potential that decreases with the number of methyl substituents ( E 1 =+0.33, +0.24, and +0.15 V vs. SCE for 1 – 3 , respectively). The monooxidized dicopper(II) π‐radical cation species 3 ′ prepared by the chemical oxidation of 3 with bromine exhibits intense metal‐to‐ligand charge‐transfer (MLCT) transitions in the visible and near‐IR ( λ max =595 and 875 nm, respectively) regions together with a rhombic EPR signal with a seven‐line splitting pattern due to hyperfine coupling with the nuclear spin of the two Cu II ions. Density functional (DF) calculations for 3 ′ evidence a characteristic iminoquinonoid‐type short‐long‐short alternating sequence of CN and CC bonds for both tetramethyl‐ para ‐phenylenediamidate bridges and a large amount of spin density of negative sign mainly delocalized along each of the four benzene C atoms directly attached to the amidate N atoms, which is in agreement with a fully delocalized π‐stacked monoradical ligand description. Hence, the spins of the two Cu II ions ( S Cu =1/2) that are antiparallel aligned in 3 (OFF state) become parallel in 3 ′ (ON state). Further developments may be then envisaged for this new permethylated dicopper(II) paracyclophane with a redox noninnocent ligand as a prototype for molecular magnetic electroswitch.