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

Double-stranded anionic dinuclear copper(II) metallacyclic complexes of the paracyclophane type [Cu2L2](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 (1a-3a), lithium(I) (1b-3b), and tetraphenylphosphonium salts (1c-3c). The X-ray crystal structures of 1a and 3c show a parallel-displaced -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 (phi) of 58.72(7) and 73.67(5)degrees for 1a and 3c, respectively). Variable-temperature (2.0-300K) 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 128-144cm(-1) for 1a-c, 2a-c, and 3a-c, respectively; H=-JS(1)xS(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 +0.15V vs. SCE for 1-3, respectively). The monooxidized dicopper(II) -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 ((max)=595 and 875nm, 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 -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.