Double-stranded anionic dinuclear copper(II) metallacyclic complexes of the paracyclophane type [Cu2L2]4 have been prepared by the CuII-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’-paraphenylenebis(oxamate) (Me4ppba; 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 p-stacked conformation with a smaller deviation from perpendicularity between the two benzene rings and the basal planes of the square planar CuII ions when increasing the number of methyl substituents (average dihedral angles (f) of 58.72(7) and 73.67(5)8 for 1a and 3c, 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 cm1 for 1a–c, 2a–c, and 3a–c, respectively; H= JS1S2). Cyclic voltammetry (CV) measurements show a reversible oneelectron oxidation of the double polymethyl- substituted para-phenylenediamidate bridging skeleton at a relatively low formal potential that decreases with the number of methyl substituents (E1=+0.33, +0.24, and +0.15 V vs. SCE for 1–3, respectively). The monooxidized dicopper(II) p-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 (lmax=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 CuII 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 pstacked monoradical ligand description. Hence, the spins of the two CuII ions (SCu=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.
Dicopper(II) Metallacyclophanes with Electroswitchable Polymethyl-Substituted p-Phenylene Spacers
ARMENTANO, Donatella;
2013-01-01
Abstract
Double-stranded anionic dinuclear copper(II) metallacyclic complexes of the paracyclophane type [Cu2L2]4 have been prepared by the CuII-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’-paraphenylenebis(oxamate) (Me4ppba; 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 p-stacked conformation with a smaller deviation from perpendicularity between the two benzene rings and the basal planes of the square planar CuII ions when increasing the number of methyl substituents (average dihedral angles (f) of 58.72(7) and 73.67(5)8 for 1a and 3c, 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 cm1 for 1a–c, 2a–c, and 3a–c, respectively; H= JS1S2). Cyclic voltammetry (CV) measurements show a reversible oneelectron oxidation of the double polymethyl- substituted para-phenylenediamidate bridging skeleton at a relatively low formal potential that decreases with the number of methyl substituents (E1=+0.33, +0.24, and +0.15 V vs. SCE for 1–3, respectively). The monooxidized dicopper(II) p-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 (lmax=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 CuII 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 pstacked monoradical ligand description. Hence, the spins of the two CuII ions (SCu=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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
