Dicopper(II) Metallacyclophanes with Electroswitchable Polymethyl‐Substituted para‐Phenylene Spacers. Issue 36 (22nd July 2013)

• Record Type: Journal Article
• Authors: Ferrando‐Soria, Jesús; Castellano, María; Ruiz‐García, Rafael; Cano, Joan; Julve, Miguel; Lloret, Francesc; Ruiz‐Pérez, Catalina; Pasán, Jorge; Cañadillas‐Delgado, Laura; Armentano, Donatella; Journaux , Yves; Pardo, Emilio
• Is Part Of: Chemistry. Volume 19:Issue 36(2013)
• Page Start: 12124
• Page End: 12137
• Publication Date: 2013-07-22
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201204484
• Languages: English
• ISSNs: 0947-6539

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>Double‐stranded anionic dinuclear copper(II) metallacyclic complexes of the paracyclophane type [Cu<sub>2</sub>L<sub>2</sub>]<sup>4−</sup> have been prepared by the Cu<sup>II</sup>‐mediated self‐assembly of different <italic>para</italic>‐phenylenebis(oxamato) bridging ligands with either zero‐, one‐, or four‐electron‐donating methyl substituents (L=<italic>N</italic>, <italic>N</italic>′‐<italic>para</italic>‐phenylenebis(oxamate) (ppba; <bold>1</bold>), 2‐methyl‐ <italic>N</italic>, <italic>N</italic>′‐<italic>para</italic>‐phenylenebis(oxamate) (Meppba; <bold>2</bold>), and 2, 3, 5, 6‐tetramethyl‐ <italic>N</italic>, <italic>N</italic>′‐<italic>para</italic>‐phenylenebis(oxamate) (Me<sub>4</sub>ppba; <bold>3</bold>)). These complexes have been isolated as their tetra‐<italic>n</italic>‐butylammonium (<bold>1 a</bold>–<bold>3 a</bold>), lithium(I) (<bold>1 b</bold>–<bold>3 b</bold>), and tetraphenylphosphonium salts (<bold>1 c</bold>–<bold>3 c</bold>). The X‐ray crystal structures of <bold>1 a</bold> and <bold>3 c</bold> 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<sup>II</sup> ions when increasing the number of methyl substituents (average dihedral angles (<italic>ϕ</italic>) of 58.72(7) and 73.67(5)° for <bold>1 a</bold> and <bold>3 c</bold>, 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 <italic>para</italic>‐phenylene spacers (−<italic>J</italic>=75–95, 100–124, and 128–144 cm<sup>−1</sup> for <bold>1 a</bold>–<bold>c</bold>, <bold>2 a</bold>–<bold>c</bold>, and <bold>3 a</bold>–<bold>c</bold>, respectively; <italic><bold>H</bold></italic>=−<italic>J<bold>S</bold></italic><sub>1</sub>×<italic><bold>S</bold></italic><sub>2</sub>). Cyclic voltammetry (CV) measurements show a reversible one‐electron oxidation of the double polymethyl‐substituted <italic>para</italic>‐phenylenediamidate bridging skeleton at a relatively low formal potential that decreases with the number of methyl substituents (<italic>E</italic><sub>1</sub>=+0.33, +0.24, and +0.15 V vs. SCE for <bold>1</bold>–<bold>3</bold>, respectively). The monooxidized dicopper(II) π‐radical cation species <bold>3</bold>′ prepared by the chemical oxidation of <bold>3</bold> with bromine exhibits intense metal‐to‐ligand charge‐transfer (MLCT) transitions in the visible and near‐IR (<italic>λ</italic><sub>max</sub>=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<sup>II</sup> ions. Density functional (DF) calculations for <bold>3</bold>′ evidence a characteristic iminoquinonoid‐type short‐long‐short alternating sequence of CN and CC bonds for both tetramethyl‐<italic>para</italic>‐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<sup>II</sup> ions (<italic>S</italic><sub>Cu</sub>=1/2) that are antiparallel aligned in <bold>3</bold> (OFF state) become parallel in <bold>3</bold>′ (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.</p> </abstract>