EPR Characterisation of Cu^II Complexes of Poly(propylene imine) Dendromesogens: Using the Orienting Effect of a Magnetic Field

Abstract

Liquid-crystalline derivatives of poly(propylene imine)dendrimers of the 0th, 1st and 2nd generations, complexed with copper(II) ions, were studied by EPR spectroscopy. The structures of copper (II) complexes with different Cu(II) loadings x per dendrimer ligand L (x = Cu/L) were determined. At the lowest concentration, the Cu(II) ions form monomeric complexes with approximately square-planar N2O2 coordination of both carbonyl oxygen and amido nitrogen atoms. At higher copper content, two kinds of Cu(II) complex sites with different geometries exist. The orienting effect of a high magnetic field was used to investigate the structure and magnetic properties of the copper(II) complexes. This effect, for the first time in dendrimers, allowed the resolution of five nitrogen super-hyperfine lines on g(z) components with the unusual coupling constant of a(Nz)= 35.9 x 10(-4) cm(-1). The combination of the magnetic parameters and the orienting effect indicates the presence of a monomeric complex with pseudotetrahedral N2O2 coordination of the Cu(II) ion, as well as a "dimer" structure with fivefold coordination, presumably due to an N3O2 environment. Higher copper loadings lead to increased exchange coupling between the complex sites.

Magnetic Properties of Poly(propylene imine)–Copper Dendromesogenic Complexes: An EPR Study

“…Figure 1 a shows the EPR spectra at room temperature for the dry sample and for the water-containing samples after exposure to water for 2, 4 and 6 days. It can be seen that the dry complex 2 (x = 7.3) exhibits one symmetrical line (which corresponds to "signal C" in our previous paper [26] ). On increasing the water content, we observe a shift of the resonance position of the symmetrical line towards lower fields, and the g 0 factor changes its value from 2.170 for the 3) with different water content: (c) dry sample; (d) sample exposed to water for 2 days; and (a) sample exposed to water for 4 and 6 days.…”

“…[21][22][23][24][25]27] In our previous paper, we presented the results of an investigation of Cu II complexes with liquid-crystalline (LC) PPI dendrimer ligands of the first (ligand 1) and second (ligand 2) generations (see Scheme 1). [26] All the complexes obtained exhibit hexagonal columnar mesophases. [26] According to their ligand generation, [a] Copper(II) complexes formed by coordination of the Cu II ion with liquid-crystalline poly(propylene imine) dendrimer ligand (L) of the first (complex 1) and second (complex 2) generations with various Cu II contents (x = Cu/L) have been studied by electron paramagnetic resonance (EPR) spectroscopy.…”

“…[26] All the complexes obtained exhibit hexagonal columnar mesophases. [26] According to their ligand generation, [a] Copper(II) complexes formed by coordination of the Cu II ion with liquid-crystalline poly(propylene imine) dendrimer ligand (L) of the first (complex 1) and second (complex 2) generations with various Cu II contents (x = Cu/L) have been studied by electron paramagnetic resonance (EPR) spectroscopy. The existence of a redox-active blue complex 1 (x = 1.9) and the copper(II) nitrate electron transfer associated with the valence tautomerism are revealed for the first time in copper-based dendrimers.…”

“…Previously [26] we identified the local structure and geometry of different kinds of copper(II) complexing sites in PPI dendrimers using the orienting effect of the magnetic field. However, our attempt to obtain precise information on the local environment of the Cu II ion in green complex 1 (x = 1.9) failed, since the super-hyperfine (SHF) structure of the dendrimer ligand was not resolved for this compound, and we could only make a supposition on the copper(II) environment.…”

Metallomesogens: Retrospectives and Perspectives

“…[134]. Заслуживает внимания и развиваемое направление, связанное с синтезом и исследованием ЖК металлодендримеров [135][136][137].…”