Evaluation of exchange interaction in binuclear copper(II) chelates of amide-based cyclophanes

Abstract

Temperature dependence of the electron paramagnetic resonance (EPR) spectra of binuclear Cu(II) complexes with amide-based cyclophanes, Cu 2 (bis-EDTAPDN), Cu2(bis-EDTANAP) and Cu2(bis-EDTABPE), in 60% methanol (pH ca. 10) has been studied at a frequency of 9.43 GHz in the temperature range of 77-4.2 K; the cyclophanes are macrocyclic compounds in which four amide bonds link two ethylenediaminetetraacetate (EDTA) units and two units of the aromatic diamine 1,4-phenylenediamine (DPN), 1,5-diaminonaphthalene (NAP) or bis(4-aminophenyl) ether (BPE). Exchange integrals found from the EPR measurements are ca. 70-105 cm -', 22 cm -' and J < 0, for Cu2(bis-EDTAPDN), Cu 2 (bis-EDTANAP) and Cu2(bis-EDTABPE), respectively; the sign of J is defined to be positive for antiferromagnetic interaction. The variation of exchange interactions in these compounds is related to the nature of the bridging organic groups.

Structure and Magnetism of Dinuclear Copper(II) Metallacyclophanes with Oligoacenebis(oxamate) Bridging Ligands: Theoretical Predictions on Wirelike Magnetic Coupling

“…In this respect, two aspects deserve to be mentioned. First, the − J values for the lithium and tetraphenylphosphonium salts of complex 2a (− J = 20.5−20.7 cm -1 ) are similar to that found in the only other reported example of dicopper(II) metallacyclophane with a double 1,5-naphthalenediamidate bridge (− J = 22.0 cm -1 ) (Table S1, entry 10) 11d. Second, the comparable or even larger − J values for the lithium and tetraphenylphosphonium salts of complex 3b (− J = 21.2−23.0 cm -1 ) are quite remarkable given the larger Cu−Cu separation through the double 2,6-anthracenediamidate bridge (see below).…”

“…This clearly evidences the greater efficiency of the oligoacene spacers compared to the oligo- p -phenylene ones in the transmission of the exchange interaction, as expected from their better π-conjugation. Once again, this fact illustrates the large influence of the nature of the extended π-conjugated aromatic spacer (substitution pattern, conformation, or presence of heteroatoms) on the magnetic coupling through π-type exchange pathways for this family of dicopper(II) amide-based metallacyclophanes (Table S1, entries 9−18). , …”

“…The variation of the ligand spacer may control both the overall structure and the magnetic behavior of the metallacyclophane complex, and the influence of different factors such as the topology and conformation of the bridging ligand can then be investigated in a systematic way. In particular, the problem of long-range magnetic coupling in dinuclear copper(II) metallacyclophanes has been addressed by several research teams through the use of diverse coordinating group substituted aromatic bridging ligands, either acyclic or macrocyclic, with more or less satisfactory results (Table S1, Supporting Information). − There is no simple correlation between the magnetic coupling and the intermetallic distance for the reported examples of structurally characterized dicopper(II) metallacyclophanes with amine-, imine-, and amide-based 11 aromatic bridges (Figure ). In general, for those dicopper(II) metallaazacyclophanes with extended aromatic spacers of comparable Cu−Cu distances, the magnitude of the magnetic coupling in the amide series is more than 4-fold greater than the largest one in the amine and imine series.…”

“…In general, for those dicopper(II) metallaazacyclophanes with extended aromatic spacers of comparable Cu−Cu distances, the magnitude of the magnetic coupling in the amide series is more than 4-fold greater than the largest one in the amine and imine series. With one notable exception,10d weak antiferromagnetic couplings (− J values smaller than 2.2 cm -1 ) have been observed for the dicopper(II) metallacyclophanes with amine- and imine-based aromatic bridges possessing rather long intermetal distances ( r values larger than 7.0 Å) (Table S1, entries 1−8). ,10b,c,e On the contrary, Inoue et al have reported moderate to strong antiferromagnetic couplings in the related dicopper(II) metallacyclophanes with amide-based aromatic bridges (− J values up to 105 cm -1 ) (Table S1, entries 9−11) 11d. It appears that the exchange interaction between the Cu II ions involves predominantly π-type orbital pathways in the amide series, whereas it is mainly transmitted through the σ-electron system of the aromatic bridge in the amine and imine series, with little or no contribution from the π-electron system.…”

Dicopper(II) Metallacyclophanes with Oligo(p-phenylene-ethynylene) Spacers: Experimental Foundations and Theoretical Predictions on Potential Molecular Magnetic Wires

“…Dicopper­(II) metallacyclophanes emerge as ideal model systems for the study of the transmission of EE interactions between paramagnetic metal centers containing one unpaired electron ( H = − J S 1 · S 2 with S 1 = S 2 = S Cu = 1/2) through potential molecular magnetic wires, both from experimental and theoretical viewpoints . The variation of the ligand spacer in the metallacyclic entity may control the overall structure and the magnetic properties and then, the influence of different factors such as the topology and conformation of the bridging ligand can be investigated in a systematic way. − So, the problem of long-range magnetic coupling in dicopper­(II) metallacyclophanes have been addressed by several research teams through the use of different coordinating group substituted amine-, imine-, and amide-based aromatic bridging ligands, with more or less satisfactory results.…”