Magnetic properties of Ln^III-Cu^II 15-metallacrown-5 dimers with terephthalate (Ln^III = Pr, Nd, Sm, Eu)

Metathesis of the isomorphous 15-metallacrown-5 complexes {[LnCu₅(GlyHA)₅(SO₄)(H₂O)_6.5]}₂(SO₄).6H₂O (Ln^III = Pr, Nd, Sm, Eu; GlyHA^2- = glycinehydroxamate) with excess p-bdc^2- (p-bdc^2- = terephthalate) gave pairs of isomorphous complexes {[LnCu₅(GlyHA)₅(CO₃)(H₂O)₄]₂(p-bdc)}.15H₂O [Ln = Pr (1) and Nd (2)] and {[LnCu₅(GlyHA)₅(SO₄)(H₂O)_x]₂(p-bdc)}.yH₂O [Sm (3) x = 5, y = 12; Eu (4), x = 4, y = 14] as outcomes of partial sulfate replacement. Complexes 1-4 are discrete dimers with two 15-metallacrown-5 blocks {LnCu₅}³⁺, a bridging p-bdc^2- and a bidentate capping anion on each Ln³⁺. The anion caps in 1 and 2 are carbonates from atmospheric CO₂ capture, while in 3 and 4 the anion caps are sulfates. Between 3 K and 300 K, intrametallacycle antiferromagnetic exchange interactions amongst the Cu²⁺ dominate the magnetic properties of 1-4, while for the Ln³⁺, spin-orbit coupling and crystal field effects are significant, the former being greater for Sm³⁺ and Eu³⁺. The experimental χ_MT vs. T data were fitted with an additive model reflecting the dominance of the Cu..Cu superexchange, while Ln³⁺..Cu²⁺ exchange interactions were taken into account through a molecular field approach. The best-fit values (-2J_Cu-Cu) for the intercopper superexchange interactions range from 88 to 120 cm^-1; the g-factors, spin-orbit coupling (λ) and axial crystal field splitting (Δ) parameter values are typical for Ln^III. (copyright 2018 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)