The concept of “molecular magnetic sponges” was introduced for the first time in 1999 by the creative imagination of the late Olivier Kahn. It refers to the exotic spongelike behavior of certain molecule-based materials that undergo a dramatic change of their magnetic properties upon reversible dehydration/rehydration processes. Here we report a unique example of a manganese(II)–copper(II) mixed-metal–organic framework of formula [Na(H₂O)₄]₄[Mn₄{Cu₂(mpba)₂(H₂O)₄}₃]⋅ 56.5 H₂O (1) (mpba=N,N′-1,3-phenylenebis(oxamate)). Compound 1 possesses a 3D Mn^II₄Cu^II₆ pillared layer structure with mixed square and octagonal pores of approximate dimensions 1.2×1.2 nm and 2.1×3.0 nm, respectively, hosting a large amount of crystallization H₂O molecules and hydrated Na^I countercations as guests. It reversibly switches from a crystalline hydrated phase with long-range ferromagnetic ordering at a rather high critical temperature (T_c) of 22.5 K to an amorphous dehydrated phase with T_c as low as 2.3 K, which is accompanied by a breathing-type dynamic effect involving a large crystal volume (ca. 45 %) and color changes after water desorption/adsorption. The combination of both the open-framework structure and the spongelike optical, mechanical, and magnetic switching behavior in this new class of oxamato-based porous magnets offers fascinating possibilities in designing multifunctional materials for host–guest molecular sensing.

A series of neutral oxamato-bridged heterobimetallic chains of general formula [MCu(L^x)₂(S)₂]⋅p S⋅q H₂O [p=0–1, q=0–2.5; L¹=N-2,6-dimethylphenyloxamate, S=DMF with M=Mn (1 a) and Co (1 b); L²=N-2,6-diethylphenyloxamate, S=DMF with M=Mn (2 a) and Co (2 b) or S=DMSO with M=Mn (2 c) and Co (2 d); L³=N-2,6-diisopropylphenyloxamate, S=DMF with M=Mn (3 a) and Co (3 b) or S=DMSO with M=Mn (3 c) and Co (3 d)] were prepared by treating the corresponding anionic oxamatocopper(II) complexes [Cu(L^x)₂]²⁻ (x=1–3) with M²⁺ cations (M=Mn and Co) in DMF or DMSO as the solvent. The single-crystal X-ray structures of 2 a and 3 a reveal the occurrence of well-isolated, zigzag, oxamato-bridged manganese(II)–copper(II) chains. The intrachain Cu⋅⋅⋅Mn distances across the oxamato bridge are 5.3761(7) and 5.4002(17) Å for 2 a and 3 a, respectively, whereas the shortest interchain Mn⋅⋅⋅Mn distances are 9.4475(16) and 8.1649(14) Å for 2 a and 3 a, respectively. All of these M^IICu^II chains (M=Mn and Co) exhibit 1D ferrimagnetic behaviour with moderately strong intrachain antiferromagnetic coupling between the square-planar Cu^II and octahedral high-spin M^II ions across the oxamato bridge [−J=31.4–35.2 and 33.4–44.8 cm⁻¹, respectively; H=∑_i−JS_M,i(S_Cu,i+S_Cu,i−1)]. Only the Co^IICu^II chains show slow magnetic relaxation effects characteristic of single-chain magnets (SCMs). Analysis of the magnetic relaxation dynamics of 3 d shows a thermally activated mechanism (Arrhenius law dependence) with values of the pre-exponential factor (τ₀=2.6×10⁻⁹ s) and activation energy (E_a=7.7 cm⁻¹) that are typical of SCMs. In contrast, two relaxation regimes are observed for 2 d in different temperature regions (τ₀=3.2×10⁻¹⁰ s and E_a=24.7 cm⁻¹ for T<4.5 K and τ₀=3.2×10⁻¹⁴ s and E_a=37.5 cm⁻¹ for T>4.5 K).

A new series of neutral oxamato-bridged M^IICu^II chiral chains of general formula [MCuL^x(S)_m(H₂O)_n]⋅aS⋅bH₂O [L¹=(M)-1,1′-binaphthalene-2,2′-bis(oxamate) with M=Mn (1 a) and Co (1 b); L²=(P)-1,1′-binaphthalene-2,2′-bis(oxamate) with M=Mn (2 a) and Co (2 b)] and the analogous racemic chains of formula [MCuL³(S)_m(H₂O)_n]⋅aS⋅bH₂O [L³=1,1′-binaphthalene-2,2′-bis(oxamate) with M=Mn (3 a) and Co (3 b)] have been prepared by reaction of the corresponding dianionic oxamatocopper(II) complex [Cu(L^x)]²⁻ with Mn²⁺ or Co²⁺ cations in either dimethylformamide (DMF) or dimethyl sulfoxide (DMSO). Solid circular dichroism (CD) spectra of the bimetallic chain compounds were recorded to establish their chiral and enantiomeric nature. They exhibit maximum positive and negative Cotton effects, each pair of enantiomeric chains being non-superimposable mirror images. The crystal structures of the Mn^IICu^II (1 a–3 a) and the Co^IICu^II (1 b and 2 b) chain compounds were solved by single-crystal X-ray diffraction methods. Our attempts to obtain X-ray quality crystals of 3 b were unsuccessful. The values of the shortest interchain Mn⋅⋅⋅Mn and Co⋅⋅⋅Co distances are indicative of a good isolation of neighbouring chains in the crystal lattice, which is caused by the bulky aromatic ligand. Although all the Mn^IICu^II and Co^IICu^II chains exhibit ferrimagnetic behaviour (−J_MnCu=18.9–26.6 cm⁻¹ and −J_CoCu=19.5–32.5 cm⁻¹), only the enantiopure Co^IICu^II chains (1 b and 2 b) show slow magnetic relaxation at low temperatures (T_B=0.6–1.8 K), which is a characteristic of single-chain magnets (SCMs) and is related to the magnetic anisotropy of the high-spin Co^II ion. Analysis of the SCM behaviour of 1 b and 2 b, based on Glauber’s theory for an Ising one-dimensional system, shows a thermally activated mechanism for the magnetic relaxation (Arrhenius law dependence). The energy barriers (E_a) to reverse the magnetisation direction are 8.2 (1 b) and 8.1 cm⁻¹ (2 b), whereas the pre-exponential factor (τ₀) is 1.9×10⁻⁸ (1 b) and 6.0×10⁻⁹ s (2 b). Interestingly, the racemic Co^IICu^II chain analogue, 3 b, showed no evidence of SCM behaviour.