Quantum Tunneling and Quantum Phase Interference in a [Mn^II₂Mn^III₂] Single-Molecule Magnet

[Mn4(hmp)6(H2O)2(NO3)2](NO3)2·2.5H2O (1) has been synthesized from the reaction of 2-hydroxymethylpyridine (Hhmp) with Mn(NO3)2·4H2O in the presence of tetraethylammonium hydroxide. 1 crystallizes in the triclinic P1̄ space group with two crystallographically independent centrosymmetrical [Mn4(hmp)6(H2O)2(NO3)2]2+ complexes in the packing structure. Four Mn ions are arranged in a double-cuboidal fashion where outer Mn2+ are heptacoordinated and inner Mn3+ are hexacoordinated. dc magnetic measurements show that both Mn2+···Mn3+ and Mn3+···Mn3+ interactions are ferromagnetic with Jwb/kB = +0.80(5) K, and Jbb/kB = +7.1(1) K, respectively, leading to an ST = 9 ground state. Combined ac and dc measurements reveal the single-molecule magnet (SMM) behavior of 1 with both thermally activated and ground-state tunneling regimes, including quantum phase interference. In the thermally activated regime, the characteristic relaxation time (τ) of the system follows an Arrhenius law with τ0 = 6.7 × 10-9 s and Δeff/kB = 20.9 K. Below 0.34 K, τ saturates indicating that the quantum tunneling of the magnetization becomes the dominant relaxation process as expected for SMMs. Down to 0.04 K, field dependence of the magnetization measured using the μ-SQUID technique shows the presence of very weak inter-SMM interactions (zJ‘/kB ≈ −1.5 × 10-3 K) and allows an estimation of D/kB at −0.35 K. Quantum phase interference has been used to confirm the D value and to estimate the transverse anisotropic parameter to E/kB = +0.083 K and the ground-state tunnel splitting ΔLZ = 3 × 10-7 K at Htrans = 0 Oe. These results rationalize the observed tunneling time (τQTM) and the effective energy barrier (Δeff).