Slow relaxation of the magnetization in a {CoIIIMnIII} heterometallic brick-wall network

Abstract

The use of the cyanide-bearing dicobalt(III) complex (PPh4)2[Co2III(μ−2,5-dpp)(CN)8] as a metalloligand towards [Mn(salen)(H2O)]ClO4 afforded the heterobimetallic two-dimensional compound of formula [{MnIII(salen)}2{(μ-NC)4Co2III(μ−2,5-dpp)(CN)4}]n (1) [PPh4+ = teraphenylphosphonium cation, 2,5-dpp = 2,5-bis(2-pyridyl)pyrazine and H2salen = N,N’-ethylenebis(salicylideneimine)] whose structure has been determined by single crystal X-ray diffraction. Compound 1 exhibits a neutral brick-wall structure, where each [Co2III(μ−2,5-dpp)(CN)8]2− unit adopts a tetrakis-monodentate bridging mode towards four {MnIII(salen)}+ fragments through four of its eight cyanide ligands. Each cobalt(III) ion is six-coordinate with one pyrazine- and one pyridine-nitrogen atoms of the 2,5-dpp molecule and four cyanide-carbon atoms describing a somewhat distorted octahedral surrounding. The two crystallographically independent manganese(III) ions are also six-coordinate in an elongated octahedral environment with two imino-nitrogen and two phenoxo-oxygen atoms occupying the equatorial positions and two cyanide-nitrogen atoms filling the axial sites. Variable-temperature direct- (1.9–300 K) and alternating-current (2.0–5.0 K) magnetic measurements of 1 show the occurrence of magnetically isolated manganese(III) ions with a local negative zero-field splitting (D = −5.87 cm−1) and slow magnetic relaxation, this compound being a new example of field-induced Single-Ion Magnet (SIM). The analysis of the ac magnetic data of 1 under an applied dc field of 5000 G reveals that the value of the energy barrier (Ea) to reverse the magnetization direction and that of the pre-exponential factor (τ0) are 10.8 cm−1 and 2.7 × 10−8 s, respectively. These values compare well with those reported for the few examples of manganese(III)-based SIMs nowadays.