Slow Relaxation of the Magnetization in a {Co IIIMn III} Heterometallic Brick-Wall Network

Abstract

The use of the cyanide-bearing dicobalt(III) complex (PPh 4 ) 2 [Co 2 III (m-2,5-dpp)(CN) 8 ] as a metalloligand towards [Mn(salen)(H 2 O)]ClO 4 afforded the heterobimetallic two-dimensional compound of formula [{Mn III (salen)} 2 {(m-NC) 4 Co 2 III (m-2,5-dpp)(CN) 4 }] n (1) [PPh 4 + = teraphenylphosphonium cation, 2,5-dpp = 2,5-bis(2-pyridyl)pyrazine and H 2 salen = 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 [Co 2 III (m-2,5-dpp)(CN) 8 ] 2- unit adopts a tetrakis-monodentate bridging mode towards four {Mn III (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 5000 G reveals that the value of the energy barrier ( E a ) to reverse the magnetization direction and that of the pre-exponential factor (t 0 ) are 10.8 cm -1 and 2.7 x 10 -8 s, respectively. These values compare well with those reported for the few examples of manganese(III)-based SIMs nowadays.