Raising the spin-reversal barrier in cyano-bridged single-molecule magnets:: Linear MnIII2MIII(CN)6 (M = Cr, Fe) species incorporating [(5-Brsalen)Mn]+ units

Reactions between K-3[M(CN)(6)] and [Mn(5-Brsalen)(H2O)(2)](+) (5-Brsalen = N,N'-ethylenebis(5-bromosalicylidene)aminato dianion) in a mixture of methanol and water afford the compounds K[(5-Brsalen)(2)(H2O)(2)Mn2M(CN)(6)](.)2H(2)O, with M = Cr (1) or Fe (2). The two compounds are isostructural, each containing a molecular cluster with a linear Mn-III-NC-M-III-CN-Mn-III core and tetragonally elongated coordination about the Mn-III centers. Magnetic data indicate the presence of weak exchange interactions within the clusters, giving rise to ground states of S = 5/2 and 9/2 with significant zero-field splitting. Despite the proximity of spin-excited states, ac susceptibility data reveal frequency-dependent out-of-phase signals characteristic of single-molecule magnets with spin-reversal barriers of U-eff = 16 and 25 cm(-1), respectively.