The synthesis and characterizations of a family of isomorphous [Mn^III₂M^III₄L₂(μ₄-O)₂(N₃)₂(CH₃O)₂(CH₃OH)₄(NO₃)₂]·2H₂O (M = Y(1), Gd(2), Tb(3), Dy(4)) are reported, where H₄L = N,N′-dihydroxyethyl-N,N′-(2-hydroxy-4,5-dimethylbenzyl)ethylenediamine. They were obtained from the reactions of H₄L with M(NO₃)₃·6H₂O, Mn(ClO₄)₂·6H₂O, NaN₃ and NEt₃ in a 1:1:1:2:2 molar ratio. The core structure consists of a Mn₂M₄ unit. The four M^III ions that are held together by two μ₄-bridging oxygen atoms form a butterfly M₄ moiety. The M₄ core is further connected to the two five-coordinate trigonal-bipyramidal Mn^III ions via one μ₄-O²⁻, two alkyloxo and one methoxo triple bridges. Magnetic susceptibility measurements indicate the presence of intramolecular antiferromagnetic interactions in complex 2, and overall intramolecular ferromagnetic interactions in complexes 3 and 4. The alternating current (AC) magnetic susceptibility studies revealed that complexes 3 and 4 showed frequency-dependent out-of-phase signals, which indicates that they exhibit slow relaxation of the magnetization.