From an <i>S</i>_T = 3 Single-Molecule Magnet to Diamagnetic Ground State Depending on the Molecular Packing of Mn^IIIsalen-type Dimers Decorated by <i>N</i>,<i>N</i><i>‘</i>-Dicyano-1,4-naphthoquinonediiminate Radicals

Two manganese(III) tetradentate Schiff-base dimers to which N,N‘-dicyano-1,4-naphthoquinonediiminate (DCNNQI) radicals are attached have been selectively synthesized by varying the solvents used in the reactions: [Mn2(5-MeOsaltmen)2(DCNNQI)2]·MeOH (1) and [Mn2(5-MeOsaltmen)2(DCNNQI)2]·2CH2Cl2·2CH3CN (2) [5-MeOsaltmen2- = N,N‘-(1,1,2,2-tetramethylethylene)bis(5-methoxysalicylideneiminate)]. These two complexes share the same molecular core, [(DCNNQI•-)−MnIII−(O)2−MnIII−(DCNNQI•-)], where −(O)2− is a biphenolate bridge in the out-of-plane dimerized [Mn2(5-MeOsaltmen)2]2+ moiety. However, their packing arrangements are completely different. Whereas complex 1 is found to be relatively isolated, strong intermolecular dimerization of the DCNNQI moieties (with the nearest contact being ∼3.0 Å) is observed in 2, forming a one-dimensional chain of [−MnIII−(O)2−MnIII−(DCNNQI•-)2−]∞. The magnetic susceptibility of 1 can be modeled with an [S = 1/2, 2, 2, 1/2] four-spin system including strong antiferromagnetic MnIII/DCNNQI radical coupling (JMn/rad/kB = −23 K) and ferromagnetic MnIII/MnIII coupling through the biphenolate bridge (JMn/Mn/kB = +2.0 K). These interactions lead to an ST = 3 ground state that possesses significant uniaxial anisotropy (DS=3/kB = −2.1 K). Low-temperature ac and dc magnetic data of 1 reveal its single-molecule magnet behavior with quantum tunneling of the magnetization. By contrast, 2 possesses the diamagnetic ground state induced by dominating MnIII−MnIII antiferromagnetic interactions mediated by the diamagnetic DCNNQI dimers and/or π−π contact along the b axis.