From an S_T = 3 single-molecule magnet to diamagnetic ground state depending on the molecular packing of Mn^IIIsalen-type dimers decorated by N,N′-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-MeOsaltmen^2- = N,N′-(1,1,2,2-tetramethylethylene)bis(5- methoxysalicylideneiminate)]. These two complexes share the same molecular core, [(DCNNQI^•-)-Mn^III-(O)₂-Mn ^III-(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 [-Mn^III-(O)₂-M^III-(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 Mn^III/DCNNQI radical coupling (J _Mn/rad/k_B = -23 K) and ferromagnetic Mn ^III/Mn^III coupling through the biphenolate bridge (J _Mn/Mn/k_B = +2.0 K). These interactions lead to an S _T = 3 ground state that possesses significant uniaxial anisotropy (D_S=3/k_B = -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 Mn^III-Mn^III antiferromagnetic interactions mediated by the diamagnetic DCNNQI dimers and/or π-π contact along the b axis.