Supramolecular Interactions Direct the Formation of Two Structural Polymorphs from One Building Unit in a One-Pot...

Two polymorphs of supramolecular isomers, a discrete dimer and a zig-zag chain, having the same chemical composition, [Mn(Hbit)Cl₂ ] (Hbit=1-methyl-2-(1H-1,2,3-triazol-4-yl)-1H-benzo[d]imidazole), were obtained solvothermally in a one-pot synthesis. The isomers differ in a number of ways: orange blocks versus pale-yellow needles, triclinic P1‾ versus orthorhombic Pbcn, double μ₂ -Cl versus alternate single and triple μ₂ -Cl, coordination number 5 versus 6, and antiparallel versus parallel near-neighbor orientation of Hbit. The packing in each case is driven by the supramolecular interactions, H-bonds (N-H⋅⋅⋅Cl, C-H⋅⋅⋅Cl) and π⋅⋅⋅π overlaps, calculated to be in the range 20-36 kcal mol^-1 . Calculations gave a difference of only 2 kcal mol^-1 in favor of the dimer, which confirms with the observation of principally the dimer at short reaction time. ESI-MS spectra of the dissolved crystals reveal the same fragments with similar distributions. The presence of two fragments at m/z 286.96 [Mn^IV (Hbit)Cl-2H]⁺ and 323.94 [Mn^III (Hbit)Cl₂ ]⁺ indicates that [Mn(Hbit)Cl₂ ] is the building unit in both cases; thus, the different orientations of the ligands lead to the two polymorphs stabilized by the respective supramolecular interactions. Importantly, the chain form represents the first example with alternate single and triple μ₂ -Cl bridges. The magnetic interactions are weakly antiferromagnetic in both cases, with J in the range 0.07-0.34 cm^-1 ; however, high-field EPR analysis reveals moderate magneto-anisotropy with D=0.26(1) cm^-1 , E=0.06(1) cm^-1 and D=0.17(1) cm^-1 , E=0.03(1) cm^-1 , respectively.