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

ABSTRACT

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 [MnIV (Hbit)Cl-2H]+ and 323.94 [MnIII (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.