Structural and Magnetic Studies of Two-Dimensional Solvent-Free Manganese (II) Complexes Prepared via Ligand Exchange Reaction under Solvothermal Conditions

Systematic investigation of the ligand exchange reactions between manganese(II) acetate and benzoic acid under solvothermal conditions led to the isolation of crystalline complexes {Mn₅(OC(O)CH₃) ₆(OC(O)C₆H₅)₄}^∞ (1) and {Mn₅(OC(O)CH₃)₄(OC(O)C₆H ₅)₆}_∞ (2) in high (i.e., >90%) yields. The complexes are characterized structurally as 2-D honeycomb-like sheets comprised of edge-shared Mn₁₂ loops with some noteworthy differences as follows. First, buckling of the 2-D sheet in 1 is not observed for 2, presumably as a consequence of additional intersheet phenyl groups in the latter. Second, complex 1 is comprised of only six-coordinate Mn(II), while 2 has both pseudo-octahedral and distorted trigonal bipyramidal coordinate metal ions. Third, while complex 2 exhibits π-stacking interactions with intersheet phenyl-phenyl contacts of 3.285 and 3.369 Å, 1 exhibits no such bonding. Antiferromagnetic exchange is observed with Weiss constants (θ) of -28 and -56 K and Neel temperatures of 2.2 and 8.2 K for complexes 1 and 2, respectively. The paramagnetic transition at higher temperatures for complex 2 may be attributed to π-π exchange through phenyl groups in adjacent layers. Preliminary gas sorption studies (76 K) indidate preferential adsorption of H₂ versus N₂ for complex 1 only.