A unique single carboxylate-bridged spin-frustrated chiral Mn(ii) metallatriangle

Abstract

The reaction between Hmbpymca ligand (prepared in situ from the hydrolysis of 5-methyl-4-cyano-bispyrimidine with NaOH and further neutralization with 2 M HCl) and Mn(ClO₄)₂·4H₂O in 1 : 1 molar ratio afforded the triangulo-trimanganese(II) complex [Mn₃(bpymca)₃(H₂O)₆]Cl₃·6H₂O 1. The chloride anions in this complex come from the HCl used in the neutralization process. The molecular structure of 1 consists of cationic molecular triangles [Mn₃(μ-mbpymca)₃(H₂O)₆]³⁺ with C₃ symmetry, chloride anions and crystallization water molecules, all of them involved in an extensive network of hydrogen bonds, leading to a chiral network. Within the [Mn₃(μ-mbpymca)₃(H₂O)₆]³⁺ cations, seven-coordinated Mn(II) ions are bridged by both oxygen atoms of the carboxylate groups and exhibit a MnO₅N₂ compressed pentagonal bipyramidal coordination environment. The temperature dependence of the magnetic susceptibility shows the presence of weak antiferromagnetic interactions between Mn^II ions mediated by the carboxylate group of the mbpymca ligand and the existence of a 3D antiferromagnetic ordering below 4 K, which has its origin in the AF inter-trimer exchange interactions mediated by the strong hydrogen bonds present in the crystal of 1. The experimental magnetic susceptibility data above 7 K could be satisfactorily fitted to the theoretical analytical expression derived from the spin Hamiltonian H = −J(S₁S₂ + S₁S₃ + S₂S₃) with J = −0.782(3) cm⁻¹ and g = 2.092(3). The model predicts a degenerate ground state with an S = 1/2, which is typical of triangular trimetallic spin frustrated systems containing metal with non-integer spins. DFT calculations were performed on the molecular structure as found in the solid state to support the experimental J value and the Mn–O_carb–Mn as the primarily exchange pathway.