Coordination pyramid inversion, angular cluster core and homochiral single-stranded helix formation in polynuclear Mn(ii) assemblies supported by a bicompartmental salen-type ligand based on an o-xylylenediamine platform

Abstract

Although salen-type compartmental ligands have a rich history, these fascinating compounds still attract considerable attention due to the wide potential applications that their complexes can exhibit. By varying the flexibility of the spacer group, connecting two NO-chelating salicylideneamine moieties, and adjusting the type of anchored additional binding sites or co-ligands used, the formation of 3d metal clusters with the desired nuclearity can be achieved. In this work, we report the synthesis of two new binuclear {Mn^II₂} (1-Mn₂Cl₃ and 1-Mn₂Cl₂) clusters and one angular (pseudo-linear) trinuclear {Mn^II₃} (1₂-Mn₃Cl₂) cluster, supported by an o-xylylenediamine Schiff base derivative, bearing o-methoxy-substituted salicylideneamine units and acting as a bicompartmental ligand. The structures of prepared Mn-based complexes were established by single crystal X-ray diffraction (SCXRD), revealing the role of o-xylylenediamine platform on the diversity of observed coordination motifs. For the first time, in the case of {Mn^II₂} clusters, displaying a similar coordination motif, it was established that the Mn atom, accommodated within the N₂O₂ coordination pocket, can adopt two different orientations of the MnN₂O₂Cl coordination pyramid relative to the o-xylylene spacer group. Additionally, the simultaneous effect of H-bonding, electrostatic and CH/π interactions between the anionic 1-Mn₂Cl₃ cluster and the NHEt₃ cation species leads to the formation of helices, stacked in a homochiral crystal with the polar chiral P4₁ space group, which is reported for the first time for complexes based on salen-type ligands. Finally, the formation of the trinuclear complex 1₂-Mn₃Cl₂, exhibiting an unusual angular motif of the cluster core, stabilized by the ligand structure and intramolecular CH/π-bonding, is observed.