Enhancement of the coordinating flexibility in a Schiff–Mannich combo ligand: forced generation of a new NiII–Ophenoxo–LnIII–Oalkoxo–LnIII array (Ln = Gd, Tb, Dy and Ho)

3d–4f coordination complexes derived from appropriately designed ligand backbones offer great perspectives to construct functional magnetic materials. Here we have reported a ligand design strategy by introducing flexible ethanol amine arms in a conventional salen type ligand by Schiff base and Mannich reaction and shown how this Schiff–Mannich combo ligand approach afforded a series of NiIILnIII2 (Ln = Gd, Tb, Dy and Ho) complexes. We have reacted the ligand with NiII to generate a metalloligand and by means of secondary assembly of this metalloligand with LnIII salts, the above mentioned NiIILnIII2 series have been realized. As far as our knowledge goes, this NiII–LnIII–LnIII topology has not been reported in the literature to date. In-depth structural analysis of the ligand and the complexes has been carried out and this structural understanding will enable us to correlate the structure–property relationship in a better way. Magnetic studies indicated that the DyIII analogue exhibits the slow relaxation of magnetization behavior under 800 Oe external dc field with energy barrier 2.94 K.