Bi-Compartmental Schiff-Base with Peripheral Ester Functionalization: Synthesis and Magnetic Behavior of Bimetallic Zn-Ln Complexes (Ln = Dy, Tb, Gd): Bi-Compartmental Schiff-Base with Peripheral Ester Functionalization: Synthesis and Magnetic Behavior of Bimetallic Zn-Ln Complexes (Ln = Dy, Tb, Gd)