Spectroscopic characteristic (FT-IR, 1 H, 13 C NMR and UV–Vis) and theoretical calculations (MEP, DOS, HOMO-LUMO, PES, NBO analysis and keto–enol tautomerism) of new tetradentate N,N′-bis(4-hydroxysalicylidene)-1,4-phenylenediamine ligand as chelating agent for the synthesis of dinuclear Co(II), Ni(II), Cu(II) and Zn(II) complexes