Tetradentate Schiff base chelating ligand. Prepn: A. T. Mason, Ber. 20, 267 (1887); and spectral analysis of Cu(II) complex: G. V. Panova et al., J. Gen. Chem. USSR 53, 1452 (1983). Prepn of transition metal complexes: P. Pfeiffer et al., Ann. 505, 84 (1933); and their physicochemical properties: A. Z. El-Sonbati et al., Transition Met. Chem. 17, 66 (1992); M. M. Bhadbhade, D. Srinivas, Inorg. Chem. 32, 6122 (1993). Mass spectra of transition metal complexes: S. M. Schildcrout et al., ibid. 34, 4117 (1995). Thermodynamic properties of salen and its metal complexes: F. Lloret et al., Inorg. Chim. Acta 189, 195 (1991). Use of Mn(III) complex as catalyst in epoxidation of olefins: K. Srinivasan et al., J. Am. Chem. Soc. 108, 2309 (1986); as specific DNA cleaving agent: D. J. Gravert, J. H. Griffin, J. Org. Chem. 58, 820 (1993).