Spectroscopic and magnetic properties and structure of a five-coordinate, O2-binding cobalt(II) Schiff base complex and of the copper(II) analogue

The cobalt(II) complex CoL and the copper(II) complex CuL of the pentadentate O2N2N′ Salen-type Schiff base ligand H2L =N, N′-bis(3-t-butyl-2-hydroxy-5-methylbenzyliden)-1, 7-diamino-4-methyl-4-azaheptane were prepared, characterized and subjected to single crystal X-ray analysis. The {MO2N2N′} coordination core forms a trigonal bipyramid differing in the degree of distortion, as expressed by the O-M-O angles 138.62(8)° [O-Co-O] and 155.21(7)° [O-Cu-O]. The distances between the metal and the aliphatic tertiary nitrogen atoms N′ are markedly different, 2.123(2) [Co-N′] and 2.300(2)Å[Cu-N′]. In contrast to normal Salen complexes the imine nitrogen atoms are located trans to each other. Solution room temperature electron spectra show a broad absorption in the near-IR range at 6350 (CoL) and 8200 (CuL) cm−1. The temperature dependence of the effective magnetic moment for high-spin CoL (S =3/2) shows a maximum at 50 K, which is reproduced by the following set of magnetic parameters, reflecting the zero-field splitting; g1 = 1.68, g1 = 1.89, g = 3.21, D/hc = −38.9cm−1, E/hc = 1.7cm−1, α110 = 0, zJ/hc = −1.00cm−1. Variable temperature electron paramagnetic resonance experiments prove that CoL, dissolved in toluene, N, N-dimethylformamide or pyridine, binds dioxygen reversibly.