Abstract/Summary

Three new heterometallic Cu(II)–Mn(II) complexes, [{(CuL)2Mn}2(μ1,5-N(CN)2)(CH3CN)2](ClO4)3 (1), [(CuL)2Mn(N(CN)2)2]·(H2O) (2), and [(CuL)2Mn(μ1,5-N(CN)2)2]n (3), have been synthesized using a Cu(II)-metalloligand of an asymmetrically dicondensed Schiff base ligand (where H2L = N-α-methylsalicylidene-N′-salicylidene-1,3-propanediamine). Complex 1 was formed when the ratio of [CuL]/Mn(ClO4)2/NaN(CN)2 was 2:1:1, whereas complexes 2 and 3 were obtained with a 2:1:2 ratio of the same reactants on varying the reaction conditions. Single-crystal structural analyses reveal that complex 1 possesses a hexanuclear structure in which two (CuII)2MnII units are connected by one μ1,5-N(CN)2– bridge, 2 is a discrete trinuclear species with two terminally coordinated N(CN)2– ions to the Mn(II), whereas complex 3 is a polymeric form of 2 with μ1,5-N(CN)2– bridges between Cu(II) and Mn(II) centers. The thermal variations of dc magnetic susceptibilities suggest that all three complexes (1–3) are antiferromagnetically coupled with comparable exchange coupling constants (−25.4, −22.8, and −22.0 cm–1, respectively) which are expected from the Cu–O–Mn angles. All the complexes show biomemitic phenoxazinone synthase-like activity for the aerial oxidation of o-aminophenol to amino phenoxazinone. The turnover numbers (kcat) for the process are 4966, 2021, and 1107 h–1 for complexes 1–3 respectively. The mass spectral evidence on intermediates suggests that the cooperative activity of the two different metal ions, i.e., coordination of substrate to Mn(II) and shuttling of oxidation state of Cu between I and II, is possibly operative in the oxidation process. The highest catalytic activity of 1 is attributed to the presence of one coordinating solvent molecule to Mn(II).