Anion dependent formation of linear trinuclear mixed valance cobalt(III/II/III) complexes and mononuclear cobalt(III) complexes of a pyrazole derived ligand - Synthesis, characterization and X-ray structures

Trinuclear mixed oxidation state Co(III/II/III) complexes with a different coordination geometry at each cobalt center of a pyrazole derived Schiff base and mononuclear cobalt (III) complexes of another pyrazole containing Schiff base having N-donor atoms have been prepared and structurally characterized. Reaction of 5-methyl-3-formylpyrazole (MPA) and 1,3-diamino-2-propanol (DAP) with CoX2 center dot 6H(2)O (X = Cl, Br) in a 2:1:1 ratio produces unusual trinuclear mixed-valent complexes [Co-3(L-1)(2)Cl-4] (1) and [Co-3(L-1)(2)Br-4] (2) [where H2L1 = 1, 3-bis(5-methyl-3-formylpyrazolylmethinimino)propane-2-ol], and the same reaction when carried out with CoX2.6H(2)0 (X = CH3COO-, ClO4- NO3-, BF4-) produces mononuclear [Co(L-1)(H2O)(2)]X center dot H2O, of which only [Co(L-1)(H2O)(2)]CH3COO center dot H2O (3) was isolated. in contrast, the reaction of 5-methyl-3-formylpyrazole (MPA) and 1,3-diamino-2-propanol (DAP) with CoX2 center dot 6H(2)O (where X = ClO4-NO3-, BF4-) in a 1:1:1 ratio produces mononuclear Schiff base complexes [Co(L)(2)] ClO4 (4), [Co(L)(2)] NO3 (5) and [Co(L)(2)] BF4 (6) [where HL = 1-(5-methyl-3-formylpyrazolylmethinimino)-3-aminopropane-2-ol], whereas when X = Cl and Br a hygroscopic intractable material was isolated. Among the above species, 1 and 4 have been crystallographically characterized. In 1, three cobalt atoms are arranged in a linear fashion and are bridged through pyrazolate rings. The terminal diamagnetic low spin cobalt(III) ions with a N4Cl2 chromophore are in a distorted octahedral environment. These two outer cobalt(III) complexes individually act as bidentate chelating ligands and encapsulate the central cobalt(II) ion, which adopts a distorted flattened, tetrahedral geometry with a N-4 chromophore. The interatomic separations are 3.794 angstrom (Co1 center dot center dot center dot Co2) and 3.791 angstrom (Co2 center dot center dot center dot Co3). In 4, the cobalt(III) atom is also in a distorted octahedral geometry with a N-6 chromophore. Variable temperature (300-2 K) magnetic susceptibility measurements have been carried out for 1 and 2. Zero field splitting of the tetrahedral cobalt(II) ion has been noticed. The EPR spectrum of 1 and 2 at low temperature is associated with a Kramer doublet +/- 1/2 arising from the S = 3/2 ground state. Some of the complexes are further characterized by UV-Vis, IR and electrochemical studies. The IR spectrum of 3 shows the typical vibration of ionic acetate at 1577 cm(-1) (V-asym) and 1397 cm(-1) (V-sym). (C) 2007 Elsevier Ltd. All rights reserved.