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

Abstract

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 CoX 2 · 6H 2O (X = Cl, Br) in a 2:1:1 ratio produces unusual trinuclear mixed-valent complexes [Co 3(L 1) 2Cl 4] ( 1) and [Co 3(L 1) 2Br 4] ( 2) [where H 2L 1 = 1, 3-bis(5-methyl-3-formylpyrazolylmethinimino)propane-2-ol], and the same reaction when carried out with CoX 2 · 6H 2O ( X = CH 3 COO - ,ClO 4 - NO 3 - , BF 4 - ) produces mononuclear [Co(L 1)(H 2O) 2]X · H 2O, of which only [Co(L 1)(H 2O) 2]CH 3COO · H 2O ( 3) was isolated. In contrast, the reaction of 5-methyl-3-formylpyrazole (MPA) and 1,3-diamino-2-propanol (DAP) with CoX 2 · 6H 2O (where X = ClO 4 - NO 3 - , BF 4 - ) in a 1:1:1 ratio produces mononuclear Schiff base complexes [Co(L) 2] ClO 4 ( 4), [Co(L) 2] NO 3 ( 5) and [Co(L) 2] BF 4 ( 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 N 4Cl 2 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 Å (Co1⋯Co2) and 3.791 Å (Co2⋯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 ( ν asym) and 1397 cm −1 ( ν sym).