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 Sachindranath Pal a , Anil Kumar Barik b , Samik Gupta c , Somnath Roy c , Tarak Nath Mandal c , Arijit Hazra c , Mohamed Salah El Fallah d, * , Ray J. Butcher e , Shie-Ming Peng f , Gene-Hsiang. Lee f , Susanta Kumar Kar c, * a Department of Chemistry, Sree Chaitanya College, Habra, North 24 Parganas, West Bengal, India b Department of Chemistry, St. Paul’s C.M. College, 33/1, Raja Rammohan Roy Sarani, Calcutta 700 009, India c Department of Chemistry, University College of Science, 92 A.P.C. Road, Calcutta 700 009, India d Departament de Quı ´mica Inorga ` nica, Facultat de Quı ´mica, Universitat de Barcelona, Martı ´ i Franque `s 1-11, 08028 Barcelona, Spain e Department of Chemistry, Howard University, 2400 Sixth Street, N.W., Washington, DC 200 59, USA f Department of Chemistry, National Taiwan University, Taipei 106, Taiwan, ROC Received 4 July 2007; accepted 20 September 2007 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 Æ 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 H 2 L 1 = 1, 3-bis(5-methyl-3-formylpyrazolylmethinimino)propane-2-ol], and the same reaction when carried out with CoX 2 Æ 6H 2 O ðX ¼ CH 3 COO  ; ClO 4  NO 3  , BF 4  Þ produces mononuclear [Co(L 1 )(H 2 O) 2 ]X Æ H 2 O, of which only [Co(L 1 )(H 2 O) 2 ]CH 3 COO Æ H 2 O (3) was isolated. In contrast, the reaction of 5-methyl-3-formylpyrazole (MPA) and 1,3-diamino-2-propanol (DAP) with CoX 2 Æ 6H 2 O (where X ¼ 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 hygro- scopic 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 4 Cl 2 chromophore are in a distorted octahedral environment. These two outer cobalt(III) complexes individually act as bidentate che- lating 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 A ˚ (Co1Co2) and 3.791 A ˚ (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 elec- trochemical studies. The IR spectrum of 3 shows the typical vibration of ionic acetate at 1577 cm 1 (m asym ) and 1397 cm 1 (m sym ). Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Trinuclear mixed oxidation state pyrazole bridged cobalt complexes; Schiff bases; X-ray structures; Magnetic properties; EPR spectra 0277-5387/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2007.09.037 * Corresponding authors. Tel.: +91 033 24322936; fax: +91 033 23519755. E-mail addresses: Salah.elfallah@qi.ub.es (M.S. El Fallah), skkar_cu@yahoo.co.in (S.K. Kar). www.elsevier.com/locate/poly Available online at www.sciencedirect.com Polyhedron 27 (2008) 357–365