Mercaptoethanesulfonic acid studies with nickel(II) complexes of tetra- and hexadentate ligands containing pyridyl groups: synthesis, structure, spectra and redox behavior T. Pandiyan a, *, V.M. Consuelo-Estrada b , R. Moreno-Esparza a , L. Ruiz-Ramı ´rez a a Facultad de Quı ´mica, Universidad Nacional Auto ´noma de Me ´xico (UNAM), Ciudad Universitaria, Coyoaca ´n, 04510 Me ´xico D.F., Mexico b Facultad de Quı ´mica, Universidad Auto ´noma de Estado de Me ´xico, Toluca, Mexico Received 15 March 2002; accepted 10 July 2002 Abstract Nickel(II)complexes of N ,N ,N ?N ?-tetrakis(2-pyridylmethyl)ethylenediamine (L 1 ), N ,N ,N ?N ?-tetrakis(2-pyridylmethyl)-1,2-dia- minopropane (L 2 ), N ,N ,N ?N ?-tetrakis(2-pyridylmethyl)-1,3-diaminopropane (L 3 ), N ,N ?-bis(2-pyridylmethyl)ethylenediamine (L 4 ), N ,N ?-bis(2-pyridylmethyl)-1,2-diaminopropane (L 5 ) and N ,N ?-bis(2-pyridylmethyl)-1,3-diaminopropane (L 6 ) were prepared and their spectroscopic and redox behaviors studied. The distorted octahedral geometry was determined for [NiL 2 ](ClO 4 ) 2 by using X- ray crystallography. The electronic spectral bands ( 3 A 2g 0/ 3 T 2g and 3 A 2g 0/ 1 E g transitions) of NiN 6 and NiN 4 O 2 were analyzed; it shows that the former complexes possess well-resolved bands compared to the latter compounds. In addition, the ligand field parameter Dq found for NiN 4 O 2 complexes was much lower than that of NiN 6 compounds; thereby, a more positive redox potential was detected for NiN 4 O 2 complexes in electrochemistry over the NiN 6 compounds. Further, the coordination studies of 2- mercaptoethanesulfonic acid as a simulator of coenzyme M reductase (CoM) with NiN 4 or NiN 6 chromophores were discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Ni(II) complexes; Nephelauxetic effects; NiN 6 chromophores; Redox potentials 1. Introduction The chemistry of metal complexes with chelate ligands is interesting to study in order to mimic the metabolic or catabolic process of biological systems. Although the coordination chemistry of various elements with hex- adentate ligands containing pyridyl groups was con- siderably studied, the corresponding chemistry of these ligands has not been extended to nickel ion. It is also a metal used to coordinate both hard and soft donor ligands; besides, its coordination ability allows a variety of geometries and oxidation states to be encompassed, with reactivity ranging from that in biological systems to organometallic chemistry. For example, the structural examination of the following compounds, M(tpen) [tpen /N ,N ,N ?N ?-tetrakis(2-pyridylmethyl)ethylene- diamine; M /Fe(II), Cu(II), Co(III), Mo(V), W(V)] [1  / 5]; M(tppn) [tppn /N ,N ,N ?N ?-tetrakis(2-pyridyl- methyl)-1,2-diaminopropane, M /Fe(II), Co(III), Eu(III), Mo (V), W(V)] [1,3,5  /8]; M(tptn) [tptn / N ,N ,N ?N ?-tetrakis(2-pyridylmethyl)-1,3-diaminopro- pane, M /Fe(II), Co(III)] [1,3] and M(bmped) [bmped /N ,N ?-bis(2-methylpyridyl)ethane-1,2-dia- mine, M/Mn(III, IV), Cu(II), Cr(III)] [9  /12] was established. However, the nickel compounds being used with these tetra-/hexadentate ligands containing pyridyl has not been significantly investigated, although there have been numerous reports published with different perspectives of the nickel(II) bound to ligands containing the amine nitrogen or thioether sulfur as coordinating atoms [13  /22]. The reported studies in- dicate that the nickel ion is an essential component for the metabolic enzyme process of acetogenic and metha- nogenic organisms [23]. Furthermore, the involvement of organonickel species has been shown to be an important factor in the mechanisms of metalloenzyme reactions such as methyl-S -coenzyme M reductase, * Corresponding author. Fax:  /52-5-622 3712 E-mail address: pandiyan@servidor.unam.mx (T. Pandiyan). Inorganica Chimica Acta 343 (2003) 79  /89 www.elsevier.com/locate/ica 0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0020-1693(02)01194-5