Reductive electrochemical study of Ni(II) complexes with N 2 O 2 Schiff base complexes and spectroscopic characterisation of the reduced species. Reactivity towards CO Fernando Azevedo, Cristina Freire *, Baltazar de Castro CEQUP/Departamento de Quı ´mica, Faculdade de Cie ˆncias, Universidade do Porto, Rua do Campo Alegre, 687, 4169-007 Porto, Portugal Received 20 November 2001; accepted 16 April 2002 Abstract Reductive electrochemical properties of series of nickel(II) complexes with salen ligands, which have different diimine bridges and substituents in the aldehyde moiety have been studied in several solvents (CH 3 CN, dmf and (CH 3 ) 2 SO). In order to assess the relative importance of the Ni(I) and Ni(II) anion radical species, the reduced species have been characterised by combining EPR and UV /Vis spectroscopy. The results have shown that complexes with aliphatic diimine bridges are reduced to four-coordinate Ni(I) species with a B 1 g (d xy ) 1 ground state, whereas those with aromatic diimine bridges are reduced to square /planar Ni(II) anion radical species that rapidly dimerise. None of the reduced species was found to bind pyridine, imidazole and triphenylphosphine, but in the presence of the stronger p-acceptor ligand CO, new Ni(I) species were formed that, and on the basis of EPR data, can be formulated as five-coordinate complexes with a B 1g (d xy ) 1 ground state, [NiL × /CO]  . These new species are more stable than the parent complexes as confirmed by the more positive E 1/2 values as a consequence of the extensive p delocalisation M0/CO. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Salen ligands; Ni(I) complexes; Nickel(II) complexes; EPR 1. Introduction The chemistry of polydentate nickel(I) complexes has attracted attention since they can act as powerful catalysts on chemical or electrochemical reduction of electrophiles, such as alkyl and aryl halides [1  /3] and carbon dioxide [4]. The ability of the starting nickel(II) complex to form upon one-electron transfer a nickel(I) complex, rather than the anion radical of the ligand, appears as a key point for obtaining an efficient catalysis. Nickel(I) complexes are expected to react with electrophiles by transfer of their metal centred unpaired electron in an inner-sphere fashion, being more efficient and selective than Ni(II) anion radicals, which function as an outer-sphere electron donor due to the delocalised nature of the ligand based unpaired electron. Salen ligands can easily stabilise low and high oxidation states of nickel and reduced [Ni(salen)] is used in the electro-reduction of alkyl and aryl halides [1a,1c,1d]. By introducing substituents in the ligand it is possible to modulate the potential at which the reduc- tion occurs, and to control concomitantly the catalytic properties of the complexes. As salen has the desirable characteristic of being readily subject to systematic modification of its electronic and steric properties by synthetic approaches, we have prepared a series of nickel(II) complexes with salen derivatives that have different diimine bridges and substituents in the alde- hyde moieties (Scheme 1). The reductive behaviour of the resulting complexes was studied in several solvents and the reduced species characterised by combining electrochemical, EPR and UV  /Vis spectroscopy in order to assess the relative importance of the Ni(I) and Ni(II) anion radical species. As our goal is to use these complexes as catalysts, we report also the reactivity of the reduced species towards p acceptor Lewis bases. Some of the complexes have already been prepared and * Corresponding authors. Tel.:  /351-22-6082-890; fax:  /351-22- 6082-959 E-mail address: acfreire@fc.up.pt (C. Freire). Polyhedron 21 (2002) 1695  /1705 www.elsevier.com/locate/poly 0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII:S0277-5387(02)01025-2