Ground state modulation in nickel(III) chemistry by controlling axial ligation in complexes with N3O2 pentadentate ligands

The accessibility of the +3 oxidation state for nickel(II) complexes with N 3O2 pentadentate Schiff base ligands prepared by condensation of naphthalde hyde or 3,5- dichlorosalicylaldehyde with triamines (Scheme 1) has been ass essed in several solvents, by combining cyclic voltammetry and EPR spectros copy of the oxidised metal complexes. The data show that trien/Metrien-base d ligands act in pentadentate fashion and allow for the stabilisation of el ectrochemically and chemically generated nickel(III) complexes in all solve nts used. The complexes, formulated as [(NiL)-L-III(solv)](+), are low-spin and have a (2)A(1), [a d(z)(2) + b d(x)(2)-(2)(y)], ground state, with a g reater than or equal to b. For dien-based ligands, steric constraints preve nt coordination of the amine nitrogen atom of the pentadentate ligand to th e metal centre, thus enforcing a fourfold coordination on the ligand. The r esulting complexes can be oxidised to nickel(III) species, but only when us ing iodine as oxidant, in DMF or (CH3)(2)SO. The complexes are formulated a s [(NiL)-L-III(solv)(2)](+), and are low-spin and have a (2)A(1), [a d(z)(2 ) + b d(x)(2)-(2)(y)], ground state, with a much greater than b. The bindin g of pyridine and cyanide ion to Ni-III trien/Metrien-base complexes was st udied by EPR, The data obtained show that these polydentate ligands provide a flexible coordination sphere, and that by varying the ratio equatorial/a xial ligand field it is possible to obtain Ni-III in three different ground states. For pyridine adducts, the same ground state as the parent complex was observed with Metrien-based ligands, while for trien-based ligands an i nverted ground state ((2)A(1), [a d(epsilon)(2) + b d(x)(2)-(2)(y)], with a much less than b) was observed, as the latter provides a higher axial liga nd field. On the other hand, coordination of cyanide adducts induces a chan ge in the principal molecular axis of the Ni-III complexes, in which the cy anide ions occupy equatorial positions, and in which a (2)A(1), [a d(z)(2) + b d(x)(2)-(2)(y)], ground state, with a much greater than b, is observed.