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Ground State Modulation in Nickel(III) Chemistry by Controlling Axial Ligation in Complexes with N 3 O 2 Pentadentate Ligands
Author(s) -
Pinho Dora,
Gomes Paula,
Freire Cristina,
de Castro Baltazar
Publication year - 2001
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/1099-0682(200106)2001:6<1483::aid-ejic1483>3.0.co;2-x
Subject(s) - chemistry , pyridine , denticity , ligand (biochemistry) , nickel , electron paramagnetic resonance , coordination sphere , steric effects , ground state , coordination complex , metal , coordination geometry , stereochemistry , crystallography , molecule , medicinal chemistry , organic chemistry , nuclear magnetic resonance , biochemistry , physics , receptor , hydrogen bond , quantum mechanics
The accessibility of the +3 oxidation state for nickel(II) complexes with N 3 O 2 pentadentate Schiff base ligands prepared by condensation of naphthaldehyde or 3,5‐dichlorosalicylaldehyde with triamines (Scheme 1) has been assessed in several solvents, by combining cyclic voltammetry and EPR spectroscopy of the oxidised metal complexes. The data show that trien/Metrien‐based ligands act in pentadentate fashion and allow for the stabilisation of electrochemically and chemically generated nickel(III) complexes in all solvents used. The complexes, formulated as [Ni III L(solv)] + , are low‐spin and have a 2 A 1 , [ a d z 2+ b d x 2 −y 2], ground state, with a ⩾ b . For dien‐based ligands, steric constraints prevent coordination of the amine nitrogen atom of the pentadentate ligand to the metal centre, thus enforcing a fourfold coordination on the ligand. The resulting complexes can be oxidised to nickel(III) species, but only when using iodine as oxidant, in DMF or (CH 3 ) 2 SO. The complexes are formulated as [Ni III L(solv) 2 ] + , and are low‐spin and have a 2 A 1 , [ a d z 2+ b d x 2 −y 2], ground state, with a >> b . The binding of pyridine and cyanide ion to Ni III trien/Metrien‐based complexes was studied by EPR. The data obtained show that these polydentate ligands provide a flexible coordination sphere, and that by varying the ratio equatorial/axial 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 inverted ground state ( 2 A 1 , [ a d z 2+ b d x 2 −y 2], with a << b ) was observed, as the latter provides a higher axial ligand field. On the other hand, coordination of cyanide adducts induces a change in the principal molecular axis of the Ni III complexes, in which the cyanide ions occupy equatorial positions, and in which a 2 A 1 , [ a d z 2+ b d x 2 −y 2], ground state, with a >> b , is observed.