Abstract:
Substitution behavior of the labile aqua ligand in four mononuclear ruthenium(II)
terpyridyl complexes with diferent auxiliary N (pyridine) (Ru1), N^N (2,2′-bipyridyl (Ru2), 2′-(2-pyridyl)quinoline (Ru3), 2,2′-biqunoline (Ru4) ligands was investigated using three nucleophiles; thiourea, 1,1-dimethylthiourea and 1,1,3,3-tetramethylthiourea. The efect of concentration and temperature on the substitution
behavior of the complexes were studied under pseudo-frst order conditions using
UV–Vis spectrophotometer. The second order rate constants (k2) of the aqua complexes decreased in the order: Ru4>Ru3>Ru1>Ru2. The results showed that the
rate of substitution of the aqua ligand increased with an increase in the π-surface
area of the N^N bidentate auxiliary ligands. This is attributable to an upsurge in
π-back-donation and electrophilicity of the complexes as the π-extension of the auxiliary ligands increases. Ru2 is less reactive than Ru1 due to the increased steric hinderance introduced by the 2,2′-bipyridyl bidentate auxiliary ligand in Ru2 compared
to Ru1 which has two independent trans pyridines. From computational results,
it was observed that as the aromatic surface area of the auxiliary ligand increased
from Ru1–Ru4, the HOMO–LUMO gap decreased accordingly. Consequently, the
chemical softness and electrophilicity of the complexes increased accordingly. This
is corroborated by the decrease in pKa values of the complexes as one moves from
Ru1 to Ru4. As a result, the nucleophilic attack becomes facile from Ru1 to Ru4.
All the reactions follow an associative interchange mechanism as indicated by the
positive activation enthalpy and negative activation entropy. The crystal structure
of bipyridylterpyridylthiourearuthenium(II) perchlorate show that the substitution
product obtained is stable.
Keywords Substitution · Ruthenium(II) · Kinetics · Auxilliary ligands ·
Nucleophiles
