Abstract:
The rates of substitution of chloro ligands from a series of
ruthenium(II) complexes, [Ru(j3
-L)(PPh3)Cl2] (L ¼ 2,20
:60
,200-terpyridine, 1; 40
-(4-methylphenyl)-2,20
:60
,200-terpyridine, 2; 4,40
400-tri-tertbutyl-2,20
:60
,200-terpyridine, 3; 40
-(4-chlorophenyl)-2,20
:60
,200-terpyridine, 4; 4-chloro-2,20
:60
,200-terpyridine, 5 and 2,6-bis(2-pyrazolyl)-
pyridine, 6), by thiourea nucleophiles was investigated under
pseudo-first-order conditions in methanol as a function of nucleophile concentration and temperature. The chloro ligands were
substituted in two steps and the reactivity trend was 4 > 5 >
2 > 1 > 6. Complexes 2 and 3 having donor substituents on the
2,20
:60
,200-terpyridine backbone experience a trans-effect making
them more reactive than 1. Complexes 4 and 5 are more reactive
than 1 due to enhanced p-back-bonding brought about by electron-withdrawing substituents on their 2,20
:60
,200-terpyridine backbones. The reactivity of 4 is higher than 5 due to greater electron
acceptor-ability of the chlorophenyl substituent than the chloro
substituent in 5. The 2,6-bis(pyrazolyl)pyridine ligand in 6 retards
the reactivity of the complex compared to 1 due to the cis-donor
effect of the pyrazole. The reactivity of the complexes is associative for all nucleophiles in step one and only thiourea in step
two. The substitution reactions proceed by a steady changeover
from an associative interchange mechanism (Ia) to a dissociative
interchange (Id) mechanism on increasing steric hindrance.
