Electronic Tuning of the Lability of Pt(II) Complexes through π-Acceptor Effects. Correlations between Thermodynamic, Kinetic, and Theoretical Parameters

π-Acceptor effects are often used to account for the unusual high lability of [Pt(terpy)L](2-n)+ (terpy = 2,2‘:6‘,2‘ ‘-terpyridine) complexes. To gain further insight into this phenomenon, the π-acceptor effect was varied systematically
by studying the lability of [Pt(diethylenetriamine)OH2]2+ (aaa), [Pt(2,6-bis-aminomethylpyridine)OH2]2+ (apa), [Pt(N-(pyridyl-2-methyl)-1,2-diamino-ethane)OH2]2+ (aap), [Pt(bis(2-pyridylmethyl)amine)OH2]2+ (pap), [Pt(2,2'-bipyridine)(NH3)(OH2)]2+ (app), and [Pt(terpy)OH2]2+ (ppp). The crystal structure of the apa precursor [Pt(2,6-bis-aminomethylpyridine)Cl]Cl·H2O was determined. The substitution of water by a series of nucleophiles, viz. thiourea,
N,N-dimethylthiourea, N,N,N‘,N‘-tetramethylthiourea, I-, and SCN-, was studied under pseudo-first-order conditions
as a function of concentration, pH, temperature, and pressure, using stopped-flow techniques. The data enable an
overall comparison of the substitution behavior of these complexes, emphasizing the role played by the kinetic cis
and trans π-acceptor effects. The results indicate that the cis π-acceptor effect is larger than the trans π-acceptor
effect, and that the π-acceptor effects are multiplicative. DFT calculations at the B3LYP/LACVP** level of theory
show that, by the addition of π-acceptor ligands to the metal, the positive charge on the metal center increases,
and the energy separation of the frontier molecular orbitals (ELUMO − EHOMO) of the ground state Pt(II) complexes
decreases. The calculations collectively support the experimentally observed additional increase in reactivity when
two π-accepting rings are adjacent to each other (app and ppp), which is ascribed to “electronic communication”
between the pyridine rings. The results furthermore indicate that the pKa value of the platinum bound water molecule
is controlled by the π-accepting nature of the chelate system and reflects the electron density around the metal
center. This in turn controls the rate of the associative substitution reaction and was analyzed using the Hammett
equation.