Tuning Reactivity of Platinum(II) Complexes

Chapter 6
Tuning Reactivity of Platinum(II) Complexes with Parametalated
Pyridine Spacer Groups: A kinetic and Mechanistic Study
6.0 Abstract
The substitution reactions of dinuclear Pt(II) complexes containing two platinum
centres connected by a pyridine bridging ligand of variable length, viz. [{cis–
Pt(OH2)(NH3)2}2–μ–L] +4, L = 4,4’-bis(pyridine)sulphide (Pt1), 4,4’-
bis(pyridine)disulphide (Pt2) and 1,2-bis(4-pyridyl)ethane (Pt3), were investigated
with S-donor nucleophiles (TU, DMTU and TMTU) and anionic nucleophiles (SCN ¯ , I ¯ and
Br ¯) under pseudo first-order conditions as a function of nucleophile concentration and
temperature, using stopped-flow and UV-Vis spectrophotometric methods. In addition,
spectrophotometric acid-base titrations were performed to determine the pKa values of
the platinum bound aqua ligands. The pKa values of the aqua ligands, namely; Pt1 (pKa1:
4.86; pKa2: 5.53), Pt2 (pKa1: 5.19; pKa2: 6.42), Pt3 (pKa1: 5.04; pKa2: 5.45) show a direct
correlation between the Pt---Pt distance and acidity of the coordinated water molecules.
The substitution reactions of the dinuclear Pt(II) complexes with a series of nucleophiles
occurred in two sequential steps. The second-order rate constants for the simultaneous
displacement of aqua ligands in the first step, decreased in the order Pt2 > Pt3 > Pt1.
The DFT calculations for the corresponding Pt1 complex clearly demonstrated that the
structure of the complex is distorted due to non–planarity of the ligand, where
repulsions from the two lone pairs on S atom and the pyridine rings prevents Pt1 to
exhibit a perfect C2v symmetry. This distortion is the reason for the lower reactivity of
Pt1 as the aerial attack of a nucleophile on the Pt(II) centre is sterically hindered. The
complexes Pt2 and Pt3, both adopt C2h molecular point group symmetry. The lability of
Pt3 is 2 times slower than that of Pt2. This could be explained by the higher value of the
dipole moment of Pt2, which exhibits greater electrophilicity at the metal centre and can
account for acceleration of nucleophilic substitution process as compared to Pt3. These
reactions show that a small change on the structure of the bridging ligand influences the
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