Influence of the bridging azine ligand on the rate of ligand substitution in a series of dinuclear platinum(II) complexes

Abstract

AbstractA series of azine‐bridged dinuclear platinum(II) complexes of the type [{trans‐Pt(NH3)2(OH2)}2(μ‐azn)](ClO4)4(where azn = pyrazine (pzn,Pt1), 2,3‐dimethylpyrazine (2,3‐pzn,Pt2), and 2,5‐dimethylpyrazine (2,5‐pzn,Pt3)) were synthesized to investigate the influence of the bridging azine ligand on the reactivity of the platinum(II) centers. The pKavalues of the complexes were determined via acid–base titration, and the rate of substitution of the aqua moiety by a series of neutral nucleophiles, viz. thiourea (TU), 1,3‐dimethyl‐2‐thiourea (DMTU), and 1,1,3,3‐tetramethyl‐2‐thiourea (TMTU), was determined under pseudo‐first‐order conditions as a function of concentration and temperature using standard spectrophotometric techniques. The introduction of the methyl groups to the bridging azine linker inPt2andPt3leads to a moderate increase in the pKavalues obtained for the first and second deprotonation steps, respectively, as a result of the increased σ‐donor capacity of the bridging azine ligand trans to the aqua moiety. A comparison of the rate constants,k1andk2, at 298 K, obtained for the substitution of the aqua moieties fromPt1,Pt2, andPt3by TU, shows that the introduction of the σ‐donating methyl groups on the bridging azine ligand inPt2andPt3results in a corresponding decrease in the reactivity, by ca. five times for the first substitution step and ca. 10 times for the second substitution step. Density functional theory calculations at the B3LYP/LACVP** level of theory for the complexes demonstrate that the introduction of electron‐donating methyl groups results in (i) increased steric hindrance over the metal centers and (ii) decreased the positive charge on the metal center and increases energy separation of the frontier molecular orbitals (EHOMO–ELUMO) of the ground‐state platinum(II) complexes, leading to a less‐reactive metal center. © 2011 Wiley Peiodicals, Inc. Int J Chem Kinet 43: 161–174, 2011