Specific chelate tuning of the substitution kinetics of platinum(II) complexes in aqueous solution

Both reaction steps observed for the substitution of water by thiourea in the complexes [Pt(en)(OH2)(2)](2+) and [Pt(phen)(OH2)(2)](2+) (en = ethylenediamine, phen = 1,10-phenanthroline) were investigated under pseudo-first-order conditions using the stopped-flow technique, The substitution of the second water molecule in each complex was also studied under high pressure, The observed pseudo-first-order rate constants k(obs) (s(-1)) obeyed the equation k(obs)(1,2) = k(1,2)[tu] (tu = thiourea), where "1" and "2" refer to the first and the second substitution reactions, respectively. Kinetic parameters associated with the substitution process are: k(en)(1) (25.0 degrees C, pH = 3.0, 1 = 0.1 M) = 25.6 M-1 s(-1), Delta H-# = 51 kJ mol(-1), Delta S-# = -48 J K-1 mol(-1); k(en)(2) (same conditions)= 12.1 M-1 s(-1), Delta H-# = 30 kJ mol(-1), Delta S-# = -124 J K-1 mol(-1), Delta V-# = -7 cm(3) mol(-1); k(phen)(1) (25.0 degrees C, pH = 1.0, I = 0.1 M) = 2900 M-1 s(-1), Delta H-# = 41 kJ mol(-1), Delta S-# = - 41 J K-1 mol(-1); k(phen)(2) (same conditions) = 1170 M-1 s(-1), Delta H-# = 37 kJ mol(-1), Delta S# = -61 J K-1 mol(-1), Delta V-# = -5 cm(3) mol(-1), The temperature and pressure dependence of all the processes studied suggest an associative substitution mechanism. The hydroxo-bridged dinuclear complex [{Pt(phen)(mu-OH)}(2)](2+) is formed from [Pt(phen)(OH2)(2)](2+) in aqueous solution unless the solution is very dilute and highly acidic. The X-ray structure of [{Pt(phen)(mu-OH)}(2)](F3CSO3)(2) . 2 H2O was determined, It belongs to the triclinic space group P (1) over bar and has one formula unit in the unit cell. The unit cell dimensions are a = 7.126(5), b = 9.665(5), c = 12.774(7) Angstrom; alpha = 71.85(5), beta = 85.52(5), gamma = 73.12(5) deg; V = 799.9(8) Angstrom(3). The structure was solved with the Patterson method and refined to R = 0.061. A square planar coordination of the platinum centers is observed, with no deviations from planarity but distortions due to the small bite angle of phen and the four-membered ring. No significant lengthening of the Pt-O bond [mean value: 2.03(1) Angstrom] is observed in comparison with [{Pt(NH3)(2)(mu-OH)}(2)](2+).