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Specific Chelate Tuning of the Substitution Kinetics of Platinum(II) Complexes in Aqueous Solution
Author(s) -
Fekl Ulrich,
van Eldik Rudi
Publication year - 1998
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/(sici)1099-0682(199803)1998:3<389::aid-ejic389>3.0.co;2-q
Subject(s) - chemistry , associative substitution , aqueous solution , thiourea , ethylenediamine , substitution reaction , kinetics , reaction rate constant , platinum , stereochemistry , medicinal chemistry , crystallography , inorganic chemistry , catalysis , organic chemistry , physics , quantum mechanics
Both reaction steps observed for the substitution of water by thiourea in the complexes [Pt(en)(OH 2 ) 2 ] 2+ and [Pt(phen)(OH 2 ) 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 1,2 obs = 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 1 en (25.0°C, pH = 3.0, I = 0.1 M) = 25.6 M −1 s −1 , Δ H # = 51 kJ mol −1 , Δ S # = −48 J K −1 mol −1 ; k 2 en (same conditions) = 12.1 M −1 s −1 , Δ H # = 30 kJ mol −1 , Δ S # = −124 J K −1 mol −1 , Δ V # = −7 cm 3 mol −1 ; k 1 phen (25.0°C, pH = 1.0, I = 0.1 M) = 2900 M −1 s −1 , Δ H # = 41 kJ mol −1 , Δ S # = −41 J K −1 mol −1 ; k 2 phen (same conditions) = 1170 M −1 s −1 , Δ H # = 37 kJ mol −1 , Δ S # = −61 J K −1 mol −1 , Δ 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)(μ‐OH)} 2 ] 2+ is formed from [Pt(phen)(OH 2 ) 2 ] 2+ in aqueous solution unless the solution is very dilute and highly acidic. The X‐ray structure of [{Pt(phen)(μ‐OH)} 2 ](F 3 CSO 3 ) 2 · 2 H 2 O was determined. It belongs to the triclinic space group P 1 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) Å; α = 71.85(5), β = 85.52(5), γ = 73.12(5) deg; V = 799.9(8) Å 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) Å] is observed in comparison with [{Pt(NH 3 ) 2 (μ‐OH)} 2 ] 2+ .