Kinetics of ligand substitution in platinum(II) complexes: A study on the concept of nucleophilic discrimination
Affiliation:
1. Chemistry Department, College of Science, Taibah University, Madinah, P.O. Box 344, Saudi Arabia;2. Chemistry Department, Faculty of Science, Sohag University, Sohag-82534, Egypt;3. Department of Medical Biochemistry, Faculty of Medicine, Sohag University, Egypt;4. Department of Medical Biochemistry, Taibah University, Saudi Arabia;5. Department of Chemistry, Faculty of Applied Science, Umm-Al-Qura University, Makkah, Saudi Arabia
Abstract:
Several platinum(II) complexes of the general type [Pt(OND)X] have been prepared and characterized, the ligand (OND) representing the phenolate anion of the tridentate Schiff bases N-(2- diethylaminoethyl)-salicylaldimine (D = NEt2), N-(2- ethylaminoethyl)-salicylaldimine (D = NHEt) and N- (3-thia-n-pentyl)-salicylaldimine (D= SEt) and X= Cl, NO3. As shown by conductimetric studies the nitrato complexes [Pt(OND)NO3] dissociate completely in methanol according to:
![></figure> with a variety of neutral and anionic nucleophiles Y in methanol at 20 °C and constant ionic strength, <em>I</em>= 0.2 M (NaClO<sub>4</sub>). The substitution follows a one- term rate law, <em>v</em> = k<sub>obs</sub>[Pt(OND)(H<sub>2</sub>O)<sup>+</sup>] = <em>k</em><sub>Y</sub>[Y]- [Pt(OND)(H<sub>2</sub>O)<sup>+</sup>]. The <em>k</em><sub>Y</sub> data obtained for 13 (D = NEt<sub>2</sub>) and 7 (D = NHEt; SEt) different nucleophiles Y cannot be adequately correlated with their <em>n</em><sub>pt</sub><sup>0</sup> values according to the well-known relationship log <em>k</em><sub>Y</sub> = <em>sn</em><sub>pt</sub><sup>0</sup> + log <em>k</em><sub>s</sub>. The deviations are strongest for large and bulky nucleophiles such as Y=Ph<sub>3</sub>P, Bu<sub>3</sub>P, Ph<sub>3</sub>As, I<sup>-</sup> and for D = NEt<sub>2</sub>, from which it is concluded that steric crowding hinders the formation of the 5-coordinate transition state. The rate reducing steric <em>cis</em>-effect observed is of the order <em>k</em><sub>Y</sub>(D = NEt<sub>2</sub>):<em>k</em><sub>Y</sub>(D = NHEt):<em>k</em><sub>Y</sub>(D = SEt) = 1:35:63 for small nucleophiles Y and as large as 1:192:2640 for Y = Ph<sub>3</sub>P. The introduction of substituents X in the salicylaldehyde ring in <em>ortho</em> (X<sup>3</sup>), <em>meta</em> (X<sup>4</sup>) and <em>para</em> position (X<sup>5</sup>) to the phenolic oxygen proves the existence of rather small electronic effects (X<sup>4</sup>, X<sup>5</sup>) and much stronger steric effects of bulky substituents X<sup>3</sup>, neighboring the donor oxygen.With the standard substrate <em>trans</em>-[Ptpy<sub>2</sub>Cl<sub>2</sub>] some new <em>n</em><sub>pt</sub><sup>0</sup> values were determined, namely for <em>N</em>, <em>N</em>′- dimethylthiourea (<em>n</em><sub>pt</sub><sup>0</sup> = 7.02), <em>N</em>, <em>N</em>′ -diphenylthlourea (<em>n</em><sub>pt</sub><sup>0</sup> = 7.19), <em>N</em>, <em>N</em>, <em>N</em>′, <em>N</em>′-tetramethylthiourea (<em>n</em><sub>pt</sub><sup>0</sup> = 6.05) and for the pseudo-halide dicyanoamide ion, N(CN)<sub>2</sub><sup>-</sup> (<em>n</em><sub>pt</sub><sup>0</sup>= 3.05). The <em>n</em><sub>pt</sub><sup>0</sup> value for the pseudo-halide tricyanomethanide, ion, C(CN)<sub>3</sub><sup>-</sup>, was estimated to be 3.03.</td>
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