Probing the influence of the Tp' ligand on reactions of platinum(II) and platinum(IV).

摘要

Transformations of platinum(II) to platinum(IV) using organic reagents have been studied. A tridentate ligand system, hydridotris(3,5-dimethylpyrazolyl)borate - Tp', was shown to promote reactions at the platinum center. Three types of organic reagents have been investigated: alkynes, carbon monoxide, and isonitriles.;Reaction of the Tp'PtMe fragment with terminal alkynes results in eta 2-binding of the alkyne. Oxidative addition of the alkyne occurs upon gentle heating. Kinetic studies indicate that electron donating groups on the alkyne increase the rate of oxidative addition and that approaching the transition state increases the entropy. The proposed mechanism of oxidative addition proceeds through a transition state in which the third arm of the Tp' ligand is dechelated, a process that would be made easier by an electron donating group on the alkyne. In contrast, the reaction of alkynes (both terminal and internal) with the Tp'PtPh fragment does not produce eta2-bound alkyne adducts. Instead, phenyl migration to the alkyne followed by ortho C-H bond activation occurs resulting in formation of unsaturated metallacycles.;Conversion of Tp'Pt(Me)(CO) to the acyl complex, Tp'Pt(C(Me)=O)MeH has been accomplished by nucleophilic attack with MeLi followed by addition of HCl. The Pt(IV) acyl complex reacts with adventitious water in the presence of CO to produce the acyl dihydride product, Tp'Pt(C(Me)=O)H2, from a proposed metal mediated water gas shift reaction. Upon addition of acid, formation of dihydrogen is observed. An analogous carboxamido complex, Tp'Pt((C=O)NHR)Me2, has also been synthesized. Deprotonation of the carboxamido ligand followed by acidification produces Tp'PtMe2H, presumably after elimination of free isocyanate.;Reaction of the Tp'PtMe fragment with 2,6-dimethylphenyl isonitrile produces a four-coordinate sigma-bound isonitrile complex. This complex is susceptible to nucleophilic attack at the carbon of the isonitrile, producing an iminoacyl moiety. Protonation at the nitrogen of the iminoacyl complex results in an aminocarbene complex. The availability of the lone pair at the nitrogen of the free pyrazole arm of the Tp' ligand provides access to rare six-coordinate iminoacyl and aminocarbene complexes.