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Synthesis, Structure and Some Catalytic Applications of Platinum Complexes with Terphenyl Phosphine Ligands

AuthorsOrtega, Laura
AdvisorPelosso, Ricardo; Carmona, Ernesto
Issue Date2016
PublisherCSIC - Instituto de Investigaciones Químicas (IIQ)
Universidad de Sevilla
AbstractChapter I contains synthetic, structural and reactivity studies on Pt(II) complexes bearing the bulky dimethylterphenylphosphines PMe2ArXyl2 (L1), PMe2ArDipp2 (L2) or PMe2ArTipp2 (L3) (see page 13 for the structural formulae of these phosphines). It comprises two main sections. The first is dedicated to compounds of the xylyl substituted terphenylphosphine (L1). Despite the bulkiness of this ligand, bis(phosphine) Pt(II) complexes could be synthesised. Since the ligand contains -C-H bonds, cyclometallation reactions could be achieved to form seven-membered platinacycles. Kinetic studies on the cyclometallation reaction were also developed. Two different Pt(II) precursors were used in this section, namely cis-[PtCl2(COD)] and cis-[PtMe2(SMe2)2]. The newly prepared complexes included bis(chloride) and bis(methyl) derivatives with different molecular geometry, i.e. trans-[PtCl2(PMe2ArXyl2)2] and cis-[PtMe2(PMe2ArXyl2)2], as well as some cyclometallated complexes. The protonation reactions of some of these platinum organometallics were investigated by variable temperature NMR spectroscopy. The second section of Chapter I describes a variety of Pt(II) complexes of ligands L2 and L3. These ligands are able to stabilize low-coordinated species that feature formally a 14-electron count. In order to counterbalance the metal coordinative and electronic unsaturation, these compounds feature a relatively weak Pt···Carene interaction with the ipso carbon atom of a flanking aryl ring of the terphenyl group of the phosphine.
A series of neutral Pt(II) complexes were characterised by common techniques (NMR spectroscopy and single crystal X-ray diffraction) for a variety of monoanionic ligands, such as Me-, Cl-, H- and SiR3-. New compounds in this section comprise the methyl derivatives cis-[PtMe2(PMe2Ar’)] of ligands L2 and L3, as well as the chloro-methyl species cis-[Pt(Me)Cl(PMe2ArDipp2)] and some hydride-silyl complexes, cis-[Pt(H)(SiR3)(PMe2Ar’)], among others. The secondary interaction offered by the phosphine was easily replaced by various Lewis bases alike CO or PR3. In addition, cationic complexes were also investigated. The important steric protection provided by the bulky terphenylphosphines allowed abstraction of one or the two chloride ligands of the complex cis-[PtCl2(PMe2ArDipp2)] with formation of mono- and bis-cationic complexes with weakly bound molecules of poorly coordinating solvents completing the metal coordination sphere. Some of the Pt(II) cations investigated were generated by protonation of cis-[PtMe2(PMe2ArDipp2)], among them the dication cis-[Pt(PMe2ArDipp2)(S)2]2+, isolated as the bis(triflimidate) salt (NTf2-). This complex contains only one firmly bound 2-P,C phosphine ligand and two molecules of poor donor solvents like dichloromethane, diethylether or water (adventitious), and may be therefore viewed as a source of the [Pt(PMe2ArDipp2)]2+ fragment. Another, in our opinion interesting complex that was fully characterised in this work is the monocation cis-[PtMe(S)(PMe2ArDipp2)]+ (the analogous PMe2ArTipp2 derivative was also studied although only for comparative structural analysis). We studied the reactivity of PMe2ArDipp2 complex towards some small inorganic and organic molecules. Specifically, the reaction with C2H2 yielded the four-coordinate allyl complex [Pt(3-C3H5)(PMe2ArDipp2)]+ through a reactive vinylidene intermediate, as demonstrated by isotopic labelling studies using C2D2. Computational studies on this reaction and on the related process involving C2H4 were also developed. To complete this section, we deemed appropriate some catalytic studies using these cations as catalyst precursors. The reactions embraced a variety of alkyne hydroarylations, along with the hydroamination of some terminal alkynes
Chapter II is dedicated to the study of Pt(0) complexes of the terphenylphosphine ligands L2 and L3. Since it is well known that Pt(0)-PR3 complexes are one of the most important families of organometallic catalysts, it was though mandatory disclosing the structural and reactivity properties of Pt(0) derivatives of these bulky phosphines, including some catalytic applications. The core of these investigations are Pt(0)-PMe2Ar’-olefin complexes of the types [Pt(C2H4)n(PMe2ARDipp2)], where n = 1 or 2. Since the mono(ethylene) complexes feature relatively poor thermal stability, the analogous complexes [Pt(CH2=CHtBu)(PMe2Ar’)] of the bulkier 3,3-dimethylbut-1-ene olefin were prepared and characterised. As discussed in a forthcoming section of this Thesis, these complexes were generated by two different methods. The first consisted in the Zn powder reduction of the corresponding cis-[PtCl2(PMe2Ar’)] precursor in the presence of the alkene, while the second involved reductive elimination of HSiEt3 from the hydride-silyl predecessor induced by the olefin. A chelating alkene such as bis(allyl)ether, (CH2=CHCH2)2O, was also utilised successfully to generate complexes of this kind. Besides the structural characterisation of the new compounds, some reactivity studies were carried out, in particular the reaction with CO, that yielded an interesting triplatinum cluster, namely [Pt3(μ-CO)3(PMe2ArDipp2)3], demonstrating that the Pt(0)-PMe2Ar’-olefin complexes are a useful source of the reactive [Pt(PMe2Ar’)] fragment. Low temperature protonation studies made evident the formation of cis-[Pt(H)(olefin)(PMe2Ar’]+ species, which rearranged by migratory insertion of the olefin into the Pt—H bond and behave as precatalysts for olefin dimerisation reactions. In addition, the Pt(0)-PMe2Ar’ complexes were shown to undergo oxidative addition reactions. This reactivity was demonstrated with the formation of platinum(II) allyl complexes when complex [Pt(C2H4)2(PMe2ArDipp2)] was treated with BrCH2C(R)=CH2 (R = H, Me). In the same way as in Chapter I, Chapter II ends with catalytic studies based on the use of some of the new Pt(0) complexes as precatalysts for the hydrosilylation of terminal alkynes. As discussed later, the hydrosilylation of alkyl-substituted terminal alkynes was accomplished with high activity and E-selectivity, while for arylacetylenes a strong dependence of the catalytic activity with electronic effects on the para-substituents was disclosed
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