Coordination of Alkenes at a Highly Electrophilic Site. New Dicationic Platinum(II) Complexes:  Synthesis, Structure, and Reactions with Nucleophiles

Abstract

phinomethyl)pyridine) have been prepared and characterized by 1 H and 13 C NMR spectroscopy, and the molecular structure of the ethylene complex has been determined by X-ray diffraction analysis. Nucleophilic attack at the coordinated double bond takes place more readily than for known neutral and monocationic species, and a variety of protic nucleophiles NuH (MeOH, H 2 O, aromatic amines) give the corresponding -functionalized σ-alkyl complexes [Pt(PNP)(CHRCHR′Nu)]BF 4 also in the absence of auxiliary bases. In the case of the styrene derivatives an unusual decomposition pathway gives the stable alkenyl complex [Pt(PNP)(CHdCHPh)]BF 4 . A competitive equilibrium process between substitution and addition was revealed by reacting ethylene with the isolated aniline complex [Pt(PNP)-(PhNH 2 )](BF 4 ) 2 . The nucleophilic addition product was found to be thermodynamically favored over the substitution product more than in the case of the analogous palladium complexes.

Enhancing Electrophilic Alkene Activation by Increasing the Positive Net Charge in Transition‐Metal Complexes and Application in Homogeneous Catalysis

“…Figure 3. The signal of the coordinated ethylene is shifted upfield to much lesser extent for the dicationic Pd II and Pt II complexes 22 a and 23 a [3,4] (Dd = À34 and À45 ppm) relative to that of the monocationic Rh I complex 2 a [22] (Dd = À64 ppm). This is consistent with the reduced electronic density due to the weaker p back-donation.…”

“…[3,4] Halide abstraction from [M(PNP)X]X (20, 21: M= Pd, Pt; X = Cl, I) with two equivalents of AgBF 4 in the presence of an excess of the alkene gave the respective alkene complexes 22 and 23 in high yields, see Scheme 4. They are stable enough as solids and in solution, and could be characterized by NMR spectroscopy and X-ray crystal structure analysis.…”

“…While the two coupling constants J C-Rh in the monocationic rhodium(i) styrene complex 2 b are rather similar (cf. [4] indicates a reduced interaction for Pt À CHR than PtÀCH 2 .…”

“…In this paper the concept of enhancement of the electrophilic alkene activation by increasing the positive net charge in transition-metal complexes is described. A case study of dicationic palladium(ii) [3] and platinum(ii) [4] alkene complexes and their reactivity towards weakly basic nucleophiles will be discussed. A new catalytic pathway for CÀC bond formation, which has arisen from those reactivity studies, [5] exhibits a nice example for the benefit of this concept for the development of new homogeneous catalysts.…”

Neutral Tridentate PNP Ligands and Their Hybrid Analogues: Versatile Non‐Innocent Scaffolds for Homogeneous Catalysis

“…[84] As such, they described the facile deprotonation of the PNP backbone at one of the methylene spacers, concomitant with the dearomatization of the pyridine ring, in the absence of CO. [85] Vitagliano and co-workers have shown that the dicationic complex [Pt (12 Ph )] (73) is a suitable starting compound for stoichiometric reactions that involve nucleophilic attack of alcohols or amines onto coordinated alkenes (e.g., ethene). [86,87] In a related study (Scheme 29), they demonstrated that the same complexes can act as catalysts for the codimerization of ethene and internal alkenes (also known as the hydrovinylation reaction). [88] The Pd analogue was subsequently shown to catalyze the same co-dimerization activity, but cyclopropanation was observed as a significant side reaction.…”