A Pincer‐Type Anionic Platinum(0) Complex

Abstract

Pincer-type complexes constitute a large family of compounds that have attracted much recent interest. Among these compounds, aryl-anchored, d8 pincer complexes of the type [M II (LCL')] (M = Ni, Pd, Pt; L = neutral ligand such as phosphine, amine, dialkyl sulfide) are a major group that plays important roles in organometallic reactions and mechanisms, catalysis, and in the design of new materials.[1] In contrast, and to our knowledge, no complexes of this type with the metal in the zero oxidation state have been prepared. Such d 10 [M 0 (LCL')] complexes with neutral L ligands and an "anionic" aryl anchor would be anionic, and would be expected to possess distinctly different properties to neutral d 10 (M 0 ) complexes. We chose to utilize bulky bis-chelating pincer-type ligands in this study as they have been shown to be effective in stabilizing reactive species and have led to unusual complexes. [1] We have recently shown that reduction of PCP-type Pd II complexes [Pd(X)C 6 H 3 (CH 2 PiPr 2 ) 2 ] (X= Cl, trifluoroacetate) with sodium metal results in collapse of the pincer system, leading to formation of the diamagnetic binuclear complex [Pd{C 6 H 3 (CH 2 PiPr 2 ) 2 } 2 Pd], which contains a 14-electron linear Pd 0 moiety and a completely nonplanar "butterfly"-type 16-electron Pd II moiety.[2] Oxidation of the binuclear complex, or its reaction with organic halides, regenerates the original mononuclear framework. [2] To avoid collapse of the pincer system upon reduction we decided to use a PCP-Pt II complex with the hope that the more diffuse nature of the Pt orbitals (compared with Pd) might stabilize the reduced metal center.[3] In addition, increasing the steric bulk of the pincer phosphine ligand might protect the reduced metal center against intermolecular reactions and might lead to a rare monometallic Pt Herein we report the preparation, characterization, and computational study of the first thermally stable, monometallic anionic Pt 0 complex. The reactivity of this electron-rich, 16-electron PCP-type anionic Pt 0 complex shows that it is a Brønsted base and an effective electron-transfer reagent that is capable of C À F activation under exceedingly mild conditions.Reduction of the bulky PCP-Pt II complex 1 (Scheme 1) [6] with sodium in dry [D 8 ]THF at room temperature overnight led to a dramatic color change from colorless to dark red. A multinuclear NMR spectroscopy study of the reaction solution revealed that reduction of complex 1 had taken place to give quantitative formation of the diamagnetic 16-electron planar PCP-Pt 0 anion 2 (Scheme 1). Thus, the resonance of 1 at d = 66.7 ppm ( 1 J Pt,P = 2893 Hz) in the 31 P{ 1 H} NMR spectrum is replaced by a signal for the new complex 2 at d = 120.5 ppm ( 1 J Pt,P = 3874 Hz). The 31 P{ 1 H} NMR spectrum of the solution confirmed that this transformation is quantitative. In addition, the 195 Pt{ 1 H} NMR spectrum revealed that the diagnostic triplet of 1 at d = À4105 ppm ( 1 J P,Pt = 2893 Hz) is replaced by a new triplet for 2 at d = À4034 ppm ( 1 J P,...

Cleavage of unreactive bonds with pincer metal complexes

“…In rhodium(I) and iridium(I) chemistry, an oxidative addition pathway for C-C bond activation has been discussed extensively, while a substitution process has been put forward for platinum(II) 60 complexes (Scheme 6). Theoretical considerations support this general picture, 29 yet it should be noted that despite the difference in terminology, the overall reaction coordinate of both substitution and oxidative addition are highly similar.…”

“…32 The transoid bidentate chelation of the donor groups in M is pivotal for entailing C-C bond activation. When using 60 monophosphine ligand precursors that lack a second donor group, C-H bond activation occurs selectively and no C-C bond cleavage has been observed. 33 With the related PCO pincer ligand precursor 19, C-H activation is kinetically preferred at room temperature (Scheme 9).…”

“…Arenium structures have been proposed to result from a 1,2-sigmatropic shift of the alkyl group from the metal center in the (alkyl)(aryl) complex 27 to the aryl carbon in 28 (Scheme 11), thus relating structures J and L directly (cf Scheme 7). 60 Spectroscopic identification of this migration has been achieved by incorporating electron-donating substituents on the aryl ring (R = OH, OMe), 38 a well-known concept in electrophilic aromatic substitution. Such a modification increases the reactivity of the ipso carbon and of the platinum 65 center in 26 and allows (alkyl)(aryl) complexes to be stabilized with a variety of different alkyl substituents other than MeI.…”

“…While significant progress in cross-coupling 55 catalysis has been achieved and reviewed, 6 perhaps the most remarkable results have been accomplished in the activation and cleavage of typically unreactive bonds, such as C-H, N-H, and O-H bonds, and also C-C and C-heteroatom bonds. This Perspective aims to overview the most significant 60 advances, both in terms of mechanistic understanding and catalytic performance of pincer metal complexes in cleaving such unreactive bonds. An excellent review has appeared recently on PCP iridium-catalyzed dehydrogenation of alkanes and amine boranes, 7 and these reactions are thus not discussed 65 in any depth here.…”

Anionic d8 Alkyl Hydrides – Selective Formation and Reactivity of Anionic cis‐PtII Methyl Hydride

“…A 16 e -anionic Pt 0 complex that rapidly activates C-F bonds was recently reported by us. [19] Figure 3. DFT optimized structure for the reductive elimination of methane from 3*.…”