Walking Metals in d⁸⋅⋅⋅d⁸ Hetero‐bimetallic Complexes: An Original Dynamic Phenomenon

Abstract

In the course of our investigations on polymetallic complexes derived from 1,3-bis(thiophosphinoyl)indene (Ind(Ph(2)P=S)(2)), we observed original fluxional behavior and report herein a joint experimental/computational study of this dynamic process. Starting from the indenylidene chloropalladate species [Pd{Ind(Ph(2) P=S)(2)}Cl](-) (1), the new Pd(II)···Rh(I) hetero-bimetallic pincer complex [PdCl{Ind(Ph(2) P=S)(2)}Rh(nbd)] (2; nbd=2,5-norbornadiene) was prepared. X-ray crystallography and DFT calculations substantiate the presence of a d(8)···d(8) interaction. According to multinuclear variable-temperature NMR spectroscopic experiments, the pendant {Rh(nbd)} fragment of 2 readily shifts in solution at room temperature between the two edges of the SCS tridentate ligand. To assess the role of the pincer-based polymetallic structure on this fluxional behavior, the related monometallic Rh complex [Rh{IndH(Ph(2) P=S)(2)}(nbd)] (3) was prepared. No evidence for a metal shift was observed in that case, even at high temperature, thus indicating that inplane pincer coordination to the Pd center plays a crucial role. The previously described Pd(II)···Ir(I) bimetallic complex 4 exhibited fluxional behavior in solution, but with a significantly higher activation barrier than 2. This finding demonstrates the generality of this metal-shift process and the strong influence of the involved metal centers on the associated activation barrier. DFT calculations were performed to shed light onto the mechanism of such metal-shift processes and to identify the factors that influence the associated activation barriers. Significantly different pathways were found for bimetallic complexes 2 and 4 on one hand and the monometallic complex 3 on the other hand. The corresponding activation barriers predicted computationally are in very good agreement with the experimental observations.

Mechanistic Aspects of Aryl–Halide Oxidative Addition, Coordination Chemistry, and Ring‐Walking by Palladium

“…Likewise, Nebra et al. observed the shift of a rhodium fragment between the arms of a bimetallic (1,3‐bis(thiophosphinoyl)indenyl)palladium complex across the ligand π system 16…”

“…[15] Likewise, Nebra et al observed the shift of ar hodiumf ragment between the arms of ab imetallic (1,3-bis-(thiophosphinoyl)indenyl)palladium complex across the ligand p system. [16] One of the questionst hat remain is whether the reactiono f [Pd(PEt 3 ) 4 ]w ith halogenated substrates will reflect that of platinum or nickel;w ill the rate-determining step be similar to the platinum or nickel system and can we control the selectivity of the reaction? In this study,w eobserved that h 2 -coordination of [Pd(PEt 3 ) 2 ]t oaC =Co raC Cm oiety is kinetically favored and followed by arate-determining aryl halide bond activation.…”

Generation of Mono- and Bimetallic Palladium Complexes and Mechanistic Insight into an Operative Metal Ring-Walking Process

“…Metal–arene interactions and many metal complexes exhibiting fluxional behavior have been studied for decades including seminal studies by the late Basolo in the 1960s. − The motivation behind those fundamental studies is often to gain mechanistic insight, which, in turn, might lead to obtaining more efficient and selective synthetically relevant carbon–carbon coupling reactions. However, direct mechanistic studies are often hampered by short-living intermediates, and therefore, advanced computational methods have provided much valuable information.…”

Metal–ligand–Lewis acid multi-cooperative catalysis: a step forward in the Conia-ene reaction

“…In the case of the (SCS) Pd complex, the Lewis acid additive may interact with the electron-rich indenediide motif as well as the sulfur atoms and thereby interfere with the Pd–ligand cooperativity. 19 A series of triflate salts were evaluated as LA additives to [(SCS)Pd] 2 in the cyclization of the β-keto,ester 2a in 4/1 chloroform/HFIP as solvent. Initial screenings were performed at 60 °C with 1 mol% Pd and 20 mol% LA (Fig.…”