Two‐Electron Redox Chemistry and Reversible Umpolung of a Gold–Antimony Bond

Abstract

Two-electron redox processes occurring at the core of dinuclear metal complexes are important because of their implication in a number of chemical transformations ranging from the photocatalytic production of dihydrogen [1] to the oxidative functionalization of CÀH bonds. [2] Two-electron redox processes are also well documented for binuclear gold complexes [3] as well as for gold/rhodium, [4] gold/iridium [5] and gold/platinum [6] heterobimetallic complexes. One of the common features displayed by all of these complexes relates to their ability to sustain two-electron redox chemistry without decomposition of the bimetallic core. As part of our interest in fundamental aspects of main-group chemistry, we have recently questioned whether similar processes could also be observed at the bimetallic core of main-group/transitionmetal complexes.Triarylstibines are soft s-donor ligands which have been incorporated in a variety of late transition-metal complexes. [7] They are also well known to undergo facile two-electron oxidation when exposed to oxidizing agents such as halogens. [8] While there is precedent for dialkylstibido transitionmetal compounds (type A) to undergo oxidative addition at the antimony center, [9] no analogous reactivity has been reported for stibine metal complexes of type B. With the discovery of main-group-based, two-electron redox platforms in mind, we have now decided to study the synthesis and oxidation of complexes of type B. Herein, we report the first incarnation of this idea in the case of a gold-antimony compound.To prevent dissociation of the gold-antimony bimetallic core upon oxidation, we chose to work with a stibine ligand whose interaction with the gold center would be supported by pendant donor ligands. These considerations led us to focus on the triphosphanylstibine ligand [(o-(Ph 2 P)C 6 H 4 ) 3 Sb] (referred to as L) [10] which was allowed to react with [AuCl(tht)] in CH 2 Cl 2 /acetone to produce 1-Cl as a pale yellow precipitate (Scheme 1). At À55 8C in CDCl 3 , the 31 P{ 1 H} NMR spectrum of 1-Cl displays two signals at d = + 39.5 and À7.4 ppm in a 2:1 intensity ratio thus suggesting that only two of the three phosphine arms of the ligand are coordinated to the gold atom. Upon elevation of the temperature, these two peaks coalesce into one signal at d = + 24.4 ppm indicating the onset of a rapid exchange. Line shape analysis of the 31 P{ 1 H} NMR spectra of 1-Cl over the À55 8C-+ 22 8C temperature range followed by an Eyring analysis afforded DH°= + 16.7 (AE 0.9) kcal mol À1 and DS°= + 16.6(AE3.8) cal mol À11 K À11 , suggesting a dissociative exchange of the phosphine arms. The ESI mass spectrum of this complex shows a peak at m/z 1101.1180 amu corresponding to the [LAu] + ion. A final clarification of the structure of this compound was derived from a single-crystal diffraction study which confirmed the coordination of two of the phosphorus atoms to the gold chloride fragment (Figure 1). [11] Inspection of the P-Au-P and P-Au-Cl angles (AE = 359.468) shows that these prima...

Coordination- and Redox-Noninnocent Behavior of Ambiphilic Ligands Containing Antimony

“…In a first attempt to access such species, we targeted a gold complex featuring the tris(phosphinyl)stibine ligand 6. 34 Reaction of (tht)AuCl with 6 afforded compound 15-Cl (Scheme 11), which underwent a clean antimony-centered oxidation upon treatment with PhICl 2 , leading to complex 16. The formation of 15-Cl was found to be reversible by treatment of 16 with excess NaI as the reductant to afford the gold iodide 15-I.…”

σ‐Accepting Properties of a Chlorobismuthine Ligand

“…[11] The two phosphine arms of the complex are coordinated to the gold center in a trans arrangement, placing the bismuth atom only at 2.9738(17) from the gold atom. [18] Unlike in these complexes, however, the Group 15 element remains trivalent rather than pentavalent. [12] In fact, the BiÀAu distance of 2 only exceeds the sum of the covalent radii of the two elements (2.75-2.84 ) [13] by 4.7-8.1 %, thus suggesting the presence of a bond between these two centers.…”

Structural Evidence for Pnictogen-Centered Lewis Acidity in Cationic Platinum-Stibine Complexes Featuring Pendent Amino or Ammonium Groups

“…The chemistry of stibines is attracted a renewed interest because of applications in material science and catalysis [ 1 , 2 , 3 , 4 ]. Although a parallel is often drawn with the ubiquitous phosphine ligands, stibines display several atypical traits and can, for example, undergo redox reactions without dissociation of the coordinated metal [ 5 , 6 , 7 , 8 , 9 , 10 ]. Moreover, while phosphines mostly act as two-electron donor spectator ligands, stibines have been shown to display Lewis acidic properties, even when coordinated to a transition metal [ 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 ].…”