Mechanistic Aspects of Redox‐Induced Assembly and Disassembly of S‐Bridged [2M‐2S] Structures

Abstract

Sulfur-bridged binuclear structures [2M-2S] play a pivotal role in a variety of chemical processes such as bond breaking and formation and electron transfer. In general, structural persistence is deemed essential to the respective function but owing to the lack of a suitable molecular model system, the current understanding of the factors that control the thermodynamic and kinetic stability of [2M-2S] cores clearly is limited. This work reports a series of binuclear complexes of nickel derived from a 1,4-terphenyldithiophenol ligand platform that is ideally suited for mechanistic work to overcome this limitation. Redox-induced assembly and disassembly of S-bridged [2M-2S] fragments have been investigated at the molecular level. As part of an extended square scheme, metastable binuclear structures that are significant mechanistically have been identified, characterized, and their reactivity studied quantitatively. Electronic properties that are inherent to [2M-2S] structures and determine thermodynamic and kinetic stability are differentiated from steric effects imposed by co-ligands.

Understanding Factors that Control the Structural (Dis)Assembly of Sulphur-Bridged Bimetallic Sites

“…All manipulations of air-and moisture-sensitive compounds were carried out under an atmosphere of dry argon using standard Schlenk or glove box techniques. Literature procedures were followed for the preparation of (Ph 3 P) 2 NiN(SiMe 3 ) 2 , ferrocenium salts, and thiophenol ligands [24,26,27]. General information on the purification of solvents and other reagents is provided in Supplementary Material.…”

“…As shown in Figure 2, compound [Ni 2 (µ-S) 2 ] + has been prepared in form of its N(SO 2 CF 3 ) 2 , NTf 2 , salt following a two-step procedure that starts from mononuclear complex Ni-2S. Complex Ni-2S is isostructural to the PMe 3 ligated congener [24] in the solid state and in solution. 1 H and 13 C{ 1 H} NMR spectra in CDCl 3 solution display N-line splitting patterns for the carbon nuclei at methyl and the quaternary position of the phenyl group of the coligand, which corroborates trans-disposition of PMe 2 Ph at nickel.…”

“…A previous study revealed that all redox state changes are electrochemically reversible at potential sweep rates ν ≤ 2000 mV•s −1 in 0.1 M nBu4NPF6/1,2-C6H4F2 solution at 24 °C [24]. The formation of [Ni2(µ-S)2] + from [2{κ-S-Ni}] + is thermodynamically highly favored on account of a difference of ~1 V between respective formal redox potentials [24]. However, the markedly higher rate of formation of [Ni2(µ-S)2] + appears to result from a kinetic effect.…”

“…Single crystal X-ray diffraction (XRD) analysis confirmed the skeletal connectivity of [Ni2(µ-S)2] + as drawn inFigure 2. Despite moderate resolution of diffraction data impaired structure refinement to a high level of accuracy, PMe2Ph ligated [Ni2(µ-S)2] + is isostructural to its PMe3 congener[24]; pertinent data are provided in the supplementary material. Short Ni-C distances of ~2.01 Å point to a η 2 :η 2 -coordination of the 1,4-disubstituted π-system to the nickel atoms.…”

“…Quantitative UV-VIS Spectroscopic Study of Oxidation-Induced Disassembly of [Ni 2 (µ-S) 2 ] + Probing the reaction of [Ni 2 (µ-S) 2 ] + and [Fc"] + by NMR spectroscopy revealed that [Ni 2 (µ-S) 2 ] 2+is short-lived even at T ≤ −60 • C. Rapid formation of [2{κ-S-Ni}] 2+ under these conditions contrasts the reactivity of the PMe 3 ligated congener that reacts noticeably at T ≥ −40 • C[24]. For this reason, the oxidation reaction was monitored by UV-VIS spectroscopy at T = −90 • C in CH 2 Cl 2 solution.…”