Molecular and Electronic Structure of Square-Planar Gold Complexes Containing Two 1,2-Di(4-tert-butylphenyl)ethylene-1,2-dithiolato Ligands:  [Au(²L)₂]^1+/0/1-/2-. A Combined Experimental and Computational Study

From the reaction of in situ generated 1,2-di(4-tert-butylphenyl)ethylene-1,2-dithiol, ²LH₂, and Na[AuCl₄]·2H₂O in 1,4-dioxane, green brown crystals of diamagnetic [N(n-Bu)₄][Au^III(²L)₂] (1) were obtained. As shown by cyclic voltammetry, 1 is a member of an electron-transfer series comprising the dianion [Au^II(²L)₂]^2-, the monoanion [Au^III(²L)₂]^-, the neutral species [Au^III(²L^•)(²L)]⁰ ↔ [Au^III(²L)(²L^•)]⁰, and the monocation [Au^III(²L^•)₂]⁺. (²L^•)^1- represents the π radical anion (S_rad = ¹/₂) of the one-electron oxidized closed-shell dianion (²L)^2-. Oxidation of 1 in CH₂Cl₂ with ferrocenium hexafluorophosphate affords green, paramagnetic microcrystals of [Au^III(²L^•)(²L)] ↔ [Au^III(²L)(²L^•)] (2) (S = ¹/₂). Complexes 1 and 2 have been characterized by X-ray crystallography. Both species possess square-planar monoanions and neutral molecules, respectively. From the oxidation reaction of 1 or [N(n-Bu)₄][Au^III(³L)₂] with 2−3 equiv of [NO]BF₄ in CH₂Cl₂, a green solution of [Au^III(²L^•)₂]⁺ and green microcrystals of [Au^III(³L^•)₂]BF₄ (3) were obtained, respectively; (³L)^2- represents the dianion 1,2-di(4-diphenyl)ethylene-1,2-dithiolate, and (³L^•)^1- is its π radical monoanion. The electronic structures of this series of gold species have been elucidated by UV−vis, EPR spectroscopies, and DFT calculations. It is shown computationally by density functional theoretical (DFT) methods that the electronic structure of [Au^III(¹L^•)₂]⁺ is best described as a singlet diradical (S_t = 0); the ligand mixed valency in the neutral species 2 is of class (III) (delocalized); the monoanion in 1 contains a Au^III ion and two closed-shell dianionic ligands; and the corresponding dianions [Au(L)₂]^2- are best described as an intermediate Au^II/Au^III species with a metal−ligand delocalized SOMO (25% Au 5d, 75% 3p of four S atoms). (¹L)^2- is the dianion 1,2-di(phenyl)ethylene-1,2-dithiolate, and (¹L^•)^1- is the π radical monoanion. The neutral species [Pd^II(²L^•)₂] (4) has also been synthesized and characterized by X-ray crystallography. Its electronic structure is the same as described for [Au^III(¹L^•)₂]⁺ (singlet diradical), whereas that of the monoanion [Pd^II(²L^•)(²L)]^- ↔ [Pd(²L)(²L^•)]^- corresponds to that of the neutral gold complex 2. Anodic oxidation of the analogous monoanion [Au^III(mnt)₂]^-, where mnt = maleonitriledithiolate, gave the neutral complex [Au(mnt)(mnt^•)] (E_1/2 = 0.91 V vs Fc⁺/Fc). The optical and EPR spectroscopies of [Au(mnt)(mnt^•)] were consistent with those observed for the corresponding di(tert-butylphenyl)ethylenedithiolate complex 2.