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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

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posted on 2007-02-19, 00:00 authored by Swarnalatha Kokatam, Kallol Ray, Joseph Pap, Eckhard Bill, William E. Geiger, Robert J. LeSuer, Philip H. Rieger, Thomas Weyhermüller, Frank Neese, Karl Wieghardt
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][AuIII(2L)2] (1) were obtained. As shown by cyclic voltammetry, 1 is a member of an electron-transfer series comprising the dianion [AuII(2L)2]2-, the monoanion [AuIII(2L)2]-, the neutral species [AuIII(2L)(2L)]0 ↔ [AuIII(2L)(2L)]0, and the monocation [AuIII(2L)2]+. (2L)1- represents the π radical anion (Srad = 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 [AuIII(2L)(2L)] ↔ [AuIII(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][AuIII(3L)2] with 2−3 equiv of [NO]BF4 in CH2Cl2, a green solution of [AuIII(2L)2]+ and green microcrystals of [AuIII(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 [AuIII(1L)2]+ is best described as a singlet diradical (St = 0); the ligand mixed valency in the neutral species 2 is of class (III) (delocalized); the monoanion in 1 contains a AuIII ion and two closed-shell dianionic ligands; and the corresponding dianions [Au(L)2]2- are best described as an intermediate AuII/AuIII 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 [PdII(2L)2] (4) has also been synthesized and characterized by X-ray crystallography. Its electronic structure is the same as described for [AuIII(1L)2]+ (singlet diradical), whereas that of the monoanion [PdII(2L)(2L)]- ↔ [Pd(2L)(2L)]- corresponds to that of the neutral gold complex 2. Anodic oxidation of the analogous monoanion [AuIII(mnt)2]-, where mnt = maleonitriledithiolate, gave the neutral complex [Au(mnt)(mnt)] (E1/2 = 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.