Redox Non-innocence of Thioether Crowns: Elucidation of the Electronic Structure of the Mononuclear Pd(III) Complexes [Pd([9]aneS3)2]3+ and [Pd([18]aneS6)]3+

The Pd­(II) complexes [Pd([9]­aneS3)2]­(PF6)2·2MeCN (1) ([9]­aneS3 = 1,4,7-trithiacyclononane) and [Pd([18]­aneS6)]­(PF6)2 (2) ([18]­aneS6 = 1,4,7,10,13,16-hexathiacyclooctadecane) can be oxidized electrochemically or chemically oxidized with 70% HClO4 to [Pd([9]­aneS3)2]3+ and [Pd([18]­aneS6)]3+, respectively. These centers have been characterized by single crystal X-ray diffraction, and by UV/vis and multifrequency electron paramagnetic resonance (EPR) spectroscopies. The single crystal X-ray structures of [PdIII([9]­aneS3)2]­(ClO4)6·(H3O)3·(H2O)4 (3) at 150 K and [Pd([18]­aneS6)]­(ClO4)6·(H5O2)3 (4) at 90 K reveal distorted octahedral geometries with Pd–S distances of 2.3695(8), 2.3692(8), 2.5356(9) and 2.3490(6), 2.3454(5), 2.5474(6) Å, respectively, consistent with Jahn–Teller distortion at a low-spin d7 Pd­(III) center. The Pd­(II) compound [Pd([9]­aneS3)2]­(PF6)2 shows a one-electron oxidation process in MeCN (0.2 M NBu4PF6, 293 K) at E1/2 = +0.57 V vs. Fc+/Fc assigned to a formal Pd­(III)/Pd­(II) couple. Multifrequency (Q-, X-, S-, and L-band) EPR spectroscopic analysis of [Pd([9]­aneS3)2]3+ and [Pd([18]­aneS6)]3+ gives giso = 2.024, |Aiso(Pd)| = 18.9 × 10–4 cm–1; gxx = 2.046, gyy = 2.041, gzz = 2.004; |Axx(Pd)| = 24 × 10–4 cm–1, |Ayy(Pd)| = 22 × 10–4 cm–1, |Azz(Pd)| = 14 × 10–4 cm–1, |axx(H)| = 4 × 10–4 cm–1, |ayy(H)| = 5 × 10–4 cm–1, |azz(H)| = 5.5 × 10–4 cm–1 for [Pd([9]­aneS3)2]3+, and giso = 2.015, |Aiso(Pd)| = 18.8× 10–4 cm–1; gxx = 2.048 gyy = 2.036, gzz = 1.998; |axx(H)| = 5, |ayy(H)| = 5, |azz(H)| = 6 × 10–4 cm–1; |Axx(Pd)| = 23× 10–4 cm–1, |Ayy(Pd)| = 22 × 10–4 cm–1, |Azz(Pd)| = 4 × 10–4 cm–1 for [Pd([18]­aneS6)]3+. Both [Pd([9]­aneS3)2]3+ and [Pd([18]­aneS6)]3+ exhibit five-line superhyperfine splitting in the gzz region in their frozen solution EPR spectra. Double resonance spectroscopic measurements, supported by density functional theory (DFT) calculations, permit assignment of this superhyperfine to through-bond coupling involving four 1H centers of the macrocyclic ring. Analysis of the spin Hamiltonian parameters for the singly occupied molecular orbital (SOMO) in these complexes gives about 20.4% and 25% Pd character in [Pd([9]­aneS3)2]3+ and [Pd([18]­aneS6)]3+, respectively, consistent with the compositions calculated from scalar relativistic DFT calculations.

本研究涉及的二价钯（Pd(II)）配合物[Pd([9]aneS3)₂](PF₆)₂·2MeCN（1，其中[9]aneS3=1,4,7-三硫杂环壬烷）与[Pd([18]aneS6)](PF₆)₂（2，其中[18]aneS6=1,4,7,10,13,16-六硫杂环十八烷），可通过电化学氧化或70%高氯酸（HClO₄）化学氧化，分别得到对应的三价钯物种[Pd([9]aneS3)₂]³+和[Pd([18]aneS6)]³+。上述配合物及氧化产物均通过单晶X射线衍射、紫外-可见（UV/vis）光谱及多频电子顺磁共振（EPR）光谱进行了表征。在150 K下测得的[Pd(III)([9]aneS3)₂](ClO₄)₆·(H₃O)₃·(H₂O)₄（3），以及在90 K下测得的[Pd([18]aneS6)](ClO₄)₆·(H₅O₂)₃（4）的单晶X射线结构显示，二者均呈现畸变八面体几何构型，Pd–S键长分别为2.3695(8)、2.3692(8)、2.5356(9) Å与2.3490(6)、2.3454(5)、2.5474(6) Å，该结果与低自旋d⁷三价钯（Pd(III)）中心的姜-泰勒（Jahn–Teller）畸变特征一致。二价钯配合物[Pd([9]aneS3)₂](PF₆)₂在乙腈体系（0.2 M四丁基六氟磷酸铵[NBu₄PF₆]，293 K）中，相对于二茂铁+/二茂铁（Fc+/Fc）对的半波电势E₁/₂为+0.57 V，对应单电子氧化过程，可归属于形式上的Pd(III)/Pd(II)氧化还原电对。对[Pd([9]aneS3)₂]³+与[Pd([18]aneS6)]³+的多频（Q波段、X波段、S波段及L波段）EPR光谱分析结果如下：对于[Pd([9]aneS3)₂]³+，其giso=2.024，|Aiso(Pd)|=18.9×10⁻⁴ cm⁻¹；gxx=2.046，gyy=2.041，gzz=2.004；|Axx(Pd)|=24×10⁻⁴ cm⁻¹，|Ayy(Pd)|=22×10⁻⁴ cm⁻¹，|Azz(Pd)|=14×10⁻⁴ cm⁻¹，|axx(H)|=4×10⁻⁴ cm⁻¹，|ayy(H)|=5×10⁻⁴ cm⁻¹，|azz(H)|=5.5×10⁻⁴ cm⁻¹。对于[Pd([18]aneS6)]³+，其giso=2.015，|Aiso(Pd)|=18.8×10⁻⁴ cm⁻¹；gxx=2.048，gyy=2.036，gzz=1.998；|axx(H)|=5×10⁻⁴ cm⁻¹，|ayy(H)|=5×10⁻⁴ cm⁻¹，|azz(H)|=6×10⁻⁴ cm⁻¹；|Axx(Pd)|=23×10⁻⁴ cm⁻¹，|Ayy(Pd)|=22×10⁻⁴ cm⁻¹，|Azz(Pd)|=4×10⁻⁴ cm⁻¹。二者的冷冻溶液EPR光谱在gzz区域均表现出五重线超超精细分裂结构。结合密度泛函理论（DFT）计算的双共振光谱测量结果表明，该超超精细分裂可归因为与大环配体上四个¹H中心的通过键耦合作用。对上述配合物中单占据分子轨道（SOMO）的自旋哈密顿参数进行分析可知，[Pd([9]aneS3)₂]³+与[Pd([18]aneS6)]³+的SOMO中分别含有约20.4%和25%的钯原子轨道特征，该结果与标量相对论DFT计算得到的组分占比一致。