Controlling One-Electron vs Two-Electron Pathways in the Multi-Electron Redox Cycle of Nickel Diethyldithiocarbamate
Controlling One-Electron vs Two-Electron Pathways in the Multi-Electron Redox Cycle of Nickel Diethyldithiocarbamate
复制标题
控制二乙基二硫代氨基甲酸镍多电子氧化还原循环中的单电子与双电子途径
DOI：
10.1021/acs.inorgchem.1c01699
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发表时间：
2021
影响因子：
4.6
通讯作者：
Farnum, Byron H.
中科院分区：
文献类型：
--
作者：
Mazumder, Md. Motiur;Burton, Andricus;Richburg, Chase S.;Saha, Soumen;Cronin, Bryan;Duin, Evert;Farnum, Byron H.
NiII（dtc）2（其中dtc-为N，N-二乙基二硫代氨基甲酸酯）在乙腈中的独特氧化还原循环在从NiII（dtc）2→ [NiIV（dtc）3]+氧化时显示2 e-氧化还原化学，但在从[NiIV（dtc）3]+→ NiIII（dtc）3→ NiII（dtc）2还原时显示1 e-氧化还原化学。这种循环的根本原因在于四配位的NiII（dtc）2和六配位的[NiIV（dtc）3]+之间发生的结构变化。循环伏安法（CV）实验表明，这些1 e-和2 e-途径可以通过添加吡啶基配体（L）的电解质溶液进行控制。具体而言，这些配体的加入导致1 e-配体耦合电子转移（LCET）氧化还原波，其产生吡啶结合的Ni（III）络合物、[NiIII（dtc）2（L）]+和[NiIII（dtc）2（L）2]+的混合物。虽然配合物不能被分离，但在4-甲氧基吡啶存在下使用化学氧化剂的电子顺磁共振（EPR）测量证实了反式-[NiIII（dtc）2（L）2]+的形成。密度泛函理论计算也被用来支持吡啶配位Ni（III）配合物的形成，通过结构优化和EPR参数的计算。发现LCET过程的可逆性取决于吡啶配体的碱度和CV实验的扫描速率。对于强碱性吡啶（例如，4-甲氧基吡啶）和/或快速扫描速率，实现了高可逆性，允许[NiIII（dtc）2（L）x]+直接还原回NiII（dtc）2+xL。对于弱碱性吡啶（例如，3-溴吡啶）和/或慢扫描速率，[NiIII（dtc）2（L）x]+不可逆地衰变形成[NiIV（dtc）3]+。使用CV的详细动力学研究表明，[NiIII（dtc）2（L）]+和[NiIII（dtc）2（L）2]+由于两种物质之间的小平衡而通过平行途径衰减。配体解离的速率常数（[NiIII（dtc）2（L）2]+→ [NiIII（dtc）2（L）]++ L）沿着[NiIII（dtc）2（L）]+和[NiIII（dtc）2（L）2]+物种的分解，随着吡啶配体的吸电子特性而增加，表明吡啶解离可能是这些络合物分解的限速步骤。这些研究确立了沿着2 e-氧化途径动力学捕获1 e-中间体的一般趋势。
The unique redox cycle of NiII(dtc)2, where dtc–isN,N-diethyldithiocarbamate, in acetonitrile displays 2e–redox chemistry upon oxidation from NiII(dtc)2→ [NiIV(dtc)3]+but 1e–redox chemistry upon reduction from [NiIV(dtc)3]+→ NiIII(dtc)3→ NiII(dtc)2. The underlying reasons for this cycle lie in the structural changes that occur between four-coordinate NiII(dtc)2and six-coordinate [NiIV(dtc)3]+. Cyclic voltammetry (CV) experiments show that these 1e–and 2e–pathways can be controlled by the addition of pyridine-based ligands (L) to the electrolyte solution. Specifically, the addition of these ligands resulted in a 1e–ligand-coupled electron transfer (LCET) redox wave, which produced a mixture of pyridine-bound Ni(III) complexes, [NiIII(dtc)2(L)]+, and [NiIII(dtc)2(L)2]+. Although the complexes could not be isolated, electron paramagnetic resonance (EPR) measurements using a chemical oxidant in the presence of 4-methoxypyridine confirmed the formation oftrans-[NiIII(dtc)2(L)2]+. Density functional theory calculations were also used to support the formation of pyridine coordinated Ni(III) complexes through structural optimization and calculation of EPR parameters. The reversibility of the LCET process was found to be dependent on both the basicity of the pyridine ligand and the scan rate of the CV experiment. For strongly basic pyridines (e.g., 4-methoxypyridine) and/or fast scan rates, high reversibility was achieved, allowing [NiIII(dtc)2(L)x]+to be reduced directly back to NiII(dtc)2+xL. For weakly basic pyridines (e.g., 3-bromopyridine) and/or slow scan rates, [NiIII(dtc)2(L)x]+decayed irreversibly to form [NiIV(dtc)3]+. Detailed kinetics studies using CV reveal that [NiIII(dtc)2(L)]+and [NiIII(dtc)2(L)2]+decay by parallel pathways due to a small equilibrium between the two species. The rate constants for ligand dissociation ([NiIII(dtc)2(L)2]+→ [NiIII(dtc)2(L)]++ L) along with decomposition of [NiIII(dtc)2(L)]+and [NiIII(dtc)2(L)2]+species were found to increase with the electron-withdrawing character of the pyridine ligand, indicating pyridine dissociation is likely the rate-limiting step for decomposition of these complexes. These studies establish a general trend for kinetically trapping 1e–intermediates along a 2e–oxidation path.