Redox-Active Bis(phenolate) N-Heterocyclic Carbene [OCO] Pincer Ligands Support Cobalt Electron Transfer Series Spanning Four Oxidation States
Redox-Active Bis(phenolate) N-Heterocyclic Carbene [OCO] Pincer Ligands Support Cobalt Electron Transfer Series Spanning Four Oxidation States
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DOI：
10.1021/acs.inorgchem.7b01906
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发表时间：
2017-10-16
影响因子：
4.6
通讯作者：
Soper, Jake D.
中科院分区：
文献类型：
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作者：
Harris, Caleb F.;Bayless, Michael B.;Soper, Jake D.
报道了一种新的低配位Co配合物家族，该配合物由三个氧化还原-非无害三齿[OCO]钳形双酚酸n杂环碳（NHC）配体支撑。结合实验和计算数据表明，电荷中性的四坐标配合物的最佳配方是Co（II）中心与封闭壳[OCO]（2-）离子结合，通式为[(OCO)Coll]（其中L为溶剂衍生的MeCN或THF）。[OCO)(CoL)-L-II]配合物的循环伏安图显示，在低于1.2 V vs Fc(+)/Fc的电位下可发生三种氧化，对应于形式上的Co(V)物质的生成，但真正的物理/光谱氧化态要低得多。化学氧化提供了咪唑啉nhc衍生配合物的单和指示，通过计算、磁和光谱方法，包括单晶x射线衍射进行了检验。单离子配合物的金属和配体氧化态是不明确的；数据与公式一致，即[(OCO)- o- S)Co-III(THF)(2)]（+）含有一个闭合壳体[(OCO)- o- S]（2-）二酚酸盐配体与S = 1 Co（III）中心结合，或[(ococii)- o- S -Co中心点（THF）(2)]（+）具有一个低自旋Co（II）离子与单阴离子[（OCO中心点）- o- S]（-）铁磁偶联，包含一个分布在[OCO]框架上的单个未配对电子。该反应最好描述为[((OCO0)-O-s)Co-II(THF)(3)](2+)，具有一个未配对的电子定位在d(7) Co（II）中心和一个双氧化、电荷中性、闭壳(OCO0)-O-s配体。这些综合数据首次为[OCO]配体氧化还原活性提供了明确的结构证据。值得注意的是，改变NHC主链的不饱和程度会使配体的氧化电位变化高达400 mV。讨论了这种意想不到的转变的可能的化学起源，以及[OCO]钳形配体在贱金属介导的有机金属偶联催化中的潜在效用。
A new family of low-coordinate Co complexes supported by three redox-noninnocent tridentate [OCO] pincer type bis(phenolate) N-heterocyclic carbene (NHC) ligands are described. Combined experimental and computational data suggest that the charge-neutral four-coordinate complexes are best formulated as Co(II) centers bound to closed-shell [OCO](2-) dianions, of the general formula [(OCO)Coll] (where L is a solvent-derived MeCN or THF). Cyclic voltammograms of the [OCO)(CoL)-L-II] complexes reveal three oxidations accessible at potentials below 1.2 V vs Fc(+)/Fc, corresponding to generation of formally Co(V) species, but the true physical/spectroscopic oxidation states are much lower. Chemical oxidations afford the mono- and dications of the imidazoline NHC-derived complex, which were examined by computational and magnetic and spectroscopic methods, including single-crystal X-ray diffraction. The metal and ligand oxidation states of the monocationic complex are ambiguous; data are consistent with formulation as either [((OCO)-O-s)Co-III(THF)(2)](+) containing a closed-shell [(OCO)-O-s](2-) diphenolate ligand bound to a S = 1 Co(III) center, or [((OCOCoII)-O-s-Co-center dot(THF)(2)](+) with a low-spin Co(II) ion ferromagnetically coupled to monoanionic [(OCO center dot)-O-s](-) containing a single unpaired electron distributed across the [OCO] framework. The dication is best described as [((OCO0)-O-s)Co-II(THF)(3)](2+), with a single unpaired electron localized on the d(7) Co(II) center and a doubly oxidized, charge-neutral, closed-shell (OCO0)-O-s ligand. The combined data provide for the first time unequivocal and structural evidence for [OCO] ligand redox activity. Notably, varying the degree of unsaturation in the NHC backbone shifts the ligand-based oxidation potentials by up to 400 mV. The possible chemical origins of this unexpected shift, along with the potential utility of the [OCO] pincer ligands for base-metal-mediated organometallic coupling catalysis, are discussed.