Feil Christoph M, Hettich Thomas D, Beyer Katharina, Sondermann Christina, Schlindwein Simon H, Nieger Martin, Gudat Dietrich
Institute for Inorganic Chemistry , University of Stuttgart , Pfaffenwaldring 55 , 70550 Stuttgart , Germany.
Department of Chemistry , University of Helsinki , P.O. Box 55, 00014 Helsinki , Finland.
Inorg Chem. 2019 May 6;58(9):6517-6528. doi: 10.1021/acs.inorgchem.9b00737. Epub 2019 Apr 24.
N-Heterocyclic phosphenium (NHP) and nitrosonium (NO) ligands are often viewed as isolobal analogues that share the capability to switch between different charge states and thus display redox "noninnocent" behavior. We report here on mixed complexes [(NHP)M(CO) (NO)] (M = Fe, Cr; n = 2, 3), which permit evaluating the donor/acceptor properties of both types of ligands and their interplay in a single complex. The crystalline target compounds were obtained from reactions of N-heterocyclic phosphenium triflates with PPN[Fe(CO)(NO)] or PPN[Cr(CO)(NO)], respectively, and fully characterized (PPN = nitride-bistriphenylphosphonium cation). The structural and spectroscopic (IR, UV-vis) data support the presence of carbene-analogue NHP ligands with an overall positive charge state and π-acceptor character. Even if the structural features of the M-NO unit were in all but one product blurred by crystallographic CO/NO disorder, spectroscopic studies and the structural data of the remaining compound suggest that the NO units exhibit nitroxide (NO) character. This assignment was validated by computational studies, which reveal also that the electronic structure of iron NHP/NO complexes is closely akin to that of the Hieber anion, [Fe(CO)(NO)]. The electrophilic character of the NHP units is further reflected in the chemical behavior of the mixed complexes. Cyclic voltammetry and IR-SEC studies revealed that complex [(NHP)Fe(CO)(NO)] (4) undergoes chemically reversible one-electron reduction. Computational studies indicate that the NHP unit in the resulting product carries significant radical character, and the reduction may thus be classified as predominantly ligand-centered. Reaction of 4 with sodium azide proceeded likewise under nucleophilic attack at phosphorus and decomplexation, while super hydride and methyl lithium reacted with all chromium and iron complexes via transfer of a hydride or methyl anion to the NHP unit to afford anionic phosphine complexes. Some of these species were isolated after cation exchange or trapped with electrophiles (H, SnPh) to afford neutral complexes representing the products of a formal hydrogenation or hydrostannylation of the original M═P double bond.
N-杂环磷鎓（NHP）和亚硝鎓（NO）配体通常被视为等瓣类似物，它们具有在不同电荷状态之间切换的能力，因此表现出氧化还原“非无辜”行为。我们在此报告混合配合物[(NHP)M(CO)(NO)]（M = Fe，Cr；n = 2，3），其允许评估两种类型配体的供体/受体性质及其在单个配合物中的相互作用。结晶目标化合物分别由三氟甲磺酸N-杂环磷鎓与PPN[Fe(CO)(NO)]或PPN[Cr(CO)(NO)]反应制得，并进行了全面表征（PPN = 氮化物-双三苯基磷鎓阳离子）。结构和光谱（红外、紫外可见）数据支持存在具有整体正电荷状态和π受体特征的卡宾类似物NHP配体。即使除一种产物外所有产物中M-NO单元的结构特征因晶体学CO/NO无序而模糊，但光谱研究和其余化合物的结构数据表明NO单元表现出硝酰基（NO）特征。这一归属通过计算研究得到验证，计算研究还表明铁NHP/NO配合物的电子结构与希贝尔阴离子[Fe(CO)(NO)]的电子结构密切相似。NHP单元的亲电性质在混合配合物的化学行为中进一步体现。循环伏安法和红外-SEC研究表明配合物[(NHP)Fe(CO)(NO)]（4）经历化学可逆的单电子还原。计算研究表明所得产物中的NHP单元具有显著的自由基特征，因此该还原可主要归类为以配体为中心的还原。4与叠氮化钠的反应同样在磷上的亲核进攻和解离下进行，而超氢化物和甲基锂通过将氢化物或甲基阴离子转移到NHP单元与所有铬和铁配合物反应，得到阴离子膦配合物。其中一些物种在阳离子交换后被分离或用电亲试剂（H，SnPh）捕获，得到代表原始M═P双键的形式氢化或氢锡化产物的中性配合物。
