Department of Chemistry, University of California, Berkeley, Berkeley 94720, California, USA.
J Am Chem Soc. 2013 Jul 31;135(30):11266-73. doi: 10.1021/ja404476m. Epub 2013 Jul 16.
The (2,2,6,6-tetramethylpiperidin-1-yl)oxyl radical (TEMPO) has been employed for an extensive range of chemical applications, ranging from organometallic catalysis to serving as a structural probe in biological systems. As a ligand in an organometallic complex, TEMPO can exhibit several distinct coordination modes. Here we use ultrafast time-resolved infrared spectroscopy to study the reactivity of TEMPO toward coordinatively unsaturated 16- and 17-electron organometallic reaction intermediates. TEMPO coordinates to the metal centers of the 16-electron species CpCo(CO) and Fe(CO)4, and to the 17-electron species CpFe(CO)2 and Mn(CO)5, via an associative mechanism with concomitant oxidation of the metal center. In these adducts, TEMPO thus behaves as an anionic ligand, characterized by a pyramidal geometry about the nitrogen center. Density functional theory calculations are used to facilitate interpretation of the spectra and to further explore the structures of the TEMPO adducts. To our knowledge, this study represents the first direct characterization of the mechanism of the reaction of TEMPO with coordinatively unsaturated organometallic complexes, providing valuable insight into its reactions with commonly encountered reaction intermediates. The similar reactivity of TEMPO toward each of the species studied suggests that these results can be considered representative of TEMPO's reactivity toward all low-valent transition metal complexes.
（2,2,6,6-四甲基哌啶-1-基）氧自由基（TEMPO）已广泛应用于化学领域，从有机金属催化到作为生物系统中的结构探针。作为有机金属配合物中的配体，TEMPO 可以表现出几种不同的配位模式。在这里，我们使用超快时间分辨红外光谱研究 TEMPO 与配位不饱和的 16 电子和 17 电子有机金属反应中间体的反应性。TEMPO 通过缔合机制与金属中心的同时氧化，与 16 电子物种 CpCo(CO)和 Fe(CO)4 以及 17 电子物种 CpFe(CO)2 和 Mn(CO)5 的金属中心配位。在这些加合物中，TEMPO 因此表现为阴离子配体，其氮中心呈金字塔形几何形状。密度泛函理论计算用于促进对光谱的解释，并进一步探索 TEMPO 加合物的结构。据我们所知，这项研究代表了对 TEMPO 与配位不饱和有机金属配合物反应机制的首次直接表征，为其与常见反应中间体的反应提供了有价值的见解。TEMPO 对每种研究物种的相似反应性表明，这些结果可以被认为代表了 TEMPO 对所有低价过渡金属配合物的反应性。
