Competitive major carbon-carbon bond activation (CCA) and minor carbon-hydrogen bond activation (CHA) channels are identified in the reaction between rhodium(II) meso-tetramesitylporphyrin [Rh(II)(tmp)] (1) and 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) (2). The CCA and CHA pathways lead to formation of [Rh(III)(tmp)Me] (3) and [Rh(III)(tmp)H] (5), respectively. In the presence of excess TEMPO, [Rh(II)(tmp)] is regenerated from [Rh(III)(tmp)H] with formation of 2,2,6,6-tetramethyl-piperidine-1-ol (TEMPOH) (4) via a subsequent hydrogen atom abstraction pathway. The yield of the CCA product [Rh(III)(tmp)Me] increased with higher temperature at the cost of the CHA product TEMPOH in the temperature range 50-80 degrees C. Both the CCA and CHA pathways follow second-order kinetics. The mechanism of the TEMPO carbon-carbon bond activation was studied by means of kinetic investigations and DFT calculations. Broken symmetry, unrestricted b3-lyp calculations along the open-shell singlet surface reveal a low-energy transition state (TS1) for direct TEMPO methyl radical abstraction by the Rh(II) radical (SH2 type mechanism). An alternative ionic pathway, with a somewhat higher barrier, was identified along the closed-shell singlet surface. This ionic pathway proceeds in two sequential steps: Electron transfer from TEMPO to [Rh(II)(por)] producing the [TEMPO]+ [RhI(por)]- cation-anion pair, followed by net CH3+ transfer from TEMPO+ to Rh(I) with formation of [Rh(III)(por)Me] and (DMPO-like) 2,2,6-trimethyl-2,3,4,5-tetrahydro-1-pyridiniumolate. The transition state for this process (TS2) is best described as an SN2-like nucleophilic substitution involving attack of the d(z)2 orbital of [Rh(I)(por)]- at one of the C(Me)-C(ring) sigma* orbitals of [TEMPO]+. Although the calculated barrier of the open-shell radical pathway is somewhat lower than the barrier for the ionic pathway, R-DFT and U-DFT are not likely comparatively accurate enough to reliably distinguish between these possible pathways. Both the radical (SH2) and the ionic (SN2) pathway have barriers which are low enough to explain the experimental kinetic data.

中文翻译：

---

RhIIMetalloradical 对 2,2,6,6-Tetramethyl-piperidine-1-oxyl 的碳-碳键活化：结合实验和理论研究

在铑 (II) 内消旋四甲基卟啉 [Rh(II)(tmp)] (1) 和 2,2 之间的反应中确定了竞争性主要碳-碳键活化 (CCA) 和次要碳-氢键活化 (CHA) 通道,6,6-四甲基-哌啶-1-氧基 (TEMPO) (2)。CCA 和 CHA 途径分别导致 [Rh(III)(tmp)Me] (3) 和 [Rh(III)(tmp)H] (5) 的形成。在过量 TEMPO 存在下，[Rh(II)(tmp)] 从 [Rh(III)(tmp)H] 再生，形成 2,2,6,6-四甲基-哌啶-1-醇 (TEMPOH) (4) 通过随后的氢原子提取途径。CCA 产品 [Rh(III)(tmp)Me] 的产量随着温度升高而增加，但 CHA 产品 TEMPOH 在 50-80 摄氏度的温度范围内。CCA 和 CHA 途径都遵循二级动力学。通过动力学研究和DFT计算研究了TEMPO碳-碳键活化的机制。破坏对称性，沿开壳单线态表面的无限制 b3-lyp 计算揭示了低能过渡态 (TS1)，用于通过 Rh(II) 自由基（SH2 型机制）直接提取 TEMPO 甲基自由基。沿着封闭壳单线态表面确定了另一种离子途径，其屏障稍高。该离子途径分两个连续步骤进行：电子从 TEMPO 转移到 [Rh(II)(por)]，产生 [TEMPO]+ [RhI(por)]- 阳离子-阴离子对，然后是净 CH3+ 从 TEMPO+ 转移到 Rh (I) 形成 [Rh(III)(por)Me] 和（DMPO 样）2,2,6-三甲基-2,3,4,5-四氢-1-吡啶鎓酸盐。该过程的过渡态 (TS2) 最好描述为 SN2 样亲核取代，涉及攻击 [Rh(I)(por)]- 的 d(z)2 轨道在 C(Me)-C 之一[TEMPO]+ 的（环）sigma* 轨道。尽管计算出的开壳自由基途径的势垒比离子途径的势垒略低，但 R-DFT 和 U-DFT 不太可能相对准确，无法可靠地区分这些可能的途径。自由基 (SH2) 和离子 (SN2) 途径都具有足够低的势垒来解释实验动力学数据。R-DFT 和 U-DFT 不太可能相对准确，无法可靠地区分这些可能的途径。自由基 (SH2) 和离子 (SN2) 途径都具有足够低的势垒来解释实验动力学数据。R-DFT 和 U-DFT 不太可能相对准确，无法可靠地区分这些可能的途径。自由基 (SH2) 和离子 (SN2) 途径都具有足够低的势垒来解释实验动力学数据。