Mealli Carlo, Ienco Andrea, Peruzzini Maurizio, Manca Gabriele
Istituto di Chimica dei Composti Organometallici - Consiglio Nazionale delle Ricerche (CNR-ICCOM), Via Madonna del Piano 10, 50019, Sesto Fiorentino, FI, Italy.
Dalton Trans. 2018 Jan 2;47(2):394-408. doi: 10.1039/c7dt04034b.
A detailed mechanism of the I-induced transformation of white phosphorus into PI emerges from a DFT analysis. This multi-step process implies that at any stage one P-P and two I-I bonds cleavages, associated with the formation of two P-I bonds plus an in situ generated brand new I molecule. Significant electron transfer between the atoms is observed at any step, but the reactions are better defined as concerted rather than redox. Along the steepest descent to the product, no significant barrier is encountered except for the very first P activation, which costs +14.6 kcal mol. At the atomic level, one first I molecule, a typical mild oxidant, is first involved in a linear halogen bonding interaction (XB) with one P donor, while its terminal I atom is engaged in an additional XB adduct with a second I. Significant electron transfer through the combined diatomics allows the external I atom of the dangling I grouping to convey electrons into the σ* level of one P-P bond with its consequent cleavage. This implies at some point the appearance of a six-membered ring, which alternatively switches its bonding and no-bonding interactions. The final transformation of the PI diphosphine into two PI phosphines is enlightening also for the specific role of the I substituents. In fact, it is proved that an organo-diphosphine analogue hardly undergoes the separation of two phosphines, as reported in the literature. This is attributable to the particularly high donor power of the carbo-substituted P atoms, which prevents the concertedness of the reaction but favors charge separation in an unreactive ion pair.
密度泛函理论（DFT）分析揭示了碘诱导白磷转化为碘化磷的详细机制。这个多步过程意味着在任何阶段，一个P-P键和两个I-I键发生断裂，同时形成两个P-I键以及一个原位生成的全新碘分子。在任何步骤中都观察到原子间有显著的电子转移，但这些反应更宜定义为协同反应而非氧化还原反应。沿着最陡下降路径生成产物时，除了第一步磷的活化需要 +14.6 kcal/mol 的能量外，没有遇到显著的能垒。在原子层面，一个典型的温和氧化剂——碘分子首先与一个磷供体形成线性卤素键相互作用（XB），而其末端碘原子与另一个碘形成额外的XB加合物。通过结合的双原子分子进行的显著电子转移，使得悬垂碘基团的外部碘原子将电子传递到一个P-P键的σ*轨道，从而导致该键断裂。这意味着在某个时刻会出现一个六元环，其键合和非键合相互作用会交替切换。碘化磷二膦转化为两个碘化磷膦的最终过程，对于碘取代基的特定作用也具有启发性。事实上，如文献报道，有机二膦类似物很难发生两个膦的分离。这归因于碳取代磷原子具有特别高的给电子能力，这阻止了反应的协同性，但有利于在无反应性离子对中进行电荷分离。
