Kinetics and Thermodynamics of Small Molecule Binding to Pincer-PCP Rhodium(I) Complexes

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• 作者：Mark D. Doherty ; David C. Grills ; Kuo-Wei Huang ; James T. Muckerman ; Dmitry E. Polyansky ; Rudi van Eldik ; Etsuko Fujita
• 刊名：Inorganic Chemistry
• 出版年：2013
• 出版时间：April 15, 2013
• 年：2013
• 卷：52
• 期：8
• 页码：4160-4172
• 全文大小：588K
• 年卷期：v.52,no.8(April 15, 2013)
• ISSN：1520-510X

文摘

The kinetics and thermodynamics of the binding of several small molecules, L (L = N₂, H₂, D₂, and C₂H₄), to the coordinatively unsaturated pincer-PCP rhodium(I) complexes Rh[^tBu₂PCH₂(C₆H₃)CH₂P^tBu₂] (1) and Rh[^tBu₂P(CH₂)₂(CH)(CH₂)₂P^tBu₂] (2) in organic solvents (n-heptane, toluene, THF, and cyclohexane-d₁₂) have been investigated by a combination of kinetic flash photolysis methods, NMR equilibrium studies, and density functional theory (DFT) calculations. Using various gas mixtures and monitoring by NMR until equilibrium was established, the relative free energies of binding of N₂, H₂, and C₂H₄ in cyclohexane-d₁₂ were found to increase in the order C₂H₄ < N₂ < H₂. Time-resolved infrared (TRIR) and UV鈥搗is transient absorption spectroscopy revealed that 355 nm excitation of 1鈥揕 and 2鈥揕 results in the photoejection of ligand L. The subsequent mechanism of binding of L to 1 and 2 to regenerate 1鈥揕 and 2鈥揕 is determined by the structure of the PCP ligand framework and the nature of the solvent. In both cases, the primary transient is a long-lived, unsolvated species (蟿 = 50 鈥?800 ns, depending on L and its concentration in solution). For 2, this so-called less-reactive form (LRF) is in equilibrium with a more-reactive form (MRF), which reacts with L at diffusion-controlled rates to regenerate 2鈥揕. These two intermediates are proposed to be different conformers of the three-coordinate (PCP)Rh fragment. For 1, a similar mechanism is proposed to occur, but the LRF to MRF step is irreversible. In addition, a parallel reaction pathway was observed that involves the direct reaction of the LRF of 1 with L, with second-order rate constants that vary by almost 3 orders of magnitude, depending on the nature of L (in n-heptane, k = 6.7 脳 10⁵ M^鈥? s^鈥? for L = C₂H₄; 4.0 脳 10⁶ M^鈥? s^鈥? for L = N₂; 5.5 脳 10⁸ M^鈥? s^鈥? for L = H₂). Experiments in the more coordinating solvent, THF, revealed the binding of THF to 1 to generate 1鈥揟HF, and its subsequent reaction with L, as a competing pathway.