斯巴泰因
化学
结晶学
铑
位阻效应
配体(生物化学)
立体化学
核磁共振波谱
有机化学
催化作用
生物化学
受体
作者
Antonio G. De Crisci,Vincent T. Annibale,Gordon K. Hamer,Alan J. Lough,Ulrich Fekl
出处
期刊:Dalton Transactions
[The Royal Society of Chemistry]
日期:2010-01-01
卷期号:39 (11): 2888-2888
被引量:7
摘要
A cationic rhodium-sparteine complex, [((−)-sparteine)Rh(η4-COD)]+ (1+; COD = 1,5-cyclooctadiene) was obtained, isolated as its tetrafluoroborate salt (1BF4), and characterized using X-ray crystallography and multinuclear (1H, 13C) NMR spectroscopy. This is the first structurally characterized sparteine complex of rhodium. The Rh–N bonds are unusually long (2.214(3) and 2.242(3) Å), apparently due to steric repulsion between COD and sparteine. 1H NMR exchange experiments (EXSY) demonstrate a dynamic process that results in an overall 180° rotation of the COD methine protons in solution (CD2Cl2) with a first-order rate constant of 460 s−1 at the coalescence temperature (314 K) and interpolated rate constant of 150 s−1 at 298 K. Temperature-dependent NMR studies yield ΔH‡ = 13.0 ± 0.3 kcal mol−1, ΔS‡ = −5 ± 1 cal mol−1 K−1, such that ΔG‡298 = 14.3 ± 0.3 kcal mol−1. DFT studies (B3LYP) indicate that the loosely bound (−)-sparteine ligand rotates through a pseudo-tetrahedral transition state where both ligands are rotated approximately 90° relative to each other. While both ligands remain bound (η4-COD, κ2-sparteine), bonding to sparteine is weakened much more than bonding to COD in the transition state. DFT computed ΔG‡298 and ΔS‡ values (15.55 kcal mol−1 and −2.67 cal mol−1 K−1, respectively) agree very well with the experimental values. Attempts to find alternative mechanisms involving partial dechelation of COD and (−)-sparteine yielded slightly higher barriers along with positive ΔS values for intermediate formation.
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