Department of Chemistry and Biochemistry, Duquesne University, 600 Forbes Avenue, 308 Mellon Hall, Pittsburgh, Pennsylvania 15282, USA.
Inorg Chem. 2012 Nov 5;51(21):11917-29. doi: 10.1021/ic3018198. Epub 2012 Oct 10.
Kinetic and mechanistic studies of atom transfer radical addition (ATRA) catalyzed by copper complexes with tris(2-pyridylmethyl)amine (TPMA) ligand were reported. In solution, the halide anions were found to strongly coordinate to Cu(I)(TPMA) cations, as confirmed by kinetic, cyclic voltammetry, and conductivity measurements. The equilibrium constant for atom transfer (K(ATRA) = k(a)/k(d)) utilizing benzyl thiocyanate was determined to be approximately 6 times larger for Cu(I)(TPMA)BPh(4) ((1.6 ± 0.2) × 10(-7)) than Cu(I)(TPMA)Cl ((2.8 ± 0.2) × 10(-8)) complex. This difference in reactivity between Cu(I)(TPMA)Cl and Cu(I)(TPMA)BPh(4) was reflected in the activation rate constants ((3.4 ± 0.4) × 10(-4) M(-1) s(-1) and (2.2 ± 0.2) × 10(-3) M(-1) s(-1), respectively). The fluxionality of Cu(I)(TPMA)X (X = Cl or Br) in solution was mainly the result of TPMA ligand exchange, which for the bromide complex was found to be very fast at ambient temperature (ΔH(‡) = 29.7 kJ mol(-1), ΔS(‡) = -60.0 J K(-1) mol(-1), ΔG(‡)(298) = 47.6 kJ mol(-1), and k(obs,298) = 2.9 × 10(4) s(-1)). Relatively strong coordination of halide anions in Cu(I)(TPMA)X prompted the possibility of activation in ATRA through partial TPMA dissociation. Indeed, no visible differences in the ATRA activity of Cu(I)(TPMA)BPh(4) were observed in the presence of as many as 5 equiv of strongly coordinating triphenylphosphine. The possibility for arm dissociation in Cu(I)(TPMA)X was further confirmed by synthesizing tris(2-(dimethylamino)phenyl)amine (TDAPA), a ligand that was structurally similar to currently most active TPMA and Me(6)TREN (tris(2-dimethylaminoethyl)amine), but had limited arm mobility due to the rigid backbone. Indeed, Cu(I)(TDAPA)Cl complex was found to be inactive in ATRA, and the activity increased only by opening the coordination site around the copper(I) center by replacing chloride anion with less coordinating counterions such as BF(4)(-) and BPh(4)(-). The results presented in this Article are significant from the mechanistic point of view because they indicate that coordinatively saturated Cu(I)(TPMA)X complexes catalyze the homolytic cleavage of carbon-halogen bond during the activation step in ATRA by prior dissociation of either halide anion or TPMA arm.
报道了铜配合物与三（2-吡啶甲基）胺（TPMA）配体催化原子转移自由基加成（ATRA）的动力学和机理研究。在溶液中，卤化物阴离子被发现与[Cu（I）（TPMA）]（+）阳离子强烈配位，这通过动力学、循环伏安法和电导率测量得到了证实。利用苄基硫氰酸盐测定的原子转移平衡常数（K（ATRA）= k（a）/k（d））对于 Cu（I）（TPMA）BPh（4）（（1.6±0.2）×10（-7））比 Cu（I）（TPMA）Cl（（2.8±0.2）×10（-8））复合物大约大 6 倍。Cu（I）（TPMA）Cl 和 Cu（I）（TPMA）BPh（4）之间的这种反应性差异反映在活化速率常数上（（3.4±0.4）×10（-4）M（-1）s（-1）和（2.2±0.2）×10（-3）M（-1）s（-1），分别）。Cu（I）（TPMA）X（X=Cl 或 Br）在溶液中的流动性主要是 TPMA 配体交换的结果，对于溴化物配合物，在环境温度下发现其交换速度非常快（ΔH（‡）=29.7 kJ mol（-1），ΔS（‡）=-60.0 J K（-1）mol（-1），ΔG（‡）（298）=47.6 kJ mol（-1），k（obs，298）=2.9×10（4）s（-1））。卤化物阴离子在 Cu（I）（TPMA）X 中的强配位促使通过部分 TPMA 解离在 ATRA 中进行活化。事实上，在存在多达 5 当量的强配位三苯基膦的情况下，观察到 Cu（I）（TPMA）BPh（4）的 ATRA 活性没有明显差异。Cu（I）（TPMA）X 中臂的解离可能性进一步通过合成三（2-（二甲氨基）苯基）胺（TDAPA）得到证实，该配体在结构上与目前最活跃的 TPMA 和 Me（6）TREN（三（2-二甲基氨基乙基）胺）相似，但由于刚性骨架，臂的移动性有限。事实上，发现 Cu（I）（TDAPA）Cl 配合物在 ATRA 中没有活性，只有通过用配位性较弱的抗衡离子（如 BF（4）（-）和 BPh（4）（-）取代氯离子来打开铜（I）中心周围的配位位点，其活性才会增加。本文中提出的结果从机理角度来看是重要的，因为它们表明配位饱和的 Cu（I）（TPMA）X 配合物通过预先解离卤化物阴离子或 TPMA 臂，在 ATRA 的活化步骤中催化碳-卤键的均裂。
