Hofmann Andreas, Jaganyi Deogratius, Munro Orde Q, Liehr Günter, van Eldik Rudi
Institute for Inorganic Chemistry, University of Erlangen-Nürnberg, Egerlandstr. 1, Germany.
Inorg Chem. 2003 Mar 10;42(5):1688-700. doi: 10.1021/ic020605r.
pi-Acceptor effects are often used to account for the unusual high lability of Pt(terpy)L (terpy = 2,2':6',2' '-terpyridine) complexes. To gain further insight into this phenomenon, the pi-acceptor effect was varied systematically by studying the lability of Pt(diethylenetriamine)OH(2) (aaa), Pt(2,6-bis-aminomethylpyridine)OH(2) (apa), Pt(N-(pyridyl-2-methyl)-1,2-diamino-ethane)OH(2) (aap), Pt(bis(2-pyridylmethyl)amine)OH(2) (pap), Pt(2,2'-bipyridine)(NH(3))(OH(2)) (app), and Pt(terpy)OH(2) (ppp). The crystal structure of the apa precursor [Pt(2,6-bis-aminomethylpyridine)Cl]Cl.H(2)O was determined. The substitution of water by a series of nucleophiles, viz. thiourea, N,N-dimethylthiourea, N,N,N',N'-tetramethylthiourea, I(-), and SCN(-), was studied under pseudo-first-order conditions as a function of concentration, pH, temperature, and pressure, using stopped-flow techniques. The data enable an overall comparison of the substitution behavior of these complexes, emphasizing the role played by the kinetic cis and trans pi-acceptor effects. The results indicate that the cis pi-acceptor effect is larger than the trans pi-acceptor effect, and that the pi-acceptor effects are multiplicative. DFT calculations at the B3LYP/LACVP level of theory show that, by the addition of pi-acceptor ligands to the metal, the positive charge on the metal center increases, and the energy separation of the frontier molecular orbitals (E(LUMO) - E(HOMO)) of the ground state Pt(II) complexes decreases. The calculations collectively support the experimentally observed additional increase in reactivity when two pi-accepting rings are adjacent to each other (app and ppp), which is ascribed to "electronic communication" between the pyridine rings. The results furthermore indicate that the pK(a) value of the platinum bound water molecule is controlled by the pi-accepting nature of the chelate system and reflects the electron density around the metal center. This in turn controls the rate of the associative substitution reaction and was analyzed using the Hammett equation.
π-受体效应常被用于解释Pt(terpy)L（terpy = 2,2':6',2''-三联吡啶）配合物异常高的活性。为了更深入地了解这一现象，通过研究Pt(二亚乙基三胺)OH(2)（aaa）、Pt(2,6-双氨甲基吡啶)OH(2)（apa）、Pt(N-(吡啶-2-甲基)-1,2-二氨基乙烷)OH(2)（aap）、Pt(双(2-吡啶甲基)胺)OH(2)（pap）、Pt(2,2'-联吡啶)(NH(3))(OH(2))（app）和Pt(terpy)OH(2)（ppp）的活性，系统地改变了π-受体效应。测定了apa前体[Pt(2,6-双氨甲基吡啶)Cl]Cl·H(2)O的晶体结构。在准一级条件下，使用停流技术研究了一系列亲核试剂（即硫脲、N,N-二甲基硫脲、N,N,N',N'-四甲基硫脲、I(-)和SCN(-)）取代水的反应，该反应是浓度、pH、温度和压力的函数。这些数据能够对这些配合物的取代行为进行全面比较，强调了动力学顺式和反式π-受体效应所起的作用。结果表明，顺式π-受体效应大于反式π-受体效应，且π-受体效应具有相乘性。在B3LYP/LACVP理论水平上的密度泛函理论（DFT）计算表明，通过向金属添加π-受体配体，金属中心的正电荷增加，基态Pt(II)配合物的前沿分子轨道的能量间距（E(LUMO) - E(HOMO)）减小。这些计算共同支持了实验观察到的当两个π-接受环彼此相邻时（app和ppp）反应性的额外增加，这归因于吡啶环之间的“电子通信”。结果还表明，铂结合水分子的pK(a)值受螯合体系的π-接受性质控制，并反映了金属中心周围的电子密度。这反过来又控制了缔合取代反应的速率，并使用哈米特方程进行了分析。
