Department of Chemistry, Queen's University, Kingston, Ontario, Canada K7L 3N6.
Inorg Chem. 2009 Dec 7;48(23):11425-33. doi: 10.1021/ic9015965.
Two sets of dinuclear Zn(II) complexes were prepared to determine the effect of the presence of oxyanionic bridging groups between the metal centers on the catalytic activity toward the methanolysis of the RNA analogue 2-hydroxypropyl-4-nitrophenyl phosphate (HPNPP, 2). The Zn(II)2 complexes of bis(di-(2-pyridylmethyl)amino)-m-xylene (6) and 2,6-bis(di-(2-pyridylmethyl)amino)-4-methylphenol (7) were compared to assess the effect of a bridging phenoxide ligand, while the Zn(II)2 complex of 1,3-bis-N1-(1,5,9-triazacyclododecyl)-propan-2-ol (8) was prepared to determine the effect of the 2-propoxy group compared to the previously studied complex of 1,3-bis-N1-(1,5,9-triazacyclododecyl)-propane (4). Detailed kinetic studies of the cleavage of 2 including k(obs) vs [catalyst] plots and (s)(s)pH-rate profiles were performed for each system along with potentiometric titration experiments to determine the acid dissociation constants for the catalytically relevant groups. The results show that inclusion of the phenoxy bridging group in 7:Zn(II)2 reduces the second-order catalytic rate constant (k2(cat)) for cleavage of 2 by a factor of 160 relative to that of 6:Zn(II)2, while the incorporation of a propoxy group in 8:Zn(II)2 reduces its efficacy by 3.7 x 10(4) times relative to 4:Zn(II)2. Energetics calculations reveal that 6:Zn(II)2 offers a 3.7 kcal/mol greater stabilization of the reaction transition state for the cleavage of 2 than does 7:Zn(II)2 and that 4:Zn(II)2 affords 6.5 kcal/mol greater transition state stabilization than does 8:Zn(II)2. The analyses show that the reduction in the transition state stabilization experienced with the complexes having permanently bridging oxyanion groups stems almost entirely from a weaker binding of the phosphate and catalyst, and a reduced catalytic rate constant. These results indicate that the presence of a bridging oxyanion ligand between the metal centers, a common structural element required for the successful formation of many small molecule dinuclear catalysts that show cooperative activity in water, significantly impairs the catalytic efficiency for cleavage of 2.
两组双核 Zn(II) 配合物被制备出来，以确定金属中心之间存在含氧阴离子桥连基团对催化 RNA 类似物 2-羟丙基-4-硝基苯膦酸酯 (HPNPP，2) 甲醇解反应的活性的影响。比较了双（二-（2-吡啶基甲基）氨基）-间二甲苯（6）和 2,6-双（二-（2-吡啶基甲基）氨基）-4-甲基苯酚（7）的 Zn(II)2 配合物，以评估桥连酚氧配体的影响，而 1,3-双-N1-（1,5,9-三氮杂环十二烷）-丙烷-2-醇（8）的 Zn(II)2 配合物则用于确定与之前研究过的 1,3-双-N1-（1,5,9-三氮杂环十二烷）-丙烷（4）相比，2-丙氧基的影响。对每个系统进行了包括 k(obs) 与 [催化剂] 关系图和 (s)(s)pH-速率曲线在内的 2 断裂的详细动力学研究，并进行了电位滴定实验以确定催化相关基团的酸离解常数。结果表明，在 7:Zn(II)2 中包含苯氧桥连基团将 2 的断裂的二级催化速率常数 (k2(cat)) 降低了 160 倍，而在 8:Zn(II)2 中包含丙氧基将其效力降低了 3.7 x 10(4) 倍相对于 4:Zn(II)2。能垒计算表明，6:Zn(II)2 为 2 的断裂提供了 3.7 kcal/mol 的反应过渡态更大的稳定性，而 4:Zn(II)2 则为 6.5 kcal/mol 的过渡态提供了更大的稳定性比 8:Zn(II)2。分析表明，具有永久性桥连含氧阴离子基团的配合物的过渡态稳定性降低几乎完全是由于磷酸酯和催化剂的结合较弱，以及催化速率常数降低。这些结果表明，金属中心之间存在桥连含氧阴离子配体，这是许多在水中表现出协同活性的小分子双核催化剂成功形成所必需的常见结构元件，显著降低了 2 的断裂的催化效率。
