We have prepared a series of mononuclear fac and mer isomers of Ru(II) complexes containing chelating pyrazolyl-pyridine ligands, to examine their differing ability to act as hydrogen-bond donors in MeCN. This was prompted by our earlier observation that octanuclear cube-like coordination cages that contain these types of metal vertex can bind guests such as isoquinoline-N-oxide (K = 2100 M−1 in MeCN), with a significant contribution to binding being a hydrogen-bonding interaction between the electron-rich atom of the guest and a hydrogen-bond donor site on the internal surface of the cage formed by a convergent set of CH2 protons close to a 2+ metal centre. Starting with [Ru(LH)3]2+ [LH = 3-(2-pyridyl)-1H-pyrazole] the geometric isomers were separated by virtue of the fact that the fac isomer forms a Cu(I) adduct which the mer isomer does not. Alkylation of the pyrazolyl NH group with methyl iodide or benzyl bromide afforded [Ru(LMe)3]2+ and [Ru(Lbz)3]2+ respectively, each as their fac and mer isomers; all were structurally characterised. In the fac isomers the convergent group of pendant –CH2R or –CH3 protons defines a hydrogen-bond donor pocket; in the mer isomer these protons do not converge and any hydrogen-bonding involving these protons is expected to be weaker. For both [Ru(LMe)3]2+ and [Ru(Lbz)3]2+, NMR titrations with isoquinoline-N-oxide in MeCN revealed weak 1 : 1 binding (K ≈ 1 M−1) between the guest and the fac isomer of the complex that was absent with the mer isomer, confirming a difference in the hydrogen-bond donor capabilities of these complexes associated with their differing geometries. The weak binding compared to the cage however occurs because of competition from the anions, which are free to form ion-pairs with the mononuclear complex cations in a way that does not happen in the cage complexes. We conclude that (i) the presence of fac tris-chelate sites in the cage to act as hydrogen-bond donors, and (ii) exclusion of counter-ions from the central cavity leaving these hydrogen-bonding sites free to interact with guests, are both important design criteria for future coordination cage hosts.
我们制备了一系列含有螯合
吡唑基-
吡啶配体的 Ru(II) 配合物的单核 fac 和 mer 异构体,以检查它们在 MeCN 中作为氢键供体的不同能力。这是由我们之前的观察得出的,即包含这些类型
金属顶点的八核立方体配位笼可以结合
异喹啉-N-氧化物(MeCN 中的 K = 2100 M−1)等客体,对结合有重大贡献客体的富电子原子与笼内表面上的氢键供体位点之间的氢键相互作用,该氢键供体位点由靠近 2+
金属中心的一组会聚的
CH2 质子形成。从 [Ru(LH)3]2+ [LH = 3-(2-
吡啶基)-1H-
吡唑] 开始,由于面异构体形成 Cu(I) 加合物,而 mer 则分离了几何异构体。异构体则不然。
吡唑基 NH 基团用甲基
碘或苄基
溴进行烷基化,分别得到 [Ru(LMe)3]2+ 和 [Ru(Lbz)3]2+,分别为它们的面异构体和体异构体;所有这些都具有结构特征。在面式异构体中,侧链 – R 或 –
CH3 质子的会聚基团定义了氢键供体袋;在单体异构体中,这些质子不会聚集,并且涉及这些质子的任何氢键预计会较弱。对于 [Ru(LMe)3]2+ 和 [Ru(Lbz)3]2+,在 MeCN 中用
异喹啉-N-氧化物进行 NMR 滴定,显示客体和客体之间存在弱 1 : 1 结合 (K ≈ 1 M−1)复合物的 fac 异构体与 mer 异构体不存在,证实了这些复合物的氢键供体能力的差异与其不同的几何形状相关。然而,与笼状复合物相比,较弱的结合是由于阴离子的竞争而发生的,阴离子可以自由地与单核复合物阳离子形成离子对,而在笼状复合物中不会发生这种情况。我们得出的结论是(i)笼中存在 fac 三螯合物位点作为氢键供体,以及(ii)从中心腔中排除反离子,使这些氢键位点可以自由地与客体相互作用,都是未来协调笼宿主的重要设计标准。