Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK.
Dalton Trans. 2014 Jan 7;43(1):71-84. doi: 10.1039/c3dt52479e.
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(L(H))3 [L(H) = 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(L(Me))3 and Ru(L(bz))3 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(L(Me))3 and Ru(L(bz))3, 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.
我们制备了一系列单核面和经异构的 Ru(II) 配合物，这些配合物包含螯合吡唑基-吡啶配体，以研究它们在 MeCN 中作为氢键供体的不同能力。这是受我们之前的观察结果的启发，即包含这些类型金属顶点的八核立方配位笼可以结合客体，如异喹啉-N-氧化物（在 MeCN 中的 K = 2100 M(-1)），与笼内表面的氢键供体位点之间的电子富原子与笼内表面的氢键供体位点之间存在显著的结合贡献，形成收敛的一组 CH2 质子接近 2+ 金属中心。从 Ru(L(H))3 [L(H) = 3-(2-吡啶基)-1H-吡唑]开始，通过 fac 异构体形成 Cu(I)加合物而 mer 异构体不形成的事实来分离几何异构体。用甲基碘或溴化苄对吡唑基 NH 基团进行烷基化，得到 Ru(L(Me))3和 Ru(L(bz))3，分别为其 fac 和 mer 异构体；均进行了结构表征。在 fac 异构体中，收敛的-CH2R 或-CH3 质子组定义了氢键供体口袋；在 mer 异构体中，这些质子不收敛，并且涉及这些质子的任何氢键预计会较弱。对于 Ru(L(Me))3和 Ru(L(bz))3，在 MeCN 中用异喹啉-N-氧化物进行 NMR 滴定表明，客体与配合物的 fac 异构体之间存在弱 1:1 结合（K ≈ 1 M(-1)），而 mer 异构体不存在，这证实了这些配合物的氢键供体能力存在差异与它们的不同几何形状有关。与笼状配合物相比，这种弱结合是由于抗衡阴离子的竞争所致，抗衡阴离子可以自由地与单核配合物阳离子形成离子对，而在笼状配合物中不会发生这种情况。我们得出结论，（i）笼状中存在 fac 三螯合位点作为氢键供体，以及（ii）从中心腔室中排除抗衡离子，使这些氢键供体位点自由与客体相互作用，都是未来配位笼主体的重要设计标准。
