Paul Rowena L, Argent Stephen P, Jeffery John C, Harding Lindsay P, Lynam Jason M, Ward Michael D
School of Chemistry, University of Bristol, UK.
Dalton Trans. 2004 Nov 7(21):3453-8. doi: 10.1039/B409809A. Epub 2004 Sep 16.
Reaction of the bis-bidentate bridging ligand L(3), in which two bidentate chelating 3(2-pyridyl)pyrazole units are separated by a 3,3'-biphenyl spacer, with Co(II) salts affords tetranuclear cage complexes of composition [Co(4)(L(3))(6)]X(8)(X =BF(4), ClO(4), PF(6) or I(-)) in which four 6-coordinate Co(II) ions in an approximately tetrahedral array are connected by six bis-bidentate bridging ligands, one spanning each of the six edges of the Co(4) tetrahedron. In every case, X-ray crystallography reveals that the 'apical' Co(II) ion has a fac tris-chelate geometry, whereas the other three Co(II) ions have mer tris-chelate geometries, resulting in (non-crystallographic)C(3) symmetry for the cages; that this structure is retained in solution is confirmed by (1)H NMR spectroscopy of the paramagnetic cages. In every case one of the anions is located inside the central cavity of the cage, with the remaining seven outside. We found no clear evidence for an anion-based templating effect. The cage superstructure is sufficiently large to leave gaps in the centres of the faces through which the internal and external anions can exchange. Variable-temperature (19)F NMR spectroscopy was used to investigate the dynamic behaviour of the cages with X =BF(4) and PF(6) in MeCN solution: in both cases two separate signals, corresponding to external and internal anions, are clear at 233 K which have coalesced to a single signal at room temperature. Analysis of the linewidth of the minor signal (for the internal anion) at various temperatures below coalescence gave an activation energy for anion exchange of ca. 50 kJ mol(-1) in each case, a figure which suggests that anion exchange can occur via a conformational rearrangement of the cage superstructure in solution rather than opening of the cavity by cleavage of metal-ligand bonds.
双齿桥连配体L(3)（其中两个双齿螯合的3-(2-吡啶基)吡唑单元由一个3,3'-联苯间隔基隔开）与Co(II)盐反应，得到组成式为[Co(4)(L(3))(6)]X(8)（X = [BF(4)]⁻、[ClO(4)]⁻、[PF(6)]⁻或I⁻）的四核笼状配合物，其中四个呈近似四面体排列的六配位Co(II)离子通过六个双齿桥连配体相连，每个配体跨越Co(4)四面体的六条边中的一条。在每种情况下，X射线晶体学研究表明，“顶端”的Co(II)离子具有面式三螯合几何构型，而其他三个Co(II)离子具有经式三螯合几何构型，从而使笼状结构具有（非晶体学的）C(3)对称性；顺磁性笼状配合物的¹H NMR光谱证实了该结构在溶液中得以保留。在每种情况下，其中一个阴离子位于笼状结构的中心腔内，其余七个在外部。我们没有发现基于阴离子的模板效应的明确证据。笼状超结构足够大，以至于在面的中心留下间隙，内部和外部阴离子可以通过这些间隙进行交换。变温¹⁹F NMR光谱用于研究X = [BF(4)]⁻和[PF(6)]⁻的笼状配合物在乙腈溶液中的动力学行为：在两种情况下，对应于外部和内部阴离子的两个单独信号在233 K时清晰可辨，而在室温下合并为一个信号。对合并温度以下不同温度下较小信号（对于内部阴离子）的线宽分析得出，每种情况下阴离子交换的活化能约为50 kJ mol⁻¹，这一数值表明阴离子交换可以通过溶液中笼状超结构的构象重排发生，而不是通过金属-配体键的断裂打开空腔来实现。
