Cubes, Squares, and Books: A Simple Transition Metal/Bridging Ligand Combination Can Lead to a Surprising Range of Structural Types with the Same Metal/Ligand Proportions

Reaction of two structurally related bridging ligands L26Py and L13Ph, in which two bidentate chelating pyrazolyl-pyridine units are connected to either a 2,6-pyridine-diyl or 1,3-benzene-diyl central group via methylene spacers, with first-row transition metal dications, results in a surprising variety of structures. The commonest is that of an octanuclear coordination cage [M8L12]X16 [M = Co(II) or Zn(II); X = perchlorate or tetrafluoroborate] in which a metal ion is located at each of the eight vertices of an approximate cube, and one bis-bidentate bridging ligand spans each edge. The arrangement of fac and mer tris-chelate metal centers around the inversion center results in approximate (non-crystallographic) S6 symmetry. Another structural type observed in the solid state is a hexanuclear complex [Co6(L13Ph)9](ClO4)12 in which the six metal ions are in a rectangular array (two rows of three), folded about the central Co−Co vector like a partially open book, with each metal−metal edge containing one bridging ligand apart from the two outermost metal−metal edges which are spanned by a pair of bridging ligands in a double helical array. The final structural type we observed is a tetranuclear square [Ni4(L26Py)6](BF4)8, with the four Ni−Ni edges spanned alternately by one and two bridging ligand such that it effectively consists of two dinuclear double helicates cross-linked by additional bridging ligands. A balance between the “cube” and “book” forms, which varied from compound to compound, was observed in solution in many cases by 1H NMR and ES mass spectrometry studies.

将两种结构相近的桥联配体L26Py与L13Ph与第一过渡系金属二价阳离子进行反应，可得到结构类型出人意料的配位产物。这两种配体均包含两个双齿螯合吡唑基吡啶单元，二者分别通过亚甲基间隔基连接至2,6-吡啶二基或1,3-苯二基中心骨架。其中最常见的产物为八核配位笼[M₈L₁₂]X₁₆（M为Co(II)或Zn(II)，X为高氯酸根或四氟硼酸根）：该结构中近似立方体的8个顶点各排布一个金属离子，每条棱边均由一个双双齿桥联配体桥连。围绕反演中心排布的面式（fac）与经式（mer）三螯合金属中心，使该笼状结构具有近似的非晶体学S₆对称。固态下观察到的另一种结构类型为六核配合物[Co₆(L13Ph)₉](ClO₄)₁₂：6个金属离子呈矩形阵列排布（两行各3个），沿中心Co-Co矢量折叠呈半开书本状；除最外侧的两条金属-金属边由双螺旋排列的一对桥联配体桥连外，其余每条金属-金属边均由单个桥联配体桥连。我们观测到的第三种结构类型为四核正方形配合物[Ni₄(L26Py)₆](BF₄)₈：4条Ni-Ni边交替由单个桥联配体与两个桥联配体桥连，整体可视为由额外桥联配体交联的两个双核双螺旋单元。在多数体系的溶液中，通过¹H核磁共振波谱与电喷雾质谱研究可观察到"立方体"与"书本"两种结构间的平衡，且该平衡因配合物种类不同而存在差异。