Helical Metallohost−Guest Complexes via Site-Selective Transmetalation of Homotrinuclear Complexes

We have designed a new type of bis(N2O2) chelate ligand that affords a C-shaped O6 site on
the metalation of the N2O2 sites. UV−vis and 1H NMR titration clearly showed that the complexation between
H4L and zinc(II) acetate affords 1:3 complex [LZn3]2+ via a highly cooperative process. Although the O6-recognition site of the dinuclear metallohost [LZn2] is filled with the additional Zn2+, the O6 site can bind a
guest ion with concomitant release of the initially bound Zn2+. The novel recognition process “guest
exchange” took place quantitatively when rare earth metals were used as a guest. In the case of alkaline
earth metals, selectivity of Ca2+ > Sr2+ > Ba2+ ≫ Mg2+ was observed. On the other hand, the transmetalation
did not take place at all when alkali metals were used for the guest. Accordingly, the trinuclear complex
[LZn3]2+ is excellent in discriminating charge of the guest ions. The metallohost−guest complexes thus
obtained have a helical structure, and the radius d and winding angle θ of the helix depend on the size of
the guest. The La3+ complex has the smallest θ (288°), and the Sc3+ complex has the largest θ (345°).
Because the radius and winding angles of helices are tunable by changing the guest ion, the helical
metallohost−guest complexes are regarded as a molecular spring or coil. Consequently, site-specific metal
exchange of trinuclear complex [LZn3]2+ described here will be utilized for highly selective ion recognition,
site-selective synthesis of (3d)2(4f) trimetallic complexes, and construction of “tunable” metallohelicenes.

我们设计了一种新型双(N₂O₂)型螯合配体(bis(N₂O₂) chelate ligand)，该配体在N₂O₂位点发生金属化后可构建C形O₆配位位点。紫外-可见光谱(UV−vis)与氢核磁共振(¹H NMR)滴定实验清晰证实，配体H₄L与乙酸锌(II)(zinc(II) acetate)经由高协同配位过程生成1:3型配合物[LZn₃]²⁺。尽管双核金属主体[LZn₂]的O₆识别位点被额外结合的Zn²+占据，但该O₆位点仍可结合客体离子，并伴随初始结合的Zn²+解离。当以稀土金属作为客体时，这种新型"客体交换"识别过程可定量发生。针对碱土金属，观测到其选择性顺序为Ca²+ > Sr²+ > Ba²+ ≫ Mg²+。而当客体为碱金属时，完全未发生金属转移反应。由此可见，三核配合物[LZn₃]²+在辨别客体离子电荷方面性能优异。所得到的金属主体-客体配合物均具有螺旋结构，螺旋半径d与缠绕角θ随客体离子尺寸变化而可调。其中镧(III)配合物的缠绕角最小（288°），钪(III)配合物的缠绕角最大（345°）。由于螺旋的半径与缠绕角可通过改变客体离子进行调控，这类金属螺旋主体-客体配合物可被视为分子弹簧或分子线圈。综上，本文所报道的三核配合物[LZn₃]²+的位点特异性金属交换反应，有望应用于高选择性离子识别、(3d)₂(4f)型三金属配合物的位点选择性合成，以及构建"可调谐"金属螺旋体。