Can aromatic interactions control the coordination geometry of zinc complexes? Structural evidence and a possible mechanism for the conversion of trigonal-bipyramidal to octahedral compounds
作者:Nicole Niklas、Olaf Walter、Frank Hampel、Ralf Alsfasser
DOI:10.1039/b204125c
日期:2002.8.29
Aromatic interactions are well known to participate in the structural stabilization of biological macromolecules. We have utilized this motif as an unconventional new means to control the stereochemistry of synthetic zinc complexes. The aromatic amino acid L-phenylalanine has been applied as a building block in the chiral ligand bpaAc–Phe–OMe (N,N-bis(2-picolyl)aminoacyl-(S)-phenylalanine-methylester). Zinc(II) complexes of this ligand are always trigonal–bipyramidal in the solid state. This is confirmed by the X-ray structures of [(bpaAc–Phe–OMe)Zn(OTf)]+ (1b, OTf = Triflate), [(bpaAc–Phe–OMe)Zn(H2O)]2+ (2b), and [(bpaAc–Phe–OMe)Zn(pz)]2+ (3b). In contrast, the octahedral complexes
[(bpaAc–Gly–OEt)Zn(pz)(OTf)]+ (3a) and [(bpaAc–Gly–OEt)Zn(N-Meim)(H2O)]2+ (4a) have been obtained with the related glycine derived ligand bpaAc–Gly–OEt. A comparison with the five-coordinate structures of [(bpaAc–Gly–OEt)Zn(OTf)]+ (1a) and [(bpaAc–Gly–OEt)Zn(Cl)]+ (5) allows an appreciation of effects brought about by (1) the coligand (chloride, triflate, pyrazole, N-methylimidazole) and (2) the non-coordinating amino acid side chain (benzyl in bpaAc–Phe–OMe). It is shown that the pyrazole complex 3b adopts an unusual tense five-coordinate geometry which is stabilized by weak aromatic interactions. The structure represents
an analogue of the possible transition state between five-coordinate trigonal-bipyramidal and six-coordinate octahedral complexes in our series.
众所周知,芳香族相互作用参与生物大分子的结构稳定。我们利用这个基序作为一种非常规的新方法来控制合成锌配合物的立体化学。芳香族氨基酸 L-苯丙氨酸已被用作手性配体 bpaAc-Phe-OMe(N,N-双(2-吡啶甲基)氨基酰基-(S)-苯丙氨酸甲酯)的构建单元。该配体的锌(II)配合物在固态下始终是三方-双锥体。 [(bpaAc–Phe–OMe)Zn(OTf)]+ (1b, OTf = Triflate)、[(bpaAc–Phe–OMe)Zn(H2O)]2+ (2b) 的 X 射线结构证实了这一点,和 [(bpaAc–Phe–OMe)Zn(pz)]2+ (3b)。相比之下,八面体配合物
[(bpaAc–Gly–OEt)Zn(pz)(OTf)]+ (3a) 和 [(bpaAc–Gly–OEt)Zn(N-Meim)(H2O)]2+ (4a) 已通过相关公式获得甘氨酸衍生配体 bpaAc-Gly-OEt。与 [(bpaAc–Gly–OEt)Zn(OTf)]+ (1a) 和 [(bpaAc–Gly–OEt)Zn(Cl)]+ (5) 的五配位结构进行比较,可以了解所带来的影响大约由 (1) 共配体(氯化物、三氟甲磺酸酯、吡唑、N-甲基咪唑)和 (2) 非配位氨基酸侧链(bpaAc-Phe-OMe 中的苄基)组成。结果表明,吡唑配合物3b采用了不寻常的紧张五配位几何结构,该几何结构通过弱芳香相互作用而稳定。该结构代表
我们系列中五配位三方双锥体和六配位八面体配合物之间可能的过渡态的类似物。