Coordination number incommensurate cluster formation.: 8.: Dynamic isomerization of a supramolecular tetrahedral M4L6 cluster
Coordination number incommensurate cluster formation.: 8.: Dynamic isomerization of a supramolecular tetrahedral M4L6 cluster
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DOI：
10.1021/ja984046s
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发表时间：
1999-05-05
影响因子：
15
通讯作者：
Raymond, KN
中科院分区：
文献类型：
--
作者：
Beissel, T;Powers, RE;Raymond, KN
双异羟肟酸配体N-二-N-（4-甲基苯基）异羟肟酸盐（E）形成M4 E6型四面体簇合物（M = Ga（III），Fe（III））。这些和其他几个四面体金属簇的合成说明了一个一般的方法来设计的超分子金属簇的基础上的无公度配位数的相互作用。在每种情况下，刚性间隔物将双齿单元分开，并排除形成除目标金属配位物质之外的金属配位物质。对于这里描述的Ga 4 E6簇，每个顶点是手性金属中心（Delta或Lambda），其产生具有T（Delta Delta Delta Delta或Lambda Lambda）、C-3（Delta Delta Delta或Lambda Lambda）或S-4（Lambda Lambda Lambda）对称性的簇。刚性配体间隔基是双峰的，在任一端容纳混合或纯手性金属中心，但一旦形成就锁定络合物的手性。因此，所有三种异构体都在溶液中可见，它们的相互转化，尽管仍在NMR时间尺度上，但明显慢于类似单分子异羟肟酸盐络合物的异构化。对于T、C3和S-4异构体，异构体在水溶液中的分布分别为4%、58%和38%。对于每个异构化步骤，通过金属中心的非离解三角扭曲发生的相互转化的势垒为58 kJ mol（-1）。介绍了配体及其铁、镓配合物的合成。化合物Ga 4 E6。18 DMF（DMF =二甲基甲酰胺）在I4（1）/a中结晶，Z = 8，24.0738（2）埃，c = 68.5828（5）埃。全矩阵细化的CCD检测器上收集的数据与7710个观测值和576个变量给出了一个R因子吨Fl为0.089。两个晶体学上独立的簇是化学等价的，都位于4个特殊位置。具有相同和相反手性的金属中心之间的Ga到Ga距离分别为9.0和8.8埃。观察到两种不同的配体构象：一种是桥接纯手性金属中心，另一种是混合手性中心。它们几乎相等的稳定性解释了所见的簇异构体的混合物（T，C-3，S-4）。该配体耦合簇中的金属顶点，从而显著增加用于λ-δ互变的过渡态自由能，但不耦合用于λ或δ基态的手性。
The bis-hydroxamate ligand isophthal-di-N-(4-methylphenyl)hydroxamate (E) forms tetrahedral clusters of the type M4E6 (M = Ga(III), Fe(III)). The syntheses of these and several other tetrahedral metal clusters have illustrated a general approach to the design of supramolecular metal clusters based on incommensurate coordination number interactions. In each case, rigid spacers separate bidentate units and preclude formation of metal coordination species other than the one targeted. For the Ga4E6 cluster described here each vertex is a chiral metal center (Delta or Lambda) that generates clusters with T (Delta Delta Delta Delta or Lambda Lambda Lambda Lambda), C-3 (Delta Delta Delta Delta or Lambda Lambda Lambda Lambda), or S-4 (Lambda Lambda Lambda Lambda) symmetry. The rigid ligand spacer is bimodal, accommodating either mixed or homochiral metal centers at either end, but locks in the chirality of the complex once formed. Therefore all three isomers are seen in solution and their interconversion, although still on the NMR time scale, is significantly slower than isomerization of similar unimolecular hydroxamate complexes. The distribution of the isomers in aqueous solution for the T, C3, and S-4 isomers is 4, 58, and 38%, respectively. The barrier to the interconversions, which occur through a nondissociative trigonal twist at the metal centers, is 58 kJ mol(-1) for each of the isomerization steps. The syntheses of the ligand and corresponding iron and gallium complexes are described. The compound Ga4E6. 18 DMF (DMF = dimethylformamide) crystallizes in I4(1)/a with Z = 8, 24.0738(2) Angstrom, and c = 68.5828(5) Angstrom. Full-matrix refinement of data collected on a CCD detector with 7710 observations and 576 variables gave an R factor ton Fl of 0.089. Two crystallographically independent clusters are chemically equivalent, both lying on 4 special positions. The Ga-to-Ga distances between metal centers with like and opposite chiralities are 9.0 and 8.8 Angstrom, respectively. Two different ligand conformations are observed: one bridging homochiral metal centers and the other mixed chiral centers. Their nearly equal stability explains the mix (T, C-3, S-4) Of cluster isomers seen. This ligand couples the metal vertices in the cluster so as to increase significantly the transition state free energy for Lambda-Delta interconversion but does not couple the chirality for the Lambda or Delta ground state.