Dynamic Isomerization of a Supramolecular Tetrahedral M4L6 Cluster1

Abstract

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 (Δ or Λ) that generates clusters with T (ΔΔΔΔ or ΛΛΛΛ), C3 (ΔΔΔΛ or ΛΛΛΔ), or S4 (ΔΔΛΛ) 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 S4 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 I41/a with Z = 8, a = 24.0738(2) Å, and c = 68.5828(5) Å. Full-matrix refinement of data collected on a CCD detector with 7710 observations and 576 variables gave an R factor (on F) of 0.089. Two crystallographically independent clusters are chemically equivalent, both lying on 4̄ special positions. The Ga-to-Ga distances between metal centers with like and opposite chiralities are 9.0 and 8.8 Å, 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, C3, S4) of cluster isomers seen. This ligand couples the metal vertices in the cluster so as to increase significantly the transition state free energy for Λ−Δ interconversion but does not couple the chirality for the Λ or Δ ground state.