Dynamic isomerization of a supramolecular tetrahedral M4L6 cluster

The bis-hydroxamate ligand isophthal-di-N-(4-methylphenyl)hydroxamate (E) f orms tetrahedral clusters of the type M4E6 (M = Ga(III), Fe(III)). The synt heses of these and several other tetrahedral metal clusters have illustrate d a general approach to the design of supramolecular metal clusters based o n incommensurate coordination number interactions. In each case, rigid spac ers separate bidentate units and preclude formation of metal coordination s pecies other than the one targeted. For the Ga4E6 cluster described here ea ch vertex is a chiral metal center (Delta or Lambda) that generates cluster s with T (Delta Delta Delta Delta or Lambda Lambda Lambda Lambda), C-3 (Del ta 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 c hirality of the complex once formed. Therefore all three isomers are seen i n solution and their interconversion, although still on the NMR time scale, is significantly slower than isomerization of similar unimolecular hydroxa mate 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. Th e syntheses of the ligand and corresponding iron and gallium complexes are described. The compound Ga4E6. 18 DMF (DMF = dimethylformamide) crystallize s 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 observ ations and 576 variables gave an R factor ton Fl of 0.089. Two crystallogra phically independent clusters are chemically equivalent, both lying on 4 sp ecial 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 a nd the other mixed chiral centers. Their nearly equal stability explains th e 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 stat e free energy for Lambda-Delta interconversion but does not couple the chir ality for the Lambda or Delta ground state.