Coordination incommensurate cluster formation .4. Rearrangement reactions in dinuclear triple helicates

A series of dinuclear M(III) (M = Fe or Ga) catecholate complexes has been prepared using bisbidentate catecholate ligands (L). The products contain discrete, dinuclear M-2(L)(3)(6-) anions featuring pseudo-octahedral coordination centers. The helical nature of the dinuclear complexes has been established by CD spectroscopy and X-ray crystallography. The salt (N(CH3)(4))(6)Ga-2(L-3)(3) (L-3 = N,N'-bis(2,3-dihydroxy-4-carbamoylbenzoyl)-1,4-phenylene-diamine) has been characterized by X-ray diffraction; crystals are hexagonal, space group <P(3)over bar 1c> with unit cell dimensions a = 14.283(2) Angstrom, c = 42.966(2) Angstrom, V = 7591 Angstrom(3), and Z = 2. Variable-temperature H-1 NMR experiments demonstrate that the configuration inversion of the enantiomers of K6Ga2(L-4)(3) (L-4 = N,N-bis(2,3-dihydroxy-4-(isopropylcarbamoyl)benzoyl) 1,4-phenylenediamine) and K6Ga2(L-5)(3) (L-5 = N-(2,3-dihydroxy-4-(isopropylcarbamoyl)benzoyl)-N'-(2,3-dimethoxy-4-(methylcarbamoyl)benzoyl)-1,4-phenylenediamine) is facile in D2O Or DMSO-d(6). The mechanism of inversion has been probed by dynamic NMR spectroscopy, using the complex K6Ga2(L-5)(3) which exists in two isomeric forms in solution, cis- and trans. The intramolecular inversion of the dinuclear helicates occurs without cis-trans isomerization and proceeds by independent trigonal twisting of each metal center, affording the heterochiral meso complex as an intermediate. The free energy of activation for the inversion of K6Ga2(L-4)(3) in D2O at p[D] = 12.1 is Delta G(298)(double dagger) = 79(2) kJ mol(-1), with Delta H = 75(2) kJ mol(-1) and Delta S-double dagger = -12(6) J mol(-1) K-1. Under slightly acidic conditions a proton-assisted pathway becomes dominant and the rate of inversion shows a second-order dependence in [D+]. The heterochiral meso complex of Ga-2(L-4)(6)(3-) is shown to be a transient kinetic intermediate in the (Lambda,Lambda) <-> (Delta,Delta) inversion process of the helicate complex.