Rearrangement Reactions in Dinuclear Triple Helicates¹

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(CH 3 ) 4 ) 6 Ga 2 ( L 3 ) 3 ( L 3 = N, N ‘-bis(2,3-dihydroxy-4-carbamoylbenzoyl)-1,4-phenylenediamine) has been characterized by X-ray diffraction; crystals are hexagonal, space group P 3̄1 c with unit cell dimensions a = 14.283(2) Å, c = 42.966(2) Å, V = 7591 Å 3, and Z = 2. Variable-temperature 1 H NMR experiments demonstrate that the configuration inversion of the enantiomers of K 6 Ga 2 ( L 4 ) 3 ( L 4 = N, N ‘-bis(2,3-dihydroxy-4-(isopropylcarbamoyl)benzoyl)-1,4-phenylenediamine) and K 6 Ga 2 ( 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 D 2 O or DMSO- d 6 . The mechanism of inversion has been probed by dynamic NMR spectroscopy, using the complex K 6 Ga 2 ( 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 K 6 Ga 2 ( L 4 ) 3 in D 2 O at p[D] = 12.1 is Δ G ⧧ 298 = 79(2) kJ mol - 1, with Δ H = 75(2) kJ mol - 1 and Δ S ⧧ = −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 (Λ,Λ) ↔ (Δ,Δ) inversion process of the helicate complex.