Rearrangement Reactions in Dinuclear Triple Helicates1

Abstract

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 M2(L)36- 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)6Ga2(L3)3 (L3 = N,N‘-bis(2,3-dihydroxy-4-carbamoylbenzoyl)-1,4-phenylenediamine) has been characterized by X-ray diffraction; crystals are hexagonal, space group P3̄1c with unit cell dimensions a = 14.283(2) Å, c = 42.966(2) Å, V = 7591 Å3, and Z = 2. Variable-temperature 1H NMR experiments demonstrate that the configuration inversion of the enantiomers of K6Ga2(L4)3 (L4 = N,N‘-bis(2,3-dihydroxy-4-(isopropylcarbamoyl)benzoyl)-1,4-phenylenediamine) and K6Ga2(L5)3 (L5 = N-(2,3-dihydroxy-4-(isopropylcarbamoyl)benzoyl)-N‘-(2,3-dimethoxy-4-(methylcarbamoyl)benzoyl)-1,4-phenylenediamine) is facile in D2O or DMSO-d6. The mechanism of inversion has been probed by dynamic NMR spectroscopy, using the complex K6Ga2(L5)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(L4)3 in D2O at p[D] = 12.1 is ΔG⧧298 = 79(2) kJ mol-1, with ΔH = 75(2) kJ mol-1 and ΔS⧧ = −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 Ga2(L4)63- is shown to be a transient kinetic intermediate in the (Λ,Λ) ↔ (Δ,Δ) inversion process of the helicate complex.

Rearrangement Reactions in Dinuclear Triple Helicates1

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Abstract

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