Syntheses, systematic potentiometry and structural studies of 26-membered hexaaza-diphenolate-based macrocyclic diiron complexes

Homodinuclear ferrous complexes with a new 26-membered macrocyclic ligand: 3,6,10,18,22,25-hexaaza-31,32-dihydroxy-14,29-dimethyl-tricyclo[25,3,1,1,11,17]dotriaconta-1(30),12,14,16(32),27,28-hexaene (L or BTBP), have been synthesized and characterized with elemental analysis, NMR, FAB MS and X-ray. The ligand maintains preorganized dinuclear integrity for iron(II) complexes, while facilitating the formation of non-bridged diiron centers. Potentiometric equilibrium studies indicate that a variety of protonated, mononuclear and dinuclear complexes are formed with Fe(II) and Fe(III) from p[H] 2 through 12 in aqueous solution. The stability constants and species distribution as a function of p[H] of the 1:1, 1:2 and 1:1:1 [ligand: Fe(II)/Fe(III) or ligand:Fe(II):Fe(III)] complexes were determined in KCl supporting electrolyte (μ=0.100 M) at 25 °C. Dinuclear iron(II) complex easily forms μ-peroxo adduct at 1 atm oxygen atmosphere with the oxygenation constant log K[O2]=7.74. The neutral homodinuclear complexes [Fe2C28H44N6O2(CO3)2)]·12(H2O) forms triclinic crystals, of space group P, with a=8.318(10) Å, b=9.443(12) Å, c=14.066(18) Å, α=73.925(2), β=73.510(2), γ=87.775(2)° and Z=1. Each iron(II) is complexed by half of the coordination sites of the dinucleating macrocycle, and the anionic carbonate enters into the coordination sphere and completes a six-coordinated octahedron.