Syntheses, systematic potentiometry and structural studies of 26- membered hexaaza-diphenolate-based macrocyclic diiron complexes Deyuan Kong, Joseph Reibenspies, Jianggao Mao, Arthur E. Martell *, Abraham Clearfield Department of Chemistry, TEXAS A&M University, College Station, TX 77842-3012, USA Received 19 March 2002 Abstract 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 (m /0.100 M) at 25 8C. Dinuclear iron(II) complex easily forms m-peroxo adduct at 1 atm oxygen atmosphere with the oxygenation constant log K [O 2 ] /7.74. The neutral homodinuclear complexes [Fe 2 C 28 H 44 N 6 O 2 (CO 3 ) 2 )] × /12(H 2 O) forms triclinic crystals, of space group P/ ¯ 1; with a /8.318(10) A ˚ , b /9.443(12) A ˚ , c /14.066(18) A ˚ , a /73.925(2), b /73.510(2), g /87.775(2)8 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. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Hexaazamacrocyclic ligand; Iron; Dinuclear complexes; Potentiometry 1. Introduction Nonheme diiron-containing enzymes: ferritin [1], methane monooxygenase (MMO) [2,3], ribonucleotide reductase (RNR) [4 /9], and stearoyl-ACP D 9 -desaturase [10] have been extensively investigated [11  /15]. With diiron clusters at active sites, these enzymes activate O 2 for different biological functions. A common reaction between reduced diiron(II) form and O 2 to yield a diiron(III) peroxo adduct was investigated. And a diiron peroxo species was thought to be the first intermediate to be detected on the metabolic pathway for dioxygen utilization. RNR protein from E. coli which participate in the generation of a tyrosyl radical has non-heme diiron centers in its R2 subunit has been thoroughly studied. It has been reported that this protein functions through Fe(II)Fe(II), Fe(III)Fe(III)-peroxo, Fe(III)- Fe(IV), and Fe(III)Fe(III)-oxo states [4  /11]. The bio- chemical importance of diiron(II) unit has stimulated investigations at the level of synthetic inorganic model complexes during the past decade and well documented by Lippard [16  /18] and Que’s groups [19  /22]. Oxyge- nation of synthetic non-heme diiron(II) complexes usually gives the oxo-bridged diiron(III) species as final products. Some diiron(III) peroxo adducts have been isolated and studied in detail with kinetic tech- niques [23 /25]. Diphenolate-based macrocyclic tetraimine ligands were well studied [26  /28]. Nag et al. first reported the reduced diphenolate-based macrocyclic tetraamine li- gands could coordinate to homodiiron(III) complexes, and heterodinuclear complex Fe(III) /M (M /Co(III), Zn(II), Mn(II), Ni(II)) containing m-OAc  as a bridged ligand [29,30]. In the present report, we describe the syntheses and characterization of diiron complexes with * Corresponding author. Tel.:  /1-979-845 5055; fax:  /1-979-845 4719 E-mail address: martell@mail.chem.tamu.edu (A.E. Martell). Inorganica Chimica Acta 340 (2002) 170  /180 www.elsevier.com/locate/ica 0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0020-1693(02)01067-8