SELECTIVE BLOCKING OF COORDINATION MODES IN 1,3,5-TRIAMINO-1,3,5-TRIDEOXY-CIS-INOSITOL - ENFORCED FORMATION OF A LOW-SPIN IRON(III) HEXAAMINE COMPLEX

[Fe(tmca)(2)](3+) and [Co(tmca)(2)](3+) (tmca = ll-cis-2,4,6-trimethox ycyclohexane-1,3,5-triamine) were prepared and characterized by elemen tal analyses, NMR, and FAB(+) mass spectrometry and by spectroscopic m ethods (UV-vis). Magnetic susceptibility measurements established a lo w-spin d(5) electron configuration for the Fem hexaamine complex. In a queous solution, cyclic voltammetry revealed a quasi reversible one el ectron reduction for both complexes with M-III/M-II redox potentials o f +0.16 and -0.30 V (vs SHE), respectively. The two compounds [M(tmca) (2)]Cl-3 . 5H(2)O . EtOH (C20H58Cl3MN6O12, M = Fe, Co) are isostructur al. They both crystallize in the triclinic space group P (1) over bar, Z = 2. Fe complex: a 10.338(3) Angstrom, b = 13.062(4) Angstrom, c = 14.042(4) Angstrom, alpha = 80.20(2)degrees, beta = 72.07(2)degrees, g amma = 69.11(2)degrees. Co complex: a = 10.307(3) Angstrom, b = 13.066 (2) Angstrom, c = 14.029(2) Angstrom, alpha = 80.14(1)degrees, beta = 72.18(2)degrees, gamma = 69.55(2)degrees. The ligand tmca is compared with the unsubstituted 1,3,5-triamino-1,3,5-trideoxy-cis-inositol (tac i) and the N-methylated ,5-trideoxy-1,3,5-tris(dimethylamino)-cis-inos itol (tdci). Due to the methyl groups on the oxygen donors, tmca binds both Fe-III and Co-III ions exclusively via nitrogen donors whereas t he previously reported [Fe(tdci)(2)](3+) had an FeO6 coordination envi ronment. Equilibration of a 1:1 mixture of [Fe(tdci)(2)](3+) and [Fe(t mca)(2)](3+) in aqueous solution resulted in the quantitative formatio n of the metathesis product [Fe(tmca)(tdci)]Cl-3 . 15H(2)O (O3N3 coord ination). Crystal data: C21H78Cl3FeN6O21, trigonal, space group R3c, a = 15.158(2) Angstrom, c = 33.591(7) Angstrom, Z = 6. The differences in the structural properties of the low-spin hexaamine [Fe(tmca)(2)](3 +) complex and the high-spin [Fe(tdci)(2)](3+) and [Fe(tmca)(tdci)](3) complexes are discussed.