Divalent first-row transition metal complexes of the rigid pendant-arm ligand 1,4,7-tris(2-aminophenyl)-1,4,7-triazacyclononane

摘要

The rigid pendant-arm macrocyclic ligand 1,4,7-tris(2-aminophenyl)-1,4,7-triazacyclononane (L~1) has been synthesized. It reacts with divalent first row transition metal perchlorate salts to yield complexes of general formula [M~(II)(L~1)][ClO_4]_2 (M = Fe~(II), Ni~(II), Cu~(II) or Zn~(II)). Single crystal X-ray crystallography indicated that [Fe~(II)(L~1)][ClO_4]_2 and [Ni~(II)(L~1)][ClO_4]_2 are isostructural with both complex cations consisting of distorted pseudo-octahedral six-co-ordinate metal centres with three macrocyclic N-donors and three anilino donors. The mean Fe-N bond length in [Fe~(II)(L~1)][ClO_4]_2 was found to be 2.10 A, which is intermediate between the normal ranges of high and low spin Fe~(II)N_6 species. Magnetometry and EPR measurements indicated that single crystals of [Fe~(II)(L~1)][ClO_4]_2 were predominately low spin, with approximately 5% of a low spin iron(III) species present in the lattice. It is postulated that this latter species is the mono anilido complex [Fe~(III)(L~(1-H))][ClO_4]_2. The crystal structures of [Ni~(II)(L~1)][ClO_4]_2 indicated a short mean Ni-N bond length of 2.09 A. the ligand field strength for this material was found to have a very high value of 12,330 cm~(-1) (B = 850 cm~(-1)) and is compared to that of some related trigonal Ni~(II)N_6 species. The crystal structure of [Cu~(II)(L~1)][ClO_4]_2 was also determined and indicated a distorted six-co-ordinate metal centre. The X-band EPR spectrum of this complex in magnetically dilute solid solution indicated a predominantly axial species although a slight rhombic distortion may be present. The variable temperature ~1H NMR of [Zn~(II)(L~1)][ClO_4]_2 in CD_3CN solution revealed a non-fluxional six-co-ordinate complex. Electrochemical analyses of the complexes indicated only irreversilble oxidation and reduction processes for [Ni~(II)(L~1)][ClO_4]_2 and [Cu~(II)(L~1)][ClO_4]_2. [Fe~(II)(L~1)][ClO_4]_2 displays at least two reversible oxidation processes with the first at +0.181 V (vs. Fc-Fc~+) being assigned as a metal centred Fe~(II) -> Fe~(III) process and the second at +0.475 V as a ligand centred process.