Late first-row transition metal complexes of a 17-membered piperazine-based macrocyclic ligand: structures and magnetism
Abstrakt
A 17-membered piperazine-based macrocyclic ligandL(diProp)(1,5,13,17,22-pentaazatricyclo[15.2.2.17,11]docosa-7,9,11(22)-triene) was resynthesized in high yield by using a linear pump. Its Mn(ii), Fe(ii), Co(ii) and Ni(ii) complexes of the general formula [MnLdiProp(ClO4)(2)] (1), [FeLdiProp(CH3CN)](ClO4)(2)(2), [CoLdiProp(CH3CN)](ClO4)(2)(3), [NiLdiProp](ClO4)(2)(4) were prepared and thoroughly characterized.
X-ray diffraction analysis confirmed that Mn(ii) complex1has capped trigonal prismatic geometry with a coordination number of seven, Fe(ii) and Co(ii) complexes2and3are trigonal prismatic with a coordination number of six and Ni(ii) complex4has square pyramidal geometry with a coordination number of five. The decrease of the coordination number is accompanied by a shortening of M-N distances and an increase of torsion of the piperazine ring from the equatorial plane.
Magnetic measurement reveals moderate anisotropy for4and rather large magnetic anisotropy for2and3(axial zero-field splitting parameterD(Ni) = 9.0 cm(-1),D(Fe) = -14.4 cm(-1),D(Co) = -25.8 cm(-1), together with rather high rhombicity). Co(ii) complex3behaves as a field-induced SMM with a combination of Raman and direct or Orbach and direct relaxation mechanisms.
Obtained magnetic data were extensively supported by theoretical CASSCF calculations. The flexibility and rather large 17-membered macrocyclic cavity of ligandL(diProp)could be responsible for the variation of coordination numbers and geometries for the investigated late-first row transition metals.