Structural, magnetic, redox and theoretical characterization of seven-coordinate first-row transition metal complexes with a macrocyclic ligand containing two benzimidazolyl N-pendant arms

Abstract

Structural, magnetic and redox properties of pentagonal bipyramidal late-first-row transition metal complexes of a benzimidazole-based macrocyclic ligand were thoroughly investigated with help of theoretical calculations.

Magnetic anisotropy of transition metal and lanthanide ions in pentagonal bipyramidal geometry

“…The [Co(L5 R )] n + complexes based on the heptadentate ligand L5 R , where R consists of two arms allowing the coordination of specific groups in the apical positions of Co( ii ), are a good illustration. 52 Changing the apical ligands from pyridine to benzimidazole induced an increase of D from 34 to 40 cm −1 , in apparent contradiction with the decrease anticipated for stronger σ donor groups. The origin of this effect has been found in structural distortions that alter the axial and equatorial ligand fields.…”

“…A typical example is [Mn(L3)(H 2 O) 2 ]·2Cl for which extensive X and Q band EPR studies performed in the solid state for a diluted sample at 300 K and 77 K, led to an axial crystal-field parameter D = 0.07 cm −1 . 51 For [Mn(L5 R )] 2+ (R = 2-benzimidazolylmethyl), 52 a combined analysis of low temperature magnetic behavior and theoretical calculations resulted in negative D values (−0.30 and −0.12 cm −1 , respectively). Moreover, D was found rather insensitive to the nature of the ligands in the apical positions for the series of complexes [Mn(L5 H )X 2 ] with X = Cl − , Br − , I − , N 3 − , or NCS − .…”

“…For R = carboxylate the magnetic anisotropy was characterized by D = −9.6 cm −1 whereas a positive value of 7.9 cm −1 was obtained with R = benzimidazole. 52,75 For the latter, the positive value for D was attributed to a distortion of the coordination polyhedron with respect to the PBP geometry. This example well illustrates that the energy difference between the d xz and the d yz orbitals is also related to structural considerations.…”

“…A theoretical investigation pointed also to a strong effect of the equatorial ligand field, the magnetic anisotropy becoming more negative as the ligand field is weaker. 52…”

Magnetic Properties of Fe(II) Complexes of Cyclam Derivative with One p-Aminobenzyl Pendant Arm

“…The best fit was found for the parameters g = 2.24, D = 17.8 cm −1 , E/D = 0.33, zj = −0.24 cm −1 for complex 1 and g = 2.17, D = 20.9 cm −1 , E/D = 0.31, zj = 0.24 cm −1 for complex 2 ( Figure 5). The obtained ZFS-parameters were close to those found in compound [Fe(L-(NH 2 ) 2 )]Cl 2 •2.5H 2 O (D = -17.7 cm −1 , E/D = 0.31) [25] except for the sign of the D-parameter, which could be most likely related to the changes (symmetry and strength) in the ligand field, because, as we showed in another paper, the sign of the D-parameter is very sensitive to the modulation of the ligand field [37]. Such relatively high magnetic anisotropy is typical for HS Fe II octahedral complexes [38].…”