Dynamics of Spin-state Interconversion and Cooperativity for Ferric Spin-crossover Complexes in the Solid State 3

Alternative Title

Bis[N-(2-(benzylamino)ethyl)salicylaldiminato]iron(III) complexes


The spin-crossover phenomenon is examined for a number of complexes with the composition [Fe(X-SalBzen)2]Y, where X-SalBzen is the N3O ligand derived from the Schiff-base condensation of N-benzylethylenediamine and X-substituted salicylaldehyde and Y is Cl- NO3- or BPh4-. The single-crystal X-ray structure of [Fe(3-allyl-SalBzen)2]NO3 has been determined at room temperature by using heavy-atom, least-squares methods, in conjunction with data measured on a four-circle diffractometer, to give R = 0.097 and RW = 0.109 for 4810 observed (F°2 > 3σ(F°2)) reflections. The compound crystallizes in the monoclinic space group P21/c with four formula weights in a cell having the dimensions a = 17.887 (4) Å, b = 21.686 (6) Å, c = 21.436 (11) Å and β = 99.71 (5)° Two crystallographically independent [Fe(3-allyl-SalBzen)2]+ cations are present. The same meridional configuration of two N3O tridentate ligands is found for both of the cations, where the oxygen atoms on the two N3O ligands are located cis relative to each other and the imino nitrogen atoms are trans. The metal-ligand atom distances in the two different cations are different and indicate that one cation is a low-spin iron(III) complex and the other iron(III) cation is intermediate between low and high spin (i.e., a spin-crossover complex). The difference in the average metal-ligand bond lengths between the two different cations is 0.046 Å. Variable-temperature Mössbauer, EPR, and magnetic susceptibility results are in agreement with the view that [Fe(3-allyl-SalBzen)2]NO3 consists of equal amounts of low-spin cations and cations that are spin crossover with a description of 67% low-spin and 33% high-spin at room temperature. [Fe(3-OEt-SalBzen)2] BPh4·CH3CN was found to undergo a spin-crossover transformation that is gradual and relatively complete with a high-spin mole fraction of 0.93 at 300 K, decreasing to 0.03 at 10 K. Slow paramagnetic relaxation for both the high- and low-spin states of [Fe(3-OEt-SalBzen)2]-BPh4·CH3CN results in sharp, well-resolved EPR signals, but extremely broad Mössbauer absorptions. Even with a 43% 57Fe-enriched sample of this compound, the signals in the Mössbauer spectrum are too broad to tell if separate high- and low-spin signals are present at temperatures above 190 K. © 1985 American Chemical Society.



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© 1985 American Chemical Society (ACS), All rights reserved.

Publication Date

01 Jan 1985