Spin crossover Fe(ii) complexes of a cross-bridged cyclam derivative

Abstract

A cross-bridged cyclam derivative containing two 2-pyridylmethyl pendant arms (L = 4,11-bis((pyridin-2-yl)methyl)-1,4,8,11-tetraaza-bicyclo[6.6.2]hexadecane) was synthesized by dialkylation of the cross-bridged cyclam with 2-chloromethylpyridine. A series of Fe(II) complexes with L and different counter-anions of the formulas [Fe(L)][FeCl₄]·H₂O (1·H₂O), [Fe(L)]Cl₂·4H₂O (2·4H₂O), [Fe(L)](BF₄)₂·0.5CH₃CN (3·0.5CH₃CN) and [Fe(L)](BPh₄)₂·CH₃OH (4·CH₃OH) was prepared and thoroughly characterized. In all the cases, the [Fe(L)]²⁺ cation adopts a cis-V configuration with a distorted octahedral geometry, and with the FeN₆ donor set. The magnetic measurements within the temperature interval of 5–400 K revealed the spin crossover (SCO) behaviour of all the complexes with the transition temperature T_1/2 increasing with the counter anion in the order BF₄^– (3) < [FeCl₄]^2– (1) < BPh₄^– (4). However, the SCO process was complete in the case of compound 3 only, with T_1/2 = 177 K, which proceeded after removal of co-crystallized CH₃CN molecules accompanied by a change of the crystallographic phase. The SCO behaviour of 3 was also confirmed by a single crystal X-ray analysis providing the average 〈Fe–N〉 distances of 2.086 Å at 120 K and 2.197 Å at 293 K typical of low-spin, and high-spin Fe^II complexes, respectively. The obtained results clearly showed that the nature of the counter anion and the presence/absence of co-crystallized solvent molecule(s) significantly affected the temperature as well as the abruptness of the spin transition. This is the first report of SCO behaviour observed for iron complexes containing a cross-bridged cyclam derivative.