Magnetic, ⁵⁷Fe Mössbauer, and IR Monitoring of the Thermal Spin Transition in a New Family of Iron(II) Spin‐Transition Complexes

Abstract

A new family of iron(II) tetrafluoroborate and perchlorate spin-crossover compounds has been synthesised and is discussed. The iron(II) ion is surrounded by six 1-ethyltetrazole ligands, which are halogen-substituted on the C2 atom of the ethyl group. The spin-crossover temperatures T 1/2 are high compared to the unsubstituted (1-alkyltetrazole)iron(II) complexes. The shape of the spin-transition curve (i.e. γ HS vs. T) varies largely over the different complexes and appears to be influenced neither by the crystal packing, nor by the elec-

On the Role of Intermolecular Interactions on Structural and Spin‐Crossover Properties of 2D Coordination Networks [Fe(bbtr)3]A2 (bbtr=1,4‐bis(1,2,3‐triazol‐1‐yl)butane; A=ClO4−, BF4−)

“…This choice was justified by the observation that the spin transition is present in iron(II) complexes of 1-substituted tetrazoles with perchlorate or tetrafluoroborate as counteranions. [25,35,36] Spin crossover is exhibited by monomeric complexes based on alkyl, [35,37] cycloalkyl [38] and fluorinated alkyl tetrazoles [36,39] as well as polymeric systems containing bis- [22][23][24] and tris-tetrazoles [26] as bridging ligands. In the present study, the magnetic behaviour of 1·BF 4 differs completely from that observed for the perchlorate analogue.…”

Low‐Spin State Structure of [Fe(chloroethyltetrazole)6](BF4)2 Obtained from Synchrotron Powder Diffraction Data

“…[11,16] The series member [Fe(chloroethyltetrazole) 6 ](BF 4 ) 2 (1) shows a two-step spin transition between 300 and 90 K and it was hypothesized that two high-spin (HS) fractions of Fe II may exist with different crystallographic positioning and behavior during the spin transition. [11] A crystal structure determination of the HS structure from powder diffraction data revealed Fe II to be in a special position, but no evidence was found for different HS Fe II fractions.…”

“…[11,16] The series member [Fe(chloroethyltetrazole) 6 ](BF 4 ) 2 (1) shows a two-step spin transition between 300 and 90 K and it was hypothesized that two high-spin (HS) fractions of Fe II may exist with different crystallographic positioning and behavior during the spin transition. [11] A crystal structure determination of the HS structure from powder diffraction data revealed Fe II to be in a special position, but no evidence was found for different HS Fe II fractions. [17] The plateau in the transition curve has also been explained by the assumption that extra thermodynamic stabilization takes place when 50 % of the Fe II ions are in the HS state and 50 % in the low-spin (LS) state, [18] and in two recent articles this stabilization was attributed to the existence of regular structures of alternating HS and LS molecules.…”

“…[1,[6][7][8][9][10] A wide range of SCOs have been observed experimentally, for example, gradual complete, two-step transitions, transitions with a residual high-spin species at low temperature, and hysteresis loops. [11][12][13] Thermal SCO in coordination compounds of 3d transition metals has been well known for many years and, in particular, the compounds [Fe(alkyltetrazole) 6 ]X 2 (X = BF 4 À , ClO 4 À , CF 3 SO 3 À ) have been investigated in detail since their discovery in the early eighties, [14][15] because of their wide variety of SCO behavior.…”

High‐Spin‐ and Low‐Spin‐State Structures of [Fe(chloroethyltetrazole)6](ClO4)2 from Synchrotron Powder Diffraction Data

“…[1,[4][5][6][7][8][9] A wide range of spin crossover has been observed experimentally, for example, gradual complete, two-step transitions, incomplete transitions with a residual high-spin (HS) species at low temperature, and hysteresis loops. [10][11][12][13] Most of the studies of spin crossover have concentrated on iron(ii) compounds. [14,15] Thermal spin crossover in coordination compounds of the form [Fe(alkyltetrazole) 6 ](X) 2 (with X = BF 4 À , ClO 4 À ) were first described by Franke.…”