Spin cross over: The spin cross-over (SCO) phenomenon is a reversible magnetic switching between low-spin (LS) and high-spin (HS) states due to the modulation of ligand field strengths depending on the temperature, light and pressure.

It is the effect which describes that changing the spin state(LS HS) interconversion with external perturbations such as temperature (T), pressure (P) and electromagnetic radiation (hυ). For iron (II)complexes, TSCO involves the change in spin states with respective temperature. At high temperature complex is in high spin state (S=2) and low temperature complex is in low spin state (S=0). [1]

Fe (II)

The spin transition temperature (T1/2) is around or above room temperature great interest in molecular electronics and Magnetic Resonance Imaging (MRI) scanning.

N BrBr

COOCH3

N BrN

COOC2H5

NNN

COOC2H5

N Si

NN

COOC2H5

NNN

N NN

N

COOC2H5

NNN N

NN

NH2

NNN N

NN

I

1 2 3

47

a b

g e

NNN N

NN

COOHf

N OHHO

COOH

h

56

8

NN

NNN

NNN N

NN

N

i

c

N N

NN

NNNN

NN

N N

N

N

NN N

N

n

j , kL1

L1

x-

I; [Fe(L1)n]2nClO4.2xH2OII; [Fe(L1)n]2nBF4.6H20

Were X-; (ClO4)2- or (BF4)2-

Scheme 1: Syntheses of ligand (L1 and they polymers), Reagents and conditions: (a) POBr3/ Br2/ P2O5/ CH3OH/ 18 h/ 180 ºC (b) (i) K/ diglyme / pyrazole/ 3 d/ 110 ºC (ii) HCl/ C2H5OH/ Conc.H2SO4 (c) TMSA/ [Pd(PPh3)4]/ CuI/ THF/ (C2H5)3N/ 4h/ RT (d) KF/Methanol/ 30 min/ r.t. (e) Butyl azide/ Sodium ascorbate/ Copper(II) sulphate/ 10h/ RT (2M) (f) LiOH/ THF/ HCl (g) (COCl)2/ NaN3/ TFA/ K2CO3 (h) NaNO2/ Conc.HCl/ aq. KI, (i) Suzuki Conditions: [Pd(PPh3)4], K2CO3, 1,4-bisphenyleneboronicacid in 1,4-dioxane at 80 °C (j)Fe(ClO4)2/DCM/MeOH, (k)Fe(BF4)2/DCM/MeOH.