Designed intramolecular blocking of the spin crossover of an Fe() complex

A ligand derived from 1,3bpp (2-(pyrazol-1-yl)-6-(pyrazol-3-yl)-pyridine) has been prepared to prove that the spin crossover (SCO) of an Fe( ii ) complex can be blocked by means of intramolecular interactions not related to the crystal field. Calculations show that the blocking is caused by the energy penalty incurred by the rotation of a phenyl ring, needed to avoid steric hindrance upon SCO. Ligand intramolecular interactions trap an Fe( ii ) complex in the high spin state, in a case of thermodynamic trapping rather than kinetic.