Role of molecular strain on the solid-state synthesis of coordination compounds from iron(II) chloride tetrahydrate and 1,10-phenanthroline under mechanical stress

A powdered crystalline mixture of iron(II) chloride tetrahydrate (FeCl₂·4H₂O) and 1,10-phenanthroline (phen) was subjected to mechanical stressing in Ar by a planetary ball-mill. By milling for 3h, the mixture turned completely non-crystalline. New IR absorption bands appeared simultaneously at 358cm^-1 due to Fe-N(phen) stretching and at 207cm^-1 due to N-Fe-N(phen) bending for [Fe(phen)₃]²⁺. Only a doublet peak due to [Fe(phen)₃]Cl₂·nH₂O was detected in the Mössbauer spectrum. All these results indicate consistently the formation of [Fe(phen)₃]Cl₂·nH₂O by milling with an almost quantitative yield. Disproportionation of the hydrated water molecules was observed after milling FeCl₂·4H₂O alone for 3h. The molecular strain triggers a solid-state exchange reaction between H₂O and phen at the contact point of two dissimilar solid particles under mechanical stressing, and promotes formation of [Fe(phen)₃]Cl₂·nH₂O in the solid state.