A Solid-Gas Route to Polymorph Conversion in Crystalline [FeII( 5-C5H4COOH)2]. A Diffraction and Solid-State NMR Study.

Cryst. form I (monoclinic) and form II (triclinic) of ferrocene dicarboxylic acid [Fe(5-C5H4COOH)2] have been employed in solid-gas reactions at room temp. with the gaseous bases NH3, NH2(CH3), and NH(CH3)2. The two crystal forms behave in exactly the same way in the solid-gas reaction, generating the same products, identified as the anhyd. cryst. salts [NH4]2[Fe(5-C5H4COO)2] (1), [NH3CH3]2[Fe(5-C5H4COO)2] (2), and [NH2(CH3)2]2[Fe(5-C5H4COO)2] (3). Interestingly though, all these crystals revert via vapor release exclusively to the metastable cryst. form I. Starting materials and products have been investigated by single-crystal and powder diffraction and by 13C, 15N CPMAS and 1H MAS methods