A solid-gas route to polymorph conversion in crystalline [Fe-II(eta(5)-C5H4COOH)(2)]. A diffraction and solid-state NMR study

Crystalline form I (monoclinic) and form II (triclinic) of ferrocene dicarboxylic acid [Fe(eta(5)-C5H4-COOH)(2)] have been employed in solid-gas reactions at room temperature 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 anhydrous crystalline salts [NH4](2)[Fe(eta(5)-C5H4COO)(2)] (1), [NH3CH3](2)[Fe(eta(5)-C5H4COO)(2)] (2), and [NH2(CH3)(2)](2)[Fe(eta(5)-C5H4COO)(2)] (3). Interestingly though, all these crystals revert via vapor release exclusively to the metastable crystalline form I. Starting materials and products have been investigated by single-crystal and powder diffraction and by C-13, N-15 CPMAS and H-1 MAS methods