Structure and Polymorphism of M(thd)3 (M = Al, Cr, Mn, Fe, Co, Ga, and In)

Formation, crystal structure, polymorphism, and transition between polymorphs are reported for M(thd)3, (M = Al, Cr, Mn, Fe, Co, Ga, and In) [(thd) = anion of H(thd) = C11H20O2 = 2, 2, 6, 6-tetramethylheptane-3, 5-dione]. Fresh crystal-structure data are provided for monoclinic polymorphs of Al(thd)3, Ga(thd)3, and In(thd)3. Apart from adjustment of the MOk bond length, the structural characteristics of M(thd)3 complexes remain essentially unaffected by change of M. Analysis of the MOk, OkCk, and CkCk distances support the notion that the MOkCkCkCkOk ring forms a heterocyclic unit with sigma and contributions to the bonds. Tentative assessments according to the bond-valence or bond-order scheme suggest that the strengths of the sigma bonds are approximately equal for the MOk, OkCk, and CkCk bonds, whereas the component of the MOk bonds is small compared with those for the OkCk, and CkCk bonds. The contours of a pattern for the occurrence of M(thd)3 polymorphs suggest that polymorphs with structures of orthorhombic or higher symmetry are favored on crystallization from the vapor phase (viz. sublimation). Monoclinic polymorphs prefer crystallization from solution at temperatures closer to ambient. Each of the M(thd)3 complexes subject to this study exhibits three or more polymorphs (further variants are likely to emerge consequent on systematic exploration of the crystallization conditions). High-temperature powder X-ray diffraction shows that the monoclinic polymorphs convert irreversibly to the corresponding rotational disordered orthorhombic variant above some 100150 degrees C (depending on M). The orthorhombic variant is in turn transformed into polymorphs of tetragonal and cubic symmetry before entering the molten state. These findings are discussed in light of the current conceptions of rotational disorder in molecular crystals.