Criteria for Liquid Crystal Formation in 5-Alkoxy-, 5-Alkylamino-, and 5-Alkanoyl-tropolone Complexes of Transition Metals (Cu^II, Zn^II, Ni^II, Co^II, UO₂^VI, VO^IV). The First Uranium Metallomesogen. Crystal Structure of Bis(5-hexadecyloxytropolonato)copper(II)

The mesophase properties of bis(5-alkoxytropolonato)copper(II) complexes are reported and interpreted in terms of d-layer spacings, which indicate considerable intertwining of the alkyl chains. The mesogenic properties depend on the nature of the 5-substituent:  to be mesogenic, it must be alkoxy with n > 8, and may not be 5-alkanoyl or 5-alkylamino. The nature of the metal is also important:  to be mesogenic, it may be Cu^II or UO₂^VI but not Ni^II, Zn^II, Co^II, or VO^IV. The crystal structure of bis(5-hexadecyloxytropolonato)copper(II) shows how, even in the crystalline state, molecular packing is dominated by the interactions between alkyl chains, together with copper−oxygen interactions. In the Cu^II complexes, strong interactions occur between Cu atoms and O atoms of neighboring molecules. These are stronger than alkyl−alkyl interactions and therefore dominate alkyl−alkyl interactions between neighbors which otherwise govern the possibility and nature of mesogenic properties. The Cu···O interactions disrupt alkyl−alkyl chain interactions enough to produce a mesophase; such interactions do not obtain in the T_d Zn^II and Co^II complexes, and indeed, these complexes are not mesogenic. Likewise, alkyl chain interactions by the oxygens of the UO₂^VI lead to a mesophase, and U constitutes the heaviest metal ever placed into a mesogenic molecule. In the case of VO^IV, analogous synthetic steps from VOSO₄ led to a new series of binuclear complexes [(VO)₂(SO₄)(5-alkoxytropoloato)₂].