Polymorphism Driven by π–π Stacking and van der Waals Interactions: Preparation and Characterization of Polymorphic Vanadium Crystals of [V V O(Hacshz)(OEt)] and [V IV (Hacshz) 2 ]

Polymorphic mono‐oxo mono‐alkoxo square‐pyramidal [V V O(Hacshz)(OEt)] (where Hacshz 2– is a doubly deprotonated dianionic acetylacetosalicylhydrazone; thick plate‐shaped dark‐greenish‐brown crystals for 1a and block‐shaped dark‐reddish‐brown crystals for 1b ) and mononuclear trigonal‐prismatic [V IV (Hacshz) 2 ] (block‐shaped dark‐bluish‐brown crystals for 2a and triangular‐shaped dark‐violet crystals for 2b ) were synthesized and characterized. The crystals were prepared by simple addition of vanadium( III ) acetylacetonate to an ethanol solution of salicylhydrazide (H 3 shz). Depending on the ratio of the ligand to the metal ion, the extent of exposure to molecular oxygen, and the filtration of the intermediate, various polymorphs could be obtained as both mixtures and pure forms. A common π–π stacked dimeric species was observed in both polymorphs 1a and 1b . The dimeric species behaves as a common repeating motif, but in different interaction modes. A second type of π–π interaction, in addition to van der Waals interactions, was observed in crystals of 1a , while only van der Waals interactions were observed in crystal 1b . Another type of π–π‐stacked dimeric vanadium( IV ) species again behaves as a repeating motif in crystals of 2a and 2b , but in different interaction modes. The thermal properties, such as the decomposition behavior and melting points, of the two different polymorphs of the same vanadium complexes are very similar but not identical. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)