The Structure of (t-butylpyrazole)2VOCl2

A V=O--V head-to-tail bridged vanadyl chain with a helical twist





Like many similar compounds, the Scifff base complex N,N'-propylenebis(salicyldiminato)oxovandium(IV) is the classic example, these vanadyl complexes aggregate to form head-to-tail linked chains in the solid (but are monmeric in solution). An interesting twist in this compound not seen in others is a pronounced helical superstructure surrounding the oxovandium core. Neighboring complexes in the chain are related to one another by an approximate 45° rotation about the chain axis. This helical twist occurs to accomodates hydrogen bonding between pyrazole protons of one complex and chloride ions of the neighbor.

The two-fold rotation symmetry of the individual complexes combined with the twist angle generates a four complex repeat unit for the chain. One consequence of this repeat unit is the apparent relction condition 00l, l=4n found in addition to an n-glide reflection condition. These two conditions do not coorespond to any space group, but careful examination of the first reveals weak violations that prodcue a true reflection condition00l, l=2n which, when combined with the n-glide, yields the tetragonal space group P42/n. This is an interesting structure to use as an illustration of the need of correctly identifying the space group. There is a fair amount of space group ambiguity at the start, but once the correct space group is chosen, the solution falls right out. It also illustrates the danger in rejecting violations of systematic absences. The violations observed here are weak enough to be dismissed as arising from double reflection (as is often done for weak violations), and their significance is easily missed.

Literature reference: Mohan, M., Bond, M. R., Otieno, T., and Carrano, C. J. "Oxovanadium Pyrazole Complexes: Synthesis and Structure" Inorg. Chem. 1995, 34, 1233-1242.

Companion Structure: None

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Last changed December 6, 1996

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