Solid-state photochemistry. Part 1. Nature of the stereocontrol in the photodimerisation of dibenzylideneacetone by UO2 2+ ion: crystal and molecular structure of trans-dichlorobis(trans,trans-dibenzylideneacetone)dioxouranium(VI) and of its acetic acid solvate

Abstract

In order to define the orientation of the ligand trans,trans-dibenzylideneacetone (dba) before photodimerisation, two complexes, [(UO2)Cl2(dba)2](3) and [(UO2)Cl2(dba)2]·2CH3CO2H (4), were prepared and their crystal structures determined from three-dimensional X-ray data by Patterson and Fourier methods. Crystals of (3) are monoclinic, space group C2/c, with Z= 4 in a cell of dimensions (at 20 °C)a= 12.397(5), b= 11.587(3), c= 21.950(5)A, and β= 97.89(2)°; least-squares refinement for 1 697 independent reflections reached R 0.025. Crystals of (4) are triclinic, space group P, with Z= 1 in a cell of dimensions (at –155 °C)a= 11.851(2), b= 10.023(3), c= 11.668(3)A, α= 103.57(2), β= 129.20(2), and γ= 103.48(2)°; least-squares refinement for 3 023 reflections reached R 0.035. The solvated form (4) is very much more sensitive to light than the unsolvated form (3). In (4), two pairs of ethylenic bonds in adjacent molecules are 3.50(1) and 3.51(1)A apart, and the parallelograms formed by these bonds have angles of 97.7(4) and 104.4(5)°. In (3), only one CC bond is adjacent to another in a neighbouring molecule, with C ⋯ C 4.09(1)A and CC ⋯ C angle 123.3(4)°. The in-termolecular geometry found for (4) is evidently more favourable for dimerisation than that in (3) and explains their different photosensitivities.