An X-ray, spectroscopic and semiempirical quantum-mechanical study on complexes of thiones and selones with molecular diiodine.

The crystal structures of four neutral charge-transfer molecular compounds formed between 5,5-dimethylimidazolidine-2,4-dithione 1, 5,5-dimethyl-2-thioxoimidazolidin-4-one 2 and 5,5-dimethyl-2-selenoxoimidazolin-4-one 3 and molecular diiodine have been determined. While all donors form 1:1 complexes, 1 also gives a 1:2 adduct when crystallized from a solution containing an excess of molecular diiodine. The compound 1.I2 is monoclinic [a = 6.681(1), b = 22.212(4), c = 7.854(1) angstrom, beta = 98.89(1)-degrees, Z = 4, space group P2(1)/n; R = 0.024] and consists of discrete moieties of the molecular formula with molecular diiodine bonded to the sulfur at C(2). The compound 1.2I2 is monoclinic [a = 14.757(2), b = 6.136(1), c = 16.284(3) angstrom, beta = 96.33(1)-degrees, Z = 4, space group P2(1)/c; R = 0.028] and consists of discrete moieties of the molecular formula with the two molecules of diiodine bonded to the two sulfur atoms. An extended network of interactions between the iodine atoms determines the crystal packing. Also 2.I2 is monoclinic [a = 13.183(3), b = 6.106(1), c = 14.550(2) angstrom, beta = 113.29(1)-degrees, Z = 4, space group P2(1)/n; R = 0.024] and consists of discrete moieties of the molecular formula with the molecular diiodine bonded to the sulfur at C(2). The compound 3.I2 is triclinic [a = 8.034(3), b = 8.769(4), c = 9.805(5) angstrom, alpha = 63.44(1), beta = 67.56(4), gamma = 67.01(4)-degrees, Z = 2, space group P1BAR; R = 0.038] and contains discrete moieties of the molecular formula with the diiodine strongly bonded to the selenium atom. In all the structures the diiodine is almost coplanar with the ring plane. Quantum-mechanical calculations (PM3) have been carried out on 1.I2 and 2.I2 in order to clarify the discrepancy between the S ... I bond distances in the two structures and the corresponding interaction in CH2Cl2 Solution, measured as spectrophotometric stability constants. The Fourier-transform Raman frequencies of nu(I-I) are correlated with the lengthening of the I-I bond distances. The C-13 NMR chemical shifts of the 1:1 complexes for the two sulfur derivatives are also reported