Element-element bonds. XI - Chain formation in crystalline dimethyldichalkogenanes

The structures of dimethyldichalkogenanes (H3C)(2)E-2 liquid at room temperature have been determined by single-crystal x-ray diffraction {E = S, alpha-form 1a, P2(1)/c; Z = 4; a = 860.9(5); b = 680.0(4); c = 845.9(4) pm; beta = 94.23(4)degrees; -141 +/- 2 degrees C; beta-form 1b. P2(1)/c; Z = 4: a = 518.3(3); b = 1445.6(10); c = 670.9(3) pm; beta = 107.30(5)degrees; -141 +/- 2 degrees C; E Se, 2, Fdd2; Z = 16; a = 2236.6(5); b = 1759.0(9); c = 520.9(2) pm; -120 +/- 2 degrees C; E = Te, 3, C2/c; Z = 4; a = 1260.8(4); b = 849.1(4); c = 561.0(3) pm; beta = 107.04(6)degrees; -130 +/- 2 degrees C}. As a remarkable result intermolecular (EE)-E-.. interactions indicated by the ratio Q = ((EE)-E-..)/(E-E) increase gradually in strength when dimethyldisulfane with ordinary van der Waals contacts (1a, Q = 1.86; 1b, Q = 1.84) is compared with the selenium (2.. Q = 1.54) and the tellurium derivatives (3, Q = 1.38). Taking these (EE)-E-.. contacts into account, an association of the binuclear molecules to pairs can be detected for polymorph 1b, whereas zigzag chains are encountered in the remaining crystal structures (1a, E-(EE)-E-.. = 95 degrees and 174 degrees; 2, 156 degrees and 161 degrees; 3, 151 degrees). Additionally, by a careful inspection several crystal structures already published by other authors turn out to exhibit similar phenomena. Furthermore, the molecular packing modes of alpha-dimethyldisulfane (1a) and elementary chlorine resemble each other, whereas the beta-polymorph 1b and the isotypic crystal structures of beta-dibromoand dichlorodiselane as described by Kniep, Korte und Mootz constitute a pair of polytypoid stacking variants. A similar relationship could be established between the structures of dimethyldiselane (2) and the homologous ditellane 3. In comparison with numerous literature data the intermolecular interactions observed with these dimethyldichalkogenanes have to be classified as only weak.