Crystal structures of three sterically congested disilanes

In the three sterically congested silanes, C₂₄H₃₈Si₂ (1) (1,1,2,2-tetra-isopropyl-1,2-di-phenyl-disilane), C₂₄H₃₄Br4Si₂ (2) [1,1,2,2-tetra-kis-(2-bromo-propan-2-yl)-1,2-di-phenyl-disilane] and C₃₂H₃₈Si₂ (3) (1,2-di-tert-butyl-1,1,2,2-tetra-phenyl-disilane), the Si-Si bond length is shortest in (1) and longest in (2), with (3) having an inter-mediate value, which parallels the increasing steric congestion. A comparison of the two isopropyl derivatives, (1 and 2), shows a significant increase in the Si-C(ipso) distance with the introduction of bromine. Also, in the brominated compound 2, attractive inter-molecular Br⋯Br inter-actions exist with Br⋯Br separations ca 0.52 Å shorter than the sum of the van der Waals radii. In compound 2, one of the bromo-isopropyl groups is rotationally disordered in an 0.8812 (9):0.1188 (9) ratio. Compound 3 exhibits 'whole mol-ecule' disorder in a 0.9645 (7):0.0355 (7) ratio with the Si-Si bonds in the two components making an angle of ca 66°.