Molecular Accordion: Vapoluminescence and Molecular Flexibility in the Orange and Green Luminescent Crystals of the Dimer, Au₂(μ-bis-(diphenylphosphino)ethane)₂Br₂

Solutions containing the components Au⁺, dppe (dppe is bis(diphenylphosphino)ethane), and Br^– in a 1:1:1 ratio can produce three different types of crystals: type A, orange luminescent solvates of the dimer Au₂(dppe)₂Br₂ (Au₂(μ-dppe)₂Br₂·2(OSMe₂), Au₂(μ-dppe)₂Br₂·2(OCMe₂), Au₂(μ-dppe)₂Br₂·2(CH₂Cl₂), Au₂(μ-dppe)₂Br₂·2(HC(O)NMe₂)); type B, green luminescent solvates of the same dimer (Au₂(μ-dppe)₂Br₂·(NCMe) and Au₂(μ-dppe)₂Br₂·0.5(C₄H₁₀O)); and type C, orange luminescent solvates of a polymer ({Au(μ-dppe)Br}_n·0.5(C₄H₁₀O) and {Au(μ-dppe)Br}_n·(CH₂Cl₂)). Some crystals of types A are solvoluminescent. Exposure of type A crystals of Au₂(μ-dppe)₂Br₂·2(OCMe₂) or Au₂(μ-dppe)₂Br₂·2(CH₂Cl₂) to air or vacuum results in the loss of the orange luminescence and the formation of new green luminescent crystals. Subsequent exposure of these crystals to acetone or dichloromethane vapor results in the reformation of crystals of type A. The dimeric complexes in crystals of types A and B are all centrosymmetric and share a common ring conformation. Within these dimers, the coordination geometry of each gold center is planar with a P₂Br donor set. In other respects, the Au₂(μ-dppe)₂Br₂ molecule is remarkably flexible and behaves as a molecular accordion, whose dimensions depend upon the solvate content of a particular crystalline phase. In particular, the dimer Au₂(μ-dppe)₂Br₂ is able to accommodate Au···Au separations that range from 3.8479(3) to 3.0943(2) Å, and these variations along with alterations in the Au–Br distances and in the P–Au–P angles are the likely causes of the differences in the luminescence properties of these crystals.