Inorganic topochemistry. Vapor-induced solid state transformations of luminescent, three-coordinate gold(i) complexes

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Abstract
The solid state interconversions of four different crystalline compounds containing the Au2(dppe)2I2 unit (dppe is bis-(diphenylphosphino)ethane) have been observed and characterized through X-ray diffraction and emission spectroscopy. Treatment of AuI in acetone with solid dppe yields the crystalline polymorphs: α-Au2(μ-dppe)2I2·2OCMe2 (1) with orange emission and β-Au2(μ-dppe)2I2·2OCMe2 (2) with green emission. Both polymorphs contain dimeric molecules with three-coordinate gold(I) centers that are further apart in the orange emitting (1) (Au⋯Au distance, 3.6720(2) Å) than in the green emitting (2) (Au⋯Au distance, 3.3955(2) Å). The acetone molecules do not bond to the gold centers in either polymorph. Crystals of α-Au2(μ-dppe)2I2·2OCMe2 (1) and β-Au2(μ-dppe)2I2·2OCMe2 (2) undergo reversible, single-crystal to single-crystal transformations. Exposure of (2) to acetone vapor converts it into (1), while (1) is converted into (2) by exposure to air for a short time. Upon loss of acetone, α-Au2(μ-dppe)2I2·2OCMe2 (1) and β-Au2(μ-dppe)2I2·2OCMe2 (2) are converted into a microcrystalline powder (3) with a green emission. Remarkably, exposure of (3) to acetone vapor converts it into acetone-free Au2(μ-dppe)2(μ-I)2 (4), which displays orange emission. The X-ray crystal structure of Au2(μ-dppe)2(μ-I)2 (4) shows that each gold is in a highly distorted tetrahedral environment with bonds to two phosphorus and two iodine atoms. The transformations between these four types of crystals are notable because of the unusual role of vapors in promoting structural changes within solids that do not necessarily change composition. Thus, the reversible interconversion of α-Au2(μ-dppe)2I2·2OCMe2 (1) and β-Au2(μ-dppe)2I2·2OCMe2 (2) occurs as vapor-stimulated single-crystal-to-single-crystal transformation between polymorphs. Likewise, the irreversible transformation of microcrystalline (3) into Au2(μ-dppe)2(μ-I)2 (4) appears to be another case of a transformation of one polymorph into another.

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