A Luminescent Gold Ring That Flips Like Cyclohexane

Abstract

Coordination-directed self-assembly of metal centers and multidentate ligands has successfully created many discrete inorganic molecules with intricate supramolecular architectures. [1] Among these compounds are metallacycles [2] which are not only aesthetically pleasing but potentially functional as catalysts, sensors, and in molecular electronics. [3] The modular character of the compounds allows their physical properties to be tailored readily and previous studies of metallacycles focused mainly on their photoluminescence and electrochemistry. [2b,c, 3, 4] On the other hand, our understanding of the fluxionality of metallacycles is still relatively limited. Given the rich fluxionality of organic rings and cyclophanes, [5] it is surprising that analogous behavior is not widely observed in their inorganic counterparts. This paucity could be a result of the rigidity of many metallacycles. Outstanding examples of metallacycle fluxionality are the PtS 5 ring inversion in [NH 4 ] 2 [Pt(S 5 ) 3 ] and the cyclooctanelike inversion of the eight-membered ring [Pt 2 Me 4 -(m-Me 2 PCH 2 Me 2 ) 2 ] reported by Riddell et al. [6a] and Puddephatt and co-workers, [6b] respectively. Herein we report the synthesis of 9,10-bis(diphenylphosphino)anthracene (PAnP), a ligand designed to form emissive cyclophanes with lowvalent metal centers. Subsequent treatment of PAnP with Au I ions produced a fluorescent trinuclear gold ring that has structure and fluxionality reminiscent of cyclohexane.PAnP (see Scheme 1) was synthesized from the reaction of dilithioanthracene and PPh 2 Cl. A similar monodentate ligand 9-diphenylphosphinoanthracene was first synthesized by Mingos and co-workers. [7] PAnP shows a singlet at d À 21.9 in the 31 P{ 1 H} NMR spectrum and its structure was confirmed by X-ray crystallography (see Supporting Information). Prolonged reflux of PAnP and Me 2 SAuCl (1:1) in methanol produced a yellow solution from which a trinuclear Au I complex [Au 3 (PAnP) 3 ][ClO 4 ] 3 (1) was isolated. Single-crystal X-ray analysis shows that the cation of 1 is a ring comprising three bridging PAnP ligands and three gold atoms (Figure 1). [8] The three anthracenyl units are nearly perpendicular to the ring, the diameter of which, taken as the Figure 1. ORTEP drawing of 1´0.5 Et 2 O; a) top and b) side view. H atoms, ClO 4 ions, Ph rings, and Et 2 O are omitted for clarity (thermal ellipsoids are set at the 50 % probability level). Selected bond lengths [] and angles [8]: