The Use of Polarizable [AuX₂(CN)₂]⁻ (X = Br, I) Building Blocks Toward the Formation of Birefringent Coordination Polymers

The [nBu4N][AuX2(CN)2] (X = Br, I) salts were synthesized and structurally characterized. Both feature square-planar [AuX2(CN)2]− anions, with trans cyano and halo ligands, which aggregate via halogen−halogen interactions. The aggregation of [AuX2(CN)2]− units results in the parallel alignment of all of the Br−Au−Br moieties in the anions along the [110] and [11̅0] directions. Two crystal habits of [nBu4N][AuBr2(CN)2] were grown: with (11̅0) and (001) as the primary faces. The birefringence in the (11̅0) plane was found to be Δn = 0.051(4) and was <0.03 in the (001) plane. Using the [AuBr2(CN)2]− unit, [M(phen)2][AuBr2(CN)2]2 (M = Fe, Ni), [Ni(terpy)2][AuBr2(CN)2]2, [Fe(terpy)2][AuBr2(CN)2][ClO4], and [Cu(phen)2(NO3)][AuBr2(CN)2] (phen = 1,10-phenanthroline, terpy = 2,2′;6′,2′′-terpyridine) were synthesized and structurally characterized: they formed ionic structures with coordinatively saturated metal cations and structurally aligning Br···Br interactions between the [AuBr2(CN)2]− anions. A molecular complex, Cu(terpy)[AuBr2(CN)2]2, was prepared, as well as the coordination polymer, [Ni(en)2(AuBr2(CN)2)][AuBr2(CN)2]·MeOH (en = ethylenediamine). The structure consists of layers of chains of Ni(en)2(AuBr2(CN)2)+ units and chains of unbound [AuBr2(CN)2]− units formed via Br···Br interactions; a Δn = 0.131(3) was measured. The Δn values were related to the supramolecular structures in terms of the relative intermolecular alignment of Br−Au−Br and NC−Au−CN bonds. These measurements both demonstrate the utility of the Au−Br bonds in enhancing birefringence and show that the contribution of the M−CN units to the overall birefringence of cyanometallate coordinations polymers is non-negligible.