American Chemical Society, Inorganic Chemistry, 14(47), p. 6353-6363, 2008
DOI: 10.1021/ic800425f
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The reaction of Pb(ClO4)2·xH2O, an ancillary ligand L, and two equivalents of Au(CN)2- gave a series of crystalline coordination polymers, which were structurally characterized. The ligands were chosen to represent a range of increasing basicity, to influence the stereochemical activity (i.e., p-orbital character) of the Pb(II) lone pair. The Pb(II) center in [Pb(1,10-phenanthroline) 2][Au(CN)2]2 (1) is 8-coordinate, with a stereochemically inactive lone pair; all 8 Pb-N bonds are similar. The Au(CN)2- units propagate a 2-D brick-wall structure. In [Pb(2,2′-bipyridine)2][Au(CN)2]2 (2), the 8-coordinate Pb(II) center has asymmetric Pb-N bond lengths, indicating moderate lone pair stereochemical activity; the supramolecular structure forms a 1-D chain/ribbon motif. For [Pb(ethylene-diamine)][Au(CN)2] 2 (3), the Pb(II) is only 5-coordinate and extremely asymmetric, with Pb-N bond lengths from 2.123(7) to 3.035(9) Å; a rare Pb-Au contact of 3.5494(5) Å is also observed. The Au(CN)2- units connect the Pb(ethylenediamine) centers to form 1-D zigzag chains which stack via Au-Au interactions of 3.3221(5) Å to yield a 2-D sheet. 207Pb MAS NMR of the polymers indicates an increase in both the chemical shielding span and isotropic chemical shift with increasing Pb(II) coordination sphere anisotropy (from δiso = -2970 and Ω = 740 for 1 to δiso = -448 and Ω = 3980 for 3). The shielding anisotropy is positively correlated with Pb(II) p-character, and reflects a direct connection between the NMR parameters and lone-pair activity. Solid-state variable-temperature luminescence measurements indicate that the emission bands at 520 and 494 nm, for 1 and 2, respectively, can be attributed to Pb → L transitions, by comparison with simple [Pb(L)2] (ClO4)2 salts. In contrast, two emission bands for 3 at 408 and 440 nm are assignable to Au-Au and Pb-Au-based transitions, respectively, as supported by single-point density-functional theory calculations on models of 3.