Closed Uranyl–Dicarboxylate Oligomers: a Tetranuclear Metallatricycle with Uranyl Bridgeheads and 1,3-Adamantanediacetate Linkers
Abstract
In the presence of NH$_4$$^+$ and either PPh$_4$$^+$ or PPh$_3$Me$^+$ cations, 1,3-adamantanediacetic acid (H$_2$ADA) reacts with uranyl ions under solvo-hydrothermal conditions to give the complexes [NH$_4$]$_2$[PPh$_4$]$_2$[(UO$_2$)$_4$(ADA)$_6$] (1) and [NH$_4$]$_2$[PPh$_3$Me]$_2$[(UO$_2$)$_4$(ADA)$_6$] (2), both of which contain a tetranuclear metallatricycle built from two 2:2 rings including convergent ligands, linked by two additional ligands in extended conformation defining a third, larger ring. While the ammonium cations are closely associated with the 2:2 rings through triple hydrogen bonding, the large PPh$_4$$^+$ or PPh$_3$Me$^+$ cations are more loosely bound to each of the two faces of the larger ring. In contrast, the complex [H$_2$NMe$_2$][PPh$_3$Me][(UO$_2$)$_2$(ADA)$_3$]·H$_2$O (3), in which dimethylammonium replaces ammonium cations, crystallizes as a two-dimensional network with honeycomb {6$^3$ } topology, albeit with very distorted, elongated hexagonal cells. These and previous results show that both NH$_4$$^+$ and PPh$_4$$^+$ or PPh$_3$Me$^+$ cations are essential to the formation of the metallatricycle. The role of the flexibility imparted to ADA$^{2–}$ by the acetate arms, when compared to the more rigid 1,3-adamantanedicarboxylate (ADC$^{2–}$), is also discussed. All three complexes are luminescent, with quantum yields of 0.06, 0.06, and 0.09 for 1, 2, and 3, respectively. The vibronic fine structure apparent on the emission spectra gives maxima positions typical of species in which the uranyl ion is chelated by three carboxylate groups.
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