Triple-armed aliphatic tricarboxylic acids as sources of ligands for uranyl ion: influence of bridgehead functionalization

Abstract

The two triple-armed species tris(2-carboxyethyl)nitromethane (H₃tcenm) and tris(2-carboxyethyl)phosphine (H₃tcep) have been used to synthesize seven uranyl ion complexes under (solvo)-hydrothermal conditions and in the presence of various structure-directing cations. The three carboxylate groups chelate three different cations and the nitro group is uncoordinated in [H₂NMe₂][UO₂(tcenm)]·3H₂O (1), [C(NH₂)₃][UO₂(tcenm)]·0.5H₂O (2) and [PPh₃Me][UO₂(tcenm)] (3), which crystallize as diperiodic coordination polymers with the hcb topology and minor variations in shape depending on the counterion. The two isomorphous complexes [UO₂(tcenm)M(bipy)₂][UO₂(tcenm)]·3H₂O, with M = Ni (4) or Cu (5) and bipy = 2,2′-bipyridine, display the same arrangement, with a M(bipy)₂²⁺ group bridging two adjoining carboxylate donors in one uranyl equatorial plane. [(UO₂)₂(tcenm)₂Cu(R,S-Me₆cyclam)]·2H₂O (6), where R,S-Me₆cyclam = 7(R),14(S)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, is the only triperiodic framework in the series, with the tcs topology resulting from Cu^II pillaring diperiodic, uranyl-based sql networks. H₃tcep is oxidized in situ to give the phosphine oxide H₃tcepo, which is partially deprotonated in [UO₂(Htcepo)] (7); the phosphine oxide and the two carboxylate groups are coordinated, and the diperiodic, three-dimensional network formed has the point symbol {6⁶} and the vertex symbol 6₂·6₂·6₃·6₆·6₄·6₄, with the rings involved in Hopf links formation. Only complexes 3 and 7 are significantly emissive in the solid state, with photoluminescence quantum yields of 9%, and the emission maxima positions are in agreement with the number of uranyl equatorial donors.