Role of N‑Donor Sterics on the Coordination Environment and Dimensionality of Uranyl Thiophenedicarboxylate Coordination Polymers

Thiophene
2,5-dicarboxylic acid (TDC) was reacted with uranyl acetate
dihydrate and one (or none) of six N-donor chelating ligands (2,2′-bipyridine
(BPY), 4,4′-dimethyl-2,2′-bipyridine (4-MeBPY), 5,5′-dimethyl-2,2′-bipyridine
(5-MeBPY), 6,6′-dimethyl-2,2′-bipyridine (6-MeBPY),
4,4′,6,6′-tetramethyl-2,2′-bipyridine (4,6-MeBPY),
and tetrakis­(2-pyridyl)­pyrazine (TPPZ) to result in the crystallization
of seven uranyl coordination polymers, which were characterized by
their crystal structures and luminescence properties. The seven coordination
polymers, Na2[(UO2)2­(C6H2O4S)3]·4H2O (1), [(UO2)4­(C6H2O4S)5­(C10H8N2)2]·C10H10N2·3H2O (2), [(UO2)­(C6H2O4S)­(C12H12N3)] (3), [(UO2)­(C6H2O4S)­(C12H12N3)]·H2O (4), [(UO2)2­(C6H2O4S)3]·(C12H14N2)·5H2O (5), [(UO2)3­(CH3CO2)­(C6H2O4S)4]­(C14H17N2)3·(C14H16N2)·H2O (6), and [(UO2)2­(C6H2O4S)3]­(C24H18N6) (7), consist of either
uranyl hexagonal bipyramidal or pentagonal bipyramidal coordination
geometries. In all structures, structural variations in the local
and global structures of 1–7 are
influenced by the positions (or number) of methyl groups or pyridyl
rings on the N-donor species, thus resulting in a wide diversity of
structures ranging from single chains, double chains, or 2-D sheets.
Direct coordination of N-donor ligands to uranyl centers is observed
in the chain structures of 2–4 using
BPY, 4-MeBPY, and 5-MeBPY, whereas the N-donor species participate
as guests (as either neutral or charge balancing species) in the chain
and sheet structures of 5–7 using
6-MeBPY, 4,6-MeBPY, and TPPZ, respectively. Compound 1 is the only structure that does not contain any N-donor ligands
and thus crystallizes as a 2-D interpenetrating sheet. The luminescent
properties of 1–7 are influenced
by the direct coordination or noncoordination of N-donor species to
uranyl centers. Compounds 2–4 exhibit
typical UO22+ emission upon direct coordination
of N-donors, but its absence is observed in 1, 5, 6, and 7, when N-donor species
participate as guest molecules. These results suggest
that direct coordination of N-donor ligands participate as chromophores,
thus resulting in possible UO22+ sensitization.
The lack of emission in 1, 5, 6, and 7 may be explained by the extended conjugation
of the TDC ligands within their structures.