Solid-state carbon-13 nuclear magnetic resonance investigations of bismuth citrate complexes and crystal structure of Na2[Bi2(cit)2]·7H2O

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Abstract
The complex Na2[Bi2(cit)2]·7H2O (H4cit = 3-carboxy-3-hydroxypentane-1,5-dioic acid) crystallized during reactions of the antiulcer drug ranitidine bismuth citrate with the tripeptide glutathione (γ-L-Glu-L-Cys-Gly) at low pH. X-Ray analysis showed one [Bi(cit)]– fragment per asymmetric unit with Bi3+ chelated to a terminal carboxylate group; citrate also binds, in tridentate mode, to the bismuth of an equivalent [Bi(cit)]– unit (related to the first by a C2 axis)via one oxygen donor from each of the remaining two carboxylate groups and the alkoxy group. Both ends of the resultant dimeric anion {Bi(µ-cit)2Bi}2– bind to adjacent dimers, related by n-glide plane symmetry, via double carboxylate bridges, to form a continuous polymeric anionic chain [{Bi(µ-cit)2Bi}n]2n– running throughout the crystal. In this way each bismuth atom achieves six-co-ordination [Bi–O 2.210(10)–2.505(10)Å] with a nido-pentagonal-bipyramidal geometry, the vacant axial site providing evidence for a stereochemically active lone pair and the second axial site being occupied by the alkoxy donor. The parallel polyanionic chains are linked by anion–cation interactions [Na ⋯ O (cit) 2.37–2.50 Å] and by bonds of largely ionic character (Bi ⋯ O 3.003–3.095 Å) to give the overall three-dimensional solid-state structure which incorporates seven water molecules per dianionic subunit. The solid-state cross-polarization magic angle spinning 13C NMR spectrum of this complex is assigned with the aid of dipolar-dephasing and inversion-recovery cross-polarization experiments, and compared to those of ranitidine bismuth citrate and bismuth citrate Bi(Hcit). The IR and solid-state 13C NMR data suggested that the alkoxy group is protonated in Bi(Hcit) but deprotonated in ranitidine bismuth citrate, and that the latter contains similar dimeric units to Na2[Bi2(cit)2]·7H2O. The general modes of aggregration of dimeric [{Bi(µ-cit)2Bi}n]2n– units and the relevance to antiulcer activity are discussed.

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