Synthesis and structure of a lead(II)–citrate: {Na(H2O)3}[Pb5(C6H5O7)3(C6H6O7)(H2O)3]·9.5H2O

The reaction of lead(II) nitrate with trisodium citrate Na3(C6H5O7) in a 1:22.5 ratio at pH 4.8 provides crystals of {Na(H2O)3}[Pb5(H2O)3(C6H5O7)3(C6H6O7)]·9.5H2O (1). The structure of 1 is two-dimensional and exhibits five distinct Pb(II) sites and four different modes of citrate bonding. The five lead sites all display hemidirected coordination geometries, that is, irregular distribution of neighboring oxygen atoms resulting in obvious gaps in the coordination spheres. Consequently, the lead coordination geometries exhibit proximal bonding to a number of oxygen donors, as well as distal interactions with nearest neighbors. The coordination numbers vary from 8 to 10, with ‘5+3’, ‘5+4’, ‘6+4’ and ‘7+3’ coordination modes where the first number refers to the proximal ligands and the second to the distal set. The four crystallographically distinct citrate groups include three with deprotonated carboxylate groups (C6H5O7)3− and one with a single protonated carboxyl group (C6H6O7)2. The citrate ligands bridge 3, 5, 7 and 7 lead sites. Three of the citrate groups exhibit tridentate chelation coordination to a lead site through two carboxylate oxygen donors and the hydroxyl groups. One citrate group projects an uncoordinated –OH group and a pendant protonated carboxyl group into the interlamellar domain. This latter carboxyl group coordinates to a sodium cation, which exhibits five coordinate geometry defined by three aqua ligands and the carbonyl oxygen of the –CO2H groups in the basal plane and a citrate –OH donor in the apical position.

The reaction of Pb(NO3)2 with trisodium citrate at pH 4.8 yields the two-dimensional compound, {Na(H2O)3}[Pb5(C6H5O7)3(C6H6O7)(H2O)3]·9.5H2O. The structure exhibits five distinct Pb(II) sites and four unique citrate coordination environments.Figure optionsDownload as PowerPoint slideHighlights
► Metal–citrate chemistry is relevant to biological transport by CitMHS proteins.
► Synthesis at pH 8.5 provides a novel Pb(II)–citrate complex.
► The 2-D structure of the complex is unique with both coordinated and uncoordinated –OH groups.
► There are five distinct Pb environments and four citrate environments.