Synthesis and structure of a lead(II)-citrate: {Na(H_2O) _3}[Pb_5(C_6H_5O_7) _3(C_6H_6O_7)(H_2O) _3]·9.5H_2O

摘要

The reaction of lead(II) nitrate with trisodium citrate Na _3(C_6H_5O_7) in a 1:22.5 ratio at pH 4.8 provides crystals of {Na(H_2O)_3}[Pb_5(H _2O)_3(C_6H_5O_7) _3(C_6H_6O_7)]·9.5H_2O (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 (C_6H _5O_7)~(3-) and one with a single protonated carboxyl group (C_6H_6O_7)~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 -CO_2H groups in the basal plane and a citrate -OH donor in the apical position.