The Effect of pH on the Dimensionality of Coordination Polymers

Hydrothermal reactions of simple alkaline salts or their hydroxides with 3,5-pyrazoledicarboxylic acid (H3pdc)
yielded seven new compounds. At a lower pH level three one-dimeinsional structures [Ca(Hpdc)(H2O)4]·2H2O
(1), [Ca(Hpdc)(H2O)4]·H2O (2), and [Ba(H2pdc)2(H2O)4]·2H2O (6) were obtained by evaporation of the solutions
resulting from hydro(solvo)thermal reactions of MCl2 (M = Ca, Ba) with H3pdc in water (1, 6) or in water/Et3N
(2) at 150 °C for 3 days. Crystal structures of 1 and 2 contain zigzag chains of metal centers bridged by a single
Hpdc2- ligand, whereas structure 6 consists of linear chains of metal centers bridged by two H2pdc- ligands. A
dimer molecule [Sr(H3pdc)(H2pdc)2(H2O)3]2·2(H3pdc)·4H2O (4) was obtained from a similar hydrothermal reaction
using Sr(ClO4)2·6H2O instead of MCl2. This compound contains [2+2] metallomacrocycles. At higher pH levels
(pH = 4−6), the three-dimensional polymers [M(Hpdc)(H2O)] (Ca 3, Sr 5, Ba 7 ) were isolated by reactions of
MCl2 (M = Ca, Sr, Ba) with H3pdc in water/Et3N or in M(OH)2 (M = Ca, Sr, Ba) with H3pdc in water under
hydro(solvo)thermal conditions (150 °C, 3 days). Calcium and strontium are seven- and nine-coordinated in 3
and 5, respectively; barium is nine- and ten-coordinated in 7. It was observed that the increase in pH resulted in
a higher connectivity level of ligands, which in turn leads to a higher dimensionality of the crystal structures. The
correlation between the structures and pH values will be discussed. Crystal data:  for 1, monoclinic, space group
P21/n (No. 14), with a = 8.382(2), b = 12.621(3), c = 11.767(2) Å, β = 98.91(3)°, Z = 4; for 2, 3, and 5,
monoclinic, space group P21/c (No. 14), Z = 4, a = 7.711(2), b = 15.574(3), c = 9.341(2) Å, β = 96.73(3)°, Z
= 4 (2), a = 6.616(1), b = 12.654(3), c = 8.782(2) Å, β = 103.65(3)°, Z = 4 (3), a = 9.213(2), b = 12.088(3),
c = 6.196(2) Å, β = 98.96(3)° (5); for 4 and 7, triclinic, space group P 1̄ (No. 2), with a = 11.263(2), b =
11.460(3), c = 12.904(2) Å, α = 71.54(3), β = 98.96(3), γ = 89.03(3)°, Z = 1 (4), a = 7.107(1), b = 9.780(2),
c = 11.431(2) Å, α = 74.69(3), β = 73.39(3), γ = 85.29(3)°, Z = 2 (7); for 6, monoclinic, space group C2/c
(No. 15), with a = 20.493(4), b = 6.708(1), c = 15.939(3) Å, β = 123.56(3)°, Z = 4.