The effect of pH on the dimensionality of coordination polymers

Hydrothermal reactions of simple alkaline salts or their hydroxides with 3,5-pyrazoledicarboxylic acid (H(3)pdc) yielded seven new compounds. At a lower pH level three one-dimeinsional structures [Ca(Hpdc)(H2O)(4)]. 2H(2)O (1), [Ca(Hpdc)(H2O)(4)].H2O (2), and [Ba(H(2)pdc)(2)(H2O)(4)]. 2H(2)O (6) were obtained by evaporation of the solutions resulting from hydro(solvo)thermal reactions of MCl2 (M = Ca, Ba) with H(3)pdc in water (1, 6) or in water/Et3N (2) at 150 degreesC for 3 days. Crystal structures of 1 and 2 contain zigzag chains of metal centers bridged by a single Hpdc(2-) ligand, whereas structure 6 consists of linear chains of metal centers bridged by two H(2)pdc(-) ligands. A dimer molecule [Sr(H(3)pdc)(H(2)pdc)(2)(H2O)(3)](2).2(H(3)pdc). 4H(2)O (4) was obtained from a similar hydrothermal reaction using Sr(ClO4)(2). 6H(2)O 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 H(3)pdc in water/Et3N or in M(OH)(2) (M = Ca, Sr, Ba) with H(3)pdc in water under hydro(solvo)thermal conditions (150 degreesC, 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 rum 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 p2(1)/n (No. 14), with a = 8.382(2), b = 12.621(3), c = 11.767(2) Angstrom, beta = 98.91(3)degrees, Z = 4; for 2, 3, and 5, monoclinic. space group P2(1)/c (No. 14). Z = 4, a = 7.711(2), b = 15.574(3), c = 9.341(2) Angstrom, beta = 96.73(3)degrees, Z = 4 (2), a = 6.616(1), b = 12.653(3). c = 8.782(2) Angstrom, beta = 103.65(3)degrees, Z = 4(3), a = 9.213(2), b = 12.088(3), c = 6.196(2) Angstrom, beta = 98.96(3)degrees (5); for 4 and 7, triclinic, space group P (1) over bar (No. 2), with a = 11.263(2), b = 11.460(3), c = 12.904(2) Angstrom, alpha = 71.54(3), beta = 98.96(3), gamma = 89.03(3)degrees, Z = 1 (4), a = 7.107(1), b = 9.780(2), c = 11.431(2) Angstrom, alpha = 74.69(3), beta = 73.39(3), gamma = 85.29(3)degrees, 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) Angstrom, beta = 123.56(3)degrees, Z = 4.