1-DIMENSIONAL, 2-DIMENSIONAL, AND 3-DIMENSIONAL POLYMERIC FRAMES IN THE COORDINATION CHEMISTRY OF AGBF4 WITH PYRAZINE - THE FIRST EXAMPLE OF 3 INTERPENETRATING 3-DIMENSIONAL TRICONNECTED NETS

The reactions of AgBF4 with pyrazine (pyz) in ethanol have led to the isolation of four polymeric coordination products. Using a 1:1 salt/pyz molar ratio, the polymeric species [Ag(pyz)](BF4)(1) was obtained, while on performing the reaction with a salt/pyz ratio of 1:2, after the initial precipitation of 1, a crystalline product formed, having a composition corresponding to [Ag-2(pyz)(3)](BF4)(2) (2). The crystals of 2 presented two different morphologies: the largely dominant species (2a) is air stable, while the other one (2b) decomposes when removed from the mother solution. With higher pyz/salt ratios another unstable species was isolated, with composition [Ag(pyz)(3)](BF4) (3). Single-crystal X-ray analyses of the four products have been carried out; these have shown that 1 and 3 are 1-D polymers, with linear and zigzag chains, respectively, and have revealed very interesting structural features for the polymorphic 2a and 2b species, both containing three-connected Ag+ ions. The structure of 2a consists of 2-D 3-C frames formed by six-membered rings of pyz-bridged metal ions in the chair conformation, adjacent pairs of rings being connected in a way reminiscent of the disposition of the rings in cis-decaline, thus resulting in markedly undulated layers. More surprisingly, 2b contains three translationally equivalent interpenetrating 3-D 3-C frameworks, each being topologically similar to the silicon frame in alpha-ThSi2, an archetypal structure containing one of the three simplest 3-D 3-C nets. The structure of each individual 3-D frame is comprised of layers of parallel -Ag-pyz-Ag-pyz- zigzag chains joined by pyz bridges along the axis normal to the layers; the chains rotate by 55.4 degrees about the normal axis on passing from one layer to the adjacent one and then turn back to the previous direction on further passing to the successive layer (in a ABAB sequence). This is, to our knowledge, the first structure presenting three interpenetrating 3-D 3-C frameworks. Compound 1 is orthorhombic, space group Pmcb (No. 55), with a = 7.136(3) Angstrom, b = 7.217(1) Angstrom, c = 14.855(4) Angstrom, Z = 4, final R = 0.0266 [F-0 > 4 sigma(F-0)]. The crystals of 2a are monoclinic, space group P2(1)/n (No. 14) with a = 8.385(1) Angstrom, b = 11.785(1) Angstrom, c = 9.797(1) Angstrom, beta = 102.15(1)degrees, Z = 4, final R = 0.0255 [F-0 > 4 sigma(F-0)]. Compound 2b is orthorhombic, space group Fddd (No. 70), with a = 11.579(2) Angstrom, b = 13.272(3) Angstrom, c = 25.304(5) Angstrom, Z = 16, final R = 0.0267 [F-0 > 4 sigma(F-0)]. Compound 3 gives triclinic crystals, space group P $($) over bar$$ 1 (No. 2), with a = 7.994(5) Angstrom, b = 10.253(3) Angstrom, c = 10.768(3) Angstrom, alpha = 90.04(2)degrees, beta = 102.63(5)degrees, gamma = 111.47(5)degrees, Z = 2, final R = 0.0417 [F-0 > 4 sigma(F-0)]. The Ag three-coordination both in 2a and in 2b is strictly planar and intermediate between T-shaped and trigonal. In 3 the silver atoms display a distorted tetrahedral geometry, being bonded to two bridging and two unidentate pyz ligands.