(C4H12N2)[Zn2(PO4)(HPO4)(H2PO4)], a layered zinc phosphate with intercalated N-methylpropane-1,3-diaminium cations

# 2005 International Union of Crystallography Printed in Great Britain – all rights reserved The title compound, catena-poly[[N-methylpropane-1,3diaminium [ -phosphato-( -hydrogen phosphato)( -dihydrogen phosphato)dizincate(II)]], {(C4H14N2)[Zn2(PO4)(HPO4)(H2PO4)]}n, consists of macroanionic [Zn2(PO4)(HPO4)(H2PO4)] 2 layers and intercalated (C4H12N2) 2+ cations. The layers are built up from ZnO4 and PO4/HPO4/H2PO4 tetrahedra that result in small channels of approximate diameter 3.7 Å within the layers. Framework-to-framework O—H O and template-to-framework N—H O hydrogen bonds are important in stabilizing the structure.


Comment
Zinc phosphates have become an important class of openframework materials due to their large structural variety and capability of forming chain, layer and framework structures (Rao et al., 2001;Natarajan, 2002;Norquist & O'Hare, 2004). Generally, these solids have been synthesized through a synthetic route requiring hydrothermal conditions and the presence of organic amines which act as templates.
The title compound, (I) (Fig. 1), has been obtained in the presence of a 3-methylaminopropylamine (MPA) template. In the asymmetric unit of (I), there are two crystallographically distinct Zn atoms and three P atoms, all of them being tetrahedrally coordinated. The Zn-O distances (Table 1) are in the range 1.911 (3)-1.965 (4) Å , in accordance with literature values (Harrison, 2001;Guo et al., 2005). All of the P-O distances that lie in the range 1.499 (4)-1.547 (3) Å correspond to P-O-Zn bridges, whereas those in the 1.560 (4)-1.566 (4) Å range were assigned to the terminal P-OH groups (Guo et al., 2005). In addition, P3-O7 is an unprotonated terminal bond with a length of 1.545 (4) Å , which is greater than the typical P O distance. The P3-O7 bond may be lengthened because O7 accepts a short strong hydrogen bond (Table 2) from an O8-H8 grouping in an adjacent layer. Overall, this results in the presence of phosphate, hydrogen phosphate and dihydrogen phosphate groups in (I). The fourmembered rings (i.e. four tetrahedral centres), as primary building units of the structure of (I), are formed by a link of the -Zn1-O6-P3-O1-Zn2-O2-P2-O5-atoms. Neighbouring rings are connected by two oxygen bridges (P3-O9-Zn1 and Zn2-O10-P2). Such a ring connection gives rise to a channel of about 3.7 Å diameter. Adjacent columns are linked into a layer by the Zn1-O3-P1-O4-Zn2 bridges. The negative charge of the zinc phosphate layers is compensated by diprotonated diamine molecules, which lie between the layers parallel to the ring channels. The protonated diamine molecules interact with the zinc phosphate layers through N-HÁ Á ÁO hydrogen bonds (Table 2) with NÁ Á ÁO separations from 2.823 (6) to 3.089 (5) Å . The interlayer O-HÁ Á ÁO hydrogen bonds result in OÁ Á ÁO separations ranging from 2.480 (5) to 2.580 (5) Å .
Very recently a similar zinc phosphate structure was reported by Jensen et al. (2005). The use of a different organic amine, namely N,N 0 -dimethylethylendiamine, in that synthesis led to a structure with very similar unit-cell dimensions and the same inorganic zinc phosphate layers, but a different arrangement of organic cations and consequently a higher structural symmetry (space group P2/n).

Experimental
A mixture having a relative molar composition of Zn(OAc) 2 : 5.5H 3 PO 4 :2MPA:100H 2 O, where Zn(OAc) 2 is zinc acetate dihydrate (Aldrich) and MPA is a 98% 3-methylaminopropylamine solution (Fluka) was prepared by successive additions of phosphoric acid and MPA to a solution of zinc acetate in water with vigorous stirring. Crystallization, which was performed hydrothermally at 437 K for 2 d, gave parallelepiped-shaped crystals of (I).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.