Received 17 March 2004
The catena-arsenite chain anion, [AsO2]nn-: (H3NCH2CH2NH3)0.5[AsO2] and NaAsO2 (revisited)
The title compounds contain the catena-arsenite [AsO2]nn- unit, in which the AsIII atom is pyramidally coordinated to one terminal and two bridging O atoms, resulting in an infinite anionic chain. Ethylenediammonium catena-arsenite, (C2H10N2)0.5[AsO2], is the first example of this anion in the company of an organic cation. The ethylenediammonium species interact with the [AsO2]- chains by way of N-HO hydrogen bonds. The structure of sodium catena-arsenite, Na[AsO2] [Menary (1958). Acta Cryst. 11, 742-743], has been redetermined to yield more reliable geometrical parameters. The As-O distances are normal and the Na+ cation is seven-coordinate [Na-O = 2.285 (4)-3.063 (4) Å] in a distorted capped trigonal prismatic geometry.
The [AsO3]3- arsenite group shows a distinctive pyramidal geometry, due to the stereochemically active lone pair of electrons on the AsIII species, with an electron configuration of [core]4s24p1. This geometry is quite distinct from the tetrahedral coordination invariably displayed by the [AsVO4]3- arsenate group. A number of minerals and synthetic compounds containing isolated pyramidal [AsO3]3- ions are known, examples being reinerite, Zn3(AsO3)2 (Ghose et al., 1977), and the unusual arsenite-chloride finnemanite, Pb5(AsO3)3Cl (Effenberger & Pertlik, 1979).
Arsenite groups may polymerize (or condense) via vertices into extended units, the simplest example of this being the [As2O5]4- diarsenite group, which is found in paulmooreite, Pb2As2O5 (Araki et al., 1980). In ludlockite, PbFe4(As5O11)2 (Cooper & Hawthorne, 1996), as many as five AsO3 units are fused together into [As5O11]7- units. The polymerization of arsenite groups results in the catena-arsenite chain anion, [AsO2]- (or [AsO2]nn-), which was first definitively characterized by Zemann (1951) in the mineral trippkeite, CuAs2O4. A few years later, the same anion was found in the synthetic compound NaAsO2 by Menary (1958). The trippkeite structure was redetermined to improved precision by Pertlik (1975), who also showed that the two synthetic lead catena-arsenite chlorides Pb(AsO2)Cl and Pb2(AsO2)3Cl contain the same chain anion (Pertlik, 1988), as does the mineral leiteite, ZnAs2O4 (Ghose et al., 1987).
We describe here the structure of ethylenediammonium catena-arsenite, (H3NCH2CH2NH3)0.5[AsO2], (I), which is the first example of a catena-arsenite chain accompanied by organic cations. We also describe the redetermined structure of NaAsO2, (II).
Compound (I) (Fig. 1) shows (H3NCH2CH2NH3)2+ cations and anionic [AsO2]- chains. The geometrical parameters for the complete ethylenediammonium cation, which is generated by twofold symmetry from the unique atoms, are normal. The catena-arsenite chain is built up from three distinct atoms, with atom O1 forming the terminal As-O bond and atom O2 acting as the bridging atom. As expected, the geometry around As is pyramidal, with the As atom displaced from the least-squares plane of the basal O atoms by 0.886 (2) Å. Interestingly, the most prominent peak (1.11 e Å-3) in the final difference Fourier map for (I) is 0.74 Å from As, approximately where the lone pair of electrons is presumed to be located, and could thus correspond to a real chemical feature. As found in other well determined catena-arsenites (Pertlik, 1975; Ghose et al., 1987), the terminal As-OT bond in (I) [1.705 (3) Å] is distinctly shorter than the average of the bridging bonds [mean As-OB = 1.812 (2) Å]. The OB-As-OB bond angle is significantly smaller than the OB-As-OT bond angles (Table 1).
As well as van der Waals and electrostatic forces, the organic cations and the chain anion in (I) interact by way of N-HO hydrogen bonds (Table 2). Two of the three H-N moieties make short near-linear hydrogen bonds to arsenite O-atom acceptors, whilst the third N-H group is bifurcated to two arsenite O acceptor atoms (sum of D-HA bond angles about atom H1 = 359°). Overall, OT accepts three hydrogen bonds and OB accepts one. These interactions help to define a structure (Fig. 2) in which the catena-arsenite chains propagate along  (generated by the 21 screw axis), crosslinked along  by OH-N-HO bonds. Interchain linking along  is via the backbone of the organic moiety. The intrachain AsAsi separation in (I) is 3.1991 (4) Å [symmetry code: (i) 1 - x, y - , - z].
The structure of (II) (Fig. 3) is more or less the same as that determined by Menary (1958) using film methods, but with improved standard uncertainties. The Na+ cation is coordinated to seven O atoms (mean Na-O = 2.623 Å), all of which are parts of neighbouring anionic [AsO2]- chains. The resulting NaO7 polyhedron approximates to a distorted capped trigonal prism. The Na bond valence sum (BVS) of 1.00 (Brown, 1996) is exactly in agreement with the expected value. The As geometry is again pyramidal, with the As atom displaced from the least-squares plane of the basal O atoms by 0.912 (3) Å. The As-O distances [As-OT = 1.684 (4) Å and mean As-OB = 1.822 (3) Å] are similar to those found for (I). As in (I), the OB-As-OB bond angle is significantly smaller than the OB-As-OT bond angles (Table 3). By comparison, Menary's (1958) results (As-OT = 1.600 Å, and As-OB = 1.810 and 1.947 Å) indicated a much greater degree of distortion about As.
In the unit-cell packing in (II) (Fig. 4), the catena-arsenite chains propagate along , as generated by b-glide symmetry, resulting in an intrachain AsAsii separation of 3.2121 (7) Å [symmetry code: (ii) - x, + y, z]. The face- and edge-sharing NaO7 groups are sandwiched between the [AsO2]- chains and crosslink them in the a direction, resulting in neutral (001) slabs of stoichiometry NaAsO2. The AsIII lone-pair electrons appear to be directed into the inter-slab region. The shortest interblock AsO and AsAs contacts are 3.762 (3) and 3.6844 (7) Å, respectively. This is quite reminiscent of the situation in ludlockite (Cooper & Hawthorne, 1996), in which the [As5O11]7- units face each other.
The geometrical parameters for the [AsO2]- units in (I) and (II) are broadly consistent with the equivalent data for CuAs2O4 and ZnAs2O4. In particular, the As-OB bond lengths are clustered in the narrow range of 1.806 (2)-1.829 (3) Å. The As-OT bond lengths show somewhat greater variability, which might be due to the different bonding situations of the O atoms in question: the OT atom in (I) [1.705 (3) Å] only accepts hydrogen bonds, whereas the OT atom in CuAs2O4 (1.765 Å) is also bonded to two Cu atoms. However, there are also some significant differences. For example, the OB-As-OB bond angle of 100.3° in CuAs2O4 is significantly larger than the OB-As-OT bond angle (95.9°), which is the reverse of the situation for (I), (II) and ZnAs2O4 (Ghose et al., 1987).
| || Figure 1 |
A view of a fragment of (I), drawn with 50% probability displacement ellipsoids. H atoms are drawn as small spheres of arbitrary radii and hydrogen bonds are indicated by dashed lines. [Symmetry codes: (i) - x, y, 1 - z; (ii) 1 - x, y - , - z; (iii) x, y - 1, z.]
| || Figure 2 |
The unit-cell packing in (I), projected on to (010) (normal to the catena-arsenite chain direction). Hydrogen bonds are indicated by dashed lines.
| || Figure 3 |
A view of a fragment of (II), drawn with 50% probability displacement ellipsoids. Note that atoms O1, O2 and O2v represent a face shared between the AsO3 and NaO7 polyhedra. [Symmetry codes are as in Table 3; additionally: (vii) - x, y + , z.]
| || Figure 4 |
A polyhedral representation of the unit-cell packing in (II), projected on to (010). The NaO7 polyhedra are shown with light shading and the AsO3 groups are represented by AsO3E tetrahedra (dark shading), where the dummy atom E (very dark shading), placed 1.0 Å from As, represents the lone pair of electrons. The catena-arsenite chains propagate towards the viewer.
For (I), a mixture of As2O3 (1 g), ethylenediamine (0.5 g) and water (10 ml) was heated to 353 K in a plastic bottle for 48 h. Upon cooling, the resultant solids were filtered off, yielding some plate-shaped crystals of (I) accompanied by substantial amounts of undissolved or recrystallized As2O3. We have not yet succeeded in making (I) in purer form. For (II), a commercial sample (Sigma Chemical Co.) of NaAsO2 was recrystallized from methanol, in which it is sparingly soluble. The resulting crystal quality is poor.
For (I), the H atoms were placed in calculated positions (C-H distances in the range 0.96-0.98 Å and N-H distances of 0.89 Å) and refined by riding, allowing for free rotation of the rigid NH3 group about the C-N bond. The constraint Uiso(H) = 1.2Ueq(attached atom) was applied in all cases. For (II), the maximum difference peak was 0.82 Å from As.
For both compounds, data collection: SMART (Bruker, 1999); cell refinement: SAINT (Bruker, 1999); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997) and ATOMS (Dowty, 1999); software used to prepare material for publication: SHELXL97.
Supplementary data for this paper are available from the IUCr electronic archives (Reference: BC1044 ). Services for accessing these data are described at the back of the journal.
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