Comment

The design of coordination-based materials in which the topology is extended from zero-dimensional (i.e. discrete complexes) into one-, two- or three-dimensional, is a topical and interesting field of research. Since the seminal paper by Hoskins & Robson (1990), where diamondoid-type hybrid frameworks were engineered based upon the simple structures of cadmium and zinc cyanides, the field has expanded rapidly. This was motivated by the unusual architectures and the potential applications of such frameworks (Batten & Robson, 1998; Janiak, 2003; Mori et al., 2004; Moulton & Zaworotko, 2001; Rowsell & Yaghi, 2004). In order to control, at least partially, the occurrence of `supramolecular isomerism' (Moulton & Zaworotko, 2001), these hydrid crystalline materials are now being constructed using a typical modular approach: rigid and highly robust coordination-based cores are used as secondary building units (SBUs) for the construction of multidimensional metal-organic frameworks (MOFs) (Yaghi et al., 1998).

During the course of our work on crystalline organic-inorganic hybrid materials (Paz et al., 2002, 2002a,b,c; Paz & Klinowski, 2003, 2004a,b,c; Paz, Shi et al., 2004), we came across N-(phosphonomethyl)iminodiacetic acid (H₄pmida), an organic molecule which, despite possessing several types of chelating functional groups, is relatively unexplored in the construction of MOFs (Fan et al., 2004; Gutschke et al., 1999; Mao et al., 2002; Pei et al., 2004; Song et al., 2004), as confirmed by a search in the Cambridge Structural Database (Version 5.25 of November 2003; Allen, 2002; Allen & Motherwell, 2002). This molecule forms with V⁴⁺ centres centrosymmetric dimeric [V₂O₂(pmida)₂]^4- anionic units, as originally reported by Crans et al. (1998); this anionic dimer is a rather convenient SBU owing, on the one hand, to its predictable self-assembly in aqueous media (particularly when hydrothermal synthetic approaches are employed) and, on the other, to the several terminal O atoms which can bound to a large variety of metal centres through various coordination modes. We recently reported the first three-dimensional frameworks containing [V₂O₂(pmida)₂]^4- anionic units, through their combination with transition metal centres (namely, Cd²⁺ and Co²⁺) and the bridging 4,4'-bipyridine organic ligand (Paz, Shi et al., 2004). We report here the first two-dimensional framework containing these anionic units: [Co(H₂O)₆]{[Co(pyr)(H₂O)₂][V₂O₂(pmida)₂]}·2(H₂O), (I) (where pyr is pyrazine).

The title compound, (I), crystallizes in the triclinic space group P, with all its primary building blocks having inversion symmetry: the [V₂O₂(pmida)₂]^4- anionic unit has its centroid located at the inversion centre (, 0, 0); both Co1 and Co2 are themselves located at inversion centres, with the former positioned in the middle of the ab plane, at (, , 0), and the latter in the centre of the unit cell, at (, , ); the pyrazine bridging organic ligand, as for the [V₂O₂(pmida)₂]^4- anionic unit, is also centrosymmetric, with its centroid located at (0, , 0).

The geometry of the [V₂O₂(pmida)₂]^4- anionic unit is very similar to that previously reported for related compounds (Crans et al., 1998; Paz, Shi et al., 2004), with the pmida^4- anionic ligands encapsulating the V⁴⁺ centres inside three five-membered chelate rings (average bite angle of ca 77°, Table 1). The coordination polyhedron of these metal centres resembles a highly distorted octahedron, {VNO₅}, with the oxo group [V=O = 1.593 (2) Å] markedly showing its trans influence in the rather long V-N distance [2.354 (2) Å]. The phosphonate groups establish the physical bridges between adjacent V⁴⁺ centres (Fig. 1a), leading to a V1V1^v distance of 5.149 (2) Å [symmetry code: (v) 1 - x, -y, 1 - z].

Both cobalt(II) centres adopt Jahn-Teller-distorted octahedral cooordination geometries, as depicted in Fig. 1 and Table 1. For Co1, the equatorial plane is formed by two water molecules plus two O-donor atoms from phosphonate groups (Fig. 1a), with an average Co-O distance of ca 2.07 Å; pyr ligands, axially coordinated, complete the coordination, with Co-N bond lengths of 2.218 (2) Å (Table 1). Co2 is coordinated only by water molecules, with O3W and O4W forming the equatorial plane (the average Co-O bond length is 2.06 Å) and O2W located 2.144 (2) Å from the metal centre. The cis octahedral angles are within the ranges 88.04 (6)-91.96 (6) and 87.84 (6)-92.16 (6)° for Co1 and Co2, respectively (Table 1).

The centrosymmetric [V₂O₂(pmida)₂]^4- anionic unit is bound to two adjacent Co1 metal centres via the trans-uncoordinated P-O bonds, imposing a Co1Co1^viii separation of 10.140 (2) Å [symmetry code: (viii) x, -1 + y, z] (Fig. 1a). The repetition of this bridging motif leads to the formation of a one-dimensional anionic ribbon (running along the [010] direction), {[Co(H₂O)₂][V₂O₂(pmida)₂]}_n^2n-, as depicted in Fig. 2(a). Centrosymmetric bridging pyr ligands are axially coordinated to these Co1 centres (Fig. 2b), establishing physical connections between adjacent ribbons and further imposing a Co1Co1^vii separation of 7.220 (1) Å [symmetry code: (vii) 1 + x, y, z]. Such an arrangement leads to the formation of a decorated two-dimensional {[Co(pyr)(H₂O)₂][V₂O₂(pmida)₂]}_n^2n- anionic layer with distances between Co1 atoms defined by the a- and b-axis lengths. The charge of this layer is compensated by the presence of hexaaquacobalt(II) cations, [Co(H₂O)₆]²⁺ (Fig. 1b), which also act as space-filling groups, located in the available spaces resulting from the parallel packing, along the [001] direction, of individual anionic layers (Fig. 3). Each {[Co(H₂O)₂][V₂O₂(pmida)₂]}_n^2n- layer is further interconnected to the [Co(H₂O)₆]²⁺ cations via a series of very strong and highly directional hydrogen-bond interactions, also involving the water molecules of crystallization (Fig. 4 and Table 2).

Figure 1 Schematic representation of (a) a portion of the anionic two-dimensional {[Co(pyr)(H2O)2][V2O2(pmida)2]}n2n- layer and (b) the interlayer [Co(H2O)6]2+ cations, showing the labelling scheme for selected atoms in the asymmetric unit. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2 Schematic representation of the construction of the anionic two-dimensional {[Co(pyr)(H2O)2][V2O2(pmida)2]}n2n- layers, which lie in the ab plane of the unit cell.

Figure 3 Alternation along the [001] direction of anionic two-dimensional {[Co(pyr)(H2O)2][V2O2(pmida)2]}n2n- layers (in green and orange), which are intercalated by [Co(H2O)6]2+ cations (in grey). H atoms have been omitted for clarity.

Figure 4 Perspective view of the crystal packing of the title compound, viewed along the (a) [100] and (b) [010] directions of the unit cell. Hydrogen bonds are represented as green dashed bonds and H atoms have been omitted for clarity.

Experimental

Chemicals were readily available from commercial sources and were used as received without further purification: N-(phosphonomethyl)iminodiacetic acid hydrate (H₄pmida, 97% Fluka), pyrazine (98% Fluka), vanadium(IV) oxide sulfate pentahydrate (99% Sigma-Aldrich) and cobalt acetate tetrahydrate (99.0% Fluka). Syntheses were typically carried out in PTFE-lined stainless steel reaction vessels (ca 40 cm³), under autogenous pressure and static conditions in a preheated oven at 373 K. Reactions took place over a period of 4 d, after which the vessels were removed from the oven and left to cool to ambient temperature before opening. The title compound proved to be air- and light-stable, and insoluble in water and common organic solvents such as methanol, ethanol, acetone, dichloromethane, toluene, dimethyl sulfoxide and chloroform. The title compound was synthesized from a mixture containing VOSO₄·5H₂O (0.40 g), CoC₄H₆O₄·4H₂O (0.34 g), H₄pmida (0.27 g), pyrazine (0.10 g) and NaOH (0.20 g) in distilled water (ca 10 g). The mixture was stirred at ambient temperature for 30 min, yielding a suspension with a molar composition of 2.1:1.2:1.0:1.1:4.2:467, which was transferred to the reaction vessel. After the reaction, a large quantity of light-blue single crystals of the title compound were isolated by vacuum filtration, washed with copious amounts of distilled water (ca 3 × 50 ml), and then air-dried at ambient temperature.

Crystal data

C14H20CoN4O18P2V2·CoH12O6·2H2O Mr = 958.15 Triclinic, a = 7.2200 (14) Å b = 10.140 (2) Å c = 12.080 (2) Å = 93.79 (3)° = 103.21 (3)° = 104.21 (3)° V = 827.6 (3) Å3 Z = 1 Dx = 1.922 Mg m-3 Mo K radiation Cell parameters from 5000 reflections = 2-22.5° = 1.73 mm-1 T = 298 (2) K Prism, blue 0.44 × 0.33 × 0.14 mm

Data collection

Kuma KM-4 CCD diffractometer scans Absorption correction: numerical(SADABS; Sheldrick, 1996)Tmin = 0.354, Tmax = 0.724 14552 measured reflections 3994 independent reflections 3510 reflections with I > 2(I) Rint = 0.033 max = 28.8° h = -9 9 k = -13 13 l = -16 16

Refinement

Refinement on F2 R[F2 > 2(F2)] = 0.025 wR(F2) = 0.068 S = 1.08 3994 reflections 260 parameters H atoms treated by a mixture of independent and constrained refinement w = 1/[2(Fo2) + (0.0369P)2 + 0.2906P] where P = (Fo2 + 2Fc2)/3 (/)max = 0.001 max = 0.62 e Å-3 min = -0.46 e Å-3 Extinction correction: SHELXL97 Extinction coefficient: 0.0134 (13)

Table 1 Selected geometric parameters (Å, °) Co1-O1i 2.044 (2) Co1-O1W 2.093 (2) Co1-N2 2.218 (2) Co2-O2W 2.144 (2) Co2-O3W 2.063 (2) Co2-O4W 2.061 (2) V1-O2ii 1.996 (2) V1-O3 1.957 (2) V1-O4 2.041 (2) V1-O6 2.030 (2) V1-O8 1.593 (2) V1-N1 2.354 (2) O1-P1 1.508 (2) O2-P1 1.534 (2) O3-P1 1.529 (2) O1-Co1 2.044 (2) P1-C1 1.829 (2) O1-Co1-O1Wi 89.40 (6) O1-Co1-O1W 90.60 (6) O1i-Co1-N2 91.96 (6) O1-Co1-N2 88.04 (6) O1Wi-Co1-N2 88.11 (7) O1W-Co1-N2 91.89 (7) O3W-Co2-O2W 88.56 (6) O3W-Co2-O2Wiii 91.44 (6) O4W-Co2-O2Wiii 87.84 (6) O4W-Co2-O2W 92.16 (6) O4W-Co2-O3Wiii 91.43 (7) O4W-Co2-O3W 88.57 (7) O2ii-V1-O4 165.00 (5) O2ii-V1-O6 84.32 (6) O2ii-V1-N1 89.08 (6) O3-V1-O2ii 91.38 (6) O3-V1-O4 90.11 (6) O3-V1-O6 154.21 (5) O3-V1-N1 80.43 (6) O4-V1-N1 76.45 (6) O6-V1-O4 87.87 (6) O6-V1-N1 74.10 (5) O8-V1-O2ii 100.68 (8) O8-V1-O3 105.45 (7) O8-V1-O4 93.29 (8) O8-V1-O6 100.33 (7) O8-V1-N1 168.34 (7) O1-P1-O3 112.77 (8) O1-P1-O2 111.06 (7) O3-P1-O2 110.17 (7) O1-P1-C1 110.75 (8) O3-P1-C1 104.04 (7) O2-P1-C1 107.73 (8) P1-O1-Co1 136.12 (7) P1-O3-V1 126.89 (7) Symmetry codes: (i) -x+1, -y+1, -z+2; (ii) -x+1, -y, -z+2; (iii) -x+1, -y+1, -z+1.

Table 2 Hydrogen-bond geometry (Å, °) D-HA D-H HA DA D-HA O1W-H1CO2i 0.84 (2) 1.98 (1) 2.7489 (19) 154 (2) O1W-H1DO2W 0.84 (2) 2.09 (1) 2.878 (2) 160 (3) O2W-H2CO5 0.84 (2) 2.00 (1) 2.798 (2) 159 (2) O2W-H2DO7iv 0.84 (2) 1.93 (1) 2.736 (2) 166 (2) O3W-H3AO5 0.84 (2) 1.87 (1) 2.672 (2) 159 (2) O3W-H3BO6v 0.84 (2) 1.95 (2) 2.7801 (18) 173 (2) O4W-H4CO5Wiii 0.84 (2) 1.86 (1) 2.698 (2) 175 (3) O4W-H4DO7vi 0.84 (2) 1.86 (1) 2.689 (2) 167 (2) O5W-H5BO4 0.84 (2) 2.14 (2) 2.893 (2) 151 (3) O5W-H5AO5vii 0.84 (2) 2.54 (2) 3.311 (3) 157 (3) Symmetry codes: (i) -x+1, -y+1, -z+2; (iii) -x+1, -y+1, -z+1; (iv) x, y+1, z; (v) -x+1, -y, -z+1; (vi) -x, -y, -z+1; (vii) x+1, y, z.

Non-H atoms were located from difference Fourier maps calculated in successive least-squares refinement cycles. The C-bound H atoms were placed in idealized positions and refined as riding with C-H = 0.93-0.97 Å and U_iso = 1.2U_eq(C). H atoms of water molecules were located in difference Fourier maps, and refined with the O-H and HH distances restrained to 0.84 (1) and 1.37 (1) Å, respectively, to ensure chemically reasonable geometry, with U_iso fixed at 1.5U_eq(O).

Data collection: CrysAlis CCD (Kuma, 1999); cell refinement: CrysAlis RED (Kuma, 1999); data reduction: CrysAlis RED; program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXTL (Bruker, 2001); molecular graphics: DIAMOND (Brandenburg, 2001); software used to prepare material for publication: SHELXTL.