Crystal structures of tetramethylammonium (2,2′-bipyridine)tetracyanidoferrate(III) trihydrate and poly[[(2,2′-bipyridine-κ2 N,N′)di-μ2-cyanido-dicyanido(μ-ethylenediamine)(ethylenediamine-κ2 N,N′)cadmium(II)iron(II)] monohydrate]

The cyanide complex [N(CH3)4][Fe(2,2′-bipy)(CN)4]·3H2O (2,2′-bipy is 2,2′-bipyridine) was synthesized as a building block for the construction of a new two-dimensional cyanide-bridged Fe–Cd bimetallic coordination polymer, [Fe(2,2′-bipy)(CN4)Cd(en)2]·H2O, in which ethylenediamine (en) adopts both bridging and chelating coordination modes.


Chemical context
Over the past several decades, hexacyanidometallate anions, [M(CN) 6 ] nÀ (n = 2-4), have been used extensively as building blocks for the design and construction of a large number of high-dimensional cyanide-bridged bimetallic coordination polymers because of their ability to act as multidentate ligands to link numerous metal atoms through all six cyanide groups (Ohba & O " kawa, 2000;Smith et al., 2000;Berlinguette et al., 2005). The highly insoluble three-dimensional Prussian blue and its more soluble Prussian blue analogues are perhaps the ISSN 2056-9890 best known examples of this class of compounds, which are obtained by reacting the building block [M(CN) 6 ] 3with octahedrally coordinated transition metal ions (Buser et al., 1977). The inclusion of a bidentate chelating ligand (L) such as 2,2 0 -bipyridine (2,2 0 -bipy) or 1,10-phenanthroline (1,10-phen) in cyanide-containing building blocks of general formula [M(L)(CN) 4 ] nÀ (n = 2, 3) instead of [M(CN) 6 ] nÀ has been a recent development in the field of low-dimensionality cyanidebridged bimetallic coordination compounds (Lescouë zec et al., 2001;Lazarides et al., 2007). The aromatic ligand L does not just block two coordination sites of the central atom, to yield one-and two-dimensional polymeric compounds, but also helps to stabilize the assembly as well as stabilizing the dimensionality of the three-dimensional supramolecular structures through aromaticstacking interactions Toma et al., 2004). It is also known that the non-coordinating nitrogen atoms of the cyanide groups can act as hydrogen-bond acceptors to self-assemble into various supramolecular architectures (Xiang et al., 2009

Structural commentary
The asymmetric unit of (I) consists of one [Fe(2,2 0 -bipy)-(CN) 4 ] À anion, one disordered tetramethylammonium cation, [N(CH 3 ) 4 ] + and three water molecules, as displayed in Fig. 1. The Fe III ion is coordinated by two nitrogen atoms from one 2,2 0 -bipy ligand and four cyanide carbon atoms in a distorted octahedral geometry. This distortion around the metal atom is defined by the sum of the octahedral angular deviations from 90 (AE), in which the trigonal distortion angle = 0 for a perfect octahedron (Marchivie et al., 2005). In (I), AE for twelve bond angles, viz, 5C-Fe-C, 6C-Fe-N and 1N-Fe-N, is 41.03 , confirming a distorted octahedral geometry around the central Fe III ion. Another factor accounting for the distortion form ideal octahedral geometry of the Fe III atom is the acute angle subtended by the chelating 2,2 0 -bipy ligand, viz. N5-Fe1-N6 = 81.14 (11 The asymmetric unit of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 35% probability level. Dashed lines indicate O-HÁ Á ÁO hydrogen bonds. Covalent bonds in the major and minor parts of the disordered are shaded differently and H atoms have been omitted for clarity. The labelling scheme A and B applied to the aromatic rings is used to identify the rings in the subsequent discussion.

Supramolecular features
The three-dimensional supramolecular structure in (I) is the result of combinations of intermolecular interactions including aromaticstacking and hydrogen bonds. As can be seen in Fig. 4, pairs of [Fe(2,2 0 -bipy)(CN) 4 ] À molecules are linked together through the parallel pyridyl rings of the 2,2 0 -bipy ligands to generate a graphite-like layers parallel to the ab plane. Within the sheets, the neighbouring pyridyl moieties related by an inversion centre are in a head-to-head arrangement with centroid (C g ) to centroid distances of 4.005 (3) Å [interplanar angle = 0.0 (4) ] and 3.903 (3) Å [interplanar angle = 0.0 (3) ] for rings AÁ Á ÁA i and BÁ Á ÁB ii [symmetry codes: (i) Àx, 2 À y, 1 À z; (ii) 1 À x, 1 À y, 1 À z], respectively. The Fe III Á Á ÁFe III separations along thestacking of parallel rings AÁ Á ÁA i and rings BÁ Á ÁB ii are 8.2821 (12) and 8.4572 (13) Å , respectively. The adjacent pyridyl rings A and B iii [symmetry code: (iii) x À 1, y, z] related by translation parallel to the a axis are arranged alternately in a head-to-tail manner with a C g Á Á ÁC g distance of research communications The structures of the molecular entities in (II), showing the atomnumbering scheme. Displacement ellipsoids are drawn at the 35% probability level. The pyridine ring labelled C is discussed in the text.

Figure 3
A view of the layer structure of (II) along the b axis. 2,2 0 -Bipy molecules and H atoms bonded to C and N atoms of the en ligands have been omitted for clarity.

Figure 4
A view of the two-dimensional anionic [Fe(2,2 0 -bipy)(CN) 4 ] À graphitelike sheet structure in (I), parallel to the ab plane, withinteractions shown as dashed lines. H atoms have been omitted for clarity.
3.865 (2) Å [interplanar angle = 1.51 (12) ] and an Fe III Á Á ÁFe III separation of 6.8690 (9) Å . A notable feature of (I) is the self-assembly of the tetrameric (H 2 O) 4 and hexameric (H 2 O) 6 subunits into (H 2 O) 10 units [the dihedral angle between the best plane of the (H 2 O) 4 and (H 2 O) 6 subunits is 55.2 (2) ]; neighbouring units are further joined together, giving rise to ladder-like water chains running parallel to the a axis. As can be seen from Fig. 5, the water molecules at O1, O1 i , O2, and O2 i (for symmetry code see Table 1) form centrosymmetric cyclic tetrameric units through classical O-HÁ Á ÁO hydrogen bonds with an R 4 4 (8) ring motif according to graph-set notation. In this unit, each water monomer acts as a single donor and a single acceptor of hydrogen bonds, and the four water molecules are perfectly coplanar (mean deviation of all non-hydrogen atoms = 0.00 Å ). The average OÁ Á ÁO distance in (I) is 2.805 Å . This value is comparable to the average distances for the gas-phase water tetramer (D 2 O) 4 (2.78 Å ; Liu et al., 1996), liquid water (2.85 Å ; Belch & Rice, 1987) and other tetrameric water units in the solid state (2.81 Å ; Tao et al., 2004, and 2.83 Å ;Long et al., 2004). The average OÁ Á ÁOÁ Á ÁO angle is 90 , which is similar to those of the cyclic water tetramer found in liquid water and in the crystal host of metal-organic frameworks, [Cu(adipate)(4,4-bipy)]Á2H 2 O (Long et al., 2004) and [Cd 3 (pbtz) 3 (DMF) 4 (H 2 O) 2 ]Á4DMFÁ4H 2 O (Tao et al., 2004).
The hexameric water unit has crystallographically imposed inversion symmetry. The six water molecules O1 i , O1 ii , O2, O2 iii , O3, and O3 iii (for symmetry codes, see Table 1) are almost coplanar with a mean deviation of 0.025 Å . Similar to the situation in the tetrameric water unit, each water molecule acts as both a single hydrogen-bond donor and acceptor, and is simultaneously involved in classical O-HÁ Á ÁO interactions, leading to a cyclic R 6 6 (12) hydrogen-bonding motif with an average OÁ Á ÁO distance of 2.786 Å . This value is slightly shorter than the average distance for the tetrameric unit and liquid water; however, it is comparable with the distance in ice I h (2.74 Å ; Eisenberg & Kauzmann, 1969) and water trapped in a metal-organic framework (2.78 Å ; Ghosh & Bharadwaj, 2003). The average OÁ Á ÁOÁ Á ÁO angle in the planar hexameric unit is 120 , deviating considerably from the corresponding value of 109.3 in hexagonal ice (Fletcher, 1970). Another remarkable feature in (I) is that the ladder-like water chains are incorporated with the aromaticstacking graphite-like layers through classical O-HÁ Á ÁN hydrogen bonds involving the lattice water molecules (O1 and O3) and the N atoms of the cyanido groups (N1 and N4), forming an R 4 4 (12) ring motif. In addition, the [N(CH 3 )] + cations lie above and below the water chains and take part in the formation of weak C-HÁ Á ÁO hydrogen bonds with the water molecule.

Refinement
Crystal data, data collection, and structure refinement details are summarized in Table 3. H atoms bonded to C and N atoms were placed at calculated positions and refined using a ridingmodel approximation, with C-H = 0.93 (aromatic), 0.96 (methyl) or 0.97 (methylene) Å and N-H = 0.89 Å , and with U iso (H) = 1.5U eq (C) for methyl groups and 1.2U eq (C, N) otherwise. For (I), the water-H atoms were located in a difference Fourier map and refined with distance restraints: O-H = 0.84 (1) Å and HÁ Á ÁH = 1.39 (2) Å with U iso (H) = 1.5U eq (O). For (II), the water-H atoms were refined with restraints of O-H = 0.82 (1) Å with U iso (H) = 1.5U eq (O). The tetrametylammonium cation in (I) exhibits rotational positional disorder in three of the methyl groups, and was refined with occupancy factors of 0.440 (6) for C16A, C17A and C18A, and 0.560 (6) for atoms C16B, C17B, and C18B. Anisotropic displacement parameters of all atoms were restrained using enhanced rigid-bond restraints (RIGU command, s.u.'s 0.001 Å 2 ; Thorn et al., 2012). The restraint SADI was also used for the disordered tetrametylammonium cation.

(I) Tetramethylammonium (2,2′-bipyridine-κ 2 N,N′)tetracyanidoferrate(III) trihydrate
Crystal data (C 4  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.72 e Å −3 Δρ min = −0.59 e Å −3 Special details Experimental. Absorption correction: SADABS-2014/4 (Bruker,2014/4) was used for absorption correction. wR2(int) was 0.0760 before and 0.0587 after correction. The Ratio of minimum to maximum transmission is 0.9266. The λ/2 correction factor is 0.00150. Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. ( (Bruker,2014/5) was used for absorption correction. wR2(int) was 0.0955 before and 0.0483 after correction. The Ratio of minimum to maximum transmission is 0.8480. The λ/2 correction factor is 0.00150. Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.  (2)