Crystal structure of catena-poly[[gold(I)-μ-cyanido-[di-aqua-bis-(2-phenyl-pyrazine)-iron(II)]-μ-cyanido] dicyanidogold(I)].

In the title polymeric complex, {[Fe(CN)2(C10H8N2)2(H2O)2][Au(CN)2]} n , the FeII ion, which is located on a twofold rotation axis, has a slightly distorted FeN4O2 octa-hedral geometry. It is coordinated by two phenyl-pyrazine mol-ecules, two water mol-ecules and two di-cyano-aurate anions, the Au atom also being located on a second twofold rotation axis. In the crystal, the coordinated di-cyano-aurate anions bridge the FeII ions to form polymeric chains propagating along the b-axis direction. In the crystal, the chains are linked by Owater-H⋯Ndi-cyano-aurate anions hydrogen bonds and aurophillic inter-actions [Au⋯Au = 3.5661 (3) Å], forming layers parallel to the bc plane. The layers are linked by offset π-π stacking inter-actions [inter-centroid distance = 3.643 (3) Å], forming a supra-molecular metal-organic framework.

Chemical context

The design of functional materials based on coordination compounds is an important area of current scientific research. For example, metal–organic frameworks (MOFs), which consist of n class="Chemical">metal ions and organic ligand linkers, are studied intensively. Fe-based coordination polymers with N-donor bridging ligands are well known as compounds with switchable spin states (Niel et al., 2003 ▸; Gural’skiy et al., 2016 ▸; Kucheriv et al., 2016 ▸). This phenomenon is called spin crossover and can be observed in complexes of 3d 4–3d 7 metal ions. Applying external stimuli, such as temperature, pressure, magnetic field, light irradiation or adding a guest can affect this kind of the compound and change their properties significantly (Gütlich & Goodwin, 2004 ▸). The synthesis and crystallographic characterization of these complexes are of current inter­est because of the bis­tability of their magnetic, electrical, mechanical and optical properties (Senthil Kumar & Ruben, 2017 ▸). The parameters of these transitions could be controlled through a wide variety of available organic ligands and co-ligands. Complexes with metallo­cyanate bridges as co-ligands to N-bridging ligands form one of the largest family of spin-crossover compounds (Muñoz & Real, 2011 ▸). Here we report on a new one-dimensional polymeric compound that is similar in its structure to switchable cyano­metallates. It employs 2-phen­yl­pyrazine as a ligand and Au(CN)2− as co-ligands, while coordinated H2O mol­ecules stabilize the FeII ions in the high-spin state.

Structural commentary

The structure of the title compound features a one-dimensional chain motif that runs parallel to the crystallographic b axis (Figs. 1 ▸ and 2 ▸). The compound crystallizes in the monoclinic space group C2/c. Selected bond distances and bond angles are given in Table 1 ▸. The coordination sphere of the FeII cation, atom Fe1, which is located on a twofold n class="Disease">rotation axis, has a distorted octa­hedral environment [FeN4O2]. It includes two 2-phenyl­pyrazine N atoms [Fe1—N3 = 2.223 (5) Å] in axial positions, and two N atoms of cyano bridges and two water O atoms of water mol­ecules [Fe1—O1 = 2.122 (4) Å] in equatorial positions. The two CN− anions bridge the FeII and AuI cations [Fe1⋯Au1 = 5.244 (3) Å] to form a one-dimensional polymeric structure with bond lengths Fe1—N1 = 2.107 (5) Å and Fe1—–N2 = 2.117 (6) Å (Fig. 1 ▸ and Table 1 ▸). The FeII octa­hedral distortion parameter (the sum of the moduli of the deviations from 90° for all cis-bond angles) is Σ|90 - Θ| = 8.53°, where Θ are the cis-N—Fe—O and cis-N—Fe—N angles in the coordination environment of the FeII atom.

Figure 1

A fragment of the mol­ecular structure of the title compound, with the atom labelling Displacement ellipsoids are drawn at the 50% probability level. The Au1⋯Au2 inter­action [3.5661 (3) Å] is shown as a dashed line. [Symmetry codes: (i) x, y − 1, z; (ii) x − 1, y − 1, z − 1; (iii) −x − 1, y, −z + ; (iv) x, y + 1, z].

Figure 2

A view along the a axis of the crystal packing of the title compound. The hydrogen bonds (Table 2 ▸) and aurophillic inter­actions as shown as dashed lines. For clarity, the C-bound H atoms have been omitted.

Table 1

Selected geometric parameters (Å, °)

Au1—C1	1.975 (7)	Fe1—N2	2.117 (6)
Au1—C2i	1.988 (7)	Fe1—O1	2.122 (4)
Au2—C13	1.988 (6)	Fe1—N3	2.223 (5)
Fe1—N1	2.107 (5)
C1—Au1—C2i	180	N2—Fe1—N3	89.60 (10)
C13ii—Au2—C13	180	O1—Fe1—N3	90.09 (16)
N1—Fe1—N2	180	C1—N1—Fe1	180
O1—Fe1—O1iii	176.73 (19)	C2—N2—Fe1	180
N3—Fe1—N3iii	179.19 (19)	N1—C1—Au1	180
N1—Fe1—O1	91.63 (9)	N2—C2—Au1iv	180
N2—Fe1—O1	88.37 (9)	N5—C13—Au2	175.8 (7)
N1—Fe1—N3	90.40 (10)

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) .

Supra­molecular features

The crystal packing features different types of weak inter­actions (see Table 2 ▸ and Figs. 2 ▸ and 3 ▸). The free di­cyano­aurate anions are linked to the polymeric chains by On class="Chemical">water—H⋯N hydrogen bonds [O1—H1A⋯N5v = 2.02 Å and O1—H1B⋯N5vi = 2.18 Å; Table 2 ▸], and by aurophillic inter­actions [Au1⋯Au2 = 3.566 (2) Å], forming layers parallel to the bc plane. The layers are then linked via offset π–π inter­actions involving a pyrazine ring as an acceptor and a phenyl ring as a donor of electron density, forming a supra­molecular metal–organic framework [Cg1⋯Cg2 = 3.643 (3) Å, where Cg1 and Cg2 are the centroids of the N3/N4/C3–C6 and C7–C12 rings, respectively; α = 3.8 (3)°, inter­planar distances = 3.466 (2) and 3.510 (2) Å, offset = 0.976 Å, symmetry code (i): −x + , −y + , −z + 1].

Table 2

Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1—H1A⋯N5v	0.86	2.02	2.851 (6)	165
O1—H1B⋯N5vi	0.85	2.18	3.023 (6)	178

Symmetry codes: (v) ; (vi) .

Figure 3

A view along the b axis of the crystal packing of the title compound. The O—H⋯N hydrogen bonds and Au⋯Au inter­actions are shown as dashed lines. For clarity, the C-bound H atoms have been omitted.

Database survey

A survey of the Cambridge Structural Database (Version 5.38; Groom et al., 2016 ▸) confirmed that the structure of the title complex has not been reported previously and revealed 41 Fe–Au Cn class="Chemical">N-bridged frameworks supported axially by different co-ligands. There are 37 compounds with an octa­hedral FeN6 environment. The coordination spheres of such compounds are formed by pyridine-azine ligands, substituted pyridines, saturated and substituted pyrazines, and pyrimidine (Clements et al., 2016 ▸; Arcís-Castillo et al., 2013 ▸; Agustí et al., 2008 ▸; Clements et al., 2014 ▸, Kosone & Kitazawa, 2016 ▸; Niel et al., 2003 ▸). Nine such compounds have a stable low- or high-spin state and another 28 are complexes with a switchable spin state. There are also four compounds with an environment formed by the N atoms of organic ligands and water O atoms. The only compound with an FeN5O environment contains a pyridine-based N-donor ligand (Xu et al., 2014 ▸), while three compounds have an FeN4O2 octa­hedral geometry. The bidentate bridging organoselenium triazole ligand and two different pyridine-based ligands were used to obtain these latter complexes (Seredyuk et al., 2007 ▸; Xu et al., 2014 ▸).

Synthesis and crystallization

Crystals of the title compound were prepared by the slow diffusion method between three layers in a 10 ml tube. The first layer was a solution of K[Au(CN)2] (0.0058 g, 0.02 mmol) in n class="Chemical">water (2.5 ml), the second was a mixture of water/aceto­nitrile (1:2, 5 ml) and the third layer was a solution of 2-phenyl­pyrazine (0.0078 g, 0.05 mmol) and [Fe(OTs)2]·6H2O (0.0101 g, 0.02 mmol) (OTs = p-toluene­sulfonate) in aceto­nitrile (2.5 ml) with 0.3 ml of water. After two weeks, yellow crystals grew in the second layer; these were collected and maintained under the mother solution until measured.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3 ▸. The hydrogen atoms were placed in their expected calculated positions (C—H = 0.93 Å) and refined as riding with U iso(H) = 1.2U iso(C). The idealized n class="Chemical">OH2 group was fixed using an AFIX 7 command that allowed the H atoms to ride on the O atom and rotate around the bond.

Table 3

Experimental details

Crystal data
Chemical formula	[AuFe(CN)2(C10H8N2)2(H2O)2][Au(CN)2]
M r	902.26
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	293
a, b, c (Å)	18.5306 (13), 10.4541 (3), 14.2522 (9)
β (°)	107.509 (7)
V (Å3)	2633.0 (3)
Z	4
Radiation type	Mo Kα
μ (mm−1)	11.70
Crystal size (mm)	0.3 × 0.3 × 0.1
Data collection
Diffractometer	Rigaku Xcalibur Eos
Absorption correction	Multi-scan (CrysAlis PRO; Rigaku OD, 2015 ▸)
T min, T max	0.292, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	7203, 3265, 2410
R int	0.031
(sin θ/λ)max (Å−1)	0.667
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.038, 0.084, 1.04
No. of reflections	3265
No. of parameters	173
H-atom treatment	H-atom parameters constrained
Δρmax, Δρmin (e Å−3)	1.37, −1.45

Computer programs: CrysAlis PRO (Rigaku OD, 2015 ▸), SHELXS97 (Sheldrick, 2008 ▸), SHELX2018 (Sheldrick, 2015 ▸), OLEX2 (Dolomanov et al., 2009 ▸), Mercury (Macrae et al., 2008 ▸), PLATON (Spek, 2009 ▸) and publCIF (Westrip, 2010 ▸).

Crystal structure: contains datablock(s) I, Global. DOI: 10.1107/S2056989019009678/su5503sup1.cif

Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989019009678/su5503Isup2.hkl

CCDC reference: 1938914

Additional supporting information: crystallographic information; 3D view; checkCIF report

[AuFe(CN)2(C10H8N2)2(H2O)2][Au(CN)2]	F(000) = 1680
Mr = 902.26	Dx = 2.276 Mg m−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 18.5306 (13) Å	Cell parameters from 2258 reflections
b = 10.4541 (3) Å	θ = 2.3–30.8°
c = 14.2522 (9) Å	µ = 11.70 mm−1
β = 107.509 (7)°	T = 293 K
V = 2633.0 (3) Å3	Plate, clear light yellow
Z = 4	0.3 × 0.3 × 0.1 mm

Rigaku Xcalibur Eos diffractometer	3265 independent reflections
Radiation source: fine-focus sealed X-ray tube, Enhance (Mo) X-ray Source	2410 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.031
Detector resolution: 8.0797 pixels mm-1	θmax = 28.3°, θmin = 2.3°
ω scans	h = −17→24
Absorption correction: multi-scan (CrysAlis PRO; Rigaku OD, 2015)	k = −7→13
Tmin = 0.292, Tmax = 1.000	l = −18→16
7203 measured reflections

Refinement on F2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.038	Hydrogen site location: mixed
wR(F2) = 0.084	H-atom parameters constrained
S = 1.04	w = 1/[σ2(Fo2) + (0.0308P)2 + 0.2566P] where P = (Fo2 + 2Fc2)/3
3265 reflections	(Δ/σ)max = 0.001
173 parameters	Δρmax = 1.37 e Å−3
0 restraints	Δρmin = −1.45 e Å−3

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.

	x	y	z	Uiso*/Ueq
Au1	0.500000	0.51420 (2)	0.750000	0.03916 (11)
Au2	0.500000	0.500000	0.500000	0.04886 (13)
Fe1	0.500000	1.01389 (8)	0.750000	0.0304 (3)
O1	0.4511 (3)	1.0197 (3)	0.8669 (2)	0.0440 (10)
H1A	0.461953	0.949471	0.898996	0.066*
H1B	0.474896	1.072647	0.910236	0.066*
C12	0.2099 (3)	0.8548 (6)	0.3359 (4)	0.0498 (16)
H12	0.177175	0.921680	0.337514	0.060*
N3	0.3852 (3)	1.0154 (3)	0.6414 (3)	0.0372 (11)
N1	0.500000	0.8124 (5)	0.750000	0.0381 (16)
C1	0.500000	0.7031 (6)	0.750000	0.0347 (18)
N4	0.2472 (3)	1.0277 (4)	0.4918 (3)	0.0487 (13)
C3	0.3652 (3)	0.9310 (4)	0.5669 (4)	0.0366 (13)
H3	0.398880	0.865572	0.565374	0.044*
C9	0.3032 (4)	0.6538 (5)	0.3300 (4)	0.0556 (17)
H9	0.335120	0.585689	0.328588	0.067*
C13	0.4723 (4)	0.3157 (6)	0.4880 (4)	0.0519 (17)
C7	0.2772 (3)	0.8418 (5)	0.4104 (4)	0.0355 (12)
C6	0.3334 (3)	1.1041 (5)	0.6411 (4)	0.0453 (15)
H6	0.343207	1.163949	0.691614	0.054*
N5	0.4606 (3)	0.2077 (5)	0.4804 (3)	0.0572 (15)
N2	0.500000	1.2164 (5)	0.750000	0.0465 (19)
C10	0.2365 (4)	0.6684 (6)	0.2558 (4)	0.0558 (18)
H10	0.222790	0.609994	0.204193	0.067*
C8	0.3230 (3)	0.7399 (5)	0.4067 (4)	0.0448 (15)
H8	0.368242	0.728887	0.456743	0.054*
C5	0.2662 (4)	1.1083 (5)	0.5674 (4)	0.0518 (16)
H5	0.231568	1.171427	0.570352	0.062*
C11	0.1910 (4)	0.7682 (6)	0.2584 (4)	0.0553 (17)
H11	0.146242	0.779119	0.207499	0.066*
C4	0.2977 (3)	0.9364 (4)	0.4927 (4)	0.0336 (12)
C2	0.500000	1.3240 (7)	0.750000	0.045 (2)

	U11	U22	U33	U12	U13	U23
Au1	0.0509 (2)	0.01133 (13)	0.0543 (2)	0.000	0.01452 (16)	0.000
Au2	0.0638 (3)	0.02992 (17)	0.0467 (2)	0.00972 (13)	0.00732 (17)	−0.00062 (12)
Fe1	0.0484 (7)	0.0125 (4)	0.0289 (5)	0.000	0.0094 (5)	0.000
O1	0.069 (3)	0.0265 (17)	0.035 (2)	0.0036 (17)	0.0132 (19)	−0.0003 (15)
C12	0.041 (4)	0.062 (4)	0.045 (4)	−0.003 (3)	0.011 (3)	−0.002 (3)
N3	0.045 (3)	0.025 (2)	0.041 (3)	0.0041 (18)	0.011 (2)	0.0012 (18)
N1	0.055 (5)	0.010 (2)	0.045 (4)	0.000	0.010 (3)	0.000
C1	0.038 (5)	0.026 (4)	0.041 (4)	0.000	0.013 (3)	0.000
N4	0.052 (3)	0.044 (2)	0.047 (3)	0.012 (2)	0.011 (2)	0.001 (2)
C3	0.043 (4)	0.023 (2)	0.042 (3)	−0.001 (2)	0.010 (3)	−0.003 (2)
C9	0.070 (5)	0.045 (3)	0.051 (4)	−0.004 (3)	0.018 (3)	−0.010 (3)
C13	0.071 (5)	0.041 (3)	0.040 (3)	0.012 (3)	0.011 (3)	−0.001 (3)
C7	0.036 (3)	0.034 (3)	0.037 (3)	−0.002 (2)	0.013 (2)	0.003 (2)
C6	0.058 (4)	0.034 (3)	0.044 (3)	0.011 (3)	0.015 (3)	−0.003 (2)
N5	0.083 (4)	0.040 (3)	0.047 (3)	0.006 (3)	0.017 (3)	−0.002 (2)
N2	0.075 (6)	0.018 (3)	0.041 (4)	0.000	0.010 (4)	0.000
C10	0.065 (5)	0.055 (4)	0.047 (4)	−0.023 (3)	0.016 (3)	−0.015 (3)
C8	0.045 (4)	0.043 (3)	0.041 (3)	−0.004 (2)	0.005 (3)	−0.008 (2)
C5	0.060 (4)	0.037 (3)	0.060 (4)	0.019 (3)	0.021 (3)	0.004 (3)
C11	0.042 (4)	0.079 (5)	0.037 (4)	−0.016 (3)	0.001 (3)	−0.006 (3)
C4	0.037 (3)	0.029 (2)	0.036 (3)	0.000 (2)	0.012 (2)	0.008 (2)
C2	0.067 (6)	0.019 (3)	0.046 (5)	0.000	0.014 (4)	0.000

Au1—C1	1.975 (7)	N4—C5	1.329 (7)
Au1—C2i	1.988 (7)	N4—C4	1.333 (7)
Au2—C13ii	1.988 (6)	C3—C4	1.376 (7)
Au2—C13	1.988 (6)	C3—H3	0.9300
Fe1—N1	2.107 (5)	C9—C10	1.373 (7)
Fe1—N2	2.117 (6)	C9—C8	1.377 (7)
Fe1—O1	2.122 (4)	C9—H9	0.9300
Fe1—O1iii	2.122 (4)	C13—N5	1.149 (7)
Fe1—N3	2.223 (5)	C7—C8	1.374 (7)
Fe1—N3iii	2.223 (5)	C7—C4	1.492 (7)
O1—H1A	0.8564	C6—C5	1.368 (7)
O1—H1B	0.8479	C6—H6	0.9300
C12—C7	1.380 (6)	N2—C2	1.125 (8)
C12—C11	1.390 (7)	C10—C11	1.349 (8)
C12—H12	0.9300	C10—H10	0.9300
N3—C6	1.333 (6)	C8—H8	0.9300
N3—C3	1.344 (6)	C5—H5	0.9300
N1—C1	1.143 (8)	C11—H11	0.9300
C1—Au1—C2i	180	N3—C3—C4	123.4 (5)
C13ii—Au2—C13	180	N3—C3—H3	118.3
N1—Fe1—N2	180	C4—C3—H3	118.3
O1—Fe1—O1iii	176.73 (19)	C10—C9—C8	120.2 (6)
N3—Fe1—N3iii	179.19 (19)	C10—C9—H9	119.9
N1—Fe1—O1	91.63 (9)	C8—C9—H9	119.9
N2—Fe1—O1	88.37 (9)	N2—C2—Au1iv	180
N1—Fe1—O1iii	91.63 (9)	N5—C13—Au2	175.8 (7)
N2—Fe1—O1iii	88.37 (9)	C8—C7—C12	118.2 (5)
N1—Fe1—N3	90.40 (10)	C8—C7—C4	122.0 (5)
N2—Fe1—N3	89.60 (10)	C12—C7—C4	119.8 (5)
O1—Fe1—N3	90.09 (16)	N3—C6—C5	120.9 (5)
O1iii—Fe1—N3	89.89 (16)	N3—C6—H6	119.6
N1—Fe1—N3iii	90.40 (10)	C5—C6—H6	119.6
N2—Fe1—N3iii	89.60 (10)	C11—C10—C9	119.3 (6)
O1—Fe1—N3iii	89.89 (16)	C11—C10—H10	120.3
O1iii—Fe1—N3iii	90.09 (16)	C9—C10—H10	120.3
Fe1—O1—H1A	108.0	C7—C8—C9	121.2 (5)
Fe1—O1—H1B	109.9	C7—C8—H8	119.4
H1A—O1—H1B	100.6	C9—C8—H8	119.4
C7—C12—C11	120.1 (6)	N4—C5—C6	124.0 (5)
C7—C12—H12	120.0	N4—C5—H5	118.0
C11—C12—H12	120.0	C6—C5—H5	118.0
C6—N3—C3	115.3 (5)	C10—C11—C12	121.1 (6)
C6—N3—Fe1	123.0 (4)	C10—C11—H11	119.5
C3—N3—Fe1	121.6 (4)	C12—C11—H11	119.5
C1—N1—Fe1	180	N4—C4—C3	120.7 (5)
C2—N2—Fe1	180	N4—C4—C7	117.1 (5)
N1—C1—Au1	180	C3—C4—C7	122.3 (5)
C5—N4—C4	115.6 (5)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1A···N5v	0.86	2.02	2.851 (6)	165
O1—H1B···N5vi	0.85	2.18	3.023 (6)	178