4,4′-Bi(1H-pyrazol-2-ium) tetra­chlorido­aurate(III) chloride: a three-dimensional cationic cooperite-like framework with multiple pyrazolium–chloride hydrogen-bonding inter­actions

In the title compound, (C₆H₈N₄)[AuCl₄]Cl, the 4,4′-bi(1H-pyrazol-2-ium) dication, denoted [H₂bpz]²⁺, is situated across a centre of inversion, the [AuCl₄]⁻ anion lies across a twofold axis passing through Cl—Au—Cl, and the Cl⁻ anion resides on a twofold axis. Conventional N—H...Cl hydrogen bonding [N...Cl = 3.109 (3) and 3.127 (3) Å, and N—H...Cl = 151 and 155°] between [H₂bpz]²⁺ cations (square-planar node) and chloride anions (tetra­hedral node), as complementary donors and acceptors of four hydrogen bonds, leads to a three-dimensional binodal four-connected framework with cooperite topology (three-letter notation pts). The framework contains channels along the c axis housing one-dimensional stacks of square-planar [AuCl₄]⁻ anions [Au—Cl = 2.2895 (10)–2.2903 (16) Å; inter­anion Au...Cl contact = 3.489 (2) Å], which are excluded from primary hydrogen bonding with the [H₂bpz]²⁺ tectons.

S1. Comment

Strong N—H···X hydrogen bonding involving organic cations and halometallate species provides a range of characteristic motifs which can be used as supramolecular synthons for the developing field of ionic solids (Crawford et al., 2004). In particular, cis-MCl₂···HN interactions, which dominate the structure of pyridinium salts containing [PdCl₄]^2- and [PtCl₄]^2- (Lewis & Orpen, 1998), have been widely explored for the construction of periodic chain-like structures based upon the alternation of halometallates and 4,4'-bipyridinium linkers (Adams et al., 2006). However, the reliability of such halometallate/NH supramolecular synthons was insufficient in the case of less nucleophilic anions, such as tetrachloridoaurate(III). The hydrogen bonding in the prototypic (PyH)[AuCl₄] salt is very weak and does not even prevent disorder of the pyridinium cations (Adams & Strähle, 1982), while systems based on the bifunctional 4,4'-bipyridinium (Zhang et al., 2006) and 1,2-bis(4-pyridinium)ethane and trans-1,2-bis(4-pyridinium)ethene (Bourne & Moitsheki, 2008) tectons commonly exhibit crystallization of mixed anion chloride–tetrachloridoaurate(III) (1:1) species, in which the halometallates are excluded from bonding to the NH donors at all.

Therefore, a very illustrative example may be anticipated when considering doubling the NH donor functionality, as provided by symmetric 4,4'-bipyrazolium [H₂bpz]²⁺ dications. Firstly, the multivalency of the pyrazolium donor and the proper and suitable orientation of the two adjacent NH donor sites could enhance the selectivity of the supramolecular interactions (Lukashuk et al., 2011). This was best demonstrated with a special kind of supramolecular synthon employing double cis-MCl₂···pyrazolium bonding ([PdCl₄]^2- and [Cu₂Cl₆]^2-; Boldog et al., 2009), and it was interesting to query whether the less nucleophilic [AuCl₄]^- could be used in a similar manner. Secondly, the combination of cationic multiple NH-donors and Cl^- and [AuCl₄]^- anions (which is typical within the pyridinium series) suggests the structural versatility of the system, concurrent N—H···Cl bonding, and new possiblities for designing hydrogen-bonded architectures by varying the NH:Cl^- ratio. In particular, the characteristic dimeric pyrazolium/chloride pattern (Boldog et al., 2009) could presumably be expanded with the generation of the framework by the simple introduction of an additional [H₂bpz]²⁺ `spacer'. In this context, we have prepared the title new organic–inorganic hybrid salt, [H₂bpz][AuCl₄]Cl, (I).

In compound (I), the organic dication is situated across a centre of inversion, the chloride anion lies on a twofold rotation axis, and the [AuCl₄]^- anion lies across a twofold rotation axis passing through atoms Cl3, Au1 and Cl4 (Fig. 1). The [AuCl₄]^- anion has a standard square-planar geometry with four virtually identical Au—Cl distances (Table 1). The [H₂bpz]²⁺ cation is planar to within ±0.008 (3) Å. The equivalence of the N1H and N2H sites is indicated by uniform angles at the nitrogen atoms [C1—N1—N2 = 109.0 (3) and C3—N2—N1 = 109.3 (3)°; Domasevitch, 2008], while in neutral 4,4'-bipyrazole these parameters are differentiated according to clearly distinguishable types: C—N—N(H) = 104.6 (2) and C—N(H)—N = 112.0 (2)° (Boldog et al., 2001).

Interion interactions occur by means of extensive N—H···Cl and C—H···Cl hydrogen bonding, while manifesting an appreciable discrimination of the halometallate hydrogen-bond acceptor in favour of the chloride anion. This agrees with the relatively poor hydrogen-bond acceptor ability of tetrachloridoaurate(III) (Calleja et al., 2001) and exactly parallels the behaviour of [AuCl₄]^- towards 4,4'-bipyridinium and its extended analogues (Zhang et al., 2006; Bourne & Moitsheki, 2008). Therefore, the [AuCl₄]^- anion is excluded from the primary bonding, which exists in the form of directional N–H···Cl1 interactions utilizing [H₂bpz]²⁺ cations as fourfold donors and the chloride anions as complementary acceptors of four N—H···Cl bonds. These are actually uniform [N···Cl 3.109 (3) and 3.127 (3) Å] (Table 2) and are typical for chloride salts of azole and azine bases. However, they are certainly weaker than those observed for the simple [H₂bpz]Cl₂ salt featuring discrete cyclic patterns {(Hpz)₂Cl₂} [N···Cl 3.0308 (13) and 3.0444 (13) Å; Boldog et al., 2009]. This motif also remains intact for (I): the same cyclic patterns are integrated into infinite polyspirane chains along the c axis by sharing Cl1 vertices (Fig. 2).

The bifunctional [H₂bpz]²⁺ tectons extend this geometry in two other dimensions, giving rise to a three-dimensional cationic framework of composition {[H₂bpz]Cl}_nⁿ⁺, with channels of ca. 5 × 5 Å running down the c direction (Fig. 3). Such evolution of the motif (one-dimensional {[H₂bpz]Cl₂}_n to three-dimensional {[H₂bpz]Cl}_nⁿ⁺), while preserving the pyrazolium–chloride supramolecular synthon, occurs with a doubling of the [H₂bpz]²⁺:Cl^- ratio under a partial substitution of the Cl^- acceptors for weakly nucleophilic [AuCl₄]^- charge-compensators. Considering the Cl1 anions as tetrahedral nodes and the organic dications as square-planar four-connected nodes, the entire topology of the hydrogen-bonded cationic framework is a binodal 4,4-coordinated net with a point Schläfli symbol {4².8⁴} (topological type of cooperite; three-letter notation pts). Thus, the hydrogen-bond acceptor capacity of the chloride anion towards the pyrazolium cations is sufficient for the generation of four-connected nodes for the net, with a nearly tetrahedral geometry of the environment (H···Cl1···H = 78–123°), and the resulting framework clearly imprints the local geometry at two types of nodes. It is worth noting that the highest NH-donor:Cl^- ratio found for pyridinium cations does not exceed 3:1 {for example, in (PyH)₃[FeCl₄]₂Cl; Shaviv et al., 1992}, and therefore the pyrazolium and related five-membered azolium species could possess an even wider potential for the design of ionic crystals, as may be compared with pyridinium–chloride systems (Goldberg et al., 2006).

The channels of the structure of (I) provide sufficient space for the location of [AuCl₄]^- anions, which form infinite stacks by long secondary interactions at both axial sites [Au1···Cl3^viii = 3.5815 (3) Å; symmetry code: (viii) -x, -y + 1, -z + 1] (Figs. 3, 4), similar to what was observed for the 1-(ethoxy)propylimidium salt [Au···Cl 3.489 (2) Å; Potts et al., 1991]. Very weak hydrogen bonding between [AuCl₄]^- and [H₂bpz]²⁺ is indicated by a set of H···Cl contacts (Desiraju & Steiner, 1999) in the range 2.86–2.93 Å (Table 2). These may be attributed to a bifurcated C—H···Cl bond [shortest distance C3···Cl4^iv = 3.426 (3) Å; symmetry code: (iv) x + 1/2, y - 1/2, z + 1] and the longer branch of a bifurcated N—H···Cl bond [N2···Cl4^iv = 3.449 (4) Å] (Fig. 4), which are slightly shorter than in the (PyH)[AuCl₄] salt [C(N)···Cl = 3.592 (3) Å; Adams & Strähle, 1982].

In summary, the framework structure and hydrogen-bonding patterns in (I) provide new insights into the design of ionic solids based upon cationic NH donors and halogenide and halometallate species. The multiple N—H···Cl^- hydrogen bonding of a potentially `tetradentate' [H₂bpz]²⁺ tecton allows the assembly of a multidimensional hydrogen-bonded framework, similar to the contruction of coordination polymers, while exploiting N-coordination to the metal ions. Variations in the [H₂bpz]²⁺:acceptor ratio, feasible with the incorporation of additional low-nucleophilic anions ([AuCl₄]^- versus Cl^- hydrogen-bond acceptors), allow control over the dimensionality of the framework.

Related literature

For related literature, see: Adams & Strähle (1982); Adams et al. (2006); Boldog et al. (2001, 2009); Bourne & Moitsheki (2008); Calleja et al. (2001); Crawford et al. (2004); Desiraju & Steiner (1999); Domasevitch (2008); Goldberg et al. (2006); Lewis & Orpen (1998); Lukashuk et al. (2011); Potts et al. (1991); Shaviv et al. (1992); Zhang et al. (2006).

S2. Experimental

4,4'-Bipyrazole (bpz) was prepared by a multistage synthesis starting from butyne-1,4-diol (Boldog et al., 2001). For the preparation of (I), solutions of bpz (0.0268 g, 0.2 mmol) in 20% HCl (4 ml) and HAuCl₄ (prepared from 0.078 g, 0.4 mmol of gold metal) in 20% HCl (2 ml) were mixed, filtered and allowed to evaporate at room temperature for a period of 20–30 d. Small orange prisms of the product (yield 0.061 g; 60%, based on the organic component) were filtered off, washed with few drops of 10% HCl and dried in air. Elemental analysis, calculated: C 14.12, H 1.58, N 10.98%; found: C 14.30, H 1.64, N 11.07%.

S3. Refinement

All H atoms were located from difference maps and were then refined as riding with the angles constrained; N—H = 0.87 Å and C—H = 0.94 Å, with U_iso(H) = 1.5U_eq(N) or 1.2U_eq(C).

Computing details

Data collection: IPDS Software (Stoe & Cie, 2000); cell refinement: IPDS Software (Stoe & Cie, 2000); data reduction: IPDS Software (Stoe & Cie, 2000); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures

	Fig. 1. The structure of (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 40% probability level. The dashed line indicates the N—H···Cl hydrogen bond. [Symmetry codes: (i) -x + 1/2, -y + 1/2, -z + 1; (ii) -x, y, -z + 1/2.]
	Fig. 2. The pyrazolium–chloride hydrogen-bonding motif, leading to chains along the c direction. Note the centrosymmetric {Hpz2Cl2} cyclic patterns integrated within the present connectivity. C-bound H atoms have been omitted for clarity. [Symmetry codes: (iii) -x + 1, -y + 1, -z + 2; (vi) -x + 1, y, -z + 3/2; (vii) x, -y + 1, z - 1/2.]
	Fig. 3. A view of the three-dimensional hydrogen-bonded pts framework, down the c axis, showing how the tetrachloridoaurate(III) stacks are incorporated inside the channels. N atoms are shaded grey and dotted lines indicate hydrogen bonding.
	Fig. 4. The linear stack of [AuCl4]^- anions involving secondary axial Au···Cl coordinations, and its weak hydrogen-bond interactions with [H2bpz]^2+ cations, constituting the framework. H atoms are shown as small spheres of arbitrary radii. [Symmetry codes: (ii) -x, y, -z + 1/2; (viii) -x, -y + 1, -z + 1; (ix) -x + 1/2, -y + 3/2, -z + 1.]

4,4'-Bi(1H-pyrazol-2-ium) tetrachloridoaurate(III) chloride

Crystal data

(C6H8N4)[AuCl4]Cl	F(000) = 944
Mr = 510.38	Dx = 2.509 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 12.5768 (8) Å	Cell parameters from 6076 reflections
b = 16.0166 (9) Å	θ = 3.2–30.0°
c = 7.1617 (5) Å	µ = 11.86 mm^−1
β = 110.533 (10)°	T = 213 K
V = 1350.99 (17) Å^3	Prism, orange
Z = 4	0.11 × 0.11 × 0.09 mm

Data collection

Stoe IPDS diffractometer	1967 independent reflections
Radiation source: fine-focus sealed tube	1798 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.025
ϕ oscillation scans	θmax = 30.0°, θmin = 3.2°
Absorption correction: numerical [X-RED (Stoe & Cie, 2001) and X-SHAPE (Stoe & Cie, 1999)]	h = −17→17
Tmin = 0.356, Tmax = 0.415	k = −22→22
6076 measured reflections	l = −10→10

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.027	H-atom parameters constrained
wR(F^2) = 0.060	w = 1/[σ^2(Fo^2) + (0.0433P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 1.00	(Δ/σ)max = 0.001
1967 reflections	Δρmax = 1.69 e Å^−3
76 parameters	Δρmin = −1.31 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0026 (2)

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 F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Au1	0.0000	0.569379 (12)	0.2500	0.02604 (9)
Cl1	0.5000	0.56587 (7)	0.7500	0.0310 (2)
Cl2	0.18636 (8)	0.57062 (7)	0.45569 (15)	0.0374 (2)
Cl3	0.0000	0.42638 (9)	0.2500	0.0370 (3)
Cl4	0.0000	0.71237 (10)	0.2500	0.0529 (4)
N1	0.3603 (3)	0.4007 (2)	0.7011 (5)	0.0335 (6)
H1N	0.3790	0.4532	0.7159	0.050*
N2	0.3931 (3)	0.3429 (2)	0.8438 (5)	0.0341 (7)
H2N	0.4379	0.3515	0.9660	0.051*
C1	0.2941 (3)	0.3648 (2)	0.5315 (5)	0.0299 (7)
H1	0.2612	0.3917	0.4078	0.036*
C2	0.2826 (3)	0.2816 (2)	0.5701 (4)	0.0251 (6)
C3	0.3464 (3)	0.2700 (3)	0.7700 (5)	0.0306 (7)
H3	0.3551	0.2195	0.8409	0.037*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Au1	0.02773 (11)	0.02806 (12)	0.02129 (10)	0.000	0.00728 (6)	0.000
Cl1	0.0376 (5)	0.0217 (5)	0.0220 (5)	0.000	−0.0042 (4)	0.000
Cl2	0.0291 (4)	0.0430 (5)	0.0366 (4)	−0.0001 (3)	0.0070 (3)	0.0000 (4)
Cl3	0.0371 (6)	0.0269 (6)	0.0496 (7)	0.000	0.0183 (5)	0.000
Cl4	0.0725 (10)	0.0279 (7)	0.0329 (6)	0.000	−0.0132 (6)	0.000
N1	0.0331 (14)	0.0279 (15)	0.0374 (15)	−0.0081 (12)	0.0099 (12)	−0.0118 (13)
N2	0.0291 (13)	0.0390 (18)	0.0258 (13)	−0.0033 (13)	−0.0008 (11)	−0.0104 (12)
C1	0.0306 (15)	0.0271 (17)	0.0273 (15)	−0.0020 (12)	0.0042 (12)	−0.0023 (12)
C2	0.0251 (13)	0.0246 (16)	0.0206 (13)	−0.0028 (12)	0.0016 (10)	−0.0032 (12)
C3	0.0308 (15)	0.0328 (19)	0.0219 (14)	−0.0014 (13)	0.0012 (12)	−0.0004 (13)

Geometric parameters (Å, º)

N1—N2	1.332 (5)	C2—C3	1.387 (4)
N1—C1	1.338 (4)	C2—C2^i	1.459 (6)
N1—H1N	0.8700	C3—H3	0.9400
N2—C3	1.331 (5)	Au1—Cl2	2.2895 (10)
N2—H2N	0.8700	Au1—Cl2^ii	2.2895 (10)
C1—C2	1.379 (5)	Au1—Cl4	2.2903 (16)
C1—H1	0.9400	Au1—Cl3	2.2903 (14)
Cl2—Au1—Cl2^ii	179.00 (5)	N1—N2—H2N	125.3
Cl2—Au1—Cl4	89.50 (3)	N1—C1—C2	108.0 (3)
Cl2^ii—Au1—Cl4	89.50 (3)	N1—C1—H1	126.0
Cl2—Au1—Cl3	90.50 (3)	C2—C1—H1	126.0
Cl2^ii—Au1—Cl3	90.50 (3)	C1—C2—C3	105.9 (3)
Cl4—Au1—Cl3	180.0	C1—C2—C2^i	127.3 (4)
N2—N1—C1	109.0 (3)	C3—C2—C2^i	126.8 (4)
N2—N1—H1N	125.5	N2—C3—C2	107.8 (3)
C1—N1—H1N	125.5	N2—C3—H3	126.1
C3—N2—N1	109.3 (3)	C2—C3—H3	126.1
C3—N2—H2N	125.3
C1—N1—N2—C3	−1.6 (5)	N1—N2—C3—C2	1.1 (4)
N2—N1—C1—C2	1.4 (4)	C1—C2—C3—N2	−0.2 (4)
N1—C1—C2—C3	−0.7 (4)	C2^i—C2—C3—N2	179.5 (4)
N1—C1—C2—C2^i	179.6 (4)

Symmetry codes: (i) −x+1/2, −y+1/2, −z+1; (ii) −x, y, −z+1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1N···Cl1	0.87	2.32	3.127 (3)	155
N2—H2N···Cl1^iii	0.87	2.32	3.109 (3)	151
N2—H2N···Cl4^iv	0.87	2.93	3.449 (4)	120
C3—H3···Cl4^iv	0.94	2.86	3.426 (3)	120
C3—H3···Cl2^v	0.94	2.93	3.846 (4)	164

Symmetry codes: (iii) −x+1, −y+1, −z+2; (iv) x+1/2, y−1/2, z+1; (v) −x+1/2, y−1/2, −z+3/2.