The layered compound poly[μ₂-4,4′-bipyridyl-di-μ₂-chlorido-mercury(II)]

The title compound, [HgCl₂(C₁₀H₈N₂)]_n, features two-dimensional [HgCl₂(4,4′-bipy)]_n neutral networks (4,4′-bipy is 4,4′-bipyridine), based on an octa­hedral Hg atom coordinated by four μ₂-Cl atoms and two μ₂-4,4′-bipy ligands in trans positions, yielding a HgCl₄N₂ octa­hedron. The structure has mmm symmetry about the Hg atoms, with most of the atoms on at least one mirror plane, but the unsubstituted C atoms of the 4,4′-bipy rings are disordered across a mirror plane. Photoluminescent investigations reveal that the title compound displays a strong emission in the green region, which probably originates from a ligand-to-ligand charge-transfer transition.

S1. Comment

Being an important class of inorganic–organic hybrid materials, metal halide–bipy systems (bipy is bipyridine) have attracted increasing attention in recent years, not only for their intrinsic aesthetic appeal, but also for their various potential applications. 4,4'-Bipy is a common linear synthon used in supramolecular syntheses. Many structures of metal halide–bipy materials have been reported (Figgis et al., 1983; Hu et al., 2003; Lu et al., 1998). However, group 12 (IIB) metal halide–bipy materials are relatively rare. We describe here the synthesis and characterization of [HgCl₂(4,4'-bipy)]_n, (I).

X-ray diffraction analysis reveals that the title compound features two-dimensional [HgCl₂(4,4'-bipy)]_n neutral networks, as shown in Fig. 1. The structure has high symmetry, with most of the atoms (except for atoms C1 and C2) lying on at least one mirror plane. The divalent metal center Hg^II, which is on an mmm site, has a slightly distorted octahedral coordination with four µ₂-Cl atoms and two bridging 4,4'-bipy ligands in trans positions, yielding edge-shared HgCl₄N₂ octahedra. Each 4,4'-bipy ligand also has crystallographic mmm symmetry about its mid-point. The unsubstituted C atoms of the 4,4'-bipy rings, C1 and C2, sit slightly off a mirror plane and so are disordered about it. Thus the 4,4'-bipy ligand is twisted with an angle between the planes of the rings of 33.69 (19)°. The HgCl₄N₂ octahedra interconnect to each other via two µ₂-Cl atoms, forming a linear inorganic chain running along the [001] direction. These chains are bridged by µ₂-4,4'-bipy ligands to form inorganic–organic hybrid two-dimensional layers, which lie parallel to the (010) plane (Fig. 1). The atoms in adjacent layers are offset from one another by one-half of the length of the a axis. In this way, the layers stack in an –ABAB– mode along the b axis to yield the three-dimensional structure (Fig. 2).

It is noteworthy that group 12 (IIB) metal halide–bipy materials are relatively rare and, to our knowledge, only two halide–4,4'-bipy compounds have been reported, viz. HgI₂–4,4'-bipy (Niu et al., 2003; Morsali & Zhu, 2006) and ZnI₂–4,4'-bipy (Fan & Wu, 2006). Both of these compounds exhibit a one-dimensional structure, in contrast to the two-dimensional structure of the title compound. For HgI₂–4,4'-bipy and ZnI₂–4,4'-bipy, each 4,4'-bipy ligand bridges two HgI₂ or ZnI₂ groups to form a one-dimensional zigzag single chain. In both structures, the I atoms are terminally coordinated to the metal centers, while in the title compound the Cl atoms act as bridging atoms. For the title compound, the Hg atom is in an octahedral coordination environment, while in the two diiodide compounds, the metal atoms are in tetrahedral coordination environments.

The solid-state emission spectrum of the title compound was investigated at room temperature (Fig. 3). The fluorescence spectrum shows that the title compound exhibits a broad and strong emission with a maximum wavelength of 492 nm upon photo-excitation at 398 nm, which is red-shifted by 54 nm compared with that of pure 4,4'-bipy (Fig. 3). The emission of (I) can probably be assigned to the ligand-to-ligand charge-transfer transition from the highest occupied molecular orbital of the Cl atom to the lowest occupied molecular orbital of the 4,4'-bipy moiety. Thus, this compound may be a candidate in green-light luminescent materials.

Related literature

For related literature, see: Fan & Wu (2006); Figgis et al. (1983); Hu et al. (2003); Lu et al. (1998); Morsali & Zhu (2006); Niu et al. (2003).

S2. Experimental

HgCl₂ (0.1 mmol, 27 mg), 4,4'-bipyridine (0.2 mmol, 31 mg), ethanol (1 ml) and distilled water (3 ml) were loaded into a Teflon-lined stainless steel autoclave (25 ml) and kept at 473 K for 3 d. After the mixture had been slowly cooled to room temperature at a rate of 8 K h^-1, colorless crystals suitable for X-ray analysis were obtained.

S3. Refinement

All H atoms were placed geometrically and refined using a riding model, with C—H distances of 0.93 Å and with U_iso(H) values of 1.2U_eq(C).

Computing details

Data collection: WinAFC (Rigaku Corporation, 2002); cell refinement: WinAFC; data reduction: CrystalStructure (Rigaku Corporation, 2002); program(s) used to solve structure: SHELXTL (Siemens, 1994); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. The layered structure of the title compound, showing the atom labels for the asymmetric unit and only one of the possible orientations of the disordered 4,4'-bipy rings. Displacement ellipsoids are drawn at the 30% probability level. H atoms have been omitted for clarity.
	Fig. 2. The packing of (I), showing the stacks of layers. H atoms have been omitted for clarity.
	Fig. 3. Solid-state emission and excitation spectra of (I) (red lines) and pure 4,4'-bipyridine (green lines) at room temperature. Solid lines: emission spectra; dashed lines: excitation spectra.

poly[µ₂-4,4'-bipyridyl-di-µ₂-chloro-mercury(II)]

Crystal data

[HgCl2(C10H8N2)]	Dx = 2.602 Mg m^−3
Mr = 427.67	Mo Kα radiation, λ = 0.71073 Å
Orthorhombic, Cmmm	Cell parameters from 24 reflections
a = 11.6289 (17) Å	θ = 4.8–61.0°
b = 12.179 (2) Å	µ = 14.55 mm^−1
c = 3.8538 (7) Å	T = 293 K
V = 545.81 (16) Å^3	Block, colorless
Z = 2	0.16 × 0.09 × 0.05 mm
F(000) = 392

Data collection

Rigaku AFC-7R diffractometer	323 reflections with I > 2σ(I)
Radiation source: rotating-anode generator	Rint = 0.021
Graphite monochromator	θmax = 25.4°, θmin = 3.4°
ω–2θ scans	h = −4→14
Absorption correction: ψ scan (Psi in WinAFC; Rigaku Corporation, 2002)	k = 0→14
Tmin = 0.571, Tmax = 1.000	l = −1→4
338 measured reflections	30 standard reflections every 5 reflections
323 independent reflections	intensity decay: none

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)] = 0.019	H-atom parameters constrained
wR(F^2) = 0.045	w = 1/[σ^2(Fo^2) + (0.0286P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 1.09	(Δ/σ)max = 0.001
323 reflections	Δρmax = 0.90 e Å^−3
35 parameters	Δρmin = −1.11 e Å^−3
0 restraints	Extinction correction: SHELXTL, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0066 (3)

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Hg1	0.0000	0.0000	0.0000	0.02671 (7)
Cl1	0.0000	−0.16166 (10)	0.5000	0.0309 (3)
N1	−0.1961 (3)	0.0000	0.0000	0.0268 (10)
C1	−0.2552 (4)	−0.0908 (4)	0.0875 (12)	0.0359 (18)	0.50
H1A	−0.2151	−0.1539	0.1492	0.043*	0.50
C2	−0.3741 (4)	−0.0925 (3)	0.0879 (13)	0.0334 (19)	0.50
H2A	−0.4128	−0.1566	0.1480	0.040*	0.50
C3	−0.4359 (4)	0.0000	0.0000	0.0263 (12)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Hg1	0.01574 (11)	0.02877 (13)	0.03562 (15)	0.000	0.000	0.000
Cl1	0.0344 (6)	0.0228 (5)	0.0355 (6)	0.000	0.000	0.000
N1	0.0184 (18)	0.0245 (19)	0.037 (2)	0.000	0.000	0.000
C1	0.0251 (18)	0.029 (2)	0.053 (5)	0.0024 (18)	−0.006 (2)	0.007 (2)
C2	0.0234 (18)	0.0234 (19)	0.053 (5)	−0.0026 (17)	0.001 (2)	0.005 (2)
C3	0.024 (2)	0.026 (2)	0.029 (2)	0.000	0.000	0.000

Geometric parameters (Å, º)

Hg1—N1	2.280 (4)	C1—H1A	0.9300
Hg1—Cl1	2.7549 (9)	C2—C3	1.379 (5)
N1—C1	1.345 (5)	C2—H2A	0.9300
C1—C2	1.383 (6)	C3—C3^i	1.491 (10)
N1^ii—Hg1—N1	180	N1—C1—C2	121.6 (4)
Cl1^ii—Hg1—Cl1	180	N1—C1—H1A	119.2
Cl1^iii—Hg1—Cl1	91.23 (4)	C2—C1—H1A	119.2
Cl1^iv—Hg1—Cl1	88.77 (4)	C3—C2—C1	120.5 (4)
N1—Hg1—Cl1	90	C3—C2—H2A	119.7
Hg1^v—Cl1—Hg1	88.77 (4)	C1—C2—H2A	119.7
C1—N1—C1^vi	118.5 (5)	C2^vi—C3—C2	117.2 (5)
C1—N1—Hg1	120.7 (3)	C2—C3—C3^i	121.4 (2)
N1^ii—Hg1—Cl1—Hg1^v	90.0	Cl1—Hg1—N1—C1^viii	118.7 (2)
N1—Hg1—Cl1—Hg1^v	−90.0	Cl1^iii—Hg1—N1—C1^vi	61.3 (2)
Cl1^iii—Hg1—Cl1—Hg1^v	0.0	Cl1^iv—Hg1—N1—C1^vi	−118.7 (2)
Cl1^iv—Hg1—Cl1—Hg1^v	180.0	Cl1^ii—Hg1—N1—C1^vi	−27.4 (2)
Cl1^iii—Hg1—N1—C1^vii	−152.6 (2)	Cl1—Hg1—N1—C1^vi	152.6 (2)
Cl1^iv—Hg1—N1—C1^vii	27.4 (2)	C1^vii—N1—C1—C2	−80.9 (4)
Cl1^ii—Hg1—N1—C1^vii	118.7 (2)	C1^viii—N1—C1—C2	31.1 (7)
Cl1—Hg1—N1—C1^vii	−61.3 (2)	C1^vi—N1—C1—C2	0.2 (3)
Cl1^iii—Hg1—N1—C1	−118.7 (2)	Hg1—N1—C1—C2	−179.8 (3)
Cl1^iv—Hg1—N1—C1	61.3 (2)	N1—C1—C2—C3	−0.5 (7)
Cl1^ii—Hg1—N1—C1	152.6 (2)	C1—C2—C3—C2^viii	−29.8 (7)
Cl1—Hg1—N1—C1	−27.4 (2)	C1—C2—C3—C2^vi	0.2 (3)
Cl1^iii—Hg1—N1—C1^viii	27.4 (2)	C1—C2—C3—C2^vii	81.3 (4)
Cl1^iv—Hg1—N1—C1^viii	−152.6 (2)	C1—C2—C3—C3^i	−179.8 (3)
Cl1^ii—Hg1—N1—C1^viii	−61.3 (2)

Symmetry codes: (i) −x−1, −y, −z; (ii) −x, −y, −z; (iii) −x, −y, −z+1; (iv) x, y, z−1; (v) x, y, z+1; (vi) x, −y, −z; (vii) x, y, −z; (viii) x, −y, z.