catena-Poly­[[bis­(hexa­fluoro­acetyl­acetonato-κ²O,O′)­zinc(II)]-μ-4,4′-bi­pyridine-κ²N:N′]

The structure of the title compound, catena-poly[[bis(1,1,1,5,5,5-hexafluoropentane-2,4-dionato-κ²O,O′)zinc(III)]-μ-4,4′-bipyridine-κ²N:N′], [Zn(C₅HF₆O₂)₂(C₁₀H₈N₂)]_n, con­sists of polymeric chains, running in two perpendicular directions, organized as planes normal to the tetragonal axis. The elemental unit of the chains is the zinc(II) coordination polyhedron bisected by a twofold symmetry axis, and thus only half of the unit is independent. The octahedral coordination geometry of the metal centre is composed of two oxy­gen-chelating (symmetry-related) hexa­fluoro­acetyl­acetonate groups and two translationally related 4,4′-bi­pyridine groups, which act as connecting agents in the polymer structure. The stabilization of this architecture of chains and planes is associated with a number of weak C—H...O and C—H...F hydrogen bonds.

S1. Comment

Coordination polymers with exo-bidentate (divergent) bridging ligands, such as 4,4'-bipyridine, have been prepared via self-assembly reactions, and when the ligand-to-metal ratio is 1:1, one-dimensional polymers are produced (Roesky et al., 2003). The zinc(II) species Zn(hfac)₂·2H₂O (hfac is the hexafluoroacetylacetonate anion) exhibits a high affinity towards divergent bis(4-pyridyl)-based ligands (L), giving crystalline polymeric compounds of formula [Zn(L)(hfac)₂] (Ellis et al., 2000; Matsuda et al., 2001; Horikoshi et al., 2002). The coordination environment around the metal centres is 4 + 2 octahedral, with either a cis (Ellis et al., 2000) or a trans (Matsuda et al., 2001; Horikoshi et al., 2002) configuration of the coordinated 4-pyridyl rings, forming helical and linear infinite chains, respectively. To our knowledge, the only known crystal structure of a one-dimensional polymer containing the bridging ligand 4,4'-bipyridine and the hfac anion is that of the manganese(II) complex cis-[Mn(4,4'-bipyridine)(hfac)₂]. In this compound, the structure consist of zigzag chains in which the metal centre is bridged by the 4,4'-pyridyl ligand (Shen et al., 1998; Plater et al. 2000). We report here the structural behaviour of the new 4,4'-bipyridine-based coordination polymer trans-[Zn(4,4'-bipyridine)(hfac)₂]_n, (I). This polymer contains linear chains, ordered in a parallel fashion to produce stacking layers at 90° to one another. A plausible explanation of this somewhat unusual design is given by considering the enhanced acidity of all types of CH moieties, due to the presence of N and F atoms in the ligands (Thalladi et al., 1998), which produce a combination of intrapolymeric (Baird et al., 1999) and interpolymeric (Dong et al., 1999; Dautel & Fourmigue, 2000; Desiraju, 1996; Thalladi et al., 1998) C—H···O and C—H···F hydrogen bonds.

The complex crystallizes in the tetragonal space group P4₃2₁2. The structure consists of polymeric chains running in two perpendicular directions, viz. along [110] (at z=0 and 1/2) and along [−110] (at z=1/4 and 3/4) (Fig. 1). These chains are built up around a family of twofold axes that bisect the elemental Zn octahedra, thus rendering only half of the units independent. These octahedra? are formed by two symmetry-related hexafluoracetylacetonate groups binding the cation through both O atoms in a chelating mode and determining the basal coordination plane (Fig. 2). The apical sites are in turn occupied by the opposite N atoms from two translationally related 4,4'-bipyridine moieties, which thus act as connecting links giving rise to the polymeric chains.

The resulting polyhedra are quite regular; the four equatorial O atoms deviate from planarity by less than 0.04 Å, leaving the cation 0.067 Å away along the twofold axis perpendicular to the plane. The basal bonds span the tight range 2.079 (3)–2.092 (3) Å, while the apical bonds are longer and differ slightly more [2.142 (4) and 2.191 (4) Å]. The basal angles, in turn, deviate from ideal values by less than 2%. In spite of the internal regularity of the innermost coordination shell, the hexafluoracetylacetonate groups (the non-fluorine core is planar within 0.015 Å) bind in a slightly slanted way, subtending an angle of 14.5° to the basal plane and thus giving the Zn environment a concave shape. The two planar groups of the 4,4'-bipyridyl ligand are rotated with resepct? to one another by 35.5°, the ligand binding through atom N1B being almost exactly parallel to one of the `vertical' coordination planes.

As expected from the lack of efficient `H-atom donors' of the OH or NH type, there are no strong hydrogen-bonding interactions in the structure. However, the presence of N– and F-containing ligands tends to enhance the acidity of the CH moieties, favouring the formation of a number of C—H···X (X = O and F) contacts, which are largely responsible for packing stabilization. For the benefit of clarity in the following discussion, we define three different `classes' of bonds, grouped according to their role in the three-dimensional connectivity and with no further signification regarding type or quality (see Table 2). The first four (class A contacts; Fig. 2) are intrachain interactions, which serve to inhibit the otherwise free rotation of the CF₃ groups (via two C—H···F bonds involving the hfac methyne H atom and two F atoms from two adjacent electron-withdrawing CF₃ groups)? and the 4,4'-bipy bases (via two C—H···O contacts involving aromatic CH groups and hfac O atoms). In this group of weak hydrogen bonds, all D—H···A angles are small because of the restrains imposed by the intramolecular geometry. Similar values, however, have been reported (Baird et al., 1999) for comparable interactions.

The second group (class B; Fig. 1) involves one C—H···F and one C—H···O interaction, which link the chains to one another [a similar case was reported by Dong et al. (1999)], thus forming two-dimensional arrays parallel to (001) (see discussion below). The third group (class C, in fact only one C—H···F interaction; Fig. 3) contributes to the interplanar stability.

The relative importance of the `in plane' [parallel to (001)] and `out of plane' [normal to (001)] interactions is clearly reflected in the different interchain spacings resulting from the action of these interactions; when traversing the structure along a base diagonal ([110] or [−110] directions), the mean chain separation is a/1.4142 or ~5.7 Å. This value is much smaller than that found when traversing the structure along the c axis (mean separation c/4 or ~9.1 Å). This fact justifies the description of the packing scheme as being formed by alternate planes parallel to (001), those at z = 0 and 1/2 having their `weave' running along [110], and those at z = 1/4 or 3/4 running in the orthogonal [−110] direction.

It appears that the peculiar stacking of the two-dimensional structures enables a closer approach of planes, allowing some extremely weak, but nonetheless stabilizing, C—H···F contacts (class C in our terminology) to form. This argument receives further support when the above structure is compared with those of the related acetylacetonate complexes [M(4,4'-bipyridine)(acac)₂]_n (M = Co^II and Cu^II; acac is acetylacetonate; Ma et al., 2001; Shi and Xu, 1985), where the replacement of –CF₃ groups by –CH₃ moities produces polymeric chains running only in one direction. Thus it seems that, in the hexafluoroacetylacetonate complex [Zn(4,4'-bipyridine)(hfac)₂]_n, the chains try to accumulate? in such a way as to optimize the viability of C—H···F interactions.

S2. Experimental

For the synthesis of trans-[Zn(4,4'-bipyridine)(hfac)₂], a methanol solution (15 ml) of Zn(hfac)₂·2H₂O (0.052 g, 0.10 mmol, Aldrich) was added into a solution of 4,4'-bipyridine (0.016 g, 0.10 mmol, Merck) in methanol (3 ml). The solution was stirred for 0.5 h and then the white crystalline precipitate was washed with methanol (3 × 2.5 ml) and dried in vacuo. Crystals suitable for single-crystal X-ray diffraction were obtained by recrystallization from methanol. Analysis calculated for C₂₀H₁₀N₂F₁₂O₄ Zn: C 37.79, H 1.59, N 4.41%; found: C 37.75, H 1.68, N 4.63%.

S3. Refinement

H atoms attached to C atoms were placed at calculated positions and allowed to ride on their parent atoms. Different C—H distances were used for refinement [C—H = 0.96 Å; SHELXL97 (Sheldrick, 1997) default] and for the hydrogen-bonding description (C—H = 1.08 Å; expected value). In all cases, U_iso(H) values were taken to be 1.2U_eq(C). The structure was refined as a racemic twin with a ratio of 0.70 (3):0.30 (3).

Computing details

Data collection: SMART-NT (Bruker, 2001); cell refinement: SMART-NT; data reduction: SAINT-NT (Bruker, 2000); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Sheldrick, 2000) and Mercury (Bruno et al., 2002); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. : A packing view of (I) along the c axis, showing the organization of chains into planes parallel to (001). The plane at z ~0.25 is drawn using heavy lines; the plane at z ~0.50 is represented by lighter lines. Note that chains in one plane are perpendicular to those in the neighbouring chain. Class B hydrogen bonds, linking chains into planes, are represented by broken lines; these are only shown in the top plane, for the sake of clarity. For symmetry codes refer to Table 2.
	Fig. 2. : A molecular diagram of (I), showing how the chains are formed. The atomic numbering scheme is shown only in the independent part of the coordination polyhedron, which is drawn with displacement ellipsoids at the 40% probability level. Hydrogen-bonding interactions within the chain (class A) are shown as broken lines. For symmetry codes refer to Table 2.
	Fig. 3. : A packing view of (I) along a base diagonal, at right angle to the direction used in Fig. 1, showing in projection the two-dimensional structures parallel to (001). The planes at z ~0.25 and 0.75 are shown using heavy lines, (chains running vertically in the plane of the figure); lighter lines represent the plane at z ~0.50 (chains coming out of the plane of the figure). Class C C—H···F contacts, linking planes, are shown as broken lines. For symmetry codes refer to Table 2.

(I)

Crystal data

[Zn(C10H2F12O4)2(C10H8N2)]	Dx = 1.773 Mg m^−3
Mr = 635.67	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, P43212	Cell parameters from 234 reflections
a = 8.0612 (4) Å	θ = 3.1–22.3°
c = 36.648 (3) Å	µ = 1.16 mm^−1
V = 2381.5 (3) Å^3	T = 297 K
Z = 4	Plates, colorless
F(000) = 1256	0.30 × 0.28 × 0.12 mm

Data collection

Bruker SMART CCD area-detector diffractometer	2791 independent reflections
Radiation source: fine-focus sealed tube	2134 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.070
ϕ and ω scans	θmax = 28.2°, θmin = 2.2°
Absorption correction: multi-scan SADABS in SAINT-NT (Bruker, 2000)	h = −10→10
Tmin = 0.71, Tmax = 0.87	k = −10→10
20037 measured reflections	l = −46→46

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)] = 0.048	H-atom parameters constrained
wR(F^2) = 0.132	w = 1/[σ^2(Fo^2) + (0.089P)^2 + 0.1176P] where P = (Fo^2 + 2Fc^2)/3
S = 0.94	(Δ/σ)max = 0.002
2791 reflections	Δρmax = 0.37 e Å^−3
180 parameters	Δρmin = −0.23 e Å^−3
0 restraints	Absolute structure: Flack (1983)
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.30 (3)

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.

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

	x	y	z	Uiso*/Ueq
Zn1	0.92847 (5)	0.07153 (5)	0.7500	0.0426 (2)
O1A	1.0488 (4)	0.1809 (3)	0.70550 (7)	0.0536 (7)
O2A	0.7958 (3)	−0.0736 (3)	0.71330 (7)	0.0515 (6)
C1A	1.1637 (7)	0.2118 (8)	0.64819 (15)	0.0833 (16)
C2A	1.0437 (5)	0.1259 (5)	0.67388 (12)	0.0513 (10)
C3A	0.9487 (5)	−0.0020 (6)	0.66037 (12)	0.0618 (11)
H3A	0.9636	−0.0330	0.6353	0.074*
C4A	0.8331 (5)	−0.0885 (5)	0.68078 (13)	0.0539 (10)
C5A	0.7398 (8)	−0.2275 (8)	0.66157 (17)	0.0887 (18)
F1A	1.3180 (4)	0.1609 (7)	0.65587 (12)	0.1368 (17)
F2A	1.1636 (7)	0.3700 (5)	0.65229 (12)	0.151 (2)
F3A	1.1435 (5)	0.1754 (5)	0.61356 (8)	0.1065 (12)
F4A	0.7785 (10)	−0.3696 (6)	0.67550 (16)	0.203 (3)
F5A	0.7612 (7)	−0.2409 (7)	0.62685 (10)	0.1430 (17)
F6A	0.5842 (5)	−0.2213 (8)	0.66705 (15)	0.188 (3)
N1B	0.7406 (3)	0.2594 (3)	0.7500	0.0478 (10)
N2B	0.1206 (4)	0.8794 (4)	0.7500	0.0519 (11)
C1B	0.6038 (6)	0.2450 (6)	0.73051 (13)	0.0676 (14)
H1B	0.5904	0.1480	0.7156	0.081*
C2B	0.4784 (6)	0.3626 (6)	0.73037 (14)	0.0709 (14)
H2B	0.3794	0.3439	0.7164	0.085*
C3B	0.4960 (4)	0.5040 (4)	0.7500	0.0432 (10)
C4B	0.3643 (4)	0.6357 (4)	0.7500	0.0488 (12)
C5B	0.1986 (5)	0.5964 (4)	0.74720 (14)	0.0592 (11)
H5B	0.1646	0.4826	0.7453	0.071*
C6B	0.0835 (5)	0.7190 (5)	0.74708 (14)	0.0631 (12)
H6B	−0.0312	0.6885	0.7448	0.076*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Zn1	0.0349 (2)	0.0349 (2)	0.0580 (3)	0.0077 (3)	−0.0051 (2)	−0.0051 (2)
O1A	0.0546 (17)	0.0423 (14)	0.0641 (17)	−0.0010 (12)	0.0020 (14)	−0.0048 (13)
O2A	0.0420 (14)	0.0443 (15)	0.0682 (17)	0.0059 (12)	−0.0044 (13)	−0.0140 (15)
C1A	0.081 (4)	0.093 (4)	0.076 (3)	−0.017 (3)	0.004 (3)	−0.005 (3)
C2A	0.042 (2)	0.051 (2)	0.061 (2)	0.0087 (17)	−0.0015 (18)	−0.0056 (18)
C3A	0.061 (3)	0.064 (2)	0.060 (2)	0.004 (2)	0.001 (2)	−0.018 (2)
C4A	0.045 (2)	0.045 (2)	0.072 (3)	0.0079 (19)	−0.0051 (18)	−0.012 (2)
C5A	0.094 (4)	0.086 (4)	0.086 (4)	−0.027 (3)	−0.005 (3)	−0.036 (3)
F1A	0.060 (2)	0.218 (5)	0.132 (3)	−0.028 (2)	0.014 (2)	−0.002 (3)
F2A	0.239 (6)	0.078 (2)	0.135 (3)	−0.052 (3)	0.091 (4)	0.001 (2)
F3A	0.109 (3)	0.148 (4)	0.0632 (17)	−0.027 (2)	0.0155 (17)	−0.0026 (19)
F4A	0.323 (9)	0.077 (3)	0.210 (6)	−0.069 (4)	−0.084 (6)	−0.029 (3)
F5A	0.157 (4)	0.169 (4)	0.103 (3)	−0.078 (3)	0.018 (3)	−0.071 (3)
F6A	0.081 (3)	0.285 (7)	0.197 (4)	−0.075 (4)	0.014 (3)	−0.139 (5)
N1B	0.0387 (13)	0.0387 (13)	0.066 (3)	0.0076 (17)	−0.0086 (18)	−0.0086 (18)
N2B	0.0382 (14)	0.0382 (14)	0.079 (3)	0.0089 (17)	−0.0024 (19)	−0.0024 (19)
C1B	0.056 (3)	0.049 (2)	0.098 (3)	0.024 (2)	−0.029 (2)	−0.028 (2)
C2B	0.055 (3)	0.059 (3)	0.098 (4)	0.023 (2)	−0.042 (3)	−0.023 (3)
C3B	0.0350 (13)	0.0350 (13)	0.060 (3)	0.0101 (19)	−0.0073 (17)	−0.0073 (17)
C4B	0.0364 (15)	0.0364 (15)	0.074 (3)	0.010 (2)	0.004 (2)	0.004 (2)
C5B	0.0385 (18)	0.0312 (18)	0.108 (3)	0.0007 (15)	−0.012 (2)	0.002 (2)
C6B	0.0382 (19)	0.0382 (18)	0.113 (4)	0.0073 (17)	−0.015 (3)	0.005 (2)

Geometric parameters (Å, º)

Zn1—O2A	2.079 (3)	C5A—F4A	1.292 (8)
Zn1—O2A^i	2.079 (3)	N1B—C1B	1.319 (5)
Zn1—O1A	2.092 (3)	N1B—C1B^i	1.319 (5)
Zn1—O1A^i	2.092 (3)	N2B—C6B	1.331 (4)
Zn1—N1B	2.142 (4)	N2B—C6B^i	1.331 (4)
Zn1—N2B^ii	2.191 (4)	N2B—Zn1^iii	2.191 (4)
O1A—C2A	1.241 (5)	C1B—C2B	1.386 (5)
O2A—C4A	1.235 (5)	C1B—H1B	0.9600
C1A—F2A	1.284 (7)	C2B—C3B	1.356 (5)
C1A—F3A	1.313 (6)	C2B—H2B	0.9600
C1A—F1A	1.340 (7)	C3B—C2B^i	1.356 (5)
C1A—C2A	1.517 (7)	C3B—C4B	1.501 (6)
C2A—C3A	1.376 (6)	C4B—C5B	1.377 (4)
C3A—C4A	1.384 (6)	C4B—C5B^i	1.377 (4)
C3A—H3A	0.9600	C5B—C6B	1.356 (5)
C4A—C5A	1.522 (6)	C5B—H5B	0.9600
C5A—F6A	1.271 (7)	C6B—H6B	0.9600
C5A—F5A	1.289 (7)
O2A—Zn1—O2A^i	176.08 (15)	C3A—C4A—C5A	117.0 (4)
O2A—Zn1—O1A	88.36 (11)	F6A—C5A—F5A	107.0 (6)
O2A^i—Zn1—O1A	91.52 (11)	F6A—C5A—F4A	102.1 (6)
O2A—Zn1—O1A^i	91.52 (11)	F5A—C5A—F4A	106.5 (6)
O2A^i—Zn1—O1A^i	88.36 (11)	F6A—C5A—C4A	112.7 (5)
O1A—Zn1—O1A^i	176.55 (16)	F5A—C5A—C4A	116.9 (5)
O2A—Zn1—N1B	91.96 (8)	F4A—C5A—C4A	110.5 (5)
O2A^i—Zn1—N1B	91.96 (8)	C1B—N1B—C1B^i	116.1 (5)
O1A—Zn1—N1B	91.73 (8)	C1B—N1B—Zn1	121.9 (2)
O1A^i—Zn1—N1B	91.73 (8)	C1B^i—N1B—Zn1	121.9 (2)
O2A—Zn1—N2B^ii	88.04 (8)	C6B—N2B—C6B^i	116.3 (4)
O2A^i—Zn1—N2B^ii	88.04 (8)	C6B—N2B—Zn1^iii	121.8 (2)
O1A—Zn1—N2B^ii	88.27 (8)	C6B^i—N2B—Zn1^iii	121.8 (2)
O1A^i—Zn1—N2B^ii	88.27 (8)	N1B—C1B—C2B	123.5 (4)
N1B—Zn1—N2B^ii	180.0	N1B—C1B—H1B	118.2
C2A—O1A—Zn1	124.2 (3)	C2B—C1B—H1B	118.3
C4A—O2A—Zn1	123.7 (3)	C3B—C2B—C1B	119.8 (4)
F2A—C1A—F3A	109.6 (6)	C3B—C2B—H2B	120.1
F2A—C1A—F1A	106.3 (5)	C1B—C2B—H2B	120.1
F3A—C1A—F1A	104.5 (5)	C2B—C3B—C2B^i	117.2 (5)
F2A—C1A—C2A	112.3 (5)	C2B—C3B—C4B	121.4 (2)
F3A—C1A—C2A	114.8 (5)	C2B^i—C3B—C4B	121.4 (2)
F1A—C1A—C2A	108.8 (5)	C5B—C4B—C5B^i	116.9 (4)
O1A—C2A—C3A	128.5 (4)	C5B—C4B—C3B	121.6 (2)
O1A—C2A—C1A	113.3 (4)	C5B^i—C4B—C3B	121.6 (2)
C3A—C2A—C1A	118.2 (4)	C6B—C5B—C4B	119.8 (3)
C2A—C3A—C4A	123.9 (4)	C6B—C5B—H5B	120.1
C2A—C3A—H3A	118.0	C4B—C5B—H5B	120.1
C4A—C3A—H3A	118.0	N2B—C6B—C5B	123.6 (4)
O2A—C4A—C3A	129.5 (4)	N2B—C6B—H6B	118.2
O2A—C4A—C5A	113.5 (4)	C5B—C6B—H6B	118.2

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C3A—H3A···F3A	0.96	2.36	2.730 (6)	102
C3A—H3A···F5A	0.96	2.36	2.739 (6)	102
C1B—H1B···O2A	0.96	2.44	3.064 (5)	123
C6B—H6B···O2A^iii	0.96	2.64	3.115 (5)	111
C2B—H2B···F1A^iv	0.96	2.71	3.430 (6)	132
C5B—H5B···O2A^v	0.96	2.81	3.620 (5)	143
C3A—H3A···F6A^vi	0.96	2.92	3.771 (6)	148

Symmetry codes: (iii) x−1, y+1, z; (iv) x−1, y, z; (v) −y, −x+1, −z+3/2; (vi) x+1/2, −y−1/2, −z+5/4.