A second polymorph of 1,1,4,4-tetra­methyl­piperazinium penta­bromo­thallate(III)

A new polymorph of the title compound, (C₈H₂₀N₂)[TlBr₅], contains cations located about crystallographic centres of inversion and trigonal-bipyramidal anions which have a C₂ axis passing through the equatorial plane of the anion. The anion has the least distorted geometry seen so far in any structure possessing this anion and the axial Tl-Br bonds are about 0.13 Å longer than the equatorial Tl-Br bonds, consistent with related structures. The anion in the initially reported polymorph has lower symmetry and a greater distortion of the trigonal-bipyramidal coordination.

S1. Comment

The pentacoordinate [TlBr₅]^2- anion was characterized only recently as an almost regular trigonal-bipyramidal species in its 1,1,4,4-tetramethylpiperazinium, (I), and N,N'-diethyltriethylenediammonium salts (Linden, Nugent et al., 1999). In both cases, the Br_ax—Tl—Br_ax angles (ax is axial) were somewhat distorted from linearity and the Tl—Br_ax bond distances were quite asymmetric, but significantly longer than the Tl—Br_eq ones (eq is equatorial). Thus, the anion in these two compounds was shown to have a similar structure to that found for [FeCl₅]^2-, which also has a quaternized ammonium counter cation (James et al., 1995), but is different from the distorted square pyramidal [InCl₅]^2- anion found in its tetraethylammonium salt (Joy et al., 1975). During an evaluation of currently available commercial diffractometers having CCD detectors, data sets were collected from a series of crystals with known `difficult' structures (Linden, 1999). One of the compounds chosen was the title compound, (I). A crystal from the original batch was selected and found to exist as a different form, (Ib), to that found in the original structure determination (Ia; Linden, Nugent et al., 1999). It is unknown whether both forms existed simultaneously in the original batch at the time of crystallization, or if this second form evolved from the former structure during the three years that the original crystals were stored.

The original structure determination for the title compound found that the space group was P2₁/c with two symmetry-independent 1,1,4,4-tetramethylpiperazinium cations sitting across centres of inversion and one unique [TlBr₅]^2- anion on a general position. In contrast, polymorph (Ib) crystallizes in space group C2/c with one unique cation possessing crystallographic C_i symmetry and one unique anion with crystallographic C₂ symmetry (Fig. 1). Although the unit-cell volumes of the two polymorphs are quite similar, as are the lengths of the b axes and the β angles, the lengths of the a and c axes show significant differences and no transformation matrix can be found which would interconvert the two unit cells, thus excluding the possibility that the lower symmetry polymorph was simply defined in the wrong space group.

The twofold axis through the [TlBr₅]^2- anion of (Ib) passes through the equatorial plane of the anion, one of the equatorial Br atoms and the Tl atom. As a result, both axial Tl—Br bonds are equal in length and the coordination geometry of the anion forms quite a regular trigonal bipyramid, although the symmetry does not constrain it to be a perfect one. Indeed, the axial Tl—Br bonds are slightly distorted from linear geometry by 2.968 (17)° (Table 1) and are also about 0.13 Å longer than the equatorial Tl—Br bonds.

The structures of just four other compounds containing almost regular trigonal-bipyramidal [TlBr₅]^2- anions are reported in the literature (Linden, Nugent et al., 1999; Linden et al., 2002; Reid et al., 1999), although other much more highly distorted [TlBr₅]^2- species are known (Linden, James et al., 1999; Linden et al., 2002). Of these four compounds, two possess [TlBr₅]^2- anions with crystallographic symmetry, namely C₂ symmetry in the N,N'-diethyl-N,N,N',N'-tetramethyl-1,2-ethylenediammonium salt (etmeen; Linden et al., 2002) and C_2v symmetry in the [Mn(15-crown-5)(H₂O)₂]²⁺ salt [Mn(crown); Reid et al., 1999]. The geometric parameters for each of these anions are included in Table 1 and show that the anion in (Ib) has the least distorted trigonal-bipyramidal geometry of the three structures. The Br_ax—Tl—Br_ax bonds in the anion of the etmeen salt show the greatest deviation from linearity [7.25 (6)°], although these distortions are still quite small. While the Tl—Br bond lengths in this latter salt are very similar to those of (Ib), the axial Tl—Br bonds in the Mn(crown) salt are significantly longer than in (Ib) and the equatorial bonds are correspondingly shorter, the difference between the axial and equatorial bond lengths now being about 0.34 Å.

The two reported structures in which the [TlBr₅]^2- anions do not possess any crystallographic symmetry are those of the first polymorph of the title compound, (Ia), and the N,N'-diethyltriethylenediammonium salt (Linden, Nugent et al., 1999). The Br_ax—Tl—Br_ax angles are distorted from linearity by 2.67 (4)° in (Ia), and by 10.3 (2) and 6.3 (2)° for the two symmetry-independent anions in the latter structure. Thus, the distortions from linearity are similar to those observed in (Ib) and the other structures in which the anions have crystallographic symmetry. The lengths of the equatorial Tl—Br bonds of these two compounds, being about 2.59 Å, are also similar to those of (Ib) and are shorter than the axial Tl—Br bonds, as expected. However, in contrast to (Ib) and the other symmetrical anions, the lengths of the axial Tl—Br bonds in these two compounds show distinct asymmetry. In (Ia), these distances are 2.840 (1) and 2.737 (1) Å, while for the N,N'-diethyltriethylenediammonium salt, they are 2.914 (4) and 2.706 (4) Å, and 2.915 (5) and 2.725 (5) Å for the two symmetry-independent anions, respectively. This asymmetry leads to a significant distortion of the trigonal bipyramid in that the Tl—Br bonds involving the equatorial Br atoms are bent at the Tl atom away from the closer of the two axial Br atoms towards the more distant one, resulting in a deviation of some Br_ax—Tl—Br_eq angles by up to 10° from the ideal value of 90° and a deviation of the Tl atom from the equatorial plane of Br atoms by between 0.10 and 0.18 Å (Linden, Nugent et al., 1999). Such asymmetry and distortions of the trigonal bipyramid are not observed in the structure of (Ib) and the Tl atom lies of necessity on the equatorial plane.

The crystal packing consists of alternating layers of cations and anions stacked parallel to the (100) plane.

S2. Experimental

The title compound was prepared as described previously (Linden et al., 1999) [Linden, James··· or Linden Nugent··· ?] and crystallized by slow evaporation of its solution in concentrated HBr (m.p. 494–497 K).

S3. Refinement

The absorption correction was based on a comparison of the intensities of equivalent reflections in the highly redundant data as described by Blessing (1995). Attempts at applying a numerical absorption correction yielded significantly inferior results, presumably because the multi-faceted nature of the crystal inhibited the development of an accurate description of the crystal shape. The largest seven peaks of residual electron density (from 2.22 down to 1.00 e Å^-3) were all within 1.2 Å of the Tl or Br atoms. The methyl H atoms were constrained to an ideal geometry with C—H distances of 0.98 Å and U_iso(H) = 1.5U_eq(C), but each group was allowed to rotate freely about its C—N bond. All other H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms with C—H distances of 0.99 Å and U_iso(H) = 1.2U_eq(C).

Computing details

Data collection: KappaCCD Server Software (Nonius, 1999); cell refinement: DENZO-SMN (Otwinowski & Minor, 1997); data reduction: DENZO-SMN; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPII (Johnson, 1976); software used to prepare material for publication: SHELXL97.

Figures

Fig. 1. The structure of (Ib) drawn with 50% probability displacement ellipsoids and showing the atom-numbering scheme. H atoms are represented by circles of arbitrary size. [Symmetry codes: (i) 2 - x, y, 3/2 - z; (ii) 1/2 - x, 3/2 - y, 1 - z.]

1,1,4,4-tetramethylpiperazinium pentabromothallate(III)

Crystal data

(C8H20N2)[TlBr5]	F(000) = 1352
Mr = 748.15	Dx = 2.766 Mg m^−3
Monoclinic, C2/c	Melting point: 495 K
Hall symbol: -C 2yc	Mo Kα radiation, λ = 0.71073 Å
a = 16.0822 (3) Å	Cell parameters from 2872 reflections
b = 9.2422 (2) Å	θ = 2.9–36.4°
c = 13.2718 (2) Å	µ = 20.09 mm^−1
β = 114.3928 (8)°	T = 173 K
V = 1796.56 (6) Å^3	Prism, yellow
Z = 4	0.26 × 0.23 × 0.21 mm

Data collection

Nonius KappaCCD diffractometer	2740 independent reflections
Radiation source: fine-focus sealed tube	2467 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.080
ϕ scans, and ω scans with θ offsets (θ OK? or κ ?)	θmax = 30.5°, θmin = 3.4°
Absorption correction: multi-scan (SORTAV; Blessing, 1995)	h = 0→22
Tmin = 0.074, Tmax = 0.105	k = 0→13
18998 measured reflections	l = −18→17

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)] = 0.029	H-atom parameters constrained
wR(F^2) = 0.068	w = 1/[σ^2(Fo^2) + (0.0247P)^2 + 10.0125P] where P = (Fo^2 + 2Fc^2)/3
S = 1.06	(Δ/σ)max = 0.001
2740 reflections	Δρmax = 2.22 e Å^−3
77 parameters	Δρmin = −2.26 e Å^−3
0 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.00183 (8)

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
Tl1	1.0000	0.80493 (2)	0.7500	0.01561 (8)
Br1	0.81335 (3)	0.81259 (4)	0.65966 (3)	0.01924 (11)
Br2	0.99722 (3)	0.66359 (6)	0.58000 (4)	0.03579 (14)
Br3	1.0000	1.08881 (7)	0.7500	0.0536 (3)
N1	0.2298 (2)	0.6460 (4)	0.4072 (3)	0.0154 (6)
C1	0.1895 (3)	0.6316 (4)	0.4907 (3)	0.0173 (8)
H1A	0.1290	0.6794	0.4615	0.021*
H1B	0.1800	0.5277	0.5010	0.021*
C2	0.2515 (3)	0.8032 (4)	0.3989 (3)	0.0165 (8)
H2A	0.2831	0.8121	0.3491	0.020*
H2B	0.1937	0.8584	0.3657	0.020*
C3	0.3140 (3)	0.5528 (4)	0.4365 (4)	0.0203 (8)
H3A	0.3613	0.5847	0.5074	0.030*
H3B	0.3367	0.5618	0.3787	0.030*
H3C	0.2984	0.4516	0.4423	0.030*
C4	0.1603 (3)	0.5956 (5)	0.2966 (3)	0.0216 (8)
H4A	0.1855	0.6053	0.2411	0.032*
H4B	0.1050	0.6547	0.2748	0.032*
H4C	0.1452	0.4940	0.3018	0.032*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Tl1	0.01864 (12)	0.01199 (11)	0.01715 (12)	0.000	0.00833 (9)	0.000
Br1	0.0181 (2)	0.0174 (2)	0.0214 (2)	−0.00016 (14)	0.00735 (16)	0.00235 (14)
Br2	0.0289 (2)	0.0509 (3)	0.0340 (3)	−0.0146 (2)	0.0194 (2)	−0.0256 (2)
Br3	0.0223 (3)	0.0123 (3)	0.0876 (6)	0.000	−0.0159 (4)	0.000
N1	0.0148 (15)	0.0118 (14)	0.0183 (16)	−0.0009 (12)	0.0056 (13)	−0.0001 (12)
C1	0.0175 (18)	0.0153 (18)	0.022 (2)	−0.0042 (15)	0.0105 (16)	−0.0009 (15)
C2	0.0187 (19)	0.0113 (17)	0.0195 (19)	−0.0019 (14)	0.0079 (16)	0.0018 (14)
C3	0.0193 (19)	0.0174 (19)	0.026 (2)	0.0053 (15)	0.0109 (16)	0.0022 (16)
C4	0.022 (2)	0.0196 (19)	0.019 (2)	−0.0019 (16)	0.0046 (16)	−0.0035 (16)

Geometric parameters (Å, º)

Tl1—Br1	2.7350 (4)	C1—H1B	0.99
Tl1—Br2	2.5915 (5)	C2—H2A	0.99
Tl1—Br3	2.6237 (7)	C2—H2B	0.99
N1—C1	1.504 (5)	C3—H3A	0.98
N1—C4	1.505 (5)	C3—H3B	0.98
N1—C2	1.509 (5)	C3—H3C	0.98
N1—C3	1.513 (5)	C4—H4A	0.98
C1—C2^i	1.504 (6)	C4—H4B	0.98
C1—H1A	0.99	C4—H4C	0.98
Br1—Tl1—Br1^ii	177.032 (17)	C1^i—C2—N1	112.8 (3)
Br1—Tl1—Br2	90.585 (14)	C1^i—C2—H2A	109.0
Br1—Tl1—Br2^ii	90.910 (14)	N1—C2—H2A	109.0
Br1—Tl1—Br3	88.516 (9)	C1^i—C2—H2B	109.0
Br2—Tl1—Br2^ii	119.46 (3)	N1—C2—H2B	109.0
Br2—Tl1—Br3	120.270 (14)	H2A—C2—H2B	107.8
C1—N1—C4	108.6 (3)	N1—C3—H3A	109.5
C1—N1—C2	108.8 (3)	N1—C3—H3B	109.5
C4—N1—C2	109.1 (3)	H3A—C3—H3B	109.5
C1—N1—C3	111.9 (3)	N1—C3—H3C	109.5
C4—N1—C3	107.4 (3)	H3A—C3—H3C	109.5
C2—N1—C3	111.0 (3)	H3B—C3—H3C	109.5
C2^i—C1—N1	113.1 (3)	N1—C4—H4A	109.5
C2^i—C1—H1A	109.0	N1—C4—H4B	109.5
N1—C1—H1A	109.0	H4A—C4—H4B	109.5
C2^i—C1—H1B	109.0	N1—C4—H4C	109.5
N1—C1—H1B	109.0	H4A—C4—H4C	109.5
H1A—C1—H1B	107.8	H4B—C4—H4C	109.5
C4—N1—C1—C2^i	−172.6 (3)	C1—N1—C2—C1^i	53.8 (4)
C2—N1—C1—C2^i	−53.9 (4)	C4—N1—C2—C1^i	172.1 (3)
C3—N1—C1—C2^i	69.0 (4)	C3—N1—C2—C1^i	−69.7 (4)

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