Tetra­bromo(2,6-di­methyl­pyridine-N)titanium(IV), a twinned crystal ­structure

The reaction of 2,6-di­methyl­pyridine with TiBr₄ affords the title compound, [TiBr₄(C₇H₉N)], which is the first example of a neutral TiBr₄L complex (L is a singly bonded ligand). The environment around the Ti atom can be described as a somewhat distorted trigonal bipyramid, with the nitro­gen base occupying an equatorial position. The crystal was a non-merohedral twin.

S1. Comment

Titanium tetrahalides are strong Lewis acids and form Lewis acid–base adducts with electron-pair donors. These reactions have been of scientific interest for a long time (Emeléus & Rao, 1958; Rao, 1960; Hensen et al. 1997). Two representative examples of such adducts are the structures of TiCl₄(pyridine)₂ (Mazo et al., 1987) and bis(µ-dichloro)hexachlorobis(N-trimethylsilyl-imidazol-3-yl)dititanium (Hensen, Lemke & Näther, 1997). However, no neutral pentacoordinated TiX₄L complexes (X is a halogen atom and L is a singly-bonded ligand) have yet been described. We present here the first example of such a complex, (I), with X = Br and L = 2,6-dimethylpyridine.

The asymmetric unit of (I) consists of two half molecules, in which the Ti1, N1 and C4 atoms of molecule 1 and the Ti1A, N1A and C4A atoms of molecule 2 are located on a special position of site symmetry 2. The two molecules (Fig. 1) are nearly identical; a least-squares fit of all non-H atoms gives an r.m.s. deviation of 0.026 Å. The environment of the titanium centres can be described as trigonal bipyramidal, with the nitrogen base occupying an equatorial position. According to the valence-shell electron-pair repulsion (VSEPR) model (Haaland, 1989; Gillespie & Robinson, 1996), an axial position of the base ligand should be expected. However, steric effects force this residue into the equatorial plane. The methyl groups of the pyridine ligands cause the equatorial N—Ti—Br angles to be widened, whereas the equatorial Br—Ti—Br angles are decreased (Table 1). The angles between the two axial Br ligands (Br1 and its symmetry equivalent) show that these atoms are displaced from the ideal linear arrangement in the direction of the aromatic ring. In contrast, the equatorial Br atoms lie nearly exactly in the plane of the aromatic ring and the axial Br—Ti bonds are almost perpendicular to the plane of the aromatic ring. The structure displays an interesting packing motif (Fig. 2); the two symmetry independent molecules lie on different parallel lines in a head-to-tail manner. One line is located on the twofold rotation axis coinciding with the b axis (0,y,0), whereas the other one is situated on the twofold rotation axis with coordinates 0,y,1/2. The displacement between the two molecules in the direction of the b axis (as defined by the difference in y coordinates between equivalent atoms) is approximately 0.46. The angle between the pyridyl rings of the two symmetry independent molecules is 66.6 (2)°.

S2. Experimental

Because of the extreme susceptibility of the titanium halides to hydrolysis, all operations were carried out under an inert gas atmosphere. 2,6-Dimethylpyridine (9.45 mmol, 1.1 ml) was added to a solution of TiBr₄ (0.96 ml, 9.11 mmol) in hexane (30 ml). The red precipitate was washed with hexane and dried. Sublimation at 353 K yielded suitable crystals after approximately two weeks.

S3. Refinement

Using all reflections initially found on the first frames for determination of a preliminary cell, it turned out that several reflections with systematic deviations from integral indices did not fit to the derived cell. This result was a strong indication of twinning. However, these reflections could be indexed when the orientation matrix was transformed using the matrix (100/010/0.11,0,1). Thus, the transformation for the reflection indices of the two twin components is: h_twin = −h, k_twin = −k, l_twin = l+h/9. This means that most of the reflections are sufficiently separated for a satisfactory data collection, but exact overlap occurred for the reflections with h = 9 m (with m integer). Since the reflections with |h| = 1,8,10 showed only partial overlap and their correct intensity cannot be derived they were discarded from the refinement process. For refinement, the data were read in via HKLF5 and an additional variable was introduced (using the BASF command) describing the fractional contributions of the two twin components for the reflections with h = 9 m; the ratio refined to 0.202 (3)/0.798 (3). All H atoms were located by difference Fourier synthesis and refined with fixed individual displacement parameters [U(H) = 1.5U_eq(C_methyl) or U(H) = 1.2U_eq(C_aromatic)] using a riding model with C—H(aromatic) = 0.95 Å or C—H(methyl) = 0.98 Å. The methyl groups were allowed to rotate about their local threefold axis.

Computing details

Data collection: SMART (Siemens, 1995); cell refinement: SMART; data reduction: SAINT (Siemens, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XP in SHELXTL (Siemens, 1995).

Figures

	Fig. 1. Perspective view (not the relative orientation in the crystal) of the independent molecules of (I) with the atom-numbering scheme; only symmetry independent atoms are labelled. Displacement ellipsoids are at the 50% probability level.
	Fig. 2. Packing diagram of (I) viewed onto the a/c plane.

Crystal data

[TiBr4(C7H9N)]	F(000) = 880
Mr = 474.69	Dx = 2.462 Mg m^−3
Monoclinic, C2	Mo Kα radiation, λ = 0.71073 Å
a = 11.6811 (2) Å	Cell parameters from 8192 reflections
b = 9.8679 (2) Å	θ = 1–25°
c = 11.1300 (2) Å	µ = 13.11 mm^−1
β = 93.482 (1)°	T = 133 K
V = 1280.56 (4) Å^3	Plate, red
Z = 4	0.40 × 0.20 × 0.05 mm

Data collection

Siemens CCD three-circle diffractometer	2002 independent reflections
Radiation source: fine-focus sealed tube	1798 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.073
ω scans	θmax = 26.5°, θmin = 1.8°
Absorption correction: empirical (using intensity measurements) (SADABS; Sheldrick, 1996)	h = −14→14
Tmin = 0.077, Tmax = 0.560	k = −12→12
10417 measured reflections	l = 0→13

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)] = 0.034	H-atom parameters constrained
wR(F^2) = 0.068	w = 1/[σ^2(Fo^2) + (0.0257P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 1.00	(Δ/σ)max = 0.001
2002 reflections	Δρmax = 0.61 e Å^−3
124 parameters	Δρmin = −0.59 e Å^−3
1 restraint	Absolute structure: Flack (1983)
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.03 (2)

Special details

Experimental.

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.

Coverage of the unique set is 100% complete to at least 25.0° in θ. Crystal decay was monitored by repeating the initial frames at the end of data collection and analyzing the duplicate reflections.

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

	x	y	z	Uiso*/Ueq
Ti1	0.5000	0.50712 (19)	1.0000	0.0157 (4)
Br1	0.62173 (6)	0.46822 (7)	1.18188 (7)	0.0231 (2)
Br2	0.62667 (7)	0.65623 (8)	0.90897 (7)	0.0252 (2)
N1	0.5000	0.2861 (8)	1.0000	0.0134 (18)
C2	0.5799 (6)	0.2151 (8)	0.9421 (7)	0.0199 (18)
C21	0.6665 (7)	0.2981 (8)	0.8765 (7)	0.0236 (18)
H21A	0.6264	0.3508	0.8122	0.035*
H21B	0.7218	0.2372	0.8416	0.035*
H21C	0.7071	0.3598	0.9334	0.035*
C3	0.5801 (7)	0.0769 (8)	0.9386 (7)	0.0213 (18)
H3	0.6349	0.0304	0.8942	0.026*
C4	0.5000	0.0042 (12)	1.0000	0.023 (3)
H4	0.5000	−0.0920	1.0000	0.027*
Ti1A	0.5000	0.96534 (19)	0.5000	0.0151 (4)
Br1A	0.39673 (7)	0.92539 (7)	0.68047 (7)	0.0210 (2)
Br2A	0.36461 (7)	1.11412 (8)	0.40588 (7)	0.02206 (19)
N1A	0.5000	0.7448 (9)	0.5000	0.0151 (18)
C2A	0.4120 (6)	0.6738 (8)	0.4433 (7)	0.0204 (17)
C21A	0.3150 (6)	0.7575 (8)	0.3819 (7)	0.0201 (17)
H21D	0.2832	0.8186	0.4408	0.030*
H21E	0.2546	0.6967	0.3489	0.030*
H21F	0.3450	0.8109	0.3165	0.030*
C3A	0.4099 (7)	0.5344 (8)	0.4421 (7)	0.0216 (18)
H3A	0.3477	0.4871	0.4023	0.026*
C4A	0.5000	0.4641 (12)	0.5000	0.023 (3)
H4A	0.5000	0.3678	0.5000	0.028*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ti1	0.0159 (11)	0.0177 (10)	0.0131 (10)	0.000	−0.0025 (8)	0.000
Br1	0.0217 (4)	0.0320 (5)	0.0148 (4)	−0.0041 (3)	−0.0056 (3)	0.0019 (3)
Br2	0.0290 (5)	0.0254 (4)	0.0213 (5)	−0.0067 (4)	0.0026 (3)	0.0044 (3)
N1	0.013 (5)	0.018 (5)	0.009 (4)	0.000	−0.004 (3)	0.000
C2	0.012 (4)	0.028 (4)	0.020 (4)	−0.003 (3)	0.001 (4)	−0.003 (3)
C21	0.018 (4)	0.021 (4)	0.034 (5)	0.001 (3)	0.013 (4)	−0.001 (4)
C3	0.023 (5)	0.020 (4)	0.020 (4)	0.006 (3)	−0.008 (3)	0.001 (3)
C4	0.030 (7)	0.023 (6)	0.015 (6)	0.000	−0.005 (5)	0.000
Ti1A	0.0145 (9)	0.0171 (10)	0.0138 (9)	0.000	0.0017 (7)	0.000
Br1A	0.0203 (4)	0.0293 (5)	0.0139 (4)	0.0017 (3)	0.0041 (3)	0.0001 (3)
Br2A	0.0232 (4)	0.0236 (4)	0.0189 (4)	0.0057 (3)	−0.0025 (3)	0.0022 (3)
N1A	0.017 (5)	0.018 (5)	0.010 (4)	0.000	0.002 (4)	0.000
C2A	0.012 (4)	0.025 (4)	0.024 (4)	0.003 (3)	0.003 (3)	−0.004 (3)
C21A	0.015 (4)	0.030 (4)	0.015 (4)	0.001 (3)	−0.004 (3)	−0.002 (3)
C3A	0.017 (4)	0.021 (4)	0.026 (4)	−0.002 (3)	−0.001 (4)	0.002 (4)
C4A	0.031 (7)	0.017 (6)	0.022 (6)	0.000	0.007 (5)	0.000

Geometric parameters (Å, º)

Ti1—N1	2.181 (8)	Ti1A—N1A	2.176 (9)
Ti1—Br2	2.3585 (14)	Ti1A—Br2A^ii	2.3564 (14)
Ti1—Br2^i	2.3585 (14)	Ti1A—Br2A	2.3564 (14)
Ti1—Br1^i	2.4326 (8)	Ti1A—Br1A^ii	2.4372 (8)
Ti1—Br1	2.4326 (8)	Ti1A—Br1A	2.4372 (8)
N1—C2^i	1.361 (9)	N1A—C2A	1.367 (9)
N1—C2	1.361 (9)	N1A—C2A^ii	1.367 (9)
C2—C3	1.364 (11)	C2A—C3A	1.376 (11)
C2—C21	1.522 (11)	C2A—C21A	1.529 (11)
C3—C4	1.391 (10)	C3A—C4A	1.387 (9)
C4—C3^i	1.391 (10)	C4A—C3A^ii	1.387 (9)
N1—Ti1—Br2	128.60 (4)	N1A—Ti1A—Br2A^ii	128.54 (4)
N1—Ti1—Br2^i	128.60 (4)	N1A—Ti1A—Br2A	128.54 (4)
Br2—Ti1—Br2^i	102.81 (8)	Br2A^ii—Ti1A—Br2A	102.92 (8)
N1—Ti1—Br1^i	80.92 (5)	N1A—Ti1A—Br1A^ii	80.69 (5)
Br2—Ti1—Br1^i	95.23 (3)	Br2A^ii—Ti1A—Br1A^ii	96.59 (4)
Br2^i—Ti1—Br1^i	96.07 (3)	Br2A—Ti1A—Br1A^ii	94.98 (3)
N1—Ti1—Br1	80.92 (5)	N1A—Ti1A—Br1A	80.69 (5)
Br2—Ti1—Br1	96.07 (3)	Br2A^ii—Ti1A—Br1A	94.98 (3)
Br2^i—Ti1—Br1	95.23 (3)	Br2A—Ti1A—Br1A	96.59 (4)
Br1^i—Ti1—Br1	161.84 (9)	Br1A^ii—Ti1A—Br1A	161.38 (9)
C2^i—N1—C2	118.0 (9)	C2A—N1A—C2A^ii	118.3 (9)
C2^i—N1—Ti1	121.0 (5)	C2A—N1A—Ti1A	120.8 (5)
C2—N1—Ti1	121.0 (5)	C2A^ii—N1A—Ti1A	120.8 (5)
C3—C2—N1	122.0 (8)	N1A—C2A—C3A	122.0 (7)
C3—C2—C21	121.5 (7)	N1A—C2A—C21A	116.5 (7)
N1—C2—C21	116.4 (7)	C3A—C2A—C21A	121.5 (7)
C2—C3—C4	119.9 (8)	C2A—C3A—C4A	118.9 (8)
C3—C4—C3^i	117.9 (11)	C3A^ii—C4A—C3A	119.9 (11)
Br2—Ti1—N1—C2^i	179.8 (4)	Br2A^ii—Ti1A—N1A—C2A	177.9 (4)
Br2^i—Ti1—N1—C2^i	−0.2 (4)	Br2A—Ti1A—N1A—C2A	−2.1 (4)
Br1^i—Ti1—N1—C2^i	−90.7 (4)	Br1A^ii—Ti1A—N1A—C2A	−91.1 (4)
Br1—Ti1—N1—C2^i	89.3 (4)	Br1A—Ti1A—N1A—C2A	88.9 (4)
Br2—Ti1—N1—C2	−0.2 (4)	Br2A^ii—Ti1A—N1A—C2A^ii	−2.1 (4)
Br2^i—Ti1—N1—C2	179.8 (4)	Br2A—Ti1A—N1A—C2A^ii	177.9 (4)
Br1^i—Ti1—N1—C2	89.3 (4)	Br1A^ii—Ti1A—N1A—C2A^ii	88.9 (4)
Br1—Ti1—N1—C2	−90.7 (4)	Br1A—Ti1A—N1A—C2A^ii	−91.1 (4)
C2^i—N1—C2—C3	1.5 (6)	C2A^ii—N1A—C2A—C3A	0.1 (6)
Ti1—N1—C2—C3	−178.5 (6)	Ti1A—N1A—C2A—C3A	−179.9 (6)
C2^i—N1—C2—C21	178.9 (8)	C2A^ii—N1A—C2A—C21A	179.6 (8)
Ti1—N1—C2—C21	−1.1 (8)	Ti1A—N1A—C2A—C21A	−0.4 (8)
N1—C2—C3—C4	−2.9 (11)	N1A—C2A—C3A—C4A	−0.1 (11)
C21—C2—C3—C4	179.8 (6)	C21A—C2A—C3A—C4A	−179.7 (6)
C2—C3—C4—C3^i	1.4 (5)	C2A—C3A—C4A—C3A^ii	0.1 (5)

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