Tetrabromo(2-methylpyridine-N)titanate(IV)

The reaction of 2-methyl­pyridine with TiBr₄ affords tetra­bromo(2-methyl­pyridine-N)­titanate(IV), C₆H₇Br₄NTi. The environment around the Ti atom can be described as a slightly distorted trigonal bipyramid, with the nitro­gen base occupying an equatorial position. The crystal structure of the title compound is isomorphous with tetra­chloro(2-methyl­pyridine-N)­titanate(IV).

S1. Comment

Titanium tetrahalides are strong Lewis acids and form Lewis acid–Lewis base adducts with electron-pair donors. These reactions have been of scientific interest over a long period of time (Emeléus & Rao, 1958; Rao, 1960; Hensen, Pickel et al., 1997). Ti^IV affords adducts with different ligands. Two representative examples 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, neutral pentacoordinated TiX₄L complexes (X = halogen atom, L = single-bonded ligand) have just recently been described (Hensen et al., 2000a,b). We present in this work another example, (I), of this kind of compound. \scheme

The environment of the titanium centre can be described as a trigonal bipyramid 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 ligands should be expected. However, steric reasons force these residues into the equatorial plane.

The two axial Ti—Br bonds display nearly the same lengths and are significantly longer than the equatorial Ti—Br bonds. The methyl group shows two steric effects: the first one is that the equatorial Ti—Br bond adjacent to the methyl group is markedly longer than the other equatorial Ti—Br bond. The second effect is that the equatorial N—Ti—Br angle is widened whereas the equatorial Br—Ti—Br angle is decreased (Table 1). The angles involving the two axial Br ligands show that these atoms are displaced from the ideal linear arrangement in the direction of the aromatic ring. On the other hand, 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 is isomorphous with tetrachloro(2-methylpyridine-N)titanate(IV) (Hensen et al., 2000b). A least-squares fit of the two molecules excluding the halogen and H atoms gives an r.m.s.d. of 0.036 Å and the bond lengths and angles involving the halogen substituents show the same geometric features as in the title compound [Ti—Cl_ax 2.289 (1) and 2.297 (1) Å, Ti—Cl_eq 2.211 (1) Å, Ti—Cl_eq(adjacent to CH₃) 2.253 (1) Å]. The Ti—N bond length is a little shorter than in the three comparable compounds: 2.185 (3) Å in tetrachloro(2-methylpyridine-N)titanate(IV), 2.190 (2) Å in tetrachloro(2,6-dimethylpyridine-N)titanate(IV) (Hensen et al., 2000b), and 2.179 (7) and 2.199 (8) Å in the two molecules in the asymmetric unit of tetrabromo(2,6-dimethylpyridine-N)titanate(IV) (Hensen et al., 2000a).

S2. Experimental

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

S3. Refinement

All H atoms were located by difference Fourier synthesis 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 aromatic C—H = 0.95 or methyl C—H = 0.98 Å. The methyl group was allowed to rotate about its 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); software used to prepare material for publication: SHELXL97.

(I)

Crystal data

C6H7Br4NTi	F(000) = 848
Mr = 460.67	Dx = 2.647 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
a = 6.7416 (1) Å	Cell parameters from 6306 reflections
b = 9.8984 (2) Å	θ = 1.0–25.0°
c = 17.3338 (3) Å	µ = 14.52 mm^−1
β = 91.820 (1)°	T = 173 K
V = 1156.12 (4) Å^3	Block, red
Z = 4	0.25 × 0.20 × 0.20 mm

Data collection

Siemens CCD three-circle diffractometer	2159 independent reflections
Radiation source: fine-focus sealed tube	1587 reflections with I > 2σ(I)
Highly oriented graphite crystal monochromator	Rint = 0.086
ω scans	θmax = 26.2°, θmin = 2.4°
Absorption correction: empirical (using intensity measurements) The data collection nominally covered a sphere of reciprocal space, by a combination of seven sets of exposures; each set had a different ϕ angle for the crystal and each exposure covered 0.3° in ω. The crystal-to-detector distance was 6 cm. Coverage of the unique set for all structures 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.	h = −8→8
Tmin = 0.026, Tmax = 0.055	k = −12→12
15542 measured reflections	l = −21→20

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.055	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.106	H-atom parameters constrained
S = 1.09	Calculated w = 1/[σ^2(Fo^2) + (0.0402P)^2 + 4.5123P] where P = (Fo^2 + 2Fc^2)/3
2159 reflections	(Δ/σ)max < 0.001
110 parameters	Δρmax = 0.98 e Å^−3
0 restraints	Δρmin = −0.92 e Å^−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.

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
Ti1	0.7481 (2)	0.26630 (15)	0.42258 (8)	0.0184 (4)
Br1	1.05923 (14)	0.17272 (9)	0.45641 (5)	0.0297 (3)
Br2	0.58777 (15)	0.31811 (10)	0.53559 (5)	0.0346 (3)
Br3	0.59407 (14)	0.04955 (9)	0.39896 (6)	0.0290 (3)
Br4	0.88825 (14)	0.48695 (9)	0.39882 (5)	0.0285 (3)
N1	0.5880 (10)	0.3157 (7)	0.3175 (4)	0.0216 (17)
C2	0.6561 (12)	0.2868 (8)	0.2483 (5)	0.0183 (19)
C21	0.8543 (14)	0.2160 (10)	0.2442 (5)	0.030 (2)
H21A	0.9498	0.2587	0.2804	0.044*
H21B	0.9029	0.2228	0.1917	0.044*
H21C	0.8388	0.1206	0.2580	0.044*
C3	0.5478 (14)	0.3269 (9)	0.1821 (5)	0.029 (2)
H3	0.5975	0.3080	0.1326	0.035*
C4	0.3717 (14)	0.3927 (10)	0.1881 (6)	0.032 (2)
H4	0.2974	0.4185	0.1430	0.039*
C5	0.3017 (14)	0.4219 (9)	0.2606 (6)	0.031 (2)
H5	0.1801	0.4689	0.2663	0.037*
C6	0.4123 (14)	0.3814 (9)	0.3232 (5)	0.027 (2)
H6	0.3648	0.3998	0.3731	0.033*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ti1	0.0187 (9)	0.0193 (8)	0.0170 (8)	0.0001 (7)	−0.0001 (6)	−0.0001 (7)
Br1	0.0230 (5)	0.0308 (5)	0.0348 (6)	0.0041 (4)	−0.0054 (4)	0.0022 (4)
Br2	0.0443 (6)	0.0356 (6)	0.0244 (5)	0.0046 (5)	0.0118 (4)	−0.0016 (4)
Br3	0.0269 (5)	0.0224 (5)	0.0379 (6)	−0.0058 (4)	0.0017 (4)	0.0011 (4)
Br4	0.0332 (5)	0.0217 (5)	0.0303 (5)	−0.0065 (4)	−0.0054 (4)	0.0026 (4)
N1	0.021 (4)	0.020 (4)	0.024 (4)	−0.003 (3)	0.003 (3)	−0.004 (3)
C2	0.012 (4)	0.024 (5)	0.019 (5)	−0.010 (4)	0.001 (3)	−0.007 (4)
C21	0.037 (6)	0.037 (6)	0.015 (5)	−0.011 (5)	0.002 (4)	−0.007 (4)
C3	0.033 (6)	0.033 (6)	0.021 (5)	−0.007 (5)	−0.004 (4)	0.000 (4)
C4	0.029 (6)	0.034 (6)	0.033 (6)	−0.009 (5)	−0.019 (4)	0.009 (5)
C5	0.023 (5)	0.033 (6)	0.036 (6)	−0.001 (4)	−0.008 (4)	0.009 (5)
C6	0.034 (6)	0.021 (5)	0.027 (6)	−0.006 (4)	0.003 (4)	0.002 (4)

Geometric parameters (Å, º)

Ti1—N1	2.145 (7)	N1—C6	1.357 (11)
Ti1—Br2	2.3245 (17)	C2—C3	1.398 (12)
Ti1—Br1	2.3502 (17)	C2—C21	1.513 (12)
Ti1—Br3	2.4129 (17)	C3—C4	1.362 (13)
Ti1—Br4	2.4202 (17)	C4—C5	1.386 (14)
N1—C2	1.328 (10)	C5—C6	1.358 (12)
N1—Ti1—Br2	115.5 (2)	C2—N1—C6	119.8 (7)
N1—Ti1—Br1	136.3 (2)	C2—N1—Ti1	122.6 (6)
Br2—Ti1—Br1	108.17 (7)	C6—N1—Ti1	117.6 (6)
N1—Ti1—Br3	81.74 (19)	N1—C2—C3	119.6 (8)
Br2—Ti1—Br3	97.47 (6)	N1—C2—C21	118.3 (7)
Br1—Ti1—Br3	93.88 (6)	C3—C2—C21	122.1 (7)
N1—Ti1—Br4	80.69 (19)	C4—C3—C2	120.5 (9)
Br2—Ti1—Br4	97.98 (6)	C3—C4—C5	119.4 (8)
Br1—Ti1—Br4	92.75 (6)	C6—C5—C4	118.0 (9)
Br3—Ti1—Br4	160.36 (7)	N1—C6—C5	122.8 (9)
Br2—Ti1—N1—C2	−176.9 (6)	C6—N1—C2—C21	178.8 (8)
Br1—Ti1—N1—C2	4.4 (8)	Ti1—N1—C2—C21	0.1 (10)
Br3—Ti1—N1—C2	−82.5 (6)	N1—C2—C3—C4	−1.3 (13)
Br4—Ti1—N1—C2	88.6 (6)	C21—C2—C3—C4	−178.7 (8)
Br2—Ti1—N1—C6	4.4 (7)	C2—C3—C4—C5	1.1 (14)
Br1—Ti1—N1—C6	−174.3 (5)	C3—C4—C5—C6	−0.8 (14)
Br3—Ti1—N1—C6	98.7 (6)	C2—N1—C6—C5	−1.1 (13)
Br4—Ti1—N1—C6	−90.1 (6)	Ti1—N1—C6—C5	177.6 (7)
C6—N1—C2—C3	1.3 (12)	C4—C5—C6—N1	0.9 (14)
Ti1—N1—C2—C3	−177.4 (6)