Di-μ-chlorido-bis­[dichlorido­bis(methyl­amido-κN)bis­(methyl­amine-κN)titanium(IV)]

The title compound, [Ti₂Cl₆(C₂H₆N)₂(C₂H₇N)₂], is a binuclear octa­hedral complex lying about an inversion centre. There are four different chloride environments, two terminal [Ti—Cl = 2.2847 (5) and 2.3371 (5) Å] and two bridging [Ti—Cl = 2.4414 (5) and 2.6759 (5) Å], with the Ti—Cl distances being strongly influenced by both the ligand trans to the chloride and whether or not the chloride anion is bridging between the two Ti^IV centres. The compound forms a two-dimensional network in the solid state, with weak inter­molecular C—H...Cl inter­actions giving rise to a planar network in the (10) plane.

S1. Comment

Chemical vapour deposition (CVD) is a well used technique for depositing thin films of materials (Jones & Hitchman, 2009). Industrially, films of transition metal pnictide materials are deposited using volatile metal and pnictogen precursors, but these precursors are often toxic, flammable and generally difficult to handle. `Single-source' precursors are compounds which contain a direct M—Pn bond (Pn = pnictogen) and they are usually much less toxic and much easier to handle (Cowley & Jones, 1994; Carmalt & Basharat, 2007).

As a part of recent work into the synthesis of molecular precursors for the deposition of transition metal pnictide thin films (Potts et al., 2009; Thomas, Blackman et al., 2010; Thomas et al., 2011), we recently reported an unusual ligand-exchange reaction between TiCl₄ and As(NMe₂)₃ (Thomas, Pugh et al., 2010). The reaction of one equivalent of As(NMe₂)₃ with TiCl₄ did not afford the 1:1 adduct [TiCl₄{As(NMe₂)₃}]. Instead, the compound [TiCl₃(NMe₂)(µ-NMe₂)₂AsCl], (I), was isolated as dark-green crystals. Structural characterization confirmed that an exchange of chloride and dimethylamide ligands had taken place.

Despite the fact that (I) was largely unsuitable as a single-source precursor to thin films of TiAs (owing to the lack of a direct Ti—As bond), further investigations into this unusual exchange were carried out. In an attempt to determine whether the exchange would still occur if the ligands on the metals were reversed, one equivalent of [Ti(NMe₂)₄] was reacted with AsCl₃. The title compound, (II), also a dark-green solid, was isolated from the reaction.

Compound (II) is a binuclear slightly distorted octahedral complex lying about an inversion centre, with two 14-electron Ti^IV centres. The bridging chloride ligand datively bonds trans to the dimethylamide group, with the four ligands cis to the dimethylamide group showing a slight repulsion from the strongly electronegative group (Fig. 1). The Ti₂Cl₂ ring at the heart of the molecule is planar, with atom N1 departing from this plane by 0.102 (2) Å.

The Ti—N bond lengths in (II) give a strong indication of the presence of two different types of N-containing ligand within the complex (Table 1). The large difference between the Ti—N1 and Ti—N2 bond lengths is indicative of the presence of anionic amide and neutral amine ligands. In addition, the geometry at N2 and the presence of a peak in the Fourier difference map in the expected position for an amine H atom lead to the assignment of N2 as a neutral dimethylamine ligand.

The Ti—Cl bond distances are heavily influenced by the trans effect, with the longest found for the chloride ligand trans to the dimethylamide group (Table 1). However, the fact that this is the `bridging' Ti—Cl2ⁱ [symmetry code: (i) 1 - x, 1 - y, 1 - z] bond also has a strong influence on the bond length. This is confirmed by looking at the Ti—Cl bonds which are trans to each other: the Ti—Cl2 bond is longer than the Ti—Cl3 bond. This effect can only be a result of atom Cl2 being involved in bridging between the two Ti centres, because the immediate environment around the chloride ligands is otherwise identical. The other Ti—Cl distance demonstrates the difference in trans effect between chloride and amine ligands. The Ti—Cl3 distance is the shortest of the four Ti—Cl distances and is a result of atom Cl3 being trans to a bridging chloride ligand. Even though atom Cl1 is trans to a neutral amine ligand instead of an anionic chloride, the stronger trans effect of the amine has the result of lengthening the Ti—Cl1 bond.

These effects were also noted in other structurally characterized examples of [TiCl₃L_aL_n]₂ species (L_a is an anionic amide donor and L_n is a neutral amine donor). Two types of these complexes have been reported in the Cambridge Structural Database (CSD, August 2010 update; Allen, 2002), all of which involve bidentate ligands. Two examples exist of 2-amidopyridine adducts: [Me₃SiN(2-py)TiCl₃]₂ reported by Jones et al. (2003) and [PhN(2-py)TiCl₃]₂ reported by Talja et al. (2005). Also, two aminosilylamide adducts of TiCl₃ have been reported by Buheitel et al. (1996) and Rannabauer et al. (2002), where the donor atoms are linked through an SiX₂ group (X = Cl or Me). In each case, the geometry at the metal centre is very similar to that found in (II), with only the nature of the N-containing ligands differing. For example, in each complex, the amide donor is trans to a bridging chloride ligand, resulting in a lengthening of that Ti—Cl bond.

Also present in (II) is an intramolecular hydrogen-bonding interaction between the N2—H2 group of the amine ligand and atom Cl1ⁱ (Table 2). Other weaker intermolecular methyl C—H···Cl interactions are also present in the crystal structure (see Table 2) and give rise to a two-dimensional network parallel to the (102) plane, as shown in Fig. 2.

The lack of arsenic in (II) is notable compared with (I), where it was retained. This does rule out (II) as a potential single-source precursor to thin films of TiAs. However, it may prove to be a viable precursor to thin films of TiN, owing to the presence of two N-donor ligands attached to Ti.

Related literature

For related literature, see: Allen (2002); Buheitel et al. (1996); Carmalt & Basharat (2007); Cowley & Jones (1994); Jones & Hitchman (2009); Jones et al. (2003); Potts et al. (2009); Rannabauer et al. (2002); Talja et al. (2005); Thomas et al. (2011); Thomas, Blackman, Parkin & Carmalt (2010); Thomas, Pugh, Parkin & Carmalt (2010).

S2. Experimental

A solution of AsCl₃ (0.2 ml, 2.37 mmol) in diethyl ether (20 ml) was added dropwise to a solution of Ti(NMe₂)₄ (0.6 ml, 2.53 mmol) in diethyl ether (20 ml). The reaction was stirred for 48 h and then the solvents were removed under vacuum, affording a dark-green solid (yield 93%). Crystallization occurred by layering a concentrated CH₂Cl₂ solution of (II) with hexane. Spectroscopic analysis: ¹H NMR (C₆D₆, 400 MHz, 298 K, δ, p.p.m.): 2.5 (s), 2.2 (s). Analysis, calculated for C₄H₁₃Cl₃N₂Ti: C 19.74, H 5.38, N 11.51%; found: C 19.57, H 5.51, N 11.62%.

S3. Refinement

All H atoms were clearly visible in difference maps and were refined using a riding model. The methyl H atoms were constrained to an ideal geometry, with C—H = 0.98 Å and U_iso(H) = 1.5U_eq(C), but were allowed to rotate freely about the adjacent C—N bonds. Atom H2 bonded to N2 was placed in a geometrically idealized position and constrained to ride, with N—H = 0.93 Å and U_iso(H) = 1.2U_eq(N).

Computing details

Data collection: COLLECT (Nonius, 1998); cell refinement: DENZO (Otwinowski & Minor, 1997) and COLLECT (Nonius, 1998); data reduction: DENZO (Otwinowski & Minor, 1997) and COLLECT (Nonius, 1998); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997) and Mercury (Macrae et al., 2006); software used to prepare material for publication: PLATON (Spek, 2009), WinGX (Farrugia, 1999) and enCIFer (Allen et al., 2004).

Figures

	Fig. 1. A view of (II), showing the atom-numbering scheme and the hydrogen-bonding interactions (dashed lines) between the amine N—H groups and Cl1^i. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry code: (i) -x + 1, -y + 1, -z + 1.]
	Fig. 2. A view of (II), showing the two-dimensional arrangement parallel to the (102) plane. [Symmetry codes: (i) -x + 1, -y + 1, -z + 1; (ii) x, y + 1, z; (iii) -x, y + 1/2, -z + 1/2; (iv) -x + 1, -y + 2, -z + 1.]

Di-µ-chlorido-bis[dichloridobis(methylamido-κN)bis(methylamine- κN)titanium(IV)]

Crystal data

[Ti2Cl6(C2H6N)2(C2H7N)2]	F(000) = 496
Mr = 486.83	Dx = 1.601 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 11855 reflections
a = 10.7088 (4) Å	θ = 2.9–27.5°
b = 6.7143 (3) Å	µ = 1.58 mm^−1
c = 15.7006 (5) Å	T = 120 K
β = 116.515 (2)°	Cut block, green
V = 1010.16 (7) Å^3	0.16 × 0.10 × 0.08 mm
Z = 2

Data collection

Bruker Nonius APEXII CCD camera on κ-goniostat diffractometer	2317 independent reflections
Radiation source: Bruker-Nonius FR591 rotating anode	2092 reflections with I > 2σ(I)
10cm confocal mirrors monochromator	Rint = 0.039
Detector resolution: 4096x4096pixels / 62x62mm pixels mm^-1	θmax = 27.5°, θmin = 3.6°
ϕ and ω scans	h = −13→13
Absorption correction: multi-scan (SADABS; Sheldrick, 2007)	k = −8→8
Tmin = 0.786, Tmax = 0.884	l = −20→20
9954 measured reflections

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)] = 0.028	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.062	H-atom parameters constrained
S = 1.12	w = 1/[σ^2(Fo^2) + (0.P)^2 + 1.0541P] where P = (Fo^2 + 2Fc^2)/3
2317 reflections	(Δ/σ)max = 0.001
95 parameters	Δρmax = 0.32 e Å^−3
0 restraints	Δρmin = −0.37 e Å^−3

Special details

Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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	0.29532 (3)	0.51821 (5)	0.44300 (2)	0.01379 (9)
Cl1	0.29281 (5)	0.37857 (7)	0.30590 (3)	0.02089 (12)
Cl2	0.49136 (4)	0.72771 (7)	0.45984 (3)	0.01622 (11)
Cl3	0.15258 (5)	0.28600 (7)	0.46057 (3)	0.02059 (12)
N1	0.15872 (15)	0.7093 (2)	0.38217 (11)	0.0171 (3)
N2	0.35993 (16)	0.6275 (2)	0.59128 (11)	0.0168 (3)
H2	0.4567	0.6317	0.6170	0.020*
C1	0.03618 (19)	0.7453 (3)	0.39827 (15)	0.0233 (4)
H1A	−0.0485	0.7121	0.3405	0.035*
H1B	0.0334	0.8859	0.4141	0.035*
H1C	0.0415	0.6619	0.4510	0.035*
C2	0.1584 (2)	0.8385 (3)	0.30680 (15)	0.0264 (4)
H2A	0.1694	0.9775	0.3280	0.040*
H2B	0.0698	0.8230	0.2493	0.040*
H2C	0.2357	0.8012	0.2927	0.040*
C3	0.3221 (2)	0.8364 (3)	0.59991 (15)	0.0262 (4)
H3A	0.3748	0.8809	0.6660	0.039*
H3B	0.2219	0.8446	0.5814	0.039*
H3C	0.3447	0.9218	0.5581	0.039*
C4	0.3351 (2)	0.5005 (3)	0.65953 (14)	0.0257 (5)
H4A	0.2348	0.4951	0.6410	0.038*
H4B	0.3842	0.5569	0.7236	0.038*
H4C	0.3699	0.3658	0.6590	0.038*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ti1	0.01305 (16)	0.01539 (17)	0.01466 (17)	−0.00119 (12)	0.00773 (13)	−0.00037 (13)
Cl1	0.0188 (2)	0.0277 (3)	0.0169 (2)	−0.00273 (18)	0.00864 (18)	−0.00555 (18)
Cl2	0.0154 (2)	0.0148 (2)	0.0207 (2)	−0.00089 (16)	0.01006 (18)	0.00132 (17)
Cl3	0.0193 (2)	0.0187 (2)	0.0270 (3)	−0.00443 (17)	0.01321 (19)	−0.00021 (18)
N1	0.0145 (7)	0.0186 (8)	0.0181 (8)	−0.0005 (6)	0.0072 (6)	0.0014 (6)
N2	0.0143 (7)	0.0197 (8)	0.0179 (8)	0.0004 (6)	0.0086 (6)	−0.0008 (6)
C1	0.0165 (9)	0.0240 (10)	0.0297 (11)	0.0028 (8)	0.0107 (8)	0.0013 (9)
C2	0.0269 (10)	0.0268 (11)	0.0262 (11)	0.0026 (9)	0.0125 (9)	0.0075 (9)
C3	0.0304 (11)	0.0225 (10)	0.0251 (11)	0.0035 (9)	0.0119 (9)	−0.0060 (9)
C4	0.0310 (11)	0.0327 (12)	0.0192 (10)	−0.0034 (9)	0.0166 (9)	0.0012 (9)

Geometric parameters (Å, º)

Ti1—N1	1.8576 (16)	C1—H1A	0.9800
Ti1—N2	2.2358 (16)	C1—H1B	0.9800
Ti1—Cl3	2.2847 (5)	C1—H1C	0.9800
Ti1—Cl1	2.3371 (5)	C2—H2A	0.9800
Ti1—Cl2	2.4414 (5)	C2—H2B	0.9800
Ti1—Cl2^i	2.6759 (5)	C2—H2C	0.9800
Cl2—Ti1^i	2.6759 (5)	C3—H3A	0.9800
N1—C1	1.463 (2)	C3—H3B	0.9800
N1—C2	1.466 (2)	C3—H3C	0.9800
N2—C3	1.483 (2)	C4—H4A	0.9800
N2—C4	1.483 (2)	C4—H4B	0.9800
N2—H2	0.9300	C4—H4C	0.9800
N1—Ti1—N2	96.62 (6)	N1—C1—H1A	109.5
N1—Ti1—Cl3	96.76 (5)	N1—C1—H1B	109.5
N2—Ti1—Cl3	90.63 (4)	H1A—C1—H1B	109.5
N1—Ti1—Cl1	96.95 (5)	N1—C1—H1C	109.5
N2—Ti1—Cl1	164.09 (4)	H1A—C1—H1C	109.5
Cl3—Ti1—Cl1	95.98 (2)	H1B—C1—H1C	109.5
N1—Ti1—Cl2	95.71 (5)	N1—C2—H2A	109.5
N2—Ti1—Cl2	81.19 (4)	N1—C2—H2B	109.5
Cl3—Ti1—Cl2	165.80 (2)	H2A—C2—H2B	109.5
Cl1—Ti1—Cl2	89.233 (19)	N1—C2—H2C	109.5
N1—Ti1—Cl2^i	174.37 (5)	H2A—C2—H2C	109.5
N2—Ti1—Cl2^i	79.65 (4)	H2B—C2—H2C	109.5
Cl3—Ti1—Cl2^i	87.537 (19)	N2—C3—H3A	109.5
Cl1—Ti1—Cl2^i	86.181 (18)	N2—C3—H3B	109.5
Cl2—Ti1—Cl2^i	79.619 (18)	H3A—C3—H3B	109.5
Ti1—Cl2—Ti1^i	100.381 (18)	N2—C3—H3C	109.5
C1—N1—C2	111.26 (15)	H3A—C3—H3C	109.5
C1—N1—Ti1	125.90 (13)	H3B—C3—H3C	109.5
C2—N1—Ti1	122.77 (13)	N2—C4—H4A	109.5
C3—N2—C4	109.48 (15)	N2—C4—H4B	109.5
C3—N2—Ti1	115.81 (12)	H4A—C4—H4B	109.5
C4—N2—Ti1	119.46 (12)	N2—C4—H4C	109.5
C3—N2—H2	103.2	H4A—C4—H4C	109.5
C4—N2—H2	103.2	H4B—C4—H4C	109.5
Ti1—N2—H2	103.2
N1—Ti1—Cl2—Ti1^i	176.84 (5)	Cl2—Ti1—N1—C2	42.68 (15)
N2—Ti1—Cl2—Ti1^i	81.00 (4)	N1—Ti1—N2—C3	−19.88 (14)
Cl3—Ti1—Cl2—Ti1^i	25.58 (9)	Cl3—Ti1—N2—C3	−116.75 (13)
Cl1—Ti1—Cl2—Ti1^i	−86.258 (19)	Cl1—Ti1—N2—C3	128.50 (16)
Cl2^i—Ti1—Cl2—Ti1^i	0.0	Cl2—Ti1—N2—C3	74.90 (13)
N2—Ti1—N1—C1	−58.77 (16)	Cl2^i—Ti1—N2—C3	155.87 (13)
Cl3—Ti1—N1—C1	32.65 (16)	N1—Ti1—N2—C4	114.43 (14)
Cl1—Ti1—N1—C1	129.55 (15)	Cl3—Ti1—N2—C4	17.55 (13)
Cl2—Ti1—N1—C1	−140.53 (15)	Cl1—Ti1—N2—C4	−97.2 (2)
N2—Ti1—N1—C2	124.43 (15)	Cl2—Ti1—N2—C4	−150.79 (14)
Cl3—Ti1—N1—C2	−144.15 (15)	Cl2^i—Ti1—N2—C4	−69.83 (13)
Cl1—Ti1—N1—C2	−47.25 (15)

Symmetry code: (i) −x+1, −y+1, −z+1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N2—H2···Cl1^i	0.93	2.40	3.3282 (16)	175
C1—H1B···Cl3^ii	0.98	2.92	3.822 (2)	153
C1—H1A···Cl1^iii	0.98	2.83	3.661 (2)	143

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