Bis(μ-2-tert-butylphenylimido-1:2κ2 N:N)chlorido-2κCl-(diethyl ether-1κO)(2η5-pentamethylcyclopentadienyl)lithiumtantalum(V)

In the title compound, [LiTa(C10H15)(C10H13N)2Cl(C4H10O)], the TaV atom is coordinated by a η5-pentamethylcyclopentadienyl (Cp*) ligand, a chloride ion and two N-bonded 2-tert-butylphenylimide dianions. With respect to the two N atoms, the chloride ion and the centroid of the Cp* ring, the tantalum coordination geometry is approximately tetrahedral. The lithium cation is bonded to both the 2-tert-butylphenylimide dianions and also a diethyl ether molecule, in an approximate trigonal planar arrangement. The Ta⋯Li separation is 2.681 (15) Å. In the crystal, a weak C—H⋯Cl interaction links the molecules. When compared to the 2,6-diisopropylphenylimide analogue (‘the Wigley derivative’) of the title compound, the two structures are conformationally matched with an overall r.m.s. difference of 0.461Å.

In the title compound, [LiTa(C 10 H 15 )(C 10 H 13 N) 2 Cl(C 4 H 10 O)], the Ta V atom is coordinated by a 5 -pentamethylcyclopentadienyl (Cp*) ligand, a chloride ion and two N-bonded 2-tertbutylphenylimide dianions. With respect to the two N atoms, the chloride ion and the centroid of the Cp* ring, the tantalum coordination geometry is approximately tetrahedral. The lithium cation is bonded to both the 2-tert-butylphenylimide dianions and also a diethyl ether molecule, in an approximate trigonal planar arrangement. The TaÁ Á ÁLi separation is 2.681 (15) Å . In the crystal, a weak C-HÁ Á ÁCl interaction links the molecules. When compared to the 2,6-diisopropylphenylimide analogue ('the Wigley derivative') of the title compound, the two structures are conformationally matched with an overall r.m.s. difference of 0.461Å .
Data collection: SMART (Siemens, 1995); cell refinement: SAINT (Siemens, 1995); data reduction: SAINT; program(s) used to solve structure: SHELXS86 (Sheldrick, 2008); program(s) used to refine structure: SHELXL93 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL93. this stabilization. Imido aryl substituents containing one bulky substituent in the ortho position also offer the possibility for substantial steric protection, not only due to the presence of the large substituent but also as a result of bending at the imido nitrogen. We have thus studied the bis (2 -t-butylphenylimido) chloro (η 5 -pentamethylcyclopentadienyl) tantalum(V) anion (I) with a view to comparing its structure with its previously reported 2,6-diisopropylphenylimido analogue (II) (Baldwin et al., 1993).
The 50% probability thermal ellipsoid plot of the molecular structure of (I) is given in Figure 1. Selected bond distances and angles are given in Table 1. Fractional coordinates and anisotropic displacement parameters are provided in supplementary material.
However, several geometrical differences exist between the two compounds as a result of the presence of a more bulky aryl imido substituent in this case.
In the 2,6-diisopropylphenylimido structure, the planes of the arylimido and Cp* rings are approximately parallel to each other to minimize steric repulsion between the respective isopropyl and methyl groups. The situation for the 2 -tbutylphenylimido congener is comparable, except that the single bulky tert-butyl substituent on each imido ligand is now positioned in a less congested orientation away from the [µ-Li(OEt 2 )]+ moiety, such that they point in a similar direction to the Ta-Cl vector.
This steric alleviation is shown in Figure 2, which also illustrates the overall conformational difference between the two structures. This structure overlay was generated by matching the following respective atom pairs in each molecule: Ta, N, Li, O, Cp* and phenyl C atoms (Weng et al., 2008). These atoms are conformationally matched with an overall root-mean-square difference of 0.461 Å. The geometric differences between the tert-butyl and disopropyl phenyl substituents are emphasized by the visual offset to this conformationally matched molecular fragment. A full list of individual atomic pairwise deviations from a perfect match is given in supplementary information.
supplementary materials sup-2 The Ta(1)-Li(1) distance of 2.68 (1)Å is slightly longer than in the Wigley derivative, being the only other reported. We presume that this is also consequent upon the greater steric repulsion from the tertiary butyl groups compared to the isopropyl groups of the phenylimido ligands.
The atoms in the OEt 2 fragment of the subject compound display large isotropic displacement parameters. Given the terminal nature of this fragment, significant thermal motion is likely the cause, although positional disorder cannot be excluded. In contrast, the analogous displacement parameters in the Wigley derivative appear regular, being comparable in size to other terminal carbon atoms in the main part of the structure.
The structure of (I) contains a weak C12-H12A···Cl1 interaction [H···Cl = 2.89 (2) Å, symmetry code: 3/2 -x, 1/2 + y, z; c.f. sum of van der Waals radii of H and Cl = 2.95 Å (Bondi, 1964)]. This links adjacent molecules forming chains which are almost parallel to the y-axis (see Figure 3). Adjacent chains are arranged anti-parallel to each other thus completing the three-dimensional structure. In contrast, no hydrogen-bonds or short non-bonded contacts are present in the diisopropylphenyl structure, as deduced from a search in Materials Mercury (Macrae et al., 2008).

Experimental
A solution of LiNH(2-t BuC 6 H 4 ) (1.717 g, 11.07 mmol) in Et 2 O (80 ml) was added dropwise to a stirred solution of Cp*TaCl 4 (1.267 g, 2.77 mmol) in Et 2 O (80 ml) at 0 °C. This mixture was allowed to warm up to room temperature and stirred for 24 h. The resultant yellow/brown solution was filtered from the white residue of LiCl, concentrated and cooled to -30 °C to yield long yellow crystals of (I) (yield: 1.47 g, 73%).

Refinement
A yellow rectangular crystal was mounted onto a Siemens SMART-CCD diffractometer using the oil-drop method (Kottke & Stalke, 1993).
Positional and anisotropic displacement parameters for all non-hydrogen atoms in the anionic part of the molecule were refined. Likewise, the lithium atom within the cation was refined anisotropically. The displacement parameters of the oxydiethyl group were refined isotropically. All hydrogen isotropic displacement parameters in the molecule were constrained supplementary materials sup-3 to the riding model, U iso (H) = 1.2U eq (C) except for those relating to terminal methyl group H atoms, where U iso (H) = 1.5U eq (C). Fig. 1. The molecular structure of (I). Displacement parameters are displayed at the 30% probability level. Hydrogen atoms have been omitted for clarity.  Bis(µ-2-tert-butylphenylimido-1:2κ 2 N:N)chlorido-2κCl-(diethyl ether-1κO)(2η 5pentamethylcyclopentadienyl)lithiumtantalum(V)