Crystal structure of nitridobis(trimethylsilanolato)[1,1,1-trimethyl-N-(trimethylsilyl)silanaminato]molybdenum(VI)

In the title compound, the MoVI cation is located on a mirror plane and has a distorted tetrahedral coordination geometry. The Mo N bond length is 1.633 (6) Å.

In the title compound, [Mo(C 6 H 18 NSi 2 )(C 3 H 9 OSi) 2 N], the Mo VI cation is located on a mirror plane and is coordinated by a nitride anion, a 1,1,1trimethyl-N-(trimethylsilyl)silanaminate anion and two trimethylsilanolate anions in a distorted tetrahedral geometry; the N atom and two Si atoms of the 1,1,1-trimethyl-N-(trimethylsilyl)silanaminato anionic ligand are also located on the mirror plane. The Mo N bond length of 1.633 (6) Å is much shorter than the Mo-N single-bond length of 1.934 (7) Å . No hydrogen bonding is observed in the crystal structure.

Chemical context
The title compound, nitridobis(trimethylsilanolato)[1,1,1trimethyl-N-(trimethylsilyl)ilanaminato]molybdenum, is a precursor for the preparation of nitridotris(triphenylsilanolato)molybdenum, which can generate alkylidynetris-(triphenylsilanolato)molybdenum, a superbly active catalyst for alkyne metathesis reactions (Bindl et al., 2009;Heppekausen et al., 2010). The structure of the title compound has been characterized by IR, 1 H and 13 C NMR and low resolution MS spectroscopy (Chiu et al., 1998;Bindl et al., 2009;Heppekausen et al., 2010). However, to our knowledge no crystal data have been reported because the title compound is an oil at room temperature, and is highly sensitive to air and moisture.

Structural commentary
In the crystal, the title complex ( Fig. 1) resides on a crystallographic mirror plane, therefore, the asymmetric unit consists of half of the complex. Atoms Mo1, N1, N2, Si2, Si3, C4, and C6 lie on the mirror plane. The compound is a mononuclear metal complex. The Mo VI complex adopts a slightly distorted tetrahedral geometry. The Mo1 N1 triple bond length is 1.633 (6) Å , which is shorter than the Mo N triple bond length in the B(C 6 F 5 ) 3 complex [1.696 (3) Å ; Finke & Moore, 2010), but is very close to that in the pyridine complex [1.640 (3) Å ; Chiu et al., 1998). This is reasonable because the nitrido group is the terminal group in both the tile complex and the pyridine complex, whereas the nitrido group also bonds to the boron atom in the B(C 6 F 5 ) 3 complex. The Mo1-N2 bond length [1.934 (7) Å ] is longer than that in the B(C 6 F 5 ) 3 complex [1.914 (3) Å ], but is shorter than that in the pyridine complex [1.973 (3) Å ]. The Mo1-O1 bond length [1.886 (3) Å ] is longer than those in the B(C 6 F 5 ) 3 complex [1.838 (3), 1.839 (3) Å ], but is shorter than those in the pyridine complex [1.921 (3), 1.924 (2) Å ]. It is also reasonable that the Mo1-N2 and Mo1-O1 bonds are strengthened in the B(C 6 F 5 ) 3 complex, but weakened in the pyridine complex. In the B(C 6 F 5 ) 3 complex, the Mo1 N1 bond is weakened due to the formation of a N1!B bond. Therefore, the rest of the bonds to the central Mo atom are strengthened. In the pyridine complex, however, the central Mo atom is five-coordinated with an extra bond between Mo and the nitrogen atom of pyridine. Since the Mo1 N1 triple bond is retained, the Mo1-N2 and Mo1-O1 bonds are weakened. Our assumption is that the central Mo atom has the same valence (+VI) in all three compounds.

Supramolecular features
No hydrogen bonding is observed in the crystal structure. The packing of the molecules is depicted in Fig. 2.

Synthesis and crystallization
The title compound was synthesized according to a literature method (Bindl et al., 2009) The crystal structure of the title compound. Displacement ellipsoids are drawn at the 30% probability level. [Symmetry code: (i) x, 3 2 À y, z.]

Figure 2
The packing of the title compound viewed along [101]. Displacement ellipsoids are drawn at the 30% probability level.
freshly distilled hexane (280 ml). Solid LiN(SiMe 3 ) 2 (80 mmol, 13.4 g) was added in three portions over 1 h to the suspension. The brownish green mixture was stirred at room temperature for a further 4 h. For work-up, the suspension was filtered through a pad of Celite under Ar, the brown filtrate was concentrated and the residue distilled under high vacuum to give the title compound as a light-brown oil (3.4 g, 19% yield based on Na 2 MoO 4 ). This oily product was left at 288 K for several days to give colorless crystals suitable for single-crystal X-ray diffraction. The crystals were first examined under a microscope. In order to avoid melting and reacting with air and moisture, crystals had to be submerged in several drops of inert oil cooled by ice. Then the selected crystal was quickly (less than 2 seconds) transferred to the cold nitrogen flow of the diffractometer. Initially, data collection was completed at 173 K. However, the final reduced data were not satisfactory. The unit-cell parameters were similar to those in Table 1, but R merge was around 0.1. We suspected that there might be some kind of phase transition at 173 K, but did not perform any further investigations. By setting the temperature to 248 K, we found that the single crystal was stable, and the diffraction spots/patterns appeared acceptable. Therefore, data collection was completed at 248 K.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The title complex resides on a crystallographic mirror plane. Therefore, only half of the complex is unique. Atoms Mo1, N1, N2, Si2, Si3, C4, and C6 lie on the mirror plane exactly. The Si(CH 3 ) 3 groups are highly disordered in the structure. Therefore, it is probably inappropriate to split the Si(CH 3 ) 3 group into two parts. Instead, the Si(CH 3 ) 3 groups are modeled in an ordered way, as if they are not disordered. In consequence, the Si-C bond lengths differ quite largely, and the ADPs of the methyl carbons are very eccentric. Therefore, several restraints were used including 'SADI 0.01 Si1 C1 Si1 C2 Si1 C3 Si2 C4 Si2 C5 Si3 C6 Si3 C7' (similar Si-C bond length) and 'ISOR 0.01 0.02 C1 C2 C3 C6 C7' (isotropic ADPs approximately). The C-bound H atoms were placed in calculated positions and treated as riding atoms: C-H = 0.97 Å with U iso (H) = 1.5U eq (C).  SHELXL2014 (Sheldrick, 2015); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: CIFTAB (Sheldrick, 2008).