N2 activation on a molybdenum–titanium–sulfur cluster

The FeMo-cofactor of nitrogenase, a metal–sulfur cluster that contains eight transition metals, promotes the conversion of dinitrogen into ammonia when stored in the protein. Although various metal–sulfur clusters have been synthesized over the past decades, their use in the activation of N2 has remained challenging, and even the FeMo-cofactor extracted from nitrogenase is not able to reduce N2. Herein, we report the activation of N2 by a metal–sulfur cluster that contains molybdenum and titanium. An N2 moiety bridging two [Mo3S4Ti] cubes is converted into NH3 and N2H4 upon treatment with Brønsted acids in the presence of a reducing agent.

recorded in THF or CH 2 Cl 2 at room temperature using glassy carbon as the working electrode with 0.1-0.2 M [ n Bu 4 N] [PF 6 ] as the supporting electrolyte. All potentials are referenced relative to (C 5 H 5 ) 2 Fe/[(C 5 H 5 ) 2 Fe] + (Fc/Fc + ). Elemental analyses were recorded on an Elementar vario MICRO cube instrument where crystalline samples were sealed in tin capsules under nitrogen. The ESR spectra were recorded on a JEOL JES-FA200 spectrometer. Labeled 15 N 2 was purchased from Cambridge Isotope Laboratories. Unless otherwise noted, all other compounds were purchased from common commercial sources and used without further purification. The synthesis of clusters 2 and 3 is described in Methods section of the main manuscript.

Large-scale synthesis of [Cp* 3 Mo 3 S 4 ][PF 6 ] [1] + .
Under an N 2 atmosphere, Cp*Mo(S t Bu) 3 (30.0 g, 60.2 mmol) was dissolved in CH 2 Cl 2 (500 mL) and treated with ferrocenium hexafluorophospate [(C 5 H 5 ) 2 Fe][PF 6 ] (6.64 g, 20.1 mmol) at room temperature. The resulting brown to green mixture was stirred overnight, and the volatile materials were removed under reduced pressure. The product [Cp* 3 Mo 3 S 4 ][PF 6 ] was originally isolated in 74% yield by column chromatography (Takei, I. et al. Organometallics 22, 1790(2003), but this procedure is not suitable for a multi-gram scale synthesis. Instead, the product was isolated in a slightly lower yield as a green powder, by filtration of a THF suspension of the crude material to collect the green solid, followed by repeated washing with THF (> 10 times) until the filtrate is not brown anymore (9.88 g, 10.2 mmol, 51% yield). It should be noted that the product is air-stable and sparingly soluble in THF. Therefore the collection and washing processes were carried out in air. Caution: All procedures need to be conducted in a fume hood as all volatile materials as well as the washings smell unpleasant. 1 H NMR (CDCl 3 ): δ 4 2.01 (Cp*).
The precursor Cp*Mo(S t Bu) 3 was synthesized by using the previously reported procedure (Kawaguchi, H. et al. J. Am. Chem. Soc. 119, 10346 (1997)), while on a large scale, i.e. treatment of an ice-cooled THF suspension of Cp*MoCl 4 (10 grams or more) with 4 equiv of LiS t Bu, which was generated in-situ from a THF solution of HS t Bu and a hexane solution of n BuLi, followed by stirring at room temperature for 1 h, evaporation of all volatile materials, extraction of the product with hexane, filtration, and concentration of the extract under reduced pressure, and followed by crystallization at -30 °C.

Formation of a trace amount of [K 3 (THF) 5 ][Cp* 3 Mo 3 S 4 Ti] 2 (µ-N 2 ) [3] -. A THF (2 mL)
suspension of KC 8 (56 mg, 0.414 mmol) was added to a THF (4 mL) suspension of 2 (200 mg, 0.213 mmol) at room temperature, resulting in the formation of dark-brown and black precipitates as well as a light green-brown supernatant. Further addition of a THF (6 mL) suspension of KC 8 (116 mg, 0.821 mmol) caused a color change, furnishing an intense red-brown solution. After being stirring for 5 minutes at room temperature, the mixture was filtered, and pentane (10 mL) was added to the filtrate. Leaving the solution to stand at -30 °C led to the formation of a trace amount of 4, which was obtained in the form of black crystalline blocks. Cluster 4 was characterized by means of X-ray crystallographic analysis. The corresponding 1 H NMR ( Supplementary Fig. 10) was measured using a crystalline solid sample containing a small amount of impurity. 1 H NMR (THF-d 8 ): δ 5.84 (w 1/2 = 60 Hz, Cp*).

X-ray Crystal Structure Determination
The crystallographic data and refinement parameters for 2, 3, and [3]are summarized in Supplementary Table 3. Single crystals were coated with oil (immersion Oil, type B: Code 1248, Cargille laboratories, Inc.) and mounted on loops. Diffraction data were collected at -100 ºC under a cold nitrogen stream on a Rigaku RA-Micro7 equipped with a Saturn70 CCD detector or a PILATUS 200K detector, using graphite-monochromated MoKα radiation (λ = 0.710690 Å). Six preliminary data frames were measured at 0.5° increments of ω in order to assess the crystal quality and preliminary unit cell parameters. The intensity images were also measured at 0.5° intervals of ω.
The frame data were integrated using the CrystalClear program package, and the data sets were corrected for absorption using a REQAB program. The calculations were performed with the CrystalStructure program package. All structures were solved by direct methods, and refined by full-matrix least squares. The atomic coordinates for 2, 3, and [3]have been deposited with the Cambridge Crystallographic Data Centre under reference nos. 1577330-1577332.