N2 fixation by Nature, which is a crucial process to supply bio-available forms of nitrogen, is performed by nitrogenase. This enzyme employs a unique transition metal-sulfur-carbon cluster as its active-site cofactor ([(R-homocitrate)MoFe7S9C], FeMoco), 1,2 and the sulfur-surrounded Fe atoms have been postulated to capture and reduce N2. [3][4][5][6] Whereas synthetic counterparts of FeMoco, metal-sulfur clusters, have displayed binding of N2 in a few examples, 7,8 the reduction of N2 by any synthetic metal-sulfur clusters or even by the extracted form of FeMoco 9 have remained elusive despite a near-50-year history of research. Here we show that the Fe atoms in our synthetic [Mo3S4Fe] cubes 10,11 capture an N2 molecule and catalyze N2 silylation to form N(SiMe3)3 under treatment with excess Na and Me3SiCl. These results exemplify the first catalytic N2 reduction by a synthetic metal-sulfur cluster with an Fe center supported only by S ligands. This work demonstrates the N2-reducing capability of Fe atoms in a S-rich environment, which Nature has selected to accomplish a similar purpose. This work also suggests some critical features for successful N2 reduction by metal-sulfur compounds, which serve as clues to understand the origin of N2 fixation on Earth.

Triangular [Mo3S4] clusters are known to serve as platforms to accommodate a metal atom M, furnishing cubic [Mo3S4M] clusters. In this study, three [Mo3S4] clusters supported by η5-cyclopentadienyl (CpR) ligands, [CpR 3Mo3S4]+ (CpR = C5Me4SiMe3, C5Me4SiEt3, and C5Me4H), were synthesized via half-sandwich molybdenum chlorides CpRMoCl4. In the cyclic voltammogram of the [Mo3S4] cluster having C5Me4H ligands, a weak feature appeared in addition to the [CpR 3Mo3S4]0/– redox process, indicating the interaction between [CpR 3Mo3S4]− and the [NnBu4] cation of the electrolyte, while such a feature was less significant for the C5Me4SiR3 variants. The [Mo3S4] clusters with bulky C5Me4SiR3 ligands were successfully applied as platforms to accommodate an Fe atom to furnish cubic [Mo3S4Fe] clusters. On the other hand, the corresponding reactions of the less bulky C5Me4H analogue gave complex mixtures.

A synthetic protocol was developed for a series of cubane-type [Mo S M] clusters that incorporate halides of first-row transition metals (M) from Groups 4-10. This protocol is based on the anionic cluster platform [Cp* Mo S ] ([1] ; Cp*=η -C Me ), which crystallizes when K(18-crown-6) is used as the counter cation. Treatment of in situ-generated [1] with such transition-metal halides led to the formation of [Mo S M] clusters, in which the M/halide ratio gradually changes from 1:2 to 1:1.5 and to 1:1, when moving from early to late transition metals. This trend suggests a tendency for early transition metals to tolerate higher oxidation states and adopt larger ionic radii relative to late transition metals. The properties of the [Mo S Fe] cluster 6 a were investigated in detail by using Fe Mössbauer spectroscopy and computational methods.

N2 fixation by Nature, which is a crucial process to supply bio-available forms of nitrogen, is performed by nitrogenase. This enzyme employs a unique transition metal-sulfur-carbon cluster as its active-site cofactor ([(R-homocitrate)MoFe7S9C], FeMoco), and the sulfur-surrounded Fe atoms have been postulated to capture and reduce N2. Whereas synthetic counterparts of FeMoco, metal-sulfur clusters, have displayed binding of N2 in a few examples, the reduction of N2 by any synthetic metal-sulfur clusters or even by the extracted form of FeMoco have remained elusive despite a near-50-year history of research. Here we show that the Fe atoms in our synthetic [Mo3S4Fe] cubes capture an N2 molecule and catalyze N2 silylation to form N(SiMe3)3 under treatment with excess Na and Me3SiCl. These results exemplify the first catalytic N2 reduction by a synthetic metal-sulfur cluster with an Fe center supported only by S ligands. This work demonstrates the N2-reducing capability of Fe atoms in a S-rich environment, which Nature has selected to accomplish a similar purpose. This work also suggests some critical features for successful by metal-sulfur compounds, which serve as clues to understand the origin of N2 fixation on Earth.