Nitrogen reduction by the Fe sites of synthetic [Mo3S4Fe] cubes.

Nitrogen (N2) fixation by nature, which is a crucial process for the supply of bio-available forms of nitrogen, is performed by nitrogenase. This enzyme uses a unique transition-metal-sulfur-carbon cluster as its active-site co-factor ([(R-homocitrate)MoFe7S9C], FeMoco)1,2, and the sulfur-surrounded iron (Fe) atoms have been postulated to capture and reduce N2 (refs. 3-6). Although there are a few examples of synthetic counterparts of the FeMoco, metal-sulfur cluster, which have shown binding of N2 (refs. 7-9), the reduction of N2 by any synthetic metal-sulfur cluster or by the extracted form of FeMoco10 has remained elusive, despite nearly 50 years of research. Here we show that the Fe atoms in our synthetic [Mo3S4Fe] cubes11,12 can capture a N2 molecule and catalyse N2 silylation to form N(SiMe3)3 under treatment with excess sodium and trimethylsilyl chloride. These results exemplify the catalytic silylation of N2 by a synthetic metal-sulfur cluster and demonstrate the N2-reduction capability of Fe atoms in a sulfur-rich environment, which is reminiscent of the ability of FeMoco to bind and activate N2.

Synthesis of Dinuclear Mo-Fe Hydride Complexes and Catalytic Silylation of N2.

Two transition-metal atoms bridged by hydrides may represent a useful structural motif for N2 activation by molecular complexes and the enzyme active site. In this study, dinuclear MoIV -FeII complexes with bridging hydrides, CpR Mo(PMe3 )(H)(µ-H)3 FeCp* (2 a; CpR =Cp*=C5 Me5 , 2 b; CpR =C5 Me4 H), were synthesized via deprotonation of CpR Mo(PMe3 )H5 (1 a; CpR =Cp*, 1 b; CpR =C5 Me4 H) by Cp*FeN(SiMe3 )2 , and they were characterized by spectroscopy and crystallography. These Mo-Fe complexes reveal the shortest Mo-Fe distances ever reported (2.4005(3) Šfor 2 a and 2.3952(3) Šfor 2 b), and the Mo-Fe interactions were analyzed by computational studies. Removal of the terminal Mo-H hydride in 2 a-2 b by [Ph3 C]+ in THF led to the formation of cationic THF adducts [CpR Mo(PMe3 )(THF)(µ-H)3 FeCp*]+ (3 a; CpR =Cp*, 3 b; CpR =C5 Me4 H). Further reaction of 3 a with LiPPh2 gave rise to a phosphido-bridged complex Cp*Mo(PMe3 )(µ-H)(µ-PPh2 )FeCp* (4). A series of Mo-Fe complexes were subjected to catalytic silylation of N2 in the presence of Na and Me3 SiCl, furnishing up to 129±20 equiv of N(SiMe3 )3 per molecule of 2 b. Mechanism of the catalytic cycle was analyzed by DFT calculations.

Synthesis, structures, redox properties, and theoretical calculations of thiohalide capped octahedral hexanuclear technetium(iii) clusters.

The first thiohalide µ3-capped octahedral hexanuclear technetium clusters with 24 valence electrons, [Tc6(µ3-S)8-n(µ3-Br)nBr6]n-4 [n = 1 ([Tc-S7Br]3-) and n = 2 ([Tc-S6Br2]2-)] and [Tc6(µ3-S)7(µ3-Cl)Cl6]3- ([Tc-S7Cl]3-), were synthesized and characterized. The structures of [Tc-S7Br]3-, [Tc-S6Br2]2-, and [Tc-S7Cl]3- were determined by single-crystal X-ray analysis. The Tc-Tc bond distances in [Tc-S7Br]3-, [Tc-S6Br2]2-, and [Tc-S7Cl]3- are 2.5842(6)-2.6029(6) Å (avg. 2.593(2) Å), 2.5835(10)-2.6049(10) Å (avg. 2.596(1) Å), and 2.5829(4)-2.5940(4) Å (avg. 2.587(3) Å), respectively. The capping halide and sulfide ligands in [Tc-S7Br]3-, [Tc-S6Br2]2-, and [Tc-S7Cl]3- were disordered in the crystals. The bond distances of Tc-S/Br as a function of the occupancies of the capping bromides for [Tc-S6Br2]2-, [Tc-S7Br]3-, and [Tc6(µ3-S)8Br6]4- ([Tc-S8]4-) showed a linear correlation. The one-electron reduction waves assignable to the Tc/TcIITc [Tc6(24e/25e)] process were observed for the novel complexes. Density functional theory (DFT) calculations of the hexanuclear technetium complexes showed a smaller energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the hexanuclear technetium complexes compared to those of the rhenium analogues. The electronic transitions of the new technetium complexes shifted to lower energy compared to the isotypic rhenium complexes.

Synthesis of [Mo3S4] Clusters from Half-Sandwich Molybdenum(V) Chlorides and Their Application as Platforms for [Mo3S4Fe] Cubes.

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, [CpR3Mo3S4]+ (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 [CpR3Mo3S4]0/- redox process, indicating the interaction between [CpR3Mo3S4]- 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 luminescent dicyanidonitridotechnetium(v) core with tridentate ligand coordination sites.

A novel, luminescent technetium complex, [TcN(CN)2bpa] (bpa = bis-(2-pyridylmethyl)amine), with tridentate ligand coordination sites was synthesized and characterized. Photoemission with a maximum wavelength at 666 nm was observed in the solid-state at 296 K.

Synthesis, structure and valence-trapping vs. detrapping for new trinuclear iron pentafluoro benzoate complexes: possible recognition of organic molecules by 57Fe Mössbauer spectroscopy.

New mixed-valence trinuclear iron pentafluorobenzoate complexes were synthesized. Their valence-detrapping and/or valence-trapping phenomena were studied by (57)Fe Mössbauer spectroscopy and X-ray crystallography. For [Fe3O(C6F5CO2)6(py)3]·CH2Cl2 (1), a valence-trapped state was observed at low temperatures, while the valence-detrapped state was observed at room temperature. Removal of CH2Cl2 from 1 gives the de-solvated [Fe3O(C6F5CO2)6(py)3] (2) where the valence was trapped at room temperature. The CH2Cl2-free 2 can reversibly absorb and desorb CH3CN; the process was followed by (57)Fe Mössbauer spectroscopy by monitoring valence-trapping and valence-detrapping phenomena. Organic molecules such as benzene, toluene, ethylbenzene, cumene, and xylene are also trapped by 2 and affect the iron valence states. However, small molecules such as H2O and CO2 do not affect the valence-trapped state of 2. Three xylene isomers trapped within the nano-void of 2 were distinguished by (57)Fe Mössbauer spectroscopy at room temperature.

Excited-state characteristics of tetracyanidonitridorhenium(V) and -technetium(V) complexes with N-heteroaromatic ligands.

Six-coordinate tetracyanidonitridorhenium(V) and -technetium(V) with axial N-heteroaromatic ligands, (PPh4)2[MN(CN)4L] [M = Re, L = 4-(dimethylamino)pyridine (dmap), 3,5-lutidine (lut), 4-picoline (pic), 4-phenylpyridine (ppy), pyridine (py), 3-benzoylpyridine (3bzpy), 4,4'-bipyridine (bpy), pyrazine (pz), 4-cyanopyridine (cpy), or 4-benzoylpyridine (4bzpy); M = Tc, L = dmap, lut, pic, py, pz, or cpy] were synthesized and characterized. The crystal structures of 11 complexes were determined by single-crystal X-ray analysis. All of the complexes showed photoluminescence in the crystalline phase at room temperature. The emission maximum wavelengths (λ(em)) of the rhenium complexes with dmap, lut, pic, ppy, or py were similar to one another with a quite high emission quantum yield (Φ(em)): λ(em) = 539-545 nm, Φ(em) = 0.39-0.93, and emission lifetime (τ(em)) = 10-45 µs at 296 K. The emission spectra at 77 K exhibited vibronic progressions, and the emissive excited state is characterized as (3)[(d(xy))(1)(dπ*)(1)] (dπ* = d(xz), d(yz)). On the other hand, the emission maximum wavelength of the rhenium complex with 3bzpy, bpy, pz, cpy, or 4bzpy was significantly dependent on the nature of the axial ligand in the crystalline phase: λ(em) = 564-669 nm, Φ(em) ≤ 0.01-0.36, and τ(em) = 0.03-13.3 µs at 296 K. The emission spectra at 77 K in the crystalline phase did not show vibronic progressions. The emissive excited state of the rhenium complex with bpy, pz, cpy, or 4bzpy is assignable to originate from the metal-to-N-heteroaromatic ligand charge-transfer (MLCT)-type emission with a spin-triplet type. The change in the excited-state characteristics of rhenium complexes by the N-heteroaromatic ligand is a result of stabilization of the π* orbital of the N- heteroaromatic ligand to a lower energy level than the dπ* orbitals. The emission spectral shapes of technetium complexes were almost independent of the nature of the N-heteroaromatic ligand with λ(em) = 574-581 nm at room temperature. The different emission characteristics between the pz and cpy coordinate rhenium complexes and the technetium analogues would be due to stabilization of technetium-centered orbitals compared with the rhenium ones in energy.

Photoluminescence switching with changes in the coordination number and coordinating volatile organic compounds in tetracyanidonitridorhenium(V) and -technetium(V) complexes.

Six-coordinate distorted octahedral tetracyanidonitridorhenium(V) and -technetium(V) complexes with a volatile organic compound (VOC) coordinating at the trans position of a nitrido ligand, (PPh4)2[MN(CN)4L] (M = Re, L = MeOH, EtOH, acetone, or MeCN; M = Tc, L = MeOH), and five-coordinate square-pyramidal tetracyanidonitrido complexes without an axial ligand, (PPh4)2[MN(CN)4] (M = Re or Tc), were synthesized and characterized. Single-crystal X-ray structural analysis was carried out for (PPh4)2[MN(CN)4L] (M = Re, L = MeOH, EtOH, or acetone; M = Tc, L = MeOH) and (PPh4)2[ReN(CN)4]. All complexes studied showed photoluminescence in the solid state at room temperature. Reversible luminescence switching between six- and five-coordinate rhenium(V) complexes and between the relevant six-coordinate rhenium(V) complexes except that between the MeCN and acetone complexes was achieved by exposing them to VOC vapor in the solid state at room temperature. Luminescence changes were observed from the five-coordinate technetium(V) complexes in a MeOH vapor atmosphere in the solid state. In contrast, no vapochromic luminescence was observed from the five- and six-coordinate complexes in an acetone vapor atmosphere.

Synthesis, spectroscopic and electrochemical properties, and electronic structures of octahedral hexatechnetium(III) clusters [Tc6Q8(CN)6]4- (Q = S, Se).

Chalcogenide-capped molecular octahedral hexatechnetium(III) clusters [Tc(6)Q(8)(CN)(6)](4-) {Q = S ([1](4-)), Se ([2](4-))} were prepared by the substitution of axial ligands with cyanide. The structures of the new complexes were determined by single-crystal X-ray analysis. The IR spectra of [1](4-) and [2](4-) showed a C[triple bond]N stretching band at 2114 and 2105 cm(-1), respectively. In cyclic voltammetry, [1](4-) and [2](4-) in CH(3)CN showed reversible one-electron-oxidation waves assignable to the Tc(6)(24e/23e) process at +0.99 and +0.74 V, respectively. Density functional theory (DFT) calculations on the hexatechnetium complexes showed that the highest occupied molecular orbital (HOMO) was primarily localized on a Tc(6)Q(8) core and the lowest unoccupied molecular orbital (LUMO) was completely localized on the metal orbitals. The energy level of HOMO and the redox potential of the M(6)(24e/23e) process (M = Tc, Re) were found to have a good linear relationship. Time-dependent DFT calculations showed that the substantially allowed transitions with the lowest energy were Tc(6)Q(8) core-centered transitions. The electronic structures and electronic transition features of the hexatechnetium complexes were similar to those of the hexarhenium analogues [Re(6)Q(8)(CN)(6)](4-) (Q = S, Se); however, the energy gap between the HOMO and LUMO was smaller in the hexatechnetium complexes.