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.

Solid-State End-On to Side-On Isomerization of (N═N)2- in {[(R2N)3Nd]2N2}2- (R = SiMe3) Connects In Situ LnIII(NR2)3/K and Isolated [LnII(NR2)3]1- Dinitrogen Reduction.

Examination of the reduction chemistry of Nd(NR2)3 (R = SiMe3) under N2 has provided connections between the in situ Ln(III)-based LnIII(NR2)3/K reductions of N2 that form side-on bound neutral (N=N)2- complexes, [(R2N)2(THF)Ln]2[µ-η2:η2-N2], and the Ln(II)-based [LnII(NR2)3]1- reductions by Sc, Gd, and Tb that form end-on bound (N=N)2- complexes, {[(R2N)3Ln]2[µ-η1:η1-N2]}2-, which are dianions. The reduction of Nd(NR2)3 by KC8 under dinitrogen in Et2O in the presence of 18-crown-6 (18-c-6) forms dark yellow solutions of [K2(18-c-6)3]{[(R2N)3Nd]2N2} at low temperatures that become green as they warm up to -35 °C in a glovebox freezer. Green crystals obtained from the solution turn yellow-brown when cooled below -100 °C, and the yellow-brown compound has an end-on Nd2(µ-η1:η1-N2) structure. The yellow-brown crystals isomerize in the solid state on the diffractometer upon warming, and at -25 °C, the crystals are green and have a side-on Nd2(µ-η2:η2-N2) structure. Collection of X-ray diffraction data at 10 °C intervals from -50 to -90 °C revealed that the isomerization occurs at temperatures below -100 °C. In the presence of tetrahydrofuran (THF), the dianionic {[(R2N)3Nd]2N2}2- system can lose an amide ligand to provide the monoanionic [(R2N)3NdIII(µ-η2:η2-N2)NdIII(NR2)2(THF)]1-, characterized by X-ray crystallography. These data suggest a connection between the in situ Ln(III)/K reductions and Ln(II) reductions that depends on solvent, temperature, the presence of a chelate, and the specific rare-earth metal.

The Lanthanide Contraction Is a Variable.

Upon examination of the bond distances of the recently reported series of [Ln(SST)3(THF)2] [Ln = lanthanides, SST = tris(trimethylsilyl)siloxide (OSi(SiMe3)3), and THF = tetrahydrofuran] compounds, it was found that over the Ln-series (La through Lu), the Ln-O(THF) bond changed by 0.257 Å, whereas the Ln-O(SST) bond varied by 0.164 Å. Examination of all similarly ligated Ln-O(THF) (Ln = La vs Lu) structures available in the Cambridge Structural Database (CSD) revealed that this previously unreported, increased Ln-contraction is pervasive. Further evaluations showed that this enhanced Ln-contraction also occurs for pyridine (py) in the [Ln(SST)3(py)2] family as well as the average Ln-N(py) (La vs Lu) structure distances recovered from the CSD. Additional ligands, such as halides (Cl and I) were found to display this enhanced Ln-contraction, while other species (i.e., cyclopentadienide, alkoxide, SST, and dimethyl sulfoxide) yielded a "normal" Ln-contraction (La-L vs Lu-L). Gas-phase electronic structure density functional theory calculations were carried out to evaluate the molecular orbital influence on the Ln-contraction between Ln-O(SST) and Ln-O(THF). The calculated [Ln(SST)3(THF)2] structures were found to demonstrate the same capricious Ln-contraction. Based on these studies, one can say that the variability of the Ln-contraction noted in the [Ln(SST)3(THF)2] experimental data is due to the different bonding types, ion-ion for the Ln-SST bond versus ion-dipole for the Ln-THF bond.

Synthesis and Characterization of Solvated Lanthanide Tris(trimethylsilyl)siloxides.

In an effort to develop precursors for the production of lanthanide silicate (LnSiOx) materials, the reactions of [Ln(NR2)3] (R = SiMe3) with three equivalents of tris(trimethylsilyl)silanol (H-OSi(SiMe3)3) or H-SST) in tetrahydrofuran (THF) were undertaken. The products were crystallographically characterized as [Ln(SST)3(THF)2] (where Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu). In general, these compounds are similar to the previously reported [Gd(SST)3(THF)2] complex, where each metal center of the monomeric species is found to adopt a trigonal bipyramidal (TBP; τ = av 0.95) geometry; however, the crystallographic structure solutions for these crystals invoke a much larger unit cell that reveals the complex disorder of the axial THF ligands. Using incompletely washed H-SST, the tetrahedrally (T-4) bound [Ln(SST)3(NEt3)] (Ln-NEt3 = Pr-NEt3, Ho-NEt3; NEt3 = triethylamine) compounds were isolated from the same reaction run in toluene. Rational syntheses of amine derivatives were realized by performing the same reaction with pure H-SST in toluene containing the appropriate amine and [Ln(NR2)3] with the final products identified as [Tm(SST)3(NEt3)] (Tm-NEt3) or [Tm(SST)3(NHPr2i)] (NHPr2i = di-iso-propylamine; Tm-NHPr2i). The products isolated from reactions undertaken in pyridine (py) were identified as [Ln(SST)3(py)2] (Ln-py = Ce-py, Eu-py, and Tm-py). The Ln-py structures exhibit the larger unit cell noted for the THF derivatives with each Ln adopting a TBP (τ = av 0.98) metal center possessing equatorial SST and axial py ligands. The same reaction run in toluene led to the isolation of [(η6-tol)Tm(SST)3] (Tm-tol). Multinuclear NMR (1H and 29Si) data support the retention of the solid-state structures of all of these compounds in solution. Photoluminescent measurements of Tb, Sm, Dy, and Eu were found to display emission and lifetime profiles in the visible range due to f-f transitions, consistent with trivalent lanthanide metal centers.

Pyridinium Salts of Dehydrated Lanthanide Polychlorides.

The reaction of lanthanide (Ln) chloride hydrates ([Ln(H2O)n(Cl)3]) with pyridine (py) yielded a set of dehydrated pyridinium (py-H) Ln-polychloride salts. These species were crystallographically characterized as [[py-H-py][py-H]2[LnCl6]] (Ln-6; Ln = La, Ce, Pr, Nd, Sm, Eu, Gd) or [[py-H]2[LnCl5(py)]] ((Ln-5; Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu). The Ln-6 metal centers adopt an octahedral (OC-6) geometry, binding six Cl ligands. The -3 charge is off-set by two py-H moieties and a di-pyridinium (py-H-py) ion. For the Ln-5 species, an OC-6 anion is formed by the Ln cation binding a single py and five Cl ligands. The remaining -2 charge is offset by two py-H+ cations that H-bond to the anion. Significant H-bonding occurs between the various cation/anion moieties inducing the molecular stability. The change in structure from the Ln-6 to Ln-5 is believed to be due to the Ln-contraction producing a smaller unit cell, which prevents formation of the py-H-py+ cation, leading to the loss of the H-bonding-induced stability. Based on this, it was determined that the Ln-5 structures only exist when the lattice energy is small. While dehydrated polychloride salts can be produced by simply mixing in pyridine, the final structures adopted result from a delicate balance of cation size, Coulombic charge, and stabilizing H-bonding.

Synthesis of a Nitrogenase PN -Cluster Model with [Fe8 S7 (µ-Sthiolate )2 ] Core from the All-Ferric [Fe4 S4 (Sthiolate )4 ] Cubane Synthon.

Constructing synthetic models of the nitrogenase PN -cluster has been a long-standing synthetic challenge. Here, we report an optimal nitrogenase PN -cluster model [{(TbtS)(OEt2 )Fe4 S3 }2 (µ-STbt)2 (µ6 -S)] (2) [Tbt=2,4,6-tris{bis(trimethylsilyl)methyl}phenyl] that is the closest synthetic mimic constructed to date. Of note is that two thiolate ligands and one hexacoordinated sulfide are connecting the two Fe4 S3 incomplete cubanes similar to the native PN -cluster, which has never been achieved. Cluster 2 has been characterized by X-ray crystallography and relevant physico-chemical methods. The variable temperature magnetic moments of 2 indicate a singlet ground state (S=0). The Mössbauer spectrum of 2 exhibits two doublets with an intensity ratio of 3:1, which suggests the presence of two types of iron sites. The synthetic pathway of the cluster 2 could indicate the native PN -cluster maturation process as it has been achieved from the Fe4 S4 cubane Fe4 S4 (STbt)4 (1).

Organophosphorus-Modified Lanthanide Nitrates as Potential Actinide Oxide Aerosol Surrogates.

In search of suitable simulants for aerosol uranium waste products from Plutonium Uranium Redox Extraction (PUREX) process burns, a series of lanthanide nitrate hydrates ([Ln(κ2-NO3)3·nH2O]) were dissolved in the presence of tributylphosphate (O═P(O(CH2)3CH3)3) referred to as TBP) in kerosene or triphenylphosphate (O═P(O(C6H5) referred to as TPhP) in acetone. The crystal structure of the TPhP derivatives of the lanthanide nitrate series and uranium nitrate were solved as [Ln(κ2-NO3)3(TPhP)3] (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and [U(O)2(κ2-NO3)2(TPhP)2] (U), respectively. The lanthanide-TBP, Ln, and U were further characterized using FTIR spectroscopy, 31P NMR spectroscopy, thermogravimetric analysis, and X-ray fluorescence spectroscopy. Further, thermal treatment of the lanthanide-TBP, Ln, and U using a box furnace to mimic pyrolysis conditions was found by PXRD analyses to generate a phosphate phase [LnP3O9 or UP2O7) for all systems. The resultant nuclear waste fire contaminant particulates will impact both aerosol transport and toxicity assessments.

[(MeCN)Ni(CF3)3]- and [Ni(CF3)4]2-: Foundations toward the Development of Trifluoromethylations at Unsupported Nickel.

Nickel anions [(MeCN)Ni(CF3)3]- and [Ni(CF3)4]2- were prepared by the formal addition of 3 and 4 equiv, respectively, of AgCF3 to [(dme)NiBr2] in the presence of the [PPh4]+ counterion. Detailed insights into the electronic properties of these new compounds were obtained through the use of density functional theory (DFT) calculations, spectroscopy-oriented configuration interaction (SORCI) calculations, X-ray absorption spectroscopy, and cyclic voltammetry. The data collectively show that trifluoromethyl complexes of nickel, even in the most common oxidation state of nickel(II), are highly covalent systems whereby a hole is distributed on the trifluoromethyl ligands, surprisingly rendering the metal to a physically more reduced state. In the cases of [(MeCN)Ni(CF3)3]- and [Ni(CF3)4]2-, these complexes are better physically described as d9 metal complexes. [(MeCN)Ni(CF3)3]- is electrophilic and reacts with other nucleophiles such as phenoxide to yield the unsupported [(PhO)Ni(CF3)3]2- salt, revealing the broader potential of [(MeCN)Ni(CF3)3]- in the development of "ligandless" trifluoromethylations at nickel. Proof-in-principle experiments show that the reaction of [(MeCN)Ni(CF3)3]- with an aryl iodonium salt yields trifluoromethylated arene, presumably via a high-valent, unsupported, and formal organonickel(IV) intermediate. Evidence of the feasibility of such intermediates is provided with the structurally characterized [PPh4]2[Ni(CF3)4(SO4)], which was derived through the two-electron oxidation of [Ni(CF3)4]2-.

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.

Synthesis and Characterization of Tris(trimethylsilyl)siloxide Derivatives of Early Transition Metal Alkoxides That Thermally Convert to Varied Ceramic-Silica Architecture Materials.

In an effort to generate single-source precursors for the production of metal-siloxide (MSiO x) materials, the tris(trimethylsilyl)silanol (H-SST or H-OSi(SiMe3)3 (1) ligand was reacted with a series of group 4 and 5 metal alkoxides. The group 4 products were crystallographically characterized as [Ti(SST)2(OR)2] (OR = OPr i (2), OBu t (3), ONep (4)); [Ti(SST)3(OBu n)] (5); [Zr(SST)2(OBu t)2(py)] (6); [Zr(SST)3(OR)] (OR = OBu t (7), ONep, (8)); [Hf(SST)2(OBu t)2] (9); and [Hf(SST)2(ONep)2(py) n] ( n = 1 (10), n = 2 (10a)) where OPr i = OCH(CH3)2, OBu t = OC(CH3)3, OBu n = O(CH2)3CH3, ONep = OCH2C(CH3)3, py = pyridine. The crystal structures revealed varied SST substitutions for: monomeric Ti species that adopted a tetrahedral ( T-4) geometry; monomeric Zr compounds with coordination that varied from T-4 to trigonal bipyramidal ( TBPY-5); and monomeric Hf complexes isolated in a TBPY-5 geometry. For the group 5 species, the following derivatives were structurally identified as [V(SST)3(py)2] (11), [Nb(SST)3(OEt)2] (12), [Nb(O)(SST)3(py)] (13), 2[H][(Nb(µ-O)2(SST))6(µ6-O)] (14), [Nb8O10(OEt)18(SST)2·1/5Na2O] (15), [Ta(SST)(µ-OEt)(OEt)3]2 (16), and [Ta(SST)3(OEt)2] (17) where OEt = OCH2CH3. The group 5 monomeric complexes were solved in a TBPY-5 arrangement, whereas the Ta of the dinculear 16 was solved in an octahedral coordination environment. Thermal analyses of these precursors revealed a stepwise loss of ligand, which indicated their potential utility for generating the MSiO x materials. The complexes were thermally processed (350-1100 °C, 4 h, ambient atmosphere), but instead of the desired MSiO x, transmission electron microscopy analyses revealed that fractions of the group 4 and group 5 precursors had formed unusual metal oxide silica architectures.

Synthesis, Characterization, and Nanomaterials Generated from 6,6'-(((2-Hydroxyethyl)azanediyl)bis(methylene))bis(2,4-di- tert-butylphenol) Modified Group 4 Metal Alkoxides.

The impact on the morphology nanoceramic materials generated from group 4 metal alkoxides ([M(OR)4]) and the same precursors modified by 6,6'-(((2-hydroxyethyl)azanediyl)bis(methylene))bis(2,4-di- tert-butylphenol) (referred to as H3-AM-DBP2 (1)) was explored. The products isolated from the 1:1 stoichiometric reaction of a series of [M(OR)4] where M = Ti, Zr, or Hf; OR = OCH(CH3)2(OPr i); OC(CH3)3(OBu t); OCH2C(CH3)3(ONep) with H3-AM-DBP2 proved, by single crystal X-ray diffraction, to be [(ONep)Ti( k4( O,O',O'',N)-AM-DBP2)] (2), [(OR)M(µ( O)- k3( O',O'',N)-AM-DBP2)]2 [M = Zr: OR = OPr i, 3·tol; OBu t, 4·tol; ONep, 5·tol; M = Hf: OR = OBu t, 6·tol; ONep, 7·tol]. The product from each system led to a tetradentate AM-DBP2 ligand and retention of a parent alkoxide ligand. For the monomeric Ti derivative (2), the metal was solved in a trigonal bipyramidal geometry, whereas for the Zr (3-5) and Hf (6, 7) derivatives a symmetric dinuclear complex was formed where the ethoxide moiety of the AM-DBP2 ligand bridges to the other metal center, generating an octahedral geometry. High quality density functional theory level gas-phase electronic structure calculations on compounds 2-7 using Gaussian 09 were used for meaningful time dependent density functional theory calculations in the interpretation of the UV-vis absorbance spectral data on 2-7. Nanoparticles generated from the solvothermal treatment of the ONep/AM-DBP2 modified compounds (2, 5, 7) in comparison to their parent [M(ONep)4] were larger and had improved regularity and dispersion of the final ceramic nanomaterials.

Synthesis and lanthanide coordination chemistry of phosphine oxide decorated dibenzothiophene and dibenzothiophene sulfone platforms.

Syntheses for new ligands based upon dibenzothiophene and dibenzothiophene sulfone platforms, decorated with phosphine oxide and methylphosphine oxide donor groups, are described. Coordination chemistry of 4,6-bis(diphenylphosphinoylmethyl)dibenzothiophene (8), 4,6-bis(diphenylphosphinoylmethyl)dibenzothiophene-5,5-dioxide (9) and 4,6-bis(diphenylphosphinoyl)dibenzothiophene-5,5-dioxide (10) with lanthanide nitrates, Ln(NO3)3·(H2O)n is outlined, and crystal structure determinations reveal a range of chelation interactions on Ln(III) ions. The nitric acid dependence of the solvent extraction performance of 9 and 10 in 1,2-dichloroethane for Eu(III) and Am(III) is described and compared against the extraction behavior of related dibenzofuran ligands (2, 3; R = Ph) and n-octyl(phenyl)-N,N-diisobutylcarbamoylmethyl phosphine oxide (4) measured under identical conditions.

A nitrogenase cluster model [Fe8S6O] with an oxygen unsymmetrically bridging two proto-Fe4S3 cubes: relevancy to the substrate binding mode of the FeMo cofactor.

An oxygen-encapsulated iron sulfido cluster, [(DmpS)Fe(4)S(3)O][(DmpS)Fe(4)S(3)](µ-SDmp)(2)(µ-OCPh(3)) (2; Dmp = 2,6-(mesityl)(2)C(6)H(3)), has been synthesized by the reaction of the preformed dinuclear iron thiolate/alkoxide [(Ph(3)CO)Fe](2)(µ-SDmp)(2) (1) with (1/8)S(8) and (1/4)H(2)O in toluene. In the [Fe(8)S(6)O] core, the oxygen atom bridges unsymmetrically two incomplete Fe(4)S(3) cubes, and two coordinatively unsaturated iron atoms are weakly bound to mesityl rings. Relevance of the cluster structure of 2 to the nitrogenase FeMo cofactor and its substrate binding mode is discussed.

Synthesis, structure, and electrochemical properties of [LNi(Rf)(C4F8)]- and [LNi(Rf)3]- complexes.

The new anionic nickelate complexes [(MeCN)Ni(C4F8)(CF3)]-, [(MeCN)Ni(C4F8)(C2F5)]-, [(IMes)Ni(C4F8)(CF3)]-, [(IMes)Ni(CF3)3]- (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), and [(F-NHC)Ni(Rf)3]- (F-NHC = 1,3-bis(2,4-F2Ph), 2,4,6-F3Ph- or 3,4,5-F3Ph)imidazol-2-ylidene; (Rf = CF3 or C2F5) were synthesized and structurally characterized. The electrochemical properties of all new compounds were revealed by cyclic voltammetry studies and compared to the known CF3 analogue [(MeCN)Ni(CF3)3]-. The IMes-coordinated complexes exhibited initial oxidation events that were well-separated from a second oxidation process in the cyclic voltammograms. The complexes containing F-substituted NHC ligands [(F-NHC)Ni(CF3)3]- are structurally quite similar to the IMes derivative and reveal also two separated oxidation waves in their cyclic voltammograms. The absolute potentials as well as the separation between the two waves vary with the substitution pattern, suggesting that the NHC ligand environment (NHC = N-heterocyclic carbene) is an interesting platform for the development of new redox-triggered reactions that release trifluoromethyl and perfluoroalkyl radicals upon oxidation.

A dinuclear Mo2H8 complex supported by bulky C5H2tBu3 ligands.

Hydride-bridged transition metal complexes have been found to serve as suitable precursors for the activation of small molecules without the use of reducing agents. In this study, we synthesized a dinuclear Mo2H8 complex supported by bulky C5H2tBu3 (Cp‡) ligands, Cp‡2Mo2H8 (1), from the reaction of Cp‡MoCl4 with KC8 under H2. The hydrides of complex 1 can be replaced with benzene at 60 °C to afford a µ-benzene complex Cp‡2Mo2H2(µ-C6H6) (2).

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.

Five- and Six-Coordinate 2-Methyl-2-propanethiolato Complexes of Zirconium(IV): Synthesis and Structures of [Li(DME)(3)][Zr(SCMe(3))(5)] and [(THF)Li](2)Zr(SCMe(3))(6).

Five-coordinate and six-coordinate 2-methyl-2-propanethiolato complexes of zirconium, [Li(DME)(3)][Zr(SCMe(3))(5)] (1) and [(THF)Li](2)Zr(SCMe(3))(6) (2), were obtained from the ZrCl(4)/LiSCMe(3) reaction system. The control of the Zr coordination number, by the ether ligands, THF or DME, bound to Li, is demonstrated by the conversion of 2 into 1 upon dissolution in DME. 1 and 2 were crystallographically characterized. The structures are extensively disordered. Crystal data follow: 1, hexagonal P6(3)/m, a = b = 12.496(3) Å, c = 17.561(9) Å, Z = 2, V = 2375(1) Å(3), R = 5.0%, R(w) = 6.8%; 2, trigonal R32, a = b = 11.813(3) Å, c = 28.37(1) Å, Z = 3, V = 3428(1) Å(3), R = 5.2%, R(w) = 6.4%.