NH3 formation from N2 and H2 mediated by molecular tri-iron complexes.

Living systems carry out the reduction of N2 to ammonia (NH3) through a series of protonation and electron transfer steps under ambient conditions using the enzyme nitrogenase. In the chemical industry, the Haber-Bosch process hydrogenates N2 but requires high temperatures and pressures. Both processes rely on iron-based catalysts, but molecular iron complexes that promote the formation of NH3 on addition of H2 to N2 have remained difficult to devise. Here, we isolate the tri(iron)bis(nitrido) complex [(Cp'Fe)3(µ3-N)2] (in which Cp' = η5-1,2,4-(Me3C)3C5H2), which is prepared by reduction of [Cp'Fe(µ-I)]2 under an N2 atmosphere and comprises three iron centres bridged by two µ3-nitrido ligands. In solution, this complex reacts with H2 at ambient temperature (22 °C) and low pressure (1 or 4 bar) to form NH3. In the solid state, it is converted into the tri(iron)bis(imido) species, [(Cp'Fe)3(µ3-NH)2], by addition of H2 (10 bar) through an unusual solid-gas, single-crystal-to-single-crystal transformation. In solution, [(Cp'Fe)3(µ3-NH)2] further reacts with H2 or H+ to form NH3.

Synthesis and molecular structure of pentadienyl complexes of the rare-earth metals.

The coordination chemistry of the sterically encumbered pentadienyl ligand pdl' (pdl' = 2,4-(Me3C)2C5H5) towards a series of rare-earth metals was systematically explored and the resulting metal complexes were fully characterized by several techniques including X-ray diffraction, elemental analysis, NMR spectroscopy and solid-state magnetic susceptibility studies. Three different reaction products were isolated depending on the redox-potentials and ionic radii of the metal atoms. They can be classified as (a) salt-metathesis, (b) metal reduction-ligand oxidation and (c) ligand deprotonation products. While for the larger and difficult to reduce metal ions, M = La, Ce, Pr and Nd, trivalent compounds [(η5-pdl')3M] (1-M) were isolated, for the more readily reduced metal ions the corresponding divalent compounds [[(η5-pdl')2M(thf)n] (2-M; with M = Sm (n = 2); Eu, Yb (n = 1)) were formed. A more complex structural motif was observed for the smaller and also difficult to reduce metal ions, M = Sc, Y, Gd, Tb, Dy, Ho, Er, Tm and Lu, which yielded the bimetallic complexes of the type [(pdl')(pdl'-1H)(pdl'-2H)M2(thf)2] (3-M). In these dimeric complexes the pdl' ligand acts as a result of deprotonation reactions not only as a monoanionic [pdl']-, but also as a dianionic [pdl'-1H]2- and a trianionic [pdl'-2H]3- ligand scaffold, which form unusual structural motifs including a six-membered metallacycle. Solid-state magnetic susceptibility studies revealed the expected free-ion magnetic moments at T = 300 K for all investigated compounds, whereas at lower temperatures crystal-field effects dominate. Furthermore, for 3-Gd, 3-Tb, 3-Dy and 3-Er weak ferromagnetic exchange interactions were observed at low temperature.

Monomeric Fe(iii) half-sandwich complexes [Cp'FeX2] - synthesis, properties and electronic structure.

The half-sandwich complex [Cp'Fe(µ-I)]2 (1; Cp' = η5-1,2,4-(Me3C)3C5H2) is cleaved when heated in toluene to form a cation-anion pair [{Cp'Fe(η6-toluene)}+{Cp'FeI2}-] (2), in which the two Fe(ii) atoms adopt different spin states, i.e., a low-spin (S = 0) and a high-spin (S = 2) configuration. Upon oxidation of 1 with C2H4I2, the thermally stable 15VE species [Cp'FeI2] (3) can be isolated, in which the Fe(iii) atom adopts an intermediate spin (S = 3/2) configuration. Complex 3 is an excellent starting material for further functionalizations and it reacts with Mg(CH2SiMe3)2 to form the unprecedented Fe(iii) (S = 3/2) bis(alkyl) complex [Cp'Fe(CH2SiMe3)2] (4). The respective spin states of complexes 2-4 are confirmed by single-crystal X-ray crystallography, zero-field 57Fe Mössbauer spectroscopy, and solid-state magnetic susceptibility measurements. In contrast to the related 14VE high-spin (S = 2) Fe(ii) alkyl species [Cp'FeCH(SiMe3)2], which resists the reaction with H2 as a consequence of a spin-induced reaction barrier, complex 4 reacts cleanly with H2 (8 bar) in cyclohexane to yield iron hydrides [{Cp'Fe}2(µ-H)3] (5) and [Cp'Fe(µ-H)2]2 (6) in a 1 : 4 ratio. However, when the hydrogenation of 4 is carried out in benzene, a green 19VE [Cp'Fe(η6-C6H6)] (A) intermediate is formed, which dimerizes to the bis(cyclohexadienyl)-bridged product [(Cp'Fe)2(µ2-η5:η5-C12H12)] (7). Further evidence for the intermediacy of [Cp'Fe(η6-C6H6)] (A) was gathered by X-band EPR and UV/vis spectroscopy. Interestingly, attempts to oxidize 7 with AgSbF6 proceeded via C-C bond cleavage instead of metal oxidation to form [Cp'Fe(C6H6)][SbF6] (8).

Reversible dinitrogen binding to [Cp'Fe(NHC)] associated with an N2-induced spin state change.

The 15-valence electron (VE), high-spin (S = 3/2) half-sandwich complex [Cp'Fe(IiPr2Me2)] (3; IiPr2Me2 = 1,3-di-iso-propyl-4,5-dimethylimidazol-2-yildene) reversibly coordinates N2 to form the 17VE, low-spin (S = 1/2) compound [Cp'Fe(IiPr2Me2)(η1-N2)] (4).

Reactivity studies on [Cp'Fe(µ-I)]2: nitrido-, sulfido- and diselenide iron complexes derived from pseudohalide activation.

The iron half-sandwich [Cp'Fe(µ-I)]2 (Cp' = 1,2,4-(Me3C)3C5H2, 1) reacts with the pseudohalides NCO-, SCN-, SeCN- and N3- to give [Cp'Fe(µ-NCO)]2 (2), [Cp'Fe(µ-S)]2 (3), [Cp'Fe(µ-Se2)]2 (4) and [Cp'Fe(µ-N)]2 (5), respectively. Various spectroscopic techniques including X-ray diffraction, solid-state magnetic susceptibility studies and 57Fe Mössbauer spectroscopy were employed in the characterization of these species. Mössbauer spectroscopy shows a decreasing isomer shift with increasing formal oxidation state, ranging from Fe(ii) to Fe(iv), in complexes 1 to 5. The sulfido-bridged dimer 3 exhibits strong antiferromagnetic coupling between the Fe(iii) centers. This leads to temperature-independent paramagnetism (TIP) at low temperature, from which the energy gap between the ground and the excited state can be estimated to be 2J = ca. 700 cm-1. The iron(iv) nitrido complex [Cp'Fe(µ-N)]2 (5) shows no reactivity towards H2 (10 atm), but undergoes clean reactions with CO (5 bar) and XylNC (Xyl = 2,6-Me2C6H3) to form the diamagnetic isocyanate and carbodiimide complexes [Cp'Fe(CO)2(NCO)] (7) and [Cp'Fe(CNXyl)2(NCNXyl)] (8), respectively. All compounds were fully characterized, and density functional theory (DFT) computations provide useful insights into their formation and the electronic structures of complexes 3 and 5.

Pentadienyl chemistry of the heavy alkaline-earth metals revisited.

Open-metallocenes of the heavy alkaline-earth metals [(η(5)-Pdl')2M(thf)n] (M = Ca (1), Sr (2), n = 1; M = Ba (3), n = 2; Pdl' = 2,4-tBu2C5H5) are readily prepared by salt-metathesis between MI2 and KPdl' and characterized by NMR spectroscopy and X-ray diffraction studies.