Low-Coordinate Iron(II) Amido Half-Sandwich Complexes with Large Internal Magnetic Hyperfine Fields.

The half-sandwich complex [Cp'Fe{N(dipp)(SiMe3)}] (Fe-dipp; Cp' = 1,2,4-tri-tert-butylcyclopentadienyl and dipp = 2,6-diisopropylphenyl) and the mixed metallocene [Cp'Fe{(η5-C6H3iPr2)═N(SiMe3)}] (Fe-chd) formed in the reaction between [{Cp'Fe(µ-I)}2] and [Li{N(dipp)(SiMe3)}]2 were characterized by NMR spectroscopy and X-ray diffraction analysis. Fe-dipp complements the series of low-coordinate, quasi-linear iron amido half-sandwich complexes [Cp'Fe{N(tBu)(SiMe3)}] (Fe-tBu) and [Cp'Fe{N(SiMe3)2}] (Fe-tms) reported earlier, and all three compounds were characterized in the solid state by zero-field 57Fe Mössbauer spectroscopy and magnetic susceptibility measurements, confirming their S = 2 electronic ground state. Moreover, the Mössbauer absorption spectra reveal slow paramagnetic relaxation at low temperatures with large internal magnetic hyperfine fields of Bhf = 96.4 T (Fe-dipp, 20 K), Bhf = 101.3 T (Fe-tBu, 15 K), and Bhf = 96.9 T (Fe-tms, 20 K). The magnetic measurements further confirm that the presence of significant axial zero-field splitting and slow relaxation of magnetization is detected, which is revealed even in the absence of a static magnetic field in the case of Fe-tBu. Supplementary ab initio and density functional theory calculations were performed and support the experimental data.

Synthesis and characterisation of an enantiomerically pure scandium pentadienyl complex and its application in the polymerisation of rac-lactide.

The alkyl-functionalised scandium complex [(pdl*SiMe2NtBu)Sc(thf)(CH2SiMe3)] (2) was synthesised in enantiomerically pure form and characterised by NMR spectroscopy and X-ray diffraction analysis. Complex 2 features a chiral constrained geometry ligand derived from the natural compound (1R)-(-)-myrtenal, in which the pentadienyl (pdl*) fragment coordinates in η3:η2-allyl-en fashion to the scandium atom. Compound 2 catalyses the polymerisation of rac-lactide at 30 °C and 50 °C yielding amorphous poly(lactide) with slightly heterotactic enchainment (Pm = 0.36 and 0.37). In agreement with the data obtained from GPC and DSC measurements, a chain-end control mechanism is proposed with fast chain propagation relative to the initiation, which leads to broad molecular weight distributions (D ≈ 1.80) and higher than expected molecular weights. Furthermore, chain transfer processes are observed, but only small amounts of transesterification and racemisation occur. Kinetic studies reveal a second-order dependence in rac-lactide (monomer) concentration and a first-order dependence in the concentration of catalyst 2.

Enantiomerically Pure Constrained Geometry Complexes of the Rare-Earth Metals Featuring a Dianionic N-Donor Functionalised Pentadienyl Ligand: Synthesis and Characterisation.

We report the preparation of enantiomerically pure constrained geometry complexes (cgc) of the rare-earth metals bearing a pentadienyl moiety (pdl) derived from the natural product (1R)-(-)-myrtenal. The potassium salt 1, [Kpdl*], was treated with ClSiMe2 NHtBu, and the resulting pentadiene 2 was deprotonated with the Schlosser-type base KOtPen/nBuLi (tPen=CMe2 (CH2 Me)) to yield the dipotassium salt [K2 (pdl*SiMe2 NtBu)] (3). However, 3 rearranges in THF solution to its isomer 3' by a 1,3-H shift, which elongates the bridge between the pdl and SiMe2 NtBu moieties by one CH2 unit. This is crucial for the successful formation of various monomeric C1 - or dimeric C2 -symmetric rare-earth cgc complexes with additional halide, tetraborohydride, amido and alkyl functionalities. All compounds have been extensively characterised by solid-state X-ray diffraction analysis, solution NMR spectroscopy and elemental analyses.

[(NHC)CoR2]: pre-catalysts for homogeneous olefin and alkyne hydrogenation.

A novel synthesis for dialkyl cobalt compounds [(tmeda)CoR2] is presented. In these complexes tmeda is readily replaced by an NHC or a bidentate phosphine ligand to form 3- and 4-coordinate compounds, respectively. [(ItBu)Co(CH2SiMe3)2] (ItBu = 1,3-di-tert-butylimidazolin-2-ylidene) serves as an efficient, homogeneous olefin hydrogenation pre-catalyst and allows the preparation of the novel cobalt bis(alkyne) complex [(ItBu)Co(η2-PhCCPh)2].

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).