Synthesis and Reactivity of Bis-tris(pyrazolyl)borate Lanthanide/Aluminum Heterobimetallic Trihydride Complexes.

Molecular heterobimetallic hydride complexes of lanthanide (Ln) and main-group (MG) metals exhibit chemical properties unique from their monometallic counterparts and are highly reactive species, making their synthesis and isolation challenging. Herein, molecular Ln/Al heterobimetallic trihydrides [Ln(Tp)2(µ-H)2Al(H)(N″)] [2-Ln; Ln = Y, Sm, Dy, Yb; Tp = hydrotris(1-pyrazolyl)borate; N″ = N(SiMe3)2] have been synthesized by facile insertion of aminoalane [Me3N·AlH3] into the Ln-N amide bonds of [Ln(Tp)2(N″)] (1-Ln). Thus, this is a simple synthetic strategy to access a range of Ln/Al hydrides. Reactivity studies demonstrate that 2-Ln is a heterobimetallic hydride, with evidence for the cooperative nature of 2-Ln shown by the catalytic amine-borane dehydrocoupling under ambient conditions in contrast to its monomeric counterparts.

A Second Glass Transition Observed in Single-Component Homogeneous Liquids Due to Intramolecular Vitrification.

On supercooling a liquid, the viscosity rises rapidly until at the glass transition it vitrifies into an amorphous solid accompanied by a steep drop in the heat capacity. Therefore, a pure homogeneous liquid is not expected to display more than one glass transition. Here we show that a family of single-component homogeneous molecular liquids, titanium tetraalkoxides, exhibit two calorimetric glass transitions of comparable magnitude, one of which is the conventional glass transition associated with dynamic arrest of the bulk liquid properties, while the other is associated with the freezing out of intramolecular degrees of freedom. Such intramolecular vitrification is likely to be found in molecules in which low-frequency terahertz intramolecular motion is coupled to the surrounding liquid. These results imply that intramolecular barrier-crossing processes, typically associated with chemical reactivity, do not necessarily follow the Arrhenius law but may freeze out at a finite temperature.

Reduction chemistry yields stable and soluble divalent lanthanide tris(pyrazolyl)borate complexes.

Reduction of the heteroleptic Ln(III) precursors [Ln(Tp)2(OTf)] (Tp = hydrotris(1-pyrazolyl)borate; OTf = triflate) with either an aluminyl(I) anion or KC8 yielded the adduct-free homoleptic Ln(II) complexes dimeric 1-Eu [{Eu(Tp)(µ-κ1:η5-Tp)}2] and monomeric 1-Yb [Yb(Tp)2]. Complexes 1-Ln have good solubility and stability in both non-coordinating and coordinating solvents. Reaction of 1-Ln with 2 Ph3PO yielded 1-Ln(OPPh3)2. All complexes are intensely coloured and 1-Eu is photoluminescent. The electronic absorption data show the 4f-5d electronic transitions in Ln(II). Single-crystal X-ray diffraction data reveal first µ-κ1:η5-coordination mode of the unsubstituted Tp ligand to lanthanides in 1-Eu.

Molecular and electronic structure of the dithiooxalato radical ligand stabilised by rare earth coordination.

Heterometallic rare earth transition metal compounds of dithioxalate (dto)2-, [NiII{(dto)LnIIITp2}2] (Ln = Y (1), Gd (2); Tp = hydrotris(pyrazol-1-yl)borate) were synthesised. The Lewis acidic rare earth ions are bound to the dioxolene and chemical reduction of 1 and 2 with cobaltocene yielded [CoCp2]+[NiII{(dto)LnIIITp2}2]˙- Ln = Y (3), Gd (4). The reduction is ligand-based and 3 and 4 are the first examples of both molecular and electronic structural characterisation of the dithiooxalato radical (dto)3˙-.

The semiquinone radical anion of 1,10-phenanthroline-5,6-dione: synthesis and rare earth coordination chemistry.

Reduction of 1,10-phenanthroline-5,6-dione (pd) with CoCpR2 resulted in the first molecular compounds of the pd˙- semi-quinone radical anion, [CoCpR2]+[pd]˙- (R = H, (1); R = Me4, (2)). Furthermore compounds 1 and 2 were reacted with [Y(hfac)3(thf)2] (hfac = 1,1,1-5,5,5-hexafluoroacetylacetonate) to synthesise the rare earth-transition metal heterometallic compounds, [CoCpR2]+[Y(hfac)3(N,N'-pd)]˙- (R = H, (3); R = Me4, (4)).

The modular synthesis of rare earth-transition metal heterobimetallic complexes utilizing a redox-active ligand.

We report a robust and modular synthetic route to heterometallic rare earth-transition metal complexes. We have used the redox-active bridging ligand 1,10-phenathroline-5,6-dione (pd), which has selective N,N' or O,O' binding sites as the template for this synthetic route. The coordination complexes [Ln(hfac)3(N,N'-pd)] (Ln = Y [1], Gd [2]; hfac = hexafluoroacetylacetonate) were synthesised in high yield. These complexes have been fully characterised using a range of spectroscopic techniques. Solid state molecular structures of 1 and 2 have been determined by X-ray crystallography and display different pd binding modes in coordinating and non-coordinating solvents. Complexes 1 and 2 are unusually highly coloured in coordinating solvents, for example the vis-NIR spectrum of 1 in acetonitrile displays an electronic transition centred at 587 nm with an extinction coefficient consistent with significant charge transfer. The reaction between 1 and 2 and VCp2 or VCpt2 (Cpt = tetramethylcyclopentadienyl) resulted in the isolation of the heterobimetallic complexes, [Ln(hfac)3(N,N'-O,O'-pd)VCp2] (Ln = Y [3], Gd [4]) or [Ln(hfac)3(N,N'-O,O'-pd)VCpt2] (Ln = Y [5], Gd [6]). The solid state molecular structures of 3, 5 and 6 have been determined by X-ray crystallography. The spectroscopic data on 3-6 are consistent with oxidation of V(ii) to V(iv) and reduction of pd to pd2- in the heterobimetallic complexes. The spin-Hamiltonian parameters from low temperature X-band EPR spectroscopy of 3 and 5 describe a 2A1 ground state, with a V(iv) centre. DFT calculations on 3 are in good agreement with experimental data and confirm the SOMO as the dx2-y2 orbital localised on vanadium.

Organometallic neptunium(III) complexes.

Studies of transuranic organometallic complexes provide a particularly valuable insight into covalent contributions to the metal-ligand bonding, in which the subtle differences between the transuranium actinide ions and their lighter lanthanide counterparts are of fundamental importance for the effective remediation of nuclear waste. Unlike the organometallic chemistry of uranium, which has focused strongly on U(III) and has seen some spectacular advances, that of the transuranics is significantly technically more challenging and has remained dormant. In the case of neptunium, it is limited mainly to Np(IV). Here we report the synthesis of three new Np(III) organometallic compounds and the characterization of their molecular and electronic structures. These studies suggest that Np(III) complexes could act as single-molecule magnets, and that the lower oxidation state of Np(II) is chemically accessible. In comparison with lanthanide analogues, significant d- and f-electron contributions to key Np(III) orbitals are observed, which shows that fundamental neptunium organometallic chemistry can provide new insights into the behaviour of f-elements.

Characterizing pressure-induced uranium C-H agostic bonds.

The diuranium(III) compound [UN''2]2(µ-η(6):η(6)-C6H6) (N''=N(SiMe3)2) has been studied using variable, high-pressure single-crystal X-ray crystallography, and density functional theory . In this compound, the low-coordinate metal cations are coupled through π- and δ-symmetric arene overlap and show close metal-CH contacts with the flexible methyl CH groups of the sterically encumbered amido ligands. The metal-metal separation decreases with increasing pressure, but the most significant structural changes are to the close contacts between ligand CH bonds and the U centers. Although the interatomic distances are suggestive of agostic-type interactions between the U and ligand peripheral CH groups, QTAIM (quantum theory of atoms-in-molecules) computational analysis suggests that there is no such interaction at ambient pressure. However, QTAIM and NBO analyses indicate that the interaction becomes agostic at 3.2 GPa.

Group 4 metal compounds incorporating the amide ligand, [N(SiMe2{C6H4-2-OMe})2](-).

The anisole-substituted silyl-amide anion, [N(SiMe2{C6H4-2-OMe})2](-) (L), has been used as a pincer-type ligand in coordination chemistry. X-ray diffraction data for the lithium salt shows a trimetallic structure consisting of two equivalents of Li(L) that sequester a molecule of LiCl. The potassium salt K(L) is dimeric in the solid-state with bridging amide ligands. Each structure shows chelation of both O-donor groups to the electropositive metal. In contrast, the titanium compound Ti(L)Cl3 is four-coordinate with a monodentate amide. The zirconium compound Zr(L)2Cl2 is monometallic with a six-coordinate metal and two N,O-bidentate amides.

New chemistry from an old reagent: mono- and dinuclear macrocyclic uranium(III) complexes from [U(BH4)3(THF)2].

A new robust and high-yielding synthesis of the valuable U(III) synthon [U(BH4)3(THF)2] is reported. Reactivity in ligand exchange reactions is found to contrast significantly to that of uranium triiodide. This is exemplified by the synthesis and characterization of azamacrocyclic U(III) complexes, including mononuclear [U(BH4)(L)] and dinuclear [Li(THF)4][{U(BH4)}2(µ-BH4)(L(Me))] and [Na(THF)4][{U(BH4)}2(µ-BH4)(L(A))(THF)2]. The structures of all complexes have been determined by single-crystal X-ray diffraction and display two new U(III)2(BH4)3 motifs.

Synthesis and catalytic activity of iron complexes with bidentate NHC ligands.

A family of well-defined Fe(II) complexes of the type {BnN(N-CH2(CH2)n-N'-tert-butyl-imidazole-2-ylidene)2}FeCl2 (Bn = benzyl; n = 1 (1) or 2 (2)), {BnN(N-CH2(CH2)n-N'-methylbenzimidazole-2-ylidene)2}FeCl2 (n = 1 (3) or 2 (4)) and {BnN(N-CH2CH2CH2-N'-methylbenzimidazole-2-ylidene)2}FeBr2 (5) has been synthesized. These complexes are rare examples of Fe species supported by bidentate NHC ligands. Complexes 2, 3, 4 and 5 were characterized by X-ray crystallography and in all cases a distorted tetrahedral geometry is observed around the Fe center. The magnetic data is consistent with the complexes containing non interacting high spin Fe(II) centers (S = 2) and indicates that a large zero-field splitting (D) is present. The new complexes are highly active pre-catalysts for the homo-coupling of Grignard reagents.

Expanding yttrium bis(trimethylsilylamide) chemistry through the reaction chemistry of (N2)2-, (N2)3-, and (NO)2- complexes.

The reaction chemistry of the side-on bound (N(2))(2-), (N(2))(3-), and (NO)(2-) complexes of the [(R(2)N)(2)Y](+) cation (R = SiMe(3)), namely, [(R(2)N)(2)(THF)Y](2)(µ-η(2):η(2)-N(2)), 1, [(R(2)N)(2)(THF)Y](2)(µ-η(2):η(2)-N(2))K, 2, and [(R(2)N)(2)(THF)Y](2)(µ-η(2):η(2)-NO), 3, with oxidizing agents has been explored to search for other (E(2))(n-), (E = N, O), species that can be stabilized by this cation. This has led to the first examples for the [(R(2)N)(2)Y](+) cation of two fundamental classes of [(monoanion)(2)Ln](+) rare earth systems (Ln = Sc, Y, lanthanides), namely, oxide complexes and the tetraphenylborate salt. In addition, an unusually high yield reaction with dioxygen was found to give a peroxide complex that completes the (N(2))(2-), (NO)(2-), (O(2))(2-) series with 1 and 3. Specifically, the (µ-O)(2-) oxide-bridged bimetallic complex, [(R(2)N)(2)(THF)Y}(2)(µ-O), 4, is obtained as a byproduct from reactions of either the (N(2))(2-) complex, 1, or the (N(2))(3-) complex, 2, with NO, while the oxide formed from 2 with N(2)O is a polymeric species incorporating potassium, {[(R(2)N)(2)Y](2)(µ-O)(2)K(2)(µ-C(7)H(8))}(n), 5. Reaction of 1 with 1 atm of O(2) generates the (O(2))(2-) bridging side-on peroxide [(R(2)N)(2)(THF)Y](2)(µ-η(2):η(2)-O(2)), 6. The O-O bond in 6 is cleaved by KC(8) to provide an alternative synthetic route to 5. Attempts to oxidize the (NO)(2-) complex, 3, with AgBPh(4) led to the isolation of the tetraphenylborate complex, [(R(2)N)(2)Y(THF)(3)][BPh(4)], 7, that was also synthesized from 1 and AgBPh(4). Oxidation of the (N(2))(2-) complex, 1, with the radical trap (2,2,6,6-tetramethylpiperidin-1-yl)oxyl, TEMPO, generates the (TEMPO)(-) anion complex, (R(2)N)(2)(THF)Y(η(2)-ONC(5)H(6)Me(4)), 8.

Varying the Lewis base coordination of the Y2N2 core in the reduced dinitrogen complexes {[(Me3Si)2N]2(L)Y}2(µ-η2:η2-N2) (L = benzonitrile, pyridines, triphenylphosphine oxide, and trimethylamine N-oxide).

The effect of the neutral donor ligand, L, on the Ln(2)N(2) core in the (N═N)(2-) complexes, [A(2)(L)Ln](2)(µ-η(2):η(2)-N(2)) (Ln = Sc, Y, lanthanide; A = monoanion; L = neutral ligand), is unknown since all of the crystallographically characterized examples were obtained with L = tetrahydrofuran (THF). To explore variation in L, displacement reactions between {[(Me(3)Si)(2)N](2)(THF)Y}(2)(µ-η(2):η(2)-N(2)), 1, and benzonitrile, pyridine (py), 4-dimethylaminopyridine (DMAP), triphenylphosphine oxide, and trimethylamine N-oxide were investigated. THF is displaced by all of these ligands to form {[(Me(3)Si)(2)N](2)(L)Y}(2)(µ-η(2):η(2)-N(2)) complexes (L = PhCN, 2; py, 3; DMAP, 4; Ph(3)PO, 5; Me(3)NO, 6) that were fully characterized by analytical, spectroscopic, density functional theory, and X-ray crystallographic methods. The crystal structures of the Y(2)N(2) cores in 2-5 are similar to that in 1 with N-N bond distances between 1.255(3) Å and 1.274(3) Å, but X-ray analysis of the N-N distance in 6 shows it to be shorter: 1.198(3) Å.

Isolation of (CO)1- and (CO2)1- radical complexes of rare earths via Ln(NR2)3/K reduction and [K2(18-crown-6)2]2+ oligomerization.

Deep-blue solutions of Y(2+) formed from Y(NR(2))(3) (R = SiMe(3)) and excess potassium in the presence of 18-crown-6 at -45 °C under vacuum in diethyl ether react with CO at -78 °C to form colorless crystals of the (CO)(1-) radical complex, {[(R(2)N)(3)Y(µ-CO)(2)][K(2)(18-crown-6)(2)]}(n), 1. The polymeric structure contains trigonal bipyramidal [(R(2)N)(3)Y(µ-CO)(2)](2-) units with axial (CO)(1-) ligands linked by [K(2)(18-crown-6)(2)](2+) dications. Byproducts such as the ynediolate, [(R(2)N)(3)Y](2)(µ-OC≡CO){[K(18-crown-6)](2)(18-crown-6)}, 2, in which two (CO)(1-) anions are coupled to form (OC≡CO)(2-), and the insertion/rearrangement product, {(R(2)N)(2)Y[OC(═CH(2))Si(Me(2))NSiMe(3)]}[K(18-crown-6)], 3, are common in these reactions that give variable results depending on the specific reaction conditions. The CO reduction in the presence of THF forms a solvated variant of 2, the ynediolate [(R(2)N)(3)Y](2)(µ-OC≡CO)[K(18-crown-6)(THF)(2)](2), 2a. CO(2) reacts analogously with Y(2+) to form the (CO(2))(1-) radical complex, {[(R(2)N)(3)Y(µ-CO(2))(2)][K(2)(18-crown-6)(2)]}(n), 4, that has a structure similar to that of 1. Analogous (CO)(1-) and (OC≡CO)(2-) complexes of lutetium were isolated using Lu(NR(2))(3)/K/18-crown-6: {[(R(2)N)(3)Lu(µ-CO)(2)][K(2)(18-crown-6)(2)]}(n), 5, [(R(2)N)(3)Lu](2)(µ-OC≡CO){[K(18-crown-6)](2)(18-crown-6)}, 6, and [(R(2)N)(3)Lu](2)(µ-OC≡CO)[K(18-crown-6)(Et(2)O)(2)](2), 6a.