Barium phosphidoboranes and related calcium complexes.

The attempted synthesis of [{Carb}BaPPh2] (1) showed this barium-phosphide and its thf adducts, 1·thf and 1·(thf)2, to be unstable in solution. Our strategy to circumvent the fragility of these compounds involved the use of phosphinoboranes HPPh2·BH3 and HPPh2·B(C6F5)3 instead of HPPh2. This allowed for the synthesis of [{Carb}Ae{PPh2·BH3}] (Ae = Ba, 2; Ca, 3), [{Carb}Ca{(H3B)2PPh2}·(thf)] (4), [{Carb}Ba{PPh2·B(C6F5)3}] (5), [{Carb}Ba{O(B(C6F5)3)CH2CH2CH2CH2PPh2}·thf] (6), [Ba{O(B(C6F5)3)CH2CH2CH2CH2PPh2}2·(thf)1.5] (7) and [Ba{PPh2·B(C6F5)3}2·(thp)2] (8) that were characterised by multinuclear NMR spectroscopy (thp = tetrahydropyran). The molecular structures of 4, 6 and 8 were validated by X-ray diffraction crystallography, which revealed the presence of Ba⋯F stabilizing interactions (ca. 9 kcal mol-1) in the fluorine-containing compounds. Compounds 6 and 7 were obtained upon ring-opening of thf by their respective precursors, 5 and the in situ prepared [Ba{PPh2·B(C6F5)3}2]n. By contrast, thp does not undergo ring-opening under the same conditions but affords clean formation of 8. DFT analysis did not highlight any specific weakness of the Ba-P bond in 1·(thf)2. The instability of this compound is instead thought to stem from the high energy of its HOMO, which contains the non-conjugated P lone pair and features significant nucleophilic reactivity.

Bonding analysis in ytterbium(II) distannyl and related tetryls.

The syntheses of the ytterbium(II) distannyl [Yb{Sn(SiMe3)3}2·(thf)4] (Yb-Sn) and of its digermyl analogue [Yb{Ge(SiMe3)3}2·(thf)3] (Yb-Ge) are presented. The compounds were characterised by multinuclear high-resolution solution NMR spectroscopy, including 171Yb NMR, and by X-ray diffraction crystallography. The bonding and electronic properties of the two complexes, along with those of the known ytterbium(II) disilyl derivative [Yb{Si(SiMe3)3}2·(thf)3] (Yb-Si) and those of the congeneric calcium distannyl [Ca{Sn(SiMe3)3}2·(thf)4] (Ca-Sn), were investigated in detail by DFT calculations. This analysis points at a primarily ionic Yb-tetrel bonding, with a small covalent contribution, attributed principally to the 5d(Yb) participation. This weak covalent character is found to be larger for the distannyl Yb-Sn than for its lighter Si- and Ge-derivatives. The covalent component is also found to be greater in Yb-Sn than in Ca-Sn, due to the availability of the 5d(Yb) orbitals for bonding.

Metal-metal bonded alkaline-earth distannyls.

The first families of alkaline-earth stannylides [Ae(SnPh3)2·(thf) x ] (Ae = Ca, x = 3, 1; Sr, x = 3, 2; Ba, x = 4, 3) and [Ae{Sn(SiMe3)3}2·(thf) x ] (Ae = Ca, x = 4, 4; Sr, x = 4, 5; Ba, x = 4, 6), where Ae is a large alkaline earth with direct Ae-Sn bonds, are presented. All complexes have been characterised by high-resolution solution NMR spectroscopy, including 119Sn NMR, and by X-ray diffraction crystallography. The molecular structures of [Ca(SnPh3)2·(thf)4] (1'), [Sr(SnPh3)2·(thf)4] (2'), [Ba(SnPh3)2·(thf)5] (3'), 4, 5 and [Ba{Sn(SiMe3)3}2·(thf)5] (6'), most of which crystallised as higher thf solvates than their parents 1-6, were established by XRD analysis; the experimentally determined Sn-Ae-Sn' angles lie in the range 158.10(3)-179.33(4)°. In a given series, the 119Sn NMR chemical shifts are slightly deshielded upon descending group 2 from Ca to Ba, while the silyl-substituted stannyls are much more shielded than the phenyl ones (δ 119Sn/ppm: 1', -133.4; 2', -123.6; 3', -95.5; 4, -856.8; 5, -848.2; 6', -792.7). The bonding and electronic properties of these complexes were also analysed by DFT calculations. The combined spectroscopic, crystallographic and computational analysis of these complexes provide some insight into the main features of these unique families of homoleptic complexes. A comprehensive DFT study (Wiberg bond index, QTAIM and energy decomposition analysis) points at a primarily ionic Ae-Sn bonding, with a small covalent contribution, in these series of complexes; the Sn-Ae-Sn' angle is associated with a flat energy potential surface around its minimum, consistent with the broad range of values determined by experimental and computational methods.

Bis(imino)carbazolate lead(II) fluoride and related halides.

The versatility of a bulky bis(imino)carbazolate ligand in lead(ii) chemistry is illustrated by the synthesis of a soluble, heteroleptic lead(ii) fluoride and several halide (Cl, Br and I), amide and hydrocarbyl congeners. All complexes have been structurally authenticated, and a full set of 207Pb NMR data is discussed.

Synthesis and Reactivity of Acyclic Germanimines: Silyl Rearrangement and Cycloadditions.

We report the synthesis of aromatic germanimines [(HMDS)2Ge═NAr] (Ar = Ph, Mes, Dipp; Mes = 2,4,6-Me3C6H2, Dipp = 2,6-iPr2C6H3) and an investigation into their associated reactivity. [(HMDS)2Ge═NPh] decomposes above -30 °C, while [(HMDS)2Ge═NDipp] engages in an intramolecular reaction at 60 °C. [(HMDS)2Ge═NMes] was shown to rearrange via a 1,3-silyl migration to give [(HMDS){(SiMe3)(Mes)N}Ge(NSiMe3)] in a 1:7 equilibrium mixture at room temperature. These latter germanimines react with unsaturated polar substrates such as CO2, ketones, and arylisocyanate via a [2 + 2] cycloaddition pathway.

A versatile nitrogen ligand for alkaline-earth chemistry.

The tridentate ligand 1,8-bis-(2,6-diisopropylphenyl)imino-3,6-di-tert-butyl-carbazolate, {CarbDiPP}-, has been used to prepare a variety of complexes of the large alkaline earths (Ae) calcium, strontium and barium. A complex of their smaller congener, magnesium, has also been synthesised. The range of coligands that have been utilised include alkyls, amides, halides and tetrelides. All structurally characterised complexes presented herein are stable in solution and do not undergo ligand redistribution that is otherwise well-known to pollute the chemistry of the alkaline-earth metals. Detailed structural and spectroscopic data for these compounds are discussed. They provide compelling evidence that this nitrogen ligand allows for the kinetic stabilisation of Ae complexes through optimal steric encapsulation of the voluminous metal centres. Its ease of access combined to its evident versatility make {CarbDiPP}H stand out in the portfolio of proligands that have been devised for similar purposes in the past decade. Yet, it fell short in our attempts to synthesise heteroleptic Ae-hydrides upon action of PhSiH3 onto Ae-amides, as the crystallised products showed the imine groups were prone to hydride reduction and formal hydrosilylation.

Bis(imino)carbazolate: A Master Key for Barium Chemistry.

Reported here is a readily available bis(imino)carbazole-based proligand that constitutes a convenient entry point into the challenging synthetic molecular chemistry of barium. It enables the preparation of rare or even, up to now, unknown, solution-stable heteroleptic barium complexes. The syntheses and structural features for the first molecular barium fluoride and the first barium stannylide, with an unsupported Ba-Sn bond, are described, along with other carbazolate barium species: an amide (both a remarkably stable starting material and an excellent hydrophosphination precatalyst), iodide, and silanylide. DFT analysis of bonding patterns in the barium stannylide and barium silanylide highlights a prevailingly ionic barium-tetrelide bond with a small covalent contribution.