Understanding the Coordination Chemistry and Structural and Photophysical Properties of EuII- and SmII-Containing Complexes of Hexamethylhexacyclen and Noncyclic Tetradentate Amines

Ligands play a crucial role in supporting or stabilizing the divalent oxidation state of lanthanide metals. To expand the range of ligands used to chelate divalent lanthanide ions, we synthesized and studied the structural and photophysical properties of complexes of EuII and SmII with hexamethylhexacyclen, 1,1,4,7,10,10-hexamethyltriethylenetetramine, tris[2-(dimethylamino)ethyl]amine, and tris[2-(isopropylamino)ethyl]amine as supporting ligands. Coordination of hexamethylhexacyclen, an analogue of 18-crown-6, generates sterically crowded complexes of EuII and SmII that are either seven or eight coordinate and adopt a range of geometries that differ from those of their 18-crown-6 counterparts and from those of lanthanide-containing complexes with the acyclic tetradente tertiary amine ligands included in this report. The emission spectra of EuII(hexamethylhexacyclen) show a moderate sensitivity to counterion identity and are more red-shifted compared to those of complexes of EuII with 18-crown-6 and the hexamethylated aza derivative of 2.2.2-cryptand. In addition, the morphology of hexamethylhexacyclen in [LnI(hexamethylhexacyclen)]I was found to resemble that of thermally stable alkalides of the form [M(hexamethylhexacyclen)]Na- (M = K+ or Cs+), suggesting that hexamethylhexacyclen could be an interesting ligand for strongly reducing lanthanide ions.

Rare Earth Starting Materials and Methodologies for Synthetic Chemistry

The number of rare earth (RE) starting materials used in synthesis is staggering, ranging from simple binary metal-halide salts to borohydrides and "designer reagents" such as alkyl and organoaluminate complexes. This review collates the most important starting materials used in RE synthetic chemistry, including essential information on their preparations and uses in modern synthetic methodologies. The review is divided by starting material category and supporting ligands (i.e., metals as synthetic precursors, halides, borohydrides, nitrogen donors, oxygen donors, triflates, and organometallic reagents), and in each section relevant synthetic methodologies and applications are discussed.

Toward Opto-Structural Correlation to Investigate Luminescence Thermometry in an Organometallic Eu(II) Complex

Lanthanide-based luminescent materials have unique properties and are well-studied for many potential applications. In particular, the characteristic 5d → 4f emission of divalent lanthanide ions such as Eu^II allows for tunability of the emissive properties via modulation of the coordination environment. We report the synthesis and photoluminescence investigation of pentamethylcyclopentadienyleuropium(II) tetrahydroborate bis(tetrahydrofuran) dimer (1), the first example of an organometallic, discrete molecular Eu^II band-shift luminescence thermometer. Complex 1 exhibits an absolute sensitivity of 8.2 cm^-1 K^-1 at 320 K, the highest thus far observed for a lanthanide-based band-shift thermometer. Opto-structural correlation via variable-temperature single-crystal X-ray diffraction and fluorescence spectroscopy allows rationalization of the remarkable thermometric luminescence of complex 1 and reveals the significant potential of molecular Eu^II compounds in luminescence thermometry.

Synthesis and Characterization of Divalent Samarium and Thulium N,N-Dimethylaminodiboranates

The syntheses and molecular structures of new Sm^II and Tm^II N,N-dimethylaminodiboranate (DMADB) complexes are described. Treating SmI₂(THF)₂ with Na(H₃BNMe₂BH₃) in THF results in the formation of Sm(H₃BNMe₂BH₃)₂(THF)₃ (1), which can be readily converted to Sm(H₃BNMe₂BH₃)₂(DME)₂ (DME = 1,2-dimethoxyethane) or Sm(H₃BNMe₂BH₃)₂(diglyme) by exchange with the corresponding ether. We also show that Sm(H₃BNMe₂BH₃)₂(THF)₃ can be prepared by reduction of the Sm^III compound Sm(H₃BNMe₂BH₃)₃(THF) with KC₈ and that addition of 18-crown-6 to this reaction mixture results in the formation of the Sm^II compound Sm(H₃BNMe₂BH₃)₂(18-crown-6). In a similar fashion, two new Tm^II complexes have been synthesized: treatment of TmI₂ in THF with Na(H₃BNMe₂BH₃) results in the formation of Tm(H₃BNMe₂BH₃)₂(THF)₂ and Tm(H₃BNMe₂BH₃)₂(THF)₃, which form a cocrystal. IR data and elemental analyses are reported for all the new compounds, as are their crystal structures. ¹H and ¹¹B NMR data are provided where available.

Trends in the Series of Ammine Rare-Earth-Metal Borohydrides: Relating Structural and Thermal Properties

Ammine metal borohydrides display extreme structural and compositional diversity and show potential applications for solid-state hydrogen and ammonia storage and as solid-state electrolytes. Thirty-two new compounds are reported in this work, and trends in the full series of ammine rare-earth-metal borohydrides are discussed. The majority of the rare-earth metals (RE) form trivalent RE(BH₄)₃·xNH₃ (x = 7-1) compounds, which possess an intriguing crystal chemistry changing with the number of ammonia ligands, varying from structures built from complex ions (x = 5-7), to molecular structures (x = 3, 4), one-dimensional chains (x = 2), and structures built from two-dimensional layers (x = 1). Divalent RE(BH₄)₂·xNH₃ (x = 4, 2, 1) compounds are observed for RE²⁺ = Sm, Eu, Yb, with structures varying from molecular structures (x = 4) to two-dimensional layered (x = 2, 1) and three-dimensional structures (Yb(BH₄)₂·NH₃). The crystal structure and composition of the compounds depend on the volume of the rare-earth ion. In all structures, NH₃ coordinates to the metal, while BH₄^- has a more flexible coordination and is observed as a bridging and terminal ligand and as a counterion. RE(BH₄)₃·xNH₃ (x = 7-5, 4) releases NH₃ stepwise during thermal treatment, while mainly H₂ is released for x ≤ 3. In contrast, only NH₃ is released from RE(BH₄)₂·xNH₃ due to the lower charge density on the RE²⁺ ion and higher stability of RE(BH₄)₂. The thermal stability of RE(BH₄)₃·xNH₃ increase with increasing cation charge density for x = 5, 7, while it decreases for x = 4, 6. For x = 3, the thermal stability decreases with increasing charge density, due to the destabilization of the BH₄^- group, making it more reactive toward NH₃. This research provides a large number of novel compounds and new insight into trends in the crystal chemistry of ammine metal borohydrides and reveals a correlation between the local metal coordination and the thermal stability.

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.

From Metal Hydrides to Metal Borohydrides

Commencing from metal hydrides, versatile synthesis, purification, and desolvation approaches are presented for a wide range of metal borohydrides and their solvates. An optimized and generalized synthesis method is provided for 11 different metal borohydrides, M(BH₄) _n, (M = Li, Na, Mg, Ca, Sr, Ba, Y, Nd, Sm, Gd, Yb), providing controlled access to more than 15 different polymorphs and in excess of 20 metal borohydride solvate complexes. Commercially unavailable metal hydrides (MH _n, M = Sr, Ba, Y, Nd, Sm, Gd, Yb) are synthesized utilizing high pressure hydrogenation. For synthesis of metal borohydrides, all hydrides are mechanochemically activated prior to reaction with dimethylsulfide borane. A purification process is devised, alongside a complementary desolvation process for solvate complexes, yielding high purity products. An array of polymorphically pure metal borohydrides are synthesized in this manner, supporting the general applicability of this method. Additionally, new metal borohydrides, α-, α'- β-, γ-Yb(BH₄)₂, α-Nd(BH₄)₃ and new solvates Sr(BH₄)₂·1THF, Sm(BH₄)₂·1THF, Yb(BH₄)₂· xTHF, x = 1 or 2, Nd(BH₄)₃·1Me₂S, Nd(BH₄)₃·1.5THF, Sm(BH₄)₃·1.5THF and Yb(BH₄)₃· xMe₂S (" x" = unspecified), are presented here. Synthesis conditions are optimized individually for each metal, providing insight into reactivity and mechanistic concerns. The reaction follows a nucleophilic addition/hydride-transfer mechanism. Therefore, the reaction is most efficient for ionic and polar-covalent metal hydrides. The presented synthetic approaches are widely applicable, as demonstrated by permitting facile access to a large number of materials and by performing a scale-up synthesis of LiBH₄.

Developments in the use of rare earth metal complexes as efficient catalysts for ring-opening polymerization of cyclic esters used in biomedical applications

Biodegradable polymers represent a class of particularly useful materials for many biomedical and pharmaceutical applications. Among these types of polyesters, poly(ε-caprolactone) and polylactides are considered very promising for controlled drug delivery devices. These polymers are mainly produced by ring-opening polymerization of their respective cyclic esters, since this method allows a strict control of the molecular parameters (molecular weight and distribution) of the obtained polymers. The most widely used catalysts for ring-opening polymerization of cyclic esters are tin- and aluminium-based organometallic

complexes; however since the contamination of the aliphatic polyesters by potentially toxic metallic residues is particularly of concern for biomedical applications, the possibility of replacing organometallic initiators by novel less toxic or more efficient organometallic complexes has been intensively studied. Thus, in the recent years, the use of highly reactive rare earth initiators/catalysts leading to lower polymer contamination has been developed. The use of rare earth complexes is considered a valuable strategy to decrease the polyester contamination by metallic residues and represents an attractive alternative to traditional organometallic complexes.

Metal borohydrides and derivatives – synthesis, structure and properties

A comprehensive review of metal borohydrides from synthesis to application.

Continuous cyclo-polymerisation of l-lactide by reactive extrusion using atoxic metal-based catalysts: easy access to well-defined polylactide macrocycles

Macrocyclic polylactide was synthesized by reactive extrusion polymerisation of l-lactide using lanthanide trisborohydrides as the catalysts. This is the first example of a cyclic polyester obtained directly via a continuous process.

(Iminophosphoranyl)(thiophosphoranyl)methane rare-earth borohydride complexes: synthesis, structures and polymerization catalysis

The {CH(PPh₂NSiMe₃)(PPh₂S)}⁻ligand has been used for the synthesis of divalent and trivalent rare-earth borohydride complexes. These compounds enable the ring-opening polymerization (ROP) of ε-caprolactone (CL) and trimethylene carbonate (TMC).