Cyclopentadienyl Complexes of the Alkaline Earths in Light of the Periodic Trends

A lithium-aluminium heterobimetallic dimetallocene

Homobimetallic dimetallocenes exhibiting two identical metal atoms sandwiched between two η5 bonded cyclopentadienyl rings is a narrow class of compounds, with representative examples being dizincocene and diberyllocene. Here we report the synthesis and structural characterization of a heterobimetallic dimetallocene, accessible through heterocoupling of lithium and aluminylene fragments with pentaisopropylcyclopentadienyl ligands. The Al-Li bond features a high ionic character and profits from attractive dispersion interactions between the isopropyl groups of the cyclopentadienyl ligands. A key synthetic step is the isolation of a cyclopentadienylaluminylene monomer, which also enables the structural characterization of this species. In addition to their structural authentication by single-crystal X-ray diffraction analysis, both compounds were characterized by multinuclear NMR spectroscopy in solution and in the solid state. Furthermore, reactivity studies of the lithium-aluminium heterobimetallic dimetallocene with an N-heterocyclic carbene and different heteroallenes were performed and show that the Al-Li bond is easily cleaved.

Metal-ring interactions in group 2 ansa-metallocenes: assessed with the local vibrational mode theory

Ansa-metallocenes, a vital class of organometallic compounds, have attracted significant attention due to their diverse structural motifs and their pivotal roles in catalysis and materials science. We investigated 37 distinct group 2 ansa-metallocenes at the B3LYP-D3/def2-TZVP level of theory. Utilizing local mode force constants derived from our local vibrational mode theory, including a special force constant directly targeting the metal-ring interaction, we could unveil latent structural differences between solvated and non-solvated metallocenophanes and the influence of the solvent on complex stability and structure. We could quantify the intrinsic strength of the metal-cyclopentadienyl (M-Cp) bonds and the influence of the bridging motifs on the stiffness of the Cp-M-Cp angles, another determinant of complex stability. LMA was complemented by the analysis of electronic density, utilizing the quantum theory of atoms in molecules (QTAIM), which confirmed both the impact of solvent coordination on the strength of the M-Cp bond(s) and the influence of the bridging motif on the Cp-M-Cp angles. The specific effect of the ansa-motif on the M-Cp interaction was further elucidated by a comparison with linear/bent metallocene structures. In summary, our results identify the local mode analysis as an efficient tool for unraveling the intricate molecular properties of ansa-metallocenes and their unique structural features.

Divalent ansa-Octaphenyllanthanocenes: Synthesis, Structures, and EuII Luminescence

Reductive dimerization of fulvenes using low-valent metal precursors is a straightforward one-step approach to access ethylene-bridged metallocenes. This process has so far mainly been employed with fulvenes carrying one or two substituents in the exocyclic position. In this work, a new synthesis of the unsubstituted exocyclic 1,2,3,4-tetraphenylfulvene (1), its full structural characterization by NMR spectroscopy and single-crystal X-ray diffraction, as well as some photophysical properties and its first use in reductive dimerization are described. This fulvene reacted with different lanthanoid metals in thf to provide the divalent ansa-octaphenylmetallocenes [Ln(C5Ph4CH2)2(thf)n] (Ln = Sm, n = 2 (2); Ln = Eu, n = 2 (3); and Ln = Yb, n = 1 (4)). These complexes were characterized by X-ray diffraction, laser desorption/ionization time of flight mass spectrometry, and, in the case of Sm and Yb, multinuclear NMR spectroscopy, showing the influence of the ansa-bridge on solution and solid-state structures compared to previously reported unbridged metallocenes. Furthermore, the luminescence properties of the Eu ansa complex 3 were studied in solution and the solid state, revealing significant differences with the known octa- and deca-phenyleuropocenes, [Eu(C5Ph4H)2(dme)] and [Eu(C5Ph5)2].

Fluorenyl-tethered N-heterocyclic carbene (NHC): an exclusive C-donor ligand for heteroleptic calcium and strontium chemistry

Exclusive C-donating ligands are rarely used with kinetically labile heavier alkaline earths (Ca, Sr, Ba). We report herein the aptitude of a combination of NHC with fluorenyl connected by a flexible -(CH2)2- linker as a ligand support for heteroleptic Ca- and Sr-N(SiMe3)2 and iodides. The Ca-N(SiMe3)2 complex even catalyzes the intramolecular hydroamination of aminoalkenes to showcase the effectiveness of this ligand framework.

[(Flu)-(CH2)2-(NHC)-CH2-(NHC)-(CH2)2-(Flu)]2−: an ‘all-organic’ hybrid and flexible ligand that enwraps a Ca2+ pseudo-tetrahedrally

A conformationally flexible ligand, [(Flu)-(CH2)2-(NHC)-CH2-(NHC)-(CH2)2-(Flu)]2− (L), that offers four sequential organic donor modules and enwraps a Ca2+ pseudo-tetrahedrally in a twisted ‘S’-shape is devised.