Well-shuffled: An unexpected substituent distribution reaction via alkyldiarylsilylium ions leads to a distribution of substituents. Starting from alkyldiaryl silanes, this reaction provides a facile synthetic approach to sterically highly hindered triarylsilylium ions. These silylium ions can be applied in dihydrogen activation reactions.
This paper reports on the potential of titanium compounds as building blocks for supramolecular polygons. Self-assembly reactions of low-valent titanocene units and N-heterocyclic bridging ligands lead to novel titanium-based supramolecular squares. Pyrazine (3), 4,4'-bipyridine (4), and tetrazine (5) were used as bridging ligands, and the acetylene complexes [Cp2Ti{eta2-C2(SiMe3)2}] (1) and [(tBuCp)2Ti{eta2-C2(SiMe3)2}] (2) as sources of titanocene fragments. Molecular rectangles can be synthesized by stepwise reduction of the titanocene dichlorides [Cp(2)TiCl2] and [(tBuCp)2TiCl2] and consecutive coordination of two different bridging ligands. The resulting complexes are the first examples of molecular rectangles containing bent metallocene corner units. Single-crystal X-ray analyses of the tetranuclear compounds revealed the geometric properties of the molecular polygons in the solid state. Comparison of bond lengths and angles in coordinated and free ligands reveals the reduced state of the bridging ligand in the low-valent titanium compounds. The syntheses and properties of these novel, highly air- and moisture-sensitive compounds are discussed.
As you like it: The choice of solvents and substituents at the silicon atom determine what product is formed from carbon dioxide after electrophilic activation by silyl cations (see scheme). Benzoic acid as well as the C‐1 building blocks formic acid and methanol are on the product tableau.
Two
independent synthetic routes to η2-imine titanocene
complexes were developed. On one hand side, ligand exchange reactions of bis(trimethylsilyl)acetylene
by (p-Tolyl)HCNPh (3) employing
the Rosenthal reagent Cp2Ti{η2-C2(SiMe3)2} (1) lead to Cp2Ti{η2-(p-Tolyl)CHNPh} (5), exhibiting a titanaaziridine
structure. On the other hand, the direct reductive complexation of 3 by using Cp2TiCl2 (2)
and Mg as reducing agent leads also to 5, one of the
rare known titanoceneaziridines without additional ligands. By using
the ketimine (p-Tolyl)2CNPh (4) instead of the aldimine 3, an unexpected coordination
mode was found by X-ray diffraction, exhibiting an azatitanacyclopent-4-ene
structure involving one tolyl fragment. In such a way, via the reductive
complexation of 4, employing 2 or Cp*TiCl3 (12), the 1-aza-2-titanacyclopent-4-ene complexes 6 and 13 are formed. Density functional calculations
at the M06-2X level identify these new complexes 6 and 13 as 1-aza-2-titanacyclopent-4-enes, in agreement with an
analysis based on the experimental structural parameters. A theoretical
study of the bonding between the titanocene fragment and the imine
ligand reveals that steric factors are more pronounced for titanaaziridines
and disfavor their formation compared to azatitanacyclopentenes. This
provides a rationalization for the preferred formation of titanoceneaziridines
in the case of aldimine ligands and azatitanacyclopentenes when ketimines
are applied. Whereas titanoceneaziridine 5 undergoes
insertion reactions into the Ti–C carbon σ-bond with
aldehydes, ketones, or carbodiimides to the five-membered titanacycles 20 and 21, complex 6 appears to
be inert in comparable reactions.
We report the spontaneous coupling of N-heterocycles, initiated by C-H bond activation reactions. The reaction of quinoxalines and the titanocene acetylene complex Cp2Ti{eta2-C2(SiMe3)2}, as an excellent titanocene source, results in the formation of trinuclear 1,6,7,12,13,18-hexaazatrinaphthylene (HATN) titanium complexes. These HATN titanium complexes are thermally stable but sensitive to air and moisture. A three-fold dehydrogenative C-C coupling is proposed as the main step in the presented synthetic procedure. Particularly using commercial starting materials, an efficient route for the dehydrogenative coupling of N-heterocycles, leading to multidentate ligands, has been established.
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