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.
The readily available iron carbonyl complexes, [CpFe(CO)2]2 (1) and CpFe(CO)2I (2) (Cp = η-C5H5), were found to be efficient precatalysts for the dehydrocoupling/dehydrogenation of the amine-borane Me2NH·BH3 (3) to afford the cyclodiborazane [Me2N-BH2]2 (4), upon UV photoirradiation at ambient temperature. In situ analysis of the reaction mixtures by (11)B NMR spectroscopy indicated that different two-step mechanisms operate in each case. Thus, precatalyst 1 dehydrocoupled 3 via the aminoborane Me2N═BH2 (5) which then cyclodimerized to give 4 via an off-metal process. In contrast, the reaction with precatalyst 2 proceeded via Me2NH-BH2-NMe2-BH3 (6) as the key intermediate, affording 4 as the final product after a second metal-mediated step. The related complex Cp2Fe2(CO)3(MeCN) (7), formed by photoirradiation of 1 in MeCN, was found to be a substantially more active dehydrocoupling catalyst and not to require photoactivation, but otherwise operated via a two-step mechanism analogous to that for 1. Significantly, detailed mechanistic studies indicated that the active catalyst generated from precatalyst 7 was heterogeneous in nature and consisted of small iron nanoparticles (≤10 nm). Although more difficult to study, a similar process is highly likely to operate for precatalyst 1 under photoirradiation conditions. In contrast to the cases of 7 and 1, analogous experimental studies for the case of photoactivated Fe precatalyst 2 suggested that the active catalyst formed in this case was homogeneous. Experimental and computational DFT studies were used to explore the catalytic cycle which appears to involve amine-borane ligated [CpFe(CO)](+) as a key intermediate.
The catalyst loading is the key to control the molecular weight of the polymer in the iron-catalyzed dehydropolymerization of phosphine-borane adducts. Studies showed that the reaction proceeds through a chain-growth coordination-insertion mechanism.
The
reaction of diarylalkylsilanes and -germanes with trityl cation
in the presence of a weakly coordinating anion to give the corresponding
triarylsilylium or -germylium ions was investigated. This reaction
provides a facile access to a larger range of these sterically highly
hindered Lewis acids. The factors that promote the substituent exchange
were studied, and significant effects of the substituent, of the solvent,
and of the group 14 element were revealed. A combined solid-state
XRD and NMR investigation of the tris(pentamethylphenyl) silylium
borate [(Me5C6)3Si]2[B12Cl12] disclosed the trigonal planar coordination
environment of the silicon atom in this silylium ion. NMR investigations
indicate for 2,4,6-triisopropylphenyl-substituted silylium and germylium
ions the onset of C–H···E+ three-center
interactions (E = Si, Ge) between the distant CH bond of the isopropyl
group and the positively charged group 14 element atom.
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.
The dehydropolymerization of the primary phosphine–boranes, RPH2•BH3 (1a–f) (R = 3,4‐(OCH2O)C6H3 (a), Ph (b), p‐(CF3O)C6H4 (c), 3,5‐(CF3)2C6H3 (d), 2,4,6‐(CH3)3C6H2 (e), 2,4,6‐tBu3C6H2 (f)) is explored using the precatalyst [CpFe(CO)2OTf] (I) (OTf = OS(O)2CF3), based on the earth abundant element Fe. Formation of polyphosphinoboranes [RPH‐BH2]n (2a–e) is confirmed by multinuclear NMR spectroscopy, but no conversion of 1f to 2f is detected. Analysis by electrospray ionization mass spectrometry confirms the presence of the anticipated polymer repeat units for 2a–e. Gel permeation chromatography (GPC) confirms the polymeric nature of 2a–e and indicates number‐average molecular weights (Mn) of 12 000–209 000 Da and polydispersity indices between 1.14 and 2.17. By contrast, thermal dehydropolymerization of 1a–e in the absence of added precatalyst leads to formation of oligomeric material. Interestingly, polyphosphinoboranes 2c and 2d display GPC behavior typical of polyelectrolytes, with a hydrodynamic radius dependent on concentration. The thermal transition behavior, thermal stability, and surface properties of thin films are also studied.
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