Another slice of pi: the addition of a second π-donor ligand engenders 1,2-addition and [2 + 2] cycloaddition reactivity across Nb-imido and Nb-oxo bonds.
We present a wide
range of reactivity studies focused on the rhenium(V) oxo imido complex
(DippN)(O)Re(BDI) (1, Dipp = 2,6-diisopropylphenyl and
BDI = N,N′-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate).
This complex, which was previously shown to possess a highly polarized
Re oxo moiety, has proven to be a potent nucleophile and a valuable
precursor to a variety of rare structural motifs in rhenium coordination
complexes. For example, the Re oxo moiety of 1 undergoes
[2 + 2] cycloadditions with carbodiimides, isocyanates, carbon dioxide,
and isothiocyanates at room temperature. In the case of CO2, the cycloadduct with 1 (a carbonate complex) undergoes
the facile ejection of CO2, demonstrating that this binding
process is reversible. In the case of isothiocyanate, chalcogen metathesis
with 1 takes place readily as the inclusion of a second
equivalent of substrate in the reaction mixture rapidly yields a dithiocarbamate
complex. This metathesis process was extended to the reactivity of 1 with phosphine chalcogenides, leading to the isolation of
terminal sulfido imido and selenido imido complexes. Attempts to complete
this series and generate the analogous terminal telluride led to the
formation of a bidentate tritelluride (Te3
2–) complex. Triethylphosphine could only undergo oxygen atom transfer
(OAT) with 1 under pressing thermal conditions that also
led to C–N cleavage of the BDI ligand. In contrast, OAT between 1 and CO or 2,6-xylylisocyanide (XylNC) was found to be much
more facile, proceeding within seconds at room temperature. While
the addition of excess CO led to a rhenium(III) imido dicarbonyl complex,
we found that the addition of 2 equiv of XylNC was necessary to promote
OAT, resulting in the isolation of a rare example of a stable metal
isocyanate complex. Our experimental observations of CO and XylNC
and their OAT reactions with 1 inspired a mechanistic
computational study to probe the intermediates and kinetic barriers
along these reaction pathways. Finally, we describe 1,2-additions
of both protic and hydridic substrates with the Re oxo moiety of 1, which most notably led to the syntheses of an uncommon
example of a terminal rhenium hydroxide complex and an oxo-bridged
Re–O–Zr hetero-bi-metallic complex that was generated
using Schwartz’s reagent (Cp2ZrHCl). A brief discussion
of a potential alternative route to 1 is also presented.
In this work, we exploit a confinement-induced molecular synthesis and a resulting bridging mechanism to create confined polyimide thermoset nanocomposites that couple molecular confinement-enhanced toughening with an unprecedented combination of high-temperature properties at low density. We describe a synthesis strategy that involves the infiltration of individual polymer chains through a nanoscale porous network while simultaneous imidization reactions increase the molecular backbone stiffness. In the extreme limit where the confinement length scale is much smaller than the polymer's molecular size, confinement-induced molecular mechanisms give rise to exceptional mechanical properties. We find that polyimide oligomers can undergo cross-linking reactions even in such molecular-scale confinement, increasing the molecular weight of the organic phase and toughening the nanocomposite through a confinement-induced energy dissipation mechanism. This work demonstrates that the confinement-induced molecular bridging mechanism can be extended to thermoset polymers with multifunctional properties, such as excellent thermo-oxidative stability and high service temperatures (>350 °C).
We report a series of β-diketiminate (BDI) complexes in which tautomeric rhenium(iii) hydride and rhenium(i) protio-BDI species readily interconvert between the solid and solution states.
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