Deprotonated aza-crown ligands as simple and effective alternatives to C₅Me₅ in group 3, 4, and lanthanide chemistry
Date
2017-06-26
Authors
Lee, Lawrence Way Mung
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Abstract
The ability of a deprotonated aza-crown ether to allow isolation of soluble lanthanide and
yttrium complexes has been investigated. A convenient route to these complexes has been
demonstrated by the protonolysis reactions of Ln[(N(SiMe₃)₂]₃ with 4,13-diaza-18-crown-6. NMR
spectroscopy and X-ray crystallography revealed a C₂V structure consisting of a basket shaped
geometry. The successful protonolysis route has been extended to the preparation of stable
alkyls, dialkyls, and alkyl cations of yttrium and zirconium stabilized by deprotonated aza-
crown macrocycles.
A yttrium, alkyl complex containing deprotonated diaza-18-crown-6 has been prepared by the
protonolysis route. The thermal stability and reactivity of this complex were investigated. This
alkyl reacts with terminal alkynes to produce a complex equilibrium between the colourless
monomeric and dimeric alkynides and a purple Z-butatrienediyl (ie.RC=C=C=CR²⁻) coupling product.
NMR studies demonstrate that electron poor alkynes favour coupling and that the carbon-carbon
double bond forming process is readily reversible at room temperature.
The flexibility of the deprotonated diaza-crown ligand is apparent from the isolation of both
cis and trans-zirconium dibenzyl complexes from the protonolysis of tetrabenzyl zirconium with
4,13-diaza-18-crown-6. The structure of both isomers were investigated by NMR spectroscopy and
X-ray crystallography. Both the cis and trans-isomers cleanly converted to the stable cation
either by protonolysis with [n-Bu₃NH]⁺[BPh₄]⁻ or by alkyl abstraction with B(C₆F₅)₃. The reactivity
of the alkyl cation derived from the reaction with B(C₆F₅)₃ was investigated. The reaction of this
cation with t-BuNC gave a vinylamide complex following a 1,2-proton rearrangement of an
initially formed iminoacyl.
Two members of the still rare yttrium dialkyl class of compounds were isolated using
monoanionic, deprotonated aza-crown ethers as supporting ligation. The dialkyl complexes were
synthesized by protonolysis of Y(CH₂SiMe₃)₃(THF)₂ with either aza-18-crown-6 or aza-15-crown-5. NMR
and X-ray analyses of the yttrium dialkyl supported by aza-18-crown-6 indicates a trans-dialkyl
geometry while NMR analysis of the aza-15-crown-5 analog indicates a cis-dialkyl geometry.
Reaction of the trans-dialkyl complex with CO afforded a trans-dienolate complex formed by the
migration of SiMe₃. Alkyl abstraction from the trans-dialkyl complex using B(C₆F₅)₃ allowed
generation of the first yttrium, alkyl cation.
Description
Keywords
Ligands, Coordination compounds, Rare earth metals