Deprotonated aza-crown ligands as simple and effective alternatives to C₅Me₅ in group 3, 4, and lanthanide chemistry




Lee, Lawrence Way Mung

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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.



Ligands, Coordination compounds, Rare earth metals