Synthesis, structure and reactivity of rare-earth metallacarborane alkyls [η¹:η⁵-O(CH₂)₂C₂B₉H₉]Ln(σ:η¹-CH₂C₆H₄-o-NMe₂)(THF)₂

Abstract

Rare-earth metallacarborane alkyls can be stabilized by the incorporation of a functional sidearm into both π and σ ligands. Reaction of [Me3NH][7,8-O(CH2)2-7,8-C2B9H10] with one equiv. of Ln(CH2C6H4-o-NMe2)3 gave metallacarborane alkyls [η(1):η(5)-O(CH2)2C2B9H9]Ln(σ:η(1)-CH2C6H4-o-NMe2)(THF)2 (Ln = Y (), Gd (), Er ()) via alkane elimination. They represent the first examples of rare-earth metallacarborane alkyls. Treatment of with RN[double bond, length as m-dash]C[double bond, length as m-dash]NR (R = Cy, (i)Pr) or 2-benzoylpyridine afforded the corresponding mono-insertion products [η(1):η(5)-O(CH2)2C2B9H9]Y[η(2)-(RN)2C(CH2C6H4-o-NMe2)](DME) (R = Cy (), (i)Pr ()) or [η(1):η(5)-O(CH2)2C2B9H9]Y[C5H4NC(Ph)(CH2C6H4-o-NMe2)O](THF)2 (), respectively. Complex also reacted with ArNCO or ArNC (Ar = 2,6-diisopropylphenyl, 2,6-dimethylphenyl) to give di-insertion products [η(1):η(5)-O(CH2)2C2B9H9]Y[OC([double bond, length as m-dash]NC6H3Me2)N(C6H3Me2)C(CH2C6H4-o-NMe2)O](THF)2 () or [η(1):η(5)-O(CH2)2C2B9H9]Y[C([double bond, length as m-dash]NC6H3(i)Pr2)C([double bond, length as m-dash]NC6H3(i)Pr2)(CH2C6H4-o-NMe2)](DME) (). These results showed that the reactivity pattern of the Ln-C σ bond in rare-earth metallacarborane alkyls was dependent on the nature of the unsaturated organic molecules. New complexes were characterized by various spectroscopic techniques and elemental analysis. Some were further confirmed by single-crystal X-ray analysis.

Two possible ways to combine boron and gadolinium for Gd-guided BNCT. A concept

“…Reactions with less sterically hindered electrophiles, such as N,N 0 -di-cyclohexylcarbodiimide, N,N 0 -diisopropyl-carbodiimide, 2-benzoylpyridine, 2,6-dimethylphenyl isocyanate, and 2,6-di-isopropylphenyl isocyanide result in the corresponding products of insertion in metal-carbon bond with retention of the modified dicarbollide ligand coordination. [33] The results obtained demonstrate that the presence of pendant Lewis bases is very important for the stabilization of dicarbollide complexes of lanthanides. Therefore, we expect that a reasonable design of the dicarbollide ligands, including the use of polydentate substituents or the combining of two dicarbollide moieties with flexible linker containing several Lewis base centers, as was done for synthesis of cobalt bis(dicarbollide) embedded crowns and thiacrowns, [34,35] will allow one to obtain air-and stable-gadolinium complexes that can be potentially useful for Gdguided BNCT or combined B þ Gd NCT treatment.…”

“…It is noteworthy that, according to the NMR spectroscopy data, the closo-structure of the complexes is retained in solution in pyridine. [33] Labile tetrahydrofuran ligands can be replaced by 2,4,6tri-tert-butylphenyl isocyanide while maintaining coordination of the remaining ligands with the formation of [3,3-…”