The utility of lithium versus potassium and sodium in forming new +2 ions of the rare-earth metals as well as reduced dinitrogen complexes containing (N2)2- ions via LnA3/M reactions (Ln = rare-earth metal; A = anion; M = alkali metal) has been evaluated. Under argon, Li reduces Cp'3Y (Cp' = C5H4SiMe3) in the presence of 2.2.2-cryptand (crypt) to form crystallographically-characterized [Li(crypt)][Cp'3Y] which has an EPR spectrum identical to previously reported [K(crypt)][Cp'3Y]. Under dinitrogen, [Li(crypt)][Cp'3Y] is also formed which contrasts with LnA3/M/N2 reactivity with M = Na and K that typically form complexes of (N2)2- and (N2)3- and no identifiable Ln2+ compounds. This is particularly unusual since lithium can reduce dinitrogen on its own. However, the Cp'3Y/Li/N2 reaction product, [Li(crypt)][Cp'3Y], formed in situ, eventually produces [Cp'2(THF)Y]2(μ-η2:η2-N2), which constitutes the first evidence of Ln2+ intermediates in LnA3/M reduction of dinitrogen. The late lanthanides, Tb, Dy, and Ho, display chemistry similar to yttrium in that Cp'3Ln complexes are reduced by lithium under argon to form the crystallographically-characterized [Li(crypt)][Cp'3Ln] complexes. Lithium in all four of these [Li(crypt)][Cp'3Ln] complexes displays an unusual N2O4 donor atom coordination geometry within the crypt. Reductions of Cp'3Tb and Cp'3Ho with lithium naphthalenide did not yield Ln2+ complexes. Instead, [Li(crypt)][Cp'4Tb] and [Li(crypt)][(Cp'3Ho)2(μ-H)] were crystallographically identified. The common NO5 donor atom set was found in the [Li(crypt)]+ cation of the former complex, while the cation in the latter complex had six oxygen atoms coordinated to lithium in a [Li(crypt)]+ geometry not previously reported in the literature.

Comparison of Li with K and Na in rare-earth metal reduction chemistry: Isolation of [Li(2.2.2-cryptand)]+ in unique coordination environments

253rd National Meeting (2017)

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