Synthesis and Characterization of an f‑Block Terminal Parent Imido [UNH] Complex: A Masked Uranium(IV) Nitride

Deprotonation of [U­(Tren^TIPS)­(NH₂)] (<b>1</b>) [Tren^TIPS = N­(CH₂CH₂NSiPrⁱ₃)₃] with organoalkali metal reagents MR (M = Li, R = Bu^t; M = Na–Cs, R = CH₂C₆H₅) afforded the imido-bridged dimers [{U­(Tren^TIPS)­(μ-N­[H]­M)}₂] [M = Li–Cs (<b>2a</b>–<b>e</b>)]. Treatment of <b>2c</b> (M = K) with 2 equiv of 15-crown-5 ether (15C5) afforded the uranium terminal parent imido complex [U­(Tren^TIPS)­(NH)]­[K­(15C5)₂] (<b>3c</b>), which can also be viewed as a masked uranium­(IV) nitride. The uranium–imido linkage was found to be essentially linear, and theoretical calculations suggested σ²π⁴ polarized U–N multiple bonding. Attempts to oxidize <b>3c</b> to afford the neutral uranium terminal parent imido complex [U­(Tren^TIPS)­(NH)] (<b>4</b>) resulted in spontaneous disproportionation to give <b>1</b> and the uranium–nitride complex [U­(Tren^TIPS)­(N)] (<b>5</b>); this reaction is a new way to prepare the terminal uranium–nitride linkage and was calculated to be exothermic by −3.25 kcal mol<sup>–1