Use of ¹⁵N NMR spectroscopy to probe covalency in a thorium nitride

Abstract

Reaction of the thorium metallacycle, [Th{N(R)(SiMe₂)CH₂}(NR₂)₂] (R = SiMe₃) with 1 equiv. of NaNH₂ in THF, in the presence of 18-crown-6, results in formation of the bridged thorium nitride complex, [Na(18-crown-6)(Et₂O)][(R₂N)₃Th(μ-N)(Th(NR₂)₃] (<b>[Na][1]</b>), which can be isolated in 66% yield after work-up. Complex <b>[Na][1]</b> is the first isolable molecular thorium nitride complex. Mechanistic studies suggest that the first step of the reaction is deprotonation of [Th{N(R)(SiMe₂)CH₂}(NR₂)₂] by NaNH₂, which results in formation of the thorium bis(metallacycle) complex, [Na(THF) _<i>x</i> ][Th{N(R)(SiMe₂CH₂)}₂(NR₂)], and NH₃. NH₃ then reacts with unreacted [Th{N(R)(SiMe₂)CH₂}(NR₂)₂], forming [Th(NR₂)₃(NH₂)] (<b>2</b>), which protonates [Na(THF) _<i>x</i> ][Th{N(R)(SiMe₂CH₂)}₂(NR₂)] to give <b>[Na][1]</b>. Consistent with hypothesis, addition of excess NH₃ to a THF solution of [Th{N(R)(SiMe₂)CH₂}(NR₂)₂] results in formation of [Th(NR₂)₃(NH₂)] (<b>2</b>), which can be isolated in 51% yield after work-up. Furthermore, reaction of [K(DME)][Th{N(R)(SiMe₂CH₂)}₂(NR₂)] with <b>2</b>, in THF-<i>d</i> ₈, results in clean formation of <b>[K][1]</b>, according to ¹H NMR spectroscopy. The electronic structures of <b>[1]^-</b> and <b>2</b> were investigated by ¹⁵N NMR spectroscopy and DFT calculations. This analysis reveals that the Th-N_nitride bond in <b>[1]^-</b> features more covalency and a greater degree of bond multiplicity than the Th-NH₂ bond in <b>2</b>. Similarly, our analysis indicates a greater degree of covalency in <b>[1]^-</b> <i>vs.</i> comparable thorium imido and oxo complexes.