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

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)(3)Th(mu-N)(Th(NR2)(3)] ([Na][1]), which can be isolated in 66% yield after work-up. Complex [Na] [1] 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)(x)][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)] (2), which protonates [Na(THF)(x)][Th{N(R)(SiMe2CH2)}(2)(NR2)] to give [Na][1]. 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)] (2), which can be isolated in 51% yield after work-up. Furthermore, reaction of [K(DME)][Th{N(R)(SiMe2CH2)}(2)(NR2)] with 2, in THF-d(8), results in clean formation of [K][1], according to H-1 NMR spectroscopy. The electronic structures of [1](-) and 2 were investigated by N-15 NMR spectroscopy and DFT calculations. This analysis reveals that the Th-Nnitride bond in [1](-) features more covalency and a greater degree of bond multiplicity than the Th-NH2 bond in 2. Similarly, our analysis indicates a greater degree of covalency in [1](-) vs. comparable thorium imido and oxo complexes.