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

Reaction of the thorium metallacycle, [Th{N(R)(SiMe(2))CH(2)}(NR(2))(2)] (R = SiMe(3)) with 1 equiv. of NaNH(2) in THF, in the presence of 18-crown-6, results in formation of the bridged thorium nitride complex, [Na(18-crown-6)(Et(2)O)][(R(2)N)(3)Th(μ-N)(Th(NR(2))(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)(SiMe(2))CH(2)}(NR(2))(2)] by NaNH(2), which results in formation of the thorium bis(metallacycle) complex, [Na(THF)(x)][Th{N(R)(SiMe(2)CH(2))}(2)(NR(2))], and NH(3). NH(3) then reacts with unreacted [Th{N(R)(SiMe(2))CH(2)}(NR(2))(2)], forming [Th(NR(2))(3)(NH(2))] (2), which protonates [Na(THF)(x)][Th{N(R)(SiMe(2)CH(2))}(2)(NR(2))] to give [Na][1]. Consistent with hypothesis, addition of excess NH(3) to a THF solution of [Th{N(R)(SiMe(2))CH(2)}(NR(2))(2)] results in formation of [Th(NR(2))(3)(NH(2))] (2), which can be isolated in 51% yield after work-up. Furthermore, reaction of [K(DME)][Th{N(R)(SiMe(2)CH(2))}(2)(NR(2))] with 2, in THF-d(8), results in clean formation of [K][1], according to (1)H NMR spectroscopy. The electronic structures of [1](–) and 2 were investigated by (15)N NMR spectroscopy and DFT calculations. This analysis reveals that the Th–N(nitride) bond in [1](–) features more covalency and a greater degree of bond multiplicity than the Th–NH(2) bond in 2. Similarly, our analysis indicates a greater degree of covalency in [1](–)vs. comparable thorium imido and oxo complexes.