Nitrogen activation and cleavage by a multimetallic uranium complex

Multimetallic-multielectron cooperativity plays a key role in the metal-mediated cleavage of N(2) to nitrides (N(3−)). In particular, low-valent uranium complexes coupled with strong alkali metal reducing agents can lead to N(2) cleavage, but often, it is ambiguous how many electrons are transferred from the uranium centers to cleave N(2). Herein, we designed new dinuclear uranium nitride complexes presenting a combination of electronically diverse ancillary ligands to promote the multielectron transformation of N(2). Two heteroleptic diuranium nitride complexes, [K{U(IV)(OSi(O(t)Bu)(3))(N(SiMe(3))(2))(2)}(2)(μ-N)] (1) and [Cs{U(IV)(OSi(O(t)Bu)(3))(2)(N(SiMe(3))(2))}(2)(μ-N)] (3-Cs), containing different combinations of OSi(O(t)Bu)(3) and N(SiMe(3))(2) ancillary ligands, were synthesized. We found that both complexes could be reduced to their U(iii)/U(iv) analogues, and the complex, [K(2){U(IV/III)(OSi(O(t)Bu)(3))(2)(N(SiMe(3))(2))}(2)(μ-N)] (6-K), could be further reduced to a putative U(iii)/U(iii) species that is capable of promoting the 4e(−) reduction of N(2), yielding the N(2)(4−)complex [K(3){U(V)(OSi(O(t)Bu)(3))(2)(N(SiMe(3))(2))}(2)(μ-N)(μ-η(2):η(2)-N(2))], 7. Parallel N(2) reduction pathways were also identified, leading to the isolation of N(2) cleavage products, [K(3){U(VI)(OSi(O(t)Bu)(3))(2)(N(SiMe(3))(2))([triple bond, length as m-dash]N)}(μ-N)(2){U(V)(OSi(O(t)Bu)(3))(2)(N(SiMe(3))(2))}](2), 8, and [K(4){(OSi(O(t)Bu)(3))(2)U(V))([triple bond, length as m-dash]N)}(μ-NH)(μ-κ(2):C,N-CH(2)SiMe(2)NSiMe(3))-{U(V)(OSi(O(t)Bu)(3))(2)][K(N(SiMe(3))(2)](2), 9. These complexes provide the first example of N(2) cleavage to nitride by a uranium complex in the absence of reducing alkali metals.