Nitrogen activation and cleavage by a multimetallic uranium complex.

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