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 (N3-). 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((OBu)-Bu-t)(3))(N(SiMe3)(2))(2)}(2)(mu-N)] (1) and [Cs{U-IV(OSi((OBu)-Bu-t)(3))(2)(N(SiMe3)(2))}(2)(mu-N)] (3-Cs), containing different combinations of OSi(CYBu)(3) and N(SiMe3)(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((OBu)-Bu-t)(3))(2)(N(SiMe3)(2))}(2)(mu-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((OBu)-Bu-t)(3))(2)(N(SiMe3)(2)))(2)(mu-N)(mu-eta(2):eta(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((OBu)-Bu-t)(3))(2)(N(SiMe3)(2))( N)}(mu-N)(2){U-V(OSi((OBu)-Bu-t)(3))(2)(N(SiMe3)(2))}, 8, and [K-4{(OSi((OBu)-Bu-t)(3))(2)U-V)( N))(mu-NH)(mu-kappa(2):C,N-CH2SiMe2NSiMe3){U-V(OSi((OBu)-Bu-t)(3))(2)]K(N(SiMe3)(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.