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.