Formation of μ-dinitrogen (salen)osmium complexes via ligand-induced N⋯N coupling of (salen)osmium(vi) nitrides

Treatment of [NⁿBu₄][Os^VI(N)Cl ₄] with a stoichiometric amount of H₂L (L = N,N′-bis(salicylidene)-o-cyclohexylenediamine dianion) in the presence of PF₆^- or ClO₄^- in MeOH affords [Os^VI(N)(L)(OH₂)](PF₆) 1a and [Os ^VI(N)(L)(CH₃OH)](ClO₄) 1b, respectively. The structure of 1b has been determined by X-ray crystallography and the OsN bond distance is 1.627(3) Å. In the presence of a N-donor heterocyclic ligand in CH₃CN, 1a reacts at room temperature to afford the mixed-valence μ-N₂ (salen)osmium species [(X)(L)Os^III-NN-Os ^II(L)(X)](PF₆), 2-14 (X = py 2; 4-Mepy 3; 4- ^tBupy 4; pz 5; 3-Mepz 6; 3,5-Me₂pz 7; Im 8; 1-MeIm 9; 2-MeIm 10; 4-MeIm 11; 1,2-Me₂Im 12; 2-Meozl 13; 4-MeTz 14). These complexes are formed by ligand-induced N⋯N coupling of two [Os ^VIN]⁺ to give initially [Os^III-N ₂-Os^III]²⁺, which is then reduced to give the more stable mixed-valence species [Os^III-N₂-Os ^II]⁺. Cyclic voltammograms (CVs) of 2-14 show two reversible couples, attributed to Os^III,III/Os^III,II and Os^III,II/Os^II,II. The large comproportionation constants (K_com) of (5.36-82.3) × 10¹³ indicate charge delocalization in these complexes. The structures of 3 and 14 have been determined by X-ray crystallography, the salen ligands are in uncommon cis-β configuration. Oxidations of 4 and 14 by [Cp₂Fe](PF ₆) afford the symmetrical species [(X)(L)Os^III-NN-Os ^III(L)(X)](PF₆)₂ (X = 4-^tBupy 15; 4-MeTz 16). These are the first stable μ-N₂ diosmium(iii,iii) complexes that have been characterized by X-ray crystallography.