Transition metal complexes with sulfur ligands CVI. Phosphine and azido [Fe(L)('N_HS₄')] complexes; structural characterization of the...

Title full: Transition metal complexes with sulfur ligands CVI. Phosphine and azido [Fe(L)('N_HS₄')] complexes; structural characterization of the key complex [Fe('N_HS₄')]·THF (L=PMe₃, PBu₃, PMe₂Ph, PMePh₂, N₃^-; 'N_HS₄'^2-=dianion of 2,2′-bis-(2-mercaptophenylthio)diethylamine). In search of model compounds exhibiting structural and functional characteristics of the active sites of nitrogenases the phosphine complexes [Fe(L)('N_HS₄')](L=PMe₃ (1), PMe₂Ph₂ (2), PBu₃ (3), PMePh₂ (4)), the azido complex (NEt₄)[Fe(N₃)('N_HS₄')] (5) and [Fe('N_HS₄')]·THF (6) were investigated as potential precursors for the synthesis of the hypothetical N₂ complex [{Fe('N_HS₄')}_x(N₂)] (x=1 or 2). 1, 2, 3, 5 and 6 were characterized by X-ray structure analyses. The phosphine complexes [Fe(PR₃)('N_HS₄')] have low-spin Fe(II) centers and chiral [Fe('N_HS₄')] cores. PMePh₂ was the bulkiest phosphine (cone angle 136°) which could be coordinated to the [Fe('N_HS₄')] fragment. The phosphine ligands are labile and rapidly substituted by CO but not by N₂(NEt₄)[Fe(N₃)('N_HS₄ ')] (5) is an Fe(lI) high-spin complex exhibiting an achiral [Fe('N_HS₄')] core and long iron-donor distances analogous to other high-spin [Fe(L)('N_HS₄')] complexes. The azido ligand is labile and could neither oxidatively nor thermally be transformed into an N₂ ligand. The amine donor in [Fe(CO)('N_HS₄')] was deprotonated and subsequently alkylated yielding the achiral low-spin complex [Fe(CO)('N_MeS₄')]. [Fe('N_HS₄']·THF (6) forms a kind of intermediate between high-spin and low-spin complexes, and is a key complex in explaining the reactivity and structure of [Fe(L)('N_HS₄')] complexes. 6 contains an iron center which is surprisingly only five-coordinate. The structure and distances of chiral 6 closely resemble the [Fe('N_HS₄')] cores of low-spin [Fe(L)('N_HS₄')] complexes, whereas the paramagnetism and high reactivity of 6 are typical of high-spin [Fe(L)('N_HS₄')] complexes.