Iron carbonyl complexes of 2-(phenylazo)pyridines. The first example of a six-electron, bridging 2-(phenylazo)pyridine ligand

Article abstract of DOI:10.1021/om00041a026

2-(Phenylazo)pyridines (1a-c) react with Fe₂(CO)₉ in THF to produce two major products, red brown Fe(CO)₃(L-L′) (2a-c) and green Fe₂(CO)₂(L-L′)₃ (3a-c), where L-L′ = 1a-c. A minor product with 3c is the lavender nitrene-capped complex Fe₃(CO)₉(μ₃-N(C₆H ₅)(μ₃-N(2-(5-CF₃C₅H ₄N))) (4c). 3a also is obtained from the reaction of Fe(1a)₃(PF₆)₂ and Na₂Fe(CO)₄ in THF. Hence, it is proposed that the first step in the formation of 3 from Fe₂(CO)₉ and 1 involves the disproportionation reaction Fe₂(CO)₉ + 31 → Fe(1)₃²⁺ + Fe(CO)₄^2- + 5CO. When 3c is chromatographed, it decomposes in part to 2-(2-phenylhydrazmo)-5-(trifluoromethyl)pyridine, (5-CF₃C₅H₃N)NHNHC₆H₅, in which the azo bond in 1c has been reduced to a hydrazine. Comparison of the infrared and visible spectra of 2 with those of analogous complexes of α-diimines indicates that the 2-PAP ligands are substantially better π acceptors than the α-diimines and that there is extensive mixing of the metal and ligand orbitals in the LUMO/HOMO orbitale of 2. ¹H, ¹³C, and ¹⁹F NMR data show that the three 2-PAP ligands in 3 are inequivalent. The diiron complex 3b crystallizes in the monoclinic space group P2₁/n with a = 9.904 (2) A?, b = 19.069 (4) A?, c = 18.606 (2) A?, β = 101.82 (2)°, V = 3439.4 (15) A?³, and Z = 4. The two iron atoms are in different environments. One iron atom occupies an approximate octahedral environment that consists of three pyridine nitrogen atoms and three azo nitrogen atoms from three 4-CH₃-2-PAP (1b) ligands. The second iron atom occupies a severely distorted octahedral environment that consists of two carbon monoxide carbon atoms and the four azo nitrogen atoms from two of the 4-CH₃-2-PAP ligands. Two of the 4-CH₃-2-PAP ligands bridge the two iron atoms with their azo linkages and serve as six-electron donors overall, while the other 4-CH₃-2-PAP ligand acts as a conventional nonbridging four-electron donor. The azo bond lengths of the bridging ligands are comparable to those of an N-N single bond.