IRON CARBONYL-COMPLEXES OF 2-(PHENYLAZO)PYRIDINES - THE 1ST EXAMPLE OF A 6-ELECTRON, BRIDGING 2-(PHENYLAZO)PYRIDINE LIGAND

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(mu-3-N(C6H5))(mu-3-N(2-(5-CF3C5H4N))) (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)3(2+) + Fe(CO)42- + 5CO. When 3c is chromatographed, it decomposes in part to 2-(2-phenylhydrazino)-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 alpha-diimines indicates that the 2-PAP ligands are substantially better pi-acceptors than the alpha-diimines and that there is extensive mixing of the metal and ligand orbitals in the LUMO/HOMO orbitals of 2. H-1, C-13, and F-19 NMR data show that the three 2-PAP ligands in 3 are inequivalent. The diiron complex 3b crystallizes in the monoclinic space group P2(1)/n with a = 9.904 (2) angstrom, b = 19.069 (4) angstrom, c = 18.606 (2) angstrom, beta = 101.82 (2)-degrees, V = 3439.4 (15) angstrom3, 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.