Reduction of hypervalent iodine by coordination to iron(iii) and the crystal structures of PhIO and PhIO₂

The iodine L₃-edge X-ray Absorption Near Edge Structure (XANES) of organic and inorganic iodine compounds with formal iodine oxidation states ranging from -1 to +7 shows edge energies spanning from 4560.8 eV to 4572.5 eV. These were used to calibrate the oxidation state of iodine in a unique iron complex of iodosylbenzene (PhIO), [Fe(tpena)OIPh]²⁺ (tpena^- = N,N,N'-tris(2-pyridylmethyl)ethylenediamine-N'-acetate), which was found to be +1.6. Thus the iodine oxidation state is reduced by 1.4 compared with that in precursor uncoordinated PhIO. On the basis of a combination of X-ray diffraction and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy, we have determined the unknown crystal structure of PhIO, along with a new phase of iodylbenzene (β-PhIO₂) using the Rietveld method. Analogous 1-D chains of halogen bonded [O-IO-I] motifs are the dominant supramolecular interactions between PhIO and PhIO₂ monomers in each structure respectively and the polymeric structures rationalise the general insolubility of these oxygen atom transfer reagents. A double stack of phenyl units in PhIO is found between the layers of the halogen bonded O/I chains. In the case of PhIO, C-Hπ interactions between adjacent phenyl groups result in the alternate phenyl groups lying in parallel planes. Supplementing the strong polymerizing halogen bonds, this supramolecular interaction must exacerbate the insolubility of PhIO. The pillared structure of the new rhombohedral β-PhIO₂ differs significantly from the known monoclinic lamellar phase, α-PhIO₂, described 36 years ago in which the chains form lamellar stacks [N. W. Alcock and J. F. Sawyer, J. Chem. Soc., Dalton Trans., 1980, 115-120].