SYNTHESIS, STRUCTURE, SPECTRA AND METAL REDOX OF IMIDAZOLIUM TRIS(ARYLAZO OXIMATO)IRON(II) - THE ASSOCIATION OF IMIDAZOLE WITH A TRIS CHELATE OXIMATO FACE AND ANALOGY WITH DISTAL IRON-CENTER-DOT-CENTER-DOT-CENTER-DOT-IMIDAZOLE DISPOSITION

The reaction of Na[Fe(RL)3].H2O With imidazole (im) in acidified metha nol affords the green-coloured low-spin (t2(6)) complexes Him[Fe(RL)3] , where [RL-] is [RC(NO)NNPh-] and R is Me, Ph, p-MeC6H4. The X-ray st ructure of the solvate Him [Fe(p-MeC6H4L)3].2MeOH has revealed that th e iron(II) atom is tris-chelated in facial geometry, the FeN6 coordina tion sphere being distorted octahedral. Hydrogen bonding among the oxi mato oxygen, imidazole nitrogen and methanol oxygen atoms affords a ce ntrosymmetric dimeric arrangement. The imidazole ring lies near the ox imato periphery of the tris chelate. The different oximato functions a re differently hydrogen bonded and the stronger the hydrogen bonding t he longer the Fe-N(oxime) length. The shortest non-bonded Fe... N(im) length is 4.260(6) angstrom [motif Fe-NO... N(im)], which can be compa red with the Fe ... N(distal) length in oxyhaemoglobin, 4.34 angstrom, and oxymyoglobin, 4.53 angstrom [motif Fe-OO... N(im)]. The Him[Fe(RL )3] salts have low electrical conductivity in solution, showing that s trong association persists in the dissolved state. An allowed band nea r 600 nm is assigned to the MLCT excitation dpi(Fe) --> pi(azo) on th e basis of an extended Huckel examination of a model system. A quasi-r eversible stereoretentive one-electron couple, [Fe(III)(RL)3]/[Fe(II)( RL)3-], is electrochemically observable, the EPR spectrum of electro-g enerated [Fe(RL)3] being axial (g(parallel-to), approximately 2.0; g(p erpendicular-to)j, approximately 2.1).