CYANIDE COORDINATION TO IRON(III) PORPHYRINS AND COVALENTLY-LINKED DIIRON(III) DIPORPHYRINS

The coordination of cyanide by diiron(III) diporphyrins in chloroform has been studied by H-1 NMR and H-2 NMR spectroscopy in order to determine the reaction pathway and intermediates. The diiron(III) diporphyrin of general formula XFe(III)TTPO(CH2)nOTTPFe(III)X (X = Cl-, Br-, I-;TTPO = 5-(o-(m-,p-)-hydroxy)- 10,1 5,20-tritolylporphyrin dianion) has been synthesized. Two TTPO units are linked via diether moiety (n = 2 or 3) through ortho-, meta-, or para-positions of meso phenyls. The cyanide salts tetrabutylammonium cyanide [TBA] [CN] and bis-(triphenylphosphine)nitrogen(1+) cyanide [BPN][CN] have been used in NMR titrations. The formation of a high-spin, monocyano species has been established in the first reaction step for all systems under investigation. The pyrrole resonances appear at characteristic position at 70 ppm (293 K). The electronic structures of monocyano iron(III) complexes and of high-spin, five-coordinate halide diiron(III) diporphyrins are similar. The monocyano species are converted into low-spin mu-cyano-bridged diiron(III) complexes of the intra- or intermolecular structure as determined by diporphyrin geometry. Two different types of mu-cyano bridged species have been detected by H-1 and H-2 NMP and IR spectroscopy. Reaction of diiron (III) diporphyrin and cyanide in excess results in the formation of low-spin [(CN)2Fe(III)TTPO(CH2)nOTTPFe(III)(CN)2]2- compleXes. The NMR results indicate that the effective C(s) symmetry of the diporphyrin subunits is retained in the diiron(III) diporphyrins. The paramagnetic shifts of tricyano and tetracyano species are in a typical range for low-spin iron(III) porphyrins. The magnetic interaction between two paramagnetic centers of {[(CN)Fe(III)TPO(CH2)nOTTPFe(III)(CN)](mu-CN)}- is small as reflected by paramagnetic shift of pyrrole resonances. The characteristic connectivity patterns have been determined for pyrrole resonances of low-spin iron(III) diporphyrins by means of 2D COSY and NOESY H-1 NMR.