Magnetic stabilisation

In a serendipitous fashion, a novel mixed valence tetranuclear copper(II)/copper(III) dithiocarbamate [2]catenane was prepared in near quantitative yield by partial chemical oxidation of a preformed dinuclear copper(II) naphthyl dtc macrocycle (Scheme 6).49 X-ray structure, magnetic susceptibility, ESMS and electrochemical studies all support the tetranuclear catenane dication formulation. The combination of the lability of copper(II) dtc coordinate bonds and favourable copper(II) dtc-copper(III) dtc charge transfer stabilisation effects are responsible for the high yielding formation of the interlocked... 

In a related study on the cyclopentafused pyrenes [94] in which regular ab initio methods were used (RHF/6-31G and B3LYP/6-31G ), we found that the magnetic properties suggested that the aromatic character decreases upon cyclopentafusion. The aromatic stabilisation energies were unaffected, though. 

These cytochromes contain haem a which differs from the haem of other haem-proteins in that it has an unsaturated substituent, —CHO. In accord with theoretical expectation such a substituent shifts all the absorption bands to lower energy, and increases the intensity of the a/J (especially the a) bands relative to the Soret band. Thus in this series both Fe(II) and Fe(III) haem a complexes have well-pronounced a-bands. The introduction of an aldehyde substituent is also likely to stabilise low-spin as opposed to high-spin states. Thus it is not surprising that magnetic susceptibility data on the cytochromes a show that neither the Fe(II) nor the Fe(III) forms are more than 75% high-spin (133). 

Although, as mentioned, iron(III) complexes are usually more stable than the corresponding iron(II) complexes, the tris(o-phenanthroline)iron (III) ion is much less stable than the tris(o-phenanthroline)iron(II) ion Pj8 for [Fe(phen)3] + is 14, and for (Fe(phen)3] + is 21. Magnetic measurements show the former to be a high-spin complex and the latter to be a low-spin one. The additional stabilisation conferred on the complex by the filling of an (w—l)d orbital of the cation more than overcomes the effect of the stronger electrostatic field. 

---