Binding of Phosphines to Haems

There is little information on the reactivity of haem-phosphine complexes. There is a hint that some complexes of ferrous-P450 are reactive towards 2 but little suggestion that ferric-haems are reduced by tertiary phosphines. In contrast, PH3 does appear to reduce both Fe(III) cytochrome c and Fe(III) cytochrome oxidase (see Sect. 3.8). As discussed in Sect. 2.1 there is a clear difference between the oxidative pathways of PH3 and PR3. It seems likely that PH3 can bind directly to Fe in haem sites (Fe(II) complexes of PH3 are known, and this should be investigated. 

TPP = tetraphenylporphine CP = chloroperoxidase HRP = horseradish peroxidase - the 5th ligand is probably cysteine thiolate Mb = myoglobin CAM = camphor 

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Table 15 summarises some of the data on the binding of phosphines to haems. It can be seen that phosphines bind to both ferric and ferrous porphyrins, the former slightly more strongly than the latter. In both cases the products are low-spin, showing the strong-held nature of the phosphine ligand. 

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