Metal Complexes with Tetrapyrrole Ligands, LXXII Cerium(IV) Sandwich Complexes with Porphyrin Ligands Linked by Aliphatic and Quinone‐Containing Bridges

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Abstract
Starting from 5‐(4‐hydroxyphenyl)‐10,15,20‐tris(4‐methylphenyl)porphyrin (1a), the new bis(porphyrinylphenoxy)alkanes 4a–c were synthesized by ether formation with the α,ω‐dibromoalkanes Br(CH2)nBr (n = 9,10; 3a, b) and 1,4‐bis(3‐bromopropyl)‐2,5‐dimethoxybenzene (3c). The bis‐[(porphyrinylphenoxy)propyl]hydroquinone dimethyl ether 4c derived from 3c was converted into the diporphyrin‐quinone 4e by methyl ether cleavage with BBr3 and subsequent oxidation with ammonium cerium(IV) nitrate. The bisporphyrins 4a–c, e were metalated with cerium(III) acetylacetonate to yield the new cerium porphyrin sandwich complexes 5a–c, e which carry the alkane links in vicinal positions at the porphyrin rings. 5a–c, e were characterized by UV/Vis, IR, 1H‐NMR spectroscopy and cyclic voltammetry. The quinone‐bridged cerium sandwich complex 5e contains the quinone moiety in a distinct non‐coplanar orientation relative to the porphyrin ligands of the double‐decker system. The length of the lateral chains may have a dramatic effect on the first and second oxidation potentials, E3 and E2, of the porphyrin rings: for 5b having 10 aliphatic C atoms in the chain, E3 and E2 are higher by 0.26 and 0.24 V, respectively, as compared with 5a having only 9 aliphatic C atoms. E3 and E2 for the latter are rather close to values for species with dipropylbenzene chains (i.e. in 5c and 5e) or without lateral bridges.

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