Studies on cobalt(II), nickel(II) and copper(II) derivatives of some new meso-aryl substituted octabromoporphyrins

A series of Co(II), Ni(II) and Cu(II) derivatives of a variety of porphyrins with tolyl (H(2)TxTP) and naphthyl (H(2)NxTP) moieties as meso-substituents are synthesised and characterised. H(2)TxTP has the tolyl functions bonded to meso-carbons at the o-, m- or p-positions of the tolyl moieties while H(2)NxTP has naphthyl groups bonded through their alpha- or beta-positions. Their octabromo derivatives (MTxOBP and MNxOBP) were also synthesised by electrophilic substitution at the beta-pyrrole positions. Optical spectra showed that the Q bands remain almost unaltered for both tolyl and naphthyl derivatives of the nonbrominated porphyrins while the B bands of their naphthyl derivatives are seen to be more red-shifted than their tolyl analogues. Even though a similar trend based on the meso-substituents is observed among all MTxOBP and MNxOBP derivatives, a substantial red-shift is seen for all the bromoderivatives compared to their nonbrominated species MTxTP and MNxTP. The energy difference AV associated with this red-shift of the Soret band is seen to be in the range 1700-2050 cm(-1) for Co(II) and Ni(II) derivatives, the naphthyl derivatives always having higher values. Energy level reordering due to electron-withdrawing Br-substituents, distortion bringing about non-planarity in the pi-framework resulting in HOMO-LUMO level changes and the mesomeric effect due to meso-aryl moieties can be considered to be the reasons for the observed spectral changes. The EPR spectra of the Cu(II) derivatives of bromoporphyrins have lower A(parallel to)(Cu) and A(-)(Cu) hyperfine values compared to their nonbrominated analogues, indicating enhanced delocalisation of electron spin to the octabromoporphyrinato moiety. An enhanced Cu-N sigma-covalency is seen in the Cu(II)-bromoporphyrins, as evident from their lower alpha(2) values. (c) 2005 Elsevier Ltd. All rights reserved.