Optical and magnetic resonance studies of the interaction of metallo tetraphenylporphyrins with nitrobenzofuroxan

Abstract

Metallo tetraphenylporphyrins form 1:1 molecular complexes with 4,6-dinitrobenzofuroxan. The molecular association is described in terms of π-π interaction with porphyrins functioning as donors. The association constants and thermodynamic parameters have been evaluated using optical absorption and ¹H nmr spectral methods. Based on the binding constants, the donor ability of various metalloporphyrins can be arranged in the following order: Pd(II) > Co(II) > Cu(II) > Ni(II) ˜ VO(IV) ˜ 2H > Zn(II). Electron paramagnetic resonance studies of the complexes reveal that the π-complexation results in changes in the electronic structure of the central metal ions which are reflected in the changes in the M-N σ bonding. The dipolar contribution to the acceptor proton chemical shifts in the CoTPP complex has been partitioned from ring current contributions using the shifts observed in the ZnTPP complex. The shifts, along with the line broadening ratios observed for the CoTPP complex, are used to arrive at the possible solution structures of the complexes.