Electrostatic forces that determine O,O-trans versus O,S-cis conformers in the aminated porphyrin complexes of Cd(N-NHCO-2-C4H3O-tPP)(OAc) and Cd(N-NHCO-2-C4H3S-tpp)(OAc)

Abstract

Two new diamagnetic, mononuclear and aminated porphyrin complexes of O,O-trans-Cd (3-trans) and O,S-cis-Cd (4-cis) have been synthesized and characterized by H-1, C-13 NMR spectroscopy. The crystal structures of (acetato)(N-2-furancarboxamido-i;teso-tetraphenylporphyrinato)cadmium(II) [Cd(N-NHCO-2-C4H3O-tpp)(OAc); 3-trans] and (acetato)(N-2-thiophenecarboxamido-ineso-tetraphenylporphyrinato)cadmium(II) [Cd(N-NHCO-2-C4H3S-tpp)(OAc); 4-cis] were determined. The coordination sphere around Cd2+ is a distorted square-based pyramid in which the apical site is occupied by a bidentate chelating OAc- group for 3-trans and 4-cis. The plane of three pyrrole nitrogen atoms [i.e., N(1), N(2), N(4) for 3-trans and N(1), N(2), N(3) for 4-cis] strongly bonded to Cd2+ is adopted as a reference plane 3N. The N(3) and N(4) pyrrole rings bearing the 2-furancarboxamido (Fr) and 2-thiophenecarboxamido groups in 3-trans and 4-cis, respectively, deviate mostly from the 3N plane, thus orienting separately with a dihedral angle of 33.4 degrees and of 31.0 degrees. In 3-trans, Cd2+ and N(5) are located on different sides at 1.06 and -1.49 angstrom from its 3N plane, while in 4-cis, Cd2+ and N(5) are also located on different sides at 1.04 and -1.53 angstrom from its 3N plane. An attractive electrostatic interaction between the Cd2+ and O(4)(-) atoms in furan stabilizes the O,O-trans conformer of 3. A repulsive electrostatic interaction between Cd2+ and S(1)(+) destabilizes the O,S-trans conformer of 4. Both of these repulsive and the mutually attractive interactions between S(1)(+) and O(3)(-) atoms favor the O,S-cis rotamer of 4 both in the vapor phase and in low polarity solvents. NOE difference spectroscopy, HMQC and HMBC were employed for the unambiguous assignment of the H-1 and C-13 NNIR resonances of 3-trans and 4-cis in CDCl3 at 20 and -50 degrees C. (c) 2006 Published by Elsevier Ltd.