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)

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 1H, 13C NMR spectroscopy. The crystal structures of (acetato)(N-2-furancarboxamido-meso-tetraphenylporphyrinato)cadmium(II) [Cd(N-NHCO-2-C4H3O-tpp)(OAc); 3-trans] and (acetato)(N-2-thiophenecarboxamido-meso-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° and of 31.0°. In 3-trans, Cd2+ and N(5) are located on different sides at 1.06 and −1.49 Å from its 3N plane, while in 4-cis, Cd2+ and N(5) are also located on different sides at 1.04 and −1.53 Å 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 1H and 13C NMR resonances of 3-trans and 4-cis in CDCl3 at 20 and −50 °C.

An attractive electrostatic interaction between Cd2+ and O(4)− atoms in furan stabilizes the O,O-trans conformer of 3 and a repulsive electrostatic interaction between Cd2+ and S(1)+ destabilizes the O,S-trans conformer of 4.Figure optionsDownload as PowerPoint slide