Interplay between Sn(IV) N-Confused Porphyrin Dianionic and Trianionic Forms Elucidated by Experiment and Theory

Abstract

In this report, we present the formation and the X-ray crystal structure of the [SnIVNCP(3-)Py2](SnCl3) complex upon direct metalation of the H2NCP(2-) ligand (NCP = 5,10,15,20-tetraphenyl-N-confused porphyrin) with a large excess of SnCl2 in pyridine in dry anaerobic conditions. Reaction of this complex with dry and wet (NBu4)Cl results in the formation of the anionic [SnIVNCP(3-)Cl2]− and neutral SnIVNCP(2-)Cl2 complexes, respectively, indicating the ability to manipulate the electronic structure of the NCP ligand by stabilizing the external NH proton in part via the formation of a hydrogen bond. Similarly, reaction of the [SnIVNCP(3-)Py2]+ cation with (NBu4)OH or water results in the formation of [SnIVNCP(3-)(OH)2]− and SnIVNCP(2-)(OH)2 complexes, respectively. UV–vis-NIR and magnetic circular dichroism (MCD) spectroscopies are indicative of a significantly larger Qx-Qy energy gap for the NCP(2-) core compared to the NCP(3-) ionic form. Density Functional Theory (DFT) calculations confirm that a much smaller difference between ΔHOMO and ΔLUMO values for the NCP(2-) ligand (ΔHOMO is the energy gap between Gouterman’s “a1u” and “a2u” MOs, and ΔLUMO is the energy gap between Gouterman’s “eg(x)” and “eg(y)” MOs) is responsible for quenching of the magnetic orbital moment of the Qx-band in all NCP(2-)-containing complexes, reflected in MCD signal magnitudes. Time-Dependent DFT (TDDFT) calculations on all complexes correlate well with the experimental data and reveal specific spectroscopic signatures of the NCP(2-) and NCP(3-) systems as a function of the axial ligands and intermolecular hydrogen bond.