Synthesis and characterization of the iron(III) complexes of tetra-(β,β'-tetramethylene)tetraphenylporphyrin, (TC6TPP)FeCl and (TC6TPP)FeONO2

Abstract

The synthesis, NMR and EPR spectroscopic investigation as well as two crystal structures of ( TC 6 TPP ) FeX , where X - = chloride and nitrate are reported. The crystal structure of ( TC 6 TPP ) FeCl reveals an almost equal mixture of saddled and ruffled distortion of the porphyrin as judged by the coefficients of the lowest-frequency vibrational modes (calculated from Normal-Coordinate Structural Decomposition), while ( TC 6 TPP ) FeONO 2 is mainly saddled and more distorted overall. This difference in core structure indicates high conformational flexibility of the TC 6 TPP porphyrin ligand. Overall, both ( TC 6 TPP ) FeX structures have smaller deviation from planarity as compared to five coordinate ( OMTPP ) FeCl and ( OETPP ) FeCl . Therefore, the nature and number of peripheral substituents as well as the axial ligand(s) control geometry and conformation of the porphyrins and fine-tune their spectroscopic properties. EPR data (4.2 K) indicate a predominantly high-spin ( S = 5/2, 97.3%) ground state for ( TC 6 TPP ) FeCl and less pure high-spin state ( S = 5/2, 80%) for ( TC 6 TPP ) FeONO 2. The NMR results support an ideally saddled structure or rapid switching between saddled and ruffled conformations of ( TC 6 TPP ) FeX in solution. The flexibility of the porphyrin core was addressed by using dynamic NMR spectroscopy. The following kinetic parameters for ring inversion were obtained: Δ H ‡ = 24(1) kJ . mol −1, Δ S ‡ = −37(3) J . mol −1. K −1 and Δ H ‡ = 36(1) kJ . mol −1, Δ S ‡ = 20(4) J . mol −1. K −1 for ( TC 6 TPP ) FeCl and ( TC 6 TPP ) FeONO 2, respectively. This results in low free energies of activation, Δ G 298‡ = 35(2) and 30(2) kJ.mol−1, respectively, indicating extremely high flexibility of the porphyrin core in solution (kex298 > 4.2 × 106 and 3.8 × 107 s−1).