Chromium(III) Porphyrins. Chemical and Spectroscopic Properties of Chloro-meso-tetraphenylporphinatochromium(III) in Nonaqueous Solutions

The spectroscopic and chemical behavior of the six-coordinate adducts formed by the association of chloro-meso-te-traphenylporphinatochromium(III), Cr(TPP)(Cl), and neutral oxygen, sulfur, and nitrogen donor ligands in nonaqueous solutions is described. In nonaqueous media, in the presence of a neutral ligand, Cr(TPP)(Cl) forms six-coordinate adducts of the type Cr(TPP)(Cl)(L) and Cr(TPP)(Cl)(B), where L and B denote neutral N-donor and O- or S-donor ligands, respectively. This behavior is confirmed by visible spectroscopy, spectrophotometric titrations, and conductivity data. Equilibrium constants for the substitution reaction Cr(TPP)(Cl)(B) + L ⇌ Cr(TPP)(Cl)(L) + B are reported. The N-donor ligands are found to bind much more strongly to the Cr(III) center than the O- or S-donors. The replacement of acetone by 1-methylimidazole in the complex Cr(TPP)(Cl)(acetone) in 60% acetone/toluene (v/v) at 25 °C occurs with an equilibrium constant of 4.0 × 10 ⁶ . Fitting the EPR spectra of the six-coordinate adducts obtained from frozen solutions at 78 K to the spin Hamiltonian H=β _e [g _x H _x S _x + g _y H _y S _y + g _z H _z S _z ] + D[S _z ² —%5/4] + E[S _x ² —S _y ² ], and using an assumed isotropic value for g of 1.995, results in values for |D| in the range 0.156 ± 0.012 cm ^-1 for complexes of the type Cr(TPP)(Cl)(L) and values in the range 0.232 ± 0.004 cm ^-1 for the complexes of the type Cr(TPP)(Cl)(B). The only observed exception was the CH3CN adduct for which a value of 0.208 cm ^-1 was obtained. In all cases |E| was found to be small, having a maximum value of 0.013 cm ^-1 for the complex Cr(TPP)(Cl)(acetone). The Cr(octaethylporphyrin) complexes behave similarly.