Oxomanganese(IV) Porphyrins Identified by Resonance Raman and Infrared Spectroscopy: Weak Bonds and the Stability of the Half-Filled t_2g Subshell

The Mn^IV-O stretching frequency is identified at 754 cm ^-1 in resonance Raman (RR) and infrared (IR) spectra of Mn^IV porphyrins prepared by electrooxidation of ClMn^IIITMP (TMP = tetramesitylporphyrin), ClMn^IIITPP (TPP = tetraphenylporphyrin) and ClMn^IIIOEP (OEP = octaethylporphyrin) in CH₃CN containing tetrabutylammonium hydroxide (TBA(OH)). This vibrational band establishes oxo-Mn^IV bond formation under these conditions and shows the bond to be anomalously weak. When the oxidation is carried out in CH₂C1₂ containing TBA(OH), or when ClMn^IIITMPyP (TMPyP = tetrakis(methylpyridinium)porphyrin) is electrooxidized in aqueous 1 M NaOH, the Mn-O stretching band is seen at 711 cm^-1. This band is associated with a trans OH^- adduct, as shown by its anomalous upshift, to 732 cm^-1, in D₂O; the lower frequency in H₂O is attributed to interaction with the Mn-O-H bend of the trans OH^- ligand, which is relieved when H is replaced by D. EPR spectra of all these species show strong g_⊥~ 4.0 and weak g_∥~ 2.0 signals characteristic of a high-spin d³ configuration. The Mn^IIITMPyP species in 1 M NaOH is shown to be a trans dihydroxide adduct by virtue of a 495-cm^-1 RR band identified, on the basis of its H₂¹⁸O and D₂O shifts, as the symmetric Mn-(OH)₂ stretch. The M-O stretching frequencies are compared for V^IV, Cr^IV, Mn^IV, and Fe^IV porphyrins, and the anomalous weakness of the Mn^IV-O bond is attributed to the special stability and low polarizability of the half-filled t³_2g subshell. Consequences of this weak bond for manganyl reactivity are discussed.