The MnIV-O stretching frequency is identified at 754 cm -1 in resonance Raman (RR) and infrared (IR) spectra of MnIV porphyrins prepared by electrooxidation of ClMnIIITMP (TMP = tetramesitylporphyrin), ClMnIIITPP (TPP = tetraphenylporphyrin) and ClMnIIIOEP (OEP = octaethylporphyrin) in CH3CN containing tetrabutylammonium hydroxide (TBA(OH)). This vibrational band establishes oxo-MnIV 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 ClMnIIITMPyP (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 MnIIITMPyP 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 VIV, CrIV, MnIV, and FeIV porphyrins, and the anomalous weakness of the MnIV-O bond is attributed to the special stability and low polarizability of the half-filled t32g subshell. Consequences of this weak bond for manganyl reactivity are discussed.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry