manganese(III) and manganese(II) Anions. Syntheses, Crystal Structures, and Electronic Structures

Article abstract of DOI:10.1021/ja00239a024

The syntheses and X-ray crystal structures of potassium cryptate (1+) salts of the manganese(II) and manganese(III) anions are reported.The complexes are prepared in THF by reaction of Mn(II)TPP with Cl(1-) and O2(2-), respectively.Both complexes crystallize in the space group P2₁/c with four formula units per unit cell.The unit cell of the chloride complex has dimensions a=12.834 A, b=18.999 A, c=23.313 A, β=99.02 at 133 K.The least-squares refinement led to final values R=0.062 and R_w=0.066.The manganese is pentacoordinate, bonded to four pyrrole nitrogens and a chloride in.The manganese atom is displaced 0.641 A from the least-squares plane based on the four pyrrole nitrogen atoms in the direction of the chloride ligand.The unit cell of the manganese(III)-peroxo complex has dimensions a=12.895 A, b=18.836 A, c=23.045 A, β=99.17 at 133 K.The least-squares refinement led to the final values of R=0.057 and R_w=0.067.The manganese is bonded to four pyrrole nitrogens toward the peroxo liagnd.The O-O bond distance is 1.421 (5) A, and the Mn-O distances are 1.901 (4) and 1.888 (4) A.The peroxo ligand is bound side-on to the manganese, eclipsing two of the manganese-pyrrole nitrogen bonds.The binding of dioxygen as peroxide was unexpected because the NMR and optical spectra of (1-) are typical of a manganese(II)- not a manganese(III)-porphyrin complex.Charge iterative extended Huckel calculations show that the strong bonding between the peroxide and the manganese, coupled with the large manganese out-of-plane displacement, leads to an alteration of the d orbital ordering: the highest energy d orbital is a d_yz + O2?_gz hybrid, not the usual d_x2_-y2.This result recounciles the seemingly contradictory spectroscopic and crystallographic data.Occupation of the d_x2_-y2 orbital is consistent with both the large out-of-plane displacement of the Mn atom and the NMR spectra.The high energy of the hybrid d_yz + O2?_gz orbutal accounts for the lack of multiple Soret bands in the optical spectra.