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Six-coordinate nitrito and nitrato complexes of manganese porphyrin

  • Author(s): Kurtikyan, TS
  • Hayrapetyan, VA
  • Mehrabyan, MM
  • Ford, PC
  • et al.

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© 2014 American Chemical Society. Reaction of small increments of NO2gas with sublimed amorphous layers of MnII(TPP) (TPP = meso-tetra-phenylporphyrinato dianion) in a vacuum cryostat leads to formation of the 5-coordinate monodentate nitrato complex MnIII(TPP)( η1-ONO2) (II). This transformation proceeds through the two distinct steps with initial formation of the five coordinate O-nitrito complex MnIII(TPP)(η1-ONO) (I) as demonstrated by the electronic absorption spectra and by FTIR spectra using differently labeled nitrogen dioxide. A plausible mechanism for the second stage of reaction is offered based on the spectral changes observed upon subsequent interaction of15NO2and NO2with the layered Mn(TPP). Low-temperature interaction of I and II with the vapors of various ligands L (L = O-, S-, and N-donors) leads to formation of the 6-coordinate O-nitrito MnIII(TPP)(L)( η1-ONO) and monodentate nitrato MnIII(TPP)(L)(η1-ONO2) complexes, respectively. Formation of the 6-coordinate O-nitrito complex is accompanied by the shifts of the ν(N=O) band to lower frequency and of the ν(N-O) band to higher frequency. The frequency difference between these bands δν = ν(N=O) - ν(N-O) is a function of L and is smaller for the stronger bases. Reaction of excess NH3with I leads to formation of Mn(TPP)(NH3)(η1-ONO) and of the cation [Mn(TPP)(NH3)2]+plus ionic nitrite. The nitrito complexes are relatively unstable, but several of the nitrato species can be observed in the solid state at room temperature. For example, the tetrahydrofuran complex Mn(TPP)(THF)(η1-ONO2) is stable in the presence of THF vapors (∼5 mm), but it loses this ligand upon high vacuum pumping at RT. When L = dimethylsulfide (DMS), the nitrato complex is stable only to ∼30 °C. Reactions of II with the N-donor ligands NH3, pyridine, or 1-methylimidazole are more complex. With these ligands, the nitrato complexes MnIII(TPP)(L)(η1-ONO2) and the cationic complexes [Mn(TPP)(L)2]+coexist in the layer at room temperature, the latter formed as a result of NO3-displacement when L is in excess.

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