Direct synthesis of six-coordinate nitro complexes of cobalt meso-tetraphenylporphyrinate with trans-O-donor ligands

The formation of a series of six-coordinate complexes (B)Co(TPP)(NO₂) (B is ketone, aldehyde, ether or epoxide; TPP is meso-tetraphenylporphyrin dianion) on exposure of thin films of five-coordinate Co(TPP)(NO₂) to vapors of the O-donor ligands was established by IR and UV/Vis spectroscopy (resorting to the data on isotope-substituted ¹⁵NO₂). The complex formation is accompanied by shifts of the vibration frequencies of the trans-NO₂ ligand and gives rise to new IR bands corresponding to the O-donors in the ν(C=O) or ν(C-O) region shifted with respect to these bands of free ligands. The complexes are rather stable in the solid phase but eliminate the O-donor ligand upon dissolution in an inert solvent (or upon maintenance in a vacuum for some complexes) being converted into the initial five-coordinate nitro complex. Using the IR spectra of CCl₄ solutions of the complexes containing a large excess of O-donor molecules, the equilibrium constants and the formation enthalpies of some (B)Co(TPP)(NO₂) complexes were determined, indicating weak coordination of these ligands.