Synthesis of niobium(V) porphyrinates. Crystal structures of tri-µ-oxo-bis[5,10,15,20-tetraphenylporphyrinatoniobium(V)] and acetato-oxo-(5,10,15,20-tetraphenylporphyrinato)niobium(V)–acetic acid (1/1)

Abstract

The action of NbCl₅ on a porphyrin (H₂L) followed by hydrolysis leads to a complex [(NbL)₂O₃]. When treated with acetic acid this complex in turn yields a metalloporphyrin of general formula [NbL(O)(O₂CMe)]·xMeCO₂H. The crystal structures of [{Nb(tpp)}₂O₃]·4C₂H₄Cl₂(1b) and [Nb(tpp)O(O₂CMe)]·MeCO₂H (2b)(tpp is the dianion of 5,10,15,20-tetraphenylporphyrin) have been solved by X-ray diffraction methods. Crystals of (2b) are triclinic, space group P, with a= 16.645(6), b= 11.332(4), c= 11.868(4)Å, α= 105.93(6), β= 100.54(4), γ= 105.05(4)°, and Z= 2. Crystals of (1b) are orthorhombic, space group Pccn, with a= 31.802(9), b= 11.595(5), c= 23.431 (7)Å, and Z= 4. Both structures have been determined from diffractometer data by direct and Fourier methods and refined by full-matrix least-squares calculations to R 0.053 for (2b)(4 038 reflections) and R 0.076 for (1b)(1 555 reflections). In both complexes the niobium atom is seven-co-ordinated by the four nitrogen atoms of the porphyrin and by three oxygen atoms; the co-ordination polyhedron is of the 4 : 3 type and the niobium atom lies 1.00(1) and 0.99(2)Å respectively from the 4 N plane of (2b) and (1b). A comparative study with [{Nb(tpp)}₂O₃]·CHCl₃ is made. A high-resolution n.m.r. study of both complexes shows the anisochrony of the methylenic protons of [{Nb(oep)}₂O₃] and [Nb(oep)O(O₂CMe)]·MeCO₂H induced by the out-of-plane position of the metal atom.