Nickel complexes of 21-oxaporphyrin and 21,23-dioxaporphyrin

Article abstract of DOI:10.1002/chem.19970030216

The nickel(1) and nickel(11) complexes of 5,20-bis(p-tolyl)-10,15-diphenyl-21-oxaporphyrin (ODTDPPH) and 5,10,15,20-tetraphenyl-21,23-dioxaporphyrin (O₂TPP) have been investigated. These oxa analogues of 5,10,15,20-tetraarylporphyrin, where one or two pyrrole rings are replaced by a furan moiety, have been synthesized by condensation of the respective precursors, namely 2,5-bis(arylhydroxymethyl) furan, pyrrole, and arylaldehyde. Insertion of nickel(11) into ODTDPPH or O₂TPP yielded high-spin five- and six-coordinate ([(ODTDPP)Ni¹¹Cl] and [(O₂TPP)Ni¹¹Cl₂]) complexes, which can be reduced with moderate reducing reagents. The EPR spectra of[(ODTDPP)Ni¹] and [(O₂TPP)Ni¹Cl] revealed the Ni¹ oxa(dioxa)porphyrin rather than a Ni¹¹ anion radical electronic structure. In the structures of [(ODTDPP)Ni¹¹Cl], [(O₂TPP)Ni¹¹Cl₂], and [(ODTDPP)Ni¹], determined by X-ray diffraction, the furan ring is planar and coordinates in the n¹ fashion through the trigonal oxygen atom; the nickel ion lies in the furan plane for the latter two complexes, but slightly outside it in [(ODTDPP)Ni¹¹Cl]. The Ni N and Ni O bond lengths decrease upon reduction of high-spin five-coordinate [(ODTDPP)Ni¹¹Cl] to four-coordinate [(ODTDPP)Ni¹]. The pattern of downfield pyrrole resonances in ¹H NMR spectra of [(ODTDPP)Ni¹¹Cl] and [(O₂TPP)-Ni¹¹Cl₂] has been established. The downfield positions of furan resonances are unusual for Ni¹¹ heteroporphyrins; they have been accounted for by the nearly in-plane coordination of the furan moiety as opposed to the side-on coordination found for thiophene- or selenophene-containing heteroporphyrins. An example of ion-pair formation, [(O₂TPPH)₂][Ni¹¹Cl₄], was produced from [(O₂TPP)Ni¹¹Cl₂] by acidification with HCl.