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

The nickel(I) and nickel(II) 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(II) into ODTDPPH or O2TPP yielded high-spin five- and six-coordinate ([(ODT-DPP)(NiCl)-Cl-II] and [(O2TPP)(NiCl2)-Cl-II]) complexes, which can be reduced with moderate reducing reagents. The EPR spectra of [(ODTDPP)Ni-I] and [(O2TPP)(NiCl)-Cl-I] revealed the Ni-I oxa(dioxa)porphyrin rather than a Ni-II anion radical electronic structure. In the structures of [(ODTDPP)(NiCl)-Cl-II], [(O2TPP)(NiCl2)-Cl-II], and [(ODTDPP)Ni-I], determined by Xray diffraction, the furan ring is planar and coordinates in the eta(1) 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)(NiCl)-Cl-II]. The Ni-N and Ni-O bond lengths decrease upon reduction of high-spin five-coordinate [(ODTDPP)Pi(II)Cl] to four-coordinate [(ODTDPP)NP]. The pattern of downfield pyrrole resonances in H-1 NMR spectra of [(ODTDPP)(NiCl)-Cl-II] and [(O2TPP)-(NiCl2)-Cl-II] has been established. The downfield positions of furan resonances are unusual for Ni-II heteroporphyrins; they have been accounted for by the nearly in-plant 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)][(NiCl4)-Cl-II], was produced from [(O2TPP)(NiCl2)-Cl-II] by acidification with HCl.