NICKEL-COMPLEXES OF 21-OXAPORPHYRIN AND 21,23-DIOXAPORPHYRIN

The nickel(I) and nickel(II) complexes of 5,20-bis(p-tolyl)-10,15-diph enyl-21-oxaporphyrin (ODTDPPH) and 5,10,15,20-tetraphenyl-21,23-dioxap orphyrin (O2TPP) have been investigated. These oxa analogues of 5,10,1 5,20-tetraarylporphyrin, where one or two pyrrole rings are replaced b y a furan moiety, have been synthesized by condensation of the respect ive precursors, namely 2,5-bis(arylhydroxymethyl)furan, pyrrole, and a rylaldehyde. Insertion of nickel(II) into ODTDPPH or O2TPP yielded hig h-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] r evealed 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 diffracti on, the furan ring is planar and coordinates in the eta(1) fashion thr ough the trigonal oxygen atom; the nickel ion lies in the furan plane for the latter two complexes, but slightly outside it in [(ODTDPP)(NiC l)-Cl-II]. The Ni-N and Ni-O bond lengths decrease upon reduction of h igh-spin five-coordinate [(ODTDPP)Pi(II)Cl] to four-coordinate [(ODTDP P)NP]. The pattern of downfield pyrrole resonances in H-1 NMR spectra of [(ODTDPP)(NiCl)-Cl-II] and [(O2TPP)-(NiCl2)-Cl-II] has been establi shed. The downfield positions of furan resonances are unusual for Ni-I I heteroporphyrins; they have been accounted for by the nearly in-plan t coordination of the furan moiety as opposed to the side-on coordinat ion found for thiophene- or selenophene-containing heteroporphyrins. A n example of ion-pair formation, [(O2TPPH)(2)][(NiCl4)-Cl-II], was pro duced from [(O2TPP)(NiCl2)-Cl-II] by acidification with HCl.