SYNTHESIS AND STRUCTURE OF TRANSITION-METAL BIS(PORPHYRINATO) COMPLEXES - CHARACTERIZATION OF ZR(TPP)2 AND ZR(OEP)2

Abstract

Treatment of Zr(NEt2)4 with the free-base porphyrins 5,10,15,20-ttraphenylporphyrin (H2TPP) or 2,3,7,8,12,13,17,18-octaethylporphyrin (H2OEP) gives the transition-metal bis(porphyrinato) complexes Zr(TPP)2 and Zr(OEP)2. The hafnium analogue Hf(OEP)2 may be prepared similarly from Hf(NEt2)4. The complexes have been characterized by UV-vis and H-1 NMR spectroscopy, and the molecular structure of Zr(TPP)2 has been determined crystallographically. The Zr-N distances of 2.40 (1) angstrom and the porphyrin-porphyrin interplanar spacing of 2.56 angstrom are the shortest such distances in all known M(porphyrinato)2 complexes. The cyclic voltammograms indicate that Zr(TPP)2 and Zr(OEP)2 each undergo two oxidations and two reductions; the redox potentials suggest that there is significant overlap between the pi-systems of the two porphyrin rings. Chemical oxidation of the Zr(porphyrinato)2 complexes with phenoxathiinylium hexachloroantimonate has led to the isolation of the pi-radical-cation complexes [Zr(TPP)2+][SbCl6-] and [Zr(OEP)2+][SbCl6-]. The UV-vis, near-IR, EPR, and IR spectra of these cations are consistent with oxidation of the porphyrin-porphyrin pi-system; most notable are the unusually high energy near-IR bands at 1110 and 962 nm in the TPP and OEP complexes, respectively. The high energy of these bands with respect to those of other [M(porphyrinato)2+] cations with larger metal atoms again can be rationalized on the basis of unusually strong overlap between the pi-systems of the two porphyrin rings. Crystallographic data for Zr(TPP)2.C5H12: monoclinic, space group C2/c, with a = 21.183 (3) angstrom, b = 21.263 (4) angstrom, c = 18.688 (3) angstrom, beta = 124.57 (1)-degrees, V = 6930.9 angstrom 3, Z = 4; R(F) = 0.077 and R(WF) = 0.083 for 1578 independent reflections with I > 3-sigma(I).