TRIOXYGEN-BRIDGED PORPHYRIN DIMERS WITH UNUSUAL MOLECULAR GEOMETRIES - X-RAY CRYSTAL-STRUCTURES OF )(MU-OH)3[ZR(OEP)]2)(7,8-C2B9H12) AND (MU-O)(MU-OH)2[ZR(TPP)]2

The X-ray structures of the title complexes revealed that the two zirc onium porphyrin moieties are joined either by three bridging hydroxo l igands or by one oxo and two hydroxo ligands. In both structures, the two porphyrin macrocycles are perfectly or nearly eclipsed and concomi tantly one of the Zr-O bonds eclipses one of the Zr-N bonds. Such unpr ecedented eclipsed conformation found in both structures strongly sugg ests that it is stabilized by d(pi)-p(pi) interactions between Zr and O and N atoms despite the unfavorable steric interactions.