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Two Mn2(μ‐O)2 units linked by another two oxo bridges make up the cluster core of 1 (see structure on the right; two bipyridine ligands have been omitted from both Mn1 and Mn4). The MnMn distances and magnetic properties of 1 indicate that this cluster may be a model of the reaction center for oxygen evolution in plants. The question whether the Mn4 unit has a chainlike arrangement or a cubane structure in the naturally occurring enzyme is nevertheless still unanswered.

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The action of thorium tetrachloride or uranium tetrachloride with porphyrins affords the dichloro complexes (Por)MC12 (M = Th or U). The latter derivatives react with sodium acetylacetonate giving rise to bis(acety1acetonato) complexes (Por)M(acac)l. The two series of complexes are characterized on the basis of mass spectral, IR, UV-visible, and NMR data. The crystal structure of the title compound has been determined by X-ra diffraction methods: (OEP)Th(acac),, C46Hs8N404Th; M, = 963; triclinic, Pi; a = 9.493 (7), b = 21.185 (5), c = 24.331 (7) i; (Y = 104.18 (2), B = 96.11 (4), y = 92.35 (4)O; V = 4705 A'; d,,, = 1.359 g Z = 4; T = 298 K. A total of 9019 intensities were measured on a CAD 4 Enraf-Nonius diffractometer in the w-20 scan mode with monochromatized Mo Ka radiation (1.5 < 8 < 20'). The crystal structure was refined to a conventional R(F) = 0.047 for 5323 reflections having a(Z)/Z < 0.33. The thorium atom is octacoordinated by the four porphyrin nitrogen atoms and by the four oxygen atoms of the two acetylacetonato groups.

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