Oxo-bridged manganese complexes have attracted much attention recently,2 principally because of their potential relevance to active sites in enzymes such as the Lactobacillus plantarum manganese catalase3 and the oxygen-evolving complex in photosystem I I (PSI1 OEC).4 Among structurally characterized binuclear manganese oxo complexes, the [ M n (~-o )~M n ]~+ /~+ /~+ core has become relatively common in the last several years.5 There are also examples of the following oxo-bridged binuclear 02CR)]+/2+,7 and [ M n (~-0 ) , M n ]~+ . * On the other hand, binuclear complexes in which manganese( 111) atoms are bridged by only a single oxo group are quite rare,9-ii and in the known cases, the terminal ligands are not relevant to the biological systems mentioned above.12 Reported herein are the notable structural, magnetic, and reactivity properties of a single oxo-bridged species with biologically relevant donors, [Mn20(5-N02saldien),] (1). structural types: [ M~( P -O ) ( P -O~C R )~M~]~+ /~+ ,~ [Mn(P-O)z(Pc-( I ) Abbreviations used: 5-NO2saldien = N,N'-bis(5-nitrosalicy1idene)-1,7-diamino-3-azapentane; PSI1 OEC = photosystem I1 oxygen-evolving complex; HB(pz)3 = hydridotris(pyrazo1-I-yl)borate; tacn = 1,4,7triazacyclononane; bpy = 2,2'-bipyridine. phthal = phthalocyanine; tphpn = tetrakis( 2-methylpyridyl)-2-hydroxypropane-l,3-diamine; py = pyridine. (2) (a) Christou, G. Elsevier: Amsterdam, 1987; pp ( 5 ) (a) Hagen, K. S.; Armstrong, W. H.; Hope, H. Inorg. Chem. 1988, 27, 967-969. (b) Plaskin, P. M.; Stoufer, R. C.; Mathew, M.; Palenik, G.
