Synthesis and Chemistry of Carboxylate-Bridged Polymetallic Complexes

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This award in the Inorganic, Bioinorganic, and Organometallic Chemistry Program provides continued support for research on carboxylate-bridged metal dimers by Dr. Stephen J. Lippard, Chemistry Department, Massachusetts Institute of Technology. Metalloproteins containing such units are responsible for many biological functions, including oxygen transport, methane hydroxylation, and hydrolysis of phosphate esters. In this study, three lines of investigation will be pursued: oxidation chemistry of diiron complexes as models for the active site of the hydroxylase component of methane monooxygenase; metallohydrolase chemistry that mimics urease and metalophosphatase; and sugar isomerization reactions to model D-xylose isomerase. Emphasis will be placed on designing dinucleating ligands that have sufficient internal preorganization so that they fold into shapes that create the appropriate cavities for metal binding. One class of ligands is dicarboxylates in which two Kemp's triacid anhydride-chloride moities are linked by a bridging 1,3-diaminoxylene, 1,4-diaminophthalazine, or bis(aminomethylphenyl)porphyrin group. Symmetric and unsymmetrically substituted amidinate ligands, that are isoelectronic with carboxylate, will also be investigated. Attempts will also be made to functionalize mono-carboxylate bridged systems using a multicomponent ligand with two terminal bidentate units joined by one bridging carboxylate spacer. The compounds synthesized will be tested to see the extent to which they mimic the structures, physical properties, and reactivities of the biological prototypes, including intermediates in catalytic cycles. Many biologically-important hydroxylases, hydrolases, and isomerases function because of a reaction center that contains two linked metal atoms and a surrounding carboxylate group. In order to understand the details of how these enzymes work, model compounds will be designed, synthesized, and tested for catalytic activity.