Studies of electronic communication between dimolybdenum cores joined by various bridges

Abstract

A series of metal-organic complexes which all contain two bridged dimolybdenum cores were synthesized and studied. Common building blocks involved in this series of syntheses include Mo2(DAniF)3(O2CCH3) (DAniF = N, N'-di-panisylformamidinate) and [Mo2(cis-DAniF)2(NCCH3)4](BF4)2. Bridges that were used to connect two different dimolybdenum cores in the synthesized structures include single metal complexes like ZnCl2 and Ni(acac)2 (acac = acetyl acetone), dimetal complex like Rh2(O2CCH3)4, as well as organic ligands like 1,2-dihydroxyl-4,5-dimethylaminbenzene and 1,3-dihydroxyl-2,5-dimethylaminbenzene. Several heterometallic supramolecules were obtained through self-assembly reactions. In these structures, the two dimolybdenum cores were bridged through different metal complexes; between these metal complexes and the molybdenum cores, isonicotinic acid anion acts as the key linkage. Depending on the geometry of the building blocks and their available binding site, these heterometallic supramolecules bear a variety of shapes, which include rod-like molecules with three metal centers, a squareshaped molecule with its four corners occupied by metal complexes, and also a zigzagshaped infinite metal complex chain. Although these molecules do show reversible redox peaks in electrochemistry studies, they demonstrated very poor electronic communication between the dimolybdenum centers. Possible explanations to this result may be that the dimolybdenum cores are far away from each other in these molecules (Mo2?Mo2 separation in compound 4, being 21 ?, is the longest among all dimolybdenum pairs synthesized to date) and that the calculated frontier orbital overlaps do not favor electron delocalization over the entire molecule. However, another type of molybdenum dimer of dimers where the dimolybdenum centers are united by conjugated organic ligands, namely 1,2-dihydroxyl- 4,5-dimethylaminbenzene and 1,3-dihydroxyl-2,5-dimethylaminbenzene, were also synthesized and found to bear significantly stronger electronic communication between the Mo2 centers. In fact, as electrochemistry reveals, these molecules demonstrated the greatest comproportionation constant values (Kc ~ 1014) than any other analogues synthesized so far. This interesting result is most likely due to the well conjugated linker ligands that would allow electrons on the metal centers to delocalize over the entire molecule. Computational studies of these compounds also show clear evidence of ? overlapping in their molecular frontier orbitals.