Structural Information on Supramolecular Copper(II) -Diketonate Complexes from Atomic Force Microscopy and Analytical Ultracentrifugation

Supramolecular Cu(II) complexes were prepared from two trifunctional beta-diketone ligands. The ligands (CH3Si(phacH)(3) and CH3Si(phprH)(3), represented by LH3) contain three aryl-beta-diketone moieties joined by an organosilicon group. The complexes have the empirical formula Cu3L2, as expected for combinations of Cu2+ and L3-. Several metal-organic polyhedra (MOPs) [Cu3L2](n) are possible (n = 1-10); a dodecahedron (Cu30L20; n = 10; estimated diameter of ca. 5 nm) should be the most stable because its internal bond angles would come closest to ideal values. Atomic force microscopy (AFM), performed on samples deposited from solution onto mica substrates, revealed a distribution of sample heights in the 0.5-3.0 nm range. The most commonly observed heights were 0.5-1.5 nm, corresponding to the smallest possible molecules (Cu3L2, i.e., n = 1). Some molecular cubes (Cu12L8; ca. 2.5 nm) or larger molecules or aggregates may be present as well. Equilibrium analytical ultracentrifugation (AUC) was also used to probe the compounds. A previously reported reference compound, the molecular square Cu-4(m-pbhx)(4) (M = 2241 g mol(-1)), behaved well in AUC experiments in four nonpolar organic solvents. AUC data for the new tris(beta-diketonate) MOPs [Cu3L2](n) in toluene and fluorobenzene did not agree well with the theoretical results for a single solute. The data were fit well by a two-solute model, but these results were not consistent in the two solvents used, and some run-to-run variability was noted even in the same solvent. Also, the calculated molecular weights differed significantly from those expected for [Cu3L2](n) ([Cu-3(CH3Si(phac)(3))(2)](n), multiples of 1322 g mol(-1); or [Cu-3(CH3Si(phpr)(3))(2)](n), multiples of 1490 g mol(-1)).