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

Supramolecular Cu(II) complexes were prepared from two trifunctional β-diketone ligands. The ligands (CH 3 Si(phacH) 3 and CH 3 Si(phprH) 3 , represented by LH 3 ) contain three aryl-β-diketone moieties joined by an organosilicon group. The complexes have the empirical formula Cu 3 L 2 , as expected for combinations of Cu 2+ and L 3- . Several metal-organic polyhedra (MOPs) [Cu 3 L 2 ] n are possible ( n = 1-10); a dodecahedron (Cu 30 L 20 ; 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 (Cu 3 L 2 , i.e., n = 1). Some molecular cubes (Cu 12 L 8 ; 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(β-diketonate) MOPs [Cu 3 L 2 ] 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 [Cu 3 L 2 ] n ([Cu 3 (CH 3 Si(phac) 3 ) 2 ] n , multiples of 1322 g mol -1 ; or [Cu 3 (CH 3 Si(phpr) 3 ) 2 ] n , multiples of 1490 g mol -1 ).