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

Abstract

Supramolecular Cu(II) complexes were prepared from two trifunctional β-diketone ligands. The ligands (CH₃Si(phacH)₃ and CH₃Si(phprH)₃, represented by LH₃) contain three aryl-β-diketone moieties joined by an organosilicon group. The complexes have the empirical formula Cu₃L₂, as expected for combinations of Cu²⁺ and L^3-. 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 Cu₄(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 [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 ([Cu₃(CH₃Si(phac)₃)₂]_n, multiples of 1322 g mol^-1; or [Cu₃(CH₃Si(phpr)₃)₂]_n, multiples of 1490 g mol^-1).