Synthetic strategies for expanding the scope of metallacrowns.

Abstract

Metallacrowns are a new type of inorganic molecular recognition agent. Structurally, metallacrowns are similar to crown ethers where transition metal ions and nitrogen atoms replace the methylene carbon atoms of crown ethers. In the past, metallacrowns had been formed with Mn(III), Fe(III), Ga(III), V(V) using the ligand salicylhydroxamic acid with Mn(II), Fe(III), and alkali metals encapsulated in the metallacrown core. Several types of metallacrowns had been formed, e.g., 9-MC-3, 12-MC-4, 15-MC-5. Reported herein is the first synthesis of metallacrowns with the low valent metal, Ni(II), as a 12-metallacrown-4. The solution integrity of 12-metallacrown-4 and 9-metallacrown-3 were studied using a variety of techniques; $\sp1$H NMR, $\sp{51}$V NMR, vapor pressure osmometry, UV/vis, Fab-MS and ESI-MS. The metallacrowns are intact in DMF and acetonitrile. Using a two-part synthetic strategy, a mixed ligand nickel 12-metallacrown-4 was isolated by using a new type of ligand, dipyridyl ketone oxime (Hpko). Since Ni(II) has a charge of +2, a 2:2 mixture of shi$\sp{-3}$ and pko$\sp-$ produced a neutral 12-metallacrown-4 ring. This strategy resulted in preparation of a mixed ligand metallacrown. By adding manganese(II) to a synthesis with Ni(II), shi$\sp{-3}$ and pko$\sp-$, a mixed ligand/mixed metal metallacrown was isolated and structurally characterized. An inverse zinc metallacrown was synthesized that has the positively charged zinc atoms oriented toward the center of the cavity instead of oxygen atoms. The inverse metallacrown encapsulates anions. Picoline hydroxamic acid and copper(II) were used to synthesize several planar 15-metallacrown-5 structures with the following encapsulated cations, La(III), Nd(III), Sm(III), Eu(III), Gd(III), Dy(III), Ho(III), Er(III), Yb(III), Ca(II), and UO$\sb2$(II). Chiral amino acids were converted to hydroxamic acids and used to form metallacrowns that are site differentiated on one face of the planar 15-metallacrown-5. For potential use of metallacrowns as MRI contrast agents the proton relaxivity of Gd(III) 15-metallacrown-5 in water was determined to be 9.8 M$\rm\sp{-1}s\sp{-1}$. Magnetic moments as high as 10.8 BM were observed with Dy(III) encapsulated in the center of the 15-metallacrown-5. The scope of the metallacrowns has been expanded to include a diverse array of new structure types with interesting physical properties. The insights gained here can be used to synthesize new metallacrowns. The diversity in metallacrown structures revealed in this contribution provide new avenues for the achievement of applications for metallacrowns.