The 9-MC-3 Motif

Figure 1 shows the ORTEP diagram for the first synthesized metallacrown: the 9-MC-3, [9-MC(V(V)O)N(Shi)-3]. The metallacrown is neutral since the 3+ charge of V(V)O is balanced by the oxygen atoms of the triply deprotonated ligand, salicylhydroxamic acid. Though the metal-oxygen bond is lengthened as compared to that of the carbon-oxygen bonds of the crown ether (~1.9 A for the metallacrown, 1.3 for the crown ether A), the angles of the connectivity also change making up for the change in bond length (90o, 120o, and 121o are the angles for O-V-N, N-O-V, and V-N-O, respectively, as compared to the tetrahedral angle of 109o for the crown ether). These two changes combined to give an overall size similar to that of the crown ethers. The metallacrown was made by the addition of one equivalent of vanadyl sulfate, VO(acetylacetonate)2, or VCl3 to one equivalent of H3shi in methanol in the presence of three equivalents of NaOCH3 or NaOH. This first MC did not include a cavity metal since its core was too small to encapsulate one. The vanadium metallacrown was isolated as an exploration of the biological chemistry of vanadium. The group was interested in the coordination chemistry of the VO3+ with hydroxamate ligands. Hydroxamates and catecholates are the major classes of siderophores discussed above. Those could be the basis for sequestering the vanadium metal by algae.

Figure 1 ORTEP diagram for [9-MC(V(V)O)N(shi)-3]. The first Metallacrown (no cavity metal).

The first MC to encapsulate a transition metal was Fe(III)(OAc)3[9-MCFe(III)N(Shi)-3] shown in figure 2 below. The MC formed by adding equimolar amounts of Fe(II) sulfate to H3shi in methanol in the presence of three equivalents of sodium acetate to fully deprotonate the ligand. Air oxidizes Fe(II) to Fe(III). The acetate anions neutralized the triply positive charge on the captive iron, and formed bridges between this iron and those of the metallacrown ring. By the formation of this metallacrown and the previous one, two characteristics were enhanced in the crown ether: first was the binding affinity of the oxygens, now negatively charged instead of neutral and better at binding the cation in the cavity, and second was the stability of the crown, with anions bridging the ring metals and the transition metals of the cavity.

Figure 2 ORTEP diagram for Fe(III)(OAc)3[9-MCFe(III)N(shi)-3]. The first transition metal binding metallacrown.

An inverse 9-MC-3 was synthesized. The positively charged copper, in the metallacrown is oriented towards the center instead of the electronegative oxygen atoms, and anions are selected for as a result. In this case, the cavity captivates hydroxides, while m3-tripodal sulfate binds to the three ring coppers(II).