The 12-MC-4 Structural Type


The new field expanded to include the 12-MC-4 structural motif with the first 12-MC-4, Mn(II)(OAc)2[12-MCMn(III)N(Shi)-4], 1, as shown in figure 1. This was also the first example of a mixed valent metallacrown. The MC formed in DMF (dimethylformamide) by the addition of one equivalent of Mn(II) acetate to one equivalent of the H3shi ligand, and only the ring metals were oxidized by air. Mn(II), with an ionic radius of 0.83 A, is elevated by1.2 above the ring whose cavity radius is 0.57 A. This capping position, however, is stabilized by the two bridging acetates between the ring metals and the cavity ion.

Figure 1 ORTEP diagram for Mn(II)(OAc)3[12-MCMn(III)N(shi)-4]. The first 12-MC-4 and a mixed valent metallacrown.

A new 12-metallacrown-4, Fe(III)(m-SO4)2[12-MCFe(III)N(Shi)-4], 2, was soon synthesized. The synthesis was similar to that of Fe(III)(OAc)3[9-MCFe(III)N(Shi)-3] with trichloroacetate used as the weak base instead of sodium acetate. This was also the first charged metallacrown, with the 3+ charged Fe ion in the cavity of the metallacrown not completely neutralized by the two negatively charged SO4 groups. Figure 2 below shows the monoanionic metallacrown. The cavity size is identical to that of the crown ether counterpart, 12-C-4, and is 0.6 A. Unlike 1, the Fe(III) atom in 2 is encapsulated in the plane of the ring due to similarity in ionic radius of Fe(III) and that of the cavity of the metallacrown (0.60 A, 0.64 A, respectively). Another difference between 1 and 2 is that the acetate bridges in 1 are on the same side of the MC ring (trans-syn). In 2, however, the sulfate bridges are above and below the ring (trans-anti).

Figure 2 ORTEP diagram for Fe(III)(-SO4)2[12-MCFe(III)N(shi)-4]. An anionic 12-MC-4 in which the cavity metal is in the plane of the metallacrown ring.

ORTEP diagram for (NaCl)2[12-MCMn(III)N(shi)-4] with two alkali metals bound, one above and one below the plane of the ring.

ORTEP diagram for {Na2[Na(12-MCGa(III)N(shi)-4)2(2-OH)4]} or a Gallium Metallacryptate