Chapter 4 - Jacobs University

Chapter 3 The Wheel-Shaped Cu 20
Figure 3.4 Side view of 1 showing ball-and-stick (left) and combined polyhedral/ball-and-stick (right)
representations.
The Cu 20 cluster in 1 is composed of only 3 structurally unique types of Cu 2+ ions (8 Cu1, 4 Cu2
and 8 Cu3), see Figure 3.6. All 20 copper centers are bridged to neighboring copper ions via m 3 -
oxo ligands resulting in a highly symmetrical, cage-like assembly. Based on bond valence sum
calculations all 24 bridging oxygens are monoprotonated. 18 Interestingly, the center of the cavity
(which has a diameter of around 7 Å) is occupied by a chloride ion (Figures 3.2-3.3). The
coordination numbers and geometries of Cu1, Cu2 and Cu3 are different from each other. Cu1 is
coordinated in a strongly distorted octahedral fashion and exhibits Jahn-Teller distortion with
axial elongation. The equatorial plane is composed of Cu1-O3A (1.922(14) Å), Cu1-O5A
66