Synthesis of Square Planar Cu 4 Clusters.

Template-assisted synthesis of well-defined polynuclear clusters remains a challenge for [M 4 ] square planar topologies. Herein, we present a tetraamine scaffold R L(NH 2 ) 4 , where L is a rigidified resorcin[4]arene, to direct the formation of C 4 -symmetric R L(NH) 4 Cu 4 clusters with Cu-Cu distances around 2.7 Å, suggesting metal-metal direct interactions are operative since the sum of copper's van der Waals radii is 2.8 Å. DFT calculations display HOMO to HOMO-3 residing all within a 0.1 eV gap. These four orbitals display significant electron density contribution from the Cu centers suggesting a delocalized electronic structure. The one-electron oxidized [Cu 4 ] + species was probed by variable temperature X-band continuous wave-electron paramagnetic resonance (CW-EPR), which displays a multiline spectrum at room temperature. This work presents a novel synthetic strategy for [M 4 ] clusters and a new platform to investigate activation of small molecules.