Synthesis of Square Planar Cu4 Clusters

Template-assisted synthesis of well-defined polynuclear clusters remains a challenge for [M-4] square planar topologies. Herein, we present a tetraamine scaffold L-R(NH2)(4), where L is a rigidified resorcin[4]arene, to direct the formation of C-4-symmetric L-R(NH)(4)Cu-4 clusters with Cu-Cu distances around 2.7 angstrom, suggesting metal-metal direct interactions are operative since the sum of copper's van der Waals radii is 2.8 angstrom. 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.

Automated summary

Template-assisted synthesis of [M-4] clusters was achieved using a tetraamine scaffold L-R(NH2)(4) to direct the formation of C-4-symmetric L-R(NH)(4)Cu-4 clusters. The study showed the importance of metal-metal direct interactions in cluster formation. Computational and experimental analysis indicated the presence of a delocalized electronic structure in the clusters.