A homoleptic, self-assembled [2 x 2] square Cu4 complex exhibiting intramolecular ferromagnetic exchange

A carboxylic acid-substituted picolinic hydrazone ligand (pcoap) forms a homoleptic [2 x 2] tetranuclear Cu(II)(4) complex with four six-coordinate Cu(II) centers bridged by mu-O alkoxide groups in a square grid structure. The terminal carboxylic groups complete the six-coordinate structures at each copper centre, capping off external coordination positions, with two functioning as carboxylates, and two in neutral carboxylic acid form. Despite very large Cu-O-Cu bridge angles (139.2-140.6degrees), and short Cu-Cu distances (3.97-4.05 Angstrom), which would normally be expected to propagate antiferromagnetic exchange interactions within the grid, strictly orthogonal bridging connections between the metal centers lead to intramolecular ferromagnetic exchange (J = 7.2(3) cm(-1)), and an S = 4/2 magnetic ground state.