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

Abstract

A carboxylic acid-substituted picolinic hydrazone ligand (pcoap) forms a homoleptic [2 × 2] tetranuclear Cu(II)₄ complex with four six-coordinate Cu(II) centers bridged by μ-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 (139.2–140.6°), and short Cu–Cu distances (3.97–4.05 Å), 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.