Single‐Atom O‐Bridged Urea in a Dinickel(II) Complex together with NiII₄, CuII₂ and CuII₄ Complexes of a Pentadentate Phenol‐Containing Schiff Base with (O,N,O,N,O)‐Donor Atoms

A pentadentate phenol‐containing ligand (H 3 L) with N 2 O 3 donor atoms yields Ni II 2 ( 1 ), Ni II 4 ( 2 ), Cu II 2 ( 3 ) and Cu II 4 ( 4 ) complexes, which have been structurally characterized by X‐ray diffraction. Complex 1 contains a single‐atom O ‐bridged urea. The compounds were characterized by IR, UV/Vis, mass spectrometry, electrochemistry and variable‐temperature (2−295 K) magnetic susceptibility measurements. Analysis of the susceptibility data shows antiferromagnetic interactions between the metal centers indicating a diamagnetic ground state for complexes 1 , 3 and 4 , whereas complex 2 , a tetranuclear Ni II cubane, has a complicated low‐lying magnetic structure with a non‐diamagnetic ground state. A plot of J vs. Ni−O−Ni angles for all structurally characterized Ni 4 O 4 cubane cores, including 2 , irrespective of their symmetry exhibits a large variation of J values within a small range of Ni−O−Ni angles. The electrochemistry of all complexes was investigated in detail and the ligand‐centered oxidation to a radical‐ligand is inferred from the occurrence of oxidation processes at potentials which are similar. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)