Structural variation from heterometallic heptanuclear or heptanuclear to cubane clusters based on 2-hydroxy-3-ethoxy-benzaldehyde: effects of pH and temperature

Five new Hheb complexes, HN(C₂H₅)₃·[M₄Na₃(heb)₆(μ₃-N₃)₆], (M = Ni (1), and Co (2), [Co₄(heb)₄(μ₃-OCH₃)₄(μ₁-HOCH₃)₄]·(H₂O)₂ (3), [M₇(heb)₆(μ₃-OCH₃)₆]·(ClO₄)₂ (M = Ni (4), and Co (5), Hheb = 2-hydroxy-3-ethoxy-benzaldehyde), were synthesized by reaction of hexahydrate perchlorate salt, Hheb, and NaN₃ under different temperature and pH conditions. Careful investigation of the effect of the reaction temperature and pH resulted in a series of compounds with different compositions and nuclearities. The diverse compounds obtained illustrate the marked sensitivity of the structural chemistry of Co- or Ni-Hheb ligand-like systems to synthesis conditions. Complexes 1 and 2, which are heterometallic heptanuclear anion [M₄Na₃(heb)₆(N₃)₆]⁻ clusters, are formed at a pH of 5.5 and at room temperature. At a pH of 7.5 and at room temperature, a neutral molecular cubane cluster, namely 3, is formed with a lower nuclearity. Further increase of the reaction temperature to 140 °C at the same pH resulted in formation of two heptanuclear cation [M₇(heb)₆(μ₃-OCH₃)₆]²⁺ clusters, 4 and 5. The results show that the pH and reaction temperatures play a key role in the structural control of the self-assembly process. Interestingly, heterometallic heptanuclear anion [M₄Na₃(heb)₆(N₃)₆]⁻ clusters have never been reported for the family of μ₃-N₃⁻ or μ₃-O-bridged heptanuclear clusters. The magnetic properties of 1–5 were investigated and are discussed in detail.