Microwave versus Traditional Solvothermal Synthesis of Ni7II Discs: Effect of Ligand on Exchange Reaction in Solution Studied by Electrospray Ionization-Mass Spectroscopy and Magnetic Properties

A comparative evaluation of the solvothermal method of synthesis of magnetic Ni7 discs with four different ligands using the same concentrations of reagents and temperature found microwave heating is more effective than the traditional oven one. Where the former only needs minutes, the latter needs days with an equivalence of 10 min microwave to 1 day of traditional. The size of crystals has a narrow distribution and increases with time for the microwave but is a rather wide distribution for the traditional one. Furthermore the shape of the crystals is more regular for the microwave. The four Ni7II discs of formulas [Ni7L6(μ3-OMe)6](ClO4)2 (1–3) and {[Ni7(L4)6(μ3-OMe)6][Ni(L4)2]}(ClO4)2 (4) were synthesized as green hexagonal rods from Ni(ClO4)2·6H2O in mixed MeOH/MeCN solution and salicylalde Schiff base ligands (L1 = 2-methoxy-6-(iminomethyl)phenol, L2 = 2-ethoxy-6-(iminomethyl)phenol, L3 = 2-methoxy-6-((methylimino)methyl)phenol, L4 = 2-ethoxy-6-((methylimino)methyl)phenol). X-ray structural analyses show six symmetrically positioned Ni(2) around a central Ni(1) bridged by the μ3-methoxide and surrounded by the ligand L which also isolates the discs from one another. The perchlorate sits in the interstices, and the planar [Ni(L4)2] of 4 also inserts itself between the discs. The structures of 1–3 can be regarded as ordered discotic liquid crystals. Electrospray ionization mass spectrometry of solutions showed an exchange of methoxide for hydroxide and a different distribution of [Ni7] phase with mixed (MeO/OH) core bridges, confirming a probable “step by step” substitution of MeO– by OH–. Magnetic studies indicate ferromagnetic interaction between Ni(1) and Ni(2) and possible antiferromagnetic between Ni(2) and Ni(2), resulting in a noncollinear system which only reaches half of the moment in 50 kOe at 2 K.