Tri- and Tetranuclear Nickel(II) Inverse Metallacrown Complexes Involving Oximato Oxygen Linkers: Role of the Guest Anion (Oxo versus Alkoxo) in Controlling the Size of the Ring Topology

A trinuclear oximato complex, [(NiHL1)3(μ3-O)]ClO4 (1), with inverse metallacrown 9-MC-3 topology has been synthesized using a Schiff-base ligand (H2L1) formed by condensation of ethanolamine (Hea) and diacetylmonoxime (Hdamo). The diamagnetic compound has been characterized by electrospray ionization mass spectrometry as well as by single-crystal X-ray diffraction analysis. In the solid state, the alcoholic OH group in this molecule stays away from coordination. Surprisingly in a similar chemical reaction, when intact Hea and Hdamo have been used as ligands instead of their Schiff-base forms, the product obtained is a 12-MC-4-type metallacrown, (Et3NH)[Ni4(damo)4(Hea)2(ea)2](ClO4)3 (2), with a larger cavity size needed to accommodate a pair of hydrogen-bonded (O−H···O)− anions. Unlike in 1, the alcoholic OH groups in 2 take part in metal coordination. Compound 2 on being refluxed with lithium hydroxide in methanol is converted to 1 in almost quantitative yield. This appears to be a novel reaction type, leading to contraction of a metallacrown ring size. A family of 12-MC-4 Ni4 metallacrowns in inverse topology, viz., [Ni4(damo)4(H2dea)2(Hdea)2](ClO4)2·2H2O (3), [Ni4(dpko)4(Hea)2(ea)2](ClO4)2·4H2O (4), and [Ni4(mpko)4(Hmea)2(mea)2](ClO4)2 (5), have been synthesized following a methodology similar to that adopted for 2, using different combinations of free oximes [viz., dipyridylketonoxime (Hdpko) and methylpyridylketonoxime (Hmpko)] and amino alcohols [viz., diethanolamine (H2dea), and N-methylethanolamine (Hmea)]. Crystal and molecular structures of 3−5 have been reported, each involving either a quasi (in 3) or a perfect (in 4 and 5) square plane (S4 symmetry) with four octahedral Ni centers occupying the corners, and serve as a backbone of puckered metallacrown rings that accommodate a pair of hydrogen-bonded (O−H···O)− anions. Antiferromagnetic interactions within the [Ni4] core [J/kB ≈ −20 to −27 K based on the following spin Hamiltonian: H = −2J(S1·S2 + S2·S3 + S3·S4 + S4·S1)] lead to an ST = 0 ground state for these complexes.