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, [(NiHL(1))(3)(μ(3)-O)]ClO(4) (1), with inverse metallacrown 9-MC-3 topology has been synthesized using a Schiff-base ligand (H(2)L(1)) 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, (Et(3)NH)Ni(4)(damo)(4)(Hea)(2)(ea)(2)(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 Ni(4) metallacrowns in inverse topology, viz., Ni(4)(damo)(4)(H(2)dea)(2)(Hdea)(2)(2)·2H(2)O (3), Ni(4)(dpko)(4)(Hea)(2)(ea)(2)(2)·4H(2)O (4), and Ni(4)(mpko)(4)(Hmea)(2)(mea)(2)(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 (H(2)dea), 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 (S(4) 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 [Ni(4)] core [J/k(B) ≈ -20 to -27 K based on the following spin Hamiltonian: H = -2J(S(1)·S(2) + S(2)·S(3) + S(3)·S(4) + S(4)·S(1))] lead to an S(T) = 0 ground state for these complexes.