Building Metallacrown Topology around a Discrete [M3(μ3-O)] (M = Ni(II) and Pd(II)) Core Using Oximato Oxygen Linkers: Synthesis, Structures, and Spectroscopic Characterization of a New Family of Compounds with an Inverse-9-MC-3 Motif

A family of trinuclear oximato complexes [(MIIL)3(μ3-O)]ClO4(M = Ni, 1−3; Pd, 4and 5) (HL = 2-alkylamino-3-oximobutane) involving a discrete [M3(μ3-O)]4+core have been synthesized in moderate to high yields by a simple one-pot reaction. The products were characterized by ESI-mass and 1H NMR spectroscopy as well as by single-crystal X-ray diffraction analysis of representative compounds viz., 1, 2, and 4. The oximato oxygen atoms from the ligands and the central μ3-O atom connect the metal centers, forming an inverse metallacrown topology in these complexes. In the isostructural nickel compounds (1, 2), the metal centers are situated at the vertices of an equilateral triangle with its centroid position being occupied by the μ3-O atom; the Ni−O−Ni angles vary in the range 119.0(2)−120.2(2)°. In the palladium complex 4, the geometry of the Pd3O core is better described as a regular trigonal pyramid with the metal centers and the μ3-O atom occupying the apexes; the Pd−O−Pd angles are close to 109°. The coordination square planes around the individual palladium centers bend appreciably from each other (dihedral angles vary in the range 28.62−34.53°), providing more of a bowl shape compared to the overall metallacrown topology that remains virtually planar in the nickel complexes. The μ3-oxygen atom in 4is displaced by 0.687 Å from the center of the triangular plane with corners occupied by the Pd(II) ions. The protons of the metallacrown peripheral rings in 4and 5are more deshielded compared to their nickel(II) counterparts, as revealed from their 1H NMR spectra in dichloromethane-d2solution.