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Crystal disassembly and reassembly of heterometallic NiII–TaV oxalate compounds

Four new complexes, [Ni(bpy)3]2[TaO(C2O4)3]Cl·11H2O (1), [Ni(bpy)3]2[TaO(C2O4)3]Cl·12H2O (2), [{; ; Ni(bpy)2}; ; 2(μ-C2O4)]1.5[TaO(C2O4)3]·13H2O (3) and [{; ; Ni(bpy)2}; ; 2(μ-C2O4)](NO3)2·H2C2O4 (4) (bpy = 2, 2′-bipyridine), have been prepared and investigated by IR spectroscopy and thermal analysis ; compounds 1, 2 and 4 have been characterized by single-crystal X-ray diffraction. In these Ni–Ta oxalate systems the crystal disassembly/reassembly process was observed. Stick-like single crystals of 1 disassemble in the mother liquor and again reassemble to form polyhedral single crystals of 2 having an additional water molecule. Compounds 1 and 2 crystallize in different crystal systems (P21/c vs. P212121) ; nevertheless they exhibit very similar crystal packing of the constituent structural units: the [Ni(bpy)3]2+ cations are involved in a specific intermolecular contact, quadruple aryl embrace (QAE), in a honeycomb 2D hexagonal network, whereas the [TaO(C2O4)3]3− anions and water molecules constitute a 3D hydrogen bonding sublattice. A survey of the Cambridge Structural Database was performed in order to find similar QAE contacts within the crystal structures containing the [M(bpy)3]n+ cation. In a Ni–Ta oxalate system with a different [{; ; Ni(bpy)2}; ; 2(μ-C2O4)]2+ cation environment, compound 3 was obtained. The disassembly/reassembly process occurs in the mother liquor containing crystals of 3, and after two weeks it yields crystals of 4 with different constituent units: the homodinuclear [{; ; Ni(bpy)2}; ; 2(μ-C2O4)]2+ cation remains unchanged, whereas the (oxalato)tantalate(V) anion decomposes during disassembly of 3 and NO3− takes its place as the counteranion in the reassembled crystal form of 4.

Tantalum; Nickel; Oxalate complex; Crystal transformation; Spontaneous resolution; Hydrogen bonds; Quadruple aryl embraces

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