Making an order: the concerted alignment of [MOF₅]²⁻ (M = Nb and Ta) dipolar anions in one-dimensional coordination chains sustained by tris(3,4,5-trimethyl-1H-pyrazole)copper(II)

Electronic distortions, which are inherent in the oxide fluoride anions [MOF5]2− (M = Nb and Ta), provide an origin of polar molecular arrangements for the development of new polar second-harmonic-generating, piezo-, pyro- and ferroelectric materials. It is still a challenge to expand this approach to the realm of metal–organic polymers, while insufficient control over the environment of the [MOF5]2− units results in their orientational disorder and loss of polarity. The structures of catena-poly[[tris(3,4,5-trimethyl-1H-pyrazole-κN 2)copper(II)]-μ-oxido-[tetrafluoridoniobium(V)]-μ-fluorido], [CuNbF5O(C6H10N2)3] n , (I), and its isostructural pentafluoridooxidotantalate(V) analogue, catena-poly[[tris(3,4,5-trimethyl-1H-pyrazole-κN 2)copper(II)]-μ-oxido-[tetrafluoridotantalum(V)]-μ-fluorido], [CuTaF5O(C6H10N2)3] n , (II), are the first examples of the strict orientational order of [MOF5]2− (M = Nb and Ta) in one-dimensional coordination chains. A primary factor for the exact discrimination of one orientation of the anion over the other is strong and shape-selective multiple interactions of [MOF5]2− with the inherently acentric CuL 3 2+ platform, with a set of two coordination and three N—H...F hydrogen bonds. In (I) and (II), the Cu2+ ions exhibit distorted square-pyramidal fivefold coordination formed by three pyrazole N atoms and the oxide O atom, defining the equatorial plane, and the anionic bridging F atom (which is trans with respect to the M—O bond) residing in the apical position. The inorganic bridges connect CuL 3 2+ moieties into polar zigzag chains; the bulk polarity of the structure is eliminated by an antiparallel alignment of the individual chains. These chains are further connected through C—H...F hydrogen bonding and very weak C—H...π interactions of the organic ligands.