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)

Abstract

Electronic distortions, which are inherent in the oxide fluoride anions [MOF] (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 [MOF] units results in their orientational disorder and loss of polarity. The structures of catena-poly[[tris(3,4,5-trimethyl-1H-pyrazole-κN)copper(II)]-μ-oxido-[tetrafluoridoniobium(V)]-μ-fluorido], [CuNbFO(CHN)], (I), and its isostructural pentafluoridooxidotantalate(V) analogue, catena-poly[[tris(3,4,5-trimethyl-1H-pyrazole-κN)copper(II)]-μ-oxido-[tetrafluoridotantalum(V)]-μ-fluorido], [CuTaFO(CHN)], (II), are the first examples of the strict orientational order of [MOF] (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 [MOF] with the inherently acentric CuL platform, with a set of two coordination and three N-H...F hydrogen bonds. In (I) and (II), the Cu 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 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.

Crystal structure and Hirshfeld surface analysis of 4,4′-(propane-1,3-diyl)bis(4H-1,2,4-triazol-1-ium) pentafluoridooxidovanadate(V)

“…One can anticipate that [VOF 5 ] 2À systems will show this effect to a particular extent since the vanadium ions experience a much larger out-of-centre displacement towards an apical O-ligand compared with their Nb and Ta analogues (Ok et al, 2006). This feature generates a larger dipole moment as well as mitigating against orientational disorder of the anions in crystal structures (Sharko et al, 2018). However, the supramolecular behaviour of the [VOF 5 ] 2À anions is less predictable and it is strikingly different from that of the most extensively examined Nb and Ta systems.…”