Defining the Proton Topology of the Zr₆-Based Metal-Organic Framework NU-1000

Planas, N.; Mondloch, J. E.; Tussupbayev, S.; Borycz, J.; Gagliardi, L.; Hupp, J. T.; Farha, O. K.; Cramer, C. J.
J. Phys. Chem. Lett. 2014, 5, 3716 (doi:10.1021/jz501899j).

Metal-organic frameworks (MOFs) constructed from Zr₆-based nodes have recently received considerable attention given their exceptional thermal, chemical, and mechanical stability. Because of this, the structural diversity of Zr₆-based MOFs has expanded considerably and in turn given rise to difficulty in their precise characterization. In particular it has been difficult to assign precisely where protons (needed for charge balance) reside on some Zr₆-based nodes. Elucidating the precise proton topologies in Zr₆-based MOFs will have wide ranging implications in defining their chemical reactivity, acid/base characteristics, conductivity and chemical catalysis. Here we have used a combined quantum mechanical and experimental approach to elucidate the precise proton topology of the Zr₆-based framework NU-1000. Our data indicate that a mixed node topology, [Zr₆(μ₃-O)₄(μ₃-OH)₄(OH)₄(OH₂)₄]⁸⁺, is preferred and simultaneously rules out five alternative node topologies.