The Gigantic {Ni₃₆Gd₁₀₂} Hexagon: A Sulfate-Templated “Star-of-David” for Photocatalytic CO₂ Reduction and Magnetic Cooling

Abstract

Gigantic coordination molecules assembled from a large number of metal ions and organic ligands are structurally and functionally challenging to characterize. Here we show that a heterometallic cluster [Ni₃₆Gd₁₀₂(OH)₁₃₂(mmt)₁₈(dmpa)₁₈(H₂dmpa)₂₄(CH₃COO)₈₄(SO₄)₁₈(NO₃)₁₈(H₂O)₃₀]·Br₆(NO₃)₆·(H₂O)<i>_<i>x</i></i>·(CH₃OH)_<i>y</i>, (<b>1</b>, <i>x</i> ≈ 130, <i>y</i> ≈ 60), shaped like a "Star of David", can be synthesized using a "mixed-ligand" and "sulfate-template" strategy. In terms of metal nuclearity number, <b>1</b> is the second largest 3d-4f cluster to date. In the solid state, <b>1</b> is porous after removing the lattice guests. The N₂ adsoption experiment reveals that the BET and Langmuir surface areas are 299.8 and 412.0 cm² g^-1, respectively. CO₂ adsorption at 298 K gives the amount of 45 cm³ g^-1 for <b>1</b>. More importantly, <b>1</b> is soluble in common organic solvents and exhibits high solution stability revealed by high resolution MALDI-TOF mass spectroscopy, small-angle X-ray scattering (SAXS), and low-dose transmission electron microscopy. The solubility and the potential open metal sites owing to the labile coordinating components prompted us to investigate the photocatalytic properties of <b>1</b>, which displays high selectivity and efficiency for reduction of CO₂ to CO with turnover number and turnover frequency of 29700 and 1.2 s^-1, respectively. These values are higher than most catalysts working under the same conditions, presumably due to the strong Ni-CO₂ binding effect. In addition, the large percentage of Gd(III) in <b>1</b> leads to a large magnetic entropy change (41.3 J·kg^-1·K^-1) at 2.0 K for Δ<i>H</i> = 7 T.