Trinuclear Magnesium Imidazolate Borohydride Complex

A new type of hybrid compound, combining properties of MOFs and borohydrides, was synthesized solvothermally using Mg(BH4)2 and imidazole as precursors. Material in the form of acetonitrile solvate with formula [Mg3{(Im)BH2(Im)}6(ImH)6] ·CH3CN crystallizes in the space group R¯3, having the unit cell parameters a = 15.1942(2)˚A and c = 28.3157(3)˚A as determined by single crystal X-ray diffraction. The structure was further investigated by solid state NMR and DFT quantum chemical calculations. The main feature of the structure, reported here for the first time, is a linear trinuclear complex, where octahedrally nitrogen-coordinated Mg2+ ions are bridged with {(Im)BH2(Im)}– units, forming inside voids of 4.6˚A in diameter between the magnesium ions. Polar intermolecular interactions hold the molecules in a dense 1 Page 1 of 32 ACS Paragon Plus Environment Submitted to Inorganic Chemistry 123456789 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 rhombohedral stacking, where a disordered acetonitrile molecule plays a cohesive role. The compound is stable in air and upon heating to about 160 ◦C. Using an alternative synthesis method from imidazole melt, an imidazole solvate with the formula [Mg3{(Im)BH2(Im)}6(ImH)6] ·ImH and a very similar crystal structure to acetonitrile solvate was prepared. It is stable up to 220 ◦C. Upon further heating, it transformed into a layered structure with the formula Mg(Im3BH)2, space group P¯31c, and unit cell parameters a = 8.7442(4)˚A, c = 17.643(2)˚A determined by synchrotron powder diffraction. Besides its structural novelty, two types of potentially reactive hydrogens, bonded to boron and nitrogen in the same molecule, make the material highly interesting for future investigations in the fields of energy storage applications.