An Unprecedented Cluster Unit in (BMIm)2[Hf9Cl14H8(AlCl4)6]

The reduction of HfCl4 with Li[AlH4] under ionothermal conditions using (BMIm)[AlCl4] as solvent and as reactant gives the compound (BMIm)(2)[Hf9Cl14H8(AlCl4)(6)] in moderate yield. It contains nona-nuclear metalloid cluster units with an unprecedented arrangement of the metal atoms. This can be described by a polyhedron consisting of two trigonal bipyramids, which are fused in one basal Hf atom. The edges of the Hf-9 polyhedron are bridged by Cl atoms and the apical sites Cl-bonded to [AlCl4](-) anions. These bridge the cluster units into 1D cluster strands. The metal atom polyhedron can be constructed by a step-wise distortion of a body centred cube. DFT calculations on [Hf9Cl14H(8-x)(AlCl4)(6)](2-) units for different numbers of hydrogen atoms (x=8-0) at the UPBE-D3/def2-TZVP level of theory indicate significant metal-metal bonding. These calculations supported strongly the presence of hydridic H atoms bridging triangular faces of the metal atom skeleton. The most stable cluster anion is calculated for eight H atoms present in the cluster unit.