Structural studies of ligand stabilized Ni/Ga clusters by means of vibrational spectroscopy and theoretical calculations

Abstract

AbstractIn order to explain the different catalytic activity in hydrogenation of two new intermetallic Ni/Ga clusters, [Ga7](NiCp*)6 (1A) and [NiGa6](NiCp*)6 (1B) (Cp* = C5Me5), investigations of structure–function relationship have been performed based on Raman and infrared (IR) spectroscopy and theoretical (density functional theory [DFT] and normal coordinate) calculations. Full interpretation of the Raman, far‐IR, and mid‐IR spectra of these dark colored solids has been proposed. Based on the overview of metal–Cp* complexes, all the 14 characteristic Cp*− skeletal fundamental modes have been identified. By comparison of the NiCp* stretching and tilting external modes (350–380 cm−1), their force constants, and bond lengths, cluster 1B exhibited slightly stronger metal–ligand bonding. Vibrations of Ga7 and NiGa6 cluster cores showed that the stretching wavenumbers and force constants of NiGa (100–350 cm−1) and GaGa (60–250 cm−1) bonds are higher for cluster 1B, in agreement with the shorter averaged experimental and calculated bond lengths of GaGa bonds (2.873 and 2823 Å for clusters 1A and 1B, respectively). Cluster hydrogenation experiments with H2 and D2 showed strong NiH and ND stretching features (at 1750 and 1260 cm−1, respectively), and at the same time, characteristic bands of self‐hydrogenated Cp*H and remained nondegraded clusters have been detected. The extent of HD exchange in cluster deuteration was obtained about 1.5 times more effective with cluster 1B than cluster 1A. The stronger hydrogen or deuterium uptake by cluster 1B and the more intensive self‐hydrogenation of Cp* clearly support the higher hydrogenation activity of cluster 1B compared with that of 1A.