Solid-state NMR/NQR and first-principles study of two niobium halide cluster compounds

Area of Science:

• Solid-state chemistry
• Inorganic chemistry
• Materials science

Background:

• Hexanuclear niobium halide clusters with [Nb6X12](2+) cores are diamagnetic.
• Previous solid-state NMR studies on niobium compounds showed different chemical shift trends.
• Structurally related molybdenum clusters ([Mo6Br8](4+)) exhibit similar NMR chemical shifts.

Purpose of the Study:

• To investigate the NMR/NQR parameters of niobium and halogen sites in [Nb6X12](2+) (X=Cl, Br) clusters.
• To correlate experimental NMR/NQR data with theoretical calculations (PAW/GIPAW).
• To understand the electronic structure and bonding within these niobium clusters.

Main Methods:

• Experimental solid-state Nuclear Magnetic Resonance (NMR) and Nuclear Quadrupole Resonance (NQR) techniques.
• Variable magnetic field (Bo) static broadband NMR measurements.
• Projector Augmented Wave (PAW) and Gauge-Including Projector Augmented Wave (GIPAW) computational methods.

Main Results:

• Niobium sites display large positive chemical shifts (δiso((93)Nb) 2,400–3,000 ppm), deviating from typical niobium compounds.
• Computed Electric Field Gradient (EFG) and Chemical Shift (CS) tensors show near axial symmetry at niobium sites, aligned with the cluster's four-fold axis.
• Bridging halogen sites exhibit asymmetric EFG and CS tensors, with specific orientations relative to Nb-X bonds and the cluster center.
• PAW predictions for the bromide compound showed better agreement with experimental data than for the chloride compound.

Conclusions:

• The study elucidates the unique electronic environment of niobium in these halide clusters through combined experimental and computational approaches.
• The findings provide a detailed understanding of the tensor orientations and symmetries at both niobium and halogen sites.
• The results highlight the utility of solid-state NMR/NQR and advanced computational methods for characterizing complex inorganic cluster compounds.