Metallophilicity-assisted piezochromism in a rhodium(I) dicarbonyl Schiff-base complex: structural, energetic, electronic and spectroscopic investigations under high pressure

Abstract

For the development of piezochromic materials, it is critical to understand their structure-property relationships under high-pressure (HP). In this contribution, we report the experimental HP behaviour of a fluorescent and nearly square-planar rhodium(I) dicarbonyl Schiff-base complex (Rh-4-Br) in the solid state, complemented by theoretical calculations. The crystal structure of Rh-4-Br is primarily governed by Rh⋯Rh metallophilic and C−H⋯O hydrogen-bond-type interactions. Both interaction kinds show notable structural, electronic, and energetic sensitivities to elevated pressures. HP single- crystal X-ray diffraction studies conducted between ambient pressure and about 10.5 GPa revealed metallophilic interaction enhancement with the Rh⋯Rh distance shortening by ~0.60 Å. HP-induced structural changes additionally affected the electronic properties. For instance, the electron density (ϱ(r_BCP)) at the Rh⋯Rh interaction bond critical point (BCP) increased by 0.155 e·Å⁻³, whereas a higher delocalization index (δ) and ratio of 1 < |V(r_BCP)|/G(r_BCP) < 2 (V & G – local potential & kinetic energy densities) indicated partial covalency of this interaction at the highest pressures. Strengthening of the metallophilic interaction under HP influences molecular orbitals contributing to the lowest-energy electronic transitions. Consequently, a reversible piezochromism (yellow-to-orange-to-red) under compression-decompression cycles with a bathochromic shift of ∼75 nm (in the range up to 7 GPa) and Raman blue-shift of the ν_Rh⋯Rh band by ∼18 cm⁻¹ (from 0.00(5) to 5.34(5) GPa) were observed. These findings align closely with the results of periodic density functional theory modelling under isotropic external pressures.