Tailored GAFF Parameters for Pentamethylcyclopentadienyl Rh(I/III) Complexes with α-Diimine Ligands: Validation and Solvation Studies

Rhodium(III) complexes of the general structure [(bpy)Rh(Cp*)X]^<i>n</i>+ (bpy = 2,2'-bipyridine, Cp* = pentamethylcyclopentadienyl, X = H₂O, OH^-, Cl^-) are potent catalysts in a wide range of reactions, including NAD⁺ reduction and NADH oxidation, CO₂ reduction, and hydrogenation of small organic molecules. Computational studies of these complexes require tailored force field parameters for accurate classical molecular dynamics. Here, we develop parameters compatible with the general Amber force field for [(bpy)Rh^III(Cp*)Cl]⁺ and the reduced [(bpy)Rh^I(Cp*)], an important intermediate in many applications. As the general Amber force field is unable to describe the η⁵ hapticity of the Rh-Cp* bond, we implement a σ-bonded model. The force field is validated against density functional theory reference data including bond lengths, angles, dihedrals, and potential energy profiles. Finally, we demonstrate its applicability by analyzing the aqueous solvation environment around the Rh^III and Rh^I complexes, highlighting its utility for studying structure-environment interactions.