Towards a Broadly Applicable Force Field for d6-piano Stool Complexes

Three-legged piano stool complexes are prototypical organometallic complexes relevant to a wide range of chemically relevant questions. Force field parametrization of transition-metal complexes is difficult and underdeveloped, and metal-specific force fields and software are required. Here we report our efforts to derive parameters for the conventional CHARMM and the Valbond-CHARMM force fields for d6-piano stool complexes. In Valbond-CHARMM, the usual angular term is replaced with hybrid orbital strength functions. These functions describe the energy not only of distorted bond angles around the minimum but also at very large distortions. Structure optimizations led to a good agreement between the calculated force field and the X-ray structures. They were comparable to RMSDs obtained between X-ray and DFT structures. In addition, and contrary to treating the systems with DFT, molecular dynamics simulations on the multiple nanosecond time scale are possible and allow to compute meaningful structural and energetic observables. Explicit solvent simulations of the complexes in methanol and water allow to determine the solvent distribution around the complexes. The parametrization presented here will be a useful starting point for dynamics investigations of catalysts in structurally more demanding environments.