Backbone flexibility of extended metal atom chains. Ab initio molecular dynamics for Cr3(dpa)4X2 (X = NCS, CN, NO3) in gas and crystalline phases

Recently published static DFT and CASSCF/CASPT2 calculations depicted extremely flat Potential Energy Surfaces (PESs) for the CrCr flexibility of Cr-3(dpa)(4)X-2 (X = NCS-, CN-, NO3-) extended metal atom chains (EMACs) (M. Spivak, et al., Dalton Trans., 2017, 46, 6202). We herein explore the thermal and crystal packing effects on the structure of EMACs using ab initio molecular dynamics (MD). Car-Parrinello DFT-based simulations of the isolated molecules show that thermal energy favors asymmetric arrangements of the Cr-3 chain due, in part, to the bending of the axial ligands (X) and the increased XCr distance, both of which weaken X -> Cr sigma-donation. This effect is even more prominent in the crystalline phase due to the interaction between the axial ligands of neighboring molecules in the unit cell. This could explain the typical discrepancies between the experimental and theoretical characterization of Cr-3 EMACs observed in the literature.