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

Abstract

Recently published static DFT and CASSCF/CASPT2 calculations depicted extremely flat Potential Energy Surfaces (PESs) for the Cr-Cr flexibility of Cr(dpa)X (X = NCS, CN, NO) 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 chain due, in part, to the bending of the axial ligands (X) and the increased X-Cr distance, both of which weaken X → Cr σ-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 EMACs observed in the literature.

The Origin of Magnetic Anisotropy and Single-Molecule Magnet Behavior in Chromium(II)-Based Extended Metal Atom Chains

“…The remaining π and δ orbitals contribute negligibly to the bonding, and distortion of the symmetric structure is an energetically facile process (∼1 and ∼4 kcal mol −1 for Δ d = 0.106 and 0.679 Å, respectively). More recent theoretical work on other [Cr 3 (dpa) 4 X 2 ] derivatives has depicted a similar scenario, − with very flat potential energy landscapes and a prominent role of thermal energy and crystal packing on molecular geometry . This interpretation is also consistent with the fact that solutions of 1a and [Cr 3 (dpa) 4 (N 3 ) 2 ] in dichloromethane show three 1 H NMR resonances, that is, only one less than expected for a symmetric structure over NMR time scale.…”

Trends in the Bond Multiplicity of Cr2, Cr3, and Cr2M (M = Zn, Ni, Fe, Mn) Complexes Extracted from Multiconfigurational Wave Functions

“…Oppositely, poor metal–metal bonding (e.g., symmetric structures) is a consequence of more electron-populated metal atoms coming from strong σ-donation from X. The present results combined with previous works recently developed in our group , elucidate the complete picture of the complex Cr–Cr bonding features of Cr 3 EMACs, including the puzzling symmetry–asymmetry behavior.…”

“…Table 6 also underpins the facts presented in Tables 3 and 4 and some of the conclusions outlined in previous theoretical works, 43,68 namely, strong σ-donor ligands favor symmetric EMAC structures with weaker metal−metal bonds as a consequence of more electron populated Cr atoms, which do not tend to form covalent bonds to stabilize the structure. On the other hand, when weak σ-donor X ligands are present, formation of metal−metal covalent bonds is a mechanism to accumulate electron density between metal centers that compensates for the destabilizing electrostatic interaction between them.…”

“…Oppositely, poor metal−metal bonding (e.g., symmetric structures) is a consequence of more electron-populated metal atoms coming from strong σ-donation from X. The present results combined with previous works recently developed in our group43,68 elucidate the complete picture of the complex Cr−Cr bonding features of Cr 3 EMACs, including the puzzling symmetry−asymmetry behavior. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.8b10124.Results of RASSCF calculation on compound 2, definition of the Mayer bond index, overlap of the atomic orbitals as a function of the Cr−Cr distance, xyz coordinates of all of the compounds, molecular orbitals scheme for Cr 2 and Cr 3 systems, complete bonding analysis of 2 and 13 and EBO analysis of the H2 molecule (PDF) E-mail: javier.lopez@urv.cat.…”

Band Structures of Quasi-One-Dimensional Incommensurate Helical Systems: A Case Study of Infinite Chromium Extended Metal Atom Chain

“…Extended metal atom chains (EMACs), or metal string complexes (MSCs), are potential candidates for molecular wires, electronics, − and single-molecule magnets. , This family of molecules features a linearly aligned metal atom chain in the center with possible direct metal–metal bonding. Experimentally, the length and possible combinations of different metals and ligands have long been under extensive investigations. − As for the theoretical part, electronic structures of finite EMACs are extensively calculated. − In addition, electron transport properties of a single molecular wire based on EMACs have been systematically studied both experimentally and theoretically. ,− However, for such promising molecular wires, band structures of infinite EMACs are rarely reported, to the best of our knowledge. − …”