Heterobimetallic Complexes That Point to When Bond Dissociation Energies Deviate from Computational Expectations.

Area of Science:

• Organometallic Chemistry
• Computational Chemistry
• Chemical Physics

Background:

• Accurate prediction of bond dissociation energies is crucial for understanding chemical reactivity and molecular stability.
• Density-functional theory (DFT) methods, particularly dispersion-corrected variants like DFT-D3(BJ), are widely used for such calculations.
• Heterobimetallic complexes serve as valuable models for transition states in important catalytic reactions like Sonogashira and Negishi couplings.

Purpose of the Study:

• To experimentally measure the gas-phase bond dissociation energies (BDEs) of d⁸-d¹⁰ metal-metal bonds in heterobimetallic complexes.
• To compare experimental BDEs with predictions from the DFT-D3(BJ) method and identify sources of discrepancies.
• To investigate the influence of ligand interactions and molecular orientation on the accuracy of computational predictions.

Main Methods:

• Measurement of formal, gas-phase, d⁸-d¹⁰ bond dissociation energies using threshold collision-induced dissociation (T-CID) of electrosprayed molecular ions.
• Deconvolution of bond energies from experimentally measured energy-resolved cross sections.
• Evaluation of DFT-D3(BJ) method predictions against experimental data.

Main Results:

• Significant discrepancies were observed between experimental BDEs and DFT-D3(BJ) predictions for several heterobimetallic complexes.
• Experimental BDEs were deemed reliable based on control studies and data deconvolution.
• Discrepancies were attributed not to metal-metal interactions, but to the uneven treatment of nonbonded interactions (medium-ranged correlation and London dispersion) between ligands.
• The accuracy of DFT-D3(BJ) was found to be dependent on the nature of interacting groups (e.g., carbon hybridization) and their relative orientation (e.g., face-to-face vs. edge-to-face aryl-aryl interactions).

Conclusions:

• The DFT-D3(BJ) method, while useful, can exhibit significant errors in predicting BDEs for certain heterobimetallic systems due to inadequate modeling of nonbonded interactions.
• Chemically significant errors in computational predictions arise from the treatment of dispersion and correlation effects between ligands, influenced by their orientation.
• These findings have important implications for the theoretical assessment of structure, stability, and reaction energetics in organic and organometallic chemistry, particularly concerning conformer energies and valence isomers.