Tetramethylsilane Coordination to Neutral, Heavier Alkaline Earth Metal Complexes

Area of Science:

• Organometallic Chemistry
• Coordination Chemistry
• Computational Chemistry

Background:

• Synthesis and characterization of bulky xanthene-bridged diamido-barium complex [(EtNONTCHP)Ba(η6-toluene)] 1-Ba.
• Investigation of reactivity of lighter analogues with tetramethylsilane (TMS).

Purpose of the Study:

• To synthesize and characterize novel metal complexes.
• To investigate the coordination behavior of tetramethylsilane (TMS) with alkaline earth metal complexes.
• To theoretically analyze the nature and strength of metal-TMS bonding.

Main Methods:

• Synthesis of xanthene-bridged diamido-barium, calcium, and strontium complexes.
• Solid-state structural analysis.
• Nuclear Magnetic Resonance (NMR) spectroscopy.
• Density Functional Theory (DFT) calculations, including ETS-NOCV and NBO analyses.

Main Results:

• The barium complex (1-Ba) is unreactive toward TMS, while calcium and strontium analogues (1-M) form 1:1 complexes (2-M) with TMS.
• In the solid-state, TMS ligands coordinate in a κ2-H,H-mode; however, NMR studies indicate dissociation in solution.
• DFT calculations show precursor complexes (1-M) are strong Lewis acids, coordinating TMS weakly (ΔG°298K ≈ -6.6 to -6.9 kcal mol−1).
• Bonding analysis in 2-Ca reveals dominant electrostatic interactions (≈68%), with significant contributions from dispersion (≈20%) and orbital interactions (≈12%).

Conclusions:

• Novel alkaline earth metal complexes capable of coordinating TMS have been synthesized and characterized.
• The metal-TMS bond is weak and primarily electrostatic in nature, with contributions from other forces.
• These findings provide insights into the coordination chemistry of bulky metal complexes and weak interactions.