Tetrapositive Hafnium-Diamide Complexes in the Gas Phase: Formation, Structure and Reaction

Abstract

Tetrapositive hafnium complexes in the form of Hf(TMPDA)₃⁴⁺ and Hf(TMOGA)₃⁴⁺ were produced by ESI of acetonitrile solutions of Hf(ClO₄)₄/TMPDA and Hf(ClO₄)₄/TMOGA respectively. Analogous Hf(TMGA)₃⁴⁺ and Hf(TMTDA)₃⁴⁺ were not observed when the Hf(ClO₄)₄/TMGA and Hf(ClO₄)₄/TMTDA solutions were subjected to ESI under similar conditions. Geometry optimizations on these four tetrapositive complexes revealed that the Hf(TMPDA)₃⁴⁺ and Hf(TMOGA)₃⁴⁺ complexes possess C₃ and D₃ geometries respectively with the Hf⁴⁺ center coordinated by nine atoms. Similar geometries were found for Hf(TMGA)₃⁴⁺ and Hf(TMTDA)₃⁴⁺, but both are six-coordinate complexes, which should account for their absence in the gas phase. In addition, no tetrapositive hafnium ion was observed when methanol was used as a solvent instead of acetonitrile. The much stronger affinity of Cl⁻ toward Hf⁴⁺ than ClO₄⁻ should be the reason why tetrapositive hafnium ions were not observed when HfCl₄ was used as the hafnium source. CID of the Hf(TMPDA)₃⁴⁺ and Hf(TMOGA)₃⁴⁺ complexes resulted in the formation of Hf(TMPDA)(TMPDA-H)³⁺ and Hf(TMOGA)(TMOGA-H)³⁺ respectively as the major products. The most stable structures of both tripositive hafnium products arise from the deprotonation of CH₃ cis to O_carbonyl, and the Hf(IV) center in both cases is six coordinate. Compared with the loss of protonated ligand observed in the experiments, it is much higher in energy for either Hf(TMPDA)₃⁴⁺ or Hf(TMOGA)₃⁴⁺ to lose neutral or cationic ligand on the basis of DFT calculations.