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

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 Ocarbonyl, 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.