Dynamic processes of H-atom site exchange in <i>trans </i>dihydrogen hydride complex of ruthenium

Abstract

A new dihydrogcn hydride complex of ruthenium of the type <i style="mso-bidi-font-style:normal">trans-</i>[(dppm )Ru (H)(η²-H₂)(PPh₃)₂][BF₄] (<b style="mso-bidi-font-weight:normal">1</b>) (dppm =Ph₂PCH₂PPh₂) has been prepared by protonating the precursor dihydride complex <i style="mso-bidi-font-style:normal">cis</i>- <i style="mso-bidi-font-style:normal">-</i>[(dppm )Ru (H)₂( (PPh₃)₂] using HBF₄.Et₂O. The fmmulation of (<b style="mso-bidi-font-weight:normal">1</b>) as a dihydrogen complex has been based upon the variable temperature <i>T</i>_{<span style="mso-bidi-font-style: italic">1</span>}<i> </i>measurements (<i>T</i>_{<span style="mso-bidi-font-style:italic">1</span>}<i> </i>(min) = 12.4 ms at 273 K, ClCD₂CD₂Cl, 400 MHz) and the observation of a substantial H-D coupling constant (<i style="mso-bidi-font-style:normal">J</i>(H,D) = 32 Hz, 243 K, ClCD₂CD₂Cl) for the corresponding HD isotopomer <i style="mso-bidi-font-style:normal">trans</i> <i style="mso-bidi-font-style: normal">-</i>[(dppm )Ru (H)(η²-HD) (PPh₃)₂][BF₄]. The <i>T</i>_{<span style="mso-bidi-font-style:italic">1</span>} and the H-D coupling constant measurements have been carried out in the temperature range 243- 283 K. The dihydrogen and the hydride ligands of (<b style="mso-bidi-font-weight: normal">1</b>) show dynamic behavior and undergo rapid H-atom site exchange at<i> </i>343 K. At 273 K (<b style="mso-bidi-font-weight:normal">1</b>) shows a static structure. The dynamics of (<b style="mso-bidi-font-weight:normal">1</b>) involving a trihydride intermediate has been studied by<i> </i>variable temperature NMR spectroscopy. The barrier to site exchange of the H-atom bet ween the dihydrogen with the<i> </i>hydride (∆G^ǂ) has been determined to be 14.4 kcal/mol at 303 K. Compound (<b style="mso-bidi-font-weight:normal">1</b>) has been found to be stable up to 343 K in<i> </i>solution and no loss of the H₂ ligand has been observed at that temperature; in addition, the compound is stable in solution<i> </i>at room temperature for a period of two days.