Structural characterization of surface immobilized platinum hydrides by sensitivity-enhanced ¹⁹⁵ Pt solid state NMR spectroscopy and DFT calculations

Supported single-site platinum hydride compounds are promising heterogeneous catalysts for organic transformations. Few methods exist to describe the structures of single-site Pt catalysts with atomic resolution because of their disordered structures and low Pt loadings. Here, we study the compounds formed when bis(tri-<i>tert</i>-butylphosphino)platinum, Pt(P ^<i>t</i> Bu₃)₂, is supported on dehydroxylated SiO₂ or SiO₂-Al₂O₃. First, we obtain magic angle spinning (MAS) ¹H, ³¹P and ¹⁹⁵Pt ssNMR spectra of four model Pt phosphine compounds with oxidation states of 0 or +2 and coordination numbers between 2 and 4. These compounds are analogs of potential structures present in the supported compounds. MAS ¹⁹⁵Pt ssNMR spectra were obtained using ³¹P{¹⁹⁵Pt} sideband selective <i>J</i>-resolved and <i>J</i>-HMQC experiments. The measured ¹H and ³¹P chemical shifts, ³¹P-¹⁹⁵Pt <i>J</i>-couplings and ¹⁹⁵Pt chemical shift (CS) tensors are shown to be diagnostic of oxidation state and coordination number. Room temperature ¹H ssNMR spectra of Pt(P ^<i>t</i> Bu₃)₂ supported on SiO₂ or SiO₂-Al₂O₃ show diagnostic hydride NMR signals, suggesting that Pt(P ^<i>t</i> Bu₃)₂ undergoes oxidative addition, resulting in surface hydrides and Pt-oxygen bonds to the support surface. MAS dynamic nuclear polarization (DNP) enables ³¹P{¹⁹⁵Pt} correlation NMR experiments on the supported compounds. These experiments enable the measurement of the ³¹P-¹⁹⁵Pt <i>J</i>-coupling constants and ¹⁹⁵Pt CS tensors. Combined NMR and DFT analyses suggest that the primary surface platinum species are [HPt(P ^<i>t</i> Bu₃)₂OSi] on SiO₂ and [HPt(P ^<i>t</i> Bu₃)₂]⁺[Si-O^--Al] on SiO₂-Al₂O₃. The Pt-oxygen bond length is dependent on the support and estimated as 2.1-2.3 Å and 2.7-3.0 Å for SiO₂ and SiO₂-Al₂O₃, respectively.